~ » a v4 ee ae Ua wet tN ess ¥¢ * x a. a a) a ad ar’, 2 ekg -? 2 . >» 9 me" a ote . Ps ® a ae, Pel “ae wie eo. 7 = pees 4 a a ey ea . oe 3 ee ae oo 2 ey i “he a ¥ rik Oe j ' “a + id “at ar ve ge ivs fas. Sere ’ Pc ae . i * “4 ys “sh + fo!) poe FS GOS te 4 ¥ . * ’ . A we P | “ Lar 4 a a ‘a ae » Wy " as * * * ¥ ] , vie 4 k eae | bSehtoo0 TOEO O ANNAN ANU an /“Q@ILAL mis) Ay 7 ; { an oe wh fis hip GENERAL ZOOLOGY y % & ay / 4G ox é a ~\ J ( # \ i ha os + “Le ~~ | “ wl “i id i= | hee . } - \ wor aa od j \ a 3 ~—t [Za ~ we \E an [oraoA]S H | Oe | Oe | =— (Oe H u 24 STRUCTURE AND ACTIVITIES OF CELLS ‘s]UQ]UOD IL[NIJ20 0} paiejar spunoduros [eotueyd sos Jo seyNUOJ yeINjoONNg “g°% “By apyosponuAjod Ww “Ay it HO—d—O— 1esng — aseg L—~ ify HO—d—O— 1esng — oseg 0 jay HO—d—O— sesng— aseg a (epyoajonu ev) proe oyAuapy “7 (ae3ns v) | iA, ii | proe oroydsoyd 7 i vd | zi (ALV) 278 ydsoydi1y sutsouspy ‘py Van Teere | | | OS d= Oda Od OH ae | | | ! 0 One [0 puog As1eue-ysty N Nw A ee or H—90 | | \ ON EN N a (uorynjos snoanbe a UI paztuor) auTsTH “fF i HS NaH epydedip y “Y : | | Sjeee | | fel Ned! OFF —)—O—d=0 (ptoe ourue yseyduts 343) | | O H HO 4H HOOOC@HN)*HO ‘aust 7 ll 4 H HO H H SAO (eseq ¥) Y re) “OH NSS NESE auruape H—O | | P \ | | See Eee ee + Oe are oa ae | | (Q) ist Isl OH On 25 GENERAL ZOOLOGY and magnesium (Mg'*) are indispensable cations in living systems, and phosphate (HPO,~), bicarbonate (HCO, ), chloride (Cl°), and iodine (I) anions are also necessary for normal function. In addition, traces of iron Gee Fe'*), cobalt (Car): and copper Cu, cations, as well as of a few other metals, are found. Reference will be made to specific ions during the discussion of the functions of cells. Carbohydrates are compounds containing carbon, hydrogen, and oxygen. Most of the carbohydrates can be classified as (1) monosaccharides, the simple sugars; (2) disaccharides, the compound sugars; and (3) polysaccharides, glycogen and the starches. Among the simple sugars are the five-carbon pentoses (C;H,,.O;), which are rare, and the very common_ hexoses (C,H, .O,) such as glucose (Fig. 2.8C’). The fact that the atoms of hydrogen and oxygen occur in the ratio of two to one in these sugars, as in water, 1s the reason for giving the name carbohydrate to the entire group. ‘Two hexoses linked together form a disaccharide (C,,H,,O,,); maltose is formed by the linking of two molecules of glucose (Fig. 2.8D), and sucrose is a com- bination of one molecule of glucose and one of fructose. Polysaccharides are built up of many molecules of a single simple sugar joined, as in the forma- tion of a disaccharide, by the so-called glycosidic linkage (Fig. 2.88); glucose is usually the simple sugar involved. ‘The sugars are soluble in water, but most polysaccharides are not. Carbohydrates are easily oxidized in the cell and are its chief source of the energy utilized for its work. The lipids are a heterogeneous group of organic compounds containing chiefly carbon, hydrogen, and oxygen. ‘These compounds are not soluble in water and are classified as (1) the simple lipids, including the fats and waxes; (2) the compound lipids; and (3) the steroids. Simple lipids are esters; an ester is a combination of an alcohol and an acid. Specifically, a chemically pure fat, such as palmitin, olein, and stearin, is an ester of glycerol and three molecules of a single fatty acid (Fig. 2.84). Natural fats, such as butter, lard, and tallow, are mixtures of chemically pure fats. In the cell, fats are readily oxidized, with the liberation of energy, much of which is transformed into heat. Waxes, which are also simple lipids, commonly occur as secretions of insects and mammals; some oils, such as that of the sperm whale, contain waxes. ‘They are esters of alcohols other than glycerol, to- gether with free alcohols and fatty acids. Among the compound lipids, the phospholipids, such as lecithin which is common in egg yolk, contain phos- phoric acid and a nitrogenous base, such as choline, in addition to glycerol and fatty acids (Fig. 2.8//). Another group of compound lipids known as the cerebrosides, because they are commonly found in nervous tissue, are combi- nations of a fatty acid, a nitrogen-containing alcohol, and a simple sugar. Steroids are crystalline, alcoholic compounds characterized by the presence of the complex phenanthrene ring system (Fig. 2.8H). ‘This group contains cholesterol which is of widespread occurrence, vitamin D, the sex hor- mones, and the hormones of the adrenal cortex, all of great physiological importance. 26 STRUCTURE AND ACTIVITIES OF CELLS Proteins are the most abundant group of organic compounds in the cell. All simple proteins contain carbon, hydrogen, oxygen, and nitrogen; in some, sulfur and iodine occur. Proteins may be conjugated with carbohydrates to form glycoproteins, with lipids to form lipoproteins, with nucleic acids (see below) to form nucleoproteins, with metal ions (Fes Few nC Zn**, Mn**) to form metalloproteins. Proteins are very large molecules with representative molecular weights as follows: insulin, a hormone, 12,000; pepsin, a digestive enzyme, 34,500; egg albumin, 40,000; hemoglobin, a respiratory pigment, 68,000; and myosin, a contractile fiber, 850,000. ‘There are at least 25 different amino acids (Fig. 2.8/), which are combined in varying proportions and sequences to form the uncounted kinds of protein molecules. In the formation of a simple protein, the different kinds of con- stituent amino acids (R-CH-NH,-COOH) are linked together by bonding between the nitrogen atom of the amine group (NH,) and the carbon atom in the carboxyl group (COOH); this is known as the peptide linkage (Fig. 2.8G). Two amino acids so linked are called a dipeptide (Fig. 2.84); longer chains are tri- and polypeptides. These chains are loosely coiled, and adjacent strands are held together by hydrogen bonds to make a very complex chemical structure. Chemists have, however, succeeded in determin- ing the exact kinds and sequences of the amino acids present in a few proteins; the pancreatic hormone insulin is an example. It should be noted that amino acids contain both a basic (NH,) and an acidic (COOH) group. In aqueous solutidn, these groups ionize, and the amino acid becomes an electrically polarized molecule, carrying both a posi- tive and a negative charge (Fig. 2.8J). Although the peptide linkages in a polypeptide chain involve most of the ionizable groups, the terminal NH, and COOH groups ionize when a protein is in solution, as many are within the cell. Depending on the acidity of the solvent, proteins may carry both a positive and a negative charge and behave as though electrically neutral. On the other hand, they may be partially dissociated and behave either as positive ions or as negative ones. The proteins of the cell, together with certain lipids, play a major role in maintaining the morphological features of cells, such as membranes. Different kinds of animals are characterized by specific proteins which endow them with their unique qualities. Proteins are extremely sensitive to changes in their surroundings such as heat and degree of acidity. "They may undergo modi- fication, involving the breaking of the hydrogen bonds of the molecule, which results in loss of capacity to take part in cellular reactions. Nucleic acids are a group of complex organic compounds present in small amounts in the cell. The structural unit of the nucleic acids is the nucleotide, of which only nine different ones have been isolated from the nucleic acids. These nucleotides are composed of a nitrogenous base, a pentose sugar, and phosphoric acid (Fig. 2.8L); they are linked (Fig. 2.8.V) in varying sequence to form the very large molecules of nucleic acid. Depending on the kind of pentose sugar present in the molecule, there are two classes of nucleic 27 GENERAL ZOOLOGY acid. One contains desoxyribose and is known as desoxyribonucleic acid (DNA); it is localized in the nucleus and is of major importance in inher- itance (p. 197). The other contains ribose and is called ribonucleic acid (RNA); this type is basically concerned in the synthesis of proteins, perhaps determining their pattern and thus their quality of specificity. In addition to the function of RNA in protein synthesis, nucleic acids may be conjugated with proteins to form the nucleoproteins; nucleic acid forms the core of the enormous spiraled molecule. It is known that some nucleoproteins, such as those of the chromosomes which contain DNA (p. 39), and virus particles which contain RNA, are capable of self-duplication; indeed, viruses are often considered to be living organisms. In the chromosome and virus it is the nucleic acid moiety which endows the whole with its unique functional quality. Enzymes are proteins which act as organic catalysts. A catalyst is a sub- stance, whether inorganic or organic, that accelerates a chemical reaction without affecting the character of its end products. An enzymatic protein frequently has to have another compound known as a coenzyme combined with it before it becomes effective as a catalyst; some metalloproteins func- tion as enzymes. Enzymes are effective in very small amounts and are unchanged by the reactions in which they participate. ‘The substance that is acted upon by an enzyme is known as its substrate. Enzymes catalyze reactions, reactions that would otherwise proceed very slowly, by becoming associated or combined with some particular region of the substrate mole- cules. The enzyme is an asymmetrical molecule of such a shape that ordinarily it alone can occupy a particular niche in the substrate molecule. A com- pound with molecules that are quite similar to those ,of a particular enzyme can sometimes be used to inhibit or block a reaction normally catalyzed by that enzyme; the inhibitor molecules pre-empt the substrate niches in which the enzyme musi be present if the reaction is to occur. ‘The association of enzyme with substrate apparently facilitates the transfer of electrons involved in the reaction. When the substrate molecule is changed, either broken apart or combined with another molecule, the enzyme is no longer associated. It is free to combine with another substrate molecule and catalyze its reaction. Within the cell, the end products of one enzyme-catalyzed reaction rarely accumulate; instead, they are immediately involved in other enzyme-catalyzed reactions. Expenditure of energy is required to drive certain chemical reac- tions, and in others energy is released. ‘The end products of many enzyme- catalyzed reactions which release energy are involved in reactions which require energy. Such coupled reactions make possible the continuous se- quences which characterize the metabolism of cells. Enzymes exhibit extreme specificity with respect to their substrates. It is obvious that with the multiplicity of chemical compounds occurring in cells there must be a great many enzymes. Enzymes may be thought of as ma- chines carrying out specialized operations in a factory. Each is a part of an integrated enzyme complex that is coming to be recognized as the essence of 28 STRUCTURE AND ACTIVITIES OF CELLS the living state in the cell. The enzymes of the cell maintain the steady state of utilization of food, transfer of energy, and synthesis of cellular components. They control the pace and the pattern of cellular life. Food Food is necessary for the maintenance of the structure and function of cells; it provides the chemical units needed to replace the ones that are constantly being destroyed in metabolic processes and lost by excretion (p. 82). In young organisms growth depends on proper food; and food is the fuel that provides the energy required to do the mechanical, electrical, and chemical work of the organism. If the chemical nature of the cell contents is recalled, it will be evident that food must contain water, inorganic salts, carbohydrates, lipids, and proteins. In addition, a group of compounds known as vitamins 1s necessary. Water, which is the most abundant constituent of the cell and body, is also, with the exception of oxygen, the most important. It is present as such in the so-called solid foods which are consumed and is produced in the cell by the oxidation of organic foods. Water is taken in as such by terrestrial vertebrates, and man supplements his intake with a variety of beverages. Although inorganic salts constitute only a small percentage of the chemical compounds of the organism, they are of widespread importance. Inorganic ions are components of various enzyme systems, of several vitamins and hormones, and of respiratory pigments; they are required for such processes as the conduction of nerve impulses, the contraction of muscle, and the clotting of blood. ‘The diet must contain calcium, phosphorus, sodium, potassium, magnesium, sulfur, and chlorine, which together comprise 60 to 80 per cent of the total inorganic material of the human body. In addition, traces of iron, copper, iodine, manganese, cobalt, and zinc are required. Milk is a source of calcium and phosphorus, both of which are necessary in proper proportion for formation and maintenance of bone. Phosphorus is also exceedingly important in cellular metabolism, as we shall see. Other inorganic substances are obtained from milk, from drinking water, and from plants grown on soils containing adequate amounts of minerals. To determine the energy value of various food constituents, the chemist burns or oxidizes them in a bomb calorimeter; the amount of heat given off in this complete combustion is measured in calories. A calorie is the quan- tity of heat required to raise the temperature of one gram of water one degree centigrade. Some of the energy which is released from food during cellular metabolism (p. 37) is transformed into heat which affects the body temperature. Special heat-regulatory mechanisms in warm-blooded animals, the birds and mammals, control loss of heat and make possible a relatively constant body temperature (p. 125). 29 GENERAL ZOOLOGY The hexose carbohydrates are the most efficient source of energy available in the diet. Glucose in excess of the immediate requirements of cellular metabolism can be built up into the polysaccharide glycogen and stored for future use. Some of the polysaccharides, such as the cellulose occurring in plants, cannot be digested by man and comprise part of the roughage which gives necessary bulk to the feces. Lipids in the diet provide a major source of energy for the animal. When oxidized, fats yield over twice as much energy per unit weight as other foods. Also, the oxidation of fat releases about twice as much water as other food- stuffs. This is an important factor in providing some animals with a supply of water under adverse conditions. For example, the desert-dwelling camel stores much fat and has a high level of fat metabolism. Also, the developing chick embryo, shut off for 21 days from supplies outside its eggshell, gets its necessary water from the oxidation of the lipid which makes up about 90 per cent of the dry weight of the yolk of the hen’s egg. About 1 per cent of the dietary lipid must contain what are called essential fatty acids, of which there are three. ‘These are unsaturated fatty acids found in many food oils. In their absence, various skin and nervous disorders may occur in man; the cellular basis for these disturbances is not known. Both carbohydrates and proteins can be converted to fat in cellular metabolism, and such excess or reserve food is stored in cells as fat. Proteins are an indispensable part of the diet for young and old. Growth and restoration of body proteins lost in daily activities can be accomplished only when a supply of amino acids is available. Of the 25 amino acids known to be constituents of proteins, only 8 are essential for human nutrition. ‘This means that they must be supplied to man in his dietary protein because he is unable to synthesize them from other foodstuffs at a rate necessary for normal function. It is important that all the essential amino acids be provided in the required amounts at approximately the same time. If all the amino acids required for the synthesis of a particular protein are not simultaneously present, those that are present will be degraded and lost from the so-called protein pool. Amino acids, whether essential or non-essential, cannot be stored in cells. Protein foods are not equally efficient in supplying essential amino acids; eggs, dairy products, liver, and kidney contain all of them. Vitamins are accessory dietary components which play important roles in cellular metabolism, although required in very small amounts. Members of this heterogeneous group of organic compounds are not sources of energy, nor are they used to replace worn-out cellular components. Instead, they make possible the utilization of other foodstuffs in normal metabolic sequences; some have been demonstrated to be coenzymes. ‘Thus, inadequate amounts of vitamins may result in metabolic defects which in time give rise to clinically recognizable diseases. To be classified as a vitamin for a particular kind of animal, the substance must be one which the animal requires in its food. This definition differen- tiates vitamins from hormones and enzymes, which also act in very small 30 STRUCTURE AND ACTIVITIES OF CELLS quantities but are produced in the body. When a certain compound is stated to be a vitamin, it is not to be inferred that all animals require it in their food. Certain animals can synthesize a compound which is a specific require- ment for maintenance or growth, for example, or they may have bacteria living in their digestive tracts which can synthesize it. Other kinds of animals must obtain this same substance ready-made in the food they eat; for them, it is a vitamin. Vitamins are detected in studies of the specific nutritive re- quirements of different species of animals; rats are widely used in such labora- tory studies. When these nutritional substances were first discovered, they were classified on the basis of their solubility in fats or in water. Letters were used for identification since their chemical structure was unknown. At pres- ent, the chemical formulas of all the commonly known, vitamins have been determined. They may be designated now by names which either describe the compound chemically or indicate its source or major function. The fat-soluble vitamins include those designated as A, D, E, and K. In each, two or more closely related chemical compounds have been found to give the effect originally assigned to a single vitamin. Deficiencies of the fat-soluble vitamins may result either from an inadequate diet or from a dis- turbance of the mechanism for fat absorption from the intestine. Vitamin A is found only in the animal kingdom. It may be obtained, pre- formed, from milk, butter, egg yolk, and fish-liver oils. A precursor, or provitamin, called beta-carotene, is present in green and yellow vegetables. This compound can be converted to vitamin A by the animal. ‘The physio- logical activity of vitamin A is concerned with the maintenance of the epithelia of exposed surfaces, such as epidermis of the skin and cornea of the eye. It also participates in the synthesis of visual pigments such as rhodopsin, or visual purple, a pigment of the retina of the eye necessary for vision, especially in dim light (p. 107). Deficiency of vitamin A results in excessive keratinization of the skin and cornea so that they become dry and horny, and in varying degrees of night blindness. Vitamin D is a group name for at least ten different steroids, of which two, D, and Ds, are especially important to man in the development of teeth and bone. ‘These vitamins increase the intestinal absorption of calcium and phosphorus and are required for the actual deposition of these substances in bone. Sources of this vitamin are fish-liver oils, butter, liver, and egg yolk. The content of vitamin D in milk can be increased by exposure to ultraviolet radiation which brings about the transformation of a precursor. ‘The same kind of reaction occurs in human skin when it is exposed to sunlight. A deficiency of vitamin D results in rickets in children and in a similar disorder in adults, especially older ones. In each age group the bone is inadequately calcified, and deformities may result. Vitamin E, another group of compounds, is known as the antisterility vitamin and is necessary for normal reproductive functions in the rat. The male be- comes sterile in the absence of vitamin E; the female is unable to maintain the placentas for the nourishment of the embryos, and they are aborted. An 31 GENERAL ZOOLOGY entirely different result of deficiency of E vitamins in young rats and in rabbits and guinea pigs is degeneration of skeletal muscle, or muscular dystrophy. The E vitamins are chemically known as tocopherols and are available in green leafy vegetables, whole wheat, and egg yolk. Up to the present time, a requirement for tocopherols in human nutrition has not been established. Vitamin K, the antihemorrhagic vitamin, is involved in an unknown way in the production of prothrombin in the liver and is, therefore, necessary for the normal clotting of blood (p. 64). A dietary deficiency of this vitamin, which is a naphtho-quinone, is unlikely since it has wide distribution in foods. Good sources are green leafy vegetables, tomatoes, cheese, egg yolk, and liver. In addition, bacteria normally present in man’s digestive tract synthesize vitamin K, which is then absorbed. Certain sulfa drugs and other intestinal antiseptics may cause a deficiency by reducing the number of bacteria which produce the vitamin. Also, if absorption is interfered with, deficiency may result. In the absence of vitamin K, the amount of prothrombin in the blood is reduced and clotting time is prolonged. A deficiency of vitamin K occurs in newborn infants, since the intestine is sterile and the amount of the vita- min supplied by the mother is small. Sometimes bleeding may be severe, or even fatal. Some physicians routinely give supplementary vitamin K to expectant mothers in order to prevent hemorrhagic episodes in the newborn. The first water-soluble vitamin to be identified was named vitamin B. It was soon realized that the effects attributed to vitamin B were related to a number of different substances, all of which were in some way growth-promoting. In other words, there was not a single B vitamin but, instead, what is referred to as the B complex. We shall consider some of the better-understood com- pounds in the complex, all of which are available in crystalline form. Thiamine, or vitamin B,, was the first member of the B complex to be identified. After its isolation from rice polishings, it was synthesized in 1936. ‘Thiamine is fairly widely distributed in foods, especially in whole grains, legumes, lean meat, and yeast; however, only small amounts are present in any source. .Thiamine deficiency is generally characterized by loss of appetite and consequent malnutrition. Later, neurological symptoms and dysfunction of the heart and other organs occur. It has been clearly shown that thiamine pyrophosphate is involved in the cellular metabolism of carbo- hydrate, functioning as a coenzyme called cocarboxylase (p. 36). Riboflavin, lactoflavin, vitamin B, or G, is necessary for the growth of many animals, including man, and for the preservation of health in the adult. This vitamin occurs in yeast, milk, eggs, liver, and some vegetables. Riboflavin deficiency in man is widespread and gives rise to soreness at the angles of the mouth, inflammation of the tongue, and tissue damage in several parts of the eye. The physiological role of riboflavin is correlated with its union with phosphate to form flavin nucleotides which then combine with proteins. These complex flavoprotein molecules have multiple functions in oxidative metabolism as assistants to a number of enzymes (p. 37). 32 STRUCTURE AND ACTIVITIES OF CELLS Nicotinic acid, niacin, or the antipellagric vitamin, protects man against the disease called pellagra, which was formerly very common in the Southern states and found throughout the nation. Pellagra is characterized by skin lesions, digestive disturbances, muscular weakness, and progressive impair- ment of the nervous system, often ending in insanity. In 1914 it was proved a dietary-deficiency disease. It has since been shown that pellagra probably results from scarcity of several of the B-complex vitamins, including thiamine and riboflavin. Education regarding proper diet and the medicinal use of nicotinic acid have reduced the incidence of the disease. Other animals exhibit pellagra-like symptoms; in dogs, the condition is called blacktongue. Niacin is abundant in fresh meat, liver, yeast, milk, eggs, and fish. This vitamin also functions in cellular oxidations, forming two coenzymes: co- enzyme I, or diphosphopyridine nucleotide (DPN), and coenzyme II, or triphosphopyridine nucleotide (TPN). ‘These are involved in several different steps in carbohydrate metabolism (p. 36), and DPN is essential for vision (see Fig. 4.16, p. 108). Pyridoxine, vitamin B,, was first found to be a dietary requirement for rats in which it prevented dermatitis. It was later shown to be essential for man, also, even though a deficiency disease for this vitamin has not been identified. Yeast, whole grains, egg yolk, milk, and liver are good food sources for pyridoxine. It plays an important role in various enzymatic reactions in amino acid metabolism in the cell. Pantothenic acid is so widely distributed in food that, even on a restricted diet, man apparently gets enough; a deficiency syndrome is not known. In rats, mice, and chickens, however, deficiency leads to dermatitis and wide- spread disorder in other tissues, suggesting a basic function in cellular metabolism. Pantothenic acid contributes to the formation of an exceedingly important compound known as coenzyme A. ‘This coenzyme takes part in the metabolism of carbohydrates, lipids including steroids, and proteins (p. 36). Biotin, or vitamin H, is necessary for the growth of various birds but is not a dietary requirement for mammals, in which it is supplied by the intestinal bacteria. ‘The feeding of raw egg white to mammals produces a _ biotin deficiency because the egg white combines with the biotin and renders it ineffective as a vitamin. Diarrhea, dermatitis, and nervous disorders are symptoms of biotin deficiency. A group of vitamins of the B complex is designated the folic acid group; all are related to pteroyl glutamic acid. Folic acid is essential for growth in higher animals. Fresh green leafy vegetables, kidney, and liver are rich food sources. Intestinal bacteria also contribute to the supply of folic acid. Con- sequently, administration of certain sulfa drugs which act directly on the intestinal flora may contribute to a deficiency. Anemia is the most obvious defect in animals with a folic acid deficiency. Knowledge of these growth substances has contributed to an understanding of the mechanism of action of a group of drugs, the sulfonamides, in com- bating bacterial infection. The various sulfonamides resemble para- 33 GENERAL ZOOLOGY aminobenzoic acid (PABA) which is a requirement for the synthesis of folic acid by some of the pathogenic bacteria. When the sulfa drugs are present, they block the utilization of PABA by the bacteria; folic acid is not syn- thesized, and the bacteria cease to grow. Since animal cells do not use PABA, they are not damaged by the drug. Vitamin B,, is a unique type of chemical compound, characterized by its complex ring structure and containing one cobalt atom and a single cyanide group; it is known as cyanocobalamin. It was isolated in 1948, subsequently synthesized, and its molecular structure reported in 1955. Vitamin B,, is found chiefly in foods of animal origin, where it is present in very low con- centration. Intestinal microorganisms synthesize B,,, which can then be absorbed. It has several functions, some in conjunction with folic acid, and participates in a variety of fundamental metabolic reactions. It also has a specific effect on the formation of blood cells and is often called the anti- pernicious anemia vitamin. Injections of B,, in patients with pernicious anemia usually produce dramatic relief of the anemia, as well as of the neuro- logical symptoms of the disease. In addition, vitamin B,, is essential for the growth of young mammals. Another type of water-soluble vitamin is ascorbic acid, vitamin C, or the antiscorbutic vitamin. It appears to be a dietary requirement for only a few animals, but man is one of them. Deficiency of ascorbic acid results in scurvy, characterized by bleeding through capillary walls. The cellular func- tion of ascorbic acid remains unknown, except that it participates in some way in the formation of intercellular material. Citrus fruits and tomatoes are rich sources of this vitamin, and it is abundant in paprika and in the cortex of the adrenal gland. In the light of modern investigations on the nutritional requirements of animals, it becomes evident that in order to secure a proper balancing of the diet we must do more than obtain a certain ratio between amounts of carbo- hydrates, lipids, and proteins. It is essential that the proteins contain the amino acids needed by the cells, that the inorganic constituents be present in proper concentrations, and that vitamins be furnished. Abnormal func- tions of the body conditioned by food inadequate in vitamins and minerals are serious factors in human welfare. On the whole, however, the very numerous cases of borderline malnutrition, predisposing the individual to various diseased conditions and allowing him to maintain only a low level of physical and mental efficiency, are of more importance to society as a whole. Every individual should eat abundantly of the protective foods containing vitamins. It is characteristic of the nutrition of animals, with only a few exceptions (p. 242), that they cannot build their foods from the constituent chemical elements but must make use of compounds produced by the cells of other animals or of plants. The products of metabolism of one kind of organism are not ordinarily usable, as such, by another type of organism. ‘Therefore, food, in addition to possessing the necessary qualities which have been dis- cussed, must be utilizable by the animal eating it. ‘The chemical processes 34 STRUCTURE AND ACTIVITIES OF CELLS of digestion which render food usable in cellular metabolism will be discussed in the following chapter. Cellular Metabolism Food is the fuel of the cell; the energy stored in its chemical bonds must be released by chemical reactions in order that the work of the cell can be done. ‘The release of energy during cellular metabolism does not occur as the result of a vigorous reaction of burning, as in a calorimeter, during which the energy is dissipated as heat. Instead, the release of free chemical energy during metabolism occurs by the gentle degradation of organic molecules in an orderly and conservative manner. The handling of food molecules in this way is a very complex process. Long chains of reactions, each catalyzed by a different enzyme, are necessary to break down, bit by bit, the organic com- pounds of food. We shall consider, in as simplified a way as possible, some of the essentials of dynamic cellular chemistry in order that you may gain a better understanding of the metabolic processes common to all living cells. Some of you will not have sufficient knowledge of chemistry to appreciate all the implications of the following account. You should, however, be able to follow the main line of the presentation and gain some conception of the intricate precision which characterizes life at the cellular level. If the ma- terial is too difficult for you, your teacher may suggest that you omit this section. It is necessary to recall that in some chemical reactions energy is liberated when a bond is broken and in others energy must be used to break a bond. There are a few compounds that carry unusually large amounts of energy in certain of their bonds; the breaking of such high-energy bonds releases free energy which can then be used to drive reactions requiring that work be done. This relationship implies that the compound carrying or storing energy must constantly be replaced in order that the chemical sequences of cellular metab- olism can continue. ‘There is one exceedingly reactive compound that sup- plies the great amounts of free chemical energy required to do the work of the cell. This is adenosine triphosphate, known as ATP (Fig. 2.84); when its terminal (third) phosphate bond is broken, nearly 12,000 calories of free energy become available for use, per mole of ATP. Such a bond is known as a high-energy phosphate bond. Carbohydrate has already been identified as the chief source of fuel for the cell. However, carbohydrates do not react directly with oxygen in the cell, nor do they immediately break down into carbon dioxide and water. ‘The production of these two waste products of cellular metabolism occurs in the final phase of the metabolism not only of carbohydrate but of lipid and pro- tein, as well. Either glycogen, the stored polysaccharide carbohydrate made up of many glucose molecules, or glucose itself can be used by the cell. The steps in carbohydrate metabolism are subdivided into two groups, dependent 35 GENERAL ZOOLOGY upon the absence or presence of oxygen; these are the anaerobic (without oxygen) phase and the aerobic (with oxygen) phase. Considering first the anaerobic phase, most of the reactions involve addi- tion, subtraction, or rearrangement of phosphate groups in the molecule (Fig. 2.9). The first step in carbohydrate metabolism is the addition of a phosphate group to glucose. ‘This reaction is called a phosphorylation and is catalyzed by a specific enzyme. In the course of the reaction, ATP donates its terminal phosphate group to the glucose, so that the resulting hexosephosphate isa high-energy compound. Similarly, another phosphate group from another ATP molecule is added to form a hexose diphosphate. ‘This six-carbon com- pound is then split, under the influence of another specific enzyme, to yield two molecules of a three-carbon compound designated as a triose. Each of these contains one high-energy phosphate bond and next obtains a low-energy phosphate group from the inorganic store. Under the influence of a different enzyme, two hydrogen atoms are removed from the triose diphosphate. This reaction is known as a dehydrogenation and occurs only when coenzyme I, or DPN (p. 33), is present to accept, or combine with, the released hydro- gen atoms. As a result of changes occurring in the molecule during the dehydrogenation, it now contains two high-energy phosphate bonds. In a series of reactions the three-carbon compound is converted to pyruvic acid which is the typical end product of anaerobic carbohydrate metabolism. However, some cells are capable of modifying pyruvic acid under anaerobic conditions. For example, muscle cells form lactic acid from it, and yeast cells convert it to alcohol. During the conversion of each molecule of glucose to two molecules of pyruvic acid, some free energy is harnessed by means of coupled reactions (p. 28). ‘Thus, four molecules of ATP are formed during the series of steps which make use of energy from two molecules of ATP to run the reactions; the cell has made a net gain of two ATP molecules. If glycogen, stored in the cell, is the starting material, only one molecule of ATP is required to drive the anaerobic phase so that the net energy gain is represented by three molecules of ATP. During the aerobic phase of carbohydrate metabolism, pyruvic acid is broken down into carbon dioxide and water, and the bulk of the energy originally stored in the hexose molecule is released in small packets. The first step involves removal of one molecule of carbon dioxide from the pyruvic acid to form acetic acid, a two-carbon compound; this enzymatic reaction requires the coenzyme cocarboxylase (p. 32). Next acetic acid enters into combination with coenzyme A (p. 33), in the presence of ATP, to form acetyl coenzyme A. Only then does the carbohydrate food material enter the series of reactions known as the tricarboxylic acid (TCA) cycle which is the pathway for oxidation of not only carbohydrates but lipids and proteins, as well. The reactions of the TCA cycle involve formation of citric acid, a six- carbon acid, from acetyl coenzyme A and a four-carbon acid, oxaloacetic 36 ao ©. so MS sas ATP— ADP Giteetinacel Phosphoglucomutase Carbohydrate { Glucose Glucose -6—PO,(~@) Glucose —1-PO, Phosphohexoisomerase Fructose -6 —PO,(~@®) Phosphohexokinase ATP—> ADP Fructose —1, 6-diPO, (~®) ~@®) Aldolase 2 mols. 3-phosphoglyceraldehyde (~@)<——— Glycerol <——_ Lipids Dehydrogenase DPN Dehydrogenation oH H3PO, 1, 3-Diphosphoglyceric acid (~@) Transphosphorylase ADP—> ATP. 3-Phosphoglyceric acid Phosphoglyceromutase 2-Phosphoglyceric acid \ noes 2H30 Enol phosphopyruvic acid (~@) Transphosphorylase ADP—> ATP DPN Yeast Muscle Alcohol dehydrogenase carboxylase ; dehydrogenase : Ao —— Acetaldehyde Satna Pyruvic acid Lactic acid 2 Carboxylase a K coe arboxvlase Decarboxylation co, ! Proteins ————> Some amino acids a oe acid Hg Acetylkinase Coenzyme A ATP—> ADP Fi id cycle” ; Ee Acetyl ~ coenzyme A palaliyacidioysleme Fatty acid <———— Lipid Coenzyme A NH3 Proteins ————> Some amino acids ——2—> Oxaloacetic acid Citric acid Malic oe’ dehydrogenase Z DPN eet ele 2H Malic acid Cis-aconitic acid Cytochromi System Ws 2 Fumaric acid Teseyeis Isocitric acid of Succinic Isocitric dehydrogenase dehydrogenase 2H Hydrogen TPN id i - ae Succinic acid Oxalosuccinic acid lectron OREN O2) Transfer 2H20 7 me SSA NE 7 2 a-Ketoglutaric acid Some amino acids <————— Proteins Kis Phosphorylase HPO, Glycogen {Carbohydrate KEY: Enzymes Coenzymes High-energy bonds or compounds Low-energy compounds Atoms Kind of process Organic molecules Fig. 2.9. Abbreviations: ADP -adenosine diphosphate ATP -adenosine triphosphate COz -carbon dioxide DPN -diphosphopyridine nucleotide H_ -hydrogen H,0 -water H3PQ, - phosphoric acid NH3 -ammonia @® -high-energy phosphate TPN -triphosphopyridine nucleotide ~ -high-energy bond Schema of cellular metabolism. The enzyme, coenzyme, and other necessary factors are known for each step in the sequence, but only a few of them are shown in this simplified presentation. : wre : — BYE ne 7 oar oi - we ; ay ae poate Gi ie i a i uA a TE Bir ire co. Eaves + pense (i Oe 4 Aas Li é ts ‘Are a ere Vagii< °¢¢2 9 \llinn a " oe, eines, | ty a 17) i FeO - Aw het Ao eee cyt 4 ag 7 i se zo VET val 4 avy S904) @5y'7 afi > 5 = a ets ap es no } ree? rr ere & cdl (ait fri a 2 \; : ' ae 4 il ide Fy se GCasre Dis in bit fares mT es il t a ‘ OSes tay They array 1 jMrewart yal & id Te: Fe VAG Pero kee ay ve? (o ace Arey eee ( | . om Vite it ote HRA ne ; é @iiencnal? Wott Gate > {Ren aaah oi Te iia Ts c ; -_ oe im ee wee ees iu ; image el 5 ay wre ) : a 25 ahi r en! ‘§ : yh ak ot J iotte Vere 8 ii 2?) | ) ae. 4 ie ] we _ nt oe - ra fe _ | s/ i | rf , - oa ae Vita oad hae araieqnbyialt Tints , en é ; = es T haigals, | c L a) Cn see = bad : La 0 ba ‘ e@ 4 / | ; ; = le lp om git’ | ome { : a . STRUCTURE AND ACTIVITIES OF CELLS acid. This four-carbon acid is an essential component of the TCA cycle and must be reformed to prime the system for the oxidation of a second two- carbon molecule. In the series of reactions occurring during each turn of the TCA cycle, two more molecules of carbon dioxide are split off. Such reactions are called decarboxylations and are coupled with dehydrogenations. The hydrogen atoms removed in these reactions become associated with what are known as carrier compounds which relay them through the flavoprotein (p. 32) and cytochrome transfer system to a final reaction with oxygen to form water. These reactions are referred to as biological oxidation, or cellular respiration, because in order to proceed they require oxygen. Dur- ing the reactions leading to complete oxidation of one two-carbon compound in the TCA cycle, free energy is stored in at least 16 molecules of ATP. A better appreciation of the efficiency of the cellular mechanisms for the release and subsequent storage of energy can be gained by consideration of the following balance sheet for carbohydrate metabolism. Combustion of glucose in a bomb calorimeter produces the end products carbon dioxide and water and releases energy in the form of heat: 1 glucose (C,H ,,O,) + 6 oxygen (O,) ——> 6 carbon dioxide (CO,) + 6 water (H,O) + 673,000 calories of energy (heat) In the cell, as a result of anaerobic sequences: Enzymes 1 glucose ——} 2 pyruvic acid + 58,000 calories of free energy, ATP of which 48,000 calories are stored in four gram molecules of ATP. The aerobic reactions, on the other hand, release a far greater amount of energy: Enzymes 2 pyruvic acid + 10 oxygen (O,) ——> 6 carbon dioxide (CO,) ATP + 4 water (H,O) + 420,000 calories of free energy, of which 384,000 calories are stored in 32 gram molecules of ATP. During the metabolism of a mole of glucose, approximately 480,000 calories of its total of 673,000 calories of energy are stored in ATP and thus made available for the work of the cell. The operating efficiency of the cell is, therefore, about 71 per cent; for comparison, the efficiency of modern steam turbines approaches 50 per cent. The basic units of protein and lipid foodstuffs are also metabolized in the cell in such a way that their intrinsic energy is progressively released in small amounts and much of it stored in ATP until needed. In both cases, the derivatives of initial breakdown reactions are channeled into the ‘TCA cycle for complete oxidation. ‘The processing of amino acids, the units of protein foods, involves their deamination (loss of their —NH, groups, Fig. 2.8/) 37 GENERAL ZOOLOGY followed by direct conversion to certain of the intermediates of the metabolic sequences of carbohydrate metabolism. ‘The —NH, groups are converted into ammonia (NH), the nitrogenous waste product of cellular metabolism. On the other hand, the fatty acids derived from lipid foods are initially handled in a more complex manner. ‘The fatty acid, which is a long-chain hydrocarbon molecule (Fig. 2.8/), is degraded in the fatty acid cycle which can be compared to a spiral staircase. First, the fatty acid must be “‘acti- vated” by combining with coenzyme A in the presence of ATP. Then, on each turn of the staircase, a two-carbon fragment complexed with coenzyme A and known as acetyl coenzyme A is removed from the chain. The remaining part of the fatty acid molecule is again activated by another molecule of coenzyme A, and another molecule of acetyl coenzyme A is removed. ‘This process is repeated until the entire fatty acid is degraded. Each of the molecules of acetyl coenzyme A enters into the TCA cycle for final oxida- tion. These reactions of cellular metabolism which have been described occur in animals from protozoa to man, and in plants as well. Compounds resulting from different preparatory reactions of the carbohydrates, lipids, and proteins eventually enter the TCA cycle. Here oxidation occurs, and the waste products carbon dioxide and water are formed. Cellular metabolism includes the synthetic mechanisms of anabolism as well as the degradations of catabolism. In the cell, some of the reactions described for the degradation of food compounds may be reversed, under appropriate conditions, with the formation of a new compound necessary for the living organism as the end result. Enzymes, secretions, nucleic acids, characteristic amino acids and fatty acids as well as the proteins, lipids, and compound carbohydrates are built up or synthesized in cells. Particularly important for many synthetic reactions is coenzyme A (p. 33) and, of course, ATP. Cell Division When cells were first discovered, it was thought that they arose sponta- neously by a sort of crystallization. ‘The nucleus was interpreted by some early investigators as a new cell in the process of formation. As the micro- scope was perfected and more observations were made, new cells were found to be formed as a result of the division of previously existing cells, and in no other way. Periods of division alternate with periods during which the cell is said to be in the vegetative or nutritive stage, that is, when it 1s either growing or maintaining itself as a functional unit of the organism. The cell is sometimes referred to, at this time, as a resting cell, but no designation could less adequately describe it during this period of metabolic activity. After a cell has reached a certain size, it may divide. Whether or not cell size is the only factor conditioning cell division, it is certainly a very important one. ‘The division of the cytosome is always preceded by 38 STRUCTURE AND ACTIVITIES OF CELLS division of the nucleus, which may occur by the method of amitosis or by mitosis. Amitosis. In amitosis, or direct nuclear division, the nucleus becomes somewhat elongated and constricts into two parts which are about equal in volume. The nuclei of certain types of cells may divide amitotically without division of their cytosomes and thus give rise to multinucleate cells. How- ever, the cytosome may divide after the nucleus is constricted, and two new cells are formed. ‘The distribution of nuclear components is only approxi- mately equal in this direct process of division. Such a type of division apparently occurs most often in cells that are very specialized, very old, or in some abnormal or degenerating condition. Mitosis. The typical method of nuclear division is by mitosis. It is called the indirect method because it involves changes that are more complicated than the simple constriction of amitosis. ‘The process of mitosis, which was first fully studied in animal cells by Walter Flemming in 1878, is divided for purposes of description into four continuous stages: prophase, metaphase, anaphase, and telophase. The general structure of a vegetative cell should be recalled, with particular reference to the cell center and the nucleus (p. 21). In some cells there are two centrioles during the vegetative phase; in others, only one. For this account let us consider a cell in which two centrioles are present. Among the earliest changes to occur in the prophase of mitosis is the separation of the two centrioles toward opposite sides of the nucleus (Fig. 2.10). At the same time delicate fibers become visible about the centrioles in the region of the cell center. ‘The fibers that stretch between the centrioles as they move apart are known as the spindle fibers, since they converge toward the centrioles in a typical spindle formation. Fibers called astral rays extend freely from each centriole into the surrounding cytoplasm. ‘The structure formed by the fibers and the centrioles is known as the mitotic spindle, because of the arrange- ment of the fibers that pass from one centriole to the other. The source of the fibers and their exact nature are not clear, but the reality of the mitotic spindle is indisputable (Fig. 2.7B); by suitable methods it can be isolated from the cell. While the mitotic spindle is being formed in the cytosome, delicate chromatic threads appear in the nucleus and are seen to be double; that is, two threads are found close together. Around these two genonemata, so-called because they are apparently made up of the linearly arranged hered- itary units or genes (p. 190), intensely staining chromatic material accumu- lates. These elements shorten and thicken forming chromosomes (Fig. 2.10). Chemically the chromosomes are known to contain desoxyribonucleic acid (DNA) and characteristic proteins. When chromosomes are gently disso- ciated, they are demonstrated to be composed of macromolecular granules which may be the genes, or clusters of them, held together by ionic bonds (p23) Chromosomes may be different in size and shape in the cells of an organ- ism; that is, round chromosomes and straight and bent rods occur. ‘The shape 39 GENERAL ZOOLOGY Cell center Centriole Nuclear membrane Nucleolus Vegetative Cell Prophase Prophase Astral rays Spindle fibers Sister Chromosome half-chromosomes Prophase Metaphase Anaphase Cell membrane Cytosome EV) (= ‘ Anaphase Telophase Telophase Daughter Cells Fig. 2.10. Mitosis and cell division in animal cells; diagrammatic. 40 STRUCTURE AND ACTIVITIES OF CELLS Center NS Wie Chromosome Fig. 2.11. A, structure of a chromosome; diagrammatic. Kinetochore Titer oeal B, relation of chromosomes to 7 eee the mitotic spindle; diagram- Pellicle matic. (Redrawn from F. Continuous Schrader, Mitosis, copyright Genonema fiber 1944 by Columbia University Y Press, printed by permission. ) é Matrix Chromosomal ° fiber ais A B of a rod-like chromosome depends on the position of the kinetochore, a faintly stained region of the chromosome that becomes associated with a spindle fiber (Fig. 2.11). The kinetochore is capable of duplication and resembles the centriole in a number of respects; its structure in electron micrographs has not been reported. If the kinetochore is near the end of a chromosome, it appears straight; a subterminal or median position of the kinetochore pro- duces a J- or V-shaped chromosome. No matter what the shapes and sizes of the chromosomes are, we find that there are two of each kind as they become fully condensed toward the end of the prophase. Two chromosomes that are alike in shape and size are known as homologous chromosomes. ‘The total number of chromosomes visible at the end of the prophase is the diploid number characteristic of any species. In any given kind of animal or plant, the same number of pairs of chromosomes will be found in all the cells of the body, with the exception of the mature germ cells (Fig. 2.12). In some species the males have one fewer chromosomes than the females; that is, one chromosome is unpaired. This same kind of chromosome occurs as a pair in the female. Chromosomes that differ in number in the two sexes are known as sex chromosomes (p. 209); the other chromosomes, of which there are two of each kind in both sexes, are called autosomes. Coincident with the forma- tion of the mitotic spindle and the condensation of the chromosomes, the nuclear membrane begins to disappear, first in the region next to the spindle, and the nucleolus is lost to view. ‘The chromosomes take up a position on the spindle midway between the centrioles to form the equatorial plate; at this time a spindle fiber connects the kinetochore with the centriole (Fig. 2.11). These changes mark the end of the prophase. Observations on certain kinds of living cells growing in a nutrient medium (tissue-culture conditions) indi- cate that the changes of the prophase take about 8 minutes to occur. 4] F Fig. 2.12. Chromosomes from various animals. A and B, an o6gonium and a spermatogonium, respectively, of the bug, Protenor, showing the two X-chromosomes of the female and the single X-chromosome of the male, as well as the 6 pairs of homologous autosomes; homologous members of the pairs are numbered alike: x3600. The diploid number of chromosomes in Protenor is 14 in the females and 13 in the males. C, two spermatids of Protenor, one of which has re- ceived the X-chromosome at the disjunctional division; each spermatid has one chromosome from each of the pairs of autosomes: <2900. The haploid number of chromosomes in Protenor is 6 in one-half of the spermatozoa, 7 in the other half of the spermatozoa and in all of the ova. D and E, female and male diploid groups from Drosophila in which the chromosome number is the same, 8, because the male contains a Y-chromosome: 5500. F, chromosomes of an odgonium of the frog, R. pipiens, showing 26 as the diploid number: x3000. G, chromo- somes of a human spermatogonium, showing 48 as the diploid number; the Y-chromosome is labeled: «3480. (D and E, modified from C. B. Bridges, 1916, Genetics, vol. 1; F, from C. L. Parmenter, 1925, Journal of General Physiology, vol. 8; G, from H. M. Evans and Olive Swezy, 1929, Memorrs of the University of California, vol. 9.) 42 STRUCTURE AND ACTIVITIES OF CELLS The succeeding metaphase is the stage at which the longitudinally doubled chromosomes are arranged on the equatorial plate (Fig. 2.10). “The duality that is such a conspicuous characteristic of a metaphase chromosome results from the separation of the matrix after the genonemata have separated from one another in the middle prophase. Each of the sister half-chromosomes in the equatorial or metaphase plate contains one of the two genonemata found in the prophase chromosome. ‘The doubling of the chromosome in such a way that sister half-chromosomes have equivalent genonemata is of great theoretical significance (p. 190). The mechanism of distribution of chromo- somes is regarded as a mechanism for the equal distribution of genes. The beginning of the anaphase is indicated by the separation of the halves of each chromosome, a moving of one half-chromosome toward each centriole or pole of the spindle (Fig. 2.10). What produces this movement is still un- known, but it is initiated in the region of the kinetochore, now reduplicated. As the half-chromosomes move toward the poles, careful examination reveals that each contains two genonemata. ‘This fact explains why two genonemata occur in each prophase chromosome; the genonemata persist from one period of division to the next. The exact time and the mechanism of replication of the genonemata remains unknown. It can be stated, however, that one genonema does not split to form two half-genonemata, each of which then reforms the part it has lost. Instead, each genonema in some way serves as a template upon which an exact duplicate is synthesized. When it is completed, this new genonema becomes free from its pattern. As the chromosomes near the poles of the spindle, they come to lie very close to one another. ‘This marks the beginning of the telophase, during which a new or daughter nucleus is formed from each clump of chromosomes (Fig. 2.10). A nuclear membrane appears at the periphery of the chromo- some group at each pole, and the members of the group begin to separate. The chromatin progressively loses its capacity to stain, but not uniformly; the thread-like genonemata stain after the matrix will no longer react with a dye. Thus, the nucleus of a vegetative cell is formed, and one or more nucleoli soon make their appearance. ‘The centriole, meanwhile, has reduplicated, and the spindle fibers begin to disappear. While the telophase of nuclear division is occurring, constriction of the cytosome takes place in the plane of the equatorial plate of the mitotic spindle. When constriction of the cytosome is complete, cell division is finished; the entire process requires somewhat more than 30 minutes for its completion. ‘Two cells have been formed from one by a complicated process, the most important aspect of which is the manner of distribution of the half- chromosomes, each containing equivalent genonemata. Each new cell gets exactly the same kind and amount of chromatic material. ‘The essential significance of mitosis is the equal qualitative and quantitative distribution of the hereditary material. When the daughter cells enter the vegetative phase, they soon grow to the size typical of their kind. It has been stated that this growth requires 43 GENERAL ZOOLOGY from 1 to 2 hours under favorable nutrient conditions. ‘There follows a period of the activity characteristic of the particular kind of cell, and after about 12 hours mitosis may occur again, followed by cytosomal constriction. ‘The times given here are for cells in tissue cultures; other kinds probably differ somewhat, and a variety of bodily conditions doubtless affect division rates. It should be clear that what evidence exists indicates that the chromatic threads found in each anaphase and telophase chromosome persist through the metabolic phase to the succeeding prophase, even though there is no morphological proof. The importance of this continuity of the genonemata from cell generation to cell generation will be increasingly apparent, and the changes that occur during mitosis should be clearly understood. Summary In this chapter we have described the structure of the cells which make up the bodies of animals and in which the vital processes occur. As we study many-celled animals, we shall find that major functions are associated with special structures—digestion with a digestive tract and glands, gas exchange with gills or lungs or air tubes, circulation with pulsating tubes and special fluids. Structure and function are intimately related to the organism as a whole, and so it is with cells. One of the things you may have thought about as you studied the “com- posite cell” is that all the different parts are definitely located. Many of the structures are separated from one another by membranes, just as the en- tire cell is separated from other cells by its plasma membrane. ‘These membranes are semipermeable, however, and some are porous in addition— communication is possible between the various compartments of the cell. It has been known for a long time that the chromosomes found in the nucleus of the dividing cell are the structures associated with the synthesis and transmission of the hereditary material of the cell, and so of the individual and species. In recent years, other functions of the cell have been associated with particular structures. Elucidation of the steps in cellular metabolism reveals that enzymes work as teams in carrying out the transformations of metabolism. And the inference is made that very often, perhaps always, the members of these teams occupy definite positions with reference to one an- other; space relations are important in chemical reactions. Insoluble enzymes involved in cellular oxidation are known to be located on the mitochondrial membrane; soluble ones are in its matrix. The great expanse of lipoprotein membrane in the endoplasmic reticulum, together with the obvious possibility for precise spacing of enzymes on it— like traffic directives painted on a highway—and its typical association with 44 STRUCTURE AND ACTIVITIES OF CELLS the RNA “messengers” from the nucleus open up stimulating vistas for scien- tific imagination and research. Biological science has stepped over a thresh- old into a period of exciting revelations concerning the microcosm we call the cell. 45 CHAPTER 3 METABOLISM Cerebral hemisphere Olfactory lobe Frontal sinus Hypophysis Diencephalon ; ‘ (third ventricle) Sphenoidal sinus Cerebellum Nasal cavit y Fourth ventricle Nostril inde =) T Eustachian tube Mouth cavity ; Medulla Teeth Hard palate Soft palate Epiglottis Glottis Vertebral column Vocal cord Se 7 Spinal cord Larynx Fig. 3.1. The human head, shown as if cut in the median longitudinal plane; semidiagrammatic. 46 IN VERTEBRATES If we recall the metabolic requirements of individual cells, we can im- mediately grasp the fact that the problems of maintenance and growth in many-celled animals must be complex. Living cells must be bathed with a continuous supply of essential nutrients and oxygen and be kept clean, so to speak, through a continuous removal of waste products of their living. Anal- ogies can be drawn between the needs of a community of cells and those of a community of human beings. Let us, instead, undertake a scientific analysis of the way in which vertebrates, like ourselves, manage to live. We shall do this by first looking at the structure or morphology of these animals with special reference to the parts related to maintenance problems. Most of you are dis- secting a vertebrate animal, such as the frog, in the laboratory; the brief discussion of vertebrate structure here is only intended to highlight the infor- mation you already have. Major emphasis will be on the kinds of individual tissues that exist in a vertebrate and especially on the way in which the tissues and organs function. We shall see that cells are specialized to form tissues and that special arrangements of these form the organs of the various anatomical divisions or systems of the body. The organ systems to be considered in this chapter are: (1) the digestive system, (2) the circulatory system, (3) the respiratory system, and (4) the excretory system. Organ Systems Related to Metabolism The Digestive System. The digestive system is made up of the digestive tract, which is a tube, and the attached digestive glands, the liver, the pancreas, and, in many vertebrates, the salivary glands. ‘The digestive tract is, in effect, a tunnel through which food passes as it is digested or chemically changed for use by the cells of the body. Through the lining of certain parts of the 47 GENERAL ZOOLOGY tract, substances actually enter the body by absorption. ‘The parts of the tract are the mouth cavity, pharynx, esophagus, stomach, small intestine, and large intestine. Teeth and tongue are characteristic structures of the mouth cavity, although birds do not have teeth (Fig. 3.1). Salivary glands empty their product, the saliva, into the mouth cavity in terrestrial vertebrates by way of the salivary ducts (p. 61). The mouth, or opening of the mouth cavity, is the place of food intake or ingestion, and teeth either hold the food in the mouth cavity or initiate its mechanical breakup. Various functions are performed by the tongue in different vertebrates: in frogs and lizards it serves to capture food; in mammals it functions to manipulate food, as a site for the taste buds (p. 110) and, in man, as an important adjunct to speech. No obvious landmark separates the mouth cavity and pharynx; they bear different names because of the manner of their origin during development. Both serve jointly as passageways for food and air. During evolution an important change occurred in these regions, as can be seen by comparing man with the frog. In man a horizontal partition, the hard palate, separates off an upper portion of the original undivided mouth cavity, such as is found in the frog. This upper part, the nasal cavity, opens externally by the nostrils and is exclusively an air pathway (Fig. 3.1). Posteriorly, the nasal cavity is continuous with the nasal pharynx, which is only incompletely sepa- rated from the oral pharynx by the soft palate. On each side the Eustachian tube leads from the anterior part of the pharynx to the cavity of the middle ear (Fig. 4.4, p. 90). Posteriorly, the pharynx is continuous with the esoph- agus and, ventrally, connects with the air passages through a slit-like open- ing, the glottis. “This crossing, so to speak, of the food and air paths is not a very efficient arrangement, as everyone is well aware. The esophagus varies in length with the neck of the vertebrate. Usually, it only carries the food from the pharynx to the stomach, but in birds a part of it, the crop, is expanded to serve for food storage. In some mammals, such as the ruminants, expanded regions of the esophagus form part of what is ordinarily referred to as the “stomach,” again functioning as a storehouse of food. The stomach and the small intestine, which are distinctly separated by the pyloric sphincter, are essentially comparable in structure in all vertebrates. They, together with the large intestine, are suspended in the coelom, or body cavity, by the mesenteries. ‘These are formed by two layers of the peritoneum which lines the coelom and covers the stomach and intestines (p. 69). The mesenteries serve to anchor the gut and to hold the liver, pancreas, and spleen. Mesenteries function as bridges by means of which the blood and lymph vessels, as well as nerves, reach the organs of the body cavity; they are also important fat depots (p. 77). The coelom has only two compartments in the lower vertebrates such as the frog, the pericardial cavity, containing the heart, and the pleuroperitoneal cavity. In mammals this cavity is divided into three, two pleural cavities, each con- 48 METABOLISM IN VERTEBRATES Esophagus x Lung \ / / Left atrium Right atrium Pericardial cavity Right ventricle Pleural cavity Diaphragm ; Stomach Liver Peritoneal cavity Large intestine Small intestine Appendix Fig. 3.2. The coelomic cavities and coelomic viscera of man. taining a lung, and the peritoneal or abdominal cavity (Fig. 3.2). ‘The peri- cardial cavity lies between the two pleural cavities; these three cavities occupy the chest or thoracic region of the trunk of the body. The peritoneal cavity is located in the abdominal part of the trunk. It is here that the stomach, small and large intestine, liver, and pancreas are found. Different degrees of folding and papillation occur in the stomach and small intestine in the different groups of vertebrates, related to increase of the surface of the lining epithelium. Various digestive glands are in the lining, and it is the absorptive surface. In the small intestine, there is marked varia- tion in the length which is correlated with the diet; the more concentrated the diet, the shorter the small intestine. In the frog tadpole, which eats a plant diet, the intestine is about 20 inches long, as compared with 12 inches in the insect-feeding adult. Carnivorous mammals have shorter small intes- tines than herbivorous ones, many of which have pouch-like outgrowths, or caeca, correlated with retention of large quantities of low-calorie plant food. The rudiment of such a caecum occurs in man near the junction of the small and large intestine; it is known as the appendix. The liver and pancreas originate from the embryonic gut and remain at- 49 GENERAL ZOOLOGY Intercellular space Lymph vessel Capillaries Fig. 3.3. The capillaries and lymphatics in relation to cells throughout the body; diagrammatic. The arrows indicate the direction of circulation. tached to that part which becomes the anterior portion of the small intestine, or duodenum, by way of their ducts, the bile duct and the pancreatic duct, respectively. The large intestine is very short in the frog, but it is quite conspicuous in mammals, including man, and is separated into the colon and rectum, which opens externally by way of the anus. In most of the vertebrates, as in the frog, the large intestine is not differentiated into regions. It opens into the cloaca, which is a common passageway for substances coming from the ex- cretory organs and urinary bladder, for germ cells from the reproductive organs, as well as for material from the large intestine. ‘The external opening of the cloaca is the anus. When, in the evolution of mammals, the cloaca was divided into a ventral urino-genital sinus and a dorsal rectum, a new opening was formed ventrally; the term anus is retained for the opening through which materials leave the digestive tract. The Circulatory System. All the cells of the vertebrate body have, in spite of its complexity, a common internal environment. ‘This is so because of the circulating fluids of the body, the blood and lymph, carried to all parts of the body by the circulatory system. ‘This system is divided into the blood-vascular system and lymphatic system, depending on the type of fluid carried. The blood-vascular system of vertebrates is, with few exceptions, what is 50 METABOLISM IN VERTEBRATES known as a closed system. ‘That is, as the blood courses through the body, it is confined to definite channels, in contrast to the situation in many, but not all, invertebrates. The chief parts of the blood-vascular system are the same in all vertebrates. There is a highly muscular region known as the heart, located in the pericardial cavity (Fig. 3.2). By contracting rhythmically, beating as we say, the heart forces the blood into the arteries, or vessels in which blood flows away from the heart. Arteries divide repeatedly as they pass to all parts of the body and finally end in a network of very thin-walled, small vessels known as capillaries, which are found in all organs of the body in close association with the cells (Fig. 3.3). The capillaries are also con- nected with other larger vessels, known as veins, through which the blood flows toward the heart. The arteries and veins that are seen when a verte- brate animal like the frog is dissected are, therefore, continuous with each other in organs all over the body by way of the capillaries. There is one very important exception to this statement in all vertebrates. Blood leaving the capillaries of the stomach and intestine passes into what is known as a portal vein because it carries the blood to another set of capillaries. This portal vein is called the hepatic portal vein because the capillary bed which it feeds is located in the liver. In lower vertebrates there is a second portal system; blood returns from capillaries in the hind legs to capillaries in the excretory organs by way of the renal portal veins. The heart connects the veins and arteries, so that the blood flows continu- ously away from the heart in the arteries, into the capillaries, thence into the veins, and so back to the heart. Although this statement describes a very simple circuit, such as that in the fishes, it is essentially true for all verte- brates. The differences that exist in the circuits of the different kinds of vertebrates are related to changes that occur in the heart and in some of the larger arteries and veins (Figs. 3.4, 3.5, and 3.6). In vertebrates with lungs, it is convenient to refer to the arteries and veins that supply regions of the body other than the lungs as systemic arteries and veins, in contrast to pulmonary arteries and veins supplying the lungs. Carotid artery Coeliac Caudal Anterior cardinal vein Posterior cardinal vein vein Fig. 3.4. The circulatory system in a vertebrate having a heart with one atrium and one ven- tricle, as in elasmobranch fishes; diagrammatic. The arrows indicate the direction of circulation. 51 GENERAL ZOOLOGY Dorsal aorta Hind limb Ze Subclavian Systemic artery GY artery Coeliaco- mesenteric artery Subclavian vein aN : CHtead\ Hau i Digestive Carotid vein | Hepatic artery tract artery Cutaneous NOP vein Precaval vein Right atrium Postcaval _— portal vein Renal portal vein Sinus Liver Abdominal venosus Hepatic vein vein Pulmonary Cutaneous a artery Pulmonary CLung=> artery Sew Fig. 3.5. ‘The circulatory system in a vertebrate having a heart with two atria and one ven- tricle, as in the frog; diagrammatic. ‘The arrows indicate the direction of circulation. The arrangement of the chambers of the heart and its valves controls the direction of blood flow as the heart contracts (Figs. 3.7 and 3.8). Blood leaving the heart enters the large arteries which have walls sufficiently heavy to maintain a fairly constant diameter; their walls contain, in addition to the thin lining of endothelium, elastic connective tissue, and non-striated muscle. As the arteries divide and redivide, the bore of each branch becomes smaller and the wall thinner. Of course, the total capacity or volume of the branches is far greater than that of the original artery. Capillaries are limited only Aorta Tliac artery Excretory NU) Right SS : Right atrium ventricle nom Postcaval vein Left atrium Pulmonary vein Fig. 3.6. The circulatory system in a vertebrate having a heart with two atria and two ven- tricles, as in mammals; diagrammatic. ‘The arrows indicate the direction of circulation. 52 METABOLISM IN VERTEBRATES Carotid artery > D posptemic artery in E wil) eae Pulmocutaneous arteriosus BIUELY, Opening of . Right atrium \ pulmonary vein Fig. 3.7. Heart of the frog, Opening of 23% sinus venosus with taeyventral walls tof ihe )Senulunar valve KY ventricle and atria removed. Left atrium ‘The arrows indicate the di- \ Atrioventricular rection of circulation. valve : Ventricle by the endothelium, and continued branching increases the capacity of the bed over that of the small arteries feeding it. All this is significant in con- nection with blood pressure and its rate of flow. When the heart contracts, a quantity of blood is suddenly forced into an artery which has much less capacity than the ventricle. The blood pressure in the aorta of man in a normal resting state is 90-100 mm. of mercury. As the cross-sectional area of all the branches increases, that is, as the capacity of the vessels increases, the blood pressure drops until it is only about 25-30 mm. of mercury as it enters the capillary bed. Here, the pressure falls still more, down to about 15 mm. of mercury, and the blood moves relatively slowly; this is important in connection with the exchanges that occur between blood and cells in capillary beds. Veins have walls containing some non-striated muscle in addition to the endothelial lining but have little ability to resist expansion; hence, the Left carotid artery Innominate artery Aorta Left subclavian artery Precaval vein Pulmonary artery Fig. 3.8. Heart of man, with the ventral walls of the ventricles and Right atrium Pulmonary atria removed. ‘The arrows indi- Veins cate the direction of circulation. Left atrium Semilunar valve Atrio- ventricular Postcaval vein valve ; ‘ Left ventricle Right ventricle 53 GENERAL ZOOLOGY pressure in the veins is lower than it is in arteries. Movement of blood toward the heart from the lower extremities of upright vertebrates such as man is aided by valves in the veins of the legs. During movement of the legs, the contraction of the skeletal muscles (p. 68) helps to push the blood along, with the valves preventing backflow. The heart beats about 70 times per minute in a normal resting man. At each contraction, a pulse wave moves along the arterial wall and can be felt in any large peripheral artery. However, this pulse wave moves much faster than the blood. By means of radioactive particles inserted in a peripheral vessel, it can be shown that it takes 20 to 25 seconds for blood to repass the point of insertion, that is, for a complete trip through the body. About 5 liters of blood leave the heart every minute during normally quiet living. If you realize that the cardiac output is greatly increased during muscular exercise, up to 30 liters per minute, and think of the years that the human heart can function continuously, you will begin to appreciate what an amazing organ it is. The mechanism and control of this remarkable pump will be described later (p. 117). In spite of the fact that the pressure of the blood in the capillary is rela- tively low, some filtration of the blood plasma occurs through the single- celled wall, in addition to diffusion. Some white blood cells may also move out between the cells of the lining. Thus, a fluid bridge is formed between the contents of the capillary and the nearby cells; this fluid is called the interstitial fluid or lymph. Some of it apparently re-enters the capillaries as the pressure decreases in the bed, but much of it enters the closed terminal vessels of the lymphatic system (Fig. 3.3). ‘These lymphatics, as they are called, remain thin-walled as they unite to form larger vessels which eventually empty into large veins in the region of the neck in higher vertebrates. Lymph is returned from the hind legs and trunk in a fairly large channel known as the thoracic duct. In birds and mammals, the lymphatics pass through the lymph nodes where lymphocytes are differentiated and enter the lymph (p. 66). The lymph nodes are also important organs of body defense since foreign materials, such as bacteria and venoms, are filtered out there and destroyed by macro- phages (p. 64). Contraction of muscles in all parts of the body moves the lymph through its thin-walled vessels, and valves control the direction of movement. The general function of the circulating fluids as what we may call a common carrier will become clear as the discussion progresses. ‘The several phases of this general function are as follows: (1) to carry necessary food materials and oxygen to the cells; (2) to carry the waste products of metabolism away from the cells; (3) to transfer hormones, or internal secretions (p. 95), from one part of the body to another; and (4) in warm-blooded animals, like the mammals, to transfer heat from regions of high oxidation and so aid in the maintenance of a constant body temperature (p. 125). The Respiratory System. Respiratory organs are of two kinds: gills in water-dwelling vertebrates and lungs in land dwellers. Gills are tufted or 54 METABOLISM IN VERTEBRATES 4 P/, >: ge wee UFC ae Pulmonary Bronchus We" <- as ‘ att eo we mf ones = = eh - a fey ‘ eS ie he LIS RIN, \ " J M ‘ Z & % ee Fig. 3.9. ‘Trachea, bronchi, and air passages of the lung in a mammal. _ (Photo- graph of plastic cast of the air passages of the lungs courtesy Ward’s Natural Science Establishment.) laminated outgrowths found along the sides of the pharyngeal pouches in fishes and some amphibians (see Fig. 5.31, p. 169). They are covered by a very thin layer of cells beneath which is a rich capillary bed. Lungs lie deep in the body and are connected to the pharynx by air passages; the lungs and air passages comprise the respiratory system. The opening from the pharynx into the larynx, an expanded portion of the air passages containing the vocal cords, is known as the glottis. A tube called the trachea passes, alongside the esophagus, from the larynx into the chest, or thorax. There, the trachea divides into two bronchi which enter the lungs. 55 GENERAL ZOOLOGY Red blood cell Respiratory membrane Alveolus Capillary Fig. 3.10. Electron micrograph of a section through lung of rat; 26,000. (From the original of Fig. 5, Frank N. Low, Anatomical Record, vol. 117, p. 257, printed by per- mission of the author and publishers.) The larynx, trachea, and bronchi are prevented from collapsing by cartilag- inous supports in their walls (p. 61). Lungs in lower vertebrates such as the frog are sac-like, with the inner respiratory surface area increased by a series of folds of the lining membrane. In higher vertebrates, such as man, the bronchi branch repeatedly in the lungs forming bronchioles; in the smaller bronchioles cartilaginous rings are no longer present. ‘These air passages end in small expansions known as the alveoli or air chambers (Fig. 3.9). Each alveolus is lined by a single layer of cells and, as in the gill, this rests on a rich capillary bed (Fig. 3.10). In the frog the lungs lie in the pleuroperitoneal portion of the coelom. In the course of evolution in the vertebrates, this cavity becomes partitioned by a muscular structure known, in mammals, as the diaphragm (Fig. 3.2). Now, each lung lies in a closed pleural cavity, one on each side of the heart, surrounded by the thoracic cage (p. 92). Breathing is the result of in- creasing and decreasing the volume of the pleural cavities by movements of the diaphragm and ribs. ‘The control of this mechanism will be discussed later (p. 124). The Excretory System. ‘The lungs, the skin, the liver, and the excre- tory organs are organs in which excretion occurs, but they do not make up a system of organs in the ordinary meaning of the phrase. In dividing the 56 METABOLISM IN VERTEBRATES body into systems, it is convenient to assign a particular function to a single group or system of organs. For that reason, the lungs are discussed as part of the respiratory system, the skin as part of the integumentary system, which functions as a covering for the body, and the liver as part of the digestive system. ‘The excretory organs and their ducts are, however, referred to as the excretory or urinary system. Since the ducts of the excretory organs are also used for the passage of male reproductive cells in vertebrates like the fishes and amphibians, the excretory and reproductive organs are frequently referred to as the urinogenital system. We shall not be concerned here with this dual system but shall describe the structure of the urinary system. Two different kinds of functional excretory organs, distinguished by their manner of origin during development, are found in adult vertebrates. The first of these, the mesonephros, is present in adult fishes and amphibians. It appears and disappears during the development of reptiles, birds, and mam- mals, in which a metanephros is formed and functions as the excretory organ of the adult. Although both mesonephroi and metanephroi are often called kidneys, the term should be reserved for metanephroi. ‘The excretory organs are paired structures located behind the peritoneum on the dorsal wall of the coelom and not suspended in it like the stomach and intestine. In mammals a urinary bladder, or reservoir where urine is stored before it is voided, is connected with the kidney by the ureters or excretory ducts (Fig. 3.11). Vena cava Diaphragm Adrenal gland ; Renal artery Cortex Renal vein Medulla Left kidney Pelvis Aorta Ureter Urethra Bladder Fig. 3.11. Excretory system and related structures of man. 57 GENERAL ZOOLOGY Dorsal aorta —, Glomerulus \ Renal Renal artery WS corpuscle Postcaval vein Capillaries Fig. 3.12. Nephron and _ blood vessels in the excretory organ of the frog; diagrammatic. The nephron of man is longer and highly convoluted; there is no renal portal vein. Renal portal vein Inside the kidney, the ureter expands to form the pelvis of the kidney into which many small tubules empty. These carry urine from the very numerous nephrons or excretory tubules to the pelvis. Each nephron ends in a cup known as Bowman's capsule which contains capillaries that arise from branches of the renal artery (Fig. 3.12). Each of these groups of capillaries is a glomerulus, which with its surrounding Bowman’s capsule makes up a_ renal corpuscle. The excretory tubules are lined by a single layer of cells and surrounded by a very conspicuous capillary network. In the frog these capil- laries are fed by the renal portal vein as well as by the renal arteries, but there is no renal portal vein in mammals. This completes the very brief survey of the morphology of the organ systems concerned with satisfying the metabolic requirements of the cells of the verte- brate. It is time now to become acquainted with the different kinds of cells found in the vertebrate body. The Kinds of Tissues: Histology The various systems of the body, or soma, are made up of cells, collectively known as the somatic cells. In Chapter 2 a so-called composite cell was described (p. 17). If the body of a vertebrate is examined microscopically, no cell will be found that conforms to that account; cells differ among them- selves, although they all possess certain features in common. Cells may be dissimilar in shape, position in the body, structure, and also function. Cells that are similar in structure and function make up groups known as tissues; tissues are groups of cells specialized in the same way for the performance of the same function. ‘Tissues are associated to form the organs that perform special functions. The cells as they are grouped to form tissues and organs 58 METABOLISM IN VERTEBRATES must be studied by means of the microscope. ‘This particular study of structure is known as histology, or microscopic anatomy, in contrast to gross anatomy, or the study of the organ systems by dissection. If we consider tissues first, we find that they are classified according to structure and function. There are five principal classes of tissue: epithelial, sustentative, vascular, contractile, and nervous. Epithelial Tissue. The cells of epithelial tissues are compactly placed with but a small amount of intercellular material. Their functions include the covering and protection of body surfaces, both internal and external, as well as absorption, secretion, and excretion. According to the predominating shape of the cells, this class is subdivided into squamous and_ columnar epithelium, each of which is again divided into simple or stratified, depending on whether it exists in single or multiple ijayers. The cells of simple squamous epithelium when viewed from the surface resemble tiling blocks; seen from the edge, they are very thin (Fig. 3.13A). Such epithelium is found lining the coelom; that is, it forms the peritoneum (Fig. 3.138). In stratified squamous epithelium only the outermost layers are typically flattened cells; in the deeper layers the cells are progressively more cuboidal (Fig. 3.13C). Since blood vessels do not penetrate epithelial layers, only the cells of the deeper layers Fig. 3.13. Epithelial tissues. A, simple squamous epithelium from the human mouth. 8B, simple squamous epithelium (mesothelium) from peritoneum. C, stratified squamous epithelium from the lining of the nasal cavity. D, simple columnar epithelium from the mucous membrane of the digestive tract. , pseudostratified ciliated columnar epithelium from the lining of the trachea; one cell is shown secreting a drop of mucus. F, simple ciliated columnar epi- thelium. G, glandular epithelium from the pancreas. H, goblet cell with a drop of mucus. A and B, surface views; C-H, sections at right angles to surfaces which are toward the top of the page. (A, B, and C from drawings by D. F. Robertson.) 59 GENERAL ZOOLOGY Fig. 3.14. Diagrams of glands. A, unicellular glands; the one at the left is shown extending below the surface layer of cells. B, a group of gland cells that remain in the surface layer. C, a simple alveolar gland. JD, a simple tubular gland. £, a compound tubular gland. F, a compound alveolar gland. receive abundant nourishment and consequently divide and replenish the outer layers, which die and are cast off. Stratified squamous epithelium is found in the outer layer, or epidermis, of the skin (Fig. 3.25, p. 83) and in the lining of the nasal and mouth cavities, pharynx, and esophagus of many vertebrates. In simple columnar epithelium, such as that lining most of the digestive tract, the cells are longer than they are wide and are arranged side by side (Fig. 3.13D). Stratified columnar epithelium is not common, but a modified type is found lining the trachea (Fig. 3.13#). Columnar epithelial cells sometimes have their free surfaces, that is, the surfaces exposed to the cavity they line, covered with cilia, which are very delicate cytoplasmic processes (Fig. 3.134 and F). The cilia are vibratile, and their motion removes any foreign materials from the surfaces or creates currents in adjacent fluids. ‘The epithelium of the air passages in higher vertebrates and of the roof of the frog’s mouth is an example of this variation. In the iris and retina of the eye (Fig. 4.3, p. 89, and Fig. 4.14, p. 106), epithelial cells contain pigment granules and are known as pigmented epithelium. Glands are organs essentially composed of simple columnar epithelium known as glandu- lar or secretory epithelium (Fig. 3.13G). These cells have the capacity to synthesize substances, such as enzymes or hormones, that must be present in order for specific events to occur. Such substances are called secretions. Glands may be unicellular, as the goblet cells of the intestinal epithelium (Fig. 3.13H), or multicellular (Fig. 3.14). All glands begin their formation on surfaces of the body and usually sink below the surface. In doing so they may form simple tubular glands or become flask-shaped to form simple alveolar glands, such as are found in the skin of the frog. Both types of simple glands 60 METABOLISM IN VERTEBRATES may become compound by the formation of outpocketings along their lengths. The glands related to digestion, and many others, are divided into secretory portions and the tubes, or ducts, by way of which the secretion passes out to the cavity or surface where it is used. Not all glands possess ducts; those that do not, the ductless glands, will be discussed in the next chapter (p. 95). The gastric glands, present in the mucous membrane of the stomach, and the intestinal glands that occur in the mucous membrane of the duodenum are simple tubular glands. ‘The liver isa compound tubular gland; the pancreas, a compound alveolar gland. Sustentative Tissue. The sustentative tissues, often called the connec- tive tissues, are a very heterogeneous group, classed together because they are all derived during development from the same source—the stellate mesen- chyme cells (Fig. 3.15B). In general, sustentative tissues function in sup- porting the body and connecting or binding together its parts. This group of tissues is characterized by the large amount of intercellular material produced by the cells. In the vertebrates this intercellular material is responsible for the supporting and connecting qualities. Sustentative tissue may be divided into four subclasses: connective tissue in the restricted sense, cartilage, bone, and adipose tissue. Connective tissues are of three kinds: mucous connective tissue, in which the intercellular material is gelatinous, is found in the umbilical cords of mam- mals (Fig. 3.15); reticular connective tissue, in which there is a meshwork of connective tissue cells with the interspaces filled with other types of cells, forms the framework of organs like the spleen (Fig. 3.15C); and fibrous connective tissue, in which the intercellular material is composed of fibers, is distributed widely as a binding tissue in many organs. ‘The intercellular fibers of fibrous connective tissue are of two kinds, collagenous and elastic. The collagenous or white fibers are very fine and occur in bundles, whereas elastic or yellow fibers are thicker and occur singly. Fibrous connective tissue in which both collagenous and elastic fibers occur is found in the submucosa of the digestive tract and in the dermis of the skin (Figs. 3.15D and 3.25). Fibrous con- nective tissue in which collagenous fibers predominate is found in tendons, and that containing chiefly elastic fibers is found in the walls of larger arteries and in certain ligaments (Fig. 3.15#). ‘The cells of fibrous connective tissues are spindle-shaped or irregular in outline and possess relatively little cytoplasm. The second subclass of sustentative tissues is cartilage, which is a supporting tissue. ‘The intercellular material in cartilage is sometimes hardened by impregnation with inorganic salts, chiefly those of calcium. Here the cells are more or less rounded and lie in spaces known as lacunae. When the matrix between the cells is translucent and apparently structureless, the tissue is called hyaline cartilage or gristle (Fig. 3.15G). Hyaline cartilage is found at the ends of long bones, at the ends of ribs, and in the cartilages of the nose and trachea. ‘The cartilage of the external ear contains elastic fibers in its matrix and, therefore, is known as elastic cartilage (Fig. 3.15H); that found 61 GENERAL ZOOLOGY aR yew AN Me, I J Fig. 3.15. Sustentative tissues. A, cells of mucous connective tissue, which occurs in the umbilical cords of mammals; the gelatinous intercellular material is not represented. 8B, mesenchyme cells. C, reticular connective tissue from the spleen. JD, fibrous connective tissue from the submucosa, showing both collagenous and elastic fibers. £, elastic fibers of fibrous connective tissue from the nuchal ligament of the ox; no cells are shown. F, adipose tissue, showing various stages of storage of fat drops in the cells. G, hyaline cartilage from the end of a rib, showing cells and an empty lacuna. H, elastic cartilage from the external ear, showing capsules of hyaline cartilage and elastic fibers. J, fibrous cartilage from an intervertebral disk, showing capsules of hyaline cartilage and collagenous fibers. J, bone cell lying in a lacuna. A, bone lacuna and canaliculi from dried bone. L, Haversian system in which lacunae are arranged concentrically around a central or Haversian canal; canaliculi connect the lacunae and the canal. (A, FE, H, A, and L from drawings by D. F. Robertson.) between the vertebrae has collagenous fibers in its matrix and is called fibrous cartilage (Fig. 3.15/). Bone, or osseous tissue, is characterized by its very hard matrix, which is impregnated with calcium and phosphorus salts. There is twice as much inorganic material in bone as there is organic. The long bones of the body, such as the femur, have a central marrow cavity filled with yellow or fatty bone marrow. In the much smaller marrow spaces at the ends of long bones and in the vertebrae and sternum is found the red bone marrow, which is the site of differentiation of red blood cells and of granular white blood cells. Bone marrow is not osseous tissue; it is merely contained in the cavities of bones. The bone cells lie in lacunae within the matrix (Fig. 3.15] and &). 62 METABOLISM IN VERTEBRATES A very typical arrangement is that of the Haversian system. ‘This consists of a central Haversian canal which contains an artery, a vein, and a nerve, sur- rounded by concentrically arranged rows of lacunae in communication with one another and with the central canal by means of minute spaces, the canaliculi (Fig. 3.15). Lymph circulates in these canaliculi and furnishes a passageway for foods and wastes between blood and cells. In adipose tissue there is no intercellular material, and the stellate mesen- chyme cells become transformed into rounded cells which serve to store fat (Fig. 3.15F). In fully differentiated adipose cells a large drop of fat is sur- rounded by a film of cytoplasm, which contains the nucleus. ‘The large drop of fat is formed by the coalescence of numerous finer drops that are deposited in the cytoplasm during the specialization of fat-storing cells. Adipose tissue is widely distributed in the body. Vascular Tissue. The vascular or circulating tissues are the blood and lymph and are characterized by a liquid intercellular material, the plasma. In blood, two kinds of cells are suspended in the plasma. Of these the red blood cells, or erythrocytes, contain the iron-bearing hemoglobin in combina- tion with which oxygen is carried in the blood (Fig. 3.164 and B). The average human being has about 6 liters of blood, and each cubic millimeter of it contains 41% to 5 million red blood cells. In mammals the red cells lose their nuclei during their differentiation, live only about 125 days, and must be replaced constantly. Red blood cells are differentiated in the red bone marrow in adults, and, if they are not formed in adequate numbers, the indi- vidual becomes anemic. ‘The red cells are destroyed by macrophages located along the walls of the capillaries of the spleen and liver; 7 to 10 million red cells are destroyed every second. White blood cells are frequently irregular in shape, since they are capable of amoeboid movement and migrate through the walls of capillaries and among the cells of other tissues (p. 54). Wandering white blood cells can UD ‘ o Nucleus D Fig. 3.16. Vascular tissue. A, mammalian erythrocytes arranged in rouleaux. 8B, single mammalian erythrocyte (non-nucleated). C, granular leucocytes with polymorphic nuclei; one with irregular cytosome. JD, non-granular leucocytes or lymphocytes. 63 GENERAL ZOOLOGY ingest solid particles like bacteria and other foreign bodies into their cyto- somes and function in this way in the event of infections or wounds in any part of the body. These white blood cells are known as microphages. In con- nective tissue other white blood cells, the macrophages, are the chief local defenders in what is known as the inflammatory reaction, or first line of bodily defense, at the site of an infection. There are only about 8,000 to 10,000 leuco- cytesinacubic millimeter of blood in a healthy individual, but the number may be greatly increased in illness. Enormous numbers of leucocytes are held in reserve in the bone marrow and can be quickly released to the blood stream in response to stimulation. Blood counts are effective aids in the diagnosis of disease. “Two main classes of white blood cells are distinguished, those in which the cytoplasm does not contain granules and those in which the cyto- plasm is granular. The most abundant kind of non-granular leucocyte is the lymphocyte (Fig. 3.16D), which is about the size of an erythrocyte and is differentiated in the lymph nodes and in the spleen. Another much less frequent and larger non-granular leucocyte is the monocyte. Granular leu- cocytes, all of which are larger than the other blood cells, have nuclei conspicuously irregular in shape and are called polymorphonuclear leucocytes (Fig. 3.16C). There are three kinds, distinguished by the staining reactions of the cytoplasmic granules. Granular leucocytes differentiate in the red bone marrow. It has proved very difficult to obtain information concerning the life span of white blood cells. Lymphocytes apparently rupture or dissolve in great numbers soon after they leave their places of differentiation, contributing their globulins, a kind of protein, to the blood plasma. Gamma globulin is concerned in the formation of antibodies against certain diseases. In cats and dogs it appears that the total population of white blood cells is replaced several times a day. Slightly more than half the volume of the blood is normally plasma, which is the carrier for all substances, except the oxygen, transported by the blood. Plasma contains, in addition to the blood cells, the blood platelets, which appear to be fragments of cytoplasm of uncertain origin and function. It also contains many submicroscopic constituents which are very important. One of these is a protein known as fibrinogen; when blood clots, this becomes changed to fibrin, in the meshes of which the cells are held. The clotting of blood acts to seal damaged blood vessels and prevent further loss of blood. Blood does not clot unless a vessel is ruptured, except when there has been tissue damage. In circulating blood there is a substance known as prothrombin, the formation of which depends on the presence of vitamin K (p. 32). In the presence of calcium salts, prothrombin is transformed into thrombin, which, in turn, conditions the change of fibrinogen to fibrin. ‘This reaction does not occur in a closed blood vessel because of a substance called heparin, or antithrombin. When a vessel is broken, the damaged tissue cells and also disintegrating platelets release a substance known as thromboplastin, or throm- bokinase, which neutralizes the effect of heparin. Then thrombin is formed 64 METABOLISM IN VERTEBRATES from prothrombin, and, in turn, fibrin is formed from fibrinogen; a clot occurs. When drawn blood clots in a bowl and is allowed to stand, the clot contracts, and a pale yellowish fluid is squeezed out. ‘This fluid is what re- mains of the original plasma. It is called blood serum and contains, among other things, the substances which immunize against certain diseases. One very important function of the blood is the transport of oxygen in combination with the hemoglobin of the red blood cells. Death results from loss of one-third the blood in warm-blooded animals because oxygen is no longer delivered to the cells of the body in adequate amounts. In medical practice a blood transfusion may save a life after hemorrhage or in severe anemia. When transfusion was first attempted, it was often found that the patient died as soon as blood from another person was introduced into his vessels. “Che reason was that the red cells of the donor blood clumped or stuck together; they were agglutinated as a result of a reaction between proteins of the plasma and proteins of the red cells. ‘These clumps prevented the free flow of blood in the capillaries and thus caused death. About 1900 it was discovered that the blood of any human being falls into one of four types, depending on the presence in the red cells of either substance A or B. of both A and B (AB), or of neither A nor B (O). About 46 per cent of the human race have blood of type O; 42 per cent, type A; 9 per cent, type B; and 3 per cent, type AB. The substances A and B are examples of what are called antigens. ‘The substances in the plasma which react with A and B to bring about agglutina- tion are examples of antibodies. These are what are known as normal antigens and antibodies. In an individual with type A blood the red cells contain antigen A and the antibody for A does not occur in the plasma. Such in- dividuals carry antibodies for antigen B. If blood of type B or type O is placed in the vessels of a type A individual agglutination can occur because antibodies for A are present in the plasma of type B and type O blood. In emergencies it is sometimes possible to use type O blood to transfuse a type A or type B in- dividual depending on the amount of blood required and the concentration of antibodies A or B in its plasma. Proteins, such as those of bacteria, which are foreign to a certain kind of animal and which invade its tissues are also known as antigens. ‘The tissues of the animal react to the presence of the foreign protein by producing anti- bodies that combat the eflects of the antigen. Such antibodies are called immune antibodies and become abundant in the blood plasma, where they remain for varying lengths of time and provide immunity to reinvasion by the foreign protein or antigen that conditioned their production. ‘Thus, recovery from a particular disease may result in permanent or transitory active acquired immunity to that disease. The capacity of animals to synthesize substances protective against foreign proteins can be utilized to protect human beings against diseases. If minute quantities of the pathogenic material are introduced into the body, its tissues will produce abundant antibodies, even though the amount of infective 65 GENERAL ZOOLOGY material gives only transitory symptoms of the disease. Thereafter, these antibodies are responsible for an artificial acquired immunity to the disease in question; this immunity may be permanent or may have to be renewed at intervals. Dead bacteria are used for inoculation against typhoid and para- typhoid; viruses of reduced virulence, called attenuated viruses, are used in vaccination against smallpox or inoculation against rabies. ‘The toxins, or poisonous substances, produced by the bacteria are injected in inoculation against diphtheria. Antibodies produced in other animals can be used to give passive immunity, usually of short duration, or to combat antigens in human beings in the control of disease. “Thus, antibodies, also called antitoxins, against the toxin giving symptoms of diphtheria are produced by horses inoculated with the toxins of the diphtheria bacilli and can be obtained in the serum which separates when the horse’s blood clots after being drawn. Such immune serum administered to an individual enables him to combat effectively the toxin or poison of the diphtheria bacillus before antibodies are produced in adequate quantities in his own tissues. After a wound is received under circumstances where tetanus bacilli might be present, tetanus antitoxin, also from horses, is routinely administered, in order to protect against the toxins these bacilli would produce. Soldiers in World War II were given inoculations of tetanus toxin and antitoxin in order that they might acquire artificial immunity against this rapidly fatal bacterium. It may be noted here that some individuals have what is called natural immunity to certain diseases. ‘his immunity may be the result of inheritance of a capacity to form certain antibodies, such as the natural antibodies against antigens A and B of the red blood cells. Very slight and unnoticed infection with the causal agent of the disease may also bring about what seems to be natural immunity. Immunity and resistance to disease must not be confused. Immunity is protection against a specific pathogenic agent. Resistance is non-specific and may depend on many factors. The quantity of blood in the circulatory system is important in connection with the ease of its circulation and the adequacy of supply to all regions. When blood is lost in amounts insufficient to cause death from oxygen want, a condition known as shock may result. ‘This condition can be controlled by increasing the blood volume through the addition of plasma without the red cells. ‘Typing is not necessary for such transfusions. In World War II one of the greatest contributions of science to the saving of human life was the development of methods of separating the plasma from the great quan- tities of blood donated by non-combatants, preventing it from clotting, and drying it in such a way that it would keep indefinitely. The dried powder was sent to all fronts along with triple-distilled water, with which it was mixed before use in combating shock in wounded men. Lymph differs from blood in that it does not contain erythrocytes and granular leucocytes. ‘The plasma of lymph is derived from the blood by 66 METABOLISM IN VERTEBRATES filtration through the walls of the capillaries and does not contain all the constituents of blood plasma. Lymph is important as the pathway over which pass the materials transferred between blood and the cells of the body. Contractile Tissue. Contractile tissues, made up of what are known as muscle cells, are of three kinds: non-striated, cardiac, and striated. The cytoplasm of muscle cells is characterized by the presence of numerous fine fibers, which are placed longitudinally. ‘The shortening and thickening of these muscle fibrillae result in the contraction of the individual cell and are therefore responsible for the particular function of this kind of tissue, the production of motion. Non-striated muscle cells are typically spindle-shaped with the nucleus centrally placed (Fig. 3.17A and B). ‘These cells usually occur in sheets loosely held together by fibrous connective tissue. ‘This kind of muscle is found in the wall of the digestive tract, in the urinary bladder, and in the walls of blood vessels. It is sometimes called involuntary muscle because it is not under conscious nervous control. Cardiac muscle is found only in the heart and is capable of rhythmical contractions (Fig. 3.17F). The cells are arranged in the form of a syn- cytium; that is, the cylindrical cytoplasmic units containing the nuclei are not separated from one another by membranes where they meet at their ends. Fig. 3.17. Contractile tissues. A, non-striated muscle cells in cross section; note that the cytosomes of many of the cells are not cut through in the region of the nucleus. B, isolated non-striated muscle cell from the wall of the digestive tract, showing spindle-shaped cytosome and single nucleus. C, cross section of a voluntary muscle; the striated muscle cells are held together in bundles by fibrous connective tissue. D, portion of a striated muscle cell, showing its multinucleate condition and the cross-striations; the cell has been injured near its right end in order to show the cell membrane. £, striated muscle cells, showing blood supply. F, cardiac muscle cells from the human heart. (A, B, C, and E from drawings by D. F. Robert- son; F, from E. A. S. Schafer, Essentials of Histology, copyright 1916 by Longmans, Green and Co., reprinted by permission.) 67 GENERAL ZOOLOGY ZaOR War he SO). aps. (@ Ya Ue) J, (2 ale. Y | 6 ‘ A Bu \Nosve ber Nerve fiber E Myelin sheath FE Fig. 3.18. Nervous tissue. A, typical bipolar neuron from the olfactory epithelium. 8B, transformation of a bipolar neuron into the type found in the dorsal root ganglia of spinal nerves. C, multipolar neuron, showing cytosome with numerous dendrites and a single axon. D, bundle of myelinated nerve fibers surrounded by fibrous connective tissue, as in the spinal and cranial nerves; each nerve fiber is surrounded by a myelin sheath. £, portion of a single myelinated nerve fiber; the interruption in the myelin sheath is called a node of Ranvier; a nucleus is seen in the neurilemma, or outer membrane. F, portion of an unmyelinated nerve fiber, characteristic of autonomic nerves; a nucleus of the neurilemma is shown. These units branch and unite in such a way that a network is formed. The fibrillae of cardiac muscle are made up of regions of different density so that the cytoplasm presents an irregularly striated appearance. Striated muscle is known sometimes as skeletal muscle because it is attached to the bones and by its contractions produces motion of body parts which are supported by bones (Fig. 3.17D and £). Since these muscles can be co- ordinated consciously, they are also called voluntary muscles. ‘They appear striated because the fibrillae have regions of different density, which occur at such regular intervals as to give a distinct cross-striped appearance to the cytoplasm. ‘The cells are cylindrical, sometimes very long, and each contains many nuclei; that is, the cells are multinucleate. Fibrous connective tissue serves to bind together striated muscle cells and forms sheaths that enclose great numbers of these cells which make up the visible muscles, such as the gastrocnemius or the biceps (Fig. 3.17C). These connective tissue sheaths are continuous with the tendons by means of which muscles are attached to bones. Nervous Tissue. ‘The cells of nervous tissue are differentiated in such a way that they are capable of receiving stimuli in some regions, of conducting nerve impulses from one part of the body to another, and of discharging these impulses. The general functions of nervous tissue can, therefore, be stated as reception, conduction, and discharge. ‘These activities make pos- sible the coordination of the organism as a whole and will be discussed in the next chapter. A nerve cell, or neuron, is composed of a nucleus surrounded by a relatively small cytosome, which is prolonged into two or more processes of varying lengths; these nerve fibers are of two types. Some taper along their 68 METABOLISM IN VERTEBRATES lengths and have branches which come off at varying angles; these are called dendrites. Others are seen to be of uniform, small diameter, with branches at right angles to the main fiber, and are surrounded by a myelin sheath contain- ing compound lipids (Fig. 3.18); these are the axons. Nerve fibers branch repeatedly at their ends, forming the terminal filaments. ‘The terminal fila- ments of axons, which often end in plate-like or club-like expansions, lie against the dendrites and cell bodies of other neurons or on muscle and gland cells (see Fig. 4.22, p. 114). Where there are only two cytoplasmic exten- sions, or nerve fibers, the cell is called a bipolar neuron (Fig. 3.18A and B). Cells with more than two fibers are known as multipolar neurons (Fig. 3.18C). Such cells never have more than one axon. ‘The cell bodies of neurons are sometimes found in groups, or ganglia, outside the central nervous system; other nerve cell bodies occur in the gray matter of the central nervous system (see Fig. 4.9, p. 96). When nerve fibers are bound together and surrounded by fibrous connective tissue, they form the visible nerves of the peripheral nervous system (Fig. 3.18D); nerve fibers also make up the white matter of the central nervous system. A nerve fiber is always continuous with the cytosome of a neuron. Organs. ‘The tissues that have been described illustrate the various types of specialization that cells undergo in the vertebrate body. Particular tissues are capable of performing their special functions alone, but they usually occur grouped in organs. ‘Thus, organs are groups of tissues associated for the per- formance of a special function. For example, if the wall of the small intestine of a mammal or other vertebrate is examined microscopically, it is found to consist of layers known as the peritoneum, the longitudinal and circular muscle layers, the submucosa, and the mucous membrane (Fig. 3.19). The peritoneum Longitudinal muscle layer Submucosa Capillary SHS iS 6 pre79 nl ES RAY seagone elas ‘One> Ne o,2. sO oo SS INR > Se f Gp B35 2) 2 2, 2 2 ate oy ~ fe Intestinal gland Lumen of intestine Peritoneum Circular Muscularis Lymphatic Goblet cell muscle mucosae vessel layer Fig. 3.19. The wall of the small intestine of an amphibian, in section; semidiagrammatic. 69 GENERAL ZOOLOGY consists of simple squamous epithelium and functions as a covering mem- brane. Both longitudinal and circular muscle layers are of non-striated con- tractile tissue bound together by fibrous connective tissue, and their contractions produce the muscular movements that mix the food contents of the small intestine and push them along toward the large intestine. Fibrous connective tissue, containing both collagenous and elastic fibers, is the distinguishing tissue of the submucosa and serves to support the numerous vessels carrying blood and lymph. ‘This layer also provides the elasticity essential for the expansion of the intestinal lumen, in addition to carrying the circulatory fluids necessary for absorption. The mucous membrane, functioning in secre- tion and absorption, is composed of simple columnar epithelium, which forms the lining of the tract; a layer of reticular connective tissue (lamina propria), which forms the cores of the villi; and a thin layer of non-striated muscle cells (muscularis mucosae), which is the outermost layer lying adjacent to the sub- mucosa. ‘These several tissues are associated to form the small intestine, in which digestion and absorption occur, and each tissue contributes to the func- tion of the whole organ. In addition to the grouping of cells to form tissues and of tissues to form organs, organs are associated to form the systems described in discussions of metabolism, responsiveness, and reproduction. Digestion Most of the food that a vertebrate eats, or ingests, is not in a form im- mediately utilizable by its cells. Foods, with the exception of water, inorganic salts, vitamins, and a very few lipids and monosaccharides, must be broken down into smaller units before they can be absorbed and utilized by the animal. ‘The processes of chemical breakdown of foods during their passage through the digestive tract are called digestion. “The chemical changes in the food are brought about by specific enzymes secreted by cells of the diges- tive organs. As a result of enzyme action, carbohydrates are broken down into hexose sugars, lipids are separated into fatty acids and glycerol, and proteins are reduced to amino acids. The purely mechanical activities of the digestive tract are aids to digestion. Chewing separates food into smaller masses which afford more surface area for the action of digestive enzymes. Movements of the digestive tract mix the food particles with digestive juices and propel the food mass from one region of the tract to the next. Digestion will be discussed as it occurs in the successive regions of the digestive tract. You should keep in mind the structure of the digestive system (p. 47). In the mouths of mammals the food may be torn apart or ground into fine particles by the teeth, but in many other vertebrates the food is merely held by the teeth and no mechanical disintegration occurs. ‘The frog, for instance, retains food with its teeth until it can be swallowed, and no digestive changes take place in the mouth. In man, however, the sight, the odor, or even the 70 METABOLISM IN VERTEBRATES thought of food induces the flow of saliva, a digestive juice secreted by the salivary glands; the average daily output is about 1500 ml. As the food is chewed, it is mixed with saliva which softens and lubricates the food. ‘This initiates the digestive changes and aids in swallowing. Although saliva is largely water and mucin, it contains a digestive enzyme, a salivary amylase known as ptyalin. This enzyme is most active in a neutral medium and is responsible for the partial digestion of glycogen and starch. Starch, if it has been cooked, enters the mouth in soluble form; if not, it is made soluble by the ptyalin. Soluble starch in the presence of ptyalin reacts with water to form dextrins and maltose, one of the compound sugars. ‘The food mass, with its complex carbohydrates partially digested, is carried down the esophagus by the muscular movements known as swallowing. No digestive changes occur in the esophagus; it is merely a passageway. Once the food mass is in the stomach, salivary digestion may continue for 15 to 20 minutes until the acid of the stomach penetrates the mass. Digestion of carbohydrate is resumed in the small intestine. Food is retained in the cavity of the stomach because of the contraction of the pyloric sphincter. Muscular movements of the stomach wall thoroughly mix the mass with the digestive juice of the stomach which is known as gastric juice. This juice, secreted by the gastric glands of the stomach lining, 1s strongly acidic because of the presence of hydrochloric acid. It contains pepsin, an enzyme responsible for most of the digestive activity in the stomach. Pepsin is secreted by the gland cells in an inactive form known as pepsinogen. In the presence of hydrochloric acid, pepsinogen is converted to pepsin. For its effective action this enzyme requires a strongly acid medium which is normally present in the stomach. Pepsin breaks certain of the peptide linkages of proteins, producing fragments of considerable size called pro- teoses and peptones. In the stomachs of young milk-feeding animals, including the human infant, another enzyme, called rennin, is found in the gastric juice. he action of this enzyme is to clot casein, the protein of milk, forming paracasein. Para- casein combines with calcium to form an insoluble compound which remains in the stomach for some time, permitting more prolonged digestion by pepsin. Rennin does not occur in the gastric juice of adults. Degradation of proteins does not go to completion in the stomach. Once the food is liquefied it is pushed through the pyloric sphincter, without regard for the degree of protein digestion, into the small intestine. The muscular activities of the small intestine are of two types, known as peristalsis and segmentation. In peristalsis a muscular contraction begins at the upper end of the intestine and passes with wave-like effect toward the lower end. This motion has a tendency to bring about the movement of the food mass toward the large intestine. Segmentation, on the other hand, consists of a series of contractions occurring close together and simultaneously at different levels of the intestine. This results in a pinching of the food mass into segments; and, since these segmentation contractions disappear and 71 GENERAL ZOOLOGY reappear at alternate levels, they produce a very thorough mixing of the intestinal contents. Food in the small intestine is acted upon by three digestive juices: bile, secreted by the liver and stored in the gall bladder, enters the intestine by way of the bile duct; pancreatic juice, secreted by the pancreatic acini (see Fig. 4.25, p. 120), comes from the pancreas through the pancreatic duct; and intestinal juice comes from intestinal glands in the lining of the duodenal portion of the small intestine. It is well to realize that food entering the small intestine is exposed to the simultaneous action of the various enzymes of these juices, all maximally effective in the alkaline medium of this region. Thus, digestion is a continuous process, not so neatly subdivided as our dis- cussion may suggest. Bile, although it does not contain any digestive enzymes, is an important aid to digestion and absorption. It serves chiefly as an emulsifying agent for lipids, which in its presence become divided into very fine droplets; these offer a large surface for the action of the fat-splitting enzyme. In addition, bile stimulates motility of the intestine and is an effective neutralizer of the acid food mass coming from the stomach. Pancreatic juice, which is also strongly alkaline, contains three types of digestive enzymes, acting on proteins, carbohydrates, and lipids, respectively. Because of its wide spectrum of action, pancreatic juice is by far the most important single digestive juice. In man approximately 750 ml. is secreted each day. Several enzymes in pancreatic juice act on proteins. Of these, the proteinases trypsin and chymotrypsin reduce protein, or proteoses and peptones resulting from prior pepsin digestion, to smaller fragments known as poly- peptides. Chymotrypsin also clots milk and renders its digestion more effec- tive. Both trypsin and chymotrypsin are secreted in the form of inactive precursors known, respectively, as trypsinogen and chymotrypsinogen. ‘These are converted to the active enzymes in the small intestine. Another protein- splitting enzyme, the peptidase called carboxypeptidase, acts on certain poly- peptide linkages, freeing some of the amino acids. ‘The most effective splitter of the complex carbohydrates, pancreatic amylase or amylopsin, is present in pancreatic juice. [t changes starch, glycogen, or the dextrins formed during the action of salivary amylase to maltose, a disaccharide. Finally, a pancreatic lipase, sometimes called steapsin, is found in pancreatic juice. This enzyme is an effective splitter of emulsified lipid into fatty acids and glycerol. Intestinal juice is not such a well-defined entity as the other digestive juices. It is known to contain certain peptidases, specifically aminopeptidase and dipeptidase, which act on polypeptide fragments and reduce them to amino acids. Also, there are several enzymes which complete the digestion of carbohydrates. These are disaccharidases, specifically maltase, sucrase, and lactase which split the disaccharides, yielding monosaccharides or simple sugars. The final stages of digestion of all foods occur in the small intestine. Diges- tion is normally so efficient that there is very little usable food discharged into 172 asojonay pue asoonpy asojoRles pue asoontsy asoonys) asoiong saplieyooesiqg aso7[2W sploe ourmy sproe A}4ey pue [O1edAT5 ‘uolsasIp ynoqe sory yediourd sy], ‘oz'|e “Big aseiong asey[ey| sopiydedAjog (payis—nie) sprdry aseprydediq asepydedoulmy sploe oululy sepryydedAjog ayeutoseovred UIMNtoTeD souojded pue sasosjoi1g aso}[eul pue suli}xeq sjonpoig pug sepiydedAjog (uteyo1d YIU sezelnseoo os[e utsdArj0ur1Ayo) seuojded pue ‘sasoojoid ‘sutejo1g (sutayxep ‘uadooA]3 ‘yor1e}8) soyerpAyoqied peyisnure spidry UWINIOTed YIM peutquios ‘pezey] —nseoo ute}o1d YI, SUIa}01g aseprydedAxoqie) asepiyydeg uisdArjomAyO uIsdAX], saseplieyooesiqy sesepiydeg SaSeUuld}OLg uisdo|Auy (s}[Npe Ut yueseid jou) ulus uisdeg ase[AWy spue[s [euljsoqUuy (eurTexye) orn [eutysezUT seoloued SOSBUTOJOI Spue[s O113Se4) auoNny (uas0ohT[S ‘yor1e4s) sozerpAyoq1ed) pesuey) spooy uryedyg ase[AUy sewAzUY eATISASIG spue[s ArBAr[eg seoIne dAT}sesIq jo sa0inog (eurex[e) aoint o1yeeT0Ueg (eurexTe) et (p18) aomf o1yseH ([eayneu) BATES suorjipuog Yd wnuwydoO pue seomp aAtysesiq 9UT}S}UL OSE] dUT}SOJUT [[eUIS YORUIOAS snseydosq AYLABD YINOP poRry, PAYSOSIG Jo suoidey 73 GENERAL ZOOLOGY the large intestine through the ileocaecal valve. In man the first part of the food mass enters the large intestine about 4 hours after having been eaten, and the discharge continues for about 2 hours. After being retained in the lower part of the large intestine for from 10 hours to 2 days, this undigested and undigestible material, now known as feces, is egested or defecated. ‘The amount of material egested is about 10 per cent of the amount ingested. In the large intestine of herbivorous animals there are many bacteria which di- gest cellulose, a carbohydrate present in the walls of plant cells, and produce simple sugars from it. Although some of this sugar is absorbed, it must be kept in mind that the bacteria digest this material for their own use. It is only incidentally that it affords nourishment for the animal harboring the bacteria. Bacteria in the digestive tract of many animals produce substances important for their hosts’ growth and maintenance. For example, in man intestinal bacteria produce vitamin K (p. 32) and vitamin B,, (p. 34). Some idea of the great number of bacteria comprising the intestinal flora can be had when it is stated that from one-fourth to one-half the dry weight of feces consists of bacteria. The principal facts about digestion are summarized in Figure 3.20, which should be carefully studied in tracing the breakdown of each of the classes of foods as they pass along the digestive tract. Absorption The simple nutrients which are the end products of digestion must be absorbed from the digestive tract into the circulating fluids and be dis- tributed to all the cells of the body before they can take part in cellular metabolism. Absorption may be defined as the passage of simple food com- pounds through the cells lining the digestive tract into the blood or lymph. Between the mucous membrane and the muscular coats of the tract is the sub- mucosa, a region of loosely arranged cells with interlacing fibers (Fig. 3.19). It is in this region, separated from the digestive cavity by the mucous mem- brane, that the delicate lymphatics and the thin-walled capillaries which connect arteries and veins are found (Fig. 3.21). In being absorbed, sub- stances pass through the cells of the mucous membrane and endothelium of the capillaries, as well as the small amount of lymph between the two. Two mechanisms are responsible for the phenomenon of absorption— diffusion, or movement from a region of high concentration (lumen of in- testine) to a region of lower concentration (epithelial cell of intestinal lining), and active transport. Diffusion obeys certain physicochemical laws in the living organism as in non-living systems. Active transport requires expendi- ture of cellular energy to move substances from the lumen of the intestine into the intestinal epithelium. Although certain foods, such as glucose, vitamins, water, and the inorganic salts, require no change before they are ready for absorption, they are not 74 METABOLISM IN VERTEBRATES Muscle layer Vein ONE Capillary net Aion Vi Artery ( Lymph vessel Fig. 3.21. Blood and lymph > |] 4 we We 4 vessels in the wall of the di- kX ye ees : gestive tract; diagrammatic. - A, a portion of the entire Be Lymphatics wall. 8B, capillaries in a {Lt villus, or finger-like projec- \ | : tion of the wall of the intes- Subiicias tine. C, lymphatic in a villus of the intestine. Ar- rows indicate direction of movement of blood and lymph. Lymphatic Capillaries absorbed from the stomach. Only alcohol is freely absorbed from the stomach. The small intestine is the chief organ of absorption. In the upper part of the small intestine the surface of the mucous membrane is greatly increased by folding and, in mammals, by the projection of numerous finger-like villi. In man it has been estimated that there are 5 million villi and 10 square meters of absorptive surface. Amino acids and inorganic salts pass through the epithelial lining directly into the blood stream as a result of diffusion. Glu- cose, on the other hand, must first be converted at the cell surface into glucose phosphate. ‘This phosphorylation under the direction of a specific enzyme requires energy supplied by ATP. ‘The glucose phosphate is rapidly absorbed by the epithelial cells, and glucose soon appears in the blood stream. Lipids are often not completely broken down to fatty acids and glycerol. However, under the emulsifying effect of the bile salts, lipid is dispersed into minute droplets, 0.5 micron or less in diameter. ‘These very fine droplets of lipid enter the epithelial cells and pass directly into the lymphatics of the 75 GENERAL ZOOLOGY LIVER MUSCLE Glycogen Dietary ies Absorbed Portal carbohydrates hexoses vein Lipid Energy + Lactic acid Glucose Energy + CO. + H,O ALL CELLS Fig. 3.22. Transport, storage, and use of carbohydrates. submucosa; the mechanism of their movement remains to be discovered. About a third of the dietary lipid is completely digested to form fatty acids and glycerol. Glycerol is absorbed by diffusion and enters the blood stream. The fatty acids form a complex with certain bile salts. This complex is soluble and passes by diffusion into the epithelial cells. There it is freed from the bile salt and converted by synthetic metabolism into a phospholipid; this lipid then passes into the blood stream. Absorption of the fat soluble vita- mins, A, D, and K, is also dependent on the presence of bile salts. In the absence of bile, deficiency of these vitamins may occur (p. 31) The large intestine is the chief site of water absorption. Pyridoxin, folic acid, and vitamin B,, (p. 34), which are synthesized by the bacteria of the large intestine, are also absorbed there. Transport and Storage of Food Animals eat at intervals, as food is available. The products of the inter- mittent meals are handled in such a way as to provide for the continuous metabolic requirements of the cells of the body. This involves the main- tenance of suitable levels of nutrients in the blood at all times. In the case of carbohydrates and lipids, the organism stores some of the abundant supplies available after eating; later these can be returned to the blood to satisfy metabolic demands. It is relatively easy to determine the general pattern of movement, storage, and reconversion of the absorbed carbohydrate and lipid. In the case of the amino acids, it was not until it became possible to tag them with radioactive isotopes of carbon and nitrogen that we became aware of their almost ceaseless journeys in the body. 76 METABOLISM IN VERTEBRATES The monosaccharides glucose, fructose, and galactose are absorbed into the blood in the capillaries of the wall of the small intestine and carried by way of the hepatic portal vein to the capillaries of the liver (Fig. 3.22). Here, under normal conditions, a considerable amount passes into the cells of the liver where it is converted to the complex carbohydrate glycogen for storage. Leaving the liver, the blood still contains more than the normal level of simple sugar. In muscle cells more of it is converted to glycogen, and the blood sugar level soon returns to its normal range of 70-130 mg. per 100 ml. Everywhere in the body as the blood passes through capillaries, glucose en- ters cells and undergoes the metabolic changes discussed previously (p. 35). The level of blood sugar is maintained between meals as a result of the con- version of liver glycogen to glucose which re-enters the blood. Muscle glycogen is utilized during muscle contraction and does not contribute to the blood-sugar level. “The human body contains about 200 grams of glycogen, equally distributed between the liver and muscles. Radioactive labeling of glucose reveals that a considerable amount is converted to fat and stored as such. Much of the lipid is absorbed into the lymphatic vessels in the wall of the intestine and, in man, is carried to the left subclavian vein by way of the thoracic duct (Fig. 3.23). A sample of blood plasma taken a few hours after a meal rich in fat has been eaten appears milky because of the large number of suspended fat droplets. ‘This fat is promptly stored in the fat depots of the body, such as the subcutaneous and intramuscular connective tissue and the mesenteries of the peritoneal cavity. Stored fat is a readily available source of fuel for the cells. In other words, fat does not go into dead storage; the depots are very labile. It should be noted that stored fat has important in- sulating properties in connection with the conservation of heat and main- LIVER Dietary Absorbed Portal aa. oS) lipids lipids Phospholipids Lymphatics Metaboli — Structure Stored lipids Energy + CO, + H,O ALL CELLS Fig. 3.23. ‘Transport, storage, and use of lipids. 77 GENERAL ZOOLOGY ALL CELLS Structure, Synthesis Anabolism Catabolism Metabolic pool Carbohydrate Dactay Absorbed of —— amino = proteins “acids ae amino acids Lipa LIVER Fig. 3.24. Schema showing dynamic quality of protein metabolism in mammals. In the con- tinuous metabolic turnover, proteins from the diet or from any one tissue can contribute to the protein of any other tissue or be degraded. tenance of body temperature. It also plays a protective role as filling or pack- ing material about and between organs. Some of the fatty acids and glycerol are absorbed into the blood and pass to the liver capillaries. ‘The liver is not normally a fat depot. Instead, the important function of the liver in fat metabolism is the synthesis of phospho- lipids which are returned to the blood to be used by all cells. Glycerol can be converted to glucose. Unlike the hexoses and lipids, the amino acids absorbed into the blood are not stored (Fig. 3.24). ‘They are removed by all cells in connection with the synthesis of protein for maintenance and growth. Excess amino acids can be converted to glucose and then to fat and stored; this is an irreversible reaction. Amino acids can also be converted to glucose and be used as fuel for the cell. During their conversion to glucose the amino acids are deaminated; that is, 78 METABOLISM IN VERTEBRATES they lose their -NH, or amine groups. The liver is the chief site of degrada- tion of excess or worn-out amino acids. In mammals, urea, the chief nitrog- enous waste product, is formed during this process. Birds and reptiles build the amine groups from the breakdown of amino acids into uric acid. In aquatic vertebrates, the amine groups become ammonia (NH3). Studies with labeled amino acids reveal a constant state of flux or shifting of individual molecules between the so-called structureless pool and the formed proteins throughout the body. Respiration The term respiration has been widely used to cover the so-called gaseous metabolism of the organism. In the discussion of cellular metabolism (p. 35), it was shown that oxygen was required for the final steps in the complete extraction of energy from the organic foods. The oxidative reac- tions release carbon dioxide, which is a gaseous waste product and must be removed if cellular metabolism is to continue normally. In_ unicellular organisms, oxygen enters the cell as a result of diffusion gradients between the contents of the cell and its surroundings. In multicellular organisms and especially in the complex vertebrate, a well-defined respiratory system func- tions, together with the circulatory system, to satisfy the gaseous require- ments for cellular metabolism. The discussion of respiration in terrestrial vertebrates will be broken down into three distinct, though continuous, phases. The first of these is pulmonary respiration, or external respiration as it is sometimes called. ‘This is subdivided into ventilation, the movement of the air mass in and out of the lungs, and gas exchange, the simultaneous diffusion of oxygen and carbon dioxide be- tween the alveoli of the lungs and the blood stream. ‘The second aspect of respiration to be discussed is the circulatory phase. ‘This includes gas transport between the lungs and cells in all parts of the body and gas exchange between the blood and these cells. The third phase of respiration is cellular respiration or biological oxidation, which has been discussed previously (p. 37). Air usually contains about 20 per cent oxygen, 0.04 per cent carbon dioxide, and 79 per cent nitrogen. ‘Terrestrial vertebrates live at the bottom of a sea of air, and just as in the ocean the pressure is greatest at the bottom. At sea level, the air we breathe has a pressure of 14.7 pounds per square inch. This is usually indicated by stating that the air pressure will support a column of mercury 760 mm. high. Animals must breathe air under pressure in order to live. At high altitudes, the atmospheric pressure decreases although the actual gas content varies little. At altitudes greater than 18,000 feet, where the atmospheric pressure is only 380 mm. of mercury, oxygenation will be inadequate unless corrective measures are taken. At 50,000 feet, man loses consciousness in about 14 seconds and will die unless pure oxygen is im- mediately supplied under pressure. ‘There is so little pressure at 63,000 feet 79 GENERAL ZOOLOGY that the blood will actually seem to boil. ‘The reduced atmospheric pressure at high altitudes makes it difficult or even impossible for men to breathe. Consequently, adequate air pressure must be artificially maintained in aircraft operating at high altitudes, and above 48,000 feet pilots are obliged to wear pressurized suits. With advances in physiology, as well as in aviation and space technology, new altitude records are constantly being set. At 126,000 feet, an altitude reached some time ago, 99.6 per cent of the atmospheric mass lies below. Living creatures sent into space in rockets have returned alive; mice, dogs, monkeys, and even men have now traveled successfully in space, beyond the limits of our atmosphere, in properly equipped, pressurized cap- sules. A thorough understanding of the physiology of respiration and asso- ciated circulatory functions in the normal individual, together with experi- mentation to achieve safeguards for these bodily requirements, is a necessary prerequisite to man’s conquest of outer space. Ventilation of the lungs requires muscular work. As the muscles of the chest, or thoracic cage, contract, air moves from the external environment into the air passages and lungs. In a 24-hour period of normal muscular activity, something like 10,000 liters of air are moved into the lungs of man, about 500 ml. at each inhalation. ‘The same volume is, of course, exhaled when the respiratory muscles relax and the inherent elasticity of the thoracic structures snaps the lungs back to their collapsed capacity. When it is necessary, or if an individual desires, a much greater volume of air can be moved as a result of more vigorous muscular contractions which not only pull air into but, also, actively force it out of the lungs. Under these conditions, the volume of air inhaled may amount to 4500 ml. for a single maximal muscular effort, depending on the age and size of the individual. In an adult man, about 1 liter of air always remains in the lungs in spite of maximal effort to blow out all the air. This is important because it means that any fresh air pulled into the lungs is mixed with and diluted by the partly used air that cannot be expelled. ‘Therefore, air in the alveoli of the lungs where gas exchange occurs contains only about 15 per cent of oxygen instead of the 20 per cent present in the outside air, and 5.6 per cent of carbon dioxide instead of 0.04 per cent. A survey of the multicellular animals reveals that many different structures have been evolved to capture oxygen from the atmosphere for eventual use in cellular oxidation. All have a common feature—the medium containing oxygen, be it water or air, comes in close approximation, through a very thin membrane, with blood or other body fluid. In man, this membrane is the epithelium lining the alveoli of the lungs. It has a surface area of nearly 1000 square feet, about 50 times that of the skin of the body. The respiratory membrane of man is composed of the alveolar membrane and the endothelium of the blood capillary; it is about 4 microns thick (Fig. 3.10). Free gases in a mixture exert a tension or pressure in proportion to their concentration. Ifa membrane separates two mixtures of oxygen which exert different partial pressures because the concentration is different, oxygen will 80 METABOLISM IN VERTEBRATES move from the region of higher pressure to the region of lower pressure until the pressure exerted by oxygen is equal on both sides of the membrane. ‘This is what happens during gas exchange in the lung. Oxygen in the alveolar air exerts a partial pressure of about 103 mm. of mercury. Blood entering the lung capillaries has a low concentration of oxygen exerting a partial pressure of only about 35 mm. of mercury. Consequently, oxygen diffuses across the respiratory membrane into the blood stream. ‘The time required for the blood to pass through the capillary bed of the lung allows for almost complete equilibration of oxygen. This same mechanism results in the elimination of excess carbon dioxide from the blood as it circulates through the capillaries of the lungs. During its course throughout the body, the blood has picked up the carbon dioxide formed during cellular oxidation. ‘The partial pressure of carbon dioxide in blood entering the lung capillaries is about 46 mm. of mercury as compared with a partial pressure of 38 mm. of mercury exerted by the carbon dioxide in the alveolar air. Therefore, carbon dioxide diffuses from the blood into the alveolar air, and blood leaving the lung capillaries has a carbon dioxide pressure of 38 mm. of mercury. Pulmonary gas exchange thus occurs as a result of diffusion. Blood leaving the lungs carries oxygen and carbon dioxide in concentrations equal to those in the alveolar air. ‘The first hurdle in meeting the oxygen requirements of cells far removed fromi the external supply of oxygen has been cleared. Elimination of carbon dioxide through the lungs is, likewise, very important in bodily maintenance. Apart from the obvious necessity of getting rid of a waste product of cellular oxidation, the elimination of carbon dioxide helps to maintain the normal acid-base equilibrium of the body. ‘The amount of carbon dioxide eliminated each day is equivalent in terms of acidity to a liter of hydrochloric acid. During transport the respiratory gases are mostly in chemical combination with certain constituents of the blood. ‘This makes it possible to move 100 to 150 times as much oxygen and carbon dioxide as could be moved if they were only dissolved in the blood. Each gas must, however, exist in simple solution on its way to the carrier compound and upon its release from that compound. In man about 99 per cent of the oxygen is carried in a loose sort of chemical combination with hemoglobin, an iron-containing protein found in red blood cells. ‘The combination is called oxyhemoglobin and is responsible for the red color of oxygenated blood. During normal ventilation of the lungs, an individual has about 97 per cent of his hemoglobin saturated with oxygen. With forced breathing the percentage of oxyhemoglobin rises. At high altitudes, or in certain diseased conditions, considerably less satu- ration of hemoglobin is possible, and the amount of oxygen supplied to the cells is inadequate. Corrective measures must be taken in order to obtain sufficient oxygen. Animals living at high altitudes make more red blood cells, thereby supplying more oxygen carriers. In an airplane or in case of disease, the concentration of oxygen in the air breathed can be raised. The increased 81 GENERAL ZOOLOGY concentration results in higher pressure which drives more oxygen through the respiratory membrane into the blood. At the capillary level in the tissues, hemoglobin gives up its oxygen and is known as reduced hemoglobin; this gives the blood a purplish color. Gas exchange now occurs between the blood and the cells through the endothelium of the capillaries. ‘The freed oxygen in simple solution diffuses through the lymph into the cells to take part in biological oxidation. The movement of oxygen into the cell and carbon dioxide from it obeys the rules governing gas exchange in the lungs. ‘The pressure gradient for oxygen in the tissues is from blood to cells; the gradient for carbon dioxide is from cells to blood. About 92 per cent of the carbon dioxide is carried in the blood in chemical combination, most of it as bicarbonate. ‘The red blood cells contain an enzyme which accelerates the combination of carbon dioxide and water to form carbonic acid. Carbonic acid then dissociates, and about a third of the bicarbonate ions form a salt with potassium in the red blood cell. The other bicarbonate ions diffuse out of the red blood cell and form either a sodium salt or more carbonic acid. In this way, the excreted carbon dioxide is processed by the red blood cells and then transported to the lungs. ‘There it goes into solution and is eliminated by diffusion. Excretion Continuity of chemical reactions depends on the removal of their end products. Consequently, the waste products of metabolic reactions, or excreta of the body, must be continuously removed from cells in order that metabolic reactions may continue; the waste products diffuse from the cells into the blood stream. The process of removal of waste products of metabolism from the body is called excretion; the important organs of removal are the lungs, skin, excretory organs, and liver. It will be recalled that the waste products of cellular metabolism are carbon dioxide, water, and nitrogenous compounds. The excretion of carbon dioxide in the lungs of terrestrial vertebrates has already been discussed. In aquatic animals this waste gas diffuses into the water surrounding the gills. Also, the relatively thin, moist skin of the amphibia excretes considerable amounts of carbon dioxide. Water is excreted by a number of organs. Exhaled air is moist because the lungs of air-breathing vertebrates excrete water. In the skin of many mam- mals, including man, there are sweat glands from which watery solutions pass to the outer surface of the body by way of ducts (Fig. 3.25). Sweating, how- ever, is of less importance as an excretory process than it is in the regulation of body temperature (p. 125). In the majority of vertebrates at least 50 per cent of the water is excreted in the urine through the ducts of the kidneys. The kidneys in man account for only 0.4 per cent of the total body weight. Yet they handle in 1 day about 180 liters of fluid containing in solution solids 82 METABOLISM IN VERTEBRATES Epidermis Duct Fig. 3.25. The skin of man, in section, showing char- acteristic structures; dia- grammatic. The bulb is the so-called root of the hair, where growth occurs. Blood vessels and nerves associated with the hair are found in the papilla. Capillaries are shown around the _ sweat gland at the left but are dissected away on the right. Oil gland iA Na Dermis f WS game? me \) a Qt: \ t Hair shaft FAN th Bulb 5 Papilla J we | SN Sweat gland eS weighing nearly five times as much as the kidneys. Each kidney in man is made up of approximately a million structural and functional units, the nephrons or excretory tubules (Fig. 3.12). The arrangement of the blood vessels of the nephron is unique and makes possible its function of excretion. A short arteriole carrying blood from the renal artery is continuous with the glomerulus, which is a tuft of capillaries with few anastomoses. Blood is forced through the glomerular capillaries into another arteriole of about the same diameter as the entering one. Under such circumstances, pressure filtration of the blood occurs for as long as it remains in the glomerulus; about one-fifth of the plasma entering the glomerulus is filtered out into the lumen of the nephron. All the constituents of the blood plasma except the large protein molecules can pass through the capillary wall and the epithelium of Bowman’s capsule. Blood from the outgoing arteriole of the glomerulus passes into a capillary bed which surrounds the excretory tubule, which is very long and convoluted in the kidneys of mammals. The difference in concentration between the glomerular filtrate and the blood in the capillary bed results in the reabsorp- tion by the blood of most of the material removed in the glomerulus. About 99 per cent of the water is reabsorbed, thus concentrating the filtrate which 83 Adipose tissue GENERAL ZOOLOGY is now known as urine. The amino acids and inorganic ions (Nai Ko) Cie HCO, , HPO, , and SO,>) also re-enter the blood by diffusion, as does about 40 per cent of the urea. Urea, CO(NH,)s, is the chief nitrogenous waste of mammals and results from amino acid metabolism. It is built up in the liver in a series of reactions, driven by the energy of ATP, and directed by at least seven different enzymes. Glucose moves back as a result of active transport (p. 74); that is, energy is required to move it across the membrane of the tubule. Only transitorily after a meal rich in carbohydrates, or in the diseased state of diabetes (p. 120), can glucose be found in the voided urine. Cells of the tubular membrane also add certain substances to the tubular fluid. In addition to the waste products of metabolism, certain drugs, such as penicillin, and compounds detoxified by the liver leave the body in the urine. The kidneys function in the manner indicated to preserve the constancy of the internal environment of the animal with respect to water and electrolyte balance which is necessary for optimal body function, as well as to eliminate nitrogenous waste products of metabolism. There are substances other than those we have just discussed which must also be eliminated if the organism is to remain normal. One of these is the pigment bilirubin, formed by the disintegration of hemoglobin when red blood cells die and are destroyed. Bilirubin leaves the body by way of the liver and is the pigment chiefly responsible for the color of the bile. Jaundice results if for any reason this pigment remains in the body. ‘The liver also eliminates cholesterol, which arises, in part, from the destruction of red blood cells. Certain types of gallstones are almost pure cholesterol, which is a steroid (p. 26). Various drugs, certain poisons, and metals, such as copper and iron, none of which are excreta as we have defined the term, are eliminated from the body in the bile secreted by the liver. ‘These substances are dis- solved in the bile and are carried to the large intestine, where they are found in the feces and eliminated when defecation occurs. Summary In this chapter, we have considered the ways in which multicellular animals like the vertebrates have met the metabolic requirements of the living cells of which they are composed. We have described systems of organs—digestive, circulatory, respiratory, and excretory—the functions of which, in the final analysis, are simply to provide an internal environment in which individual cells, specialized in many different ways, can maintain themselves and con- tribute to the life of the whole group. It may have occurred to you that in such a complex and interrelated series of events there must needs be some insurance of orderliness, of checks and balances, because externally the indi- 84 METABOLISM IN VERTEBRATES vidual animal appears to us as a unit. The unification or coordination of the many functions you have learned about in this chapter is the contribution of other cells and organs, the nature of which will be discussed in the next chapter. 85 Olfactory lobe sory Cerebral ics es hemisphere hemisphere Olfactory lobe Ogre Cerebral 4 Diencephalon LE Dptic lobe Optic lobe Cerebellum Cerebellum Medulla Medulla oblongata oblongata (B) Amphibian (C) Reptile (A) Fish CHAPTER RESPONSIVENESS IN VERTEBRATES Fig. 4.1. Brains of representative vertebrates, from dorsal view. Cerebral Olfactory lobe hemisphere Cerebral hemisphere Cerebellum Medulla oblongata (D) Bird: (E) Mammal (F) Human RESPONSIVENESS IN VERTEBRATES The organ systems related to metabolism perform certain functions which are necessary if individual cells are to remain alive. ‘They deliver food and oxygen to all cells and remove their waste products, providing a suitable internal environment for maintenance and function of the different tissues. The stability of the internal environment, upon which our feeling of well- being depends, requires constant adjustment of the interlocking functions. The many systems must work together, or be coordinated. If any system concerned with the metabolic needs of cells ceases to function, the organism cannot remain alive; it becomes abnormal if the activities of its organs are not correlated in the usual way. Although we are not conscious of the regulation of our internal environ- ment, we are well aware of many of the adjustments we necessarily make to our external environment. Every mechanism of regulation, adaptation, or coordination, whether conscious or unconscious, is possible because proto- plasm has the capacity of responsiveness; a cell responds by internal reaction to a stimulus, or change in its environment. ‘There are two ways of altering the immediate environment of cells in the vertebrate body. One is by means of impulses that pass along the nerves which penetrate to every part of the animal, and the other is by means of substances that circulate in the blood. Nervous coordination is brought about by the activities of the sense organs and nervous system. ‘The secretions which enter the blood from the endocrine or ductless glands make what is known as chemical coordination possible. We shall first review the structure of the systems related to coordination and then explain the ways in which they function. ‘These systems are the nervous system, together with the organs of special sense, skeletal system, muscular system, and endocrine system. ‘The skeletal and muscular systems are discussed here because so many adaptive responses to the external en- vironment involve bodily movements for which the nervous, muscular, and skeletal systems are responsible. Organ Systems Related to Coordination The Nervous System. The nervous system of vertebrates is divided for purposes of discussion into two parts: the central nervous system and the peripheral nervous system. ‘The central nervous system is composed of the brain and spinal cord. ‘The peripheral nervous system consists of the nerves which connect the brain and spinal cord with all parts of the body. The Central Nervous System. The central nervous system develops in the same way in all vertebrates. Soon after its first appearance it is found to have five regions in the brain, which can be distinguished from the spinal cord. These five regions are known, from anterior to posterior, as the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon (see Fig. 5.21, p. 156). None of these parts is lost in any vertebrate, but differences in the degree of development of certain regions, especially of the telencephalon 87 GENERAL ZOOLOGY Nasal cavity Olfactory nerve (1) Optic nerve (II) A A Shee SECUNDA = MUSC ULORUM Fig. 4.7. The muscles of man (cf. Fig. 4.6). 93 GENERAL ZOOLOGY Hypophysis (pituitary gland) Parathyroid glands Thyroid gland Pancreas Duodenum Uterus Fig. 4.8. ‘The endocrine glands of man; with the exception of organs of the reproductive system, the location of these glands is the same in both sexes. 94 RESPONSIVENESS IN VERTEBRATES Muscles are attached to rigid bones which are held together at the joints and are characterized by their ability to contract or shorten. By pulling against one another across joints, muscles make possible typical postures, as in man, and when they contract produce locomotion or move the appendages inde- pendently. Joints of the skeleton of the appendages are freely movable and are of two main types. Hinge joints, which are found between the bones of the upper and lower arm and upper and lower leg, allow movement in only one plane. Ball-and-socket joints, which are found where the bone of the upper arm articulates with the shoulder and the bone of the upper leg articulates with the hip girdle, permit a rotation of the limbs when certain muscles con- tract or pull against others. ‘The coordination of the activities of the muscles is extremely important for the animal as a whole and is brought about by the nervous system. The Endocrine System. Reference has been made to glands, or organs of secretion, which possess ducts and pass their secretions onto body surfaces (p. 61). There are a number of glands which do not have ducts by which to discharge their secretions; instead, the secretions pass directly into the blood stream. Such glands are known as the ductless glands, glands of internal secretion, or endocrine glands. ‘Their secretions are called internal secretions, endocrines, or hormones. ‘The endocrine system does not consist of a group of closely associated organs like the other systems. Instead, the endocrine glands are widely separated from one another and possess only one feature in com- mon; they pass their secretions into the blood stream. ‘The principal organs of the endocrine system are the hypophysis (pituitary gland), thyroid gland, parathyroid glands (four in man), pancreas, adrenal glands, the gonads, and, in the higher mammals, the placenta (Fig. 4.8). There are, however, several more or less isolated groups of cells, especially in the wall of the small intestine, which also produce hormones. Coordination with Special Reference to the External Environment Basic Mechanisms of Nervous Coordination. Every nervous coordi- nation is the result of a reaction by some part of the body to a stimulus. The simplest type of response to a stimulus is known as reflex action. When you touch anything hot with your finger, the muscles of your arm react to with- draw your hand. A nerve impulse passes from the point stimulated to the central nervous system and travels back to produce the contraction of the arm muscles. Another well-known illustration is the knee-jerk reflex, in which the leg is extended as a result of a sharp tap below the kneecap. In both these examples the response to the stimulus is apparent in the general region that receives the stimulus. Complete analysis of these simple reflexes shows that they are the expression of a nervous mechanism which some investigators be- lieve explains all nervous coordination. 95 GENERAL ZOOLOGY Fig. 4.9. Different regions of the central nervous system, in cross section, to show the dis- tribution of white (clear) and gray (dotted) matter; dia- grammatic. A, spinal cord; B, medulla; CC, cerebral hemispheres. The cells of the nervous system that are directly concerned with its function of coordination are known as neurons (p. 68). The neurons are arranged in such a way that the parts containing the nuclei are located in groups which constitute the gray matter (Fig. 4.9) of the central nervous system and the ganglia of the peripheral nervous system. ‘The processes which are always continuous with the main part of the cytosome around the nucleus are called nerve fibers, and bundles of them make up the white matter of the central nervous system and the nerves of the peripheral system. Many neurons are arranged in what are known as reflex arcs; the reflex arc is the cellular mechanism of reflex action. In a reflex arc the ends of the terminal filaments of the axonic process of one neuron (see Fig. 4.22) come in contact with a dendritic process or the cell body of another neuron, but there is no structural continuity between the cells. Such places of contact between different neurons are known as synapses and make possible the func- tional continuity of the nervous system. In the simplest reflex arc there may be only two neurons involved (Fig. 4.10A). ‘The stimulus is received by some specialized group of cells constituting a receptor, which is a general term for any type of sense organ. Asa result of the reception of the stimulus, what is known as a nerve impulse is established; this is conducted from the place of stimulation toward the central nervous system along a nerve-cell process (p. 68). In the simplest reflex arc the impulse will be conducted to the 96 RESPONSIVENESS IN VERTEBRATES Dorsal root White matter Gray matter Afferent neuron Dorsal root ganglion Afferent neuron Adjustor neuron Ventral root Efferent neuron Effector te Wn root anglion / c\ gang Adjustor neuron Ni a Afferent | +— neurons ! <— D Efferent neuron aS DEEL ETD t i, Efferent neuron Ventral 200 x re a Spinal nerve er BH Fig. 4.10. A, the spinal cord, in cross section, showing a pair of spinal nerves and the essen- tial parts of a reflex arc; diagrammatic. ‘The neurons necessary for the simplest type of reflex action are shown on the left, and those of a typical reflex arc are represented on the right. 8B, the spinal cord, from the side, showing some relations of neurons in reflex arcs; diagrammatic. The brain would lie to the right as the diagram is constructed. Arrows in both diagrams indicate the direction of conduction of the nerve impulses. 97 GENERAL ZOOLOGY spinal cord. ‘lhe neuron over which the impulse enters the spinal cord is a sensory or afferent neuron. ‘The cytosome of such a sensory neuron is located in the dorsal-root ganglion of the dorsal or sensory root of a spinal nerve. One of the processes of the afferent neuron enters the centrally located gray matter of the spinal cord, and its terminal filaments end in contact with other nerve cells found there. In the hypothetical case under consideration, the impulse would pass through a synapse between a process of a sensory neuron and a dendrite or the cell body of a motor or efferent neuron and leave the spinal cord by way of the axon of the same efferent neuron. ‘This nerve fiber passes into the ventral or motor root of the spinal nerve and continues to a skeletal muscle, on which it terminates. The place of contact between a nerve-cell process and a muscle cell is known as a neuromuscular junction or motor end plate (see Fig. 4.22). It is at such a place of contact that the impulse is discharged and a specific chemical compound, acetylcholine, is liberated (p. 115). ‘The muscle is stimulated by the acetylcholine and contracts. This reaction is the effect produced by the change in the environment. A muscle is known, there- fore, as the effector in a reflex arc. ‘The parts of the simplest type of reflex arc are the receptor, where the stimulus is received; the afferent neuron, over which the impulse is conducted to the spinal cord; the efferent neuron, over which the impulse is conducted away from the spinal cord and from which it is discharged; and the effector, where the reaction to the stimulus occurs. In the great majority of reflex actions the effect is produced in some part of the body other than that at which the stimulus is received. If the skin of a dog’s back is rubbed with a pointed instrument, the animal will respond by attempting to scratch the place of stimulation with its hind leg. ‘The receptors in this instance are located at the roots of the hairs of that region of the back which is stroked. ‘The afferent neurons conduct the impulses to the spinal cord over the dorsal root of the spinal nerve whose fibers extend to that region of the skin. Within the gray matter of the cord each sensory neuron has synapses with neurons of which both the cytosome and processes lie entirely inside the spinal cord. Over the processes of such neurons the nerve impulse is conducted posteriorly along the spinal cord to the level of exit of the nerves that extend to the hind legs. Here synapses occur with efferent neurons, and impulses leave the spinal cord over the ventral root of a spinal nerve and pass to muscles (effectors) that produce the scratching motion. In this type of reflex arc three kinds of neurons are concerned (Fig. 4.104). The neuron along which the impulse passes within the spinal cord is known as an adjustor neuron or interneuron. Adjustor neurons are very numerous in the central nervous system and make possible the varied reactions that a single stimulus can produce. For example, when acid is applied to the skin of a frog’s back, the first reaction is a contraction of muscles of the body wall in the region stimulated. Very soon, however, this is followed by other reactions which can be observed best in a frog from which the brain has been removed (Fig. 4.108). The fore leg on the side stimulated moves toward the location of the acid, and this reaction is followed quickly by movements of the hind 98 RESPONSIVENESS IN VERTEBRATES legs on the same side. ‘These movements tending to remove the acid are made possible by the passage of impulses over adjustor neurons which conduct them anteriorly and posteriorly to efferent neurons leading to muscles of the fore and hind legs. The reactions described occur on the side of the animal to which the acid has been applied. If under such conditions the hind leg on this side be held, the muscles of the hind leg on the opposite side will respond to the original stimulus by contracting. ‘This effect is made possible by the presence of adjustor neurons which conduct impulses from one side of the spinal cord to the other and thus bring about bilateral coordination. In examples given we have been concerned with isolated reflexes; that is, particular reflex arcs have been discussed as if they were separable from the remainder of the nervous system. Such is obviously not the actual state of affairs. During life great numbers of sensory neurons are conducting impulses to the central nervous system at all times. Within the central nervous system, fibers of a number of different sensory neurons may end on the dendritic processes or cell body of a single sensory or adjustor neuron. Different axonic processes of a single sensory or adjustor neuron may end on different adjustor or motor neurons. It has been reported that the cell body of a single motor neuron in the spinal cord of a mammal may have as many as 1800 end feet of terminal filaments on its surface. Over this apparent maze of pathways pass in orderly fashion the nerve impulses that make possible not only simple reflex actions but higher nervous coordinations as well. The factors which determine the smooth function of the nervous system are complex and not entirely understood. In an organ of special sense there are cells that are much more sensitive to a certain kind of change in the environ- ment than to any other. ‘The response of sensory cells to stimuli apparently involves a change in the permeability of their membranes, modifying the ionic or chemical environment of nerve endings in their vicinity. In some way this brings about excitation of the nerve endings so that their membranes exhibit increased permeability to sodium and potassium ions; the movement of ions results in the establishment of an electric current. ‘This excitation builds up in the nerve endings until a nerve impulse of the so-called all-or-none type is touched off and conducted along the nerve fiber in a manner to be described later (p. 112). ‘The more intense the stimulus, the more frequent the initia- tion of the impulses; the frequency varies from 50 to 100 impulses per second. So long as the excitation is adequate, impulses of uniform intensity are established and travel toward the central nervous system and into all processes of the neuron at an undiminished rate. In the terminal filaments in the region of the synapses the arrival of an impulse, lasting about 1/1000 of a second, is probably associated with the transitory production of acetylcholine (p. 115) which increases the permeability of the membrane of the dendrite or nerve-cell body of the neuron with which the synapse occurs. Local responses at the many synapses on adjustor and motor neurons vary in in- tensity and are known as graded responses in contrast to the all-or-none responses of axonic processes. As a result of the phenomenon of facilitation, 99 GENERAL ZOOLOGY graded responses in adjustor and motor neurons may be reinforced by the arrival of successive impulses on the same or different sensory neurons until the threshold for firing an all-or-none impulse is reached. When this hap- pens, the pattern of response to the stimulus begins to take shape as the con- duction pathways are selected. It is important to understand that the nerve- cell bodies in the central nervous system are kept in a constant state of excita- tion, the central excitatory state, by the arrival of successive impulses from the many sense organs. If the volleys of impulses are frequent enough, the graded responses will build up in a neuron until they trigger the firing of all-or-none impulses in its axonic process. ‘The central nervous system is functionally characterized by its readiness to respond. The mechanism of the reflex arc obviously makes possible the highest de- gree of coordination in what is known as the behavior of the animal. Sher- rington has generalized the facts of nervous coordination in his principle of the final common path. Each afferent neuron is a special pathway by which im- pulses from its particular receptor enter the central nervous system. Within the central nervous system the impulse can travel over varied paths formed by synapses between adjustor neurons and, theoretically, can produce a reac- tion in any of the effectors. ‘The efferent neurons, over which impulses travel from the central nervous system to the effectors, differ from the afferent neurons in that they are not private paths for particular impulses. It is a commonplace that many different kinds of stimuli can produce the same re- action or effect. Consider, for example, the many and varied stimuli to which man responds by walking. ‘The efferent neuron is, therefore, a final common path over which impulses established by stimulation of receptors all over the body can be discharged at a particular effector. By means of the adjustor neurons of the central nervous system, connections are made possible between all the special pathways that lead from receptive areas and these final com- mon pathways to effector regions. ‘The conduction of impulses according to this principle of the final common path establishes a mechanism for the com- plicated and varied responses that characterize nervous coordination. By means of this mechanism the animal can behave as a unit in its reactions to the changing conditions of its external environment and also maintain its internal environment within a narrow range of physiological variation. Series of reflexes, or their occurrence in sequence, are well understood in some situations and are a very important factor in reflex coordination. ‘The procedure by which a frog obtains its food involves a sequence or chain of reflexes. “The visual stimulus produced by a moving insect is followed by pro- trusion of the tongue. If the insect is captured, its contact with the roof of the mouth cavity initiates the swallowing reflexes, which occur in sequence. In the examples considered so far, the response to the stimulus has been studied with respect to the usual external conditions that produce the effect. Pavlov, a Russian physiologist, discovered that it is possible to produce what he termed conditioned reflexes. For instance, the flow of saliva is a reflex action stimulated normally by the sight of food. Under experimental condi- 100 RESPONSIVENESS IN VERTEBRATES tions a bell is rung whenever food is given to an animal. After a number of such experiments the mere ringing of the bell, without the sight of food, will result in the secretion of saliva. In this way a stimulus that originally had no effect upon the salivary glands has become associated with one to which the glands respond. As a result of this association the previously indifferent stimulus of the ringing bell becomes effective in producing the reaction of the salivary glands; a conditioned reflex has been established. Experiments and analysis of conditioned reflexes make it clear that many human reactions are the result of such correlations. Our responses to warning colors, signals, and nationally used signs and symbols are in the nature of conditioned reflexes. The same explanation holds for many more subtle and less well-understood adjustments. Localization of Function in the Nervous System. ‘The basic mech- anisms of excitation and conduction are shared by all parts of the nervous system. Different regions of the nervous system do, however, have certain distinctive functions. The general function of the peripheral nervous system is to conduct nerve impulses to and from the central nervous system. In regard to the spinal nerves, it has been pointed out that processes of afferent neurons enter the spinal cord over the dorsal roots of spinal nerves, and the processes of efferent neurons pass out along the ventral roots. ‘he spinal nerves are called mixed nerves and may be considered to represent the primitive condition of nerve trunks. Certain of the cranial nerves, as the third or oculomotor, also carry processes of both sensory and motor neurons. Other cranial nerves carry processes of but one type of neuron. ‘The eighth cranial or auditory nerve is made up entirely of processes of afferent neurons from the ear; the eleventh and twelfth cranial nerves, found in the higher vertebrates, contain processes of only efferent neurons. Finally, the autonomic nerves are entirely efferent and constitute the final common paths to glands and to non-striated muscles of the blood vessels and viscera. As has been repeatedly implied in the discussion of the reflex arc, the gen- eral function of the central nervous system is the adjustment of incoming to outgoing impulses. It is in the central system that afferent neurons have synapses with adjustor neurons, and these in turn with efferent neurons. ‘The multiplicity of connections thus made possible furnishes the most important part of the mechanism of integration. Adjustor neurons in the spinal cord are related to the simpler and _ less complicated of the reflex arcs. In the scratch reflex, for instance, adjustor neurons carry the impulse posteriorly in the spinal cord or conduct it from side to side. Impulses entering the cord over spinal nerves can also pass anteriorly to the medulla, cerebellum, and diencephalon. ‘The cytosomes of these adjustor neurons are located in the gray matter of the spinal cord; their processes, over which impulses are conducted along the cord, are to be found in the white matter. The white matter also contains groups of nerve processes which arise from the cell bodies of adjustor neurons located in 101 GENERAL ZOOLOGY the cerebral hemispheres, mesencephalon, and medulla. The gray matter of the cord is, therefore, the location of adjustor neurons which connect dif- ferent levels of the cord with one another and with parts of the brain and which conduct impulses from one side of the cord to the other. In addition, the cell bodies of efferent neurons, the processes of which pass out in the ventral roots of spinal nerves, are found in the gray matter of the cord. Simple reflexes are adjusted in the spinal cord, and impulses are conducted to and from the brain. The primitive brain, or brain stem, is composed of the telencephalon, dien- cephalon, mesencephalon, metencephalon, and myelencephalon (see Fig. 5.21, p. 156). Changes in the direction of greater brain complexity occur chiefly in the regions of the telencephalon and metencephalon, from which the cerebral hemispheres and the cerebellum, respectively, develop. ‘The cerebral hemispheres and cerebellum are the parts in which new functions are added; the functions of the brain stem remain practically constant throughout the vertebrate group. In contrast to its arrangement as a column in the spinal cord, the gray matter occurs in the brain in masses known as centers, which may be surrounded completely by white matter, as in the medulla, or form a continuous peripheral layer, as in the cerebral cortex (Fig. 4.9). It is im- possible to present here a detailed account of the functions of the parts of the brain, but the more important localizations will be given. The medulla, into which the spinal cord merges, serves as a pathway be- tween the cord and other parts of the brain. It also contains the centers that adjust the reflexes of the tongue and of breathing. In the tongue reflexes, impulses enter over processes of afferent neurons by way of the fifth and ninth cranial nerves and pass out over processes of efferent neurons by way of the twelfth nerve. ‘The adjustor neurons lie in centers within the medulla. ‘The respiratory reflex depends on the sensitivity of the respiratory center in the medulla to the amount of carbon dioxide in the blood (p. 124). Impulses are conducted over processes of efferent neurons to muscles between the ribs and in the diaphragm. ‘The rhythm and depth of breathing and other re- flexes of the viscera, pharynx, and larynx are also adjusted in the medulla. The ventral part of the metencephalon consists of fiber tracts that conduct from side to side, as well as of those connecting lower and higher levels. In the cerebellum, or dorsal part of the metencephalon, there are important muscle-coordinating centers. “These coordinations may involve the body as a whole, as when reactions occur in response to stimuli received by the organs of equilibration, the semicircular ducts of the ear. ‘The adjustments that re- sult in bilateral muscular coordinations are also made in the cerebellum. Such bilateral coordinations are chiefly those of the movements of the limbs, although the muscles of the eyes, facial expression, and mastication are believed by some investigators to be bilaterally correlated in cerebellar centers. On the dorsal surface of the mesencephalon are found the optic lobes; here are located centers in which certain important visual reflexes, such as the constriction of the pupil of the eye in response to the stimulus of light on the 102 RESPONSIVENESS IN VERTEBRATES Retina Fig. 4.11. The optic chiasma in higher verte brates; diagrammatic. retina, are adjusted. In the higher vertebrates certain reflexes following sound stimuli are also adjusted in the optic lobes. ‘The lateral and ventral regions of the mesencephalon contain groups of neurons that provide for numerous connections and nerve tracts over which impulses are relayed from one region to another. In the diencephalon are found many nerve tracts connecting centers in other parts of the brain with the cerebral cortex. The optic nerves and tracts over which impulses are conducted from the ‘retinas to the optic lobes form part of the floor and lateral walls of the diencephalon. In the lower vertebrates all the fibers from one retina cross the optic chiasma to enter the opposite optic lobe. ‘The crossing in higher vertebrates involves only the medial half of the fibers of each retina; the fibers of the lateral halves do not cross (Fig. 4.11). Certain correlations resulting from olfactory stimuli are made in the diencephalon, and impulses giving rise to pain sensations are received there. Centers concerned with the regulation of many basic vital processes such as sleep, water balance, and heat control are located in the hypothalamus or floor of the diencephalon (p. 125). Among the lower vertebrates the most important parts of the telencephalon are the centers for correlation of impulses transmitted from the olfactory organs. ‘The olfactory centers in mammals occupy the same relative position but are overshadowed by the very great growth of the dorsal part of the telencephalon to form the cerebral hemispheres. In the cerebral hemispheres, as in the cerebellum, the neurons that make up the gray matter are found in a continuous superficial layer known here as the cerebral cortex; there are more than 9 billion neurons in the human cortex. Although the cortex is con- tinuous, certain areas are known to be concerned with special functions. Impulses producing movements of the voluntary muscles are conducted from the motor centers of the cortex to opposite sides of the body; that is, if these particular areas are destroyed in one cerebral hemisphere, the animal 103 GENERAL ZOOLOGY Fig. 4.12. ‘The left cerebral hemisphere of the human brain on which sensory, motor, and associa- tion areas are indicated. In the sensory areas (dotted), one or many stimuli are interpreted, as when we identify a typewriter by its sound alone, a lead pencil by handling it, or the type and nationality of an airplane by a synthesis of fleeting visual stimuli. In the motor areas (lined), muscular activities are arranged into purposive or meaningful sequences, as in speaking. ‘The association areas (clear) are concerned with the complex correlations of memory and thought. is unable to use the voluntary muscles on the opposite side of the body. ‘The regions that coordinate movements of the principal parts of the body, from the toes to the face muscles, are known in man. Another major division of the cortex is concerned with sensory functions and contains the sensory centers to which impulses are conducted from visual, auditory, and olfactory re- ceptors, as well as from receptors of pressure, temperature, and taste stimuli. These areas have been mapped almost completely for the human cortex (Fig. 4.12). The association centers of the cortex are filled with adjustor neurons which are involved in the complicated pathways used in the mental activities of thinking and learning. Intelligence depends on the degree of development of the cerebral cortex and especially on the neurons of the association areas. An animal’s ability to profit by experience involves analysis of a situation and memory, enabling the individual to react in a way that is advantageous in a new situation. This ability is determined by the animal’s degree of intelligence, which, in turn, is limited by the number of adjustor neurons and the synapses existing between them. It is known that all the neurons that an animal will ever possess are present at a very early stage of its development. New synapses are, however, formed throughout the life of the individual and probably de- pend on the variety and intensity of the stimuli received by that individual. The sensory impulses that reach the cerebral cortex, the motor impulses that pass out from it, and the associations made in it constitute our so-called 104 RESPONSIVENESS IN VERTEBRATES consciousness. Sleep and anesthetics in some way lessen or completely block the functioning of the cerebral cortex and produce unconsciousness. By combined functions of the nervous system as a whole, the activities of the organism are correlated so that it behaves as a unit; the function of the nerv- ous system is, therefore, coordination or integration. It has been pointed out that nervous coordination depends essentially on three factors. In the first place, the organism must be able to be aware of changes in its environ- ment, that is, to receive different kinds of stimuli. Secondly, nerve impulses which are set up in response to the stimulus are conducted over the neurons of the reflex arcs. And, finally, the animal must respond to the stimulus when the impulse is discharged at some effector. We shall now describe some of the mechanisms concerned with reception, conduction, discharge, and response, especially with respect to adjustment to changes in the external environment. Reception and Conduction. ‘The eye is an organ of special sense con- taining the retina which is made up of cells receptive to changes in the environment brought about by light rays (Fig. 4.3). The cornea, lens, and fluids of the eye function as refracting surfaces which, in the normal eye, bend the light rays entering the eyeball from a distance of more than 20 feet so that they are brought to a focus on the retina (Fig. 4.13). If, during its develop- ment, the eyeball grows out of proportion to the refracting surfaces, defective vision will result. The near-sighted individual has an eyeball too long for its refracting surfaces, and light rays focus in front of the retina; objects at a dis- tance seem blurred. Contrary to popular opinion, such an individual does not see close objects any better than a normal person does. ‘The far-sighted person has an eyeball that is too short for the refracting surfaces, and light rays focus behind the retina; this, too, results in blurred images. If appro- priate corrective lenses are placed in front of the eyeball, the light rays can be brought to focus on the retina, and objects are seen distinctly. When an object is closer than 20 feet, a change in the shape of the crystal- line lens of the eye must occur in order for the light rays to be focused on Near-sighted (Myopic) | : Fig. 4.13. Eyeballs of different shapes; note | effect on the focal point for light rays travel- ing more than 20 feet. | Normal | (Emmetropic) | | | Far-sighted | | (Hyperopic) ial GENERAL ZOOLOGY Lamina vitrea , &_~ — Pigment epithelium Outer nuclear layer Outer plexi- form layer Inner nuclear layer Inner plexi- form layer Ganglion layer Nerve fibers ia Int. lim. memb. Fig. 4.14. The human retina (cf. Fig. 4.3). Left, drawing of vertical section; the region at the bottom is adjacent to the chamber of the vitreous humor; x500. M7ddle, diagram showing relationships of principal types of cells. Light passing through the lens, vitreous humor, and inner layers of the transparent retina reaches the receptor layer made up of the outer segments (cf. Fig. 4.15), or photosensitive regions, of the cones (c) and rods (7). Nerve impulses established in the region called the outer plexiform layer are conducted by way of neurons of different types, the cell bodies of which are shown at a, 6, cb, db, g, h, and pg. These impulses finally are conducted by way of the nerve fibers which form the innermost layer of the retina and which extend outward through the retina in the region of the blind spot to make up the optic nerve. Right, diagram of a Miller fiber (nucleus at A/), which functions to hold the other retinal elements together; opposite ends of the Muller fibers form the external limiting membrane (ext. lim. memb.) and the internal limiting membrane (int. lim. memb.), which lies adjacent to the vitreous humor. (From G. L. Walls, The Vertebrate Eye, copyright 1942 by Cranbrook Institute of Science, reprinted by permission. ) the retina. This accommodation involves the contraction of the ciliary muscle which releases the tension on the lens capsule and permits the lens to become more nearly spherical. After 40 years of age, there is a gradual loss of the capacity to accommodate in man; this can be corrected by wearing reading glasses. The retina contains the light-sensitive receptors known as rods and cones (Figs. 4.14 and 4.15). Rods are exceedingly sensitive to light and are responsible for vision under conditions of dim illumination. The rods of marine fishes and land vertebrates contain a light-sensitive red pigment, 106 RESPONSIVENESS IN VERTEBRATES rhodopsin, sometimes called visual purple; rhodopsin absorbs light rays and is bleached in the process (Fig. 4.16). In this way, the receptor cell is stimu- lated, and impulses are conducted to the central nervous system by way of the optic nerve. Complete resynthesis of rhodopsin can occur only in dim light or complete darkness. If there is a deficiency in vitamin A, rhodopsin cannot be reformed and night blindness results. In the rods of fresh-water vertebrates there is a purple light-sensitive pigment called porphyropsin, the synthesis of which depends on a supply of cs vitamin A,, a molecule with a structure slightly different from that of vitamin A; the reaction cycle is like that of rhodopsin. ‘The cones of land vertebrates contain a light-sensitive violet pigment, iodopsin, which has a reaction cycle like that of rhodopsin except that the protein involved is photopsin, not scotopsin. Cones of fresh-water vertebrates contain a light-sensitive blue pigment named cyanopsin and the reaction cycle is like that of porphyropsin except that the protein concerned is photopsin. Cones function under conditions of good illumination and make possible appreciation of fine detail and of color. Certain defects in cones lead to color blindness. Fig. 4.15. Visual cells of vertebrates; 1000. A, common rod of leopard frog; B, common cone of leopard frog; C, rod of man; and D, cone of man. d, oil droplet, typically embedded in e, the ellipsoid, which is probably a light-concentrating region; f, the footpiece which makes contact with processes of nerve cells in the outer plexiform layer (cf. Fig. 4.14); /, posi- tion of external limiting membrane; Ptoae < s Tee Sie m, myoid, a contractile region not Ba present in human visual cells; 1, WH nucleus; 0, outer segment or light- aie sensitive portion. (From G. L. Walls, 4 The Vertebrate Eye, copyright 1942 by Cranbrook Institute of Science, re- printed by permission.) GENERAL ZOOLOGY Rhodopsin Darkness Bleached by light Scotopsin Scotopsin AF aP : Blue light : Neo-b retinene All-trans retinene - = SS SSS = Alcohol dehydrogenase + DPN — — — ————— - | Neo-b vitamin A All-trans vitamin A Blood Pigment epithelium of retina Fig. 4.16. Diagram of the rhodopsin system in the retina of land vertebrates. The visual pig- ment rhodopsin is formed by the combination of scotopsin, a protein, and retinene, an oxidized form of vitamin A. The molecule known as vitamin A exists in a number of different physical shapes or isomers, each of which can be oxidized to a retinene molecule of the same shape. Only the form known as neo-b retinene can combine with scotopsin to form rhodopsin. When rhodop- sin absorbs light, establishing a visual impulse, and the pigment bleaches, the resulting retinene is the all-trans form. ‘This may be transformed or isomerized, in the presence of blue light, to neo-b retinene or reduced to all-trans vitamin A. Again, this may be isomerized to neo-b vita- min A or lost to the pigment epithelium of the retina (cf. Fig. 4.14) or to the blood. The reversible transformation between vitamin A and retinene is dependent on the enzyme alcohol dehydrogenase and coenzyme DPN, which contains nicotinamide, one of the B vitamins (p. 33). (Adapted from G. Wald, American journal of Ophthalmology, vol. 40, 1955.) Receptors located in the inner ear are of two types: those that respond to changes in position and make possible the maintenance of equilibrium, and those that are sensitive to sound waves and facilitate hearing (Fig. 4.4). During the evolution of vertebrates, these receptors have become localized in different parts of the inner ear. In higher vertebrates, the semicircular ducts’ and ampullae of the ear, with their associated receptors, function in maintain- ing equilibrium (Fig. 4.17). The cochlea in these vertebrates contains struc- tures which make possible response to sound waves or vibrations. Sound waves set up vibrations in the tympanic membrane or eardrum. These vibrations are transmitted across the middle ear by way of the auditory ossicles to the so-called round window of the cochlea and produce waves in its basilar membrane (Fig. 4.18). This membrane varies greatly in stiff- ness along its length, and sounds of different frequencies excite specific regions of it; the highest frequencies set up maximum vibrations in the stiffest portion which is near the window. ‘These localized movements in the basilar mem- brane bring the surfaces of the hair cells against the tectorial membrane in particular organs of Corti and establish impulses that are conducted to the ‘The term semicircular canals refers to the spaces in the skull occupied by the semi- circular ducts. 108 RESPONSIVENESS IN VERTEBRATES brain by way of the auditory nerve. How the auditory centers of the brain translate these signals into the call of a bird, a symphony, or the sounds of a city is unknown. Man can detect sounds ranging in frequency from 16 to 20,000 waves per second (middle C is 256 waves per second). Very loud sounds are felt as well as heard and sometimes cause a sense of pain. Exposure to intense sounds may lead to destruction of specific regions of the basilar membrane and so to deafness for tones of the same frequency. Vertebrates become aware of chemical substances in their environment if these substances become dissolved in the moist surface of the olfactory epithe- lium or in the secretions of the mouth cavity. The nasal epithelium contains the olfactory receptors which are actually neurosensory cells such as occur in Semicircular duct Ampulla Auditory nerve fibers Fig. 4.17. Crista of inner ear; semidiagrammatic. ‘There is a sensory area, or crista, in the ampulla of each semicircular duct (cf. Fig. 4.4). Movement of fluid in a semicircular duct, each of which is located in a different plane, brings about a change of position in the cupula in which extensions from the sensory or hair cells are embedded. ‘Thus the hair cells are stimulated, and an impulse passes over fibers of the auditory nerve toward the brain. Cristae are stimulated by changes in rate of movement of the head and especially by rotational movement. ‘The structure of the macula, the sensory area in the utricle of the vestibular portion of the inner ear, is essen- tially like that of a crista. However, the gelatinous material in which processes of the hair cells are embedded is arranged in a layer known as the otolith membrane because it contains small masses of calctum carbonate called otoliths. These respond to gravitational pull no matter what the position of the head; the consequent stimulus of the hair cell makes us aware of the position of the head even when it is not moving. ‘There is a macula in the sacculus of the vestibule, but its function has not been determined. 109 GENERAL ZOOLOGY Tectorial membrane Hair cell ASS Supporting cell Basilar membrane Auditory nerve fibers Fig. 4.18. Organ of Corti; semidiagrammatic (cf. Fig. 4.4). many lower animals (Fig. 4.194 and p. 523). When the olfactory receptors are stimulated by odors, the impulses that arise are conducted to the brain over processes of the receptor cells. “There is a rapid loss of sensitivity on continued exposure to any particular odor, but this does not interfere with responses to other odors. Taste buds, more widely distributed in aquatic vertebrates, are confined to the tongue in mammals (Figs. 4.20 and 4.198). ‘There are four different kinds of gustatory cells or taste receptors, which makes it possible for man to discriminate four sensations of taste—sweet, sour, bitter, and salty. ‘Taste and smell are inseparable in our reactions to various foods and beverages. The cutaneous sensations are touch, temperature, and pain. Receptors for touch are particularly concentrated in the finger tips and are sensitive to slight deformation of the skin in which they are located. ‘There are two types of Surface —— Surface of tongue cell “ae => Gustatory cell Supporting cell iar Flagellum a . —— Cytosome . ae NN ce i, \ » —\ Acrosome - Nucleus Mid piece Tail > D Fig. 5.8. Stages in spermiogenesis in the guinea pig. A, spermatid with early growth of flagellum from one centriole. B and C, showing the centrioles near the nucleus, the accumula- tion of cytoplasm along the flagellum, and the formation of the acrosome, which is the most anterior part of the spermatozoon. D, mature sperm, showing head, midpiece, and tail; the cytoplasmic mass has been detached and lost. (From F. Meves, 1898, Archi fiir mikroscopische Anatomie, vol. 54.) 137 GENERAL ZOOLOGY lst meiotic Odcyte spindle nucleus Sperm head Fertilization membrane A B C 2nd meiotic 1st polar 2nd pol baa nd polar body Female pronucleus Male pronucleus Perivitelline space Fig. 5.9. Meiosis during odgenesis of a roundworm, Parascaris equorum (Ascaris megalocephala bivalens). A, a primary odcyte into which a sperm carrying the haploid number of chromo- somes (two) has just penetrated. B, showing two tetrads, each containing four half-chromosomes (chromatids), on the equatorial plate of the spindle of the first meiotic division. C, showing the anaphase of the first meiotic division with two chromatids from each tetrad passing to each end of the spindle. D, showing the two pairs of chromatids on the spindle of the second meiotic division. £, showing the nucleus of the mature female germ cell (female pronucleus), the first and second polar bodies, and the nucleus of the sperm head (male pronucleus), which has been in- active since its entrance. ‘The meiotic spindles in Parascans do not have centrioles or astral rays. A perivitelline space is formed between the odcyte and the fertilization membrane which arises after the sperm enters. All figures x690. zoon is specialized for motility and contains one-half as many chromosomes as the primordial germ cells and somatic cells contain, one chromosome corre- sponding to each of their pairs. Four functional spermatozoa are derived from each primary spermatocyte. The process of o6genesis, or maturation of the female germ cell, is entirely comparable to that of spermatogenesis insofar as nuclear changes are con- cerned; the cytosomal specialization differs (Figs. 5.7 and 5.9). The un- differentiated germ cells found in the ovaries are known as oégonia and give rise to other o6gonia during the period of division. Each of these presently enters its period of growth, and the homologous chromosomes pair in synapsis and become duplicated to form tetrads. his is the time when cytosomal 138 REPRODUCTION AND DEVELOPMENT IN CHORDATES differentiation occurs in the female germ cell, or primary odcyte, as it is now called. In all female vertebrates certain of the undifferentiated germ cells form a layer around the primary odcyte. ‘This envelope, or follicle, serves a nutritive function during the storage of food, usually in the form of yolk, which occurs in the odcyte during the growth period. ‘The follicles of different vertebrates vary in thickness; those of mammals are very large (Fig. 5.2 and 5.38). Not all vertebrates store a large supply of food for the young animal that may develop from the egg if it is fertilized. “The method and place of development are correlated with the amount of food stored in the female germ cell and will be discussed later. When food storage is complete, the primary o6cyte divides to form two cells very unequal in size. ‘There is a large cell, the secondary odcyte, containing most of the food, and a very small cell, the first polar body, which has only a thin layer of cytoplasm around its nucleus. The division which produces these cells is the first meiotic or disjunctional division, and the nucleus in each cell contains half- tetrads, or dyads, just as does the nucleus of each of the secondary sperma- tocytes. At the second meiotic or equational division the first polar body divides to form two polocytes, or polar cells, of equal size, but the secondary oécyte gives rise to a small cell, the second polar body, and to the ovum, or mature female germ cell, which contains most of the food that was stored in the primary odcyte. The three polar cells and the ovum have comparable nuclei; each contains one chromosome corresponding to each homologous pair in the o6gonia and somatic cells. Only one ovum, or macrogamete, is pro- duced from each primary odcyte. The small polar cells are non-functional and die. All the cells of the vertebrate organism become differentiated to perform the activities of the particular organs of which they form a part. Specialization of the germ cells occurs much later in life than differentiation of most of the somatic cells and is limited to the period of sexual activity in the organism. Primordial germ cells are totipotent cells (p. 159). The essential difference between somatic cells and the germ cells is to be referred to their activities: the somatic cells are specialized in ways which contribute to the maintenance of the life of an individual, whereas differentiated germ cells make possible the reproduction of a new individual and so provide for continuity of the species. Both groups of cells have the same essential structure of nucleus and cytosome; both have the same requirements for life. Somatic cells contribute to the existence of the germ cells of the same generation, whereas germ cells make possible the existence of somatic cells of the succeeding generation. Reproduction Historical. Since Aristotle’s observations, in the fourth century B.c., on the developing hen’s egg, students have been interested in the origin of new individuals. Before the invention of the microscope the male germ cells could 139 GENERAL ZOOLOGY Fig. 5.10. Homunculus, as imagined to exist in the head of the spermatozoon of man. (From N. Hartsoeker, Essay de dioptrique, Paris, 1694.) not be seen. Spermatozoa were studied first in 1677 by Antony van Leeuwenhoek, a Dutch microscopist. Several observers soon associated the occurrence of spermatozoa in the seminal fluid with the phenomenon of reproduction, and some workers stated that each spermatozoon contained a fully formed, miniature individual. This tiny individual was called a homun- culus and was believed to grow, nourished by the female, until the time of birth (Fig. 5.10). Such a fantastic idea was opposed by other scientists of the eighteenth century who believed that new individuals were fully formed in the egg cells and that spermatozoa were parasitic in character and entirely un- necessary for reproduction. ‘These ideas of minute individuals encased in spermatozoa or in eggs were responsible for the Theory of Preformation, according to which development was simply the growth of a small individual preformed in the so-called germ. 140 REPRODUCTION AND DEVELOPMENT IN CHORDATES In 1824, Prévost and Dumas proved that spermatozoa are essential for the formation of new individuals by filtering the seminal fluid of male frogs before mixing it with eggs. No new individuals were formed under such conditions. Even this experiment did not establish the fact that the sperm and the egg united. It was not until 1875 that the actual penetration of an egg by a spermatozoon, followed by the union of the two nuclei, was observed independently by Hermann Fol and Oscar Hertwig in sea urchins (p. 499). When it is recalled that the Cell Theory was formulated in 1838 and 1839 (p. 11), the recognition by Hertwig and Fol that the spermatozoon and egg were cells is not surprising. With this recognition a sound interpretation of reproduction, which had baffled students for hundreds of years, was soon reached. ‘The earliest workers had lacked mechanical equipment in the form of microscopes, but the improvement in lenses was not all that was needed. Progress in science is always dependent on what scientists think about the facts they observe. ‘The conception of cells as the units of structure and function was, and is, as great a scientific tool as the microscope, and we are strikingly impressed with this fact in a study of reproduction and development. Methods of Reproduction. Reproduction, or the formation of a new individual, is accomplished in several different wavs. On the one hand, it may occur without the production of germ cells and be the result of the activity of only one individual—asexual or uniparental reproduction. On the other hand, germ cells, or gametes, may be produced by two individuals and unite in pairs—sexvual or biparental reproduction. Among the unicellular ani- mals asexual reproduction is brought about by cell division (p. 38). If this cell division produces two equal cells, it is known as fission; if the two cells are unequal in size, the process is called budding. Sometimes a process of multiple cell division or multiple fission occurs, with the result that many new individuals are produced at the same time (p. 245). Some of the simpler multicellular animals, such as the coelenterates, reproduce asexually by the methods of budding and strobilization; the flatworms undergo fis- sion (pp. 281, 299, 305, and 322). Sexual reproduction occurs throughout the Animal Kingdom by the method of syngamy, or union of two gametes to form a zygote. When the gametes are differentiated into microgametes and macrogametes, the process of syngamy is known as fertilization. Among the protozoans both isogametes and anisogametes occur, but anisogametes are typical of multicellular animals. Syngamy usually takes place in animals that produce anisogametes, but parthenogenesis, or development of a macro- gamete without union with a microgamete, sometimes occurs, notably among the rotifers and insects (pp. 346, 472, and 535). Reproduction is typically a function of adult animals; germ cells are pro- duced by mature individuals. However, in one of the amphibians, the axolotl, immature or larval animals give rise to germ cells which function in reproduction. Reproduction by immature animals is called pedogenesis and is known to occur in several invertebrate groups (p. 330). 141 GENERAL ZOOLOGY 1st polar body 2nd polar body Fertilization membrane Perivitelline space G8 ZIWS SS C D Fig. 5.11. Amphimixis and early cleavage in Parascaris equorum (Ascaris megalocephala bi- valens), a nematode. A, the male and female pronuclei have approached one another as the mitotic spindle for the first cleavage is formed; each contains two chromosomes, the haploid number. 8, the four chromosomes (diploid number) at the equatorial plate of the first mitotic spindle, seen from the side. C, the four chromosomes at the equatorial plate of the first mitotic spindle, seen from one end of the spindle. D, a two-cell stage in which both nuclei are in the late prophase of mitosis (the second cleavage). All figures «640. Fertilization. The union of an egg and spermatozoon is known as fertilization, or syngamy, and the resulting cell, which has the capacity to develop into a new individual, is called a zygote. “Two separable and very important phenomena, which were recognized almost immediately, are ob- served during fertilization. In 1875 Hertwig appreciated the fact that it is the union of spermatozoon and egg that stimulates the egg to begin its development; this aspect of fertilization is known as activation. Not until 1883, when Van Beneden studied fertilization in Parascaris, was the additional significance of the fusion of cells from two parents recognized. Fol had observed meiosis in 1875, but Van Beneden had much more favorable ma- terial and could see that the nuclei of spermatozoon and ovum contributed 142 REPRODUCTION AND DEVELOPMENT IN CHORDATES equally to the nuclear constituents of the zygote (Fig. 5.11). The union of two pronuclei, each containing one chromosome of each of the homologous pairs characteristic of the species, restores the diploid number of chromosomes and is known as amphimixis (p. 41). Each zygote, consequently, has the same number of pairs of chromosomes that each of its parents has, and each parent contributes one chromosome of each pair. Meiosis and amphimixis furnish the physical basis for an understanding of heredity (pp. 185-194). Meiosis and spermiogenesis occur in the seminiferous tubules of the testis, and the spermatozoa are mature when they are shed by the male and pene- trate the eggs. The female germ cells are not always mature when the spermatozoa enter. ‘The primary odcytes, surrounded by their follicles, grow and store nutrients in the ovary, and in some animals both meiotic divisions occur before ovulation; ova are liberated and fertilized. In other animals the primary odcyte is ovulated, and the spermatozoon enters before either meiotic division has occurred (Fig. 5.9). Meiosis begins in many vertebrates just before the time of ovulation, and the spermatozoon enters either the primary or secondary odcyte, which then completes its meiosis. When it enters an immature egg, the spermatozoon remains quiescent until odgenesis is completed. The nucleus of the spermatozoon, or male pronucleus, then be- comes rounded and vesicular like the female pronucleus before amphimixis takes place. As long ago as 1785, when Lazaro Spallanzani mixed lemon juice and vinegar, among other things, with frogs’ eggs in an attempt to stimulate them to develop, biologists sought to secure activation by artificial means. Finally, in 1899, Jacques Loeb succeeded in finding a method of artificial parthenogenesis, as the experimental activation of an egg that normally unites with a spermatozoon is called. A few years later Loeb was able to rear to maturity frogs that had developed from artificially activated eggs; these frogs had a mother but no father. It is now known that a variety of methods will initiate development in eggs that normally develop only after fertilization. George Lefevre, Sr., in 1907 used dilute organic acids (Fig. 5.12); others have used heat, shaking, pricking of the surface of frogs’ eggs, and various chemi- cal agents. Most of the studies on artificial parthenogenesis have been on the eggs of invertebrates, but in recent years attempts have been made, with some success, to activate mammalian eggs artificially. It has been stated that parthenogenesis occurs normally in some insects, notably bees and aphids, and among the rotifers. Parthenogenesis, whether normal or artificial, can be induced only when the egg cell is in a certain stage, which may be called a fertilizable condition, and corresponds only to the activation aspect of fertilization. ‘There is only one parent, and amphimixis cannot occur. Fertilization depends not only on the fertilizable condition of the egg but also on the ability of the spermatozoa to come in contact with the egg. ‘The flagellate spermatozoon of vertebrates swims by lashing its tail; fertilization can occur only in liquids. ‘The lower vertebrates, such as the frogs, typically copulate as the eggs are passed from the body of the female, so that the 143 GENERAL ZOOLOGY Polocytes Fertilization membrane A Perivitelline space B Fig. 5.12. Early development after artificial parthenogenesis in Thalassema, a worm-like invertebrate. A, meiosis has occurred within a fertilization membrane, the formation of which was conditioned by a short treatment with dilute acid. 8B, the first cleavage spindle; only the haploid number of chromosomes is present. (From G. Lefevre, 1907, Journal of Experimental Zodlogy, vol. 4.) spermatozoa are shed over the egg mass in the water. Fertilization takes place in the water, outside the body. In vertebrates that live on the land, fertilization is internal. Copulation occurs, and spermatozoa are introduced into the reproductive tract of the female, where they can swim in the seminal fluid and in the liquid filling the female ducts. ‘The spermatozoa pass into the Fallopian tubes and meet the eggs as they are ovulated. In most reptiles and birds additional food material in the form of albumen is secreted about the fertilized egg, or zygote, as it passes down the oviduct. A shell, which prevents drying and serves as a protection for the developing individual, is added before the egg is laid or passed out of the female’s body to develop (Fig. 5.4). The mammalian zygote is retained and nourished in the uterine portion of the oviducts during the developmental period. Usually only one spermatozoon penetrates an egg cell, but in some verte- brates, such as the birds, polyspermy is a normal occurrence; that is, several spermatozoa enter each egg. However, the nucleus of only one of these spermatozoa fuses with the egg nucleus in amphimixis. In 1875, Fol observed that, as soon as one spermatozoon had reached the egg, a membrane, known as the fertilization membrane, becomes separated from the surface of the egg, leaving a perivitelline space around the egg (Figs. 5.9 and 5.11). ‘The fertiliza- tion membrane was believed by many students to be a device to prevent polyspermy but is now recognized as a by-product of activation. Such a membrane is formed when eggs are artificially stimulated to develop (Fig. 5.12). ‘The spermatozoon swims actively until it comes in contact with the surface of the egg; the egg then engulfs the spermatozoon after first sending out a minute projection, the entrance cone, in which it becomes embedded (Fig. 144 REPRODUCTION AND DEVELOPMENT IN CHORDATES 5.13). The process of engulfing the spermatozoon requires only a few seconds in many animals, although it may take as long as an hour. Most often the tail of the spermatozoon is not taken into the egg; only the head containing the male pronucleus and the middle piece containing the centriole, which gives rise to the centrioles of the spindle of the zygote, are involved in fertiliza- tion (Fig. 5.8). If the tail of the spermatozoon enters the egg, it undergoes degeneration as development begins; this happens in the frog, the bat, and some other animals. Development Reproduction in the vertebrates is complete when maturation, activation, and amphimixis have taken place, when the differentiated egg and sperm cells have united. The zygote is potentially a new individual; it has the capacity to develop into an organism similar to its parents in all essential respects. “The process of development, which always follows reproduction by syngamy in the multicellular animals, consists essentially of cell division, cell localization, and cell differentiation. Growth occurs in all developing individuals and is responsible for the increase in the amount of protoplasm, which is correlated with the increase in cell numbers by continued cell divi- sion. Development is a continuous process in which a definite series of events occurs in a definite sequence under very limited environmental conditions. Food and oxygen are furnished, waste products are removed, excessive loss of water is prevented, and a rather limited temperature range is maintained. The methods of caring for the metabolic requirements of a developing in- Dect nuclei: 1st meiotic aster Entrance cone Fig. 5.13. Entrance of sperma- tozo6n and formation of first meiotic spindle in the egg of Thalassema (cf. Fig. 5.12); 820. Fertilization membrane 145 GENERAL ZOOLOGY dividual will be discussed later (p. 159), but it may be noted here that un- due evaporation is prevented during the development of terrestrial vertebrates by egg membranes and shells or by the retention of the embryo in a uterus. The range of temperature is determined in fishes, amphibians, and reptiles by the season during which the eggs are laid, and unseasonal temperatures may kill such developing eggs. Birds, of course, incubate their developing eggs; in mammals, the mother’s body regulates the temperature of the embryos in the uterus. In our discussion of. development we shall assume that all en- vironmental conditions are normal. ‘The development of a single cell into a complex, highly differentiated animal is one of nature’s marvelous pageants, a series of events that occurs in such an orderly fashion as to fill the observer with awe. The pattern of early development is correlated very closely with the amount of nutritive material stored in the odcyte during its growth period in the ovary. In the Chordata there is a wide range in the quantity and position of the yolk in eggs. ‘Vhe amphioxus (p. 550), for example, has an isolecithal egg, so called because the relatively small amount of reserve nutrients is dis- tributed almost uniformly throughout the cytosome. Among the vertebrates the frog has a telolecithal egg in which a considerable quantity of yolk is stored more abundantly in one half, the so-called vegetal hemisphere, than in the other half, or animal hemisphere, where the nucleus is always found (Fig. 5.38). A more pronounced polarization of nucleus and yolk is to be seen in the telolecithal eggs of fishes, reptiles, and birds (Fig. 5.46 and C). In such eggs the nucleus is located near the animal pole surrounded by a relatively small amount of cytoplasm forming the blastodisk, and the large cytosome is packed with yolk enclosed by a very thin layer of cytoplasm, which lies at the surface of the egg. The egg-laying mammals produce telolecithal eggs like those of reptiles; the placental mammals have isolecithal eggs in which reserve nutrients are stored in varying amounts but never in large quantity. As the sequence of events during the development of chordates is described, comparisons will be made between certain representative forms to call atten- tion to the fundamental similarity of development in all, as well as to indicate special differences. Development is a continuous process, although it can be divided into stages for purposes of discussion. In the account which follows, the early stages characterized only by cell division are described under that heading. ‘The stages during which the conspicuous mass movements of cells occur are described under the heading of cell localization; these movements separate the so-called germ layers and establish the body plan, as well as the primordia of all the organ systems. The final stages in which cytosomal differences appear in cells, with the production of tissues, are summarized under the heading of cell differentiation. Cell Division: Cleavage. Cleavage follows activation and consists of a series of cell divisions. Cell division, of course, occurs during other periods, but during cleavage it is the only visible indication of development. What 146 REPRODUCTION AND DEVELOPMENT IN CHORDATES we see externally is the constriction of the cytosome during the telophase of mitosis; the orderly separation of half-chromosomes occurs before this. Dur- ing the early divisions all the cells divide at so nearly the same rate that it appears as if the zygote were being cut with a knife into smaller and smaller parts. If the zygote is visualized as a globe with the north pole of the earth representing the animal pole of the zygote and the south pole repre- senting the vegetal pole, it may be easier to understand how the zygote is divided during cleavage. Polar body Polar body Perivitelline space Pronuclei Fertilization membrane Endoderm Gastrocoel Ectoderm Blastula Gastrocoel Ventral surface Fig. 5.14. Early development in the amphioxus. A, one-cell stage, in section; the arrow indicates the egg axis. B, eight-cell stage, from the surface. C, 32-cell stage, from the surface. JD, early blastula, in section. £, late blastula, as if cut in half. F, early gas- trula, in section. G, gastrula, from the surface. H, late gastrula in section, oriented to show dorsoventral and anteroposterior axes. (A, B, and G, from E. G. Conklin, 1933, Journal of Experimental Zodlogy, vol. 64; C-F and H from E. G. Conklin, 1932, Journal of Morphology, vol. 54.) 147 GENERAL ZOOLOGY In the amphioxus (Branchiostoma lanceolatum) cleavage is total; that is, the entire zygote is divided into two cells of the same size, and division con- tinues to produce cells that differ but little in size (Fig. 5.148 and C). ‘The cleavage pattern is like that of the frog, in which cleavage is also total but in which inequality in the sizes of cells is soon apparent. In the frog the plane of the first cleavage furrow, as the cytosomal constriction is called, passes from the animal to the vegetal pole. Usually one of the resulting cells is destined to give rise to the right side of the individual, the other to the left side. This fact has been established because of certain changes that occur in the frog’s egg after the entrance of the sperm. ‘The sperm enters at some point in the animal hemisphere, and as the second meiotic division occurs, streaming movements in the cytoplasm result in a distribution of material with reference to the plane of entrance of the sperm and the path it follows as it is carried toward the egg nucleus (Fig. 5.154 and B). These streaming move- ments of the cytoplasm, in addition to producing invisible localization of cytoplasmic regions, give rise to the gray crescent, an area from which some of the pigment is carried away and which lies approximately opposite the en- trance point of the sperm. With this visibly different region as a landmark, observations can be made concerning the fate of certain regions of the zygote during development. ‘Thus, it has been determined that the future median longitudinal plane of the embryo coincides with a plane passing through the egg axis and bisecting the gray crescent (Fig. 5.158). Since the first cleavage plane usually bisects the gray crescent, it follows that bilateral symmetry becomes apparent at this time. The second cleavage furrow likewise appears first at the animal pole and passes to the vegetal pole at right angles to the first, so that a four-cell stage results. These cells are of equal size, but cleavage now becomes unequal in the frog. The third cleavage furrow cuts each of the four cells in a plane parallel to the equator but nearer the animal than the vegetal pole. Of the resulting eight cells, the four in the animal hemisphere are smaller. ‘Two fourth cleavage furrows appear simultaneously in the smaller cells and pass through the larger cells to form a 16-cell stage. After this, two fifth cleavage furrows, one on each side of the third cleavage furrow, produce 32 cells. Indications of an irregularity of division rate can often be seen during the fourth and fifth cleavages, since the furrows pass more rapidly through the cells containing less yolk. Formation of the Blastula. As the egg divides during cleavage, the cells tend to become spherical. Since the yolk stored in the female germ cell is being utilized as the source of energy for cleavage, a small cavity appears internally as early as the eight-cell stage in the frog. This cavity is quite conspicuous after the fifth cleavage and is known as the cleavage cavity, blastula cavity, or blastocoel. The developing individual is now called a blastula, and this period in development is referred to frequently as the blastula stage. The blastula arises as a result of cell division only; no cell movements have occurred. If the egg is isolecithal and cleavage is total and 148 REPRODUCTION AND DEVELOPMENT IN CHORDATES 1st polar body 2nd polar body Blastula Sperm nucleus wm, / Egg nucleus Gray crescent Germ ring A B Cc Blastula Chordamesoderm cavity Gastrocoel Anterior end Ph aes ay _ Invagination ‘/ Epiboly \ \ Involution A, cavity ~\ Dorsal lip J NS Yolk plug : . mS - Blastopore Epo Ventral lip \ Ventral D E F surface Chordamesoderm Ectoderm Mesoderm Neural plate Notochord Neural ee = Fig. 5.15. Early development in the frog. A, egg before activation, in section. B, one- cell stage before amphimixis, in section. The first cleavage furrow would pass in the plane of the paper, through the egg axis, and bisect the gray crescent. C, late blastula, in section. D, early gastrula, in section. £, gastrula in section. , late gastrula, in longitudinal section, oriented to show dorsoventral and anteroposterior axes. G, H, and J, embryos during the formation of the notochord, the dorsal mesoderm, and the neural plate, in cross section. All figures diagrammatic; the arrow indicates the egg axis. (# and F, redrawn with modifica- tions from R. S. McEwen, Vertebrate Embryology, copyright 1931 by Henry Holt and Co., printed by permission. ) 149 GENERAL ZOOLOGY Blastula cavity Fig. 5.16. Blastulae and gastrulae of amphibians and birds. A, blastula of Triton, a sala- mander. 8B, blastula of the domestic fowl. C, gastrula of Triton. D, gastrula of the chick (cf. Fig. 5.68). (A and C, redrawn from O. Hertwig, Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere, 1890; B, redrawn from J. T. Patterson, 1910, Journal of Mor- phology, vol. 21.) approximately equal, the blastula cavity is located centrally and surrounded by cells of similar size, as in the amphioxus (Fig. 5.14D). In the amphibians, where the egg is telolecithal and cleavage is total but unequal, the blastula cavity is in the animal hemisphere and has a roof of small cells and a floor of large cells (Fig. 5.16A). Further, in the fishes, reptiles, and birds in which the egg is telolecithal and cleavage occurs only in the blastodisk, or small amount of cytoplasm surrounding the nucleus at the animal pole, the blastula cavity lies between the disk of cells and the underlying, undivided yolk mass (Figs. 5.4B and C and 5.168). The blastula cavity in forms like the amphibians and birds is filled with a solution of food material that diffuses into it from the yolk-laden cells or yolk mass of its floor. Cell division continues during the blastula stage. In the frog the cells divide parallel to the surface of the blastula so that the blastula cavity be- comes roofed by several layers of cells. An equatorial belt of cells, which is not visibly different from other parts of the animal hemisphere, is known as the germ ring and plays a very important role in later development (Fig. 5.15C). The cells formed in this region spread toward the vegetal pole. 150 REPRODUCTION AND DEVELOPMENT IN CHORDATES Since these cells are pigmented, the direction and extent of their movements can be noted; the original location of the gray crescent can still be ob- served. The establishment of the germ ring and the beginning of its shift in position mark the end of that part of development in which cell division is the characteristic event. Cell division, however, continues, and the number of cells increases greatly during the next period of development. In eggs that do not have a natural marker such as the gray crescent of some amphibian eggs, it has been possible to mark different parts of the blastula by means of non-toxic or vital dyes. It has been found that cer- tain limited areas of the blastula always move, in the normal course of development, to a specific part of the gastrula. ‘These regions whose develop- mental fate, so to speak, is already determined at the blastula stage are called presumptive areas. Cell Localization. The spread of cells formed at the original equatorial region of the zygote toward the vegetal pole is the beginning of a phase of development characterized by mass movements of cells which lead to the establishment of the so-called body plan of the chordate. During this period of cell localization masses of cells are brought into new relations with one another. The embryo is presently found to consist of three distinguishable layers of cells which have long been called the germ layers, namely, ectoderm, endoderm, and mesoderm. ‘The first part of cell localization is frequently referred to as gastrulation. Gastrulation in the amphioxus is a much simpler process than in amphibia, where yolk-laden cells are a complicating factor in the movements that occur. In the amphioxus the late blastula becomes somewhat flattened at the vegetal end, the cells of which begin to move as a group into the blastula cavity (Fig. 5.14#-H). This is known as the process of invagination. ‘The open end of the early gastrula, as the developing individual is now called, is known as the blastopore, and the cells of the germ ring form its lips. “The move- ments of the cells continue until the blastula cavity is obliterated completely and there is a new internal cavity, the gastrula cavity, gastrocoel, or archenteron, which opens externally by way of the blastopore. As a result of the shifting of cells the individual now consists of two layers of cells, an outer layer of ectoderm and an inner layer containing the primordia of the other two germ layers, which will be sorted out by subsequent movements. ‘The outer and inner layers are continuous with one another in the region of the germ ring, that is, at the lips of the blastopore. Continued cell divisions add cells to both these layers, and the gastrula becomes elongated as the germ ring de- creases in circumference. ‘This decrease in size of the germ ring is also known as the closure of the blastopore, which becomes smaller and smaller. ‘This stage in the development of chordates is suggestive of the so-called diploblas- tic, or two-layered, body plan of some coelenterates (pp. 226 and 284). Gastrulation in amphibia 1s essentially the same as in the amphioxus. ‘The yolk-laden cells tend to move toward the blastula cavity, but invagination is not the conspicuous process that it is in amphioxus (Fig. 5.15D). A more 151 GENERAL ZOOLOGY noticeable activity is the overgrowth of the cells of the vegetal hemisphere by those of the animal hemisphere, so that the pigmented area increases; the non-pigmented area decreases in extent. As the spread of the pig- mented cells occurs, the region of the gray crescent can still be identified, and at the pigmented border of this region an inward movement of cells be- gins. In other words, the cells derived from the gray crescent come to lie inside the gastrula in the region of the dorsal lip of the blastopore and in what is to be the mid-dorsal region of the embryo. The part of the germ ring from which inturning first occurs is known thenceforth as the dorsal lip of the blastopore, or the opening into the gastrocoel. Soon the inward shift of cells, or involution, occurs along the entire margin of the germ ring, which thus becomes the lips of the blastopore. In amphibia the blastopore is plugged with cells of the vegetal hemisphere which have not shifted their position (Fig. 5.15#). The overgrowth of the yolk-laden cells by pigmented cells is known as epiboly and continues, with the resulting decrease in the circum- ference of the germ ring and in the area of the yolk plug, until the yolk plug is covered and the blastopore is a minute opening. Internally, the gastrula cavity increases greatly in extent as a result of invagination and involution, while the blastula cavity decreases in size as the gastrula cavity expands. As gastrulation progresses, it is possible to distinguish dorsal and ventral sur- faces, as well as anterior and posterior ends, because of their subsequent development. Furthermore, the expansion of the gastrula cavity in the dorsal half of the gastrula and the obliteration of the blastula cavity in the animal hemisphere result in a rotation of somewhat more than 90 degrees in the posi- tion of the individual within its jelly envelopes. Instead of the animal hemisphere, the dorsal half now floats uppermost. As the blastopore closes in amphibia, the shift of cells from an external to an internal position is completed. ‘The cells which remain on the surface are the ectoderm; those which have moved in and now Iine the gastrocoel dorsally and laterally are the presumptive notochord and mesoderm. Farther down along the sides of the gastrocoel the inwardly shifted cells are the true endoderm. In the floor of the gastrula cavity these cells receive additions from the yolk-laden cells by a process of delamination, or rearrangement, to form a definite layer. Laterally, at the junction between the endoderm and the presumptive mesoderm, a separation occurs between the two. Division of cells in each region extends the layers. ‘The two sheets of endoderm move dorsally until they meet one another in the mid-dorsal line to form a con- tinuous lining for the gastrula cavity. ‘This changes the presumptive mesoderm into a middle layer, the true mesoderm, lying between the ectoderm and endoderm. Its free ventral margin on each side extends until the two sheets meet midventrally. Dorsally, the mesoderm is continuous with a median mass of cells derived by involution from the dorsal lip of the blastopore (Fig. 5.15G). This association constitutes the chordamesoderm. Presently the sheets of the mesoderm are no longer continuous with the mid-dorsal cells, which become arranged as a longitudinal cord of cells known now as the 152 REPRODUCTION AND DEVELOPMENT IN CHORDATES notochord (Fig. 5.15# and J). Somewhat later the characteristic cavity of the mesoderm appears; this is the coelom (Fig. 5.17). With the localization of the three germ layers and the formation of the archenteron and the coelom as Cavities surrounded, respectively, by the endoderm and the mesoderm, the fundamental triploblastic body plan of the chordates has been established. The establishment of the typical body plan is only the beginning of cell localizations. Within each of the three layers movements of lesser masses of cells occur and give rise to the primordia of the organ systems of the adult vertebrate. At the same time that the mesoderm and notochord are being localized, the first stage in the formation of the nervous system occurs. ‘The ectoderm over the chordamesoderm becomes thicker and is known as the neural plate (Fig. 5.15H); when this occurs the embryo is sometimes called a neurula. Along Ectoderm Neural plate Dorsal mesoderm SEBea, . Sooty pe Sey ee es 7\D lo) oF icfoy 00O., ©. C0895 S20 a0098Se 20293880 Notochord Endoderm Neural folds Vy Veet oe eeZo SLES Intermediate Neural canal saeseniain tube Neural crest ] Coelom Latera BeaZUd sien © COESROS v << 6 052 S) 09 Ps Gan Oo Re 3608 Sodas Eom oO re th Mesodermal somite Cc Fig. 5.17. The localization of the neural tube in the domestic fowl. A, B, and C, successive stages, in cross section. (Redrawn with modi- fications from M. Duval, Atlas d’embryologie, 1889.) 153 GENERAL ZOOLOGY Ectoderm Diencephalon Mesoderm Optic vesicle Optic stalk placode Fig. 5.18. The early development of the eye. A, B, and C, cross sections through the heads of chick embryos, showing successive stages in the localization of the various parts of the eye. the lateral edges of this thicker plate of neural ectoderm, where it is con- tinuous with the thinner superficial ectoderm, folds appear on the surface of the embryo. ‘These are the neural folds, which move toward the dorsal mid- line, where they meet and fuse. ‘This fusion unites not only the edges of the neural plate to form the neural tube but also the edges of the superficial ectoderm that covers the entire surface of the frog embryo and gives rise to the epidermis of the skin (Figs. 5.15/ and 5.17). During the closure of the neural folds some ectodermal cells are left between the superficial ectoderm and the neural tube; they form the neural crest. ‘The neural plate is wider at its anterior end than it is toward the blastopore, and the neural tube is consequently larger at the anterior end. ‘Thus, from a very early period, the anterior region is distinguishable as the part destined to give rise to the brain, and the posterior part is marked as the region which is to give rise to the spinal cord (Fig. 5.19). Within the brain region localized expansions produce first three and then the five brain vesicles which are characteristic of all vertebrate embryos (Fig. 5.21). The formation of the eye occurs at the time when the brain vesicles are established in vertebrates. Near the anterior end of the brain, expansions appear to the right and left; these are the optic vesicles (Fig. 5.18). Later the outer cells of these vesicles move in to produce the optic cups. At the same time the superficial ectoderm covering the optic cup thickens and folds in to form the lens vesicle, which is later cut off and forms the lens of the eye (see Figv4i35 p: 89). The first stage in the establishment of the digestive system is seen when the endoderm and archenteron are formed, although the blastopore does not persist as the posterior opening of the alimentary canal. A depression of superficial ectoderm occurs posteriorly to form the proctodeum (Fig. 5.19A) and anteriorly to form the stomodeum. ‘These pits, lined with ectoderm, be- come the most posterior part of the digestive tract and the mouth cavity, respectively (p. 48). “he liver and pancreas arise as outgrowths from the archenteron and are lined with endoderm. Closely associated with localiza- tions related to the digestive system are those of the respiratory system. In the 154 REPRODUCTION AND DEVELOPMENT IN CHORDATES pharyngeal region of the alimentary canal paired pouches extend to the sur- face ectoderm in which slits appear (Fig. 5.20). These gill slits or clefts make possible a flow of water through the pharynx in adult fishes and certain amphibians, including the tadpole of the frog. Gills are developed later in this region. Such pharyngeal pouches and at least vestigial gill slits are characteristic of all chordate embryos (Figs. 5.21 and 5.22). In terrestrial vertebrates the lungs and air tubes are also formed by an outgrowth from the endoderm lining the pharyngeal region of the primitive gut. The mesoderm undergoes many changes in the period after its localization. Most conspicuous is the proliferation that occurs along the sides of the neural tube, forming what is called the dorsal mesoderm, or epimere. Soon these cords of mesoderm become segmented and give rise to the mesodermal somites (Figs. 5.17 and 5.21). Later, after continued cell division, the cells of the somites are shifted in position. Some migrate around the notochord and neural tube and later form the vertebral column. Others make up the muscle plates, from which the striated muscles of the trunk arise, and still others form the dermis of the skin. Lateral to the dorsal mesoderm on each side there is a region called the intermediate mesoderm, or mesomere, from which Ectoderm Endoderm Neural crest Neural tube Notochord Coelom ‘ a S an ae pasaecesee Archenteron Proctodeum Liver Mesoderm D oon orsal surface Gpimalicord WOK KU Sane Pericardial cavity Fig. 5.19. Young tadpoles of the frog, to show localization of organ-system primordia. A, tadpole of about hatching age, in longitudinal section. 8B, later tadpole, in cross section; and C, later tadpole, in longitudinal section. All sections diagrammatic. (A, redrawn from T. H. Morgan, The Development of the Frog’s Egg, copyright 1912 by The Macmillan Co., printed by permission; C, redrawn from J. W. Jenkinson, Vertebrate Embryology, copyright 1913 by Oxford University Press, printed by permission. ) 155 GENERAL ZOOLOGY Gill slit Ear Neural fold Eye Nostril Heart Pronephros Blastoderm Fig. 5.20. Embryo of the torpedo, an elasmobranch, attached to its yolk sac. (Redrawn from H. E. Ziegler, Lehrbuch der verglewchenden Entwick- lungsgeschichte, 1902.) the excretory and reproductive systems are differentiated (Fig. 5.17C). ‘The remaining mesoderm is the lateral mesoderm, or hypomere, the cells of which become rearranged into an outer layer lying close to the ectoderm and an inner layer lying next to the endoderm (Fig. 5.17B and C). ‘The cavity of the hypomere lying between these two layers is the coelom (Fig. 5.195). The heart and main vessels of the circulatory system are established in the mesoderm (Figs. 5.19C' and 5.21B). Blastoderm Mesencephalon Metencephalon ” Optic ss — 4 vesicle ja, Myelencephalon—=—=~— A ; Lens vesicle : : : ae Diencephalon Brain Auditory pit ra (i ; Optic ae vesicles penn A \ ae f Telencephalon Gill slits f eas < Zegaremes pit 2 f La Be Atrium Hip ds \ wr Ventricle = \"___—+-~ Vitelline vein Sinus = Bi fing e venosus ake >— cae Mesodermal NN i —Vitelline ce ‘ i y » RY somites ‘4 ) vein Hi ie i? eX Neural Notochord ai —~ Vitelline folds je \> 7 artery (2 \ Ke ed ~~ Mesodermal \ somites Tail bud A B Fig. 5.21. Chick embryos, showing localization of organ-system primordia; dorsal view. 4, embryo after 29 hours of incubation. B, embryo after 52 hours of incubation, with its em- bryonic membranes removed. Internal structures are shown as if seen through the surface. (Redrawn from M. Duval, Aélas d’embryologie, 1889.) 156 REPRODUCTION AND DEVELOPMENT IN CHORDATES Localizations occur in different regions at the same time, but in a very orderly manner. ‘The same degree of localization will be found in any vertebrate embryo of a given species at a given age (Fig. 5.22). Under normal environmental conditions developmental processes occur with machine- like precision. Gill slits Brain vesicle Eye vesicle Allantois ea 7 Yolk sac Ae bud Hind limb bud Reptile Bird Lower jaw Ear vesicle te. ‘lil Wis ¢ Zee Heart X(( A) / 2 ; ik, NU res fi: re \ 7 4 Naris Umbilical cord Pig Man Fig. 5.22. Vertebrate embryos of comparable age. (Redrawn from B. M. Patten, Early Embryology of the Chick, copyright 1951 by Blakiston Div., McGraw-Hill Book Co., printed by permission. ) The mechanism of the cell movements that result in the shifting of the presumptive areas of the blastula to their definitive positions in the gastrula and neurula remains unknown. It is clear that the characteristic changes in 157 GENERAL ZOOLOGY LOCALIZATION DIFFERENTIATION Germ Layers Systems and Organs Tissues Nervous system Nervous tissue Eyes, ears, and nasal cavities Epidermis of the skin Epithelial tissue Lining of the proctodeum and stomodeum Ectoderm Dermis of the skin Skeletal system Wall of the digestive tract, except the lining Muscular system Circulatory system Vascular tissue Excretory system Reproductive system Epithelial tissue Lining of the coelom Sustentative tissue Contractile tissue Mesoderm Lining of the digestive system, except in the region of the stomodeum and proctodeum Lining of the lungs and air tubes Endoderm Epithelial tissue Fig. 5.23. ‘The principal localizations of cells and their differentiation during the development of the vertebrates. shape associated with localization of specific groups of cells result from in- trinsic capacities of the cells themselves. This has been shown by using enzymes to digest away the intercellular material which binds the cells of the early embryo together. Such dissociated cells will, under carefully controlled conditions, undergo the changes of shape and the movements characteristic of similar cells within the embryo. Cell Differentiation. During the various movements of masses of cells in relation to their neighbors that occur as localization takes place, no essential changes in the character of the cells can be noted. When a group of cells has finally shifted its position, however, cell differentiation, or histo- genesis, begins; that is, the cytosomal changes that give rise to the tissues of the adult occur. ‘The differentiation or specialization of the somatic cells will not be described in detail. By referring to Figure 5.23 you can correlate the localizations and differentiation of development with the anatomy and histology of the adult vertebrate. Differentiation of certain kinds of somatic cells is not confined to the developmental period. Cells wear out and are re- 158 REPRODUCTION AND DEVELOPMENT IN CHORDATES placed by others recently formed by mitosis from stocks of relatively un- specialized cells. Cells that do not become differentiated during the developmental period are known as totipotent or embryonic cells in the adult animal. Metabolic Requirements of Embryos The orderly series of changes which occurs during development depends on chemical reactions that take place in the cells, and these, in turn, on the metabolic requirements of the cell. A constant supply of food and oxygen must be available, and waste products of metabolism must be eliminated. The temperature, which is one factor conditioning the rate of metabolism, cannot vary widely, and drying must be prevented. In the frog embryo, which has been used to illustrate the course of develop- ment in vertebrates, these metabolic requirements are met simply. A large amount of food is stored in the egg, the egg is laid in water from which oxygen is obtained by diffusion, and the breeding season occurs during a period of the year when temperature conditions are favorable for development. The frog embryo within the fertilization membrane is protected from adhesions, and there are no so-called embryonic membranes. As the localization of the primordia of organ systems occurs, a U-shaped sucker appears on the ventral surface of the head of the frog embryo, and a median posterior extension foreshadows the formation of a tail. The embryo hatches by slipping out of its disintegrating jelly envelopes and becomes attached to objects in the water by means of the sucker. When the stomodeum becomes continuous with the pharynx and the tail is developed further, the individual swims and _ feeds, although it does not resemble an adult frog. Such a self-supporting but not fully developed individual is called a larva; the larva of the frog is known as a tadpole. During the tadpole stages the frog feeds upon plants and carries on gas exchange by means of its gills, of which there is first an ex- ternal and then an internal set. “Toward the end of the tadpole period in development the animal begins a metamorphosis, or change from larval to adult structure. Hind legs and then front legs appear and grow, the tail decreases in size until it disappears, and the mouth and jaws become like those of the frog. ‘The lungs become functional, and the gills disappear. In correlation with the change from a diet of plants to one of insects, the in- testine becomes much shorter during the period of metamorphosis (p. 454). Many frogs complete their metamorphosis about 3 months after hatching, but the bullfrog usually passes its first winter as a tadpole. Such a larval period in the life cycle of a vertebrate is unusual, although larval stages char- acterize the development of many invertebrates (pp. 330, 375, 455, and 4906). In fishes the telolecithal eggs are laid in water, but there 1s one important difference in development as compared with that of the frog. Cleavage is 159 GENERAL ZOOLOGY Amniotic fold Eggshell Kgg membrane Amniotic — fold Amnion ) \ Allantois Chorion hj j/ 2 Wg | SS zZ 250-5 SSS ‘\ Yolk sac 0 nail 1e) Orie, 0 078 HOLOTS Allantoic — cavity Allantois Amniotic ~ cavity Amnion Allantoic XA stalk YQ \ Yolk stalk Xl Y Y"\ ss . \J Chorion Fig. 5.24. ‘The embryonic membranes of the chick. A, B, and C, diagrams of longitudinal sections of successive stages during development. (Redrawn with modifications from T. J. Parker and W. A. Haswell, Textbook of odlogy, copy- right 1921 by Macmillan and Co., Ltd., printed by permission. ) partial, and the yolk-laden part of the zygote is not divided into cells. Correlated with this fact we find the blastoderm rapidly extending over the surface of the yolk (Fig. 5.20). The cells of the endoderm and its adjacent mesoderm form a yolk sac which eventually encloses the yolk, and numerous blood vessels in this embryonic membrane absorb the yolk as it becomes diffusible and carry it to the developing embryo, which sometimes hatches before all its yolk is used. 160 REPRODUCTION AND DEVELOPMENT IN CHORDATES Fig. 5.25. 4, chorionic vesicle of man removed from uterus, showing villi on surface; age, about 34 days. 8B, chorionic vesicle of man, opened to show embryo enclosed by amnion and attached to placenta at lower right; crown-rump length of embryo, 17.6 mm.; age, about 7 weeks. (Photographs courtesy Carnegie Institution of Washington, Department of Embryology. ) 161 GENERAL ZOOLOGY Fig. 5.26. Human fetus in uterus with chorion and amnion pulled away; the umbilical cord attaches fetus to the placenta above. The crown-rump length of the fetus is 61 mm., and its age is 10 to 11 weeks. (Photograph courtesy Car- negie Institution of Washington, Department of Embryology.) The problem of meeting the metabolic requirements is much more com- plicated for the embryo of an egg-laying terrestrial vertebrate. Certain embryonic membranes always develop in such forms. ‘They persist only dur- ing development and serve to prevent drying, furnish food and oxygen, and eliminate waste products (Fig. 5.24). A yolk sac is formed in reptiles and birds, as in fishes, and also appears as a vestigial structure in the development of mammals. In addition, the amnion with its enclosed amniotic fluid keeps the embryo moist and provides a protective cushion, and the chorion forms a protective membrane next to the shell of the egg of the reptile or bird. A fourth membrane, the allantois, is richly supplied with blood and _ lies next to the chorion. ‘The allantois is a membrane which functions in gas ex- change and excretion; it absorbs oxygen and eliminates carbon dioxide, both of which pass through the porous shells of reptiles’ and birds’ eggs. Among the mammals the problem is again somewhat different. Some mammals lay yolky eggs as do the reptiles and birds; others store a certain amount of yolk in their eggs. [The opossum, for instance, gives birth to very immature young which spend a considerable period in an abdominal brood pouch, nourished by milk from the mammary glands (see Fig. 18.310, p. 583). In the majority of mammals, however, practically no food is stored in the egg, and development is completed within the uterus. ‘This is made possible by the embryonic membranes, which are somewhat altered in func- tion. During the early part of the period of cell localization the mammalian embryo becomes closely associated with the lining of the uterus. “The amnion 162 REPRODUCTION AND DEVELOPMENT IN CHORDATES which is formed has a protective function comparable to that in reptiles and birds (Fig. 5.258). The chorion, however, is the layer next to the tissues of the mother, and in man, for example, becomes concerned directly with nutrition, gas exchange, and excretion. An allantois appears during the development of the human embryo but has no function. A rich supply of blood vessels in the chorion is connected with the vessels of the embryo’s body by way of vessels in the umbilical cord (Fig. 5.26). The chorion. is covered with villi, or finger-like processes, that extend into blood-filled spaces in the uterine wall (Figs. 5.254 and 5.27). That part of the wall of the uterus in which the young human embryo becomes embedded, together with extensions of the chorion, constitutes the placenta. In the placenta the blood of the embryo is everywhere separated from the blood of the mother by the cells of the chorion, through which diffusion of nutrients, oxygen, and waste products occurs. ‘There is no mechanism for regular exchange of blood between mother and embryo, although a certain amount of seepage probably occurs. In this connection it should be stated that the somewhat widespread ideas of prenatal impressions are entirely without foundation. ‘The attach- ment between mother and embryo is such that diffusible substances carried by the blood can pass from one to the other. ‘There is, however, no mech- anism for the transfer of emotional conditions, unless an effect on nutrition might be so produced, and no nervous connections exist whereby reactions of the mother to unpleasant sights can affect the embryo. Only metabolic con- ditions can be reflected by the development of the embryo. In very few Uterine vein Placental septum Uterine blood space UA Ly Chorion i. Umbilical cord cae Yi Amnion Umbilical vein f/ Umbilical artery 2 yy Oe) Fig. 5.27. A portion of the human placenta, in section, showing the relation between the capillaries of the embryo, which are continuous with the umbilical arteries and veins, and the blood spaces of the mother, which are continuous with the uterine arteries and veins; diagrammatic. (Redrawn from L. B. Arey, Developmental Anatomy, copyright 1930 by W. B. Saunders Co., printed by permission. ) 163 GENERAL ZOOLOGY instances is there any evidence of transfer of disease or poisons from the mother to the embryo in the uterus. ‘The microorganism causing syphilis can pass the placenta and produce the disease in the fetus, as the human embryo is called after the third month. If a woman has German measles during the first 3 months of pregnancy, the causative agent passes the placenta and brings about severe effects in the embryo. A striking illustration of another of the few disadvantages inherent in the intimate association of the embryo with its mother has been discovered. ‘This is the origin of a very serious disease (fetal erythroblastosis) which reduces the number of red blood cells in the fetus during late pregnancy. It has been found that human red cells may have an antigen called the Rh (D) substance in addition to A and B (p. 65). Whether or not these antigens are present depends on the heredity of the individual (p. 199). Individuals who lack the Rh antigen do not normally contain an antibody for it; only 15 per cent of the general population lack the Rh antigen. If a man has the Rh sub- stance and his wife lacks it, at least half and perhaps all their children will produce red cells containing the Rh substance. It appears certain that enough Rh-containing red blood cells, or fragments of them, pass from the blood vessels of the fetus to the blood of the mother in the placenta to stimulate the production of Rh antibodies by the mother. She immunizes herself against this foreign protein, the Rh substance. These antibodies, when they have become abundant enough, begin to diffuse back through the placenta into the blood of the fetus. Here they result in the destruction of the fetal red blood cells and, consequently, in excessive drain on the regions forming red cells, usually with fatal results. ‘The anti-Rh substances persist in the blood of such a woman and, if her husband is homozygous (p. 186) for Rh, make it almost impossible for her to bear a second living child. Erythroblastosis results in the death of the fetus or newborn infant in sightly more than 2 per cent of the pregnancies in the white population of this country. If a partially jaundiced child born of Rh-incompatible parents is given an exchange blood transfusion within 24 hours after birth, it may be able to recover from the effects of the excessive destruction, in utero, of its red blood cells. In an exchange transfusion, matched Rh-negative donor blood is passed, over a period of 2 to 3 hours, into the infant’s cir- culatory system through an umbilical vessel as the infant’s blood is with- drawn. Such a procedure provides the red blood cells necessary for oxygen transport until the infant’s own supply of red cells is built up in its blood- forming organs which have not been permanently damaged by the severe drain on them. The embryonic membranes are not permanent structures. When the rep- tile or bird hatches, the embryonic membranes are left in the shell. After the birth of a mammal its embryonic membranes are expelled from the uterus of the mother. The development of such structures by reptiles, birds, and mammals has made them independent of an aquatic environment during development. Amphibians that are terrestrial as adults must undergo 164 REPRODUCTION AND DEVELOPMENT IN CHORDATES their development in the water since they have no amnion and _ chorion. Embryonic membranes and their modifications must have been extremely important in the evolution of the vertebrates, especially in the origin of the mammals. Experimental Modification of Development That the orderly processes which occur during development are conditioned by a number of closely interrelated factors can be experimentally demonstrated. The genes, or hereditary units, carried by a zygote control its development, and certain combinations of genes have been demonstrated repeatedly to bring about death in animals used in experiments (p. 205). Nuclei of embryonic cells are quite similar in appearance, and when they divide all are found to contain the number of homologous chromosomes characteristic of particular species. It has long been assumed that the nuclei of somatic cells of an individual are genetically equivalent. In recent studies, nuclei from cells of blastulae and early gastrulae as well as from cells of the chordamesoderm and endoderm of late gastrulae were transplanted singly into enucleated zygotes of the frog. Depending on the age and source of the transplanted nucleus, development of the zygote varied greatly or not at all from its normal pattern. It seems, therefore, that nuclei become differentiated in some way during development and that their differences, whatever they may turn out to be, are correlated with cell localization and differentiation. The cytoplasm of the zygote is shifted in an orderly way by streaming movements after fertilization so that certain parts are located in particular cells during cleavage and carried into typical positions by later movements. If the zygote is subjected to strong centrifugal force so that the cytoplasm Fig. 5.28. Development of the frog after separation of cells and injury at the two-cell stage. A, cleavage in each part after separation of the first two cells within the jelly envelopes by constriction with a hair; each separated part gives rise to a complete embryo. 8B, a half embryo at the neural-fold stage, following injury to one of the first two cells by means of a hot needle. (A, after H. Spemann, 1914, Verhandlung der deutschen zodlogischen Gesellschaft, vol. 24; B, redrawn from W. Roux, 1888, Archiv fiir pathologische Anatomie, vol. 114.) 165 GENERAL ZOOLOGY SoeeEP ee CY) Somites . te nd) i. Plane of section ce’ shown in C Fig. 5.29. Diagram showing results of the transplantation of a piece of dorsal lip of the blasto- pore of an amphibian (shown in A) to the blastocoel of another embryo of the same age (shown in section in B). In D, the lower individual has developed under the influence of the grafted dorsal lip; the line c-c’ indicates the plane of the cut surface shown in C. In C, cells derived from the graft are shown in black; induced cells are in white. (Redrawn from Analysis of Development, edited by B. H. Willier, P. A. Weiss, and V. Hamburger, copyright 1955 by W. B. Saunders Co., printed by permission. ) is thrown out of its usual position, the course of development is not normal. This localization of cytoplasmic areas occurs more slowly in some animals than in others. If the first two cells of the frog embryo are separated, each gives rise to a whole embryo; but if one of the cells is injured, the other forms half an embryo (Fig. 5.28). In some other animals each of the first two cells will form only half an embryo if separated. | As development progresses, certain cell masses are localized and differentiate into specific organs. ‘That this differentiation is determined in part by the relationship of the cell mass to its neighboring cells can be demonstrated by changing the typical relationships of the cells. ‘The ectoderm on the ventral 166 REPRODUCTION AND DEVELOPMENT IN CHORDATES and lateral surfaces of the frog embryo normally develops into epidermis, whereas ectoderm of the mid-dorsal region gives rise to the neural plate. When the dorsal lip of the blastopore is transplanted beneath the ectoderm on the ventral or lateral regions of a frog at the gastrula stage, the ectoderm covering such a transplanted dorsal lip gives rise to a neural plate, not to epidermis (Fig. 5.29). In other words, formation of a neural plate in the mid-dorsal region is dependent on the localization of ectoderm cells in a certain relation to those cells of the dorsal lip that move internally during gastrulation. ‘This is an example of the phenomenon of induction. After localization is completely established in a region, differentiation is apparently independent of the influence of neighboring masses. ‘The optic cup, for example, can be completely removed from an embryo and transferred to a test tube containing a nutrient solution which is changed at intervals. Under such conditions differentiation of the retina will take place, although the shape of the eyeball will not be normal, since the shape of an organ de- pends on the mutual pressure of adjacent cell masses. Similar indications of independent differentiation can be obtained by grafting parts of one embryo onto other embryos in a way that provides the metabolic necessities. A spectacular demonstration of the completely intrinsic nature of differentiation has been made by separating, by the use of enzymes, the cells of certain localized presumptive areas, such as the wing bud or mesonephros of the chick embryo. When masses of such dissociated cells are cultured on a nutrient clot in a moist chamber (cf. Fig. 2.5, p. 17), the characteristic tubules of the mesonephros and cartilage of the wing bud appear in due course (Fig. 5.30). In addition to the relations between nucleus and cytoplasm and between masses of cells within the embryo, the conditions of the environment affect the course of development. Development normally occurs at a certain pace; anything that alters that rate produces an atypical embryo. ‘This fact has been demonstrated in different ways. If the temperature is altered, the rate of development will be changed. When this is done at a time when some conspicuous mass movement of cells is occurring, as during the early stages of cell localization, later development may be atypical in many respects. If the rate of metabolism is altered by decreasing the amount of food or oxygen, similar results are obtained. One such atypical effect is the production of two embryos by one zygote. Frequently such embryos are joined, but some are entirely separate; these are identical twins, which are not produced by separa- tion of cells during early cleavage but by some arrest of development at a later stage, probably during early cell localization. Another method of altering the metabolic rate is by the introduction of poisonous or unusual constituents into the environment. When certain salts are added to water in which frog embryos are developing, atypical localization of the optic vesicles gives rise to one median eye (Fig. 5.31). It becomes obvious from these and numerous other experiments that devel- opment proceeds normally when a closely interlocking set of circumstances 167 GENERAL ZOOLOGY si Mesonephric tubules i Cartilage Ps Sie ete ~ * *, Fig. 5.30. Differentiation of reaggregated dissociated cells of chick embryo. 4A, dis- sociated cells of wing bud; x720._ B, reaggregated mixture of dissociated cells of mesonephros and wing bud of 3-day chick embryo; x90. C, mesonephric tubules and cartilage differentiated in reaggregated mixture, as in B, cultured for 10 days; x79. (From J. P. ‘Trinkaus and P. W. Groves, Proceedings of the National Academy of Sciences, vol. 41, copyright 1955 by University of Chicago Press, reprinted by permission.) is normal. If any condition in this group is abnormal, the orderly sequence of developmental processes will be disturbed. Relations between nucleus and cytoplasm, cell mass and cell mass, embryo and external environment can be varied but slightly if a normal individual is to develop. In the developing animal, as in the adult, the cells carry on metabolism if certain conditions are normal. The cells are also responsive and react not only to changes in neighboring cells but also to environmental changes external to the embryo. 168 REPRODUCTION AND DEVELOPMENT IN CHORDATES The embryo is a living individual, potentially self-sufficient at every stage in development, yet dependent, as is the adult, upon external conditions for sur- vival. Many problems of development remain to be solved, and many new methods of research have been devised in the attack upon the unknown in this field of zoology. It is a fascinating and a rich field for further study. Summary This completes your study, in the vertebrate animal, of the manifestations of life deriving from the distinguishing capacities of the cell, namely metab- olism, responsiveness, and reproduction. You have seen how the different kinds of mutually dependent, specialized somatic cells perform a myriad of adaptive functions concerned with the maintenance of the life of the multi- cellular individual. Even in the embryo, as it develops from a single cell, this closely adjusted relationship between different cells and between cells and environment is maintained. Elucidation of the complexity of structure and function has gone forward for many years with the help of the Cell Theory, although at the present time, great contributions are being made by studies at the subcellular level. In spite of the close attention to detail which is required to grasp the many facts presented, it is hoped that you have been stimulated to think beyond the details, even beyond the major aspects of how the problems of living have been solved at the cellular level. What really makes these intricate mech- anisms go in such orderly ways? And how does it happen that the basic func- External gills Fig. 5.31. Embryos of Rana Medianjeye y =~ vee pifens treated briefly with alkali during the early gas- trula stage; both are the same age. A has only one eye in A the median position; B is partially doubled in the region of the spinal cord and tail. (Redrawn from Analysis of Development, edited by B. H. Willier, P. A. Weiss, and V. Hamburger, copyright 1955 by W. B. Saunders Co., printed by permission.) Sucker Yolk-filled cells 169 GENERAL ZOOLOGY tions such as digestion, respiration, circulation, excretion, and coordination, to say nothing of reproduction and development, are alike in all the animals of a certain kind? What determines the pattern; what perpetuates it? The importance of enzymes as directives in the chemical reactions of living cells has been pointed out in many discussions. Were we to delve more exhaus- tively into the “Show” of life, almost innumerable instances could be cited of the ubiquity of these uniquely sculptured protein molecules which somehow provide the special niches in which the vital reactions occur. We can go a little farther in our search for a “‘constancy factor,” a “directing agent,” a ‘set of instructions” which is not only effective among individuals of the same generation but is handed on to successive generations. ‘This leads to a consideration of the phenomena of heredity and variation which we shall next undertake. 170 Pa Ber, wins HEREDITY AND VARIATION What any individual is, what it can become, and what its descendants can be like is determined by its hereditary constitution, its genetic make-up. It is often said that like reproduces like, and this statement is true in a general sense. Frogs give rise to frogs, not to toads; and dogs reproduce dogs. How- ever, the pups of a litter are not identical with their parents or with one an- other. A new individual resembles its parents, yet differs from them. ‘The phenomena of heredity and variation go hand in hand. Heredity may be defined as the tendency of individuals to resemble their ancestors and rela- tives; variation is the tendency of organisms related by descent to differ in specific ways. New individuals arise during the process of reproduction and develop through an orderly series of changes until they reach maturity. It is obvious that whatever it is that passes from one generation to the next must determine not only the typical sequence of developmental processes but also the characteristics of the adult organism. In other words, the germ cells must carry the mechanism responsible for heredity and variation. The individual ordinarily develops under environmental conditions which are practically uniform for successive generations but which cannot be ignored in seeking the complete answer to questions concerning heredity and varia- tion. Genetics, a great subdivision of zoological science which has developed conspicuously since 1900, has for its province the subject matter relating to the facts and theories of heredity and variation. Heredity and variation can be studied by four different methods, all of which have yielded information concerning basic problems. It is possible to observe and analyze resemblances and variations from one generation to another in large groups of individuals as they are found under natural con- ditions. ‘This is the statistical method, or the method of biometry. Contrasted with such mass analysis is the observation of inheritance and variation in animals bred under experimental conditions for generation after generation. 172 Fig. 6.1. Gregor Mendel. (A portrait by Flatter from Journal of Heredity, t af 1940, vol. 31, reprinted by permission.) = This method of experimental breeding under controlled conditions that give the best environment for the organisms makes it possible to know in detail the character of heredity and variation in any particular individual, as well as to compare specifically individuals of successive generations. In order to interpret fully the result of experimental breeding it becomes necessary to study, by the method of cytology, the germ cells from which new individuals arise. ‘The greatest progress in the theory of genetics has come from correla- tion of the results obtained from experimental breeding and such study of germ cells. A fourth way of approaching the problem of the mechanism of heredity and variation is the method of experimental embryology, in which individuals of known ancestry are subjected to conditions that are not usual for their development. Comparison of results obtained from these several methods of approach has yielded considerable information concerning many facts of heredity and variation and has led to the formulation of theories of the mechanism involved. Clarification and extension of our knowledge of genetics may be expected to continue in view of the great amount of interest in research in this field. The Method of Biometry Investigators who use the biometrical method collect a great amount of observational data upon organisms under natural conditions, analyze these data by statistical methods, and formulate generalizations concerning heredity and variation that will be true for the whole group but for no particular individual. Sir Francis Galton (1822-1911) did the first serious biometrical work when he studied the relation between the height of parents and the height of offspring in over a thousand human families. The original study of 173 GENERAL ZOOLOGY Galton on human height has been supplemented by investigations of inherit- ance of eye color, mental ability, length of life, and other characteristics by Karl Pearson, Raymond Pearl, and other biometricians. Under circum- stances where experimental analysis and study of individual pedigrees are impossible, as is the case for most human characteristics, the biometrical method can determine trends and reveal suggestive correlations. ‘The results of such studies do not make possible predictions concerning the inheritance of traits by the progeny of specific crosses. Galton’s observations, although made on groups that were not subjected to experimental control, furnished the stimulus for the experimental work of Wilhelm Johannsen, a Danish botanist (1857-1927). Johannsen conceived the idea that if offspring of parents who were above or below the average were also above or below the average, respectively, as Galton had found, it might be possible to shift the average by continued selection of parents from the unusual groups. Working with beans in which self-fertilization occurs, he chose the heaviest seeds from which to raise a new generation. ‘The seeds selected weighed 0.8 gram each, and from them Johannsen obtained plants that produced seeds varying in weight from an average of 0.35 gram on some plants to an average of 0.6 gram on others. He next tried similar experiments with the seeds from single plants. ‘The selection of the smallest seed or the largest seed for planting had no effect on the size range of the seeds produced. It was not possible to grow larger and larger beans by planting the largest seeds time after time. Since cross-fertilization did not occur, the fundamental hereditary constitution was not altered throughout the experiment. Beans descended by self-fertilization from any single plant constitute what is known as a pure line. Within pure lines Johannsen found that, although environ- mental factors of temperature, moisture, or soil might affect the size range, the average weight could not be shifted by selecting either the lightest or the heaviest beans for planting. Johannsen was also able to demonstrate that in large groups of bean plants many pure lines are represented. Such large groups of individuals are known as populations. Just as each pure line varies around its average, the popula- tion varies around the average of its component pure lines. Starting with a population, it is possible to sort out, by selection of parents, lines having desired characteristics. ‘The practical breeder tends to do this in his selec- tions of breeding stock and seeds. However, pure lines are stable only if cross-breeding is prevented, which is frequently impossible. Selection within pure lines has been tested by many investigators. Experiments have been conducted on inheritance of size variation in different organisms, chemical content of potatoes and sugar beets, egg-laying capacity in poultry, time of maturity of seeds, distribution of color in coats of mammals, and numerous other characteristics. In no case has selection shifted the average about which a pure line varies. Such variations, which are known as fluctuations, are conditioned by environmental effects during development and do not in- fluence the characteristics of succeeding generations. 174 HEREDITY AND VARIATION The Method of Experimental Breeding Mendel’s Contribution. Although the statistical or biometrical method gives a survey of the average course of heredity and variation in populations and pure lines consisting of numerous individuals, it is not of value in the analysis of individual cases. Experimental breeding in a controlled environ- ment makes possible the accumulation of data on the process of heredity in all the individuals produced from generation to generation. ‘The first re- corded experiment in plant breeding was that of Camerarius in 1694. Not until Father Gregor Mendel (1822-1884) of the Augustinian Order carried out his careful work on the breeding of peas in the monastery garden at Brno (Briinn), Moravia, did this method yield results that revealed the principles of heredity (Fig. 6.1). Mendel’s success came when he followed the inheri- tance of single specific characteristics in many individuals for several genera- tions. One of his original experiments was cross-fertilization between peas with tall stems and those with dwarf stems. The use of parents that differ in one or more characteristics is known as the method of hybridization, and the offspring of such a cross-fertilization are hybrids. If the parents differ in one characteristic, such as length of stem in peas, a cross between them is called monohybridization. Mendel found that without exception the offspring produced by hybridizing purebred tall and dwarf peas were tall, no matter which parent was tall and which dwarf (Fig. 6.2). When these hybrid tall peas were crossed among themselves, three-fourths of the next generation were tall like the tall parents, and one-fourth were dwarf like their dwarf grand- parents. Dwarf stem, a characteristic which did not appear in the first filial generation (/', generation) that arose from the cross between tall and dwarf parents (P, generation), emerged unchanged in the second filial generation (f’, generation). Dwarfness in such peas was as pure as was the dwarfness of their grandparents, and these /’, dwarf peas gave rise only to dwarf peas when they were bred together. Breeding of the tall peas that constituted three- fourths of the F, generation revealed that, although these tall individuals superficially resembled one another, they were dissimilar as parents. One- third of the tall group gave rise in successive generations to tall offspring without exception. Such tall specimens, which constituted one-fourth of the total F’, generation, corresponded, in their resemblance to the purebred tall pea of the P, generation, to that quarter of the group that was like the dwarf pea of the P,;. The remaining two-thirds of the tall peas of the F, gen- eration, or one-half of the offspring of the hybrid tall peas, were like their parents. When interbred, these F, tall peas gave rise to offspring in the ratio of three tall to one dwarf. As shown in Figure 6.2, this group again breaks up, when analyzed by breeding, into three types that occur in the ratio of 1 :2:1; that is, one-fourth are pure tall peas, one-half hybrid tall peas, and one-fourth pure dwarf peas. In the case under discussion, tallness is said to be dominant to dwarfness; conversely, dwarfness is recessive to tallness. 175 GENERAL ZOOLOGY Tall Dwarf Tall x Tall E. peas peas 1 Tall Tall x Tall Dwarf EF peas peas peas peas 2 Inter- Inter- crossed > \s Tall Tall \y Tall Dwarf F peas peas peas peas 3 Inter- Inter- a “ly crossed crossed foal crossed Tall Tall Tall Tall Tall Dwarf oe Dwarf\ F peas peas peas peas peas peas oe peas 4 Fig. 6.2. Diagram to illustrate monohybridization in peas that differ in length of the stem; tall stem is dominant to dwarf stem. It has been stated that the offspring of a cross between tall and dwarf peas are tall. Superficially, they cannot be distinguished from the tall parent. Yet these tall hybrids when bred together give some tall and some dwarf offspring. The tall peas of the /, generation are all similar in appearance; but experi- mental breeding proves that some reproduce only tall peas, whereas others are like their parents in giving rise to both tall and dwarf offspring. In such cases of inheritance with dominance, it is impossible to distinguish by super- ficial examination a hybrid individual from an individual that will breed true for the dominant character. Johannsen proposed the terms phenotype, to designate individuals that look alike, and genotype, to designate individuals that breed alike. The tall peas of the F, generation constitute a phenotype 176 HEREDITY AND VARIATION which can be shown to be made up of two genotypes. On the other hand, the dwarf peas, or any individual that appears recessive, will always breed true for the recessive characteristic; the recessive phenotype is identical with the genotype. If any dominant characteristic be represented by D and any recessive char- acteristic by d, the following summary of the possible crosses can be made. (Parents) DD x DD 100 per cent DD (offspring) (Parents) dd x dd 100 percent dd (offspring) (Parents) DD x dd 100 percent Dd (offspring) (Parents) Dd x Dd 25 percent DD plus 50 per cent Dd plus 25 per cent dd_ (offspring) (Parents) DD x Dd 50 per cent DD plus 50 per cent Dd (offspring) (Parents) Dd x dd 50 percent Dd_ plus 50 per cent dd (offspring) It must be understood that large numbers of specific cases were studied in order to obtain the percentages that have been indicated as characteristic of these crosses. Figure 6.3 gives some of the actual numbers of individuals in the /, generations in Mendel’s original monohybridization experiments. Extension of Mendelian Theory. Mendel published the results of his studies in 1866, but they remained unknown until 1900, when his paper was discovered by three scientists who had independently reached the conclusions that he had stated so clearly. Since that time, his results have been con- firmed by experiments with many plants and animals. For example, when a gray mouse is crossed with a white mouse, all the offspring are gray. ‘This result indicates that gray coat color is dominant to white coat color in mice (Fig. 6.4). In the #, generation gray and white mice occur in the ratio of 3:1. Of the gray mice which constitute three-fourths of this /, generation, Number of Number of Character : i Dominants Recessives Form of seed 5,474 smooth 1,850 wrinkled 2.96 to 1 Color of seed coat 6,022 yellow 2,001 green 3.01 to 1 Length of stem | 787 tall 277 dwarf 2.84 to 1 Color of flowers / + 705 colored 224 white 3.15 to 1 Position of flowers 651 axial 207 terminal 3.14 to 1 Form of pods 882 inflated 299 constricted 2.95 to 1 Color of unripe pods 428 green 152 yellow 2.82 to 1 2.98 to 1 Fig. 6.3. Data from Mendel’s original experiments, from which was derived the 3:1 ratio characteristic of the F, generation in monohybridization. (From H. E. Walter, Genetics, copyright 1922 by The Macmillan Co., reprinted by permission. ) 177 GENERAL ZOOLOGY te aS : x iwes P, eg tern Gray mouse White mouse Gray mice F Intercrossed 1 homozygous Fy 1 homozygous gray mouse 2 heterozygous gray mice : white mouse | I Intercrossed Ne es Be 5 ee Bee, Intercrossed Intercrossed .) 2 fn ke | ise aku Homozygous white mice TTC D\ 2. fa, % 4} 4 > Aa 1 homozygous e 2 heterozygous gray mice a 1 homozygous gray mouse white mouse Fig. 6.4. Results of monohybridization of mice which differ in color of coat; gray coat color is dominant to white coat color (cf. Fig. 6.11). some are found to breed true for grayness, whereas others produce both gray and white offspring. Complete analysis by breeding reveals that 25 per cent of the F’, generation are gray and will breed true for gray coat color, 50 per cent appear gray but will not breed true, and 25 per cent are white and re- produce only white-coated individuals when interbred. Up to this point the examples used have involved inheritance with dominance. Dominance and recessiveness do not, however, characterize all cases of in- heritance. If red and white four-o’clocks (Mirabilis jalapa) are crossed, the hybrids of the F, generation have pink flowers, not red or white. When these pink-flowered hybrids are interbred, offspring occur in the ratio of 1 red :2 pink: 1 white. The individuals with red and white flowers breed true for these characteristic colors, whereas those with pink flowers always give three kinds of offspring in the typical 1:2:1 ratio. Another case of inheritance with- out dominance is the Blue Andalusian fowl (Fig. 6.5). Blue Andalusian fowls 178 HEREDITY AND VARIATION are produced by crossing a type of black fowl with a certain kind of white fowl. They are, therefore, hybrids and, as would be anticipated, do not breed true. Blue Andalusian parents yield 25 per cent black chicks, 50 per cent blue chicks, and 25 per cent white chicks. ‘The black and the white offspring breed true, but the blue offspring, like all other Blue Andalusians, will always yield 25 per cent black, 50 per cent blue, and 25 per cent white individuals. The course of inheritance for characteristics that do not exhibit dominance is in no way different, therefore, from that for characteristics in which domi- nance occurs. In-inheritance without dominance, the hybrid individuals, or those that will not breed true, can be distinguished superficially from those that will give rise to offspring like themselves. Dominance is not an essential ie) LAW 2 Splashed-white fowl Blue Andalusian 1 black fowl : 2 blue Andalusian fowls : 1 splashed-white fowl Fig. 6.5. Results of monohybridization of fowls which differ in color of feathers. ‘This is an example of inheritance without dominance, since black and white parents yield blue hybrids. In the F’, generation three visibly distinct types are produced in a 1 : 2:1 ratio. 179 GENERAL ZOOLOGY P, yee eet cas Black, smooth-coated White, rough-coated F; x i Black, rough-coated Interbred F, % it mire sak 9 black, rough-coated 2 3 white, rough-coated : 3 black, smooth-coated : 1 white, smooth-coated Fig. 6.6. Results of dihybridization in guinea pigs which differ in color of hair and quality of coat; black hair is dominant to white hair, and rough coat to smooth coat (cf. Fig. 6.15). (Rearrangement of figures from W. E. Castle, Genetics and Eugenics, copyright 1916 by Harvard University Press, printed by permission. ) feature of heredity, although it is almost universally encountered in practical breeding. ‘The behavior of the hereditary units in the germ cells is the same whether or not dominance is involved. If two individuals that differ in two characteristics are crossed, the process is known as dihybridization. In guinea pigs black hair and rough coat are dominant to white hair and smooth coat. When a black-haired, smooth-coated guinea pig is bred with a white-haired, rough-coated one, the offspring are all black-haired and rough-coated (Fig. 6.6). Whether these characteristics have been present in the male or in the female parent is not important; the combi- nation always produces individuals in which both dominant characters are seen. When these F’, hybrids are crossed, four kinds of offspring (phenotypes) result in the F, generation, in the following ratio—9 black-haired, rough- coated : 3 white-haired, rough-coated : 3 black-haired, smooth-coated : 1 white- haired, smooth-coated. Mendel’s original work on dihybrids consisted in crossing wrinkled green peas with smooth yellow peas. The offspring of such a hybridization are smooth yellow peas, a fact which indicates that smooth surface is dominant to wrinkled surface and yellow color is dominant to green color in the seeds. In the F, generation smooth yellow peas, smooth green peas, wrinkled yellow peas, and wrinkled green peas occur in the ratio of Di Soe 180 HEREDITY AND VARIATION Trihybridization is the crossing of two individuals differing in three charac- teristics. A third character that can be used in guinea pigs is length of hair, short hair being dominant to long hair. When a black, short-haired, smooth- coated guinea pig is crossed with a white, long-haired, rough-coated guinea pig, all the offspring of the /, generation are black, short-haired, and rough- coated (Fig. 6.7). Eight different kinds of individuals (phenotypes) are produced in the F, generation, in the ratio indicated—27 black, short- haired, rough-coated :9 black, short-haired, smooth-coated :9 white, short- haired, rough-coated : 9 black, long-haired, rough-coated : 3 white, short-haired, smooth-coated :3 black, long-haired, smooth-coated :3 white, long-haired, rough-coated : 1 white, long-haired, smooth-coated. In the crosses previously considered, the sex of the parent having a particu- lar characteristic has been of no significance. ‘There are, however, cases of sex-linked inheritance in which the sex of the parent that possesses a certain character modifies its distribution in the offspring. Extensive work in experi- mental breeding for the study of heredity and variation was first carried on in this country by T. H. Morgan (Fig. 6.8) and his students, who used the x P, < abides em in i r a Black, short-haired, White, long-haired, smooth-coated rough-coated F, : all black, short- haired, rough-coated Interbred ne ~Sileacoid ' 27 black, short-haired, : 9 black, short-haired, : 9 white, short-haired, ; 9 black, long-haired, rough-coated smooth-coated rough-coated rough-coated o- 3 white, short-haired, : 3 black, long-haired, 3 3 white, long-haired, : 1 white, long-haired, smooth-coated smooth-coated rough-coated smooth-coated % Fig. 6.7. Results of trihybridization in guinea pigs which differ in color and length of hair and quality of coat; black hair is dominant to white hair, short hair to long hair, and rough coat to smooth coat. (Rearrangement of figures from W. E. Castle, Genetics and Eugenics, copyright 1916 by Harvard University Press, printed by permission.) 18] GENERAL ZOOLOGY Fig. 6.8. Left, Edmund Beecher Wilson, 1856-1939. Right, Thomas Hunt Morgan, 1866-1945. (Photographs courtesy Mrs. A. F. Huettner.) fruit fly, Drosophila. An example of sex-linked inheritance may be selected from the abundant data concerning heredity in this small insect. Red eye color is dominant to white eye color in Drosophila (Fig. 6.9). When a red- eyed female is crossed with a white-eyed male, both male and female offspring of the F', generation are red-eyed. If such red-eyed individuals are inter- crossed, all the females and one-half of the males of the F, generation have red eyes, whereas one-half of the males have white eyes. The reciprocal cross, or the cross between a white-eyed female and a red-eyed male, gives very different results (Fig. 6.10). The males of the F', generation are white-eyed, and the females are red-eyed. In the /, generation red-eyed and white-eyed males and females occur in equal numbers. When the mechanism of inherit- ance is considered in the following section, sex-linked inheritance will be found to furnish additional confirmation of Mendelian principles. ‘The method of experimental breeding, first carefully used by Mendel, has yielded a vast amount of detailed information concerning the course of in- heritance of specific characteristics by particular individuals through succes- sive generations. It was clear to Mendel that the reproductive cells, which do not, of course, exhibit the characters of stems and seeds, must carry something correlated with the appearance of characters of the adult organism from generation to generation. ‘These ‘“‘somethings”’ are usually called hereditary 182 HEREDITY AND VARIATION factors, or genes. Mendel proved that a factor related to a specific expression of a character, such as dwarf stem, was not altered by association with a factor for the alternate expression of that character, such as long stem. Hereditary factors for the alternate or allelomorphic states in which characters are found to exist retain their unmodified independence even when present together in Females Males Fig. 6.9. Results of crossing a red-eyed female Drosophila with a white-eyed male. ‘The X-chromosomes are represented as carriers of the genes determining eye color; W is the symbol used for the gene for red eye color which is dominant to the gene, indicated by w, for white eye color (cf. Fig. 6.17). ‘The hook-shaped chromosome which does not contain a symbol for a gene represents the Y-chromosome of the male (cf. p. 209). This cross is the reciprocal of that shown in Fig. 6.10. (Modified from T. H. Morgan et al., The Mechanism of Mendelian Heredity, copyright 1922 by Henry Holt and Co., printed by permission. ) 183 GENERAL ZOOLOGY Females Males Fig. 6.10. Results of crossing a white-eyed female Drosophila with a red-eyed male (cf. Fig. 6.18). The X-chromosomes and the symbols for the genes are explained in the legend of Fig. 6.9, which shows the reciprocal of this cross. (Modified from T. H. Morgan et al., The Mechanism of Mendelian Heredity, copyright 1922 by Henry Holt and Co., printed by permission.) the cells of a hybrid individual and even though one factor is without effect in the presence of the other. When a hybrid produces its reproductive cells, these two allelomorphic factors, or alleles, one for each of the visible expressions of the character under consideration, must be able to undergo segregation from one another and occur alone in different reproductive cells. When random combination of gametes occurs to form individuals of the next genera- tion, new combinations of genes occur which condition the characteristic ratios of mono-, di-, and trihybridization experiments. The details of the 184 HEREDITY AND VARIATION behavior of the genes can be understood best if considered in connection with that of the chromosomes. The Method of Cytology Chromosomes as' Carriers of the Genes. When Mendel’s work was brought to light in 1900, it was well known that new individuals developed from zygotes formed by the union of ova and spermatozoa. ‘These gametes, or mature germ cells, carry the haploid number of chromosomes that is char- acteristic of the species; the diploid number is restored in the zygote (pp. 136 and 143). As mitosis occurs during development of the zygote, the chromo- somes are distributed equally to all the cells of the new animal (p. 43). The primordial germ cells of any individual contain chromosomes that can be grouped in pairs of similar size and shape. One member of each pair is of paternal and the other of maternal origin (see Fig. 5.7, p. 135). When the first meiotic division occurs, homologous chromosomes disjoin and pass into different cells. This disjunction does not involve the separation of the sets of chromosomes that came from the two parents at the time of fertilization. On the contrary, the distribution of homologous chromosomes is random, with the members of each pair of chromosomes separating independently. ‘These facts were discovered by the microscopical examination of germ cells by the methods of cytology. In 1902 W. S. Sutton called attention to the behavior of the chromosomes as furnishing a cellular mechanism for the explanation of Mendel’s results. Since that time the theory of the chromosomes as carriers of the genes has been greatly extended, and chromosomes are now considered to be the physical basis of heredity. The researches of E. B. Wilson (Fig. 6.8) and Nettie M. Stevens in 1905-1906 were important in the analysis of the numbers and types of chromosomes in male and female animals; the work of Eleanor E. Carothers in 1917 furnished evidence for the independent assort- ment of homologous chromosomes during the meiotic divisions. If the example of monohybridization between a gray and a white mouse is analyzed according to the concept that the genes are located in the chromosomes, the assumption is made that the zygote from which the gray mouse developed contained two genes for grayness, one from each of its parents, whereas the zygote from which the white mouse developed contained two genes for whiteness, one from each of its parents (Fig. 6.11). Each of these genes is regarded as being located in a separate chromosome, but the two genes of each animal are present in homologous chromosomes. When separation of the homologous chromosomes occurs at the disjunctional divi- sion during meiosis, the genes are carried into different gametes. All the gametes of the gray parent possess a single gene for gray coat color, and those of the white parent a single gene for white coat color. When fertiliza- tion occurs, each zygote obtains two genes for coat color, but one is for whiteness and one for grayness; the allelomorphic genes are now together in 185 GENERAL ZOOLOGY Zygotes from Gray White hich P mouse mouse developed Gametes of 12) 1 after ie maturation re rp Gray which F gray mouse hybrids de- velop Gametes of Fy a Zygotes from () () which Fy gen- Wee W Sal i eration de- U U velops 1 homozygous gray : 2 heterozygous gray : 1 homozygous white Fig. 6.11. A cross between a homozygous gray mouse and a homozygous white mouse (cf. Fig. 6.4). The diagram illustrates the segregation of a pair of genes when their carriers, a pair of homologous chromosomes, undergo disjunction; notice the possible combinations that can occur between gametes. IV, gene for gray coat color; w, gene for white coat color. the same zygote. A zygote of this kind is called a heterozygote, in contrast to the zygotes like those from which the parents developed, which are known as homozygotes because the genes of the pair are alike. ‘The F, individuals are spoken of as heterozygous, and the parents in this case are called homo- zygous. As can be seen from Figure 6.11, two kinds of gametes are formed when disjunction occurs in individuals of the F, generation, and three kinds of combinations of gametes with chromosomes sa genes are possible in the F’, generation. According to chance, one of these classes, the Ww combi- nation, will occur twice as frequently as either the WW or the ww. ‘The effect of the dominance of the gene for grayness is such that both homo- zygous and heterozygous gray mice look alike; the apparent or phenotypic ratio in the F, generation is 3 gray:1 white. It is necessary, therefore, to obtain offspring from particular crosses in order to differentiate genetically between individuals that exhibit a dominant characteristic, that is, to separate the genotypes of the dominant phenotype. The theoretical explanation of the results obtained when a heterozygous gray mouse is crossed with a white 186 HEREDITY AND VARIATION , Zygotes from Ww Gray x White which P, developed mouse Gametes of P. 1 Zygotes from which F 1 develops 1 heterozygous gray : 1 homozygous white Fig. 6.12. A cross between a heterozygous gray mouse and a homozygous white mouse. ‘The diagram illustrates the segregation of a pair of genes when their carriers, a pair of homologous chromosomes, undergo disjunction; notice the possible combinations that can occur between gametes. IW, gene for gray coat color; w, gene for white coat color. Zygotes from which P, developed pw 2 ON O® @®Oxr Zygotes from w which F develops 1 homozygous gray : 1 heterozygous gray Fig. 6.13. A cross between a homozygous gray mouse and a heterozygous gray mouse. ‘The diagram illustrates the segregation of a pair of genes when their carriers, a pair of homologous chromosomes, undergo disjunction; notice the possible combinations that can occur between gametes. W, gene for gray coat color; w, gene for white coat color. 187 GENERAL ZOOLOGY Fig. 6.14. Results of monohybridization of flies which differ in type of wing. The chromosomes are shown as the carriers of the genes; V is the symbol used for the gene for long wing, which is dominant to the gene, v, for vestigial wing (cf. Fig. 6.11). (Modified from T. H. Morgan et al., The Mechanism of Mendelian Heredity, copyright 1922 by Henry Holt and Co., printed by permission.) mouse, which will always be homozygous since white is recessive, is presented in Figure 6.12. ‘This is known as a back-cross with a recessive. ‘The difference between the offspring obtained in this cross and those obtained by crossing a homozygous gray with a white mouse (Fig. 6.11) furnishes the type of breed- ing test used for differentiating homozygous from heterozygous dominant individuals. The diagram in Figure 6.13 shows why indiscriminate crossing between the gray mice would fail to yield information that would enable one to distinguish homozygous gray mice with certainty. In the case illustrated 188 HEREDITY AND VARIATION all the offspring will be gray in both the F', and F, generations; the same situation would prevail if both original gray mice had been homozygous. However, if any white mice appeared in a cross between two grays, their presence would prove both parents to be heterozygous. The breeding results are explained adequately in these cases of mono- (E) Beater Long - winged, al ( Zygotes from ged, x ebony-bodied | y y which P, ©) ® et fly U U (e) developed VD] |U ie BO pe waa to long - winged, gray — bodied flies Zygotes of F, fs v © that give rise Zygotes of Fy 9 long - winged, gray - bodied 3 long-winged, ebony - bodied vestigial gray vestigial gray 3 vestigial - winged, gray - bodied vuEe vuee te ; ©) vestigial vestigial 1 vestigial - winged, gray ebony ebony - bodied Fig. 6.15. A case of dihybridization between a vestigial-winged, gray-bodied fly (Drosophila) and a long-winged, ebony-bodied fly (cf. Fig. 6.6). The diagram illustrates disjunction and independent assortment of two pairs of homologous chromosomes which carry two pairs of genes; notice the possible combinations that can occur between the gametes formed. £, gene for gray body; e, gene for ebony body; V, gene for long wing; v, gene for vestigial wing. 189 GENERAL ZOOLOGY hybridization by the assumption that allelomorphic genes conditioning the alternate expressions of the character used are carried in homologous chromo- somes and thus distributed to gametes and zygotes (Fig. 6.14). The theory of the chromosomes as the physical basis of heredity can also be used to ex- plain cases of di- and trihybridization. In Drosophila long wing (V) is domi- nant to short or vestigial wing (v), and gray body color (£) is dominant to ebony (e) body color. A fly with vestigial wings and gray body (wEE) is mated with a long-winged fly with ebony body (VVee) (Fig. 6.15). At the time of the first meiotic division the homologous chromosomes of each pair undergo disjunction and pass into different gametes. The gametes produced by the parents contain genes vE and Ve, respectively. The zygotes of the F’, generation will be Vvke and will develop into long-winged, gray-bodied flies. Four different kinds of gametes can be formed by these F', individuals, since the different pairs of homologous chromosomes assort independently when they undergo disjunction ai the first meiotic division. Segregation and independent assortment of genes yield the following four classes of gametes in any F, individual: VE, Ve, vk, and ve. Random combination of gametes con- taining such genes gives rise to 16 possible zygotes that develop into the F’, generation as shown in the checkerboard at the bottom of Figure 6.15. There are four phenotypes occurring in a 9:3:3:1 ratio and nine genotypes (VVEE, VVEe, VvEE, VuEe, VVee, Vuee, vwvoEE, vuEe, and vvee). Linkage. The discussion of the chromosomes as the carriers of the genes has been concerned so far with what could occur if each gene were carried in a separate chromosome. It was discovered by W. Bateson and R. C. Punnett in 1906 that certain characteristics were linked in inheritance. They found, in crossing a sweet pea with purple flowers and long pollen grains and a sweet pea with red flowers and round pollen grains, that the genes which came from each parent tended to remain together instead of assorting inde- pendently during meiosis. The study of inheritance of several hundred genes in Drosophila shows that they fall into four groups; the genes composing each of these groups are said to be linked (Fig. 6.16). Cytological investigation demonstrates the presence of four pairs of chromosomes in Drosophila (see Fig. 2.12D and E, p. 42), and pairs of linked genes are known to be carried by pairs of homologous chromosomes. Furthermore, there is evidence which is interpreted to indicate that genes in a chromosome are arranged like beads on a string, and relative distances between genes in the string have been com- puted (Fig. 6.16). This linear order of the genes makes the longitudinal reduplication of the chromosomes and subsequent separation of the. sister genonemata highly significant events (p. 43). The example of sex-linked inheritance described as a breeding experiment (p. 181) can be explained if it be assumed that the X-chromosomes carry the genes concerned with red and white eye color. It will be recalled that in the cells of a female there are two so-called X-chromosomes, whereas in the male only one X-chromosome is found (see Fig. 21 2Di andwil, p42 jp" ihe male Drosophila possesses one X-chromosome and a Y-chromosome which car- 190 HEREDITY AND VARIATION a“ = Se 1 (X) 2 3 4 Y>5T NW 0 yellow (B) al 0 aristaless (B)fu- =O roughoid (E) bt <0 bent (W) sc Xo + scute (H) s 1.3 star (E) ve’ | ~0.2 veinlet (V) ci Ve cubit. interr. (V) H i} \Yo+ hairy wing (W) sv ‘0.1 shaven H “1 \Ns white (E) ey 0.1 eyeless (E) \ 3.0 facet (E) NSE echinus (E) ec rb 7.5 ruby (E) dp N cv 13.7 crossveinless (V) Ge TE) cl 16.5 clot (E) Pe 20.0 cut (W) jv 19.2 javelin (H) sn 21.0 singed (H) se 26.0 sepia (E) kz 27.7 lozenge (E) hi) 26:5) hatey (EH) d 31.0 dachs (B) Vv 33.0 vermilion (E) m 36.1 miniature (W) ; D __ dichaete (H) J 41.0 jammed (W) Gg —41.0— plued (BE) s 43.0 sable (B) th 43.2 thread g 44.4 garnet (E) st’ | “44.0 scarlet (E) cepa b 48.5 black (B) D 475 deformed (E) rd\} 751.0 reduced(H) P, 48.0 pink (E) pr 54.5 purple(E) CU 50.0 curled (W) BI\| 754.8 bristle (H) <_—_ f\] 86.7 forked(H) ft’ | “55.0 light (E) _ sb} 582 _ stubble (H) B~]-—57.0_ bar (E) cn 57.5 cinnabar (E) ss7 —58.5 spineless (H) i ee) bx’ | S58 Bithomx (B) car 62.5 cornation(E) © 62.0 _ engrailed (B) sr 62.0 stripe (B) gl’| 63.0 glass (E) 67.0 vestigial(W)DI 7 66.2 delta (V) H. | 69.5 hairless (H) 70.7 ebony (B) bb~1 —66.0 bobbed (H) L 72.0 lobe (E) c 75.5 curved (W) ed 74.7 cardinal (E) ro 91.1 rough (E) hy 93.3 humpy (B) px 100.5 plexus(W) ca 100.7 claret (E) bw 100.7 brown (E) sp 107.0 speck (B) Mg 106.2 minute-g (H) Fig. 6.16. Maps of the four chromosomes of the haploid set in Drosophila; the genes located in any one chromosome and their allelomorphs constitute a linkage group. Chromosome 1 is the X-chromosome, and 4 is the small round one (cf. Fig. 2.12D, p. 42, and Fig. 6.194). Letters in parentheses indicate part of fly where effect of mutant gene is observed: B, body; E, eyes; H, bristles; V, venation of wings; W, wings. The positions of association with the spindle (cf. p. 41) are indicated by arrows. (After C. Bridges, from A. H. Sturtevant and G. W. Beadle, An Introduction to Genetics, copyright 1939 by W. B. Saunders Co., printed by permission. ) 191 GENERAL ZOOLOGY Red - eyed x White-eyed Zygotes from female male which P, developed Haas. a. . "f Red - eyed x Red -eyed Zygotes female male of Fy fom Gametes of F; 1 Zygotes of Fo Red-eyed females Red-eyed male White-eyed male Fig. 6.17. A cross between a red-eyed female Drosophila and a white-eyed male (cf. Fig. 6.9). The diagram illustrates disjunction of sex chromosomes during the formation of gametes and the possible combinations of these; the X-chromosomes carry genes for eye color. This is the reciprocal of the cross shown in Figure 6.18. X, X-chromosome carrying gene for red eye color; X, X-chromosome carrying gene for white eye color; Y, Y-chromosome which is confined to males and does not carry a gene for eye color. ries very few genes and does not leave the male line. Each ovum formed during maturation contains one X-chromosome; one-half of the sperm have an X-chromosome, the other half have a Y-chromosome. In the cross be- tween a homozygous red-eyed female Drosophila and a white-eyed male, each ovum contains an X-chromosome carrying a gene for red eye color, whereas half the sperm have an X-chromosome bearing a gene for white eye color and half have a Y-chromosome which has no gene for eye color (Fig. 6.17). Ran- dom unions of ova and spermatozoa result in red-eyed females and red-eyed males in the F, generation. The females are red-eyed because the gene for red eye color is dominant to the gene for white eye color. ‘These females are, however, heterozygous for eye color and give rise to two kinds of ova, in one of which the X-chromosome carries a gene for red eye color, and in the other the X-chromosome carries a gene for white eye color. Half the spermatozoa produced by the red-eyed males have an X-chromosome with a gene for red eye color and half have the Y-chromosome. Combinations of the gametes in 192 HEREDITY AND VARIATION a cross between a male and a female of the F, generation result in red- eyed females, red-eyed males, and white-eyed males. In this kind of inherit- ance the white-eyed characteristic of the male parent does not occur in the F, generation but reappears in one-half the males of the /, generation. ‘The reciprocal cross, in which a white-eyed female is mated with a red-eyed male, is shown in Figure 6.18. If the distribution of the X-chromosomes is followed, the reason for the difference between the offspring from these two crosses should be clear. It is to be understood that sex linkage is not an exception to Mendelian principles but confirms the theory that chromosomes carry the genes. Cytology has furnished knowledge of the behavior of chromosomes and their transmission from one generation to the next. ‘The experimental breeder has been able to explain adequately the results obtained in his breeding ex- periments by the assumption that what is present in the zygote and determines the appearance of a character in an adult organism is carried by the chromo- somes. ‘These hypothetical determiners of inherited characters are known as White- eyed y Red-eyed Zygotes from female male which P, developed Lap or A Ze Gametes of Pi Red -eyed x White- eyed Zygotes female male of F; ~ A Gametes of F 1 Zygotes of Fy Red - eyed White-eyed Red -eyed White-eyed female female male male — ve Fig. 6.18. A cross between a white-eyed female Drosophila and a red-eyed male (cf. Fig. 6.10). The diagram illustrates disjunction of sex chromosomes during the formation of gametes and the possible combinations of these; the X-chromosomes carry genes for eye color. This is the recip- rocal of the cross shown in Figure 6.17, the legend of which describes the symbols. 193 GENERAL ZOOLOGY 6 =Sy N85 Ss A Sa es ad a Ne ONE Ses inw i» CL ESQ5"" Se “Hy Dyer 7” Canoe TCE hee NeaGn oe LY: Fig. 6.19. Chromosomes of Drosophila. A, equatorial plate from an odgonium; chromosome pair IV is seen in the center of the group. 8, chromosome pair IV from a salivary gland cell; members of each pair of chromosomes are closely associated, greatly increased in size, and exhibit conspicuous banding. (From C. B. Bridges, 1935, Journal of Heredity, vol. 26.) hereditary factors, or genes. It is known that many genes are found in a single chromosome and that they are arranged in a row along the length of the gene string. In the cells of the salivary gland of Drosophila the chromo- somes are very large because of several reduplications of the genonemata with- out nuclear division (Fig. 6.19). The bands on these chromosomes are characteristic and constant in position on the members of a pair which are closely associated side by side. When certain genes are found to be missing in breeding experiments, a cytological examination of these banded chromo- somes reveals that certain bands are absent (Fig. 6.20); that is, the genes of a linkage group can be considered to occupy definite positions in a specific linear order on a pair of homologous chromosomes. Genes are distributed equally by the mechanism of mitosis to all the germ cells derived from a zygote. ‘The mechanism of disjunction and independent assortment of the pairs of homologous chromosomes, during maturation of the germ cells, and the possibilities of random combination of the germ cells furnish a cellular basis by means of which Mendelian ratios can be explained. The Method of Experimental Embryology Experimental breeding is usually carried on under the conditions most favorable for the organisms concerned. ‘The results obtained, therefore, are comparable with what might be expected to occur in uncontrolled breeding 194 HEREDITY AND VARIATION in the natural habitat. ‘This normal course of inheritance can be observed and interpreted, as has been explained. From it we learn the laws of the transmission of the hereditary units; we learn the mechanism of heredity. How does a particular complement of genes, or genome, influence the course of development of a zygote in such a way as to condition the appearance of the phenotype? What is the relation of gene to character? In this, as in many fields of study, a knowledge of what happens under abnormal or un- usual conditions may clarify our understanding of the normal situation. In the examples cited to illustrate Mendelian principles, a certain combination of genes always gives a certain type of individual. However, if the genetic combination remains the same but the environment is changed, the in- dividuals may be different (p. 167). For example, the red primrose has red flowers if kept at a temperature ranging from 15° to 20°C. A plant with the same genes, reared at a temperature of 30° to 35°C. with other environ- mental conditions unchanged, produces white flowers. If a plant with white flowers is brought into a room at 15° to 20°C., the flowers that develop later will be red. ‘The effect of the genes for color of flower is limited by the temperature of the environment in which the flowers develop. ‘That the gene is not altered is shown by the ability of the plant to cease producing white flowers and give rise to red ones when kept at a different temperature (cf. Fig. 6.21). The same type of effect has been demonstrated in Drosophila. A certain race of the fruit fly is distinguished from the normal by the fact that there are very few black bands on the abdomen. When this race is Fig. 6.20. Chromosomal aberrations seen in the paired salivary-gland chromosomes of Drosophila. A, part of the pair of X-chromosomes, one of which has lost an internal section by deletion (cf. Fig. 6.23); the bent portion of the normal partner indicates the length of the deleted section. 8, the end of a pair of X-chromosomes, one of which has lost a terminal section. (A, from T. S. Painter, 1934, Journal of Heredity, vol. 25; B, from M. Demerec and Margaret E. Hoover, 1936, Journal of Heredity, vol. 27.) 195 GENERAL ZOOLOGY Fig. 6.21. ‘The appearance of Himalayan rabbits under different temperature conditions. A, a rabbit raised at a temperature above 30°C. 8B, a rabbit raised at a temperature of about 25°C. C, a rabbit which has had its left flank artificially cooled at a temperature below 25°C. (After R. Danneel, from R. P. Wagner and H. K. Mitchell, Genetics and Metabolism, copyright 1955 by John Wiley and Sons, Inc., reprinted by \ permission. ) reared on a rich supply of moist food, the abdominal bands are almost completely absent in all individuals. ‘The same stock raised on scant, dry food exhibits normal banding of the abdomen. If a culture is started with abundant moist food which is not replenished but allowed to become dry, the individuals that develop first will show abnormal banding, and _ those that develop later will appear normal. ‘These flies are genetically the same; the difference in their appearance is conditioned by the environment in which they develop. In other words, the appearance of characters in an adult individual is dependent on the presence of certain genes in the zygote acting in a particular environment during development. The age of an individual sometimes affects the development of characters. Certain characters may not appear until the individual reaches a particular age. In other individuals a character may appear in early stages of develop- ment and be lacking in the adult. Age is, in this sense, a phase of the environment. ‘The relationship between genes and environment is shown further by the fact that in Drosophila red color is confined to the eyes and does not occur on the legs or wings. The influence of genes is likewise dependent on their association with certain other genes; evidence for the interaction of genes is clear-cut but very complicated and will not be given here. At least 25 pairs of allelomorphic genes are concerned with eye color 196 HEREDITY AND VARIATION in Drosophila. Conversely, a single pair of genes may influence more than one character. In Drosophila the genes for rudimentary wings affect characters of the legs and the number of eggs laid. Genes are the functional units that determine inheritance in organisms. At least two allelomorphic genes can be associated with the appearance of each heritable character of an individual. In many instances it is known that three or more allelic states of the gene for a particular character exist. No more than two of these multiple allelomorphs are present in a particular individual. ‘There may be many pairs of allelomorphic genes that interact to produce a given character. A single pair of genes may also influence the appearance of more than one character. Certain environmental factors during development of a character limit the effect of the genes. Microorganisms such as bacteria and molds are now widely used in studies concerning the nature of the gene and the way in which hereditary factors produce their effects; such investigations are sometimes known as physiological genetics. The possibility of exact control and the ease of modification of their environment, together with their rapid rate of reproduction, make these simple plants desirable material for genetic studies. ‘This serves to point up the fact that no fundamental differences in the mechanisms of heredity and variation have been found to exist between animals and plants, unicellular or multicellular. ‘The tentative hypothesis concerning the nature of genes proposes that they are molecules of desoxyribonucleic acid (DNA), possibly complexed with distinctive protein moieties. It should be understood that the number of structurally unique DNA molecules is, theoretically, almost un- limited, as is true of protein molecules. ‘These distinctive chemical units, the genes, are conceived to be held together in linear order by physico- chemical forces. ‘The effects of the genes arranged in these gene strings, or genonemata, are believed to be conditioned not only by their own unique chemical structure but, also, by that of contiguous hereditary units, as well as the reactant substrate outside the chromosomes. Long chain reactions occurring between the constituents of the cytoplasm in any particular region, triggered by an enzyme produced by one particular gene and modified by enzymes formed by other genes, are thought to result in the differentiation of the hereditary characteristics of organisms. You may want to refresh your memory concerning enzyme-directed chain reactions in cells by referring again to Figure 2.9, p. 36. Human Inheritance Man’s characteristics, determined by genes at more than 5000 loci, are inherited from generation to generation, as are those of other living organ- isms. ‘The course of heredity is well understood for many characteristics, and much information has been accumulated. For the inheritance of human eye color there is a pair of genes, the dominant member of which must be 197 GENERAL ZOOLOGY Normal x Color-blind Zygotes of P, woman man generation Gametes of P, Zygotes of F, generation Daughters heterozygous Sons normal for color vision but normal for color vision Zygotes of F; Gametes of F, Zygotes of Fy Normal Normal but Normal Color-blind homozygous heterozygous sons sons daughters daughters Zygotes of F, Gametes of Fo Zygotes of Fs Normal but Color-blind Normal Color-blind heterozygous daughters sons sons daughters Fig. 6.22. Inheritance of color blindness in man. The gene for a defective retina is located in the X-chromosome, which explains the crisscross transmission from a color-blind man to half the sons of his daughters. Color-blind daughters may occur if a woman heterozygous for color vision marries a color-blind man, as shown in the third cross. The symbol for a female is 9; 4 is the symbol for a male. An X-chromosome carrying a gene for normal color vision is indicated by 0; @ indicates an X-chromosome carrying a gene for color blindness; y is the Y-chromosome of the male. 198 HEREDITY AND VARIATION present if any pigment is to be deposited in the eye. Individuals who carry two recessive genes in their cells are albinos; their eyes appear pink because the blood vessels of the iris are not screened by pigment. Where pigment is deposited, another pair of genes conditions its distribution. Homozygous or heterozygous dominant individuals have a purple-black pigment behind the iris and brown pigment in front of the iris and appear brown-eyed. Homo- zygous recessive individuals have pigment only behind the iris and appear blue-eyed. The albino genes affect pigmentation of the skin and hair, as well as of the eye. Curly hair is dominant to straight hair. In color blind- ness, which is the result of an inherited defect of the retina, and in hemo- philia, a defect of the blood that prevents its clotting, the genes are sex-linked (Fig. 6.22). The production of the A, B, M, N, and Rh antigens found in human red blood cells is determined by heredity (pp. 65 and 164). The Rh (D) gene is dominant, and individuals homozygous or heterozygous for it contain the Rh antigen. In the A and B substances we have an ex- ample of multiple allelomorphs. Three allelic genes are known to condition the blood types, but only two of these genes occur in any individual. ‘They are symbolized as A (gives A antigen), a8 (gives B antigen), and a (gives neither antigen). Individuals may, therefore, be genotypically AA or Aa and have blood of type A; a’a® or aa and have blood of type B; Aa® and have blood of type AB; aa and have blood of type O. Among individuals of European stock 45 and 42 per cent, respectively, type as O and A, only 3 per cent as AB. Mental as well as physical characteristics appear to be inherited. The evidence seems to indicate that mental qualities leading to degeneracy, crime, and pauperism, as well as those yielding leadership in all social fields, may be inherited according to Mendelian principles. It is not to be understood that there are special genes determining crime or pauperism but rather that genes giving rise to defective mental equipment predispose to these undesir- able social traits. In the same way the so-called inheritance of diseases is an inheritance of morphological or physiological characteristics that render an individual more susceptible to infections. Instances of prenatal infection (p. 164) are not cases of inheritance as the term has been used in this chapter. ‘he environment in which a particular gene complex develops may limit and obscure its possibilities. ‘Thus, a given combination of genes in a human individual may produce a better adult in a favorable environment than it could in an unfavorable one. On the other hand, a good combination of genes will give rise to a better individual in a particular environment than will a poor set of genes. ‘Training is another element that is very important in the unfolding of human potentialities. An inferior inheritance with su- perior training may result in an individual better fitted for society than one with a good inheritance and no training. But no amount of training can produce anything for which the inherited capacities are not present, nor can the best of environments implant qualities if the potentialities for them are lacking in the germ plasm. In other words, heredity limits very definitely the effects of training and envirenment. 136 GENERAL ZOOLOGY Eugenics is that particular branch of applied genetics which deals with the improvement of the mental and physical characteristics of future generations of the human race. Its problems are: first, extension of our knowledge con- cerning human heredity; and, second, the education of the public in an appreciation of the meaning and application of this knowledge. ‘The first of these problems is very difficult. A knowledge of hereditary principles is best gained from controlled and repeated experiments, which obviously cannot be carried out with human beings. ‘Thus, the social tabu against marriages between near relatives is based on the knowledge that defective offspring result if recessive genes for undesirable traits are brought together. ‘This, of course, can also occur in marriages between unrelated individuals. Inbreed- ing experiments with rats conducted by Helen Dean King for many years produced an unusually vigorous stock of animals. Animals having desirable genes in a homozygous condition will be obtained by inbreeding if the de- sirable genes are present in the beginning; neither defective nor desirable genes are produced by inbreeding. In 1930 H. S. Jennings, in The Biological Basis of Human Nature, pointed out the great handicaps of eugenics in the light of modern knowledge of genetics. The phenomenon of dominance makes heterozygous individuals appear normal, although they may transmit undesirable genes. Prevention of the breeding of the socially unfit and of those afflicted with uncontrollable physical or mental handicaps, desirable as such a measure may seem, will not eliminate the heterozygous carriers. Until recently it was impossible to detect the presence of a recessive gene when it was combined with its dominant allele in a heterozygous individual, except by the breeding test. It has been found by using the technic of paper chromatography that some products of metabolism in the cells of heterozygous individuals (Drosophila and two species of plants) differ from those occurring in homozygous individuals. In paper chromatography, substances in solution are spot-dried on special grades of filter-type paper. Later, components of these substances can be separated from one another by allowing special liquids to move slowly through the paper. The dried substances redissolve and migrate during the movement of the fluid. Different chemical compounds are found to move at different rates and, thus separated, can be identified by various methods on the redried paper. By using this technic, it is possible to distinguish between fruit flies, for example, homozygous for cer- tain genes and those that are heterozygous. ‘That is, the products of metab- olism in the cells of heterozygous individuals differ from those formed in homozygous individuals; these compounds migrate at different rates in the paper. Similar technics have been used successfully to identify children carrying hereditary factors that bring about a fatal blood disease (familial primary systemic amyloidosis) in adults 30 to 40 years of age, long before there is any clinical indication of the disease. If these technics for the detection of carriers can be extended, they will prove to be of revolutionary importance in the formulation of a sound program of eugenics. 200 HEREDITY AND VARIATION Heritable Variations The genetic data so far presented indicate two general reasons why offspring are not exactly like their parents. In the first place, the environment may influence the developing young and produce a fluctuation, or somatic varia- tion. As was pointed out in connection with Johannsen’s studies on beans (p. 174), such variations are not inherited; they do not affect the germ cells. In the second place, the results of hybridization experiments show that new combinations of genes give rise to individuals differing from their parents. Disjunction and independent assortment of the pairs of chromosomes contain- ing the linked genes of the several groups give rise to gametes different in their genic content. The random combination of gametes to form zygotes can produce a great number of phenotypes and more genotypes in cases where dominance occurs. ‘The possibilities of new combinations of linkage groups are limited, however, by chance, and the same variations are produced many times. This sort of thing is sometimes said to be like dealing hands of cards. Many combinations can be dealt, but the cards themselves, which would be comparable to the chromosomes carrying groups of genes, remain unchanged and occur in the same numbers and kinds. New combinations of genes may also arise as a result of changes brought about in any linkage group by the process of crossing over. Crossing over happens when comparable regions of homologous chromosomes become ex- changed (Fig. 6.23C and D) and gives rise to unexpected classes of off- spring. If a male Drosophila with white eyes and a yellow body is mated to a female with red eyes and a gray body, all the /, offspring, both males and females, will have red eyes and gray bodies. ‘The genes for these characters are known to be located on the X-chromosome, so that the ordinary expecta- tion of F, can be easily ascertained by referring to Figure 6.17. If one of these heterozygous females with red eyes and gray bodies is then mated to a male with white eyes and a yellow body, 99 per cent of the offspring are of the expected kinds: equal numbers of males and females with red eyes and gray bodies and with white eyes and yellow bodies (Fig. 6.24). The other 1 per cent is made up of equal numbers of males and females with red eyes and yellow bodies and with white eyes and gray bodies. ‘These individuals arise from zygotes containing chromosomes in which crossing over has occurred. Crossing over has been extensively studied, and the percentage of cross-over types to be expected in given crosses is known; the amount of crossing over differs among different genes. The concept that genes are arranged in a given linear order was deduced from crossing over, and chromo- some maps showing the distances between the loci occupied by certain genes have been compiled from the data collected on crossing over (Fig. 6.16). The changes in combinations of entire linkage groups that occur from generation to generation as a result of disjunction of homologous chromosomes and subsequent combinations of gametes, as well as the changes in gene 201 GENERAL ZOOLOGY a A 1 b B a Iwo pairs of homologous chromosomes : Cc showing positions of allelomorphic genes. d D A B Crossing over: ‘The chromosomes of the pair shown in A may twist about one an- other as in C and break in the plane of the dotted line so that comparable sections are \/ Qo eh @ sl & sacs exchanged as shown in D. and break in the plane of the dotted line so i) ye ee pS that an internal section containing gene c is lost, or deleted, as shown in F. Inversion: One member of the chromo- some pair shown in A may twist on itself as in G and break in the plane of the dotted line so that the section containing genes B and C'is inverted as shown in H. (sl, Gee ts) i) [Sy ey tsp fe) Se) ers Duplication and deficiency: If one mem- ber of the chromosome pair shown in A comes to lie across the other as shown in | and a break occurs in the plane of the dotted line, the chromosome on the left in J will have a duplication and contain both gene d and gene D, and the chromosome on the right will have a deficiency of the section ee) ° AY os SOQ > o (St (eas) ————_2—__e—_3s— —_e@—2—__e—__o— —e®—_o—_e—_e- —¢—_@—_6—_2— > va containing gene D. Translocation: One member of the chro- mosome pair shown in A may come to lie Q > Q across one member of the chromosome pair o by o~ Q vs > shown in B, as seen in K. If a break occurs Deletion: One member of the chromosome pair shown in A may twist on itself as in E | | . . f in the plane of the dotted line, sections ! of non-homologous chromosomes are ex- K L changed, or translocated, as shown in L. Fig. 6.23. Crossing over and various chromosomal aberrations; diagrammatic. 202 HEREDITY AND VARIATION Non-cross-over progeny 99% Cross-over progeny 1% Fig. 6.24. ‘The effect of crossing over in a case of sex-linked inheritance. A heterozygous female Drosophila with red eyes and gray body is mated with a male which has white eyes and a yellow bedy like her father. Ninety-nine per cent of the offspring (shown at A’) are of the expected or non-cross-over types that result when the X-chromosomes at A pass into ova and combine with sperm. One per cent of the offspring (shown at B’) have unexpected combinations of eye and body colors; these develop from zygotes that have received chromosomes in which crossing over has occurred (shown at 8B). Chromosomes from the female are represented as empty, and those from the male are dotted. Y and y are the genes for gray and yellow body color; W and w for red and white eye color (cf. Fig. 6.23 Cand D). 203 GENERAL ZOOLOGY Fig. 6.25. Drosophila mutants, arising by gene mutations in chromosome II. The mu- tations are named as follows: A, balloon wing; B, vestigial wing; C, jaunty wing; D, arc wing; /, apterous, or wingless; and F, tele- scope abdomen. (From C. B. Bridges and T. H. Morgan, 1919, Carnegie Institution Pub. 278.) associations within linkage groups that arise from crossing over, are merely shifting of genes without change of their quality or quantity; that is, the genes are present in the expected numbers and unaltered in kind. When the complete results of breeding are examined, however, we find that there are changes in single genes and in groups of genes which produce noticeable changes in the characteristics of an individual and which are _ heritable. Heritable variations are the only source of new kinds of organisms, the material of organic evolution (pp. 636 and 648). Such variations fall into two classes: those arising by an alteration, or mutation, of a particular 204 HEREDITY AND VARIATION gene, and those resulting from what are known as chromosomal aberrations which give rise to changes in the numbers of genes or in their relationships to one another in the linkage groups. Gene mutations are ordinarily thought of as changes in the quality of a given part of the gene string. Hundreds of such mutations have been dis- covered in the many thousands of Drosophila that have been examined, although the number of times that any particular mutation has occurred is very small (Fig. 6.25). One gene has been recorded as mutating 4 times in the formation of 500 gametes, another 2 times in 1800 gametes. When all the genes in chromosome II of Drosophila were considered, it was found that only 30 mutations occurred in 5000 chances. Not all genes mutate with the same frequency, and very few, apparently, change often under normal con- ditions. Mutations produce changes in the structure of organisms and in the way they function. Many mutations that tend to alter function result in the death of the organism; they are called lethal mutations. Chromosomal aberrations involve parts of chromosomes, whole chromosomes, and even entire haploid sets of chromosomes. ‘The fact that losses or addi- tions of genes in the cell produce observable changes in the individual reminds us that genes ordinarily interact with one another in what must be thought of as a balanced condition. If this balance is shifted by adding or subtracting groups of genes, the effect may be to kill the individual; with other aberrations the individual may live but be unable to produce functional germ cells. Sometimes such aberrations can be handed on from one generation to the next. ‘The various types of alterations in linkage groups can be described briefly. Alteration in a linkage group may be brought about by loss of a certain region of a chromosome. ‘This is known as deletion, and an individual in which a deletion occurs will contain only one of each of the genes located in that region of the chromosome (Figs. 6.20 and 6.23 and F). Sometimes breeding results indicate that inversion has taken place; that is, a portion of the gene string has been reversed in postion (Fig. 6.23G and H). It is of considerable interest that alteration of the linear order may produce a herit- able effect different from that resulting from the same genes in their typical sequence. Occasionally, a part of one chromosome becomes attached to the other member of a pair in such a way that one chromosome has a given region duplicated, whereas its homologue is deficient for the same region and the genes it carries (Fig. 6.23/ and J). This is, in effect, an unequal crossing over. Another type of shift in linkage relations occurs when pieces of non- homologous chromosomes become interchanged (Fig. 6.23A and L). This is known as translocation. What cytological evidence there is concerning the chromosomal behavior responsible for these altered linkage relationships indicates that, when the chromosomes are in the form of long threads during the early growth period, they may come into contact with one another, stick together, and sometimes be broken when the contractions of the threads occur. 205 GENERAL ZOOLOGY It has been established by careful studies on the fruit fly, maize, and cotton that linear clusters of hereditary units known as pseudo-alleles occur. ‘These have distinct but related functions. They are assumed to have arisen by duplication and reduplication of an original limited region of the chromo- some, an assumption which rests on both cytological and genetical evidence. The repeated units may undergo mutation in different ways, thus making possible evolution of the functional units of heredity. An entirely different type of abnormal chromosomal behavior sometimes occurs during the meiotic divisions. Homologous chromosomes may fail to separate from one another and, therefore, will pass together into one cell. This is known as non-disjunction and can involve any pair of chromosomes. One resulting cell will lack a whole chromosome of the typical haploid set, whereas the other will have a haploid set plus one chromosome. Fertilization will result in some individuals which have only one chromosome of a particu- lar pair and in others which have three such chromosomes. ‘These individuals will lack one complete gene string or have an extra one. In Drosophila non- disjunction of X-chromosomes and of the smallest chromosomes (IV) is known to occur (Fig. 6.26). Non-disjunction of the X-chromosome gives rise to females and males with unexpected characteristics, since some females get both X-chromosomes from their mother and some males get an X-chromo- some from their father. Non-disjunction of chromosome pair IV_ produces haplo-IV and triplo-IV individuals differing from one another and from the normal diplo-IV fly in appearance. Entire sets of chromosomes may fail to disjoin, so that a gamete will contain the diploid rather than the usual haploid number of chromosomes. If fertilization adds a haploid set to such a diploid gamete, the zygote has three chromosomes of each kind and gives rise to a triploid individual. In Drosophila such flies are conspicuously different from the diploid or normal type. Individuals that have four or more chromosomes of each set are also known. They may arise as a result of an incomplete mitosis at the time of the first cleavage of the zygote. ‘The chromosomes undergo reduplication but do not separate, so that the number is doubled and a tetraploid individual develops. Such organisms usually are conspicuously larger than their diploid relatives. Chromosome doubling sometimes occurs when gametes from two different species of plants have united. Subsequent synapsis between pairs of homol- ogous chromosomes is thus made possible, and functional gametes may be formed. ‘This usually does not occur in hybrids between different species, such as the mule. Several entirely new species of plants are known to have been established by chromosome doubling in a hybrid, but so far no com- parable cases have been discovered among animals. Since gene mutations and chromosomal aberrations offer very interesting material for the study of the mechanism of heredity, many investigators have attempted to increase the rate at which such alterations occur. Drosophila was subjected to high and low temperatures, various nutritional modifications, and treatment with a great many chemicals, but with no appreciable change 206 HEREDITY AND VARIATION in the frequency of the appearance of mutations. It remained for H. J. Muller to show in 1925-1926 that X rays greatly increased the number of chromosomal aberrations, as well as gene mutations. Other kinds of radia- tion as, for example, radium emanations and ultraviolet light have been shown to increase the rate of mutation. Some chemical compounds, such as mustard gas, have been proved mutagenic. The manner of action of ionizing radiations, such as X rays, has been a baffling problem. Evidence increasingly supports the concept of an indirect effect rather than that of a direct hit by a particle on a gene target. Radia- tion of a living cell produces its primary effect on the solvent water, dis- rupting its molecules into H atoms and OH radicals; the H atoms may combine with O, to form HO, radicals. ‘These free radicals, OH and HO,g, are very active chemically and involve the compounds of the cell in unusual reactions. Something which is produced as a consequence of these atypical reactions acts as a chemical mutagen to modify the genetic material in some way. ‘The particular composition of the cell contents influences the response to radiations. ‘Thus radiations appear to produce their effects by indirectly distorting the normal metabolic sequences of the cell. In germ cells the genonemata become modified in such a way as to affect the heredity of future generations. It has been determined that the effects of ionizing radiations in producing mutations are directly proportional to the dose. ‘This is true whether a given i II Ill Oocytes Disjunctional division wh Polar (&) Polar bran) Polar Osan body Qn body Goan body Normal female Superfemale Mal : ale Weeae Geena (X from father) ( “Y8°'"S , Normal male Female Dies (both X’s from mother) Fig. 6.26. Diagram showing, in column I, normal disjunction of the X-chromosomes in odgenesis and the subsequent possibilities of fertilization with typical sperm; in column II, non-disjunction of X-chromosomes, both remaining in the ovum, and subsequent fertilization; and in column III, non-disjunction of X-chromosomes, both passing to the polar body, and subsequent fertilization. A is used as a symbol for a complete haploid set of autosomes. 207 GENERAL ZOOLOGY dose is received in a short time or distributed over a long period, whether continuous or intermittent. ‘That is, the biological effects of ionizing radia- tions are cumulative. ‘These facts, known for a long time, have assumed great importance for man in recent years. The use of X rays in diagnosis and treat- ment by the medical and dental professions has increased. Now, with experimental detonation of thermonuclear weapons there is unpredictable increase in radioactive fallout in the atmosphere. Radioactive waste from atomic power plants the world over may become an increasing source of con- tamination of water and air. Grave concern is felt, nationally and inter- nationally, over the adverse genetic effects on man, as well as on _ his domesticated plants and animals. Of the mutations that have been induced in experimental animals and plants, the great majority have been detrimental, many producing death. It is evident that serious attention must be given to the necessity of keeping the total amount of radiation received by every in- dividual below the critical level for genetic damage. The future of the human race is at stake. Sex Determination Animals are typically dioecious; that is, there are two sexes, which differ essentially in that the males produce microgametes, or spermatozoa, whereas the females produce macrogametes, or ova. In association with this primary distinction between males and females, we have seen that differences exist be- tween the reproductive systems (p. 128). In many animals what are known as secondary sex characters are very conspicuous distinguishing features of the sexes. For example, the gay plumage of many male birds, the vocal differences between the sexes in many vertebrates from frog to man, and in some mammals the greater growth of hair in the males, as in the lion and man, differentiate the sexes. It is true that in numerous species of vertebrates, especially among lower animals, no such secondary sex characters can be ob- served, although there may be a size difference between the sexes. Not all animals are sexually distinct; some are monoecious, or hermaphroditic, and every individual produces both microgametes and macrogametes. Some- times hermaphroditic animals produce first sperm and later eggs, or vice versa, but frequently eggs and sperm are matured at the same time. Almost all plants are monoecious. We see that sexual differentiation between in- dividuals is not by any means a universal attribute of living things. ‘The production of differentiated gametes correlated with the capacity of reproduc- tion is not dependent on the sexual differentiation of individuals. The conspicuous differences between the sexes in the higher animals have long excited the curiosity of biologists and led to attempted explanations and hopes for control. Early hypotheses were formulated in terms of the nutritive conditions under which the young developed, in spite of the obvious fact that a litter of pups or kittens, for example, contains both males and females 208 HEREDITY AND VARIATION whose development has occurred under identical conditions. In man two kinds of twins are known. Fraternal twins may be of the same or opposite sex and bear no more resemblance to one another than other brothers and sisters, but identical twins are always of the same sex. A comparable situa- tion is known in the nine-banded armadillo, which gives birth to four young, all of the same sex, and in certain insects, which produce by the method of polyembryony large numbers of young, all of the same sex. ‘These situations were clarified by study which revealed that identical twins in man and other mammals and the quadruplets of the armadillo, as well as the polyembryos of insects developed from single zygotes, had identical hereditary constitutions. Furthermore, the discovery and study of the so-called sex chromosomes led to the interpretation proposed by E. B. Wilson in 1905 that sex was determined at the time of fertilization by the chance combinations of the gametes formed. In the insect, Protenor, there are 14 chromosomes in the female but tonly 13m «the male (see Fig. 222A; By and’ C, p. 42); andiy pairs of chromosomes in the female, but only 6 pairs and an extra or odd chromosome in the male. ‘The extra chromosome of the male can be clearly seen to be similar to one of the pairs of the female, the largest pair in this instance. ‘This unpaired chromosome of the male and the comparable pair of the female are called X-chromosomes or sex chromosomes. All the other chromosomes, which occur in pairs in both males and females, are known as autosomes. If a haploid set of autosomes is designated as A, then a formula for the chromosome number of any somatic cell or primordial germ cell of a female Protenor would be 2A + 2X, where X stands for a sex chromosome, and the formula for every mature ovum would be A + X. The chromosome con- tent of somatic cells and undifferentiated germ cells of a male Protenor would be 2A + XY. One-half of the spermatozoa could be represented by A + X and the other half by A + O. Wilson pointed out that, when an ovum (A +_YX) was fertilized by one kind of spermatozoon (A + XY), the zygote would have the number of chromosomes characteristic of the female (2A + 2X). If the ovum (A + X) was fertilized by the other kind of spermatozoon (A + Q), then the zygote would have the number of chromosomes characteristic of the male (2A + X). Since by the process of mitosis during development each cell receives the same number of chromosomes that the zygote has, it can be seen how the concept arose that sex was determined by the number of X-chromo- somes present in the zygote. It was soon discovered that the male did not always differ from the female in number of chromosomes and that, although the male had only one X- chromosome, it sometimes had a Y-chromosome that segregated from the X- chromosome at the disjunctional division (see Fig. 5.7, p. 135). The Y-chromosome, like the X, is known as a sex chromosome. Drosophila males have such a Y-chromosome (see Fig. 2.12, p. 42) and produce two classes of spermatozoa, which can be represented as A + X and A + Y. Union with A + X ova yields female-producing zygotes (2A + 2X) and male-producing zygotes (2A + XY). In Protenor, which is an example of forms with YO / 209 GENERAL ZOOLOGY males, and Drosophila, which is an example of forms with XY males, the males are the digametic sex, or the sex that gives rise to two kinds of gametes with respect to the X-chromosome. The diagrammatic simplicity of such a method of sex determination became somewhat confused when the observation was made that in some species of moths and in birds the female was the digametic sex; that is, a female moth produces two kinds of ova, one with an X-chromosome and one without, whereas all the spermatozoa are alike in carrying an X-chromosome. Many facts indicate an undeniably close correlation between number of X-chromo- somes and sex, however. One of the most interesting is the occurrence of bilateral gynanders, which are male on one side and female on the other. It is clearly established that gynanders in Drosophila, for example, arise from female-producing zygotes (2A + 2X). At the time of the first division one of the X-chromosomes, either the paternal or the maternal, is lost on the mitotic spindle, so that one nucleus has 2A + 2X chromosomes and the other has 2A + X chromosomes. ‘The former gives rise to the female half, the latter to the male half of the gynander. If the X-chromosome is lost in some later division, only a limited region of the female will exhibit male characteristics. In spite of such confirmation of the sex-chromosome theory of sex determi- nation, it should be obvious that the study of inheritance has shown char- acters to depend on combinations of the genes located in the chromosomes, not on the chromosomes as such. Yet with sex, the presence of two particular chromosomes was assumed to condition the differentiation of one sex, and one of these chromosomes was assumed to condition the differentiation of the other sex. Either one of the X-chromosomes of a female could enter a male- producing zygote, and the X-chromosome of a male could pass into a female- producing zygote. No gene for maleness or femaleness has been located on the chromosome maps of Drosophila or with certainty been identified in any species, although reported by some investigators. The discovery of inter- sexes among the offspring of triploid females by C. B. Bridges in 1921 led to his formulation of the Theory of Genic Balance to explain the determination of sex. When disjunction occurs in the germ cells of triploid (34 + 3.) females, some eggs with 2A + X chromosomes and some with A + 2X are formed, among others. If a 2A + X ovum is fertilized by an A + X sperm, a 3A + 2X zygote results and develops into an intersex, or an individual that is male in some parts and female in others. ‘These male and female parts are not clearly segregated into halves or quarters as in gynanders but are completely blended, and intersexes range from almost total females to almost total males. Bridges’ theory is that sex is the result of the inter- action of many different genes, some of which are to be thought of as female determiners and others as male determiners. Both types of genes are located on all the chromosomes. However, there are more female than male deter- miners in the X-chromosomes, whereas the reverse situation is true in the autosomes. ‘The distribution and effect of these genes are such that, when a zygote has a 2A + 2X constitution, the female determiners on the X- 210 HEREDITY AND VARIATION chromosomes overbalance the male determiners on the autosomes. When the zygote has a 2A + XO or 2A + XY constitution, the male determiners on the autosomes overbalance the female determiners on a single X-chromosome; the Y-chromosome appears to carry no genes related to sex. In the 3A + 2X individuals neither set of genes overbalances the other, but both are some- what effective in molding the appearance of the intersex. This theory is strengthened by the occurrence of what are known as superfemales and super- males in which the chromosomal make-up is 2A + 3X [ovum (A + 2X) + spermatozoon (A + X)] and 3A + XY [ovum (2A + X) + spermatozoon (A + YJ], respectively. Although triploid (3A + 3X) and tetraploid (4A + 4X) individuals are females, as would be expected on the basis of an explanation in terms of genic balance, haploid individuals (A + X) are males in bees and other forms that normally produce males parthenogenetically. Haploid drosophilas have not been found, so that the theory remains untested in a crucial case. More facts are necessary before the final word can be said on the mechanism of sex determination at fertilization. We have seen an apparently satisfactory theory in terms of 2X versus YO or XV zygotes fail to explain accumulated observed facts and hence undergo modification. Such is the method of science—observation, explanation, further observation or experimentation, and modification of explanation when necessary—a_ con- tinued seeking for the whole truth. As in all development, the environment influences the differentiation of so-called sex characters. A zygote in which the genic balance is female- determining may develop into a male in an atypical environment. In 1915 Emil Witschi found that if frogs were forced to develop at a high tempera- ture, they were all males because the female-producing genotype had been overridden by environmental conditions. Sometimes reversal of sex occurs. A hen, for example, may change into a cock and produce spermatozoa. AI- though the mechanism of the overriding of the inherited constitution, the genotype, is not clear, this is only a special case of the effect of the en- vironment, as noted previously (pp. 167 and 195). The problem of sex determination is not really a special one, although it has long been so treated. It is well known by geneticists that single pairs of genes do not by themselves produce an effect in the organism. ‘They al- ways depend on the presence of other genes, on a given pattern of distribution of the cytoplasm of the zygote during cleavage and the cell movements during development, on interactions between differentiating cells, and on the chemi- cal and physical conditions of the external environment. In the ordinary course of events the genes are the part of this complex most often varied; that is, new combinations of genes occur with each fertilization, but development proceeds under practically uniform conditions in the great majority of cases. Genes initiate chains of reactions which may be environmentally modified during cell differentiation. When the environment is altered, its importance in the complex of factors determining what the individual will be, not only with respect to its sex but also its other characteristics, is appreciated. 211 GENERAL ZOOLOGY Summary The genes located in the chromosomes of the nucleus have been found to be responsible for all types of similarities between individuals related by descent, from gross morphological characteristics to single enzyme-directed metabolic reactions. In fact, insofar as we know, everything that a living cell does can be referred in the final analysis to genic initiation and direction. If these amazing self-perpetuating units were immutable, it may be supposed that all living organisms would be identical within narrow limits, reflecting environmental influence on genic expression. A great common pool of genes is obviously present in living organisms as evidenced by the many similarities that exist in the reactions of all cells. But innumerable changes have occurred through the ages that living things have existed on the earth. You will be- come acquainted with the great diversity of animal life as you continue your study of zoology. ‘The capacity of genes to become changed and to per- petuate that change has provided the material from which the environment has selected during the process of organic evolution. Not only is the past and present status of living organisms conditioned by the genes they carry, but their future potentialities are latent in the possibilities for change in the genome. Is it any wonder that many scientists work with all the resources at their command to determine the nature of the gene? 212 CHAPTER } CHAPTER y THE CLASSIFICATION The preceding chapters have presented an introduction to zoology through the study of structure and function in vertebrate animals. This introduction has illustrated the organization of vertebrates and their capacities of metab- olism, responsiveness, and reproduction, as well as the nature of development and heredity. From the standpoint of function such an introduction would suffice for animals in general; the characteristics and activities of the cells are essentially alike in the bodies of all animals. It may be said that all living things are faced with common problems of survival, and the functional solutions of these problems are similar in many diverse types of animals. From the standpoint of structure, however, study of the vertebrates alone is inadequate. There are many kinds of animals very different from vertebrates in their organization, although their general functions may be very similar. In the chapters to follow some of these different types of animals will be ex- amined, particularly as they illustrate principles of morphology, ecology, and evolution. As an introduction to this survey, we may consider briefly the diversity of animal life and some of the evidence indicating that all the diverse types will fit into an orderly system, based on structural similarities and be- lieved to indicate evolutionary relationships. Let us begin by examining the principles, methods, and implications of the classification of animals. 214 Fig. 7.1. The orders of the class Amphibia. A, order Caudata: the mud _ puppy, Necturus maculosus. B, order Apoda: a caecilian, or limbless amphibian, Siphonops annulatus. C, order Salientia: the pickerel frog, Rana palustris. (A and C, od photographs courtesy New York B . Zoological Society; B, redrawn, after Claus-Grobben, from W. Stempell, 1926, Zoologie im SSS SESE EH DSL BEI ST ET BE IIT Grundriss. ) Classification Historical. Confronted with any large array of diverse facts or forms, man finds it necessary to catalog or classify them before they are under- standable in relation to each other. This need to classify, to group like with like, has been felt since earliest times in studies of the vast numbers of living things inhabiting the earth. The first serious attempt to classify animals logically and scientifically, on the basis of similarities and differences in funda- mental characteristics, was made by Aristotle (384-322 B.c.). During later centuries other systems of classification were erected, and comparisons were drawn between the structures of animals and those of plants. These early systems were not widely accepted, and the classification that developed into the modern scheme was not formulated until the eighteenth century. Almost all the early systems suffered from the fact that relatively little was known of the fundamental characteristics of living things; these systems were based largely on superficial or artificial criteria. In general, the classification of plants progressed more rapidly than that of animals. John Ray (1628-1705) attempted to classify both animals and plants in a single system, emphasizing structure as the basis of comparison. 215 GENERAL ZOOLOGY SUBKINGDOM PROTOZOA PHYLUM PROTOZOA Subphylum Plasmodroma Class Sarcodina Class Flagellata Class Sporozoa SUBKINGDOM METAZOA PHYLUM ONYCHOPHORA PHYLUM MESOZOA PHYLUM PORIFERA PHYLUM COELENTERATA Class Hydrozoa Class Seyphomedusae Class Anthozoa PHYLUM CTENOPHORA PHYLUM PLATYHELMINTHES Class Turbellaria Class Trematoda Class Cestoda PHYLUM NEMERTINEA PHYLUM ASCHELMINTHES Class Rotifera Class Nematoda Class Gastrotricha (Other minor classes) PHYLUM ACANTHOCEPHALA PHYLUM ENTOPROCTA PHYLUM ECTOPROCTA PHYLUM PHORONIDEA PHYLUM SIPUNCULOIDEA PHYLUM ECHIUROIDEA PHYLUM MOLLUSCA Class Amphineura Class Pelecypoda Class Gastropoda Class Scaphopoda Class Cephalopoda PHYLUM ANNELIDA Class Archiannelida Class Polychaeta Class Oligochaeta Class Hirudinea Subphylum Ciliophora Class Ciliata Class Suctoria PHYLUM ARTHROPODA Subphylum Trilobitomorpha (Ext.) Subphylum Chelicerata Class Xiphosurida Class Eurypterida (Ext.) Class Pycnogonida Class Arachnida Subphylum Mandibulata Class Crustacea Class Diplopoda Class Chilopoda Class Insecta (Other minor classes) PHYLUM CHAETOGNATHA PHYLUM BRACHIOPODA PHYLUM ECHINODERMATA Subphylum Pelmatozoa Class Cystoidea (Ext.) Class Blastoidea (Ext.) Class Crinoidea Subphylum Eleutherozoa Class Asteroidea Class Ophiuroidea Class Echinoidea Class Holothuroidea PHYLUM HEMICHORDATA PHYLUM CHORDATA Division Acraniata Subphylum Urochordata Subphylum Cephalochordata Division Craniata Subphylum Vertebrata Superclass Pisces Class Agnatha Class Placodermi (Ext.) Class Chondrichthyes Class Osteichthyes Superclass Tetrapoda Class Amphibia Class Reptilia Class Aves Class Mammalia Fig. 7.2. The phyla of the Animal Kingdom and their principal subdivisions. Ray adopted the concept of the species, limiting the term to forms producing offspring like themselves when bred together; this definition has much in com- mon with the modern understanding of this important concept. However, it remained for Linnaeus (1707-1778), justly called the Father of Classification, 216 THE CLASSIFICATION OF ANIMALS to establish the basic structure of the present-day system. Linnaeus was by training more of a botanist than a zoologist, although he zealously studied animals as well as plants and developed a classification that included most of the animals known to him. His work with animals was less successful than his botanical work; nevertheless, the fact that he was able to formulate a scheme which became widely accepted, and which served as the basis for the modern system, justifies his enduring fame in the field of biological science called taxonomy (Greek, “law of arrangement’). Linnaeus’ most significant con- tributions to classification were (1) his use of structure as the basis of com- parison between forms, and (2) his establishment of binomial nomenclature, by which each organism is given a generic and a specific name. The Modern System. It is a familiar fact that animals fall into restricted groups called species (singular species, not specie), which may be approxi- mately defined as groups whose members successfully interbreed and resemble each other more than they resemble the members of other similar groups. The individuals, which are the ultimate material of classification, can thus be grouped into species. Different species with much in common form genera, similar genera form families, similar families form orders, and so on. We may begin with the individuals and species, as did early investigators; or, as the system is now established, we may begin with a larger group and follow its subdivisions until the species and individuals are reached. For example, the existence of a large group of animals known as Vertebrata was recognized after it had been discovered that a great array of animals possessed backbones com- posed of vertebrae. Later, it was found that several types of animals with- out vertebrae, and with relatively simple organization, possessed certain features in common with the vertebrates, particularly a notochord such as occurs in the early embryos of all vertebrates. The Vertebrata were then placed, along with these simpler groups, within a single phylum, one of the principal subdivisions of the Animal Kingdom. Despite the fact that this phylum, the Chordata, includes a very wide range of animal types, all chordates have certain basic features in common. ‘These common, distin- guishing characteristics include the presence, in the adult or at some stage of development, of (1) a primitive axial skeleton, the notochord, (2) gill slits, or traces of these structures, and (3) a dorsal, tubular central nervous system. The principal subdivisions of the phylum Chordata are listed in Figure 7.2. One of these subdivisions is the subphylum Vertebrata, including, among others, the class Amphibia. Within this class, the existing forms may be classified as follows: Class Amphibia (Fig. 7.1) Order Caudata, salamanders, newts, etc. Order Apoda, limbless, worm-like amphibians Order Salientia, frogs and toads. After the orders, proceeding to smaller groups, come families, then genera (singular genus), and finally species. ‘Thus, the order Salientia contains, along with some half dozen other families, the family Ranidae, which in turn includes 217 GENERAL ZOOLOGY the genus Rana and other genera. The genus Rana consists of several closely related species. The relationships of these categories may be shown as follows: Order Salientia. Family Ranidae. Genus Rana. Species Rana pipiens Schreber, the leopard frog. It will be noted that, in referring to the species, it is proper to use both generic and specific names; that is, the name of the animal commonly called the leopard frog is Rana pifiens or R. pipiens, not piprens alone. ‘The generic name is always capitalized, the specific name always written with a small letter. In the complete form, the name of the species is followed by the name of the individual who originally described it, and the date of the description is given in parentheses. Several of the species making up the genus Rana are R. catesbeiana, the bullfrog; R. clamitans, the green frog; R. sylvatica, the wood frog; R. palustris, the pickerel frog; and R. pifiens, the leopard frog. A species containing groups of animals with relatively slight but constant differences is often divided into subspecies or varieties. “The amount of difference between the individuals composing any one species can be appre- ciated only by examining a large number of specimens and making compari- sons. In general, the differences that separate subspecies or varieties within species are less than those separating species. Whether a group shall be called a variety of an existing species, described as a new species, or made the basis of a new genus depends on the judgment of the individual making the classification. The system of cataloging animals just described illustrates the principles of classification followed at the present time. Beginning with a group of ani- mals, such as a species of frog or grasshopper, we can follow its classification into larger and larger groupings until the phylum is reached, and finally the Animal Kingdom, which is coordinate with the other great group of living things, the Plant Kingdom. Conversely, if we began with the Animal King- dom, we might follow its branches into every subdivision until all the species were reached, and pass in review the diversified known forms of animal life. The modern taxonomic system is the result of years of study, in which hundreds of thousands of species have been described and arranged in accordance with their similarities to other forms. Such a system is necessary as a means of cataloging the multitudinous organisms constituting the Animal Kingdom. But classification, as it is now undertaken by biologists, is more than a cataloging system. The basis of classification is structure, and it can be confidently assumed that structural similarities are indicative of ancestral relationships. Since the acceptance of the Theory of Organic Evolution, which followed the publication of Charles Darwin’s Ongin of Species in 1859, classification has become a never-ending effort to express evolutionary rela- tionships between animals, as well as to furnish a catalog of types. For this 218 THE CLASSIFICATION OF ANIMALS purpose the functions of the parts of an organism are not important, for even in animals of the same phylum comparable parts often have entirely different functions. ‘The basic structure of the parts is significant because this struc- ture is found to be the same despite modifications correlated with different uses in different animals. ‘The fore limbs of various vertebrate animals furnish examples of modifications of comparable parts in adaptation to diverse functions. ‘The wings of birds and of bats, the fore limbs of horses, dogs, mice, frogs, monkeys, and so on, have all been modified in various directions in adaptation to various uses; but the underlying structural basis of the fore Chordata Hemichordata Echinodermata Arthropoda Se Mollusca Onychophora Brachiopoda Dipleurula? Annelida SRS Chaetognatha : ee een —__-*Kchiuroidea ENTEROCOELA SCHIZOCOELA Aschelminthes ee Acanthocephala Let \ a sium hk Nemertinea EUCOELOMATA ___ 2 —- Platyhelminthes / ACOELOMATA eae PRIMITIVE ACOEL? Ctenophora Coelenterata BILATERIA = RADIATA PLANULA-LIKE FORM? EUMETAZOA PARAZOA (Porifera) Joma MESOZOA Flagellata. ~~ > Other Protozoa Fig. 7.3. A phylogenetic tree of the Animal Kingdom, designed to show the possible interre- lationships of the various phyla. Such a figure should be interpreted as a statement of evolu- tionary probabilities and not as a precise representation of the exact course of evolutionary change. 219 GENERAL ZOOLOGY limb is similar in all. Structures in different animals which reveal, like the fore limbs of vertebrates, basic similarities in organization and in embryonic origin, are spoken of as homologous structures. In contrast, parts of animals which are of diverse origin and composition, even though they may be adapted to similar functions, are said to be analogous; the wings of birds and the wings of insects are examples often cited. In classifying animals, then, we depend not on the functional similarities of analogous parts but on the more fundamental and significant structural similarities exhibited in homologous features. At the present time, many species of animals remain to be cataloged, and the taxonomic positions of known species are undergoing constant revision as additional information and insight seem to warrant. ‘Therefore the “family tree” of living things cannot be drawn with certainty in all its features, although many lesser conclusions can be drawn with reasonable accuracy, considering the nature of the evidence. Upon this tentative basis the tree of descent shown in Figure 7.3 is presented; but before considering in detail what this figure means, we should examine the catalog of animal types now known to us. The Principal Types of Animals. In accordance with the foregoing principles of classification, by which animals are arranged in species, genera, families, and so on, zoologists have arrived at the present comprehensive system. Sweeping changes have been made, in the past, as knowledge of new types or new facts about known types have accumulated. It appears, how- ever, that classification of the major phyla and their main subdivisions now rests upon a fairly permanent basis. During the earlier years of this century there was wide acceptance, at least by American zoologists, of a system including 12 or 15 animal phyla, to which lesser groups of questionable status and affinities were appended. More recently, the tendency has been toward an increasing number of phyla, in recognition of the distinctive features of many of the smaller groups. ‘There appears to be no serious objection to the elevation of a minor group to the rank of a phylum if its characteristics are sufficiently different from those of other animals to justify the change. Hyman, whose studies have done much to elucidate possible interrelationships between invertebrate groups, * makes the following statement: ‘““A phylum should consist of closely allied animals distinguishable from any other phylum by well-defined positive char- acteristics, some of which do not exist in any other phylum or not in that particular combination. Any group of animals, however small, having such distinct characters, should be regarded as a separate phylum until evidence 'The term invertebrate is used generally to include animals without vertebrae, or backbones. The distinction between vertebrates and invertebrates, although convenient, is artificial from the taxonomic standpoint. It cuts across the phylum Chordata, some members of which are vertebrates and some invertebrates. It thus appears in contrast to the distinction between the phyla, or be- tween the Protozoa and the Metazoa. 220 THE CLASSIFICATION OF ANIMALS KINGDOM SUBKINGDOM BRANCH GRADE SERIES SUBSERIES PHYLUM Mesozoa — — — — — oe ee Mesozoa ANIMALIA— Parazoa — as = —_— —_— Porifera Radiata _____| Coelenterata METAZOA— (radial) Ctenophora Acoelomata ______——_ i Platyhelminthes (without body cavity) Nemertinea Pseudocoelomata Aschelminthes (with “false” coelom) Entoprocta Eumetazoa — [Stamnes Ectoprocta Bilateria _ Phoranides (bilateral) Sipunculoidea Echiuroidea Mollusca Annelida Onychophora Arthropoda Brachiopoda* Chaetognatha Enterocoela~-—j Echinodermata Hemichordata Chordata Schizocoela— Eucoelomata (with “true coelom”— or its remnants) *Some brachiopods are apparently schizocoelous in their mode of coelom formation, but this may be a secondarily derived characteristic. Fig. 7.4. Interrelationships among the phyla of animals, based on similarities and differences in broad, general characteristics. shall be forthcoming showing its relationship to some other phylum.” In the treatise from which this quotation was taken, Hyman lists 22 phyla; 23 are now commonly recognized (see Fig. 7.2), and an additional new phylum has recently been proposed to contain an aberrant group of animals, the Pogonophora, dredged from ooze on the deep ocean floor. Some of these phyla are small in numbers of species, and many consist of animals not easily available and hence relatively unknown except to special- ists. For the purposes of the present chapter, only the larger and _better- known phyla need to be considered in any detail. It can then be remembered that the Animal Kingdom includes certain lesser groups, which may also be classified as phyla, although even today authorities differ on many details of classification. For example, the phylum Aschelminthes has been proposed to include several problematical groups classified in different combinations by various authorities; many zoologists do not interpret the evidence for the interrelationships of these forms as justifying their inclusion in a single phylum. The list of phyla and their principal subdivisions in Figure 7.2 is inserted for reference and orientation; some minor groups have been omitted. As you proceed, the names of the larger phyla, such as Protozoa, Coelenterata, Arthropoda, and Chordata, as well as of some of their subdivisions, will become familiar. “Quoted by permission from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 by McGraw-Hill Book Co., Inc., page 32. 221 GENERAL ZOOLOGY As we have seen, an individual phylum is made up of related classes con- taining animals with similar fundamental characteristics. We assume that similarity in structure implies common ancestry, and therefore that members of the same phylum are more closely related to each other than to members of other phyla. It is significant also that the various phyla can be grouped into larger categories, on the basis of similarities and differences in large general characteristics, as shown in Figure 7.4. A study of this figure will demonstrate that just as a phylum can be subdivided into classes, orders, and lesser ranks, the Animal Kingdom can be subdivided into subkingdoms, branches, grades, series, and subseries. For purposes of grouping the phyla into these larger categories, use is made of more general and fundamental characteristics. "These include the grade of organization, the kind of sym- metry, and the presence or absence, and kinds, of body cavities. Since these features are of such a basic nature and make possible such sweeping and gen- eral distinctions within the Animal Kingdom, we may consider briefly what they involve in terms of the structure of animals. Bm Grade of organization. ‘This criterion distinguishes the kind of construc- tion, whether unicellular as in the Protozoa, or multicellular, as in all other animals. Among the multicellular forms, the Mesozoa and Parazoa are said to represent the cellular grade of construction, with little or no differentiation into tissues. All the higher animals, on the other hand, constituting the Eumetazoa, possess groups of similar cells specialized for the performance of particular functions and so are said to exemplify the tissue grade of organization. Bp Kind of symmetry. Among the Eumetazoa, two kinds of symmetry are distinguishable. One, characteristic of the Coelenterata and Ctenophora, is termed radial symmetry and is marked by the presence of a principal axis of the body, about which the parts are disposed in a radiating pattern. In con- trast to this, all the more complex metazoans possess bilateral symmetry, in which there is one median or sagittal plane dividing the body into right and left halves which are approximately mirror images of each other. ‘There are apparent correlations between forms of symmetry and ways of life in animals. Radial symmetry is primarily associated with sessile or attached animals, which are in approximately equivalent contact with their environment in all directions; in radially symmetrical animals there are commonly differentia- tions between oral and aboral ends, or between the free end and the attached end, so that an axis of polarity usually corresponds to the axis of symmetry. Bilateral symmetry, on the other hand, is generally associated with active, free-moving ways of life, and in addition to right and left sides and head and tail ends, back and belly surfaces may usually be distinguished. m Body cavities (Fig. 7.5). Some of the bilateral animals have no cavities within the body other than that of the digestive system; these are termed acoelomate. Among the metazoans which do possess a body cavity surround- ing the digestive tract, this cavity may have different characteristics and may arise in different ways. For example, the members of the series Eucoelomata 222 THE CLASSIFICATION OF ANIMALS (Fig. 7.4) possess a body cavity termed a coelom, which is lined by a layer of mesodermal tissue, the peritoneum. ‘The manner of origin or mode of forma- tion of the coelom differs in consistent ways in different kinds of eucoelomate animals, and these facts are used in distinguishing between the two subseries which can be recognized. In contrast, members of the series Pseudocoelomata have a body cavity which is not a coelom but a pseudocoel with no peritoneal lining. Considering these broad, general characteristics, the phyla may be grouped into larger subdivisions, according to a classification which seems reasonable to many zoologists. It should be pointed out that these larger categories, the branches, series, and so on, are not recognized taxonomic entities with the same standing as, for example, the species, the family, or the phylum. They are rather to be regarded as synthetic groupings, useful as aids to an under- standing of the similarities and differences, and thus the probable ancestral relationships, between the members of the various phyla. It should be realized also that any classification represents merely an opinion of experts, based on the knowledge available at a given time. Aristotle, for example, working in the fourth century B.c. with very little information other than that which he himself could accumulate, classified animals as those with red blood (essentially our vertebrates) and those without red blood (our invertebrates). Early in the nineteenth century, the great French naturalist Cuvier (1769-1832) divided the Animal Kingdom as it was known to him into four main types: Vertebrata, Articulata, Mollusca, and Radiata (Zoophyta). With increasing knowledge and increasing insight, it was later recognized that each of these groups included forms so diverse that they could not properly be classified together. Subsequent changes in Cuvier’s scheme involved the separation of the Articulata into the joint-footed Arthropoda and the worm- c-) ve Fig. 7.5. Schematic cross sections for comparison of acoelomate, pseudocoelomate, and eucoelomate plans of organization. A, acoelomate: epidermis and gastrodermis are separated by mesenchymal ‘‘parenchyma,” through which lymph-like fluid percolates. B, pseudocoelomate: the body wall encloses a cavity which contains muscular and other elements of mesodermal origin, but the cavity is not lined, or the structures covered, by a peritoneal layer. C, eucoelomate: the body cavity is a true coelom, lined throughout by a continuous sheet of mesodermal epithelium, the peritoneum; organs and structures that lie in the coelom are also covered by the peritoneal layer and are suspended by double mesodermal sheets, the mesenteries. In C, the peritoneum and mesenteries are represented by broken lines. 223 GENERAL ZOOLOGY i ) { e . PORIFERA No true symmetry Diploblastic (?) No true tissues . COELENTERATA and CTENOPHORA Radial to bilateral Diploblastic to triploblastic Tissues, no true organs 3. PLATYHELMINTHES Bilateral Triploblastic Non-segmented Acoelomate Organs and organ systems bo is . MOLLUSCA Bilateral Triploblastic Non-segmented Eucoelomate Organ systems an ANNELIDA Bilateral Triploblastic Segmented Eucoelomate Organ systems 6. ECHINODERMATA a Secondarily radial Triploblastic Non-segmented Eucoelomate Organ systems 7. CHORDATA Bilateral Triploblastic Segmented Eucoelomate Organ systems Fig. 7.6. Summary of general characteristics of major animal groups. like Vermes. Later, the Vermes were partitioned into several smaller phyla. Again, Cuvier’s Radiata were split into the phylum Echinodermata and the phylum Coelenterata, and later the phylum Porifera was separated from the coelenterates. Thus, more knowledge has made more phyla, although a limit has apparently been reached in such groups as the Chordata, Arthropoda, Mollusca, and Echinodermata, with their clearly defined characteristics (Fig. 224 THE CLASSIFICATION OF ANIMALS 7.6). There are always differences of opinion among the experts, even when the same observations are involved, and many aspects of classification must remain matters of judgment until more explicit information is forthcoming than any now available. Yet, there has come to be an increasing degree of agreement in taxonomy as knowledge of animal life and characteristics has progressed. Evolutionary Interpretation of Classification. We may now ex- amine the evolutionary implications of the natural or genetic classification, which zoologists have constructed on the basis of structural resemblance. This classification is, as we have seen, an effort to construct a family tree of animal life. If certain phyla are placed together as Eumetazoa, the group- ing means that they are regarded as closely related in ancestry. It is possible, therefore, to consider classification as a statement of evolutionary probabilities. Referring to Figure 7.4 as though it were a family tree, we may state that the first great step in evolutionary progress of animals was the divergence between forms that continued in the ancestral single-celled condition and gave rise to the Protozoa, and those that attained in some way the multicellular state and gave rise to the Metazoa. Within the latter line, the next major divergence was between animals that continued in a primitive state of dif- ferentiation, the Mesozoa and Parazoa, and forms that acquired a gut cavity, along with other complexities, and became the progenitors of all the eumeta- zoan phyla. Within this more advanced group next occurred a divergence into two stems: one, developing radial symmetry, the Radiata, and the other, with bilateral symmetry, the Bilateria. Again, within the great stem Bilateria, forms possessing a body cavity arose from ancestors which had lacked such a cavity; and this space evidently originated in various ways and became variously specialized. Finally, we have come in the course of evolution to the existing phyla with their subdivisions, and to the species and individual animals of today. Many of these animals, in their fullest, adult development, resemble forms which may be interpreted as having served as stages in the evolution of higher animals. ‘This existence, in the life cycles of complex forms, of developmental stages which resemble those of much simpler animals is considered significant in evolutionary interpretation and is the basis of the so-called Recapitulation Theory. ‘This states, in essence, that in its own developmental sequence each individual organism exhibits transitory stages which represent stages in the evolutionary history of the group to which it belongs. According to this theory, we should be able, within certain limits, to determine the phylogenetic derivation of a group of animals by studying the embryonic development of modern representatives of the group. Interpreted in this light, the existence of a unicellular stage, the zygote, in the life cycle of every sexually repro- ducing animal may be regarded as reflecting the probability that all multi- cellular animals have descended ultimately from unicellular forebears. The fact that all members of the phyla grouped as Bilateria (Fig. 7.4) exhibit in 225 GENERAL ZOOLOGY some degree the so-called gastrula stage in their life cycles may be taken to indicate that a two-layered condition, which now persists in the adult stage in the simplest coelenterates and as a developmental stage in higher animals, was characteristic of the common ancestors of Radiata and Bilateria. What- ever its evolutionary significance, the gastrula is morphologically a two- layered sac, generally comparable with the basic type of structure in simple coelenterates. Similar interpretations may be based on the appearance of fish-like stages in the development of amphibians and higher vertebrates, and on the existence of various stages of development within other phyla which are suggestive of ancestry. It should be emphasized, however, that no existing species of animal is regarded as the specific ancestor of any higher group. The grand course of evolution can be pictured if we speculate on the changes that have occurred since the differentiation of organisms into animals and plants. It is clear that the major steps in this great progression oc- curred at a very early period, since the oldest fossil remains of animals, in rocks of the Pre-Cambrian period (older than 500 million years), include types representing all the great phyla except the Chordata. The conclusions presented graphically by the family tree in Figure 7.3 are frankly specula- tive; they are based on facts of structure and development observed in living animals or are deduced from comparisons of fossil remains, interpreted in the manner just described. 226 CHAPTER 8 CHAPTER 8 UNICELLULAR ANIMALS: The Protozoa Crystalline inclusions Endoplasm Hyaline cap (ectoplasm)| Contractile vacuole Plasmasol Plastingel sheet Fig. 8.1. Amoeba: general structure; schematic optical section. (Adapted from S. O. Mast, 1925, Journal of Morphology and Physvlogy, vol. 41, printed by permission.) 228 THE PROTOZOA In the preceding discussion of the classification of animals, it was pointed out that the simplest animals are unicellular, and that on the basis of this characteristic they may be classified as members of the subkingdom Protozoa, containing only the phylum Protozoa. ‘These unicellular animals may be contrasted with all the phyla of multicellular animals constituting the sub- kingdom Metazoa. ‘The Protozoa are defined as single-celled animals, al- though some protozoans form permanent colonial aggregations which approach the simplest metazoans in complexity. “The word Protozoa (‘‘first animals,” “primordial animals”) is well chosen; although many protozoan cells are very highly specialized, the single-celled condition is considered most prim- itive of all the structural plans of animals. It seems reasonable to conclude that the protozoans have descended, without changing their unicellular state, from the primeval organisms that were also the ancestors of the Metazoa. The phylum Protozoa is divided into two subphyla: the subphylum Plasmo- droma, which: contains the class Sarcodina, the class Flagellata, and the class Sporozoa; and the subphylum Ciliophora, which includes the most complex of the protozoans, along with many simpler types, in the class Ciliata and the class Suctoria. The vast majority of protozoans are free living, but all members of the class Sporozoa are parasitic, and there are many parasitic species in other classes. Some of these parasites, such as the malaria organism, cause serious diseases in man and animals. Save for exceptional species, the Protozoa are of microscopic size; for this reason, among others, they were not clearly understood in relation to other forms of life until rather late in the history of classification. Certain of the ciliates were observed and recognizably described by Leeuwenhoek as early as 1675, and many other protozoans were discovered during the eighteenth and nineteenth centuries. The unicellular nature of these animals, and the fact that they might be compared with the cellular units of metazoans, was not recognized until 1845, after the elucidation of the Cell Theory. Although we do not now speak of digestive tracts and other organs in protozoans, as Ehrenberg did in 1838, it must be recognized that within the limits of the unicellular state some protozoans are very complex organisms. The structural complexity of many protozoans illustrates the fact that dif- ferentiation in animals may occur at the cellular level as well as in tissues or organs composed of large numbers of cells. “The component cells of meta- zoans, specialized to carry on particular functions, are physiologically un- balanced. ‘They can exist in such a state of specialization only because the many-celled organism taken as a whole is a physiologically balanced unit. The single cell constituting the body of a protozoan necessarily performs all functions, and hence it must be physiologically balanced, whatever its degree of specialization. ‘Together with the complexity of many protozoans, this con- dition of physiological balance, which enables the protozoan to be a complete and independent individual, has led some zoologists to regard the Protozoa as animals to which the cell concept does not apply. According to this view, 229 GENERAL ZOOLOGY the Protozoa should be considered as acellular, or non-cellular, animals— as organisms whose bodies are not equivalent to individual cells but are simply not subdivided into cells. Their minute size and soft bodies have made it impossible for most types of protozoans to leave any record as fossils. However, the siliceous skeletons of representatives of the group known as Radiolarians are found abundantly in Pre-Cambrian rock (considerably older than half a billion years) as well as in later deposits. These unicellular animals have thus had an extremely long evolutionary history. In this chapter, the Protozoa will be examined as a phylum of the Animal Kingdom and as animals whose unicellular organization may be contrasted with the multicellular organization described for the vertebrates. Also, the capacities of metabolism, responsiveness, and reproduction, which are charac- teristic of all living things, will be examined and compared in protozoans and in vertebrates. The Sarcodina In the class Sarcodina are included the morphologically simplest forms of Protozoa, although from an evolutionary standpoint the Flagellata as a group are probably the more primitive. A distinctive feature of the Sarcodina is the capacity to form temporary protoplasmic extensions of the body, called pseudopodia (‘‘false feet”). In the subdivision Rhizopoda, which includes creeping forms such as Amoeba, the pseudopodia are lobular or root-like, sometimes subdividing, and may frequently change their shape or be with- drawn. In the Actinopoda, which are floating forms such as Actinophrys, the processes are stiff, rod-like, and more nearly permanent. The name Sarcodina was first applied because protozoans of this class resemble undifferentiated protoplasm, which was originally termed “‘sarcode,” or flesh. Notable among the Sarcodina are the genus Amoeba and related genera, which are collectively spoken of as amoebae or amoebas. The Amoeba: General Structure. The protoplasm of an amoeba (Fig. 8.1) consists of a thin external layer, the plasmalemma, which functions as a cell membrane; a non-granular region just within, the ectoplasm; and a granular inner region, the endoplasm, in which the nucleus lies. Features of cells like those of a vertebrate are thus apparent. The larger bodies distributed in the cell body, or cytosome, are granules of various sizes, food vacuoles in which digestion occurs, a single contractile vacuole, and other vacuoles con- taining watery fluid and comparable with those found in many other cells. Also present are crystals of definite forms, which may be distinctive for particu- lar species of amoebas; oil globules; and many small inclusions ranging to the limits of microscopic visibility. The significance of these parts will be discussed as necessary in the accounts to follow. 230 THE PROTOZOA Movements and Responsiveness. ‘Vhe manner in which an amoeba moves, by the flowing of its irregularly shaped body, has attracted attention ever since the animal was studied by the early microscopists, who called it the proteus animalcule, or “‘changing little animal.” This amoeboid movement is simple in appearance, but it is surprisingly difficult to explain. Some of its features can be imitated by inanimate models, such as a drop of clove oil in a mixture of glycerin and alcohol; here changes in surface tension are respon- sible for the phenomena, and one theory assumed that similar forces were significant in amoeboid movement. However, it is now clear that the move- ments of inanimate models are not strictly comparable with those of an amoeba. Various accounts have been given of the changes to be observed in the formation of pseudopodia and in the locomotion of different species. Amoebas have been described as extending their pseudopodia like jets of water from a fountain, with a current flowing outward in the center of a pseudopod and backward on all sides. They have been described as rolling like a sac with elastic walls and fluid contents; and they have been said to “walk” upon stiff pseudopodia. Different kinds of amoebas thus move in different ways, but the formation of pseudopodia is probably fundamentally similar in all. The best and most generally applicable theory of amoeboid Gelation Solation Solation Solation B Fig. 8.2. Amoeba: cytoplasmic movements in locomotion. NV, nucleus; CV, contractile vacuole. (Adapted from S. O. Mast, 1925, Journal of Morphology and Physiology, vol, 41, printed by permission. ) 231 GENERAL ZOOLOGY Fig. 8.3. Amoeba: reactions to contact with substratum. (Adapted from H. S. Jennings, The Behavior of the Lower Organisms, copyright 1906 by Columbia University Press, printed by permission. ) movement is based on the assumption that a relatively stiff, elastic layer, the plasmagel, surrounds the cell just beneath the plasmalemma and en- closes the more fluid inner contents, or plasmasol (Fig. 8.2). Localized changes cause a temporary liquefaction of the gelatinous outer layer at the point where a pseudopod is to arise; the elasticity of the remainder of the gelatinous sheath forces the fluid endoplasm against and through such a weakened area. Within the pseudopodial lobe thus formed, the fluid endo- plasm flows peripherally and stiffens, adding to the plasmagel layer. ‘This type of movement, therefore, involves one of the fundamental capacities of the endoplasm: that of changing its physical state from gel to sol, and the reversal of this process. An amoeba, we may say, moves as a tunnel might, if the mortar of its wall became fluid at the posterior end and flowed within the tunnel to its anterior end, carrying the bricks to be laid again anteriorly by a new setting of the mortar. Just as the collective reflexes make up the behavior of multicellular organ- isms such as vertebrates (see p. 86), the movements and other reactions of an amoeba in response to stimuli, or changes in the environment, make up the behavior of this unicellular animal. We may compare the activities of the single-celled amoeba with those of a white blood cell in the vertebrate body (see p. 63), or we may think of the amoeba as an individual animal to be compared with another complete individual. In the former case, we compare cell with cell, and the parallels are obvious. In the latter comparison, we forget about the cellular organization and think only of the individual as a whole. ‘The behavior of white blood cells, when they move into certain regions and ingest such foreign bodies as bacteria, may be compared with the behavior of amoebas. The behavior of other cells in the vertebrate animal may be similarly compared, but the correspondence is less evident. Thoughtful consideration of an amoeba as an individual, reacting to its environment, and in comparison with another individual, such as the many- celled vertebrate, enables us to recognize broad factors common to each, and to state them in general terms, irrespective of the cellular organization of either animal. Nevertheless, what is called the behavior of an amoeba is 232 THE PROTOZOA based on the capacity of responsiveness, as shown by the reactions of a single independent cell. The behavior of a multicellular animal is also based on the responsiveness of cells, but such behavior involves reactions in sequence by a number of cells. Most easily demonstrable reactions of amoebas are negative, since they consist of withdrawals of pseudopodia or contraction of the cell in response to stimulation. Certain other responses, to contact, for example, are positive reactions. An amoeba dropped into water and settling slowly toward the bottom through a considerable distance may give a positive response by ex- tending pseudopodia in all directions (Fig. 8.3). If one of these pseudopodia comes into contact with a surface, such as the stem of a water plant, the amoeba may respond positively by flowing in the direction of this contact and may thus begin to move over the surface. If one of its pseudopodia is then touched with a glass needle, or if certain chemicals in solution are brought into contact with a pseudopod by means of a capillary pipette, a negative response may be indicated by withdrawal of the pseudopod. If the stimulus is sufficiently strong, the entire amoeba may contract into a globular form. Positive reactions are also involved in feeding, and these will be described in the account of metabolism which follows. Feeding and Metabolism. Amoebas feed upon other organisms, both animal and plant, and may thus be described as holozoic in their nutrition. Such a species as Amoeba proteus is essentially a beast of prey, eating what- ever it can capture, from small to relatively large protozoans and single- celled plants. The most common food of this species consists of small flagellates and ciliates, which an amoeba consumes in large numbers. _ Inges- tion involves the extension of pseudopodia about the prey, which is engulfed and transferred into the endoplasm (Fig. 8.4). A food vacuole thus originates by the enclosure of a drop of water containing one or more food bodies. ‘The feeding reactions are surprisingly complex and variable, considering the ap- parent simplicity of an amoeba. Forms such as motionless unicellular plants evoke responses different from those induced by active prey. A certain selec- NN AQ ~ AQy ® Y Y Y Y O © © © Fig. 8.4. Amoeba: ingestion of a flagellate and successive stages in the ey formation of a food vacuole. (Adapted from W. A. Kepner and W. H. Taliaferro 1913, Biological Bulletin, vol. 24, printed by permission.) 233 GENERAL ZOOLOGY tivity is exhibited by the amoeba: in the presence of two kinds of prey, equally abundant, the organism ingests the one kind which appears to be most easily digested, and rejects the other. Moreover, the responses are not fixed and mechanical but vary with the physiological state of the amoeba. In the adjustment of reaction to stimulus, and to the state of its physiology, an amoeba behaves in a manner resembling the behavior of multicellular organisms. The resemblance between the vital functions in an amoeba and those in a vertebrate may be shown by tracing the history of the ingested food. When a small flagellate, such as Chilomonas, is ingested by Amoeba proteus, the prey continues to move about for several minutes before it is killed by something within the vacuole. Meanwhile, the food vacuole, which at the outset con- tains a relatively large amount of water, shrinks by the diffusion of excess water into the cytoplasm. ‘The fluid then remaining within the vacuole be- comes alkaline, and in later stages it becomes acid. If the changes in individual vacuoles are followed, the Chzlomonas will be seen to disintegrate gradually, until, some 12 or 24 hours later, there remain only certain granules that are apparently indigestible. Fat globules are liberated from the food mass and appear in the vacuolar fluid within 2 or 3 hours, after which they gradually decrease in size and disappear. Starch grains disintegrate into a pasty mass, which disappears as the vacuole slowly decreases in volume. The disintegration of other particles and further shrinkage of the vacuole follow, until only a few granules remain. Even these remnants may pass into the endoplasm instead of being egested. Egestion occurs by the discharge of food in various stages of digestion, and of the indigestible residue of food, after all the digestible material has passed into the cytoplasm. Often several vacuoles in late stages coalesce, and the resulting mass comes into contact with the plasmalemma at or near the posterior end of the amoeba. ‘The mass is egested by rupture of this membrane. From observations such as these, it is inferred that fats, carbohydrates, and proteins are digested in the food vacuoles, presumably by specific enzymes, as in the digestive tracts of many- celled animals. It also appears that the products of digestion pass into the endoplasm as they pass into the cells lining the digestive tract of a higher animal. Alternatively, the comparison may be made with the absorption of nutrients from the surrounding lymph by cells in all parts of the vertebrate body. Thus, the digested food is ready to be assimilated. Amoebas cannot survive in water from which all dissolved oxygen has been removed, as by boiling; this demonstrates that available oxygen is necessary for the continued existence of these animals. Normally, oxygen diffuses into the cell from solution in the surrounding medium, just as it enters the cells of a vertebrate from solution in the intercellular lymph. Asin the cells of vertebrates, oxygen is essential to cellular metabolism, the series of oxidative reactions which release energy within the cell. The end products of the catabolic process are, asin the vertebrate, carbon dioxide, water, and nitrogenous compounds. ‘These are eliminated from the amoeboid cell by excretion through its surface. Ex- 234 THE PROTOZOA cretion in Amoeba may be compared, again, either with that of a component cell ofa higher animal, or with that of the higher animal as an organism. For in a vertebrate excretion occurs initially at the cellular level, waste products being released by cells into the lymph, eventually to find their way to the specialized organs of excretion through which they will ultimately be eliminated from the body. At the amoeba’s unicellular level of organization, the initial passage of wastes from the cell into the surrounding water completes the process of excretion. The contractile vacuole of Protozoa has long been considered as primarily an organelle (the intracellular counterpart of a multicellular organ) specialized for the elimination of excreta. Nitrogenous substances have actually been de- tected in samples of fluid ingeniously removed from contractile vacuoles. How- ever, the chief function of these structures probably involves the removal of excess water from the cell. As a result of the osmotic gradient between the external medium and the cell contents, water tends constantly to enter the cell, and provision must be made for its elimination. The contractile vacuole may be thought of asa pump, operating continuously to maintain the proper environ- mental conditions within the cell. "The water removed by this action contains a certain amount of excretory material in solution, and to this extent the con- tractile vacuole may be regarded as an excretory organelle. Many protozoans (for example, most marine forms) lack contractile vacuoles and must depend en- tirely on diffusion through the general cell surface for the elimination of wastes. Undoubtedly most excreta are removed from Amoeba in this manner. From these considerations it is apparent that the metabolic processes of this very simple, unicellular animal and of a complex vertebrate are essentially analogous. The fundamental requirements of both protozoan and metazoan cells are everywhere comparable: food, providing energy sources and essential basic substances; oxygen, for the combustion of nutrients in an ordered manner, releasing energy for synthetic activities; and a provision for the maintenance of favorable internal conditions by the elimination of metabolic wastes. ‘The chief differences displayed concern the fact that in each species the cell synthesizes the particular specific compounds characteristic of its kind. Life Cycle and Reproduction. ‘The life cycle, or life history, of a many- celled animal is the series of changes from egg to adult that occurs in each generation. Many protozoans, including some members of the Sarcodina, also exhibit serial changes of form which constitute their life cycles. In the com- mon amoebas, however, the life history seems to involve nothing but an endless Fig. 8.5. Amoeba: successive stages in division. (Adapted from K. von Frisch, 1952, Biologie, vol. 1, printed by permission of Bayerischer Schulbuch-Verlag. ) 235 GENERAL ZOOLOGY Fig. 8.6. Representative shelled rhizopods. 4, Arcella, and B, Difflugia, order Lobosa. C, Actinophrys, order Heliozoa. D, Globigerina, order Foraminifera. (D, redrawn from W. C. Williamson, 1858, Mono- graph on the Recent Foraminifera of Great Britain.) series of cell division by binary fission, although more complicated phenomena, such as encystment and sexual reproduction, have been described. Present in- dications are that Amoeba proteus, for example, reproduces only by binary fission (Fig. 8.5), with subsequent growth of the daughter cells to full size, continuing in the active state without syngamy or encystment. Amoebas may become smaller through starvation, or, as in some larger species, multinucleate forms may be produced by the failure of the cytosome to divide following nuclear division. The large fresh-water amoeba, Pelomyxa carolinensis, contains hundreds of nuclei produced in this way. At the time of cell division, the cytosome divides, distributing the nuclei between the resultant daughter individuals. In some of the other amoeboid forms, more complicated life cycles, with budding and encystment, have been discovered. Some of these cycles include flagellated stages, and in others, gametes and syngamy are known. Other Sarcodina. Vhe genus Amoeba, with many other free-living forms which it resembles, is placed in a subdivision of the Sarcodina known as the Rhizopoda. Other rhizopods, suchas the genera Arcella and Difflugia, possess shells with a single opening from which the pseudopodia extend (Fig. 8.6). In the Foraminifera, there is a shell composed of calcium carbonate, chitin, silica, or other materials, and the pseudopodia extend through numerous openings. With few exceptions, the Foraminifera are marine, living near the surface of the sea as well as on the bottom. ‘The shells of dead foraminiferans make up a large part of the silt that covers the ocean floor, in regions such as the deeper portions of the North Atlantic. In chalk formations in various parts of the world, some of them hundreds of feet thick, these minute shells make up as much as 70 per cent of the deposits. Another subdivision, the Actinopoda, includes the “sun animalcules,” such as Actinophrys sol (Fig. 8.6), and the Radiolaria, which are notable for their siliceous skeletons of great beauty and variety (Fig. 8.7). In some of the very ancient sedimentary rocks occur 236 THE PROTOZOA skeletons of Radiolaria and Foraminifera almost identical with those of present-day species. ‘This fact indicates that these forms, and so perhaps many other Protozoa, have remained almost unchanged for half a billion years or more. Parasitic Rhizopoda. Although most of the rhizopods are free-living animals, a smaller number of the known species are parasitic. One of these, Entamoeba histolytica, causes the serious disease of humans called amoebic dysentery, or amoebiasis. Entamoeba histolytica is found in humans chiefly in the large intestine, where the active feeding stages, or trophozoites, destroy the mucous membrane and invade the submucosa. ‘The ulcers of the in- testinal lining thus formed become infected with bacteria, as well as with amoebas. ‘The food of the parasite consists of the tissues it destroys, to- gether with large numbers of red corpuscles. In advanced cases of amoe- biasis, the parasite may gain access to the intestinal blood vessels; in the blood stream, the parasites are carried to the liver, lungs, and brain. Al- most any part of the body may be thus invaded, the amoebas then causing serious abscesses in the secondary sites. Hence, the symptoms of amoebiasis are not limited to intestinal disturbances. The trophozoites (Fig. 8.8) may be observed in the freshly discharged feces of the host. They become increasingly sluggish as the feces cool and die with- ina few hours. The feces of an infected individual also contain large numbers of encysted stages of the parasite, which are smaller cells encased in cysts and having typically four nuclei. “Vhe cysts can survive for some time out- side the host and are moderately resistant to heat and cold. ‘They can be killed by the pasteurization process in milk and by boiling in water. Viable cysts are transferred to the intestine of a new host via the mouth and digestive tract in contaminated food and drink, and to a lesser extent by other means. Reproduction in the trophozoite stage occurs by binary fission. In the en- 237 GENERAL ZOOLOGY Fig. 8.7. Skeletons of representative foraminiferans (A) and radiolarians (8B). (Photographs courtesy General Biological Supply House, Inc.) cysted form, the four nuclei are produced by two nuclear divisions, the fore- runners of cytoplasmic divisions which occur immediately after excystment. From each cyst, eight uninucleate individuals are ultimately produced, each of which subsequently grows into the large, active trophozoite form. Once regarded as a tropical disease, amoebiasis is now known to be dis- tributed throughout the world, even to the Arctic Circle. Infections are of common occurrence wherever crowded conditions are combined with in- adequate sanitary facilities; such situations may lead to the outbreak of epidemics. With increasing travel to and from parts of the world where the incidence of the disease is high, amoebiasis may cause increasing concern in this country. Fortunately, methods of prevention are well known, although not always practiced; however, curative treatment is still a problem. 238 THE PROTOZOA A variety of other amoebas inhabit the intestine of man and lower animals, probably more species than can now be recognized. Most of these appear to be relatively harmless “‘messmates,” or commensals, living within the larger animal but not markedly disadvantageous to the host. The Flagellata In the class Flagellata are included the protozoans possessing one or more flagella (singular, flagellum), or whip-like extensions of the cytoplasm, during the more representative phases of the life cycle. Flagella are primarily organelles of locomotion; in some species they also assist in feeding. Flagella are also found in many species of Sarcodina but usually only during a limited part of the life cycle. In a similar manner, amoeboid stages occur in the life cycles of many flagellates. The existence of both flagellate and amoeboid stages in a single species suggests a close relationship between Sarcodina and Flagellata. The Flagellata also exhibit a close relationship with plants, since many of the flagellate Protozoa possess chlorophyll and are sometimes indis- tinguishable from unicellular plants. Among such plant-like flagellates are species of the genus Euglena. The Euglena: General Structure. A typical euglena (Fig. 8.9) is covered by a thin pellicle, comparable with the cell wall in plant cells and often marked externally in a spiral pattern. ‘The pellicle is stiff enough to pre- serve the contours of the organism as it swims through the water but flexible enough to allow the changes of shape called euglenoid movement. ‘The anterior end of the organism bears a mouth-like notch, from which a flask-shaped cavity extends a short distance into the cell. The single flagellum protruding from this cavity arises from two branches, each of which originates in a granule, or blepharoplast. From one blepharoplast a fiber extends to the nuclear membrane. ‘The flagellum itself consists of a central axial filament, formed by the union of the two branches, and a surrounding, spirally wound Fig. 8.8. Entamoeba histolytica. A, trophozoite or vegetative form; B, a cyst. (Adapted from W. Balamuth in F. A. Brown, Jr., et al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by per- mission. ) 239 GENERAL ZOOLOGY “Gullet”’ Stigma Reservoir Contractile vacuole Fig. 8.9. Euglena vindis: general structure. Nucleus (Adapted from W. Balamuth in F. A. Brown, Jr., et al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission. ) Chloroplast Paramylum bodies sheath. In Euglena the cavity from which the flagellum extends does not function as a mouth and gullet, for in its nutrition the euglena is holophytic, like the green plants. In related flagellates which ingest and digest food, the anterior opening may more properly be called a mouth. In the euglena, in the anterior end of the cell, minute vacuoles periodically enlarge and coalesce to form a contractile vacuole, which discharges into the “gullet.” As in the A Fig. 8.10. A, euglenoid movement; note the extreme plasticity of the cytosome. 8, binary fission in Euglena virdis; the division of the nucleus (NV) involves a mitotic process. (A, 240 THE PROTOZOA amoeba, such vacuoles are believed to eliminate water from the cell, and only incidentally to serve for the expulsion of the soluble excreta which this water may contain. A mass of red pigment at the anterior end of the organism is called the stigma, or eye spot; it seems to be a light-sensitive organelle. “The nucleus lies near the center of the cell, surrounded by green chromatophores, the chloroplasts, which fill the cytoplasm. The chloroplasts contain chlorophyll and are responsible for the green color of the cell. “This chlorophyll is com- parable with that in the green cells of plants. Between the chloroplasts the most conspicuous inclusions in the cytoplasm are bodies of characteristic shape, varying between different species, composed of paramylum. ‘This 1s a complex carbohydrate related to starch, and the paramylum bodies are inter- preted as stored food reserves. ‘There is no flowing of the endoplasm as in the amoeba, although the plasticity of the cytosome is demonstrated when the euglenoid cell changes its shape. Movements and Responsiveness. Characteristic expansions and con- tractions of the cell, occurring when the euglena is not in active locomotion, are called euglenoid movements (Fig. 8.10). ‘These are not interpreted as re- lated to progressive locomotion, which is brought about by the action of the flagellum. The flagellum beats in such a way as to propel the organism in a spiral course, rotating upon its long axis. By these movements of the cell body, and by spiral swimming, the organism reacts to a variety of stimuli. The behavior with respect to light, a necessary factor in the environment of these plant-like forms, has been studied especially. Observations have shown that a euglena which has been moving toward a source of light gradually changes its direction when the direction of the light is changed and so continues to orient positively toward the light. ‘The adjust- ment involves a complex series of movements, including rotation of the cell upon its long axis; but once the orientation is accomplished, the animal con- B after A. E. Shipley, 1893, Zoology of the Invertebrata; B, adapted from R. P. Hall and T. L. Jahn, 1929, Transactions of the American Maicroscofrcal Society, vol. 48, printed by permission.) 241 GENERAL ZOOLOGY Mastigamoeba Peranema Monosiga Fig. 8.11. Representative colorless flagellates. Although Mastigamoeba produces pseudopodia, it is considered a flagellate because of the presence of a flagellum. Afonosiga, a choanoflagellate, bears a protoplasmic collar surrounding the flagellum. tinues its spiral progression in one direction. In general, the euglena responds positively to light of optimum intensity; if the light is very intense, a negative response will be exhibited. In these and other reactions the organism mani- fests the responsiveness characteristic of all cells. Nutrition and Metabolism. Possessing chlorophyll, the euglena carries on holophytic nutrition like that of green plants. It is doubtful that ingestion of food ever occurs in Euglena, although such colorless flagellates as Peranema and others do ingest small organisms through the gullet and form food vacuoles. When kept in darkness, Euglena gracilis and other green flagellates lose their green color but continue to live and reproduce for long periods. ‘This is true, however, only if certain organic compounds are present in the culture medium, to satisfy the energy requirements of the cells. Thus it has been established that the same species can maintain itself in the light by holophytic nutrition and in darkness by saprophytic or saprozoic nutrition. In the absence of light, the organism is unable to manufacture its energy-rich compounds by photo- synthesis and must depend on external sources. Life Cycle and Reproduction. As in many other protozoans, the life cycle of some species of Euglena includes an active phase, during which the organism moves about, and an encysted phase, during which it is enclosed within a cyst and is non-motile. It is questionable whether Euglena wridis ever undergoes encystment. In this species reproduction occurs by binary fission, which is typically a longitudinal division beginning at the anterior end of the cell (Fig. 8.10). In other euglenas this division may proceed in either the active or the encysted phase. So far as is known, there is no sexual reproduction 242 THE PROTOZOA in flagellates like Euglena, although the production of gametes, and syngamy, are well known in other flagellates. Other Flagellates. For convenience, the flagellates are sometimes sub- divided into two groups, the plant-like and the animal-like forms. ‘The class is so heterogeneous that such a major subdivision is of questionable value. Some of the plant-like forms are clearly very similar, structurally, to other species which lack chlorophyll. It is evident that the Flagellata are very difficult to separate from the unicellular plants, on the one hand, and from the Sarcodina, on the other; they also show some affinities with the Sporozoa, to be described later. The flagellates include many interesting forms (Fig. 8.11). In Mastigamoeba the cell is amoeboid, although the presence of a flagellum leads to its classification as a flagellate. In Peranema the cytosome is strikingly mobile, and the flagellum is generally held straight out anteriorly, vibrating only at the distal end as the animal progresses. In Monosiga and related forms, called choanoflagellates, there is a delicate protoplasmic collar sur- rounding the flagellum, and ingestion of food particles occurs in this region. Noctiluca, a dinoflagellate, is one of the organisms responsible for luminescence in the ocean. Parasitic Flagellates. Many of the Flagellata are parasitic. “The lower di- gestive tract of man and other mammals often harbors such forms, and almost any frog or tadpole will be found to have more than one species of flagellate in its large intestine. The digestive tracts of termites and wood roaches con- tain an amazing array of Hypermastigida, a group of flagellates with very numerous flagella (Fig. 8.12), which have been shown to perform an essential function in the digestion of wood by these insects; hence, these are not actually parasites in the accepted sense. The forms called trypanosomes (‘“‘awl body”’) occur in the blood of verte- brates and the digestive tracts of invertebrates (Fig. 8.13). A trypanosome Fig. 8.12. A hypermastigote flagellate, Trichonympha campanula, from the alimentary tract of a termite. The longitudinal striae represent the lines of attachment of additional hundreds of flagella, with which the anterior portion of the organism is completely covered. (Adapted from C. A. Kofoid and O. Swezy, 1919, Unwersity of Californa Publications in vacuole 3 ee Oral groove Gullet Micronucleus Fig. 8.17. Paramecium: general structure. (Adapted from L. H. Hyman, The Inverte- brates: Protozoa through Ctenophora, copy- right 1940 by McGraw-Hill Book Co., Inc., printed by permission.) Macronucleus Food vacuoles Contractile vacuole blood invaded. It has also been established that chronic infections may per- sist in the tissue cells after the blood has been cleared of parasites by treat- ment; this fact explains the frequent “relapses” often characteristic of the disease and the difficulty of effecting a final cure. The Ciliata The class Ciliata includes the protozoans in which the body is wholly or partially covered by cilia. Many of the ciliates are complex and _ highly specialized cells, whose structural complexities far exceed those found at the cellular level in metazoans. A unique feature is the almost universal separa- tion of the nuclear material into two parts, a larger macronucleus and at least one micronucleus, with important differences in function. The genus Para- meclum 18 representative, and its species have been the subject of many Investigations. The Paramecium: General Structure. If any forms can be called the omnipresent protozoans of fresh water, they are Paramecium aurelia and P. caudatum. No species of large size occur more commonly in cultures or under a wider range of conditions. Moreover, these species can be easily cultured in the laboratory and are favorable for study. The account to follow deals with P. caudatum, unless otherwise stated. The size of the individuals seen in mixed cultures varies greatly. Like other kinds of animals which have been extensively studied, P. caudatum con- sists of many races which breed true among themselves but may differ widely 248 THE PROTOZOA when one race is compared with another. Reproduction, food, and environ- mental factors also influence body size. ‘The cell is spindle-shaped, with the anterior end bluntly rounded and the posterior end more pointed (Fig. 8.17). At one side, a depression, the oral groove, passes diagonally from the anterior end to about the middle of the body, where it ends in a gullet. “The body is covered with cilia, which are of uniform length except for those at the posterior end and in the oral groove, which are slightly longer. Within the gullet the cilia are arranged in a special band-like undulating membrane. On the surface of the cell just posterior to the end of the oral groove lies the anal spot, where egestion occurs. The outermost layer of the cell is a thin, elastic pellicle, which under high magnification shows a geometric pattern (Fig. 8.18) related to the regular distribution of the cilia and trichocysts (Fig. 8.19). Beneath the pellicle is the ectoplasm, from which the cilia and trichocysts originate. ‘The trichocysts, found also in many other ciliates, are structures of problematic function. In the paramecium they appear to serve as defensive organelles; when stimu- lated, they emit long threads. In other ciliates they aid in the capture of food. Fig. 8.18. Paramecium: structure of the pellicle. This is a photomicrograph of a special preparation which demonstrates the arrangement of the plate-like elements in the pellicle. These plates, and their rela- tionships to cilia and trichocysts, are shown in greater detail in Figure 8.19. (Photo- graph courtesy General Biological Supply House, Inc.) 249 GENERAL ZOOLOGY a Cilium Fig. 8.19. Paramecium: rela- tionship of pellicular structure Pore on to ectoplasmic — organelles. trichocyst (Adapted from E. E. Lund, 1935, University of California Publications in Zoology, SN tor” i ; apa Wi vol. 39, printed by permis- sion of the University of California Press.) Basal granule Body of trichocyst The greater part of the cell is composed of the endoplasm, which is suf- ficiently fluid to allow circulation of food vacuoles and other inclusions. ‘The definite shape of the paramecium depends, therefore, on the relative rigidity of the ectoplasm and the pellicle. Within the endoplasm lies the macronucleus, related to the metabolic or vegetative activities of the cell, and the micro- nucleus, concerned with heredity and reproduction. Paramecium aurelia has two micronuclei, whereas P. multamicronucleatum has many. ‘The endoplasm also contains two contractile vacuoles, one anterior and one posterior, and the food vacuoles. Larger masses of various sorts may also be found, in addition to the very small inclusions of the cytoplasm. Movement and Responsiveness. Locomotion in Paramecium is effected by the action of the cilia, which by coordinated beating propel the animal in a spiral course. An understanding of this process involves two problems: first, that of explaining the operation of individual cilia; and second, that of accounting for the integration of the activities of the individual cilia in such a way as to provide for directed locomotion. ‘The mechanism of ciliary action is not well understood, but it is clear that the physical state or configuration of the cilium must be altered between its “effective” stroke and its “recovery” stroke. It can be observed that the cilium is relatively stiff, and moves rapidly, during the effective or driving stroke, and that it becomes relatively limp and moves more slowly as it returns to its original position during the recovery phase (Fig. 8.20). The factors governing these changes presumably result from the interaction of the basal granule, or kinetosome, of the cillum and the axial filament, which springs from the kinetosome and runs the length of the cilium. Experiments have shown that a single cilium exhibits spon- taneous movements as long as its connection with the kinetosome remains intact. Without the kinetosome, the cilium is incapable of beating. This indicates that a capacity for initiating activity resides in the kinetosome. It seems reasonable to speculate that a common physicochemical mechanism 250 THE PROTOZOA may underlie all phenomena of protoplasmic contractility, including amoeboid movement and ciliary and flagellary action, as well as muscular contraction. Paramecium is propelled in its course by the beating of its thousands of cilia, but their beat is not random. Successive ‘‘metachronal” waves of action sweep smoothly along each row of cilia from the anterior to the posterior end of the animal, indicating that the ciliary beat is coordinated by some integrat- ing activity. ‘These waves can be interrupted by making transverse incisions through the ectoplasm of a paramecium; this demonstrates conclusively that the coordinating influences are conducted longitudinally through the ectoplasm. Looking for a structural foundation for these phenomena, we find that the kinetosomes of the cilia are interconnected in longitudinal rows by fibrillar strands, all of which are related to a central ectoplasmic structure near the mouth. It is difficult to avoid the conclusion that the coordination of beating in the rows of cilia is mediated through this fibrillar system, which has therefore been termed the neuromotor apparatus; the central structure from which the strands radiate is called the motorium. One further aspect of ciliary action in Paramecium and other ciliates remains problematic. In the “avoiding reaction,” to be discussed below, the animal stops and moves abruptly backward for a short distance. This does not, however, involve merely a reversal of the metachronal waves; rather, all the cilia immediately and simultaneously reverse their actions. Investigations of this phenomenon have yielded rather equivocal answers, but it appears certain that the reversal action is not mediated through the longitudinal ectoplasmic fibrillar system. The locomotion of Paramecium is a composite of three basic movements: progression, rotation, and swerving. ‘The animal moves forward, rotating on 12 11 10 Fig. 8.20. Ciliary action. A, outline R 2 of a ciliate, showing metachronal 8 waves of ciliary beat. B, phases of 7 action of a single cilium; schematic. ; : : : 6 E, 0-4: successive stages in effective stroke; R, 5-12: stages in recovery 5 phase. (Redrawn from K. von Frisch, 4 1952, Biologie, vol. 1, printed by permission of Bayerischer Schulbuch- 3 Verlag.) Es 2 1 0 A B Zo GENERAL ZOOLOGY Fig. 8.21. Paramecium: locomotion, feeding, and the avoiding reaction. A, the spiral, rotat- ing path described in swimming freely; the stippled cones represent the areas from which food particles are drawn into the oral groove. 8, behavior of an individual encountering an obstacle; six successive positions. (Adapted from H. S. Jennings, The Behavior of the Lower Organisms, copyright 1906 by Columbia University Press, printed by permission. ) its long axis; an aboral swerving motion of the anterior end, in combination with progression and rotation, causes the animal to describe a spiral course instead of simply spinning like a rifle bullet (Fig. 8.21). Paramecium generally reacts to stimuli by movement; therefore, variations of the locomotor patterns constitute its characteristic behavior. For example, its reaction to strong chemical stimulation involves swimming backward in a 252 THE PROTOZOA spiral for a considerable distance before resuming its forward progression. More commonly, in response to weaker chemical stimulation or to such a stimulus as contact with an obstacle, the animal exhibits a characteristic avoiding reaction. ‘This involves stopping, reversing for a short distance, rolling in a cone-shaped path, and moving forward in a new direction. If this new path brings the animal again into contact with the obstacle, the pattern may be repeated indefinitely until the obstacle is cleared (Fig. 8.21). Such trial-and-error methods constitute a large proportion of the behavior of Paramecium; but by their infinite repetition, no matter how blindly, a suit- able adjustment to environmental conditions can be effected. By thus demon- strating its ability to react to changes in its surroundings, the paramecium gives evidence of a capacity of responsiveness comparable with that found in other organisms. In connection with the behavior of ciliates, the question arises whether these animals can learn from experience. ‘There are accounts of what is claimed to be “learning,” and reports of what appears to be the exercise of “choice” in the acceptance or rejection of food particles, among ciliates. ‘To the extent that these reports are valid, the behavior of ciliates may be more complex than its usual trial-and-error features indicate. Feeding and Metabolism. In feeding, the cilia of Paramecium draw a cur- rent of water against the oral surface, so that particles like bacteria, smaller protozoans, algae, and organic debris enter the gullet. By means of the cilia, and by movements of the gullet, masses of this food included in a drop of water pass into the cytoplasm and are thus ingested. ‘The food vacuoles so formed move along a definite course within the cytosome carried passively by currents in a process termed cyclosis. As in Amoeba, it is assumed that enzymes are secreted into the vacuoles and bring about digestion. ‘The products of digestion are evidently transferred into the surrounding endo- plasm, since the vacuole finally contains only material to be egested at the anal spot. ‘The observations which can be made on Paramecium are similar to those described for Amoeba, and we reason similarly from them with the aid of knowledge concerning other animals. The products of digestion, passing out of the food vacuoles, are utilized during metabolism. Cellular respiration corresponds to the process in vertebrates; oxygen enters the cell directly from the surrounding fluid, and final breakdown of the cellular constituents occurs, with transformation of energy and formation of waste products. Excretion of the wastes of metabolism occurs chiefly by diffusion over the entire surface of the cell and to some extent by means of the con- tractile vacuoles. Under suitable conditions the storage of reserves such as glycogen and fat occurs in the cytoplasm. ‘The nutrition of Paramecium is, therefore, holozoic; and its metabolism is fundamentally like that of higher animals. Life Cycle and Reproduction. ‘The life cycle of Paramecium consists of an active phase, which may continue indefinitely in a suitable medium. ‘There is no encysted phase that may be commonly observed in the laboratory, al- 253 GENERAL ZOOLOGY Fig. 8.22. Paramecium: binary fission. ‘The macronucleus simply elongates and constricts, whereas the micronucleus divides by a mitotic process. though encystment has been described. Perhaps it occurs more frequently in nature, since it is difficult to understand how any protozoan can be so uni- versally distributed in fresh water without undergoing occasional encystment to survive periods of drought. Paramecium, however, does not appear to encyst upon aquatic vegetation; it is rarely, if ever, obtained by placing such vegetation in sterile water. In the laboratory the life cycle is an endless active phase with frequent reproduction by transverse binary fission, which is an asexual process. Periodic phases of nuclear reorganization, termed endomixis, also occur. Reproduction by conjugation, or temporary union of individuals with exchange of nuclear material, may also be observed. Some strains of Paramecium, however, appear capable of maintaining themselves indefinitely, by fission and endomixis, without conjugation. In the course of binary fission, by which reproduction is accomplished, the macronucleus divides amitotically by elongation and constriction; the micro- nucleus undergoes a kind of mitosis (Fig. 8.22). As division of the two nuclei nears completion, the cell body becomes constricted and finally separates into two daughter cells of equal size. Meanwhile, one new contractile vacuole has been formed for each daughter, and a new gullet has arisen in each from the oral region of the parent cell. After separation the daughter cells usually 254 grow to full size before the next division. THE PROTOZOA Under favorable conditions there may be as many as 4 divisions, with the production of 16 individuals, in 24 hours. ‘The rate of reproduction is determined by external conditions, such as food and temperature, and by internal factors. Although Paramecium aurelia and probably other species of the same genus may live indefinitely without conjugation, this process apparently occurs under natural conditions as well as in the laboratory. In some cultures J. G. . Two individuals unite by buccal grooves. The micronuclei separate from the macronuclei. . The macronucleus begins to degenerate. The micronucleus divides. . The micronuclei divide again. Three of each four disappear. . The remaining micronuclei divide to form migratory and stationary nuclei. Exchange of migratory nuclei. . The migratory and stationary nuclei unite. . The fusion nucleus is thus formed. The individuals separate. H. Division of the fusion nucleus. I. Division, as shown. Differentiation into macro- and micronuclei and disappearance of three micronuclei. K. Cells and nuclei divide as shown to produce the original condition. Fig. 8.23. Paramecium: sequence of events in conjugation and subsequent divisions. (Adapted from H. S. Jennings, Life and Death, Heredity and Evolution in Unicellular Organisms, copyright 1920 by R. G. Badger, printed by permission of Chapman and Grimes, Inc.) 255 GENERAL ZOOLOGY maintained for long periods (Calkins, P. caudatum), it was observed that conju- gation occurred at intervals of some 200 generations. In others (Woodruff, P. aurelia), it was found that conjugation did not occur, even in many thousands of generations. ‘The details of conjugation in P. caudatum, after the two individuals have fused together in the region of the buccal grooves, are shown in Figure 8.23, which should be carefully studied. ‘The process differs remarkably from the syngamy, or permanent fusion of gametes, which occurs in many other protozoans as well as in higher animals. Syngamy involves the permanent and complete union of two cell bodies and _ their nuclei; conjugation consists of a temporary union and exchange of nuclear material, and it is known to occur only in the Ciliophora. However, the net outcome is the same in syngamy and in conjugation. The result of syngamy is a single cell with nuclear contributions from two cells. In conju- gation, two cells unite temporarily, and after they separate, each ‘“‘exconju- gant” has a nucleus of double origin. Conjugation thus appears actually to be more efficient than syngamic fertilization, because the result of conjugation is two cells, each with a new combination of chromosomes. In fertilization the outcome is a single cell, the zygote. In endomixis the macronucleus and a considerable portion of the micro- nuclear material disappear. From a single persistent micronucleus, new macro- and micronuclei are formed. ‘Thus endomixis involves a nuclear reorganization comparable with that occurring during conjugation, but endo- mixis takes place within a single individual. Endomixis and binary fission are commonly regarded as asexual methods of reproduction; conjugation is considered a sexual process, although the two members of a conjugating pair are not obviously differentiated as male and female cells. In 1937, within the species Paramecium aurelia, a number of “mating types” were discovered which are related to the occurrence of conjugation. In each of several morphological varieties of this species two mating types were recognized; individuals conjugate only with members of the opposite mating type within their own variety. Similar phenomena have since been described for other species of Paramecium, and for other ciliates as well. The exist- ence of mating types may be taken as evidence of physiological differentiation between individuals, possibly related to the kind of differentiation which has led to the development of sexual dimorphism in other animals. ‘The proto- zoan mating types, however, are not opposite sexes in the usual sense, as indicated by the fact that the transfer of nuclear material between the two individuals is mutual. The full significance of conjugation in ciliates is still unsettled. It is clear that the resulting exconjugants are individuals with new chromosomal com- plements and new characteristics, as are the zygotes which result from fertili- zation in metazoans. ‘The question that has led to endless investigation and discussion for years is whether conjugation has an important physiological effect upon what may be termed cell vitality, as measured by the rate of cell division; upon the longevity of the race; and upon normal cell activities. 256 THE PROTOZOA Do these organisms grow old and die unless they are “rejuvenated” by conju- gation? In some species such a rejuvenescence seems to occur, if the experi- mental evidence cannot be accounted for on other grounds. At the very least, conjugation, like other kinds of sexual reproduction, provides for a species the advantages of new nuclear combinations which make for greater vari- ability and adaptiveness. Other Ciliates. The class Ciliata includes most of the species of large Protozoa occurring commonly in fresh water; therefore, this class will be reviewed by listing its principal subdivisions and the names of representative genera; some of the forms listed are illustrated in Figure 8.24. Class Ciliata Subclass Protociliata—cilia of equal length covering entire cell; leaf-shaped or ellipsoidal in shape; no cytostome; parasitic in intestine of amphibians and fishes. Opalina and Protoopalina. Subclass Euciliata—ciliation and shape of cell specialized as indicated in the several orders; typically free-living, but some species in each order parasitic. Order Holotrichida—cilia of approximately equal length and uniformly cover- ing the cell in most species; with or without a cytostome; without a special adoral zone of cilia. Amphileptus, Coleps, Colpoda, Didinium, Dileptus, Frontoma, Lacrymana, Lionotus, Paramecium, Prorodon, etc. Order Heterotrichida—cilia of cell surface small or reduced in numbers as compared with the specialized ciliation of the oral region. Nyctotherus, Spirostomum, Stentor, etc. Euplotes Lacrymaria Stylonychia Spirostomum Lionotus Fig. 8.24. Representative ciliates. Zor GENERAL ZOOLOGY G/ Fig. 8.25. Representative Suctoria, mature and immature stages. A, Acneta; B, Podophrya, feeding on small ciliates; C, ciliated juvenile stage of Podophrya; D, similar stage of 7 oko- phrya. (A, adapted from W. S. Kent, 1880, Manual of the Infusona; C and D, adapted. after A. Kahl, from R. P. Hall, Protozoology, copyright 1953 by Prentice-Hall, Inc., printed by permission. ) Order Oligotrichida—cilia greatly reduced in numbers and specialized; mostly parasitic and known principally from the digestive tracts of herbivorous mammals, but some free-living representatives. Duplodimum, Hallteria, Strombidium, etc. Order Hypotrichida—cilia scattered and highly specialized for locomotion and feeding; some as sensory organelles. Cell usually flattened and with what may be termed dorsal and ventral surfaces; typically creeping forms. Euplotes, Stylonychia, etc. Order Peritrichida—cilia usually restricted to a conspicuous disc-like oral region and a basal region at opposite end; more familiar forms attached to substrate by a contractile stalk. Carchesium, Epistylis, Trichodina, Vorticella, Zoothamnum, etc. The Suctoria The class Suctoria comprises a small group of Protozoa placed in the sub- phylum Ciliophora because cilia are present during the immature, motile 258 THE PROTOZOA phase of the life cycle. During the adult, attached phase of the cycle the cilia are replaced by structures called tentacles, used in feeding. Representative genera are E-phelota, Podophrya, and others (Fig. 8.25). “The mature animal is attached to the substratum by a stalk, and its tentacles radiate from the central cell body. Small organisms coming into contact with the knob-like ends of the tentacles are held fast. Apparently the tentacles digest their way through the surface of the captive. ‘The fluid contents of the prey may be seen later streaming down through the tentacles into the body of the suc- torian, as the prey, if it is small enough to be destroyed in this manner, slowly shrivels until released as a crumpled mass. Frequently a suctorian attacks ciliates much larger than itself, such as Paramecium, which is some- times seen swimming with a Podophrya attached. Reproduction in suctorians involves cell division of a peculiar type which is usually termed budding. In this process the nuclei divide, as in Paramecium, one set of daughter nuclei being pinched off with a bud of cytoplasm into a temporary cavity within the distal end of the adult body. Within this cavity the bud gradually enlarges and develops bands of cilia. When it is released, it swims about by means of these cilia for a short time, then settles to the substrate and develops the stalk and tentacles of an adult. A process of conjugation is also known for the Suctoria. Biogenesis Historical. Some general problems intimately related to unicellular organisms may now be examined. One such problem involves the origin of living forms, or biogenesis. As a result of investigations extending over more than 200 years, it was shown during the third quarter of the nineteenth century that abiogenesis, or the spontaneous origin of organisms, does not occur. Organisms come from pre-existing organisms by the processes of reproduction. It was natural for the ancients to believe that animals such as insects, which suddenly swarmed in certain places, were produced from the mud of the fields under the influence of the sun’s rays or arose spontaneously within the decomposing carcasses in which they were found. It was even supposed that mammals arose spontaneously within the female, although under the influence of seminal fluid from the male. The higher animals were known to have parents, but the nature of the continuity between generations was not comprehended, except as the eggs of birds and reptiles produced young and mammals gave birth to living off- spring. Gradually it was recognized that smaller animals also arise from eggs. The Italian naturalist, Redi, performed experiments (1688) which showed how maggots originate in meat from eggs laid by flies. He placed meat in jars, covering some with wire gauze and some with parchment and leaving others uncovered. Flies were attracted and laid their eggs upon the meat or upon the gauze. Maggots were seen to hatch from the eggs laid on 259 GENERAL ZOOLOGY the meat and to grow as they consumed the meat. The pupal stage and the emergence of the adult flies were observed. Maggots also hatched from eggs transferred from the gauze to the meat. The meat in the parchment-covered jars decomposed without the appearance of maggots. Redi made other obser- vations on the development of insects and reached the conclusion that all spontaneous generation was presumably due to the introduction of living ‘“‘serms” from without. In 1676, the Hollander, Antony van Leeuwenhoek, discovered with the microscope, which had recently come into use as a scientific instrument, what he described as “‘little animals observed in rain, well, sea, and snow water as also in water wherein pepper had lain infused.” Among other forms of life he observed some of the larger bacteria and many protozoans. During the eighteenth century the observations of Leeuwenhoek were extended by other workers until the important types of microscopic animals became known. Although larger organisms were seen to arise from eggs or seeds, it could still be believed that microorganisms arose spontaneously if conditions were suitable. ‘This belief was not unnatural in view of the sudden appear- ance of these forms in the great numbers often observed in laboratory cultures. Some biologists from Redi onward, reasoning by analogy with higher organ- isms, believed that microorganisms arose from pre-existing forms. Others clung to belief in spontaneous generation. In spite of repeated failures to find evidence of abiogenesis, the question was reopened on theoretical grounds by Pouchet in 1859. Final Establishment of Biogenesis. ‘The work of Louis Pasteur (1822— 1895) and his contemporaries, about 1860-1864, was stimulated by this re- opening of the problem. A series of brilliant researches by this great French- man, by the German Koch (1843-1910), and by others finally showed that even the smallest organisms arise by division from parent forms. Species of protozoans and of bacteria were followed stage by stage until the life cycles of representative types were known in their active and in their resting phases. The English physicist, Tyndall, during investigations upon light about 1876, studied the ‘‘floating matter of the air’ and showed that it teems with spores and other resistant stages of microorganisms which need only settle upon a proper medium to germinate. The English surgeon, Lister (1827-1912), and others who investigated the germ theory of disease as applied to surgery demonstrated that the germs found in wounds are not generated within the body but are introduced, as the spores or the active stages of such minute organisms may be introduced into a sterile culture medium. ‘The extension of these demonstrations and of the Cell Theory completed the overthrow of abiogenesis and established biogenesis as the true explanation of the origin of new individuals. The saying of an earlier time, omne vivum ex ovo, every living thing from an egg, and a later one, omnis cellula e cellula, every cell from a cell, express the facts as now established. The long controversy over biogenesis was related throughout its history to the observation that infectious diseases spread and multiply like living organ- 260 THE PROTOZOA isms. When it was discovered that organisms living as parasites are the causative agents in many such diseases, the basis for the observed facts became apparent. Still problematical, however, were the increasing number of in- fectious diseases in which no parasitic organisms could be discovered. In these cases it appeared that “something” was present which increased like an organism during growth; this entity came to be called a virus. A virus was shown to be something invisible, capable of passing through filters which retained bacteria. It was further discovered that although a virus could be transferred from cell to cell and continue its multiplication, it was unable to grow or multiply except in the living cells in which it was found. Diseases now known to be caused by viruses include the type of sleeping sickness called encephalitis, as well as hog cholera, poliomyelitis, parrot fever, smallpox, yellow fever, and a long list of plant diseases, notably the Fig. 8.26. ‘The virus causing influenza in man. ‘The particles are normally individual spherules, but in being prepared for this electron micrograph many have clumped together or have collapsed on the plastic film used to support the preparation. The line marked Ip represents 1 micron (0.001 mm) enlarged to the same degree as the virus particles. (Electron micrograph courtesy D. E. Philpott.) 261 GENERAL ZOOLOGY mosaic disease of tobacco. In addition, many types of viruses are known which attack the cells of bacteria; these are called “bacteriophages.” With the development of the electron microscope (see Fig. 2.4, p. 16), which makes possible the visualization of particles thousands of times smaller than the smallest ones visible with the light microscope, virus particles could be “‘seen” for the first time (Fig. 8.26). An individual particle has the general characteristics of a complex molecule, and viruses can be prepared as crystalline substances. In this “‘non-living,” crystalline state they are infective for living cells of suitable hosts. Infection with a virus disease involves the introduction of a few particles into the cells of the host. ‘These particles rapidly monopolize the metabolic processes of the host cell, shunting its synthetic mechanisms from the manufacture of the normal products of the cell to the duplication of virus nucleic acids and proteins. ‘This, of course, pro- foundly affects the vitality of the host cell, and the symptoms of the disease are the direct or indirect results of such interference with cellular metabolism. Studies with bacteriophage have revealed that bacterial cells sensitive to the “phage” may be disrupted within a few hours, releasing thousands of new virus bodies. On the other hand, a virus particle entering an insensitive bacterial cell may remain quiescent, behaving more or less like a normal gene of the cell and being transmitted like a gene from one generation of the host to another. The origin and possible relationships of viruses are unknown. By some, they are interpreted as modern representatives of an extremely primitive stage in the evolution of terrestrial life. If viruses are primitive, they may well represent a stage of life at which large, complex molecules of protein and nucleic acid first evolved the ability to metabolize in the presence of the proper substrate, and to organize the surrounding materials into duplicates of their own structure. According to this hypothesis, the metabolic require- ments of viruses, once plentiful everywhere, can now be found in the proper combinations only within living cells. Viruses have been called ‘‘naked genes,” and indeed their composition and activities, particularly the phe- nomenon of self-duplication, seem to be similar to those we associate with genes. All studies agree, however, in the conclusion that viruses contain no DNA (see p. 22); as in no other organisms known, the mechanisms of heredity in viruses appear to involve only proteins and RNA. Alternatively, the viruses may be regarded as extremely degenerate forms of life, which in adaptation to parasitism have reached the ultimate in parasitic reduction. By this interpretation, viruses have become so specialized and so dependent upon conditions within host cells that when removed from this crystalline state. Whatever their ce b) environment they revert to a ‘‘non-living,’ true nature and history, viruses are clearly on the boundary line between living and non-living matter. The intensity of present research into the characteristics and properties of viruses is a measure of the potential value of these studies in terms of the conquest of disease, as well as of the increase in our knowledge of the vital processes of living things. 262 THE PROTOZOA The Protozoan Cell Returning to the Protozoa, we may conclude that the members of this phylum are single cells which exhibit the fundamental capacities of metab- olism, responsiveness, and reproduction; they are, therefore, capable of going about the business of living as individuals which are single cells. Structurally speaking, an amoeba is an animal reduced to essentially the simplest terms. It should be recalled that, alternatively, the Protozoa may be regarded as non-cellular animals, as animals whose bodies are not subdivided into cells. However, the bodies of protozoans exhibit fundamental similarities to the cells of other organisms, both in their basic structure and in their physiological activities. “The question of the nature of protozoans is largely a philosophical one, but it seems reasonable to believe that both Protozoa and Metazoa arose by evolution from a common ancestry of single-celled forms. In their descent from such ancestry, the Protozoa have undergone specialization within the limits of a single cell, except insofar as species have arisen which consist of colonial aggregations of cells or individuals. “These colonial forms will be discussed in the next chapter. The Metazoa, on the other hand, have special- ized as many-celled individuals in which there is a division of labor between cells, and hence an unbalanced physiological state for the individual cell. There are species among the Protozoa whose cellular organization is far more complex than that of any metazoan cell, because specialization within the limits of the unicellular state is the unique direction in which the Protozoa have evolved. Yet the most complex of these protozoan cells can be regarded as single cells thus specialized and need not be considered as organisms having nothing in common with the cells of metazoans. Unfortunately, there is no fossil record which shows, like the record of vertebrate evolution, how unicellular and multicellular animals arose, or from what ancestry they were derived. The record does show that protozoans such as the Radiolaria were in existence at the period represented by the oldest known rocks containing animal fossils, and that the Foraminifera are only a little younger as a group. From this fact it may be presumed that there were simpler protozoans aeons before that early period, because morphological simplicity must logically antedate such complexity as that of the shelled amoeboid forms. A one-celled animal, no matter how specialized it may be- come as a cell, is obviously simpler as an animal than a many-celled one. From this standpoint the Protozoa are the simplest of animals, and they seem to be more like the ancestors of all animal life than any other animals now in existence. 263 CHAPTER ; MULTICELLULARITY, AND THE SIMPLEST METAZOAN ANIMALS: Mesozoa and Porifera In Chapter 8 we discussed representatives of the phylum Protozoa, modern descendants of the primitive one-celled animals; these have been limited in their specializations to the same one-celled plan of organization which must have characterized their remote ancestors. Earlier, it was pointed out that a significant epoch in the evolutionary history of animals was marked by the rise of forms consisting of many cells, each cell an integral unit playing its part in the economy of the individual, and in its special activities sub- servient to the whole complex. Without question, the evolution of multi- cellularity opened vast new potentialities and made possible the development of the Animal Kingdom as we know it. In view of the fundamental similari- ties demonstrated between the vital functions of Protozoa and those of higher animals, it appears most reasonable to assume that Protozoa and Metazoa had a common ancestry, and that the progenitors of both lines were one-celled organisms. In this chapter we shall consider the manner in which many-celled animals may be thought to have arisen from these unicellular forebears, and some implications of the multicellular condition for the further progress of the Metazoa. In addition, we shall discuss briefly two of the modern phyla, the Mesozoa and the Porifera. “These groups are so unlike the true Metazoa that they have been placed in separate branches, the Mesozoa and the Parazoa 264 Zs 4 PD “BOG Fig. 9.1. Solitary and colonial plant-like flagellates. 4, Chlamydomonas, a solitary individual. B, Gonum pectorale, a plate-like colony of 16 individuals. C, Pandorina morum, a solid, mul- berry-like colony. (Adapted from G. M. Smith, Fresh-water Algae of the United States, first edition, copyright 1933 by McGraw-Hill Book Co., Inc., printed by permission.) (see Fig. 7.4, p. 221). ‘The simplicity of their organization, however, appears to present features which may have been characteristic of early metazoans. Colonial Protozoa Although the typical protozoan is a single cell, as we have seen, there are many species of Protozoa, particularly ciliates and flagellates, in which many similar cells live together in groups, or colonies, during a considerable part of the life cycle. ‘The difference between these colonial protozoans and a metazoan lies in the relationship of the individual cell to the other cells with which it is associated. In the adult metazoan the cells can be classified as somatic cells and germ cells, depending on their relation to the reproductive process. ‘This classification is not hard and fast; but in general the somatic cells are specialized for the various functions of metabolism and responsive- ness, whereas the germ cells are specialized for sexual reproduction. During asexual reproduction in a metazoan, there may be formative cells which can be called reproductive insofar as they become an important source for the cells of the new parts. In asexual phases no cells that are comparable to the germ cells, with their strictly reproductive functions, may be present. Most 265 GENERAL ZOOLOGY important in the metazoan is the fact that the various kinds of somatic cells, as well as the germ cells, are dependent on the collective activities of the metazoan individual. ‘The cell of a metazoan is a unit subordinated to the activities of the multicellular whole, which is the organism. By contrast, the protozoan cell is an independent, self-sustaining individual. In most colonial protozoans, each cell of the colony is likewise an inde- pendent individual, so far as the fundamental capacities of metabolism, responsiveness, and reproduction are concerned. In such a species the colony eventually disintegrates by separation of its units, so that each cell goes its way, encysting, dividing, conjugating, or uniting with another cell in syngamy, until a new colony is formed again from a single cell by repeated divisions and by the remaining together of the daughter cells. Every cell of the colony is, therefore, as independent functionally as though there were no colonial stage in the life cycle. Such species are manifestly no more than aggregations of independent protozoan cells, each of which is sufficient unto itself, or physiologically balanced. In a few species of colonial Protozoa, however, the distinction can be made between somatic cells, which are destined to die, and germ cells, which can in a sense continue to live by contributing their substance to the next gen- eration, if they become gametes and unite in syngamy. A comparison of these species with other Protozoa, on the one hand, and with the Metazoa, on the other, makes clear the basic continuity in reproductive processes from one end of the Animal Kingdom to the other. Such a comparison also indicates that this primitive step in differentiation may have been fundamental in lead- ing to the development of truly multicellular types, with their characteristic wide additional specializations within the somatic cell line. A series of forms in the flagellate family Volvocidae will be used for this discussion, although this family belongs among the Phytomastigina or plant- like flagellates. Of course, no inference is intended that this series of forms marks the actual pathway of the development of multicellularity in animals. Rather, this alignment represents merely an arrangement of existing organ- isms into a series showing an increase in mutual interdependence between the individuals of the colony. ‘The many-celled condition in animals may well have arisen through some analogous process. The members of the family Volvocidae are colonial forms of varying size and complexity, the colonies in each species consisting of a specific number of component individuals. ‘These individuals strongly resemble a simple, non- colonial flagellate of the genus Chlamydomonas (Fig. 9.1), which is covered by a cell wall of cellulose, bears two flagella, and contains a prominent chloro- plast, a sensitive pigmented eye spot, and two contractile vacuoles. A simple colonial form of the genus Gonium may be composed of 4 of these units, or of 16, depending on the species. In Goniwm, the individuals are arranged in a flat plate, and as in all the Volvocidae the colony is held together by a mucilaginous matrix secreted by the individuals. Aside from the fact that the colony moves as a unit, propelled by the flagella which all lie on one 266 PRIMITIVE MULTICELLULAR ANIMALS side of the plate, the cells are independent in their functions, and hence each is physiologically balanced. ‘This is indicated by their behavior in reproduc- tion: asexually, each of the cells may divide repeatedly to form a small daughter colony, and these then separate and grow to the size of the parent colony. Sexual reproduction is brought about when the individuals of the colony separate as isogametes (gametes without differences in size) and unite in pairs to form zygotes. From the zygotes new daughter colonies arise by cell division, without separation of the daughter cells. Pandorina colonies consist of 8, 16, or 32 individual cells resembling Chlamydomonas, arranged in an oval mass (Fig. 9.1). Reproduction may be asexual, as in Goniwm, when each individual divides repeatedly to form a daughter colony. Sexual reproduction also occurs, involving either the fusion of isogametes (isogamy) or syngamy between different-sized gametes (aniso- gametes); a transition to anisogamy is indicated here. Eudorina is a colony of 8, 16, 32, or even 64 individuals (Fig. 9.2). Re- production occurs as in Pandorina, except that in sexual reproduction an- isogamy is the rule, the two kinds of gametes being produced consistently in different colonies. Pleodorina, sometimes considered a variety of /udorina, exists in two forms showing interesting differences. Whereas in the colonial species just de- scribed, all the cells of the colony are capable of reproduction, in Pleodorina wlinoisensis a group of 4 individuals at one pole of the spherical, 32-cell colony are smaller and are incapable either of asexual reproduction or of forming gametes (Fig. 9.2). Only 28 of the cells may thus be said to retain the physiological balance characteristic of all the cells in a Pandorina colony, for example. In another species, Pleodorina californica, only half the 64 or 128 cells retain the ability to reproduce; the other half have become, in effect, somatic cells. Fig. 9.2. Further development of colonial plant-like flagellates. A, Eudorina elegans, a hol- low, ovoid colony of flagellated individuals; the flagella have been omitted from this figure. B, Pleodorina illinoisensis, in asexual reproduction. The 4 “‘somatic cells,” or sterile indi- viduals, remain undivided, whereas each of the other 28 individuals (“germ cells”?) produces a daughter colony. 267 GENERAL ZOOLOGY Volvox itself stands at the apex of this series. A colony of V. globator, typical of the genus, consists of some hundreds or thousands of cells, arranged in a single layer about the periphery of a hollow ball (Fig. 9.3). Each of the individuals composing the colony lies in a polygonal segment of the common matrix. In favorable preparations; each can be seen to be interconnected with its neighbors by delicate protoplasmic bridges. ‘These interconnections presumably mediate coordinating influences which enable the relatively huge colony to progress through the water in a directed fashion. ‘They also make possible the maintenance of a measure of integration of the activities of the component individuals. Reproduction in Volvox is a function of only a rela- tively few individuals, which either enlarge and sink into the central cavity to produce daughter colonies by repeated asexual divisions or similarly en- large and transform into either eggs or sperm bundles. Syngamy between these anisogametes results in the formation of a zygote which secretes about itself a heavy cyst wall and is thus enabled to survive conditions which result in the death of the parent colony. ‘The significant feature of Volvox is the fact that, as in the Metazoa, only a relatively few cells retain the power of repro- duction and can be termed germ cells. ‘The vast majority of the component cells are concerned not with the maintenance of the species through reproduc- tion but only with the maintenance of the individual (or colony) through their metabolic activities. Such a loss of physiological balance on the part of the somatic cells is characteristic of the Metazoa; it is thus possible to regard Volvox, and in the same sense Pleodorina, as multicellular organisms rather than as colonies of protozoan cells. From our accounts of the Protozoa in the preceding chapter, it is clear that there are no gross discontinuities between the vital functions of Protozoa and those of Metazoa, aside from the fact that the protozoans carry on all their activities within the confines of a single cell. Consideration of the series of colonial protozoans just described emphasizes the fact that the development of colonial organization in at least one group of modern Protozoa has progressed to the point at which, in effect, multicellularity has been attained. ‘Thus it appears that the capacity of developing multicellularity is not lacking in protozoans. We may conclude that through some similar persistence of aggregations of one-celled animals, with the gradual emergence of the char- acteristics of an integrated organism from the collective activities of its com- ponent, originally independent units, a point was reached making possible the rise of more complex many-celled animals. As previously indicated, the next step involved in this progression, after the differentiation of somatic and germinal cells, is the further specialization of somatic cells into specific struc- tural and functional types, each capable of performing with added efficiency one or a few special functions. The two phyla now to be considered, the Mesozoa and the Porifera, repre- sent the simplest metazoan types known to exist. In these it will be seen that the specialization of somatic cell types, and the division of labor among them, has progressed to a considerable degree. Unfortunately, in the absence of 268 Fig. 9.3. Volvox aureus: A, surface view of a portion of the colony; B, sectional view of the edge of a colony; C, parent colony rupturing, releasing young daughter colonies. (A and bemattens Cam anette O12. Le Volvox; C, adapted from K. von Frisch, 1952, Burologie, vol. 1, printed by permission of Bayerischer Schulbuch- Verlag.) PRIMITIVE MULTICELLULAR ANIMALS 269 GENERAL ZOOLOGY types intermediate between highly organized colonial Protozoa and_ these simple Metazoa, we must bridge the evolutionary gap with logical conjecture, reasoning from the known facts to supply the missing information. The Phylum Mesozoa All Mesozoa are parasites within the bodies of other animals. ‘They may be defined as animals consisting of an outer syncytial or cellular layer, com- monly ciliated, which encloses one or more cells giving rise to the gametes and to another type of reproductive cells called agametes. ‘The life cycle is complicated and apparently includes asexual and sexual generations, which alternate. ‘The phylum includes the single class Moruloidea, which contains the order Dicyemida and the order Orthonectida. ‘The members of this phylum are the simplest of all the truly many-celled animals. ‘This simplicity, how- ever, is perhaps an outcome of degeneration, since all Mesozoa are parasitic during the greater part of their life cycles. Parasites commonly show struc- tural simplification, as compared with their free-living relatives, and the Mesozoa may have degenerated greatly in the course of their evolution. Such modification may have gone so far that it would be impossible to identify the free-living type from which the Mesozoa have evolved, even if this type were still in existence. Many zoologists regard the Mesozoa as greatly degenerated flatworms, but there is no clear evidence for an evolutionary origin of this sort. ‘The Mesozoa constitute a small but well-defined group, important because its members possess a simpler organization than that of any other group of many-celled animals. They can, therefore, be taken at their face value as the simplest metazoans of the present day, even though they may have arisen from more complex ancestors and become simplified as a result of their parasitic existence. Structure and Life Cycle. ‘The dicyemids, as members of the Dicyemida are called, occur as parasites in the excretory organs (nephridia) of squids and octopi. They are small, elongate animals, a few millimeters in length, con- sisting of very few cells, often a total of less than 25 (Fig. 9.4). An outer layer of these cells, ciliated, encloses an inner axial cell or cells from which the reproductive cells arise. [he outer or somatic cells are differentiated into a head region and a trunk region. ‘The structure of a dicyemid is thus ex- tremely simple, but the life cycle is complex and is not known completely for any single species. Apparently, from the single axial cell, many cells are formed which are called agametes because, although they are not ova, they develop without fertilization. This is an instance, rare among Metazoa, in which a single cell, which is a germinal cell but not a gamete, is capable of producing the new generation. Development of the agametes gives rise to numbers of asexually reproducing individuals, the nematogens. Eventually, a new type of individual, the rhombogen, appears and produces still another form, the infusorigen. Remaining within its parent rhombogen, the in- 270 Fig. 9.4. Mesozoans. A through E are selected stages in the life cycle of Dicyemida, occurring in the excretory organs of cephalopod mol- lusks. F through Hare sexually reproductive phases of various Orthonectida. 4, immature dicyemid in a very young squid. 8, later stage, containing masses of cells developing from agametes to produce nematogens. CC, nematogen. D, nematogen in optical section, showing the internal cells which give rise to further generations of nematogens, or to other stages known as rhombogens and infusorigens. ££, ciliated in- fusoriform larva which leaves the host. Beyond this point the life cycle is unknown. F, fertilization in an ortho- nectid. G, mature female, and H, mature male. The larval stages of these forms are masses of cells (plasmodia) occurring in a variety of in- vertebrate hosts. (Redrawn, after various authors, from L. H. Hyman, The Inverte- brates; Protozoa through Cte- nophora, copyright 1940 by McGraw-Hill Book Co., Inc., printed by permission.) PRIMITIVE MULTICELLULAR ANIMALS 271 GENERAL ZOOLOGY Fig. 9.5. ‘Types of sponges. A, Calcarea: Scypha, three living — individuals. 13%, Hexactinellida: portion of the skeleton of a glass sponge, Euplectella. C, Mon- axonida: Mucrociona. (A and C, photo- graphs by George Lower; B, photograph by Bassett Maguire, Jr.) fusorigen develops male and female gametes. ‘These unite in syngamy, and the divisions of the zygotes thus formed give rise to numerous infusoriform larvae. ‘The larvae are liberated from the body of the parent (and grand- parent) and leave the host, but their further history is completely mysterious. They survive only brief periods in sea water and are apparently incapable of infecting squids and octopi. It thus appears that an unknown intermediate host must be involved in the cycle but, despite the efforts of many com- petent investigators, further information is unavailable. In the Orthonectida, the life cycle also includes parasitic asexual phases; these are more or less structureless, multinucleate masses of protoplasm in- habiting a variety of invertebrate hosts. In addition, free-living males and females are known which produce sperm and egg cells. ‘These cells are re- leased into the sea and unite in syngamy to form zygotes, which develop into ciliated larvae resembling those of dicyemids. ‘The larvae are infective for the invertebrate hosts, and, having gained entry, transform into the asexual form (Fig. 9.4). The Body Plan and Life Cycle. ‘To generalize, the mesozoan body consists of a small number of body cells, surrounding a parent germ cell capable of producing many agametes. ‘The agametes are single cells, each of which can reproduce a new individual. In some and perhaps all species gametes are eventually produced, so that there is an irregular alternation of sexual and asexual generations. Although the mesozoan is hardly more complicated than some colonial protozoans, it is clearly a metazoan animal. Even if its simplicity has resulted secondarily from a parasitic mode of life, it is an example of a very lowly type of metazoan. If its simplicity is really primitive, the mesozoan type is very important because it suggests a possible step in the evolution of many-celled from single-celled animals. Die PRIMITIVE MULTICELLULAR ANIMALS A significant difference in the organization of the Mesozoa as compared with that of the more advanced Eumetazoa is the nature of the inner cell layer. Only in the Mesozoa is the inner cell mass exclusively reproductive; in higher Metazoa the inner cells are concerned with nutrition. The Phylum Porifera Porifera are the simplest multicellular animals constituting a well-defined group which includes a considerable number of species. “They may be char- acterized as Metazoa with tissues of a very simple sort but without organs; with a more or less extensive system of internal cavities but without a digestive cavity or enteron; and usually with an internal skeleton. ‘The name Porifera (‘pore bearers’’) is derived from the many small openings upon the exposed surface, through which water enters on its way to the internal cavities, and the fewer large openings through which this water is expelled. The phylum Porifera includes three classes (Figs. 9.5 and 9.6): the class Calcarea, in which the skeleton is calcareous and in which are grouped the simplest sponges, such as Leucosolenia and Scypha; the class Hexactinellida, with skeletons of glass; and the class Demospongiae, in which are included the natural fibrous bath sponges and related forms. In general the skeletons of sponges are composed of minute spicules, of proteinaceous fibers as in the bath sponge, or of both spicules and fibers. ‘The spicules of sponges are often of such characteristic shapes that families and even genera may be identified by spicules alone. ‘These spicules, like the skeletons of Radiolaria and Foramini- fera (p. 236), occur in the debris upon the bottom of the ocean and are often found in sedimentary rocks. Fragments of sponges, as well as isolated masses 273 GENERAL ZOOLOGY © FOF ~ ~ & ” ie > -& , se =i —-. & * Es ’. =e j , re Fig. 9.6. Types of sponges, continued: Keratosa. A, living bath sponge, Spongia. B, a horny sponge, Hircinia, in its natural coral reef habitat. (Photographs by John F. Storr.) of spicules, occur as fossils in Pre-Cambrian and later deposits; the evolutionary history of sponges thus covers more than half a billion years, and no group of animals has a longer fossil record. All sponges are marine, except for a very few fresh-water forms, and most occur in shallow water. ‘They are typically firmly attached to the substrate at maturity, but there is a free-swimming, flagellated larva in the early develop- mental stages. Because of their attachment and manner of growth, sponges were first classified as plants. Later, because of their possession of flagellated cells with collars, they were regarded as protozoan colonies. ‘They were later grouped with the Coelenterata, and it was not until the beginning of the present century that they were finally given their current taxonomic position as members of a unique and aberrant phylum. The most familiar example of a sponge is Spongia, the fibrous skeleton of which is the natural bath sponge of commerce. A sponge of this type may be regarded as a colony of individuals, although the boundaries of these in- dividuals are indefinite. “To understand the organization of sponges, we must begin with forms much simpler than Spongza. General Structure. The Olynthus. ‘The structure of sponges is best explained by first describing the fundamental type from which all sponges have probably arisen in the evolutionary history of the phylum. ‘This type, which is called the olynthus, was originally considered an adult sponge and was given the generic name Olynthus. It is now known to be a late stage in the development of certain species. An olynthus (Fig. 9.7) is a cylindrical organism, attached at its basal end, with an opening, the osculum, at its free 274 PRIMITIVE MULTICELLULAR ANIMALS end and an inner cavity known as the spongocoel. ‘The walls of this hollow cylinder are perforated by incurrent pores (Fig. 9.8); these are intracellular canals passing through the cytoplasm of cells called porocytes. ‘The external surface, and the distal surface of the spongocoel just within the osculum, are covered by a dermal epithelium of flattened cells. The remainder of the spongocoel is lined by a single layer of flagellated cells called choanocytes because of a protoplasmic collar surrounding the base of the flagellum on each cell. Between the dermal cells and the choanocyte layer is a middle region containing the spicules, with the cells that secrete them, the scleroblasts, and connective-tissue cells (Fig. 9.9). Wandering cells called amoebocytes are most numerous in this middle region, but they may occur in any part of the body. ‘These migrate by amoeboid movements like the white blood cells of a vertebrate. ‘The archaeocytes, a large type of amoebocyte, are also found Osculum Fig. 9.7. Olynthus stage of a calcareous sponge; diagrammatic. (Redrawn from E. Haeckel, 1872, Die Kalkschwamme.) 275 GENERAL ZOOLOGY Fig. 9.8. Diagrammatic _longi- tudinal section of an asconoid sponge. (Adapted from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 Pore by McGraw-Hill Book Co., Inc., printed by permission. ) Spongocoel Choanocytes Spicules here. ‘These are considered totipotent cells and have been described as cap- able of differentiating into all the other cell types, including gametes. More Complex Sponges. ‘The simplest adult sponges, such as Lewcosolenia, arise by budding and growth from an olynthus. More complex sponges are modified in a great variety of ways, but their units of organization, the canal systems, can be derived from a type like the olynthus. A series of these modi- fications is shown diagrammatically in Figure 9.10. ‘The primary type of canal system found in the olynthus is termed the asconoid type. A second type, called syconoid, found in such sponges as Scypha, is actually derived in development by the folding of the wall of an olynthus stage and subsequent differentiation. Additional cavities are thus formed within the sponge, and the course of water from the exterior to the osculum becomes more com- plicated. ‘The openings in the surface of Scypha are not the same as the pores of the olynthus, which correspond rather to those leading from the so-called incurrent canals to the excurrent canals. In the adult Scypha these pores are no longer intracellular channels; the porocytes of the olynthus stage dis- appear during development. Other changes involve the restriction of the 276 PRIMITIVE MULTICELLULAR ANIMALS choanocytes to the linings of the excurrent canals and the extension of the dermal epithelium to line the entire spongocoel. “The homologies between the asconoid and syconoid types are clear, however. ‘The third, or leuconoid, type of canal system can be derived by folding of the wall of the syconoid type with its two sets of canals, and by the development of extensive subdermal spaces within which water circulates on its way to the flagellated chambers. ‘The most highly organized sponges have very elaborate canal systems and small, spherical flagellated chambers, but all can be compared, in the manner in- dicated, with the simple arrangement in the asconoid type. ‘These higher sponges are further complicated by increase in the number of oscula and spongocoels, each the center of a canal system, and by the indefinite growth of the entire mass. aa) oes cell Fig. 9.9. Stages in the formation of a triradiate spicule by scleroblasts in a calcareous sponge. Individual cells come together to form a “trio”; each cell then divides, and the resulting “sextet” (A) forms and finishes the spicule (B and C). Among the cells of the sextet, basal and apical cells may usually be distinguished. (Adapted from E. A. Minchin, 1898, Quarterly Journal of Microscopical Science, vol. 40.) 277 GENERAL ZOOLOGY vary t ; Re { \ \ capone be i | = eae S Spongocoel oe AY lf \ Spongocoel oa : S Es Ostium ay ey ce Incurrent oS a = o ° 3 = 4— canal fe} —— a D fe a3 Excurrent canal eS ee : E Excurrent canal D Fig. 9.10. Comparison of various types of sponge structure; diagrammatic. ‘The dark areas represent choanocyte layers; arrows indicate direction of water currents. A, asconoid type, with spongocoel completely lined by choanocytes. 8B, simple syconoid type, with choanocytes in excurrent canals only. C, more complex syconoid type, with thickened walls and narrowed openings into incurrent canals. JD, primitive leuconoid type, with excurrent canals in groups opening into branches of the spongocoel. &, more advanced leuconoid type; the choanocytes are restricted to small, spherical flagellated chambers (fc) which are considerably removed from both the outer surface and the spongocoel. (Redrawn with modifications from E. A. Minchin, in Lankester’s Treatise on Zoology, 1900.) Few sponges are symmetrical like Leucosplena and Scypha, although the radial symmetry of these genera and of the olynthus appears to be the primi- tive state from which all sponges have been derived in the evolution of the phylum. Moreover, there may be great differences in shape among the in- dividuals of the species, for the growth of a sponge is much influenced by con- ditions in the immediate environment. Metabolism. ‘The food of sponges consists of microorganisms and particles of organic detritus which enter the canal systems with the inflowing water 278 PRIMITIVE MULTICELLULAR ANIMALS and are ingested by the choanocytes, which are also responsible for main- taining the water currents. In view of the rapid growth of many sponges, the nutrition is evidently very effective. “The metabolism of sponges, which be- cause of the structure of these animals is difficult to study, is assumed to be similar to that of other animals in which it is better understood. When a sponge is exposed to a suspension of carbon particles (India ink) or carmine granules in sea water, some of the particles enter the surface open- ings with the currents of water. Later, when bits of the sponge are examined, the granules are found within certain of the cells, just as such particles are seen after ingestion by a paramecium (Fig. 9.11). Unicellular organisms have been observed undergoing ingestion in a similar manner in sponges. Diges- tion is undoubtedly intracellular, as in Protozoa. In the calcareous sponge Scypha, the food particles are ingested chiefly by the choanocytes and are then passed into nearby amoebocytes. Since the amoebocytes are migratory and are capable of differentiating into other cell types, the food may thus be distributed to all parts of the sponge. Storage of reserves, carbohydrates and fats, occurs in modified amoebocytes. Excretory processes occur at the cellular level, and soluble excreta and carbon dioxide are readily removed by the cur- rents of water passing through the canal system. An abundant supply of oxygen for respiration is provided by the same water currents. Responsiveness. ‘Vhe flow of water into the minute openings upon the sur- face, through the canal systems and flagellated chambers to the spongocoel, and out the osculum is the factor that conditions all other activities of the sponge. ‘This flow is maintained by the flagella of the choanocytes. ‘The steady and strong currents so produced, and the directed nature of the cur- rents, have been found to depend on rather precise adjustments of the rela- tive diameters of incurrent and excurrent pores and canals. In sponges, the only easily demonstrable reactions to stimuli are the closing of pores and oscula and contractions of entire masses of cells, which may temporarily obliterate the canal systems. It is to be supposed that less violent reactions to various stimuli involve slight constriction or expansion of pores and canals, to bring about physiological adjustments of the volume and velocity of water flow. ‘The presence of special contractile cells surrounding water passages has long been recognized (Fig. 9.12), but these were thought to operate as independent effectors, responding directly to stimulation without the inter- Fig. 9.11. ‘Transfer of ingested particles from choanocytes to an amoebocyte. (Re- drawn, after N. Pourbaix, from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 by McGraw-Hill Book Co., Inc., printed by permission.) 279 GENERAL ZOOLOGY Contractile cells Fig. 9.12. Mechanisms of reception, con- duction, and response in a calcareous sponge. Stimulation of choanocytes sets up nerve impulses which are conducted through a nerve net to effectors, such as contractile cells surrounding a canal. (Adapted from O. Tuzet, R. Loubatieres, and M. Pavans de Ceccatty, 1952, Comptes rendus de lacadé- mie des sciences, vol. 234.) Neurons Choanocytes vention of specialized nervous elements. Recently, students of sponges have demonstrated that a primitive type of nervous system does exist, in a wide variety of different types of sponges. ‘This system appears to consist basically of a diffuse network of typical neurons, with processes connecting the choanocytes with contractile cells surrounding parts of the canal systems (Fig. 9.12). In addition some sponges possess large and highly specialized nerve cells which are lodged singly in vacuoles within the mesenchyme, and others which appear from their structure and connections to function as neurosensory cells. ‘These new facts make it clear that coordination and re- sponsiveness in sponges depend on the activities of cells specialized for recep- tion of stimuli and conduction of impulses; presumably their functions are similar to those of the more highly developed nervous systems of more ad- vanced animals. ‘The reactions of sponges are limited and sluggish but are adequate for the needs of sponges, which are all either attached or incapable of locomotion as adults. Reproduction and Development. Most sponges appear to be monoecious (‘one house’’), or hermaphroditic, and thus are capable of producing both male and female gametes. ‘The eggs and sperms are frequently produced at different times in a single individual, however, making self-fertilization im- possible. In sexual reproduction the zygote develops within the parent into a flagellated larva which is discharged through the osculum with the outgoing water. After a brief period of free life, the larva becomes attached and under- goes a peculiar type of development, in the course of which the flagellated 280 PRIMITIVE MULTICELLULAR ANIMALS external cells move to the interior and produce the choanocytes which line the spongocoel or the flagellated chambers (Fig. 9.13). In simpler forms, the olynthus stage is passed through; in more complex sponges, development may be more direct. ‘The final stages may include much budding and growth, often leading to the formation of a large mass. ‘This budding is comparable with the asexual reproduction occurring in other multicellular animals, al- though it is often difficult to distinguish from the general process of growth. The fresh-water sponges, and some marine forms, produce internal buds, termed gemmules, which are covered with resistant membranes and can sur- vive severe conditions such as freezing and drying. In various other sponges, under adverse conditions, so-called reduction bodies are formed. ‘These con- sist of masses of amoebocytes surrounded by dermal cells. They are less resistant than gemmules, but they serve a similar purpose in preserving the species through periods during which normal life would be impossible. With the return of favorable conditions, the gemmules or the reduction bodies can produce fully developed individuals. Regeneration and Reassociation. Regeneration, by which lost parts are restored and even whole individuals are formed from small pieces, is com- monly associated with conspicuous powers of asexual reproduction. The ex- tensive budding and vegetative growth of which sponges are capable would lead us to expect greater powers of regeneration than seem to exist in these animals. Some sponges will regenerate from cuttings, and artificial propaga- tion of commercially valuable sponges has been attempted, without marked success, by planting small cuttings in favorable locations. In at least a few types of sponges there is a remarkable capacity of re- association of cells after the organization of the body has been completely disrupted. For example, pieces of Mucrociona prolifera, the common red Fig. 9.13. Flagellated — amphi- blastula larva of a_ calcareous sponge, Grantia compressa. The developing sponge reaches _ this stage before it is released from the parent. After a brief free- swimming period, it settles to the substratum and transforms into the sessile adult form. (Redrawn from O. Duboseq and O. Tuzet, 1937, Archives de zoologie expeén- mentale et générale, vol. 79.) 281 GENERAL ZOOLOGY sponge of the Atlantic coast, can be squeezed through silk bolting cloth of very fine mesh, so that the cells of the sponge are almost completely separated or dissociated. If these dissociated cells are allowed to settle upon the bottom of a dish of sea water and remain undisturbed, they immediately begin to coalesce into small multicellular masses. ‘These masses continue to fuse, forming thin encrustations over the substratum, and under favorable condi- tions large sponges like the original will eventually be formed. The phenomenon of reassociation involves very interesting problems. _ It demonstrates with remarkable clarity the relatively loose organization char- acteristic of the sponge body; what is particularly remarkable is the fact that the capacity of reforming their original organization resides in isolated cells. A similar ability to reassociate occurs to a limited extent in some simple coelenterates, and such phenomena can be demonstrated in embryonic stages of more highly specialized animals (p. 167). The loose association of cells in the sponge body, together with the rela- tively slight degree of specialization exhibited by the somatic cells of sponges, is a cogent reason for considering sponges to be at the cellular level of organi- zation, in contrast with the unicellular protozoans and the tissue-grade organ- isms of more advanced phyla. The Sponge Body Plan To speak of the sponge body raises the question of what constitutes the in- dividual in animals of this phylum. In the olynthus, or in such a sponge as Scypha, an individual with a single osculum is recognizable. In other sponges, which grow into large masses with many oscula, the entire mass may be called an individual; or, the individual may be defined as any part of the entire mass that includes an osculum and its related canal system. However we define the individual sponge, the body plan basic for the phylum is the one seen in the olynthus stage and in its simpler derivatives. In the cellular organization of the sponge body, there are tissues of a simple kind but no organs. The early stages of development are peculiar, and it is difficult to reconcile the layers of the sponge body with the embryonic germ layers of more complex animals. Finally, the sponge has no internal cavity homologous with the digestive cavity or enteron of higher animals. ‘This last feature of bodily structure, together with the primitive cellular organization and the peculiar mode of development, gives the phylum its unique position among the Metazoa. Evolutionary Significance of Mesozoa and Porifera The fact that both Mesozoa and Porifera are multicellular organisms and show some cellular differentiation warrants placing them among the Metazoa. 282 PRIMITIVE MULTICELLULAR ANIMALS However, their peculiarities of organization and their simplicity clearly in- dicate that, phylogenetically, they antedate all other members of this sub- kingdom. (In this statement we are assuming that the Mesozoa are primarily simple, and not secondarily simplified organisms.) Both may be assumed to have arisen from somewhere near the point of origin of multicellular animals from unicellular ancestors, and the history of both groups, particularly of the sponges, is marked by a certain amount of evolutionary progress. ‘This progress has advanced in different directions in the two groups, but in neither has the path led in the direction taken by more progressive animals. No metazoans other than Mesozoa, for example, possess as their only internal tissue a cell mass specialized exclusively for reproduction; none but the sponges has as its only internal cavity a system of water chambers lined by choanocytes. It therefore seems reasonable to conclude that the three simplest metazoan phyla, Mesozoa, Porifera, and Coelenterata (which will be discussed in the next chapter), represent groups which evolved independently, and at different times, from primitive multicellular ancestors. Of the three fundamental body plans characteristic of these groups, those of the Mesozoa and Porifera were evidently so limited in evolutionary possibilities that only the modern repre- sentatives of these same phyla can be traced to them. ‘The basic coelenterate plan, with an internal digestive tissue later excavated to form a_ hollow enteron, was apparently sufficiently adaptable to the needs of animals to serve as the basis for further evolution. 283 per, RADIAL ANIMALS: The Phyla Coelenterata and Ctenophora The phyla Coelenterata and Ctenophora contain the relatively simple types of animals characterized by radial to biradial symmetry and by a generally primitive plan of organization. ‘This plan centers about a single internal digestive cavity, typically with a single opening to the exterior. ‘The bodies of these animals are frequently composed of only two cellular layers—one covering the external surface, the other lining the digestive cavity. Cellular differentiation has given rise to tissues, but the organs that are present are very primitive. Although the coelenterates and ctenophores are sometimes considered as subphyla within a single phylum Coelenterata, their funda- mental differences justify placing them in separate phyla. The Phylum Coelenterata The Coelenterata are the simplest many-celled animals possessing the diges- tive cavity characteristic of all Metazoa except mesozoans and sponges. ‘The digestive cavity with its single opening, the mouth-anus, is responsible for the name Coelenterata, literally “hollow intestine.” Coelenterates may be defined as radially symmetrical animals consisting in the simplest cases of two layers of cells, epidermis and gastrodermis, separated by a non-living, non- cellular, secreted supporting lamella. ‘This is true only of the most primitive forms; all others have developed a middle layer which contains cells and is therefore comparable with the mesenchyme, of mesodermal origin, in higher animals. Although this third layer is rudimentary at best, its presence casts doubt on the common general statement that coelenterates as a group are 284 Fig. 10.1. Pelmatohydra oligactis in its normal attitude, attached to a submerged leaf, with tentacles extended. Note the attenuated, stalk-like basal portion of the trunk. (Re- drawn from L. H. Hvman, 1930, 7 ransactions of the American Microscofrcal Society, vol. 49.) diploblastic. The typical radial symmetry has in some cases become biradial, with two centers of symmetry rather than one. Coelenterates are always pro- vided with stinging capsules, the nematocysts, structures which are not known to be produced by any other animal group. ‘The organs which occur in this phylum are simple in structure and function. As compared with sponges, coelenterates exhibit more definite form and symmetry and a more advanced level of organization. The phylum includes three classes: the class Hydrozoa, containing the hydras, hydroids, hydromedusae or hydroid jellyfishes, and hydroid corals; the class Scyphomedusae, including jellyfishes of a more advanced type; and the class Anthozoa, the sea anemones, sea pens, sea fans, and true stony corals. Coelenterates are predominantly marine animals, occurring at all depths in the ocean and under a wide range of conditions, since the phylum contains both attached and free-swimming forms. ‘Typically, however, they are in- habitants of the shallow waters along shore and of the upper layers of the deeper ocean. In their feeding habits they are strictly carnivorous, capturing other animals by means of tentacles armed with nematocysts, from which paralyzing threads are emitted. Most coelenterates are attached for a con- siderable part of the life cycle, during which they commonly reproduce by budding, and colonies of innumerable individuals may be produced. In Hydrozoa a free-swimming sexual phase may alternate with the attached, asexual stage; in Scyphomedusae the free-swimming jellyfish stage constitutes the major part of the life cycle; but in Anthozoa both asexual and sexual reproduction occurs during the attached phase, for no free-swimming repro- ductive individuals are produced. The radial symmetry characteristic of coel- 285 GENERAL ZOOLOGY enterates is presumably related to the attached growth habit, since radial symmetry is commonly associated with a fixed mode of life or indicates descent from attached ancestors. The fixed growth habit, together with the radial symmetry, led early naturalists to classify the coelenterates as plants. Their animal nature became apparent to zoologists in the eighteenth century when it was recog- nized that the organisms have mouths, tentacles, and digestive cavities. How- ever, Aristotle’s view that they were intermediate between plants and animals persisted until well into the nineteenth century and is reflected by the inclu- sion of the coelenterates, with the echinoderms, in a group called Radiata or Zoophyta (“animal-plants”’). Only when it became clear that the structural organization of coelenterates is vastly simpler and more primitive than that of echinoderms was the modern phylum Coelenterata finally established. Remains of coelenterates such as hydroids and corals appear in some of the earliest fossil-bearing rocks. Even jellyfishes have left a fossil record as thin, delicate carbon-film impressions in ancient sedimentary beds. Thus, the early evolution of coelenterates long antedates the beginning of our fossil record, and we have no direct information about their antecedents. Phylogenetic speculation must be based on the facts of development and probable interrela- tionships deduced from the life cycles of modern representatives. Although they are by no means “‘typical” hydrozoans, the fresh-water hydras, long studied by zoologists, are widely distributed, easily obtainable, and relatively simple representatives of the phylum. They are also repre- sentative of the metazoans higher than sponges, for they have a digestive cavity and cell layers comparable with these features of the more advanced animals. Because of its relative simplicity, the hydra is particularly well suited to illustrate the structure of a metazoan. Accordingly, we shall ex- amine the hydra as a coelenterate, but more especially as a simple metazoan, to be compared in structure and function with the vertebrate and with the protozoan. THE CLASS HYDROZOA The Hydra: Habitat and Activities. Several species of the genus Hydra occur in the United States. Related hydras are placed in different genera because of special characteristics. A common brown form, with very long tentacles and with its trunk region differentiated into a slender basal stalk and a stouter body, has been designated Pelmatohydra oligactis (Fig. 10.1). A green hydra, deriving its distinctive color from the presence of alga-like zoochlorellae in cells lining its digestive cavity, is recognized as Chlorohydra viridissima. Unless specific exceptions are noted, the following account is applicable in general to any type of hydra. The individual hydra is usually found attached by its base to submerged objects, with its body extended. It may float freely, however; in an aquarium 286 COELENTERATA AND CTENOPHORA it is often seen attached to the surface film, hanging down into the water. Although there are no special organs of locomotion, the animal shifts its position by simple movements of the body or by a slow gliding of the base without detachment from the substratum. Hydras react positively to light of moderate intensity and so tend to collect in the lighted portion of an aquarium. ‘This reaction is especially noticeable in the green hydra. But let us defer further consideration of habits and general behavior until the struc- tures involved have been described. General Structure. ‘The body of a hydra consists of a simple two-layered tube (Fig. 10.2), the trunk, normally attached at one end, the base, and surmounted at the other by a circle of tentacles, varying in number. ‘The tentacles enclose a conical region, the hypostome, which bears at its apex the mouth. The body wall surrounds a digestive cavity or coelenteron, which extends into the tentacles. The cell layers, an outer epidermis and an inner gastrodermis, are separated from each other by a non-cellular supporting la- mella. ‘This structure corresponds functionally to an elastic skeleton. It serves as a place of attachment for the cells and gives support and elasticity Epidermis Supporting lamella Gastrodermis Bud fee AeTeAT seesouee Na AA WAZA! Coelenteron = GAWADEGAATON ie a Oar INS ¥' RE NS UII NYS a iRGchiatinnSN Fig. 10.2. Hydra: general structure, shown by a diagrammatic longitudinal section of the body. Sieh Sy 287 GENERAL ZOOLOGY Epidermis Gastrodermis Epidermis 4 Muscle fibrils Gastrodermis = : “ 74, of epidermis =) Nematocysts Supporting lamella a Interstitial - cells Epidermal gland cells Gastrodermal Supporting Food vacuoles muscle fibrils fibrils Myoneme or muscle fibril Fig. 10.3. Details of cellular structure in the hydra, as shown by cross sections through dif- ferent regions of the body. A, in the stalk region, with very vacuolated gastrodermal cells. B, through the hypostome, showing glandular gastrodermal cells. C, through the pedal disk, with glandular epidermal cells. D, through the “stomach” region, showing digestive gland cells and food vacuoles. £, detail of a large epidermal epitheliomuscular cell. (A—D, re- drawn from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 by McGraw-Hill Book Co., Inc., printed by permission; £, after J. von Gelei.) to the entire organism. In hydrozoan medusae, or jellyfish, this layer is repre- sented by a thick, watery jelly without cells, termed the mesogloea (“‘middle- jelly): Cellular Structure and Function. ‘The outer cell layer, or epidermis, is composed principally of large epitheliomuscular cells (Fig. 10.3). ‘These are so called because, in addition to their epithelial function of covering a surface, they also serve the function of contractility and make possible the movements of the animal. Each cell possesses contractile processes extending along the supporting lamella. In the epidermis, the contractile processes run lengthwise of the body tube, so that their coordinated contraction produces a shortening of the body. The corresponding muscle processes of similar cells in the gastrodermal layer extend along the circumference of the tube, at right angles to the epidermal processes, and their contraction lengthens the body by decreasing its diameter. The many changes of shape and position exhibited by the hydra are all produced by the coordinated and localized contractions of this simple muscular system. At the base of the body the epidermal cells are represented by glandular cells. ‘These produce the secretion by which the 288 COELENTERATA AND CTENOPHORA attachment of the animal is effected. Interspersed between the large epi- dermal cells of the body and tentacles are numerous small interstitial cells and cells called cnidoblasts (‘thread formers’’), derived from interstitial cells. The sensory cells and nerve cells of the epidermis will be described in the discussion of responsiveness in the hydra. The cnidoblasts, containing the nematocysts with which the hydra subdues its prey, are scattered throughout the epidermis but are most abundant upon the tentacles. Nematocysts are one of the most remarkable mechanisms in the Animal Kingdom. ‘They are not cells, but non-living cell products, which might be likened to harpoons, ready to be shot from guns and capable of paralyzing any minute animal which they may strike. ‘The presence of nema- tocysts is one of the distinguishing characteristics of all coelenterates. The bodies of these animals are soft and defenseless, except as some may with- draw within a protective skeleton; but the members of this phylum must be recognized as powerful enemies of the many small animals upon which they prey. The nematocysts of most coelenterates are not harmful to man, although those of some species may be irritating. ‘The poison of a few species can induce a violent reaction of the human skin, serious general symptoms, and even death in sensitive individuals. In the development of a nematocyst, an interstitial cell produces in_ its cytoplasm a minute structure resembling a vacuole; the early stages are so minute that the details of development are not well understood. However, the vacuole is later seen to be a capsule containing a fluid and a thread. ‘The commonly accepted theory is that the thread, hollow in its final form, arises as an ingrowth from one end of the capsule. Clearly, the nematocyst is not a cell but a capsule containing an inverted thread, which is produced and retained until discharge within the cytoplasm of its cnidoblast, a modified interstitial cell. The discharge of a mature nematocyst involves the forceful eversion of the thread, like the turning right-side-out of an inturned glove finger. The sudden entrance of water into the capsule, possibly as a result of some rapid change in osmotic relations or in the permeability of a limiting membrane, is the most plausible explanation of this reaction. ‘The greatly increased internal pressure thus forces the thread out. Cnidoblasts may undergo differentiation from interstitial cells in parts of the body remote from the tentacles, in which nematocysts are most commonly used. In this case the cnidoblast is transferred over a considerable distance, coming to rest eventually in the epidermis of the tentacle. Here it becomes oriented with its trigger-like projection, the cnidocil, protruding from the general surface of the epithelium. ‘The cnidocil is apparently the part of the cnidoblast most sensitive to external stimulation. Because of the absence of nervous connections, the cnidoblasts are con- sidered to be independent effectors, structures responding directly to stimuli and lacking nervous control. Nematocysts are little affected by purely me- chanical stimuli, such as those produced by rubbing the tentacle with a clean glass rod or with a bit of clean blotting paper. Contact with a saliva-coated 289 GENERAL ZOOLOGY Fig. 10.4. The four types of nematocysts char- acteristic of all the hydras, shown undischarged and discharged. A and A,, desmoneme; B and B,, atrichous isorhiza; C and C,, holotrichous isorhiza, with spiny tube; D and D,, stenotele, the largest and most potent type. (Redrawn from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 by McGraw-Hill Book Co., Inc., printed by permission.) glass rod, or with a piece of blotting paper soaked in any of a variety of animal fluids, however, evokes immediate discharge. This evidence suggests that the diffusion of substances emanating from the prey in some way sensi- tizes the cnidoblast so that subsequent mechanical contact with the prey induces eversion of the thread. In contrast to the reactions of the cnidoblasts as independent effectors, the muscular responses of the tentacles in the capture of food appear to be coordinated by nerve cells, numerous in the epidermis and present in smaller numbers in the gastrodermis. Among coelenterates in general, 17 different types of nematocysts have been recognized, and it has been found that the characteristics of these structures are useful in the classification of coelenterates. Some groups have only one type, and some types are found in only one or a few related groups. ‘The 290 COELENTERATA AND CTENOPHORA hydras all have four types (Fig. 10.4): a large globular kind with conspicuous spines on the enlarged basal portion of the long thread, a small globular type with its thread always spirally coiled after discharge, and two small elongate types with long, straight threads. Examination of the bodies of animals par- alyzed by hydras indicates that these nematocysts have different functions. The threads of the first type penetrate the body of the prey, even piercing the exoskeleton of small crustaceans; it is now apparent that this puncturing of the prey is very significant in the further feeding activities of the hydra (p. 295). ‘Threads of the spiral type wrap themselves about fine projections, such as the bristles of a water flea. In some manner the nematocyst exerts a paralyzing influence on the prey. This action has been attributed to a poison, which is variously considered as being ejected from the free end of the hollow thread or as simply coating the outside of the everted thread. The exact nature of this poison has not been ascertained, although the name “‘hypno- toxin” has been applied to it. It is apparently composed of several different substances, and is probably not the same throughout the phylum. The gastrodermis is the inner layer of cells lining the coelenteron. It con- sists chiefly of large cells which may bear one or two flagella (Fig. 10.3). These cells are capable of extending pseudopodia at their free ends to ingest particles of food from the coelenteron. As in the epidermis, these cells are epitheliomuscular in character; it was pointed out earlier that the arrange- ment of their muscle processes gives them an action antagonistic to that of the epidermai cells in contraction. In Chlorohydra the gastrodermal cells are crowded with green bodies regarded as unicellular plants. ‘These cells live within the gastrodermis of the hydra and presumably pass from one genera- tion to the next by transfer in the hydra’s eggs. Similar plant cells, unicellu- lar brown algae, or dinoflagellate protozoans inhabit the cells of many marine coelenterates, particularly some of the corals. The presence of such cells seems advantageous to the hydra, for in their autotrophic nutrition these simple plant or plant-like cells consume carbon dioxide and inorganic wastes and produce oxygen and organic compounds which are useful to the coelen- terate host. Similarly, an advantage would seemingly accrue to the plant cells from the abundant raw materials produced as metabolic wastes by the cells of the host, as well as from the protected environment afforded the plants. A relationship of this sort, in which both organisms involved may be thought to benefit from the association, is termed mutualism. ‘This is a par- ticular manifestation of the broader phenomenon of symbiosis, interpreted as any situation in which the existence of one organism is closely involved in the existence of another. Mutualism may be contrasted with another kind of symbiosis termed parasitism, in which the advantage of the association is all on one side, and with commensalism, in which animals are associated merely as ‘““messmates”’ without obvious advantage or disadvantage to either. The ciliated protozoans Trichodina and Kerona, often seen living on the outer surfaces of hydras, are probably commensals and not parasites. In addition to the large gastrodermal epitheliomuscular cells, there are 291 GENERAL ZOOLOGY club-shaped gland cells, with the smaller end attached to the basement mem- brane and the larger end exposed to the digestive cavity (Fig. 10.3). These are located principally in the distal third of the body (the basal end is considered proximal) and presumably are the source of enzymes effective in extracellular digestion occurring in the coelenteron. A circlet of gland cells just within the mouth secretes something that seems to activate the other gland cells, indicating that even in this simple animal a rather complicated secretory cycle may exist. Scattered sensory cells, nerve cells, and some interstitial cells also occur in the gastrodermis. The cells described in the epidermis and gastrodermis collectively constitute the somatic cells of the hydra. Cell specialization in this animal gives rise to different kinds of somatic cells, but it must be noted that the aggregation of these types into well-defined tissues has not progressed to any great extent. For example, the epitheliomuscular cells of the epidermis and gastrodermis combine the functions of epithelia and of contractile tissues, which in more advanced animals become the properties of separate and distinctly modified aggregations of cells. Organs are similarly primitive: only the tentacles may be termed organs in the accepted sense. Thus it may be stated that in the hydra there are different kinds of specialized cells, but that the differentiation of tissues and of organs is at a very low level. The interstitial cells of the hydra constitute a sizable complement of ap- parently totipotent cells, capable of specialization in a variety of ways through- out the life of the individual. For example, interstitial cells are the source of cnidoblasts required for the replacement of discharged nematocysts. “They are also important in the differentiation of a new hydra arising by budding, and in the regeneration of lost parts, for which these animals have a con- siderable capacity. Interstitial cells are, in addition, the source of the germ cells which appear at certain seasons. For all its simple organization, the hydra, with its several kinds of somatic cells, shows a great advance in cell specialization over such forms as Volvox, in which all the somatic cells are alike. Metabolism. The small animals serving as food for the hydra, after being paralyzed and held fast by the nematocysts, are brought to the mouth by the tentacles and are ingested by engulfing movements of the hypostome. ‘The mouth is capable of a surprising degree of distension to accommodate large objects of food. Soon after its ingestion the food is shifted by peristaltic contractions of the body to a position in the distal half of the coelenteron, where the early stages of digestion occur. Although no structural differentia- tion exists other than the abundance of gland cells in the distal region, there is apparently a physiological division of the coelenteron into gastric and intestinal regions; the food mass is never found in the more proximal or basal part of the cavity. The process of digestion in the hydra is twofold. Enzymes released from the gland cells bring about the disintegration of the softer parts of the food mass, liquefying it and hastening its breakdown into particles. The soluble 292 COELENTERATA AND CTENOPHORA products of this extracellular phase of digestion are absorbed directly by the gastrodermal cells. Finely divided particulate matter is ingested by pseudo- podia formed by the large gastrodermal cells and comes to lie in food vacuoles within their cytoplasm. Here the intracellular phase of digestion occurs, which is presumably entirely comparable with the process as it occurs in an amoeba. ‘The indigestible residues are cast off by the gastrodermal cells and, together with the resistant parts of the food mass in the coelenteron, are expelled through the mouth by a series of violent contractions of the body. The bottom of a culture dish near a vigorous and well-fed hydra may often be littered with the egested exoskeletons of the water fleas upon which the animal has been feeding. Although there is no circulatory system for the transfer and distribution of nutrients, it will be noted that no part of the body of the hydra is far removed from the source of food in the coelenteron. ‘The wave-like contractions of the body tube, together with the activities of the gastrodermal flagella, main- tain a circulation of the products of digestion within the coelenteron, which in recognition of its dual function is often termed the gastrovascular cavity. In colonial hydrozoans this distributive function of the common coelenteron pervading the entire branching colony is very important in the nutrition of all the component individuals. Soluble products of digestion undoubtedly reach epidermal regions by diffusion across the short intervening distances. Oxygen reaches all the cells by diffusion from the external medium. As in all animals it is used in cellular metabolism, which releases energy and pro- duces as by-products carbon dioxide, water, and nitrogenous wastes. In animals as small as the hydra, excretion probably occurs over the general body surface; there are no specialized organs of excretion. Responsiveness. All movements of the hydra are the result of contractions by the longitudinal and circular muscle processes of epidermal and gastro- dermal cells. The varied positions and shapes which the animal may assume indicate that these processes can contract and relax locally as well as over the entire body, and that they react in a coordinated fashion. In addition to extensions and contractions of body and tentacles as a whole, there are peristaltic movements; these may be very slow, or they may be rapid, as when egesta are violently expelled, or when food is quickly moved from one region of the coelenteron to another. ‘The circular muscle processes function as sphincters about the mouth and at the base of each tentacle. Locomotion occurs in a variety of ways. A hydra may move by imper- ceptible degrees, gliding upon its base. It may extend itself horizontally until the tentacles are in contact with the surface, release the base, which is then drawn to a new point of attachment, and extend the tentacles once more, repeating the process. Again, the hydra may attach the tentacles, release the base, and move by a series of slow somersaults (Fig. 10.5). It can also walk clumsily upon its tentacles with the body free and contracted. The habit of some hydras of floating at the surface with the base clinging to the surface film has already been mentioned. 293 GENERAL ZOOLOGY SAL nuke ( Bonk dal ad Ar 279-6 zs PERT ‘ Fig. 10.5. A, locomotion in Hydra. Above, ““inchworm’” locomotion, in which the distal end is applied to the substratum and the basal end drawn up to attach beside it. Below, “‘somer- saulting’” locomotion. Numbers indicate successive positions. 8B, Abraham ‘Trembley, the first serious student of hydras, with his young pupils. (A, redrawn, and B, reproduced di- rectly, from Abraham ‘Trembley, Histovre de Polypes, Leyden, 1744. This monograph contains the earliest published accounts of the details of locomotion, feeding, and reproduction in hydras, masterful experimental studies of regeneration, and the first demonstration of the animal nature of the polyps. See also Figure 12.13, p. 362.) The characteristic movements and reactions involved in feeding are well co- ordinated. When a water flea or other small organism has been captured by a single tentacle, the other tentacles usually take part in the transfer of the prey to the mouth; however, a very small animal may be caught and transfer- red to the mouth by one tentacle alone, without reactions by the others. ‘The 294 COELENTERATA AND CTENOPHORA mechanisms underlying feeding reactions have only recently been elucidated. It has been demonstrated that the hydra opens its mouth to engulf prey only after the attacked organism has been punctured by the discharge of the large, barbed nematocysts. ‘This suggests that the puncture of the prey releases some substance into the water, and that the hydra reacts to the presence of this substance by opening its mouth and executing engulfing movements of the hypostome. Following this line of reasoning, investigators have found only one specific chemical compound, reduced glutathione, which is normally present in the body fluids of animals upon which hydra feeds and will elicit the feeding reaction in hydras. Animals such as other hydras, which the hydra never attacks or attempts to ingest, do not contain this substance; but if a hydra is exposed to a minute amount of reduced glutathione in the culture medium it will engulf other hydras, or in the absence of food will open its mouth so widely that it turns inside-out. ‘The operation of the mouth and hypostome in engulfing food brought in by the tentacles is certainly a co- ordinated response, and the reactions are probably set off by certain neuro- sensory cells which are stimulated by very low concentrations of reduced glutathione emanating from injured prey. ‘The general behavior of the in- dividual is influenced by its physiological state; hydras kept for a_ short time without food are much more responsive in their feeding reactions than well-fed animals. In addition to its specific reaction to reduced glutathione, the hydra reacts to mechanical contacts—light, heat, and electrical stimulation. ‘The effective stimuli in the normal behavior of the animal are probably combinations of specific environmental changes, conditioned by the physiological state of the individual at any given time. Responses to combinations of stimuli can be more easily studied in some of the larger coelenterates. “The hydrozoan jelly- fish Gonwonemus, for example, reacts moderately to contact with objects, to objects in motion, and to chemical compounds such as acids in solution, but the reaction is greater when these stimuli are presented in combination. Thus, the tentacles of a resting Gonionemus are somewhat sensitive to the contact of a fine pipette which merely touches them or is moved along their surface, and to meat juice gently ejected against them without movement of the pipette. But when the pipette is drawn rapidly along a tentacle as the juice is ejected, the entire animal goes into action. All the tentacles twist and turn, bending toward the mouth; the hypostome turns toward the tentacle stimulated, and nematocysts are discharged. ‘This combination of stimuli is presumably like that received from a small fish or crustacean, which would normally be attacked, for the reactions are similar in the two cases. We may now turn to an examination of the cellular organization of the sensory-neuro-muscular mechanism, the physical basis for the reactions which have been described. “The epidermis contains many nerve cells, or neurons, and at least four different kinds of sensory receptor cells, all connected with the contractile processes of the large epidermal epitheliomuscular cells. In addi- tion there are neurosensory cells, so termed because they resemble nerve cells 295 GENERAL ZOOLOGY and have processes, presumably involved in sensory reception, extending to the outer surface of the epidermis. Nerve cells and receptors are also found in the gastrodermis, although in smaller numbers. The epidermal and gas- trodermal elements are generally considered as forming two networks, apparently rather sparsely interconnected by neurons traversing the support- ing lamella. ‘The richer innervation of the epidermal longitudinal muscula- ture is reflected in the greater degree of coordination of its activities, as compared with those of the gastrodermal circular musculature. The nature of the coelenterate sensory-neuro-muscular system has been the subject of much disagreement among zoologists. The controversy has centered about the question whether the network of neurons represents a morphological continuum, composed of cells whose processes are actually physically continuous with each other, or whether the extremities of adjacent neurons are merely in synaptic contact as in the nervous systems of all higher animals. Reliable evidence bearing upon this question is difficult to obtain in an animal as small as the hydra, but it is probably safe, where necessary, to extend to the hydra the results of studies on larger coelenterates. Modern research has demonstrated almost beyond question that the network is not continuous, and that nerve impulses traveling through the system must pass from cell to cell across a discontinuity, or synapse (pp. 96-100). Al- though the processes of adjacent neurons may be in contact or actually intertwine with each other, there seems to be no protoplasmic continuity between them. If this is true, it probably follows that the passage of a nerve impulse across a synapse in coelenterates, as elsewhere in the Animal Kingdom, involves the secretion by the neuron of a chemical mediator which excites the adjacent neuron. However this system may thus seem to agree, in its basic mechanism, with those of more advanced animals, there are two fundamental and interrelated differences which emphasize its primitive nature. One is the fact that this is a diffuse, net-like system, with no indication of the formation of ganglia, the aggregations of nerve cells so characteristic of the nervous systems of all higher forms. ‘he other is the fact that the neurons and synapses of coelen- terates conduct impulses in any direction with apparently equal facility; in other words, there is no indication here of polarization within the nervous system. In higher animals conduction within one neuron may occur in either direction, but the neuron is polarized in the sense that the impulse can be propagated across a synapse at only one end. ‘This is the basis of the one- way transmission characteristic of the nervous systems of animals more ad- vanced than coelenterates. In the hydra and its relatives a nerve impulse must be conducted over a long pathway, wandering along the branching proc- esses of widely spread neurons, possibly exciting inappropriate connections along the way. In contrast to this, a ganglionated system, with polarized synapses, offers the special advantages of contiguity of nerve cell bodies inter- connected by short processes, promoting the channelization of impulses with greater efficiency along definite pathways. Although the coelenterates do not 296 COELENTERATA AND CTENOPHORA 6 approach this level of specialization, functional elements constituting a “‘nerv- ous system” do occur and are somewhat more highly organized than those of sponges (p. 280). If we recall the definition of responsiveness as the capacity, inherent in living cells, of responding to stimuli or changes in the environment, it 1s apparent that the foundation of behavior in the hydra, as in a vertebrate or in a protozoan, is the responsiveness of cells. All the cells of the hydra are capable of responsiveness, but those of the sensory-neuro-muscular system are specialized in this respect. Although experiments which have demon- strated the nervous functions of larger animals cannot be performed with an animal as small as the hydra, the functions of certain cells can be inferred from their relationships and from our knowledge of the reactions of similar cells in other animals. Reception of stimuli and establishment of nerve impulses by the sensory cells, conduction and discharge of impulses by the neurons, and the resultant action of effectors such as contractile and secretory cells all occur in the hydra. If a “‘system” is defined as a ‘“‘group of organs” performing some general function, the hydra has no nervous system. Yet, we naturally speak, as in the foregoing discussion, of the ‘nervous system” in hydra. In general, the nervous mechanism of coelenterates is a receptor-effector system, in contrast with the receptor-adjustor-effector systems of higher animals (Fig. 10.6). Further comparisons of the mechanisms of coordination in animals will be deferred until after the nervous system of the earthworm has been described in Chapter 14. The organization of this mechanism in annelids is inter- mediate between that of coelenterates and that of vertebrates. Reproduction and Development. At certain seasons of the year, particu- larly in autumn, hydras reproduce by syngamy, the union of gametes. The testes are usually located on the distal half of the trunk, the ovaries near the middle. ‘Testes may appear first and ovaries later on the same animal, or both may be present together. Animals in which the same individual possesses both ovaries and testes are said to be hermaphroditic or monoecious. Cnidoblast Sensory cell Neurosensory Muscle Supporting Nerve Muscle fibril lamella cell fibril Fig. 10.6. Diagram of the relationships of the sensory-neuro-muscular system in the epidermis of Hydra. One of the nematocysts, considered independent effectors, is included. ‘The muscle fibrils occupy the basal processes of large epitheliomuscular cells. 297 GENERAL ZOOLOGY LO 3 OR ‘ or ei Spermatozoa Epidermis Spermatids ; Oogonia Gastrodermis 09 G6 Apo lte He Oe e a A, % lov A G Oscytes~ “CS Fig. 10.7. Aspects of sexual reproduction in Hydra attenuata. A, male with well-developed testes (and a young, asexual bud). 8, longitudinal section of a single testis showing stages in maturation of gametes. C, longitudinal section through a portion of an ovary, showing stages in o6genesis. D, a mature ovum resting in the remains of the ovary. £, female bearing six mature ova. fF, female bearing fertilized eggs, two of which are undergoing cleavage. (From P. Brien and M. Reniers-Decoen, 1951, Annales de la société royale coologique de belgique, vol. 82.) 298 COELENTERATA AND CTENOPHORA Monoeciousness may be the usual condition in hydras, although species in which the individuals seem to be exclusively male or female, hence dioecious, have been reported. ‘There are no secondary sexual characteristics in hydras; only by observing the testes or ovaries can the sex of an individual be determined. ‘These gonads appear as swollen protuberances from the epi- dermis in the characteristic regions (Fig. 10.7). Within them, ova or sperma- tozoa arise from interstitial cells. Fully matured spermatozoa may be seen moving actively within the testis; they are discharged by the periodic opening of the apex of the testis, which thus liberates successive swarms. ‘The sper- matozoon then swims about until it dies, or until it comes into contact with an ovum which has been exposed by the rupture of its epidermal covering. The zygote formed by the union of these two gametes undergoes cleavage and secretes about itself a shell-like cyst, or theca. Within the theca, development proceeds until an outer layer of cells, the ectoderm, and an inner solid mass, the endoderm, have been formed. ‘The embryo within its theca then becomes detached from the parent and drops to the bottom. ‘Tentacles eventually develop; the embryo breaks from its cyst, becomes attached, develops a coelenteron, forms a mouth, and so becomes a miniature hydra. Zygotes developing in late autumn pass the winter within the protective cyst. Hydras frequently produce new individuals by budding, a process referred to as asexual reproduction (Fig. 10.8). It is, essentially, reproduction by cell division. It differs, however, from the asexual reproduction of protozoans in that the mass of new cells produced is organized by some integrating in- fluence into a multicellular individual with the characteristics of the parent. There is first an accumulation of nutrient material in the gastrodermal cells at some place toward the middle of the body, and cells in the epidermis of this region divide repeatedly to form a bud-like swelling. An extension of the coelenteron grows into the bud, which then appears as a blindly ending outgrowth of the two layers of the body wall. ‘Tentacles appear as evagina- tions of epidermis and gastrodermis, and finally a mouth is formed. If food is abundant, the bud may remain attached to the parent for some time, and in exceptional cases it may rebud to form several generations in a branching system. Usually, however, the connection between parent and offspring be- comes constricted, and the bud is detached as an independent individual as soon as the tentacles and mouth become functional. Growth and Regeneration. Recent research has demonstrated the interest- ing fact that a region just below the hypostome of the hydra constitutes a zone of proliferation. In this area new cells are constantly being pro- duced by mitotic activity and added to the layers making up the body wall. There is a constant slow progression of cells downward toward the base; the oldest cells in the body make up the base itself and are gradually sloughed off at this point. ‘Thus, the base and the portions of the stalk adjacent to it are composed of aged, ‘‘exhausted” cells, whereas a continuous cycle of replacement provides young, vigorous cells in the more distal regions. Like many other types of animals with simple organization and_ well- 299 GENERAL ZOOLOGY Fig. 10.8. Asexual reproduction by budding in Pelmatohydra oligactis. (Redrawn from L. H. Hyman, 1930, Transactions of the American Microscofrcal Society, vol. 49.) developed powers of asexual reproduction, hydras and the coelenterates in general have a marked capacity for regeneration. When a hydra is cut transversely into two pieces, a new basal part appears on the piece having tentacles, and new tentacles, mouth, hypostome, and zone of proliferation develop at the distal end of the basal piece. In a few days two complete hydras will have been formed and will have gradually assumed the normal proportions. As might be expected from knowledge of its peculiar cellular constitution, the extreme basal region has very limited powers of regeneration. In more distal regions, however, regeneration and subsequent growth of new individuals occur even when the animal is cut into several small pieces. 300 COELENTERATA AND CTENOPHORA The Hydroidsand Hydromedusae. With few exceptions, among which the hydras are notable, the Hydrozoa are marine animals. ‘They are attached in at least one phase of the life cycle, and most species form colonies of individuals variously specialized in correlation with feeding, protection, and reproduction. ‘The life cycle in some species, but not in the majority, includes a free-swimming Jellyfish stage or hydromedusa. ‘The species Obelia geniculata (Fig. 10.9) is representative of those that do present this feature. In_ its hydroid (“hydra-like”) phase or generation this species is a colony contain- ing possibly thousands of individuals, or polyps, each individual comparable with a single hydra and all united as the buds of a hydra would be if they did not become detached. ‘The obelia colony in fact begins as a single polyp, which grows and buds repeatedly until it consists of many upright stems bear- ing polyps and arising from the root-like hydrorhiza (‘hydra root”) growing Medusa \\ Hydrotheca : Hydranth bud =X = I: EAI] ¢ | Hydrant QV DD 2 Gametes | ly ‘Z © 8 & <{(Medusa \ \ bude Cleavage stages SU. ‘Vs Planula Gonanth : Perisarc Blastostyle Gonotheca Coenosarc Hydrorhiza Fig. 10.9. Obelia: structure and life cycle. A, portion of a colony of the hydroid genera- tion. 8, medusa, and sexual reproduction. C, growth of a young hydroid colony from the attachment of a planula larva; numbers indicate successive stages of growth. 301 GENERAL ZOOLOGY along the surface of attachment. In the fully developed colony there are two types of individuals: (1) hydranths, which have mouths and tentacles, and coelenterons continuous with the tubular cavity pervading all the stems of the colony; and (2) gonanths, modified polyps without either mouths or tentacles, consisting chiefly of a central, rod-like blastostyle upon which are formed the buds which become free-swimming medusae. Both hydranths and gonanths are surrounded by appropriately shaped secreted containers, known respec- tively as hydrothecae and gonothecae, which are specialized portions of the perisarc surrounding the entire colony. ‘The living material, continuous throughout the colony, constitutes the coenosarc. The obelia colony, with its clearly marked dimorphism (“two forms’’), thus arises by a process of repeated budding involving also the differentiation of the two types of polyps. ‘The medusae, or jellyfishes, which constitute the products of asexual reproduction in the gonanths, are specialized in yet an- other direction—for swimming and sexual reproduction. After their release from the parent gonanth they reach sexual maturity as males or females, having either testes or ovaries. ‘The gametes produced in these gonads are released into the water, where fertilization occurs. ‘The resulting zygote develops into a ciliated, solid-bodied, two-layered, free-swimming stage, the planula, which settles to the bottom and transforms into an attached polyp from which a new colony is formed. In Obelia, sexual reproduction is carried on only by the medusa generation, and asexual reproduction is limited to the hydroid generation. ‘Thus, we may say that there are two generations which alternate; the term metagenesis is sometimes applied to such alternation of generations. ‘The medusae formed by hydroid colonies, and other similar medusae, are called hydromedusae to distinguish them from the larger, more specialized jellyfishes belonging to the class Scyphomedusae. Not all the genera of the Hydrozoa exhibit alternation of generations. Some colonial forms develop reduced or degenerate medusae, often called gonophores, which are never released but which develop gonads and produce gametes while still attached. In other genera such fixed medusae grade into special gamete-forming structures termed sporosacs; in still others, as in the solitary hydras, the gonads develop directly on the polyps, without a trace of the medusoid generation. In the opposite direction lie such forms as (1) Gonionemus, with a large, free-swimming medusa produced by a minute polyp which scarcely buds except to generate the medusa; and (2) Lirope, in which the planula develops directly into the medusa without a trace of the polyp. The entire assemblage of hydrozoans may be arranged in a regular series, with exclusively polypoid forms like Hydra at one extreme, and ex- clusively medusoid forms like Liriope at the other. In such a series Obelia, with its polypoid and medusoid generations about equally represented, lies in the center. It may be suggested that this kind of a series probably repre- sents an evolutionary sequence, but the direction of evolutionary change is a matter of conjecture. It seems illogical, however, to consider Hydra a primi- tive stem form; it is one of the very few fresh-water hydrozoans, is non- 302 COELENTERATA AND CTENOPHORA Fig. 10.10. A _ siphonophoran, Physalia. Vhe entire colony is suspended below the float. Sev- eral small fishes have been snared and killed by dactylo- zooids and are being drawn up for ingestion and digestion in gastrozooids. (Photograph by George Lower.) colonial, and shows a remarkably telescoped pattern of development, without medusae or planulae. It is preferable to assume that planula-like ancestors developed into medusae, and that the polypoid stages represent original larval forms which have persisted and become, in many cases, reproductive stages. Other Hydrozoa. Related to the hydroids and hydromedusae are the hydroid corals, or Hydrocorallinae. ‘These have a massive skeleton of car- bonate of lime, somewhat resembling the skeleton of true corals. Other interesting types are the Siphonophora, shown by the fossil record to be an extremely ancient group, of which the ‘‘Portuguese man-of-war,” Physalia, 1s the most familiar example (Fig. 10.10). Physalia is a colony of specialized individuals, sometimes spoken of as “‘persons,” having a gas-filled float sup- porting the whole. The colony closely resembles an individual organism, with organs specialized for various functions such as food-getting, digestion, repro- duction, and so on. Actually, the “organs” are individual members of the colony, each specialized to perform a particular function. Such a group may be spoken of as polymorphic, in contrast to dimorphic colonies like those of Obelia. The individuals that function as tentacles (dactylozooids) are laden with nematocysts which can affect the human skin very severely. These “persons” capture such prey as small fishes and crustaceans, which are then drawn up to the digestive polyps, or gastrozooids, near the float. ‘There are 303 GENERAL ZOOLOGY c. oes ets, Ss. 7 - ~ : » “~ . a $f é on Oe Sd 5 .coe SY ae oe A Sas oy 2 seg * o ae Fig. 10.11. A scyphomedusan jellyfish, Cyanea. (Photograph by George Lower.) also gonozooids, medusa-like individuals which are not released but produce gametes. ‘The siphonophores are considered to have diverged from the main hydrozoan stem early in its evolution. THE CLASS SCYPHOMEDUSAE Most of the jellyfishes called hydromedusae are small, like Gonionemus, or smaller. ‘The jellyfishes comprising the class Scyphomedusae are mostly of a larger size; individuals of the species Cyanea arctica (Fig. 10.11) have been recorded with a diameter of 12 feet and tentacles over 100 feet in length. The amount of solid or living material in such individuals would be small, however, because jellyfishes are composed chiefly of water. ‘The bulk of their ce substance consists of the “‘jelly,” which in these forms is a gelatinous mass conspicuously provided with cells resembling connective-tissue elements of higher animals. ‘lhe jelly itself may thus be considered as intercellular ma- terial, comparable with the fibrous substance of connective tissue or the ground substance of cartilage. In scyphomedusans specialized organs of equilibration, termed statocysts, are located at intervals around the margin of the bell; these sense organs are important in the free-swimming locomotion of jellyfishes, and similar though simpler statocysts occur also in hydromedusae. 304 COELENTERATA AND CTENOPHORA The genera Cyanea and Aurellia are representative Scyphomedusae found in North Atlantic waters. In typical cases the life cycle of a scyphomedusa consists of the following sequence (Fig. 10.12): a planula larva develops from a zygote; this larva produces an attached polyp generation, the scyphistoma, from which free-swimming medusae arise in succession by transverse budding or strobilization. “(he young medusae released from the strobila are saucer- like individuals called ephyrae; they grow and transform into adult, sexually reproducing Jjellyfishes. Asexual reproduction of the polypoid generation, by budding to produce additional polyps, has also been reported. Fig. 10.12. Structure and life cycle of Scypho- medusae. A, polyp stage, or scyphistoma, of Cyanea, developed from a planula larva, and two strobilas undergoing asexual reproduction, giving rise to many ephyrae. 8, ephyra, or immature medusa, probably of Chrysaora. C, mature jellyfish or medusa stage of Awrellia. The bell of this medusa is traversed by many gastrodermal canals, shown in only one quad- rant; the margin bears many short tentacles as well as marginal sense organs. The gonads are prominent semicircular structures borne _ be- neath the radial canals; the oral tentacles are long and flexible and are the structures chiefly used in feeding. (A, redrawn from J. J. Steenstrup, Ueber den Generationswechsel, Copen- hagen, 1842. Steenstrup was among the first to understand. the significance of alternation of generations in such animal groups as coelenterates and parasitic flatworms. B and C, redrawn from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copyright 1940 by McGraw- Hill Book Co., Inc., printed by permission.) 305 GENERAL ZOOLOGY Siphonoglyph Hh / yy i) i ve oii Retractor muscle -Acontia Pedal disk Oey e. Bothrium of scolex Tail Fig. 11.21. Immature stages in the life cycle of the broad tapeworm or fish tapeworm, Diphyllobothnum latum. A, free-swimming, ciliated coracidium larvae, which is ingested by a small crustacean such as Cyclops. B, immature, and C, mature procercoid, developing from the hexacanth in the body of the crustacean. D, plerocercoid, which develops in muscles, liver, spleen, or coelom of a fish which has eaten an infected Cyclops. The plerocercoid trans- forms into the mature, segmented tapeworm in the intestine of the final host, a fish-eating mammal, after being ingested. (Redrawn, after Rosen, from L. H. Hyman, The Invertebrates: Platyhelminthes and Rhynchocoela, copyright 1951 by McGraw-Hill Book Co., Inc., printed by permission. ) 339 GENERAL ZOOLOGY which enable each zygote, through multiplicative larval stages, to develop into numerous adults. A step in this direction, however, is seen in the life cycle of Echinococcus granulosus, which is found as a minute adult in dogs, and as a larva in cattle, sheep, swine, and occasionally man. ‘The hexacanth larvae of this worm, once they have gained access to the tissues of an inter- mediate host, transform into bladder worms. ‘The originally single inverted scolex in each of these cysticerci proliferates asexually to produce thousands of daughter cysts, attached to the inner wall of the parent bladder or floating free in its fluid contents (Fig. 11.20). ‘The cyst slowly emkarges, growing in the course of years to the size of an orange or larger. It may, if it lies in the brain, for example, bring about serious consequences to the intermediate host. From the standpoint of the parasite the advantage lies in the fact that the final host, feeding upon the flesh of an intermediate host containing one of these huge cysts, will receive a massive infestation of potential adult tape- worms. The taenioid tapeworms which have been used as examples above represent only one, and probably the most highly specialized, of several orders of Eucestoda. Parasites dangerous to man are also found in other orders, in which the life cycles and morphology of the tapeworms differ from those of the taenioids. An example is Diphyllobothnum latum, the broad tapeworm of man and other carnivores. ‘This worm possesses in each proglottid a uterine pore in addition to the genital pore. ‘Through this uterine opening eggs are emitted singly as they mature, instead of reaching the outside through the shedding of entire proglottids. The life cycle of Diphyllobothrium (Fig. 11.21) requires for its completion a small crustacean, such as Cyclops, and a fish, as intermediate hosts. ‘The stages infective for the final host are plerocercoids, encysted in the flesh, and particularly in the livers, of infected fish. A bear, a fox, a dog, or a man may become infested by eating raw fish. Records of massive infestations with tapeworms of this species have been obtained among fisherfolk in the Baltic area, who are accustomed to eating raw fish liver spread on bread. Tapeworm infestations are now comparatively rare among adult human beings in communities where sanitary precautions are practiced. Meat inspection, cold storage, proper cookery, and widespread knowledge regarding modes of infection have almost eliminated Taenia solium and 7. saginata as serious problems for the United States and Western Europe. ‘This is in con- trast to conditions prevailing before about 1850, when the life cycles of these parasites, and hence the means of preventing infestation, became known. The Phylum Nemertinea Members of the phylum Nemertinea are sometimes called ‘‘ribbon worms” because the bodies of many of them are greatly elongated and flattened. Most species are marine, although a few fresh-water and terrestrial nemer- 340 FLATWORMS Lateral blood vessel Excretory, canal Proboscis Anterior sheath proboscis sheath Ovary Intestinal Stylet of diverticula proboscis Accessory stylets Retractor muscle of proboscis Fig. 11.22. General features of a typical nemertine, Amphiporus pulcher; this is a dorsal view of a female individual. ‘The stylet is so placed that it occupies the tip of the proboscis when this is fully everted. Note the relatively simple reproductive system, consisting of a series of pouch-like ovaries, each with a separate external opening, and the longitudinal vessels of the primitive blood-vascular system. (Redrawn, after Biirger, from C. G. Goodchild in F. A. Brown, Jr., ef al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission. ) tines are known. Some of the marine forms are free-swimming (pelagic), but for the most part they live burrowing in the bottom or among the growths of animal and plant life of the ocean floor. One species lives as a commensal within the mantle cavities of bivalve mollusks. Common American forms are Cerebratulus lacteus, a burrowing species which may be several feet in length, and Tetrastemma elegans, less than an inch long. 341 GENERAL ZOOLOGY Mesenchyme 4 " ih os Nerve cells VV; A eo di. = Muscle fibers MW y Nerve ring Z ee y SL” Fig. 11.23. Pilidium larva of a nemertine; optical section, showing distribution of nerve and muscle fibers. (Redrawn from W. Salensky, 1886, erlschnift fiir wrssenschaftliche ery ue, yee SS B Shae > Pharynx On % 1b A= SS % 2s AN ? Genital 5 4 * Mee Q) i hae ete. OY os pore ey + : - 4 GY ay NY Vv i Zar} Bt) ‘ ” d Sy, ; i Bev 8ene Mouth gy! ee A Developing x a } - RP juvenile worms i ;. * ot” dd © fh) i ‘ 4 ie f : lox) Uterus > ge ; iB j “o “4'q xX) i , Op i { Be 4 § i 5 hae | A Seminal ¥ Sx) receptacle , i \ a ‘] EN Ovary SS ‘ . B A Fig. 12.5. Class Nematoda. A, adult female trichina worm, Trichinella spiralis; this stage inhabits the intestine of the host and produces thousands of juvenile worms which pass into the circulating blood and reach various tissues, chiefly muscles, where they encyst. B, encysted juvenile worm in striated muscle. (A, redrawn from C. G. Goodchild in F. A. Brown, Jr., ef al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission; B, photograph courtesy General Biological Supply House, Inc.) of the host and ingest tissue fragments and blood. They are considerably more dangerous to the host than the ascaris, occurring in more massive in- festations and causing serious loss of blood. Certain sections of the United States have undoubtedly been retarded in their development by the prev- alence of hookworm infestations among the rural populations. As a result of active campaigns by public-health authorities and others, the situation has been much improved. Other important nematode parasites of humans are the trichina worm, Trichinella spiralis (Fig. 12.5), which lives as an adult in the intestine of man and as encysted juveniles in the muscles of this host and a variety of others, including swine; and the filarial worms, such as Wuchereria bancrofti, which inhabits the lymph glands of man and requires a mosquito as an intermediate host. Details of the life cycles and pathological effects of these and many other interesting nematode parasites will be found in any textbook of parasitology. 353 GENERAL ZOOLOGY THE ASCHELMINTH BODY PLAN As stated at the outset, the phylum Aschelminthes is a very diverse group, and it is difficult to describe a body plan common to members of all the six classes. Certain characteristics are found in all the classes, however, and others occur in two or more. For instance, the nature of the body cavity is a major unifying feature. ‘The condition of the body wall, with its cuticle, simple epidermal epithelium, and muscular layers bounding the pseudocoel externally, is also comparable throughout the phylum. Shell Retractor Larval muscle hooks Spines Epidermis Fertilization membrane 355 GENERAL ZOOLOGY The tissues of acanthocephalans are largely syncytial, and as in rotifers and nematodes the number of nuclei represented in each organ of the body is relatively constant. ‘Thus, for example, although a protonephridium may consist of hundreds of flame bulbs, the entire organ contains only three nuclei. The taxonomic position of the acanthocephalans, and their possible af- finities with other groups, are matters still subject to discussion. In many of their features they seem to show relationships to various members of the phylum Aschelminthes; in others they are somewhat similar to the class Cestoda of the phylum Platyhelminthes. Because they are pseudocoelomate, we have placed them near the aschelminths. It is clear that the Acantho- cephala have behind them an extremely long history of endoparasitism, and that adaptations to this parasitic way of life have been so extensive as to mask their true relationships. The Phylum Entoprocta. The members of this small phylum were originally confused with coelenterate polyps and later included with the Ectoprocta as a class of Bryozoa, or moss animalcules. ‘They possess certain well-defined characteristics which make it logical to consider them as a separate phylum, allied to the aschelminths and acanthocephalans. ‘The phylum contains a total of about 60 species; all are marine except members of the genus Urnatella, which inhabit fresh water. With some exceptions, entoprocts are colonial forms, the colony comprising a horizontally extended stolon from which the individual members arise at irregular intervals. Each individual consists of an upright stalk with a cup-shaped calyx at its tip; the organs are contained within the calyx. The upper rim of the calyx bears a ring of ciliated tentacles, which enclose a space termed the vestibule. At one side of this upper surface, within the circlet of tentacles, lies the mouth; the anus also lies within the vestibule, at the side opposite the mouth. ‘The mouth and anus are connected by a U-shaped digestive tract. The organs of a typical entoproct are shown in Figure 12.8. Surrounding the digestive tract is a pseudocoelic space largely filled with mesenchyme and containing the reproductive organs, a central ganglion, and a symmetrical pair of protonephridia. Reproduction involves fertilization of the eggs within the oviducts; the zygotes then emerge and attach to the floor of the vestibule, where early embryonic stages are passed. Eventually, free- swimming larvae are produced, which after a short period of free life attach to the substratum and undergo metamorphosis to adulthood. Colony forma- tion follows the growth of a stolon from the base of the stalk of such a solitary individual. Like rotifers, the ciliated larvae of entoprocts have been compared with the trochophores of mollusks and annelids (pp. 375, 401), although the simi- larities seem to be highly superficial and may be accounted for on the basis of convergent evolution in response to similar environmental conditions. Probably of greater significance is the marked resemblance between certain entoproct larvae and certain types of rotifers. Since the entoprocts are of the same grade of organization as the rotifers and are like them pseudo- 356 PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA coelomate forms, they may well have evolved from an ancestral stock which also gave rise to the rotifers. Minor Eucoelomate Phyla THE PHYLUM ECTOPROCTA Members of the phylum Ectoprocta are permanently attached, bilateral metazoans; most representatives develop extensive colonies, either arborescent or encrusting. In their possession of circumoral tentacles, they bear a super- ficial resemblance to coelenterate polyps and to entoprocts, and they have been classified with both of these groups. Increased knowledge of their structure has, however, revealed that the ectoprocts are of a higher grade of organization than either entoprocts or coelenterates. Typically, each individual has a ciliated ridge surrounding the mouth and bearing many tentacles. ‘This ridge, the lophophore, is circular in marine ectoprocts and horseshoe-shaped in the fresh-water types. ‘This characteristic, among others, is a basis for the subdivision of the phylum into two classes. In all, the anus lies just outside the lophophore, and the digestive tract is consequently U-shaped. ‘This arrangement is not uncommon in sessile metazoans. Tentacles Fig. 12.8. An individual ento- a: proct, Pedicellina cernua. his in- wd Detar) | Mn3 z Mouth dividual has developed by the act he metamorphosis of a larva. From y KRY the base of its stalk a stolon is be- = Ventral ganglion ginning to form, which will grow horizontally over the substratum. From this stolon additional in- Pseudocoel dividuals will arise to form a colony. (From C. Cori in W. Kiikenthal and T. Krumbach, 1933, Handbuch der Zoologie.) Stalk Stolon Basal disc 307 GENERAL ZOOLOGY Fig. 12.9. The erect, branching colonial ectoproct, Bugula. (Photograph courtesy New York Zoological Society.) The genus Bugula, members of which are abundant along our North Atlantic coast, is representative of the arborescent marine ectoprocts (Fig. 12.9). The individuals of the colony, termed zooids, grow in double rows on upright stalks branching from the original point of attachment of a single free-swimming larva. Each zooid is encased in a cuticular skeleton, into which the lophophore with its ring of ciliated tentacles may be completely withdrawn. ‘The anus is located upon a collar-like region just below the lophophore and thus projects beyond the cuticular sheath when the lopho- phore is extended. Attached to the external surfaces of the major zooids are smaller, highly modified individuals termed avicularia. “These resemble birds’ heads; their large “beaks” are highly mobile and are opened and closed by powerful groups of muscles. By grasping and removing small objects with which they come in contact, the avicularia presumably function to keep the colony free of encrusting organisms. Internally, the U-shaped digestive tract consists of a pharynx, a stomach, a caecum, an intestine, and a rectum (Fig. 12.10). Bugula is a ciliary par- ticulate feeder, and its microscopic food is drawn into the mouth and driven into the pharynx by the action of the cilia. Here the food collects in masses which are moved through the digestive tract by the coordinated contractions of muscle fibers in the wall of the gut. Digestion occurs extracellularly, chiefly in the stomach, caecum, and intestine. Products of digestion are absorbed by the single-layered gastrodermal mucosa. Further transfer into 358 PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA the coelomic fluid probably occurs, and in the absence of a vascular system the coelomic fluid presumably serves as a circulatory medium. Excretory organs are lacking, although cells of the stomach epithelium appear to ac- cumulate waste products in the form of brown granules. In older individuals these brown concretions are conspicuous, and eventually the entire body of the organism, with the exception of the cuticular sheath, progressively degen- erates into a so-called “brown body.” The significance of this process is not entirely clear. The nervous system of the zooid is simple, consisting of a single ganglion in the region between mouth and anus, with fibers innervating sensory cells and muscle bands in the tentacles as well as other muscles in all parts of the body. The coelom, probably developed by a schizocoelous process (p. 368), is lined by visceral and parietal peritoneum connected by a mesentery (funiculus) Lophophore Exoskeleton Fig. 12.10. General structure of an encrusting ectoproct, based on AMfembrampora. A, individual with lophophore extended; B, with lophophore retracted. In both diagrams the individual is represented as though sectioned along the midsagittal plane. The peritoneal lining of the coelom, and the remnants of mesenteries and septa, are indicated by broken lines. Note the position of the anus in relation to the circlet of tentacles about the mouth. C), surface view of a portion of a colony of Membranipora tuberculata, with all individuals retracted. Note the crescentic operculum at the anterior end of each individual. (A and 8, redrawn from C. Cori in W. Kiikenthal and T. Krumbach, 1938, Handbuch der Zoologie; C, photograph by George Lower.) 359 GENERAL ZOOLOGY Brood pouch Zygote Central stomach Fig. 12.11. Bugula: structure and reproduction. A, transfer of a zygote from the coelomic cavity of a zooid into the brood pouch, where it will develop into a ciliated larva. B, in- dividual retracted and at rest; note the position of the lophophore within the vestibule. (Re- drawn, after Gerwerghagen, from C. Cori in W. Kikenthal and T. Krumbach, 1938, Handbuch der EAS ARKO) 0, Testis Qe cae Oo 0 365 GENERAL ZOOLOGY enterocoelous development indicates a position somewhere near the evolution- ary stem which culminated in the Chordata. Members of the genus Sagztta (Fig. 12.16) are pelagic forms, often very abundant in the marine plankton, swimming by means of body movements and horizontal fins. There are prehensile mouth parts consisting of a series of stout, chitinous hooks, sug- gesting the name Chaetognatha, or “bristle jaws.” A relatively large coelom is divided transversely into three compartments by septa, and there are also dorsal and ventral mesenteries linking the gut wall and the body wall. The nervous system consists of a dorsal ganglion, from which nerve cords extend as a circumpharyngeal ring to a ventral ganglion, with nerve fibers radiating to all parts of the body. ‘The animals are monoecious, and the zygotes develop into miniature adults without a ciliated larval stage. Summary In this chapter we have discussed several groups of metazoan animals which have little in common except their triploblastic, bilateral organization. Each of these groups, with the exception of the Aschelminthes, presents a consistent pattern of general characteristics sufficiently different from those of all other animals to warrant their status as separate phyla. ‘There re- mains some doubt among zoologists about the validity of erecting a phylum Aschelminthes to contain the widely different forms proposed for inclusion, of which we have discussed only two; the pseudocoelous body cavity is their chief unifying characteristic. The members of each of the phyla considered in this chapter have under- gone extensive modification in their evolution, in adaptation to particular ways of life or conditions in their environments. As evidence of their evolu- tionary histories and phylogenetic affinities there remain only the most general and fundamental characteristics. ‘These enable us to determine the approxi- mate level of organization which each type represents but not to state with any confidence the true relationships of any of these groups. We may con- clude that although they are interesting from the biological standpoint, per- haps none of them has been of great significance in relation to the evolution of the major phyla of animals. In the chapters to follow we shall discuss the major invertebrate groups— phyla which have been successful, as judged from the numbers of species and of individual organisms they contain. Generally speaking, these major groups retain sufficiently significant characteristics to permit us to establish, with some probability, their phylogenetic relationships. 366 CHAPTER rere THE PHYLUM MOLLUSCA Schizocoela and Enterocoela The major groups of eucoelomate animals constitute two great divergent stocks, distinguishable on the basis of several fundamental differences. ‘These differences involve such features as the type of cleavage, the time in develop- ment when the cellular precursors of the various organs become differentiated, and the relationship of the blastopore to the axial polarity of the embryo and the future adult. In addition, there are basic differences in the mode of formation of the mesoderm and of the coelom, and it is from this particular characteristic that the terms Schizocoela and Enterocoela are derived. In the schizocoelous forms, comprising the major phyla Mollusca, Annelida, and Arthropoda and several minor phyla, the mesoderm of the adult arises by proliferation of cells called mesoblasts, set aside early in the cleavage process. The coelom forms as a result of the development of cavities within the solid masses of mesodermal cells so produced. In the enterocoelous forms, on the other hand, represented by the major phyla Echinodermata, Hemi- chordata, and Chordata, the mesoderm appears primitively as hollow out- growths from the wall of the embryonic gastrocoel, and the coelomic cavities develop as enlargements of the spaces within these evaginations. It should be understood that from the point of divergence of these two stocks the phylogenetic tree of animals consists of two major branches, in each of which evolution has proceeded independently of the other (see Fig. 7.3, p. 219). The schizocoelous branch culminates in the great phylum Arthropoda, and the enterocoelous branch has reached its apex in the phylum 368 Ventricle Anterior aorta Auricle Stomach Pericardial cavity Pedal ganglion Anterior adductor Posterior adductor muscle muscle Cerebropleural Posterior aorta ganglion : VF i — Anus Ns ki e Excurrent Foot —waei/s.... : : as fasta : Digestive gland V——4== == — Visceral ganglion Gonad Nerve cord Nephridium Intestine Gill Fig. 13.1. Internal structure of a typical fresh-water pele- cypod, drawn as though the left valve had been removed and the body dissected from the left side; semidiagrammatic. Chordata. From the standpoint of numbers of species and individuals, as well as of adaptability to various environmental situations, each of these climax groups represents a markedly successful type of organization. It is fruitless to speculate about which is “‘higher,”’ an arthropod or a chordate, or about which of these evolutionary lines has produced the more successful type of animal. In this and subsequent chapters, we shall consider first the schizocoelous phyla, beginning with the phylum Mollusca, and then the enterocoelous animals. The Phylum Mollusca The Mollusca may be defined as bilateral, unsegmented, triploblastic animals, typically with a calcareous exoskeleton. ‘The body is divided into head, foot, and visceral regions, and a single pair of compound excretory organs is present. Except in certain specialized members of the phylum, the body is enclosed by a characteristic outgrowth of the body wall, the mantle, which is responsible for the secretion of the exoskeleton. “The word Mollusca is derived from the Latin molluscum, meaning soft, and refers to the texture of the body within the shell. The phylum consists of five classes: the class Amphineura, or chitons, typically flattened forms with segmented dorsal shell plates; the class Gastropoda, represented by the snails and slugs; the class Pelecypoda, or bivalve mollusks such as clams and mussels; the class Scapho- 369 GENERAL ZOOLOGY poda, a small group of unfamiliar marine animals called “tooth shells”; and the class Cephalopoda, the squids, cuttlefishes, and nautili. The majority of mollusks are free-living animals, adapted to a creeping or burrowing existence and provided with a protective shell into which the ex- tensible soft parts can be withdrawn. Such mollusks as the cephalopods, however, are modified for a more active, free-swimming habit. Most mollusks are marine animals and are abundant in shallow waters but not at great depths. ‘The sea seems to have been the primitive habitat, but many species of gastropods and pelecypods are found in fresh water, and gastropods are common also on land. The more representative types were classified as Mollusca by Aristotle, Linnaeus, and Cuvier, but in the early classifications this group included many animals which have since been distributed among other phyla. Many different kinds of mollusks are used as food today, and archaeological evidence indicates that shellfish were a very important item in the diet of many primitive humans. In this chapter we shall examine particularly the characteristics of bivalve mollusks such as the clam and the fresh-water mussel, as animals typical of this grade of organization, to be compared with the hydra, the planarian, the earthworm, and the vertebrate. THE CLASS PELECYPODA The Clam or Fresh-Water Mussel: General Structure and Activities. The following account is applicable to any of the common marine clams, such as Venus mercenaria, or to the fresh-water mussels such as species of the genera Lampsilis or Anodonta. ‘The shell is composed of two valves, fitted together at the dorsal side to form a hinge which is covered externally by a tough, elastic hinge ligament. ‘The dome-like part of each valve, lying near the hinge, is termed the umbo (plural, umbones); this is the oldest part of the shell, as indicated by the rings of growth which surround the umbo and mark the successive outlines of the growing shell. ‘The valves are lateral; the hinge ligament is dorsal; the gape of the valves is ventral. ‘The umbones generally lie just anterior to the midline of the shell, although between different species of bivalves this characteristic is subject to much variation. Looking at a clam with its valves intact reveals nothing of the living animal itself, except as the foot may be thrust out between the margins of the shell, or the tube-like siphons extended posteriorly. Removing the shell reveals the external surface of the clam. Closely ap- pressed to the internal surface of each valve is a flap of the mantle (Fig. 13.1). On each side, the mantle represents a sheet-like outgrowth of the dorsolateral body wall. Its functions include the secretion of the shell, the various layers of which are produced by specialized tracts of glandular cells at the edges of the mantle and on its external surface. Passing through the mantle on both sides are the fibrous masses of the anterior and posterior adductor muscles; these course directly from one valve to the other and are attached to the valves at 370 THE PHYLUM MOLLUSCA roughened areas termed muscle scars. Contraction of the powerful adductor muscles brings about closure of the valves. When the muscles relax, the elasticity of the hinge ligament causes the valves to gape ventrally. Other muscle masses, retractors and protractors, are variously developed in different species. Generally speaking, they occur in pairs both anteriorly and_ pos- teriorly, originating in the body wall of the clam and inserting near the adductor muscles on the internal surfaces of the valves. In effect, the re- tractor muscles suspend the body of the organism from the valves, and varia- tion of their states of contraction and relaxation adjusts the position of the animal within the shell. Enclosed between the two flaps of the mantle is a space, the mantle cavity, filled with circulating water. ‘The ventral edges of the mantle flaps, held closely together at the margins of the valves, effectively seal off the mantle cavity from the external environment. ‘The posterior margins of the mantle, however, are modified into two extensible tubes, the siphons, through which the mantle cavity communicates with the surrounding water. ‘The siphons may be protruded between the valves, and in Venus mercenaria they are provided with special triangular siphon retractor muscles attached to each valve. ‘Through the ventral or incurrent siphon water is drawn into the mantle cavity; through the dorsal or excurrent siphon it is conducted outward. Ventricle Rectum Left atrium Yi; TN Wey 3 {/ Suprabranchial chambers ‘= Mantle vein Typhlosole Efferent branchial vessel Afferent branchial vessel Fig. 13.2. Diagrammatic cross section of a typical fresh-water pelecypod, Anodonta, showing the heart and principal blood vessels. (From W. Stempell, 1926, Zoologie im Grundriss.) 371 GENERAL ZOOLOGY The major portion of the body, the visceral mass, occupies the central part of the mantle cavity. The foot is an extensive, flexible, muscular region attached along the ventral margin of the visceral mass and continuous with it. On each side of the body a pair of plate-like gills, each composed of rows of parallel water tubes, hangs down into the mantle cavity between the visceral mass and the mantle (Figs. 13.1, 13.2). Anterior to the gills are the labial palps, which, like the gills, are paired. ‘The mouth lies in the angle between the anterior end of the foot and the anterior adductor muscle, and the palps are connected across the midline by ridges which extend transversely and form lip-like structures above and below the mouth. In its normal position, embedded in the sandy or muddy bottom of a body of water, the animal usually lies with the median plane vertical and only the posterodorsal margins of the valves visible. Ordinarily, the valves are held slightly agape, and the siphons protrude into the water. ‘The action of cilia on surfaces bounding the mantle cavity maintains a gentle circulation of water, passing in through the ventral siphon, circulating in the mantle cavity and through the gills, and finally moving outward through the dorsal siphon. In locomotion the foot is thrust forward and either expanded at its tip or turned so that it takes a clumsy hold in the sand or mud of the bottom. The body is then drawn forward by the contraction of the powerful muscles of the foot. Structures and Functions Related to Metabolism and Responsiveness. During the following discussion, it should be borne in mind that the entire “economy” of the clam is based on the circulation of water through the mantle cavity. Powerful cilia on the mantle, on the surfaces of the visceral mass, on the gills, and on the palps function to maintain this circulation. The organism depends on these currents of water for food, for the elimination of egesta and excreta, for exchange of respiratory gases, and very often for the dispersal of gametes and zygotes. The water entering the mantle cavity through the incurrent siphon is drawn through the ostia, minute openings on the surfaces of the gills, into the water tubes, and through these vertical passages upward to the suprabranchial chambers. ‘There are four of these chambers, one lying above each gill and parallel to the longitudinal axis of the body. The suprabranchial chambers unite beneath the posterior adductor muscle to form a region known as the excurrent chamber, or cloaca, which opens externally by way of the excurrent siphon. In the course of this circulation through the gills, gaseous exchange is effected between the water and the circulating blood. Microscopic particles of food borne by the water entering the mantle cavity are sifted out by the cilia on the gills, or are entangled in sheets of mucus secreted by the mantle and the surface of the visceral mass. These food particles and food-laden sheets of mucus are moved toward the palps by specifically oriented ciliary pathways, and finally are conducted into the mouth by cilia on the palps. The palps appear to exercise a certain amount of selectivity, largely on the basis of weight: heavier particles are dropped by the palps and gather 372 THE PHYLUM MOLLUSCA ventrally in the mantle cavity. Here they are bound together by mucus and eliminated through the incurrent siphon by periodic vigorous contractions of the adductor muscles. Among the mollusks the ciliary-mucus feeding mecha- nism is best developed in the pelecypods, although some sessile gastropods have evolved a similar method. Certain tube-dwelling annelids and a few primitive chordates also depend on ciliary currents and sheets of mucus to entrap their microscopic food. The digestive system of the clam consists of mouth, esophagus, stomach, intestine, rectum, and anus (Fig. 13.1). The esophagus is short and leads directly into the stomach, which also receives the openings of a pair of branching digestive diverticula. ‘The stomach of many pelecypods is provided with another diverticulum, the style sac, which secretes and holds in its lumen a semisolid, gelatinous rod, the crystalline style. “This rod is kept in constant rotation by the cilia lining the sac, and its free end protrudes into the cavity of the stomach, where its substance gradually and continuously dissolves. This rod is essentially a mass of the secretion products of gland cells in the style sac. It contains digestive enzymes, largely amylases, which are re- leased into the stomach to function in the preliminary steps of the digestive process. Particles of partially digested food are conducted by ciliary currents into the branching lumen of the digestive diverticula, where digestion is con- tinued. Finally, particles of food are engulfed by cells lining the passages within the diverticula, and the process of digestion is completed intracellularly. Absorption occurs in the diverticula, and to some extent in the anterior parts of the intestine. The major function of the relatively long intestine and rectum appears to involve the dehydration and concentration of digestive wastes into fecal pellets or strands, which are eliminated at the anus into the outflowing streams of water in the cloaca. The well-developed circulatory system constitutes what might be called an “open” system. fae ‘ a td ar, 4 ‘zZ - Fig. 14.18. Class Hirudinea. A common fresh-water leech, Placobdella parasitica. A, dorsal aspect; B, ventral aspect. Note the anterior and posterior suckers and the division of the body into a large number of apparent segments. (From Master’s Thesis of M. H. Woods, 1940, Cornell University. ) 419 GENERAL ZOOLOGY Anterior sucker a Brain ZS &F = Circumpharyngeal ii he a connective Pharynx = 4) a \ Segmental ganglion Ovary ‘ Ss Ko Penis te Ke Vagina Fig. 14.19. Class Hirudinea. . ae Dorsal view of the medicinal leech, Caeca ee Nerve cord Fiirudo medicinalis, showing the gen- (full) : a oe cetera eral arrangement of internal or- ; Py ae 5 fe caeae yoe gans. (Redrawn, after Parker and & b = Gs Haswell, from F. A. Brown, Jr., in Caeca = LD Testes F. A. Brown, Jr., et al., Selected In- (empty) 9: \erat vertebrate Types, copyright 1950 by a" SH John Wiley and Sons, Inc., printed tall Nephridia by permission. ) Stomach \ 2 CH {KAW OS 1 RY Se \3 Ce 4H ee Intestine 9 Sa, D3? Rectum Posterior sucker Larger coelomic spaces persist in connection with the segmental nephridia and within the reproductive organs. Structurally, and to some extent functionally, the leeches show many evidences of affinities with the oligochaetes. ‘The indications are that the two groups have evolved in different directions from common, or at least similar, ancestry. Some fresh-water leeches are dangerous parasites of man and domestic animals. Some large leeches have an estimated capacity of 8 milliliters of blood. The terrestrial leeches found in many tropical countries of the Orient They occur on or near the ground, in the dank vegetation are serious pests. of the rain forests, and attach themselves to men and animals passing by. The Annelid Body Plan The type of structure found in annelids is important for comparison with the plans of organization of simpler and of more complex metazoans. In such comparative considerations the significant features of annelids involve chiefly 420 THE PHYLUM ANNELIDA (1) the simple, generalized condition of metamerism and (2) the development and relationships of the coelom and its derivatives. In comparison with the acoelomate, non-metameric turbellarians, for example, the annelid plan is considerably advanced. Comparison with a more complex metazoan, such as an arthropod or a vertebrate, shows that the relative complexity of these higher forms is largely a difference in degree. “The fundamental characteris- tics of metamerism and of the coelom are firmly established in the annelid; the more advanced structural features of arthropods and of vertebrates may be interpreted as elaborations and modifications of a basic plan already laid down in the annelids or in their ancestral stock. ‘This is the more interesting when we reflect that both segmentation and the coelom apparently originated differently and evolved independently, in the annelid-arthropod stock and in the echinoderm-chordate line (see Fig. 7.3, p. 219). In any case, in none of the metazoan phyla more advanced than the annelids are the characteristics of metamerism and coelom present in such an uncomplicated condition as in these segmented worms. Even within the phylum Annelida, evolution has involved departures from the primitive conditions in these characteristics. “The process of cephalization, the development of anteroposterior differentiation, has progressed to a con- siderable degree in the higher annelids. Along with cephalization there has been an increase in regional specialization. As a result, the originally uniform series of similar somites has become organized into a series of differentiated groups of somites, each group modified for the performance of some particular function. ‘This has involved general structure, the append- ages, and the internal organs as well, and it is perhaps best demonstrated among the sedentary, tube-dwelling polychaetes. The oligochaetes and leeches furnish examples of variant modifications of the originally undifferen- tiated and extensive coelomic pouches. In specifically delimited regions of the body these have been given over to reproductive functions; in the Hirudinea they have all but disappeared. Other annelid characteristics should be emphasized in anticipation of our discussions, in the next chapter, of the phylum Arthropoda. ‘Throughout their history, the annelids have retained and developed, as the basic locomotor mechanism, the body-wall musculature, consisting of concentric sheets of antagonistic circular and longitudinal muscle fibers. In correlation with the type of locomotion to which they are thus limited, and with their cutaneous method of respiration, the annelids have retained the thin, flexible, secreted cuticle as a protective cover for the body wall. ‘The nervous system, organized on the basis of quasi-independent segmental units, is also significant. All these characteristics, as we shall see, foreshadow special conditions to be encountered among arthropods. 421 ea oi JOINT-FOOTED ANIMALS AND The Phyla Arthropoda and Onychophora The Phylum Arthropoda Like the Annelida, the Arthropoda are bilateral, triploblastic, segmented animals. Unlike the annelids, however, the arthropods have a much reduced coelom, and the cuticle which covers the body in annelids is represented in arthropods by a hardened exoskeleton which must be periodically shed to per- mit growth. The name Arthropoda, which means “‘joint-footed,” refers to another conspicuous feature, the fact that the appendages are composed of several divisions, so hinged together as to be capable of specific movements. The taxonomy of arthropods is difficult and necessarily complicated, owing to the relatively large number of different types of animals which possess the general characteristics of the phylum but show important distinctions indi- cating varying degrees of interrelationship. ‘The scheme we shall follow, although it is not sufficiently detailed to satisfy a specialist, will nevertheless take into account many of these distinctions. We shall consider the phylum Arthropoda to be composed of three sub- phyla: the subphylum Trilobitomorpha, the subphylum Chelicerata, and the subphylum Mandibulata. The first of these is a group of extinct arthropods, represented by the Trilobita, which had a long evolutionary history dating from Pre-Cambrian times but are now known only as fossils. “The members of the subphylum Chelicerata are set apart from other arthropods because they are without antennae, or specialized sensory outgrowths from the head, 422 THEIR KIN: Fig. 15.1. Subphylum Trilobitomorpha. 4, dorsal view, and B, ventral view of a gen- eralized trilobite. [The appendages are all relatively unspecialized; the fringed lateral processes are interpreted as epipodites, which probably served for gaseous exchange. (Re- drawn from R. E. Snodgrass, Textbook of Arthropod Anatomy, copyright 1952 by Cornell University Press, printed by permission.) and the first pair of ventral appendages are modified into pincer-like struc- tures termed chelicerae. ‘The Chelicerata include four classes: the class Xiphosurida, containing “‘horseshoe crabs” of the genus Limulus; the class Eurypterida, an extinct group represented by the “‘sea scorpions” such as Eurypterus; the class Pycnogonida, peculiar, long-legged animals known as “sea spiders”; and the class Arachnida, which contains the familiar spiders and scorpions. The subphylum Mandibulata, as the name implies, includes those arthro- pods in which a pair of appendages flanking the mouth are adapted as mandi- bles, or jaws. Of the several classes grouped in this subphylum, the most important are the class Crustacea, mostly aquatic forms such as the lobster, the crayfish, and the crabs; the class Chilopoda, or centipedes, with long, many-segmented bodies and reproductive ducts which open posteriorly; the class Diplopoda, in which the segments are fused in pairs and the openings of the reproductive ducts are anterior; and the class Insecta, which are mostly terrestrial, wing-bearing arthropods with three pairs of walking legs. Famil- iar insects include flies, beetles, and butterflies. The habitats of arthropods are more diverse than those of any other phylum. Arthropods are found abundantly in the ocean, in fresh water, and on land, and most of the insects are adapted for flight. In correlation with this diversity of habitat the species present a great variety of habits and structural modifications, although the arthropod type of organization is well 423 GENERAL ZOOLOGY defined by segmentation, the exoskeleton, and the jointed appendages. No other phylum of animals approaches the arthropods in numbers of species. It has been estimated that the number of species of insects alone is well over 1,000,000, as compared with a total of some 37,500 species of chordates and 60,000 species of mollusks. If we add to the insects an estimated 30,000 species of crustaceans, 16,000 species of chelicerates, and 2000 species of chilo- pods and diplopods, the total is much larger than that of all other species of animals. We are accustomed to think of vertebrates as the dominant forms of terrestrial life at the present day; the vertebrates are far exceeded in numbers of species and numbers of individuals by the insects, which are so numerous that they literally contend with the vertebrates for possession of the earth. The arthropods are of considerable economic importance. Crustacea, such as lobsters, crabs, and shrimps, are a significant source of food. Certain in- sects produce silk, others prepare honey, and through the agency of insects many plants useful to man are pollinated. Some insects are beneficial to man in that they prey upon, or parasitize, other kinds of insects. On the other hand, the arthropods include species that destroy almost every form of vegeta- tion, others that are parasitic on man and his domestic animals, and still others that transmit the organisms of disease to man. The oldest known fossil remains of arthropods are those of trilobites (Fig. 15.1); according to the fossil record, these forms were already abundant and highly specialized in Cambrian times. It is logical to assume that these com- plex forms arose through a long Pre-Cambrian evolutionary history, and hence that more primitive and generalized arthropod types must have been in existence, possibly for some millions of years, before the beginning of our fossil record. All the modern arthropod groups had arisen and become well di- versified by the early part of the Mesozoic Era, roughly 200,000,000 years ago (see Fig. 20.1, p. 617). By this time trilobites had become extinct. Although Chelicerata undoubtedly arose earlier than Mandibulata, the two most successful and significant modern groups of arthropods are the mandi- bulate forms included in the classes Crustacea and Insecta. In the discussion to follow, we shall devote our attention chiefly to these two classes, referring more briefly to other arthropods. THE CLASS CRUSTACEA The Crustacea, for the most part aquatic animals, include the principal marine representatives of the phylum Arthropoda. During their evolution the crustaceans appear to have spread from their primitive habitat, the ocean, to fresh water and in a few cases to terrestrial life, in a manner reminiscent of the gastropod mollusks. The two principal types of crustaceans, Entomos- traca and Malacostraca, will be considered after we have examined the cray- fish as a typical crustacean. The Crayfish: Habitat and Activities. Crayfishes are abundant in streams and fresh-water ponds of all the continents and many of the larger 424 THE PHYLA ARTHROPODA AND ONYCHOPHORA Fig. 15.2. A fresh-water crayfish in its natural habitat, partially concealed under a_ stone. Note the evident utility to the animal of the elongate, sensitive antennae and antennules and of the movable compound eyes. (Photograph by Charles W. Schwartz.) islands, such as Tasmania, New Zealand, and Madagascar. ‘Their distribu- tion is limited by certain environmental factors, notably the availability of calcium carbonate in the water. Different species are adapted to various habitats: some frequent rapidly flowing streams; others are found only in stag- nant ponds or sluggish streams; still others occur in wet meadows and marshes. Species of the genera Cambarus and Orconectes are widely dis- tributed in the more temperate regions of North America east of the Rocky Mountains, and several species of the genus Astacus are found in the streams of the Pacific Slope. The account of the crayfish that follows is applicable to the common species of Cambarus and related forms. In nature, the crayfish is found crawling upon the bottom or concealed under stones (Fig. 15.2), or in burrows which many species excavate into the 425 GENERAL ZOOLOGY banks of ponds and streams. Some forms burrow for considerable distances into the banks, or into the soil of wet meadows far from open bodies of water. They construct entrances and air holes that appear as chimney-like masses of mud brought up and deposited around the openings. ‘The burrow usually terminates in a chamber below or near the waterline. In moving about upon the bottom of a stream or pond, the crayfish walks slowly forward with its great claws held in front of the body. Its common es- cape reaction is to dart backward through the water with great rapidity, pro- pelled by sudden strokes of the tail fin. As the animal glides after each stroke, the abdomen is folded under; coming to rest upon the bottom, the animal lifts and expands the abdomen in preparation for another stroke. Crayfishes respond quickly to visual stimuli; but in burrows, where little light penetrates, and in their nocturnal activities generally, various receptor organs for tactile and chemical stimuli must be more significant. Such receptors are generally distributed over the body but are most numerous on the two pairs of anten- nae. In feeding, the crayfish captures animals, such as aquatic insects and fishes, by lying in wait and seizing them with its claws. It also lives as a scavenger, feeding upon the bodies of animals found dead upon the bottom. Crayfishes are primarily aquatic animals, but in the laboratory they thrive best if kept where they can crawl out of the water, and they will often remain exposed in a moist atmosphere for hours. In their natural habitats, along a stream at night, crayfishes are sometimes seen upon the bank near the water; they occasionally make nocturnal expeditions of some length upon land, possibly in search of food. General Structure. As in all segmented animals, the body of the crayfish is composed of a series of somites. ‘These are not all similar, however, but are grouped together and modified to form definite, specialized regions of the body. ‘Three regions, the head, the thorax, and the abdomen, may be dis- tinguished externally. The head and the thorax are fused to form the so- called cephalothorax, covered dorsally and laterally by a non-segmented portion of the exoskeleton, the carapace. ‘The carapace terminates anteriorly in a pointed rostrum, and the boundary between head and thorax is marked on the carapace by a transverse cervical groove. In these anterior regions the underlying segmentation of the body, obscured dorsally by the carapace, is revealed ventrally by the segmental origin of a series of paired appendages. In the abdomen the somites are clearly demarked both dorsally and ventrally. The paired eyes and two pairs of sensory appendages, antennules and antennae, project laterally and anteriorly from their attachments beneath the rostrum. ‘The appendages about the mouth, which are modified to assist in the capture and manipulation of food, are distinguished as oral appendages, or “mouth parts” (Fig. 15.3). Of these, the mandibles and two pairs of maxillae originate from segments of the head; the following three pairs of oral appendages, the maxillipeds, originate from the three anteriormost segments of the thorax. Behind the third maxillipeds arise the great pincer-bearing appendages, the chelipeds, which constitute the first of five pairs of walking 426 THE PHYLA ARTHROPODA AND ONYCHOPHORA Fig. 15.3. Comparative aspects of selected appendages of the lobster, Homarus americanus. A, antenna; 6, second maxilla; C, second maxilliped; D, third pereiopod or walking leg; E, third pleopod or swimmeret; /, uropod. Abbreviations: ex, exopodite; end, endopodite; pro, protopodite; ep, epipodite; 4r, branchia. (Redrawn from F. H. Herrick, 1909, Bulletin of the U.S. Bureau of Fisheries, vol. 29.) 427 GENERAL ZOOLOGY legs, or pereiopods; all these are thoracic appendages. ‘The abdomen bears five pairs of delicate, paddle-like appendages, the swimmerets, which function in maintaining water circulation, and to which, in the female, the zygotes are attached at the breeding season. In the male the two anterior pairs of ab- dominal appendages are strongly modified as copulatory organs, whereas in the female the first pair is very small. In both sexes the most posterior appendages, borne laterally upon the last abdominal segment, are the fan-like uropods. A median, flap-like outgrowth of the last segment, termed the telson, bears the anus upon its ventral face. The telson is not a segment, and as it is neither paired nor jointed, it is not considered an appendage. ‘The uropods and telson together form the broad tail fin used in swimming. There are thus 19 pairs of appendages; 5 pairs belong to the head, 8 to the thorax, and 6 to the abdomen. If we assume that each somite bears a single pair of appendages, there are 19 somites in the body. ‘This count is confirmed by the appearance, during the course of development, of 19 pairs of ganglia in the central nervous system, although not all these ganglia are distinct in the adult animal. Comparing the structure of the appendages, we find that all are based on a similar plan, although some are so greatly modified that the homologies are not clearly recognizable until the developmental stages are examined. ‘The simplest appendages are the swimmerets, which in the adult show the funda- mental plan of structure: a basal protopodite with two segments, bearing at its distal end two branches, a lateral exopodite and a medial endopodite. In the pereiopods the protopodite bears in the adult only one distal branch, the endopodite, which is divided into five segments. Exopodites are present on these appendages until a late stage of development but are lost before the animal reaches adulthood. In the maxillipeds the three fundamental divi- sions, protopodite, exopodite, and endopodite, again appear. In the maxillae and mandibles it is necessary to refer to the developmental stages to deter- mine homologies. ‘The mandible, for instance, bears an exopodite during development but in the adult consists of only protopodite and endopodite. The antennae show the three fundamental parts, with the endopodite greatly elongated. ‘The antennules are similarly divided into a basal segment bear- ing two distal branches, here both elongated, but their homology with the other appendages is uncertain. he part of the head bearing the eyes and the antennules probably had an origin different from that of the somites, and hence these sensory outgrowths of the head may not be homologous with the segmental appendages. The basic structural similarity of all the true segmental appendages of the crayfish is interpreted as indicating that at some stage in the remote ancestry of these forms, the somites all bore similar, simple, biramous appendages, possibly resembling the swimmerets of the modern crayfish. As regional specialization of the body developed, the appendages underwent differential modification in adaptation to the more efficient performance of specific opera- tions. ‘The mouth parts, for example, are clearly adapted in a variety of ways 428 THE PHYLA ARTHROPODA AND ONYCHOPHORA for grasping and manipulating food; the development of the chelipeds is simi- lar in nature to that of the other pereiopods, but different in the extent to which it has been carried. In the course of these adaptive processes, various parts of the appendages have developed differently; the exopodites are often reduced in size, and some disappear during the growth stages of the individual. The segmental appendages of such crustaceans as the crayfish offer excellent material for the study of serial homology. ‘The progressive nature of the modifications, which are often completed only in the adult stage, is indicative of a certain amount of recapitulation; that is, each individual in its develop- ment more or less repeats, in abbreviated form, the long history of evolution of its ancestry. The gills are located in the thoracic region, lying in branchial chambers covered on each side by a lateral flap of the carapace (Fig. 15.4). The gills are a series of lateral outgrowths of the body wall, protected by the exten- sions of the carapace, much as the gills of a pelecypod mollusk are covered by the mantle flaps and the shell. In most crayfishes two kinds of gills are distinguished, on the basis of their points of origin: podobranchiae, arising with the non-branchial epipodites from the basal segments of the thoracic limbs; and arthrobranchiae, arising from the joints by which the thoracic limbs are attached to the body. In the lobster and in some crayfishes there are also pleurobranchiae arising from the sides of most thoracic segments. ‘Typically, each thoracic segment bears one pair of podobranchiae and two pairs of arthrobranchiae, but modifications of this plan are frequent. In concluding this general account of the external features of the crayfish, it should be emphasized that the skeleton is a continuous structure, cover- ing the entire external surface of the animal and even forming the lining of the digestive tract at its anterior and posterior ends. Even the most delicate Carapace Pericardial sinus S y Branchiopericardial BES = Su B RR canal Gonad Ai ; = D 2 Arthrobranchiae co TEN \\\ , \ per rar kha Pleurobranchia—# SS \ E B Digestive gland - Sternal artery Gut Efferent branchial : vessel Podobranchia Muscle Ox ‘ 3 or “> ; Ventral nerve cord ee: Et oe Lateral sinus Ventral thoracic artery ~ “f= Sternal sinus Fig. 15.4. Diagrammatic cross section of a crayfish at the level of the sternal artery (cf. Fig. 15.5). Arrows indicate direction of blood flow. 429 GENERAL ZOOLOGY external parts, such as the feathery gills and the hair-like sensory setae on the appendages, are covered by a thin layer of the skeleton. Like the cuticle of an annelid, the crustacean exoskeleton is a non-cellular, non-living secretion product of the cells of the epidermis. Basically, the exoskeleton is composed of varying proportions of tough scleroproteins and a characteristic nitrogen- containing polysaccharide called chitin. At points of flexure, as between somites of the abdomen and between divisions of the appendages, the skeleton is thin and flexible. In its thicker portions, as in the carapace, the organic substance of the basic skeleton is hardened by the addition of calcium carbonate. The exoskeleton of the crayfish may be progressively thickened by the deposition of additional material, but it cannot stretch laterally except in the early stages of its formation, before the calcium carbonate has been deposited. As a result of this mechanical relationship between the skeleton and the completely enclosed body, growth cannot occur by continual ad- ditions to the skeleton, as it can with the exoskeleton of a mollusk. Hence, the crayfish, like other arthropods, periodically resorbs some of the material of its skeleton and secretes a thin, new skeleton beneath the old one. ‘The old one, including the lining of both ends of the digestive tract, is then shed, or molted. In the few hours immediately following such a molt, the crayfish, covered only by the thin, elastic, flexible new skeleton, imbibes water and swells to a larger size. As the new skeleton progressively hardens during the days that follow, the animal assumes again its normal hard-shelled condition. During its soft-shelled state the crayfish is defenseless; and, since its muscles are attached only to the flexible new skeleton, its powers of movement are much impaired. It usually remains in its burrow or otherwise concealed until the skeleton has attained some degree of hardness. ‘The soft-shelled crabs, considered a table delicacy, are merely crabs captured so soon after molting that their shells have not yet hardened. Structures and Functions Related to Metabolism. ‘The digestive system consists of the digestive tract and its appended glandular organs (Fig. 15.5). A short esophagus leads from mouth to stomach, the anterior portion of which contains the gastric mill. This structure has teeth or ossicles formed by thickenings of its skeletal lining; the teeth are so arranged that they grind against each other and so complete the mastication of ingested food. Large, paired digestive glands open into the anterior region of the intestine, immedi- ately behind the stomach. Finely divided particles of food are passed through filters in the stomach and into tubular cavities within the digestive glands. These tubules are lined by an epithelium composed of several types of cells: one type secretes digestive enzymes which accelerate the breakdown of food in the tubules; another type functions in absorption of the products of diges- tion and in storage of energy reserves such as glycogen and fat. Undigested residues of food are returned into the cavity of the intestine and carried back through the long hind-gut for egestion at the anus. ‘The intestine apparently serves little or not at all in digestion and absorption; only the portion of the intestine into which the digestive glands open is lined by a mucosa derived 430 THE PHYLA ARTHROPODA AND ONYCHOPHORA Heart with ostia : ' Posterior ‘ Pericardial Sternal aorta Anterior aorta cavity artery eee Ventral abdominal cephalic artery artery Afferent \ Sternnl Pines Y branchial Efferent sinus vessel branchial Ge } A vessel Ventral ee Abdominal thoracic SoS artery Intestine Nerve cord with ganglia Fig. 15.5. General relationship of the vascular, digestive, and nervous systems of the crayfish; semidiagrammatic. A, vascular system; distributing vessels are shown in black; collecting vessels and sinuses are stippled. 8, digestive system (stippled) and nervous system (black); the large digestive glands, which occupy much of the space in the cephalothorax, have been omitted from this diagram. (Modified after K. von Frisch, 1952, Brologie, vol. 1, printed by permission of Bayerischer Schulbuch-Verlag. ) from endoderm during development. The remainder of the digestive tube is lined by tissues derived from ectoderm and is covered by a cuticle of varying thickness. The digestive glands, and the viscera generally, are surrounded by extensive cavities through which blood flows after it has left the arteries (see p. 432). These cavities, because they contain blood, are collectively termed the hemocoel. They are not homologous with the coelomic cavities of other schizocoelous forms, although the true coelomic cavities of the embryonic stages may contribute slightly to their formation. More definitely identifiable remnants of the embryonic coelom are found as sacs at the inner ends of the excretory organs, and perhaps as the cavities within the hollow ovaries and testes. 431 GENERAL ZOOLOGY The circulatory system of the crayfish, unlike that of such annelids as the earthworm, is not a “closed” system with capillaries. Rather, it resembles superficially the “open” circulatory system described for most mollusks. Branching arterial vessels conduct blood from the heart to the organs, where the arteries terminate (Fig. 15.5). Leaving the arteries, blood percolates through the tissues and collects in sinuses, or hemocoelic spaces. From the general hemocoel blood flows ventrally into the sternal sinus and the lateral sinuses with which the sternal sinus communicates. From the lateral sinuses a series of vessels constituting the branchial circulation carries blood through the gills for oxygenation. ‘The blood then passes through a system of venous channels into the pericardial sinus surrounding the heart, and re-enters the heart, when it relaxes between beats, through three pairs of valved openings, the ostia. Such an “open” system is not markedly efficient; the pressure exerted by the heart in contraction is rapidly dissipated in the large sinuses, and blood flow through the gills is relatively slow. Such a system obviously suffices for the needs of the animal, however. Blood flowing to the tissues transports to them food picked up in the digestive glands, and oxygen from the gills, and carries away carbon dioxide and nitrogenous wastes. Carbon dioxide is exchanged for oxygen in the branchial circulation, and nitrogenous wastes are removed from the blood in the ex- cretory organs. The gills and their circulation, which constitute the mechanism of gas exchange, have been adequately described. ‘The efficiency of this mechanism is enhanced by the maintenance of an external water circulation over the gills. In the living crayfish water is continually drawn under the posterior and ventral edges of the carapace into the gill chambers, where it passes forward, bathing the gills, and is ejected anteriorly. ‘These water currents are pro- duced by a specialized, paddle-like extension of the second maxilla on each side of the body. As previously mentioned, the swimmerets aid by keeping the water in motion about the posterior, ventral part of the body. The excretory organs are a pair of compound tubular structures termed the green glands. ‘These lie in the hemocoel, one on each side, just anterior to the stomach. Each (Fig. 15.6) consists of a terminal, flattened end sac, a convoluted labyrinth or cortex, and a distal tubular portion, at least partially lined by secretory epithelium, leading from the labyrinth to a small bladder. The bladder opens externally by a nephridiopore on the basal segment of the antenna. ‘The green gland is interpreted as a compound nephridium; the end sac is considered the remnant of a coelomic pouch. ‘The blood supply to these organs is copious, derived from branches of the lateral cephalic and ventral thoracic arteries. ‘The functions of the green glands are not completely under- stood. According to current interpretations, nitrogenous wastes and salts are extracted from the blood in the labyrinth and proximal parts of the tubule; as the urine thus formed flows outward through the tubule, secretory cells lining the distal portion selectively reabsorb salts and return them to the blood. ‘The comparatively watery urine remaining in the tubule is then 432 THE PHYLA ARTHROPODA AND ONYCHOPHORA Bladder Sphincter muscle 7 »—-Pore Coelomosac Bladder Coelomosac Sphincter muscle Cortex Excretory pore A B Fig. 15.6. Green gland of the crayfish; semidiagrammatic. A, the gland extended, showing relationships of the various portions. 8B, the gland in vertical section, as it lies in the hemocoel. (Redrawn from P. Marchal, 1892, Archives de zoologie expérimentale et générale, vol. 10.) passed into the bladder, which empties periodically to the exterior. It is thus apparent that through the selective activities of the secretory tubular cells, the green glands are important not only in excretion but also in salt and water balance. The Nervous System and Responsiveness. ‘Vhe general plan of the nervous system in the crayfish is similar to that of annelids (Fig. 15.7). There is a dorsal ganglionic mass, the supraesophageal ganglion, or “‘brain,” from which nerves extend to the eyes and antennae. ‘The brain also gives rise to a pair of circumesophageal connectives which pass around the esophagus and join with the subesophageal ganglion at the anterior end of the ventral nerve cord. Nerves arising from the subesophageal ganglion innervate the six pairs of oral appendages, the green glands, the esophagus, and the muscles of the anterior region of the thorax. In the course of embryonic development the subeso- phageal ganglion is formed by the coalescence of six pairs of ganglia, corresponding to the six pairs of oral appendages. The ventral nerve cord is a double structure, composed of fused, paired ganglia, joined together in a linear series by connectives. Posterior to the subesophageal ganglion there is a pair of ganglia for each somite; the metameric nature of the animal is thus revealed by the segmental organization of the nervous system, as well as by external divisions and the distribution of appendages. The antennules and antennae bear very numerous receptors, for both tac- tile and chemical stimuli. In addition to these receptors, the hair-like proc- esses upon the appendages and other parts of the body, such as the edge of the carapace, are tactile in function. Related to these tactile receptors are two statocysts, which are organs of equilibration located in the basal segment of each antennule. ‘The statocysts are sac-like invaginations from the outer surface of the appendage; they are thus lined with an exoskeletal 433 GENERAL ZOOLOGY Circumesophageal connective Subesophageal Fig. 15.7. Nervous system of a ganglion crayfish, Astacus; dorsal view. (Redrawn with modifications First thoracic from W. Schmidt, 1915, Zevt- ganglion schnift fiir wissenschafthiche Ovipositor ee Lu u——_ B Meso- Metathorax Fig. 15.15. General features of a locust, Melanoplus differentialis. A, female, lateral view; B, female, ventral view. (Redrawn from E. O. Essig, College Entomology, copyright 1942 by the Macmillan Company, printed by permission.) 445 GENERAL ZOOLOGY Fig. 15.16. Face view of a small locust. Note the prom- inent compound eyes and an- tennae, and the shield-like labrum covering the mouth parts anteriorly. (Photo- graph by Charles Walcott.) are a pair of mandibles, or chewing jaws; a pair of maxillae, jointed, leaf-like structures modified to aid in the manipulation of food; and a single lower lip, the labium, which is derived during development by the fusion of a pair of second maxillae. The mouth parts which are not segmental appendages are the labrum, or upper lip, formed as a downgrowth from the anterior surface of the head, and the hypopharynx, a median projection from the floor of the mouth which acts more or less as a tongue. Possessing mandibles and other mouth parts adapted for chewing, the locust is spoken of as a mandibulate insect. As we shall see, in other insects the same fundamental mouth parts may be modified for sucking, for piercing and sucking, and for lapping; however, the primitive insects are believed to have been mandibulate. Each of the three thoracic segments bears a pair of jointed appendages, the legs. These are all similar in structure, but the posterior or metathoracic legs are highly developed and specialized for leaping. At the distal end of each leg, a small pad and a pair of hooks provide grasping organs by which the insect obtains a firm hold upon vegetation. ‘Iwo pairs of wings are attached to the dorsolateral surfaces of the mesothoracic and metathoracic 446 THE PHYLA ARTHROPODA AND ONYCHOPHORA segments, as in most other insects. In the locust the anterior wings are usually heavy and tough and function as protective covers for the delicate, membranous metathoracic wings. In the lubber locusts both pairs of wings are greatly reduced, with a corresponding decrease in the power of flight. The wings of insects are composed mainly of exoskeleton, with a very small amount of cellular material between the upper and lower skeletal layers. They are stiffened and supported by thickened structures, called ‘“‘veins,”’ between which the wing is thin and membranous. ‘The veins also mark the pathways of channels through which blood circulates. The abdomen lacks segmental appendages, and none appears in this region during development in the locust; in certain other insects there are traces of vestigial abdominal appendages in the embryonic stages, and these persist to adulthood in some small, wingless, primitive insects. In the female locust a large ovipositor is present at the posterior end of the abdomen, surrounding the external opening of the female reproductive system. In the male an eversible penis marks the opening of the genital duct, just ventral to the anal opening at the tip of the abdomen. External openings of the respiratory system appear as the paired spiracles; one lies on each side of every thoracic segment and each of the first eight abdominal segments. In life, the spiracles are opened and closed rhythmically with the breathing movements of the body. The so-called tympanic membranes, which are the external parts of the auditory organs, are conspicuous structures like eardrums on the dorsolateral portions of the first abdominal segment of the locust. a yD A Pa AAC AG f 4 mk CAV has ge HM, Y ; ? ¢ \ P \ ? = (an ie j \ Sa \ | Rw ar \ 4 1 Labrum | Soa t > ia * j — f \& ‘ re Ve ] am, | aah o\ Gs a bss Y plat EB l=» (g N 4 ao / oY g| \74, salt EN hak Ys eg 44, 4 vi/ aL 5 f Fig. 15.17. Mouth parts of a_ locust. Mandibles (Adapted from T. Dobzhansky, Evolu- tion, Genetics, and Man, copyright 1955 cas by John Wiley and Sons, Inc., printed bie Yi Maxillae by permission. ) 447 GENERAL ZOOLOGY Esophagus Aorta Crop Ovary Heart I . —————- ntestine —— olon SS = Ss AMG it —— ee Ocellus Fwy)! mr. , Se 1 SFE Brain Nn 2 | Zs ee <\s Genital A = opening F : i Nerve Seminal Mouth Saipan Gastric Ventriculus cor d eas Optic lobe a Malpighian Oviduct Connectives tubule oso cS —SSS= —— up 5 SS Saas Subesophageal Thoracic Abdominal B ganglion ganglia ganglia Thoracic air sac Abdominal air sacs Tracheal trunks peered: Abdominal spiracles Heart spiracles Dorsal diaphragm Spiracle Salivar gland Tracheal trunk ne! ‘ . Ge 4 ; ae . D Nerve cord Ventral diaphragm Fig. 15.18. Details of some internal features of the locust. A, lateral view, showing rela- tionships of digestive, reproductive, nervous, and excretory systems. 8B, major parts of the central nervous system, ventral view. C, major parts of the tracheal system. D, cross section of the body in the thoracic region; all cross-hatched structures are tracheal branches. (Adapted from E. O. Essig, College Entomology, copyright 1942 by the Macmillan Company, printed by permission. ) Structures and Functions Related to Metabolism. ‘The internal anatomy is much the same in all species of locusts. The digestive system (Fig. 15.18) develops by specific modifications, extremely specialized in some insects, of an originally simple tube running from mouth to anus. Only the midregion of this tube, as in the crayfish, is lined by tissues derived from embryonic endoderm. The anterior and posterior regions are lined by inward extensions of the epidermis and are covered by cuticle, which is shed as the animal 448 THE PHYLA ARTHROPODA AND ONYCHOPHORA molts. ‘The buccal cavity, enclosed by the mouth parts described above, receives the ducts from the salivary glands lying in the thorax. From the buccal cavity a short esophagus leads dorsally, widening to produce a thin- walled crop which extends into the thorax. ‘The crop tapers posteriorly and joins the stomach. ‘The junction between crop and stomach is marked by six pairs of glandular outgrowths of the digestive tract known as gastric caeca; these caeca secrete a juice containing digestive enzymes. Posterior to the stomach lies the intestine, which has a narrow region, the ‘‘colon,” followed by an expanded rectum leading to the anus. ‘The division between stomach and intestine is marked by the zone of attachment of the Malpighian tubules, which are excretory organs. In feeding, the animal bites off pieces of grass and other vegetation with its mandibles, using the labrum and labium as upper and lower lips, and the palps of the maxillae and labium as tactile and gustatory organs. ‘The salivary secretion, emptied into the buccal cavity, serves as a lubricant in swallowing and as a digestive fluid acting upon food stored in the crop. Digestion may occur to some extent in the crop, but the cavity of the stomach is the principal digestive region. Absorption of products of digestion into the circulating blood occurs in the stomach and the anterior part of the intestine, and assimilation follows the diffusion of these transported nutrients into the cells of the body. As in the crayfish, the contribution of the embryonic coelom to the definitive body cavities of the locust is problematical. ‘The spaces between muscles and surrounding the viscera are hemocoels and do not, on the whole, represent a modified coelom. In the adult locust this space is largely occupied by an irregular mass of storage tissue termed the fat body. Such fat bodies make up most of the body bulk in insect larvae storing up food reserves in advance of metamorphosis. Leading from the spiracles on the thoracic and abdominal segments are the air tubes, or tracheae, which form the respiratory system. ‘The tracheae unite and branch in a complex manner and expand into conspicuous reservoirs, or air sacs, in the abdominal region (Fig. 15.18). The fine end branches of the tracheae, the tracheoles, ramify within all the tissues of the body and permit the direct delivery of atmospheric oxygen to the fluids surrounding the cells. Air must first enter the body through the spiracles, a fact which makes it possible to kill insects by clogging these openings with dust or films of oil and soapsuds. Oxygen enters the cells of the insect from the intercellular fluid and is used, as in other animals, in cellular metabolism. ‘The tracheal type of respiratory mechanism is found only among the higher arthropods and their relatives, the Onychophora. ‘he fact that many insects are capable of rapid and sustained metabolic activities, as in prolonged periods of flight, is an indication that the tracheal system has been developed to a high point of efficiency. Malpighian tubules are the chief excretory organs of insects, functioning to re- move nitrogenous wastes of metabolism from the blood and eliminate them from the body. Each tubule, of which there may be dozens or hundreds, is 449 GENERAL ZOOLOGY SN Potassium + Uric Potassium urate + Water acid bicarbonate oe Ma M Potassium + Carbon . Potassium Uric acid urate * Water dioxide bicarbonate peat ae precipitate Alk - (pH shift) ——> Acid NS Malpighian tubule _ KX WS Fig. 15.19. Schematic diagram of the general mechanism of uric acid excretion in the Mal- Lumen of Blood in hemocoel hind - gut pighian tubules of insects, based on the blood-sucking bug Rhodnius. (Adapted from V. B. Wigglesworth, 1931, Journal of Experimental Biology, vol. 8, printed by permission.) composed of large cells surrounding a tubular lumen. ‘The tubule is closed at its distal end, but the proximal end communicates with the lumen of the intestine. It is generally spirally wound about by several strands of muscle fibers. ‘The tubules lie in the hemocoel, bathed by blood, from which the cells extract wastes, chiefly in the form of salts of uric acid (Fig. 15.19). These salts are passed through the wall of the tubule into the lumen and move downward, in solution, toward the intestine. Cells in the more proximal portions of the tubule extract water and certain inorganic constituents from this “urine,” resulting in the precipitation of uric acid as crystals or concretions in the lumen of the tubule. By gentle muscular pulsations of the tubule, the precipitated mass, in the form of a paste, is emptied into the lumen of the hind-gut. Here more water is extracted, and the wastes are eliminated from the body with the feces. This excretory mechanism may be interpreted as an adaptation for the conservation of water, by eliminating wastes with the smallest possible loss of water. The development of the tracheal system has relieved the blood of a function which in most other animals is of major importance, that of distributing respiratory gases. Consequently, it is not surprising to find that the circulatory system is not extensively developed, and that the flow of blood is relatively sluggish. ‘The heart of the locust is a slender, pulsatile tube extending along the dorsal midline of the abdomen (Fig. 15.18). In each segment throughout its length, the heart is provided with a pair of valved openings, or ostia. ‘The heart is supported from the dorsolateral body wall on each side by fan-shaped groups of alary muscles and lies in a division of the hemocoel termed the pericardial sinus. ‘This space is separated from the perivisceral hemocoel by a perforated membrane, the dorsal diaphragm. Blood enters the heart from the pericardial sinus through the ostia and is driven forward, into the thorax and head, through the aorta, which terminates near the brain. From the head, 450 THE PHYLA ARTHROPODA AND ONYCHOPHORA blood flows posteriorly through the hemocoelic spaces, bathing the tissues, un- til it again reaches the abdomen, passes through the dorsal diaphragm, and re-enters the pericardial sinus and the heart. ‘There is a variety of colorless, amoeboid cells in the blood of insects, but nothing comparable to the erythro- cytes of vertebrates; the blood contains no pigments involved in the transport of oxygen. The Nervous System and Responsiveness. ‘The nervous system of the locust is arranged according to the typical arthropod plan, as described for the cray- fish. ‘There is a compound ganglionic mass in the head, termed the “brain” or supraesophageal ganglion; a pair of circumesophageal connectives; a sub- esophageal ganglion; and a ventral chain of paired, segmental ganglia with their connectives and radiating nerves (Fig. 15.18). The brain lies in the head between the compound eyes. Although it is obviously a compound structure produced by fusion of segmental ganglia, the exact number of such ganglia included cannot be determined. Large optic nerves pass into the brain from the compound eyes, smaller nerves from the ocelli, the antennae, and the labrum. In addition, small nerves extend ventrally to the frontal ganglion, from which the visceral or sympathetic nerve leads to the anterior part of the digestive tract. The circumesophageal connectives pass around the digestive tract and join the subesophageal ganglion. ‘Ihis also has a com- pound origin, apparently being formed during development by the coalescence of three pairs of segmental ganglia. From the subesophageal ganglion nerves radiate to the mandibles, the maxillae, and the labium. ‘The thorax contains three paired ganglia, corresponding to the three thoracic somites. But the posterior ganglion is comparatively large, and its nerves are distributed in a way that indicates its formation by fusion of the third thoracic and first ab- dominal ganglia. There are five ganglionic masses in the abdomen, which again represent fusions, particularly at the posterior end of the nerve cord. The adult locust, therefore, has fewer ganglia than somites, but in the embryo there is a ganglion for each somite, as in the less highly specialized arthropods. Like the majority of other insects, locusts are well equipped with sense organs. Small receptors for tactile stimuli are widely distributed over the surface of the body, and they are concentrated in especially sensitive areas, such as the antennae, the cerci at the tip of the abdomen, some of the mouth parts, and the distal segments of the legs. Olfactory stimuli also affect the basal parts of the antennae. Some insects are able to respond to very slight olfac- tory stimuli. For example, the males of certain species of moths will fly up- wind from considerable distances in response to odors emitted by the scent glands of the females. Gustatory stimuli are perceived by taste organs on the mouth parts of the locust. The ocelli are sensitive to light but are probably incapable of forming images. When the compound eyes of a locust are covered with black paint, the insect will not react to moving objects by leap- ing away, but it will find its way out of a box in which there is only one small opening to admit light. When the ocelli as well as the compound eyes are covered, the animal escapes from the box only by chance. It has been shown 451 GENERAL ZOOLOGY that some insects respond differently to different wavelengths (i.e., colors) of light, although the range of stimulation is not the same as that in humans. Ants, for example avoid violet light, as they avoid direct sunlight, and they seem not to distinguish red or orange light from darkness. Ants and honey- bees are very sensitive to ultraviolet light, but humans have no conscious per- ception of light in this portion of the spectrum. ‘The structure of the com- pound eye in the locust is similar to that described for the crayfish (p. 435). Each eye consists of a large number of visual units, the ommatidia, each of which is capable of stimulation by light from a portion of the entire visual field. Eyes of this type, as previously noted, seem especially well adapted to the perception of moving objects, which stimulate different ommatidia in succession. In the locust, the tympanic membranes are assumed to be auditory organs, more because of their structure than from any experimental evidence. Each consists of a membrane, against the inner side of which lie structures con- nected with nerves. Individual locusts react to, and so presumably “hear,” the rattling sound produced by the wings of other locusts in flight. Com- parable flight sounds are characteristic of many other insects; the buzz of a fly and the telltale whine of a mosquito are of this nature. ‘The flight sounds of mosquitoes appear to be important in mating; recordings of the sounds emitted by females in flight have been shown to attract males in considerable numbers. The auditory organs of mosquitoes are located on the antennae. The most conspicuous anatomical feature of the neurosensory mechanism in insects is the degree of cephalization—the concentration of the sense organs and ganglia with their adjustor neurons toward the head or cephalic end of the animal. ‘This fact is obviously correlated with the very active life of most insects, which subjects them to frequent and varied changes in their environments; the condition resembles the more pronounced cephalization that characterizes the vertebrates. Insects exhibit reflex actions of great complexity, involving what have been called chain reflexes in vertebrates. Many of the activities which will be mentioned in discussing other groups of insects will illustrate this statement. There is no evidence that insects have any capacity comparable to the intelligence of higher vertebrates, although they perform instinctively very involved actions, especially in connection with mating, care of young, and colonial life. The Reproductive System, Reproduction, and Development. In the account of external features the differences between male and female locusts have been described. Such differences in external characteristics are reflected in the internal details of the reproductive systems (Fig. 15.18). In the male there are two testes, which lie as a saddle-shaped mass dorsal to the intestine. The tubules leading from the testes pass ventrolaterally and unite into right and left ductus deferentes (vasa deferentia), which join at the midline be- neath the intestine to form a single ejaculatory duct traversing the penis. Accessory glands, which open into the anterior end of the ejaculatory duct, secrete a fluid apparently necessary to aid the transfer of spermatozoa from 452 THE PHYLA ARTHROPODA AND ONYCHOPHORA SP eS Fe ee” Fig. 15.20. A_ locust, Trimerotropis coeruleipes. A, mature female deposit- ing a packet of eggs in the ground, having dug a hole with her ovipositors. B,a young nymph. Note the differ- ences in proportions, and in develop- ment of the wings, between nymph and adult. (A, from E. S. Ross, Insects Close Up, copyright 1953 by Uni- versity of California Press, reprinted by permission; B, photograph by 1d, Ss IRoss)) male to female at copulation. In the female there are two ovaries, lying in a position corresponding to that of the testes in the male. Each ovary is com- posed of large numbers of tubular ovarioles in which the ova are formed. The ovarioles have a common anterior origin in the ovarial ligament, and all the ovarioles of each side unite posteriorly to form an oviduct. Passing ventrally like the ductus deferentes of the male, the two oviducts join beneath the in- testine into a median tube, the vagina, which opens externally between the valves of the ovipositor. A small, tubular appendage of the vagina, the seminal receptacle, receives and stores spermatozoa transferred by the male at copula- tion. At the time of oviposition, eggs pass singly down the oviducts and through the vagina; as they move past the opening of the seminal receptacle, they are penetrated by spermatozoa from the stored supply and so leave the vagina as zygotes. The female deposits these zygotes, called “eggs,” in masses of several dozen, placing them in the soil in holes dug beneath the sur- face by thrusting movements of the abdomen and ovipositor (Fig. 15.20). In 453 GENERAL ZOOLOGY Adult Fig. 15.21. Life cycle of an ametabolous insect. Juvenile stages Embrya ‘one temperate climates most species of locusts lay their eggs during late summer and autumn. ‘The eggs undergo a period of dormancy, or diapayse, and re- sume development only after they have been subjected to low temperature. Thus, the young do not normally hatch until the following spring. At hatch- ing, the young locust, or nymph, resembles the adult, although the head is relatively larger and the wings are very small (Fig. 15.20). The final size and normal adult proportions are reached through a series of molts. Insect Life Cycles and Metamorphosis. The life cycles characteristic of different insects range from very simple to extremely complex. Insects that on other evidence are considered among the most primitive in existence show what is probably the ancestral type of development. Eggs hatch to produce young which are exactly like the adults, except that they are smaller and are incapable of sexual reproduction. Increase in size and development of sexual maturity occur during a series of successive molts. Such a simple life cycle, which does not include metamorphosis (change of form), is spoken of as ametabolous (Fig. 15.21). A different type of life cycle is characteristic of many insects such as locusts, which are not so primitive but are of rather generalized form. The young emerging from the eggshell is like a miniature adult in most respects, but its proportions are different, and the wings are relatively undeveloped. ‘This immature individual is termed a nymph, which, like the young locust, gradually develops wings and attains adult proportions through a series of molts. This Adult Fig. 15.22. Life cycle of a paurometabolous insect. Nymphal stages Embryo 454 THE PHYLA ARTHROPODA AND ONYCHOPHORA type of cycle, involving a gradual metamorphosis, is spoken of as paurometabolous (Fig..15:22). Somewhat different from the paurometabolous forms are those insects whose eggs are laid in water and develop into aquatic nymphs, or naiads. These immature forms are strikingly different from the adult and usually possess accessory structures, such as tracheal gills, related to their aquatic habitat. During its aquatic life the naiad increases’ in size by repeated molts; at maturity it crawls out of the water and in a final molt becomes the winged adult. This cycle includes a partial metamorphosis and is termed hemimetabolous (iigliS- 23). The most highly evolved insects, belonging to what may be considered the most successful modern orders, have the most complex life cycles. ‘The young emerge from the eggs as larvae, which are completely different from the adult in structure, in habitat relations, and very often in food habits and feeding mechanisms. ‘The larva, primarily a feeding stage, stores up reserves of Adult Fig. 15.23. Life cycle of a hemimetabolous insect. Embryo energy in its fat body and other tissues in the form of fats and glycogen. After passing through a definite number of stages, each ending with a molt and an increase in size, the larva transforms into a resting stage, the pupa. At the conclusion of the molt marking the transition from larva to pupa, the general outlines of the adult body form are laid down and are often visible externally. The pupal stage may be brief, or it may last for several months. During this period the insect does not feed, and nothing enters or leaves the body except water vapor and respiratory gases. The pupa lives at the expense of reserves laid down during the larval stages, and after a longer or shorter period of quiescence, the organs of the adult begin to form. These structures are syn-— thesized from stored reserves and from materials that become available as the special organs and tissues of the larva disintegrate. Eventually, the pupal skin splits, and the adult which has been formed within it emerges. ‘This adult often has very small but perfect wings, which must immediately be in- flated to full size and allowed to harden before they can be used. One of the first acts of the adult is the ejection from the anus of a mass of nitrogenous 455 GENERAL ZOOLOGY Adult Pupa Embryo Fig. 15.24. Life cycle of a holometabolous insect. Larval stages wastes, representing excreta which have accumulated during the period of pupal life. The life cycle just described, consisting of embryonic, larval, pupal, and adult stages and including complete metamorphosis, is of the holometabolous type (Fig. 15.24). Consideration of the life cycles of insects raises significant questions con- cerning the controlling influences regulating the far-reaching sequential changes involved in molting and metamorphosis. We have already noted (p. 435) that molting and other important systemic processes in the crayfish, and in crustaceans generally, are under the control of hormonal substances, produced by certain neurosecretory cells and transported by the circulating blood. Analysis of the processes of molting and metamorphosis in insects has shown that an analogous mechanism of control operates in these animals. In general terms, a group of neurosecretory cells in the brain of the insect secretes a hormone which acts specifically to activate a structure termed the prothoracic gland, apparently homologous with the “‘Y-organ” of crustaceans (p. 435). ‘The prothoracic gland, in turn, produces a hormone which sets in motion the complex processes of growth and differentiation which lead to molting. A third endocrine organ, the corpus allatum, produces a conserva- tive factor which has been termed the juvenile hormone, the general effect of which is to prevent changes of form at molting. In the presence of this last factor, a nymph molts to become a larger nymph, not an adult; and a larva transforms into a larger larva, not a pupa. At certain times in the life cycle, the corpus allatum ceases to produce its hormone, and in paurometabolous insects the adult stage is attained; in holometabolous forms pupation occurs. A correlation between cyclical climatic changes and the timing of events in the life cycle is evident in many insects. For example, in many moths pupation occurs in late summer, and the pupa remains dormant until the following spring, when metamorphosis takes place. In some species it can be demonstrated that pupal dormancy continues indefinitely unless the insect 1s exposed to low temperature for a sufficiently long period of time. Low tem- perature, then, exerts some effect which sets in motion the initial steps in the process of metamorphosis. It has been established experimentally that the 456 THE PHYLA ARTHROPODA AND ONYCHOPHORA specific effect of low temperature is to activate neurosecretory cells in the brain of the pupa, causing them to secrete the hormone which stimulates pro- duction of the prothoracic gland hormone; this, in turn, initiates development of the adult insect within the pupal skin. Students of insect physiology are actively investigating such fundamental questions as the evolution of these control mechanisms among insects in gen- eral, the precise biochemical foundations of endocrine regulation of meta- morphosis, and related problems. Representative Orders of Insects. Opinions differ about the number of orders among which the members of the class Insecta should properly be distributed. The number generally recognized varies approximately between twenty and thirty. In this section we shall describe and discuss briefly only a few of these orders, selected as illustrating the features of both simple and more complex insects. Additional details concerning these and other orders can be found in textbooks of entomology. It will be noted that the features most commonly used in distinguishing between orders are (1) kind of life cycle; (2) nature of the wings, where these are present; and (3) differentiation of the mouth parts for different methods of feeding. A comparison of the various orders shows that the more generalized types of insects possess mandibulate mouth parts. In those which have developed other methods of feeding, involving perfection of suctorial mouth parts, the modified feeding apparatus is clearly derived from the more primi- tive mandibulate plan. In the simplest insects, such as Protura and Thysanura, wings are absent in the immature as well as in the adult stages; these and other wingless forms are sometimes grouped together as the Aptery- gota. Other insects are wingless as adults but show, by the appearance of abortive wing rudiments in the larval stages, that they have evolved from winged ancestors. ‘The presence of two pairs of functional wings is char- acteristic of the Pterygota, comprising the majority of modern insects. In Coleoptera, and to a lesser extent in Orthoptera and Hemiptera, the anterior wings are specialized as protective covers for the hind wings. ‘This, like the reduction of the posterior pair of wings in the Diptera, is regarded as a modi- fication of the more typical four-winged state. As previously stated (p. 454), the simple, ametabolous life cycles of primitive insects indicate that the more complex cycles of higher orders have developed with the evolution of the class. Order Protura (first tail)—minute, wingless forms living in humus and decaying leaves (Fig. 15.25). ‘These must be considered the most primitive of living insects; although they possess a well-defined thorax and three pairs of legs, they are without antennae, the anterior pair of legs being held in front of the head as tactile organs. Unlike the more typical insects, proturans possess vestigial appendages on the anterior abdominal somites. In the course of growth to maturity, new somites are added at the posterior end of the body; this kind of growth is not typical of insects generally but is found in more primitive arthropods and in annelids. ‘The mouth parts of proturans are mandibulate, and the life cycle is ametabolous. 457 GENERAL ZOOLOGY Fig. 15.25. Protura. A typical proturan, Acerentulus barben. Note the simple eyes and the absence of antennae. (Redrawn, after H. E. Ewing, from H. H. Ross, A Textbook of Entomology, second edition, copyright 1956 by John Wiley and Sons, Inc., printed by permission. ) Order Thysanura (tassel tail)—-generalized, wingless insects, some species of which possess such primitive characteristics as vestigial abdominal ap- pendages and paired external genital openings. Common examples are the household pests known as silverfish, and the firebrat, Thermobia (Fig. 15.26). The mouth parts are mandibulate, and the life cycle is ametabolous. Order Isoptera (equal wings)—termites. ‘These primitive insects are in- teresting from several biological standpoints, as well as from the economic point of view related to their wood-eating habit and the consequent damage to houses and other wooden structures. ‘The termites are primarily tropical forms but occur in warm temperature regions also. We have mentioned (p. 243) the array of hypermastigote flagellates which inhabit the intestinal tracts of termites and other wood-eating insects; this appears to be a mutual relationship, the insect depending on the capacity of the protozoans to digest cellulose. Although termites are structurally relatively simple insects, they have developed a social structure which to some extent parallels that of the social Hymenoptera (pp. 470-475). The social unit, or colony, is organized about a pair of functional reproductive individuals, the king and queen, responsible for the production of all the zygotes that develop into members of the society. In addition, there are variable numbers of sterile soldiers, or both sterile soldiers and sterile workers, and larger numbers of nymphs. Each of these castes exhibits morphological and behavioral specializations related to the maintenance of the colony. Experimentation has demonstrated that the numbers in each caste are maintained in constant ratio, and that the factors responsible for this coordination include specific substances which are transmitted between individuals. Briefly, some of the evidence is as 458 THE PHYLA ARTHROPODA AND ONYCHOPHORA follows. Normally, the presence of a royal pair inhibits the development of additional reproductive individuals, except in parts of the nest distant from the royal chamber. This inhibition appears to depend on the produc- tion by the king and queen of specific substances which all members obtain by ingesting the secretions or feces of the royal pair. Ifa colony is deprived of its king or its queen, supplemental reproductive individuals of the appropriate sex will develop within a short time, from among the undifferentiated nymphs. If an “orphaned” colony is separated from a normal one by wire screens which prevent all contact and all transfer of secretions, the isolated group will develop reproductive individuals in the normal manner. If only a single screen separates the groups, however, preventing transfer of secretions but allowing the members of the two colonies to touch each other with their antennae, the orphaned colony will develop supplementary reproductive individuals but kill them as fast as they are produced. ‘Thus it is evident that caste development is regulated by two sets of stimuli, one sensory, the other chemical; it also appears that the two media of information transfer operate at different levels. Similar experimentation indicates that the same kinds of cues regulate colonial organization among other social insects such as bees, wasps, and ants. In termites, as in ants, wings are developed by reproductive individuals in preparation for a seasonal swarming period. After the establishment of a new colony, the wings are discarded. The mouth parts of termites are mandibulate, and the life cycle is pauro- metabolous, with gradual metamorphosis. Order Odonata (toothed)—dragonflies and damsel flies. These are aqua- tic during the nymphal or naiad stages but give rise to aerial adults; the life cycle is thus hemimetabolous (Fig. 15.27). There are two pairs of membranous wings in the adult, and the mouth parts of all stages are mandibulate. Adult dragonflies are wonderfully efficient fliers, skimming and hovering gracefully over the surface of ponds and streams. Although superstitiously considered dangerous to man, they are in fact beneficial, destroying in- numerable small flies and mosquitoes which they capture in flight. The Fig. 15.26. Thysanura. A typical thysanuran, the firebrat, Thermobia domestica. (Redrawn, after Illinois Natural History Survey, from H. H. Ross, A Textbook of Entomology, second edi- tion, copyright 1956 by John Wiley and Sons, Inc., printed by permission.) 459 GENERAL ZOOLOGY 460 Fig. 15.27. Odonata. 4A, naiad of a dragonfly; in this immature stage the insect breathes by means of tracheal gills lining the rectum. B and C, two views of an adult dragonfly; note the large, many-faceted eyes, the stout mandibles, and the efficient wings. (Photographs by Charles Walcott.) THE PHYLA ARTHROPODA AND ONYCHOPHORA Fig. 15.28. Orthoptera. A, long-horned grasshopper, or katydid, Scuddena furcata. B, Chinese praying mantis, Paratenodera sinensis. Compare the modifications of the walking legs in these insects. (A, photograph by E. S. Ross; B, photograph by Charles Walcott.) 461 GENERAL ZOOLOGY eyes of a dragonfly are unusually large, and this may be a factor in its efficiency as a predator of other flying insects. The clumsy naiads of dragon- flies are found crawling along the bottom in fresh water; in these immature stages they are predators of small aquatic animals. Before the final molt to adulthood, they climb along the stem of a plant into the air, and after the adult emerges the cast skin of the naiad remains clinging to this support. Damsel flies, members of a different but related suborder, are smaller, more delicate of body and wing, and less efficient in flight. ‘The life cycle, way of life, and general features are very similar to those of dragonflies. Order Orthoptera (straight wings)—locusts and their relatives, grass- hoppers, katydids, crickets, cockroaches, walking sticks, and praying mantises. In all of these, the mouth parts are mandibulate, and the life cycle is paurometabolous, with gradual metamorphosis. ‘There are usually two pairs of wings, the anterior pair being modified to form thick, tough covers for the membranous posterior pair. Crickets and katydids resemble locusts sufficiently in their general external features to be recognized at once as allied forms. ‘The crickets that are most familiar are the house and field crickets of the genus Gryllus. The hind legs are elongated for jumping, as in the locust. In many species the wings are reduced in size, and some crickets are wingless. In connection with the nocturnal activities of these animals, the antennae, which are long and slender, are highly specialized as tactile organs. In males certain of the wing veins are modified for the production of sounds. ‘The mole cricket is a type with strongly modified, shovel-like anterior legs which it uses in burrowing. Katydids (Fig. 15.28) are like green grasshoppers with very long antennae; the females bear conspicuous blade-like ovipositors. Walking sticks and mantids are interesting, the former because of their striking mimicry of twigs, and the latter for their efficiency in destroying other insects. The “praying mantis’ possesses strongly modified anterior legs which form a pair of deadly pincers; as it waits for its insect prey, the mantis holds these appendages in an attitude suggestive of the folded hands of a supplicant (Fig. 15.28). Like the dragonflies, these insects are wrongly maligned as harmful; they are actually so beneficial in their capacity of insect predators that mantises and their characteristic egg masses are protected by law in many parts of this country. Order Hemiptera or Heteroptera (half-wings or different wings)—true bugs, the only insects that may properly be called “bugs.” Representative examples are the cabbage bug, the squash bug, the assassin bug, and the water boatman. ‘The mouth parts of these insects are suctorial; the wings overlap across the dorsal midline, and the anterior pair are thick at their bases and membranous at their tips. ‘The life cycle in all hemipterans is paurometabolous. The squash bug, Anasa tristis, which is a pest upon squash and pumpkin vines the country over, is perhaps best known by the disagreeable odor it emits when handled. It is representative of the true bugs with their mouth parts 462 THE PHYLA ARTHROPODA AND ONYCHOPHORA erro Fig. 15.29. Hemiptera. A cone-nose __ bug, T riatoma. Note the extended mouth parts, modified for piercing and bloodsucking, and_ the characteristically half-mem- branous wings. (Photograph by E. S. Ross.) adapted for piercing and sucking, and their wings showing the X-shaped pattern by which hemipterans are commonly recognized. ‘The adults hiber- nate over the winter, dying in the spring soon after eggs have been laid upon the sprouts of vines where the young will feed. “The nymphs, like those of the locust, are at first wingless individuals which undergo a series of molts before attaining adulthood. They feed by piercing the leaves and stems of the plant with their beak-like mouth parts and sucking the juices. Most hemipterans appear to feed upon plant juices, but the members of some families are predators upon other insects. A few, like the assassin bugs, habitually attack vertebrates, piercing the skin and sucking blood (Fig. 15.29). This is the chief means of infection and spread of the affliction of man known as Chagas’ disease, native to the American tropics. ‘The causative organism is a blood-inhabiting flagellate protozoan, Trypanosoma cruzi, which multiplies in the gut of the insect host, very much like the parasite causing African sleep- ing sickness (p. 244). Order Coleoptera (sheath wings)—widely distributed and highly varied insects known as beetles. ‘(he mouth parts are mandibulate but in the wee- vils, or snout beetles, form a piercing beak. ‘The anterior pair of wings are modified as stout covers beneath which the posterior wings are folded in a complex manner. ‘The life cycle of beetles is holometabolous. 463 GENERAL ZOOLOGY Fig. 15.30. Coleoptera. A, larva of a tiger beetle, Omus, in its burrow, alert for prey. B, adult tiger beetle, Czcimdela. Note the well- developed mandibles of both larva and adult, and the rigid, sheath-like fore wings of the adult. (From E. S. Ross, Jnsects Close Up, copyright 1953 by University of California Press, reprinted by permission. ) The Colorado potato beetle, Leptinotarsa decemlineata, like other beetles, has fore wings specialized into a pair of covers, or elytrae, which fit together so tightly along the median line that they seem at first glance to form the dorsal side of a wingless body. When the elytrae are lifted, the functional hind wings are found folded beneath. In flight, the fore wings do not vibrate but are held stretched upward and outward in a V, to clear the rapidly beating 464 THE PHYLA ARTHROPODA AND ONYCHOPHORA hind wings. Eggs of the potato beetle are fastened to leaves of the potato plant; at the close of the embryonic period the humpbacked larvae, or grubs, emerge and begin their depredations by feeding on the vines. After a suc- cession of larval stages, the mature grub crawls down from the plant and burrows into the soil before transforming into a pupa. ‘The adult emerges after a week or 10 days and resumes the feeding interrupted by the pupal stage. The history of the potato beetle is interesting; the insect is native to western North America, where in its natural state it feeds upon purple nightshade, a wild species related to the potato. When potato culture was in- troduced into the Western states, the beetle transferred its activities to the potato plant and has since spread widely, by natural means, to practically every country where the potato is grown. Reservoir populations of potato beetles may always be found in stands of purple nightshade. Fig. 15.31. Lepidoptera. Caterpillars usually possess mandibulate mouth parts like those of the moth larva shown in A. With metamorphosis, the mouth parts change to a suctorial type like those of the butterfly in B. This involves extreme modification of the maxillae in particular, which become lateral halves of a long tube, coiled at rest but capable of unrolling to reach the nectar in deep flowers. (A, photograph by Charles Walcott; B, photograph by a SaeiNoss)) GENERAL ZOOLOGY Fig. 15.32. Lepidoptera. Stages in the life cycle of a moth, Platysamia cecropia. A, group of eggs deposited on a wild- cherry leaf. 8, young first-instar larvae feeding. C, young fourth-instar larva. D, mature (fifth-instar) larva beginning the construction of a cocoon. £, very early pupa within the cocoon, shedding the skin of the last larval stage. (Photo- graphs by Charles Walcott.) Taken as a group, beetles, like butterflies and moths, present a bewildering array of species adapted for many diverse conditions. Predaceous beetles inhabit ponds and streams in both larval and adult stages, and others, like the tiger beetles (Fig. 15.30), prey as larvae or adults on any small terrestrial animals they can capture. Many beetles bore into wood, particularly in their larval stages. The Japanese beetle, Pofillia japonica, spends its larval life in the soil, feeding on grass roots and causing serious damage to turf grasses. The adult feeds voraciously on a wide variety of plants, including roses and fruit trees. Order Lepidoptera (scale wings)—butterflies and moths. In this order, the mouth parts are either suctorial or non-functional in the adult stage but mandibulate in the larvae, which are called caterpillars (Fig. 15.31). There are two pairs of membranous wings in the adult, covered with minute, over- lapping scales which are responsible for the color patterns. ‘The life cycle is always holometabolous. Among butterflies, the monarch or milkweed butterfly, Danaus menippe (=Anosia plexippus), is one of our commonest species. It ranges all over North and South America and occurs in other lands also, particularly western Europe, Australia, and the Pacific islands. Eggs of this insect are laid singly upon leaves of various milkweeds, and in a few days at summer temperatures 466 ' Aa 4 i fi uo in 7 ‘ ’ ‘ i , eu they hatch to produce minute larvae. The larvae feed on the leaves of the plant, molting several times as they increase in size during the 2 or 3 weeks of the larval period. Mature larvae are light-green caterpillars, conspicuously banded with black and yellow stripes, and with pairs of antenna-like projec- tions toward each end. ‘The molt marking the end of the larval period pro- duces the so-called chrysalis, or pupa, in which such distinctive adult features as wings and antennae are recognizable. ‘The adult emerges after 10 or 15 days, inflates its wings, and appears as the familiar flying form. In autumn the adults are killed by cold, or they migrate southward in great swarms to pass the winter in subtropical regions, returning to the north in spring. Other butterflies have different seasonal relationships, some passing the northern winter in the pupal stage, others as eggs that hatch in early spring. Moths, of which there are very many species, are usually nocturnal in their activities, whereas butterflies are typically active during the daylight hours. Moths have feathery antennae and hold their wings horizontally at rest; butterflies have slender, clubbed antennae and hold their wings vertically. Again, the pupal stage of the butterfly is typically a naked chrysalis attached to some object by a single thread; the pupae of most moths are surrounded by cocoons of silk, spun during the last larval stage. At the close of the pupal period, the pupal skin is molted within the cocoon, and the adult forces its 467 GENERAL ZOOLOGY 468 i cebe % is aA Fig. 15.33. Cecropia life cycle, continued. A, mature pupa within its cocoon. 8B, adult emerging from the cocoon. C, young adult in the process of expanding its wings. JD, adult with wings fully expanded and dried, ready for flight and mating. (Photographs by Charles Walcott.) THE PHYLA ARTHROPODA AND ONYCHOPHORA way out of the cocoon to spread and dry its wings (Figs. 15.32, 15.33). The household pest known as the clothes moth (Tzneola bisselliella) is a familiar example of this group; the silkworm, used in the Orient and in Europe for the commercial production of silk, is the larva of a large moth, Bombyx mor. Several species of giant silkworm moths are found in North America and, be- cause of the large size of their larvae and pupae, have been very useful in studies on the biochemistry and endocrine control of metamorphosis. Order Hymenoptera (membrane wings)—ants, bees, wasps, etc. ‘The mouth parts are typically mandibulate but in the adult are often modified to form a tongue-like structure adapted for lapping liquids. ‘The wings are membranous, and two pairs are usually present. ‘The life cycle is holo- metabolous, with complete metamorphosis. The Hymenoptera include a variety of types, ranging from insects of rela- tively simple habits to species with highly developed social organizations, such as honeybees and some ants. ‘The examples that follow will illustrate this diversity. Fig. 15.34. Parasitism among insects. The large caterpillar (tomato hornworm) bears many cocoons of a small hymenopteran. An adult wasp deposited her eggs on the caterpillar, and the wasp larvae fed extensively on the tissues of the host before emerging to spin their cocoons and pupate. Such a parasitized caterpillar is incapable of completing its own life cycle; thus insects parasitizing other insects play an important role in the control of many agricultural pests. (Photograph by Charles Walcott.) 469 GENERAL ZOOLOGY Parasitic hymenopterans include many species which pass their develop- mental stages as parasites within the bodies of other insects. Among these parasitic forms are ichneumon wasps and braconid wasps (Fig. 15.34), which usually deposit their eggs upon or within the bodies of a variety of other insects. After the larvae hatch, they live parasitically in the body of the host until their time of pupation; they then come to the surface and spin cocoons from which adult wasps later emerge. Caterpillars are often found covered with the minute cocoons of these parasites. Many times, an ap- parently normal cocoon of one of the giant silkworm moths will be found to contain not the pupa of the moth but that of an ichneumon wasp, whose larva has completely devoured the caterpillar which spun the cocoon. ‘The activi- ties of such parasitic wasps have sometimes helped check outbreaks or invasions of insect pests of economic importance. Non-parasitic wasps may be subdivided into solitary forms, in which there is no colonial organization, and social wasps, which live in colonies like those of bees and ants. Mud daubers of the genus Sceliphron are solitary wasps that build a nest of mud fashioned into several tubes. When one of these tubes is completed, the wasp collects small spiders, which she paralyzes with her sting and with which she fills the tube. She then deposits a single egg in the outer end of the tube before sealing it with mud. When the larva hatches, it uses the spiders as food, eventually pupating and finally emerging as a winged adult, which gnaws its way out of the tube. Only females are active in the nest-building operation; males apparently die soon after mating. Digger wasps, excavating subterranean burrows which they provision with paralyzed insects, offer another example of hunting and food-storing activities (Fig. 25 5))e Among social wasps, species of the genus Polistes represent a simple type of social organization. ‘They build nests of paper, which they make by chew- ing wood fibers and mixing them with saliva. A female Polistes, after hiber- nating through the winter, begins to construct a nest which by the end of the summer may reach a diameter of a foot or more. ‘The nest is a plate-like group of individual paper-walled cylinders, generally fastened to some support by a slender stalk. ‘The single female, or queen, which begins the construc- tion, is soon aided in tending the young and adding to the nest by other females, the infertile workers, which hatch from her eggs. The males are drones, which do not work and which die soon after mating. ‘The nests of hornets that hang from the limbs of trees are composed of a series of paper combs essentially like those of Polistes but enclosed in a common covering. Among bees there are both solitary and social species and others that show transitional stages. Thus, it is possible to establish theoretically the steps through which the highly organized honeybee colony may have evolved. In solitary species each female constructs her own separate nest, in which an egg is laid and where food is either stored or brought to the larva during its development. Some of these solitary species show a tendency toward gre- gariousness, suggesting the beginnings of social life. ‘They build many nests 470 THE PHYLA ARTHROPODA AND ONYCHOPHORA Fig. 15.35. Hymenoptera. A, a digger wasp, Bembex, beginning excavation of a burrow. B, a hunting wasp, Chlorion, preparing to transport a paralyzed nymphal locust to her burrow. ‘There she will deposit an egg upon her living but helpless prey, and the larval wasp which hatches will feed upon the locust. By seeking various kinds of insects and spiders with which to provision their burrows, the hunting wasps are a significant factor in insect control. (From E. S. Ross, Insects Close Up, copyright 1953 by University of California Press, reprinted by permission.) close together, although each nest belongs to a single individual. In others the nests are separate, but the neighbors cooperate in the construction of a common entrance. Bumblebees represent a more specialized organization, in which the females have become differentiated as fertile queens and infertile workers, and the males as drones; this is the usual situation among the social hymenopterans. A colony of honeybees may be regarded as a further development of such a colonial organization as that of bumblebees. The honeybee workers keep the hive in repair, collect nectar and pollen and modify these substances for 471 GENERAL ZOOLOGY storage as honey and beebread, tend and feed the young, care for the queen, and are in general responsible for the maintenance of the organization. ‘The queen, having once been inseminated, produces zygotes from which develop all the individuals of the colony. ‘The males, or drones, develop from un- fertilized, haploid eggs through parthenogenesis. ‘The fertilized eggs produce larvae which are always females but which may develop into either sterile workers or fertile young queens, depending on the diet with which they are fed. Future queens are fed throughout larval life on “royal jelly,” which permits full development of their genital organs. Other female larvae receive this diet only for a day or two; their genital organs fail to develop, and such physiologically castrate females become workers. ‘There is some evidence indi- cating that workers in a colony obtain some secretion from their queen which generally suppresses the production of additional queens (cf. caste production in termites, pp. 458-459). ‘The integrity of a colony with its single queen is thus maintained until the swarming period, when many workers leave the colony with the old queen, to found a new colony and build a new hive. When this happens, a part of the old colony is left behind to rear a new queen. ‘The behavior of the individuals is a marvel of precision and seeming adaptation of means to ends, although it consists of reactions based on in- herited reflexes, or instincts, which are modifiable only within very narrow limits. In addition to these innate complex behavior patterns, other phenom- ena of the life of the bee are of interest and remain to be accounted for. For example, how is it possible to explain the origin and inheritance of the highly specialized structural modifications of the workers? ‘The legs of these insects bear specifically developed combs, pollen baskets, antenna cleaners, and so on, perfectly adapted to the food-gathering and other functions of the workers (Fig. 15.36); yet the workers are sterile and never produce offspring which could inherit these traits, and the queen and the drone, which are the parents of the workers, bear none of these specialized structures. “The factors operat- ing within the body of the queen, which evoke the production of either fertilized eggs or parthenogenetic eggs, and thus either female or male progeny, are also unaccounted for. Only within recent years has some insight been gained into the means of communication between bees in a colony. Foraging scouts, when they have located a source of food, return to the hive and transmit to other workers information about the kind of food available, its abundance, and the distance and direction from the hive to the food supply. The kind of food, that is, generally the kind of flower from which it may be obtained, is communicated by both olfaction and taste. Other workers gather about the returning scout and “‘smell’’ with their antennae the characteristic fragrance of the flowers which clings to its body; similar information is transmitted with the nectar which the scout regurgitates from its honey stomach and which other workers ingest. Bees are capable of distinguishing large numbers of fragrant essential oils from flowers; they are confused by some which also smell alike to man. The distance-and-direction information is transmitted by a series of “dances” 472 THE PHYLA ARTHROPODA AND ONYCHOPHORA Worker Queen Fig. 15.36. Hymenoptera: the honeybee, Apis mellifera. A, comparative aspects of the three castes of a honeybee colony. 8B, a worker honeybee visiting a flower; note the bulging pollen baskets on the metathoracic legs of the insect, packed with pollen to be carried to the hive. These pollen baskets, along with other special modifications, are found only in the worker caste. (A, redrawn from K. von Frisch, 1953, Biologie, vol. 2, printed by permission of Bayerischer Schulbuch-Verlag; B, photograph by Charles Walcott.) performed at the entrance of the hive by the returning forager; the dances are soon joined and imitated by other workers. A so-called ‘‘round dance” is per- formed when the food is near the hive, and the abundance of the supply is indicated by the vigor of the dance. For sources many hundreds of yards, or even a mile or more, from the hive, the forager performs a more com- plicated “‘waggle dance” involving a straight passage which indicates the compass direction to be flown in seeking the food. By imitating the straight part of the dance, other workers evidently perceive the pattern of polarization of light from the sky, which is characteristic of the compass heading they must fly. Analysis and integration of polarized light are presumably functions of the well-developed compound eyes of these insects. Fantastic as these state- ments may seem, they are all based on sound experimental evidence gathered 473 GENERAL ZOOLOGY 474 Fig. 15.37. Diptera. A, larvae and pupa of a mosquito, Aédes, breathing at the surface of the water; the functional spiracles of the larva are posterior, whereas those of the pupa open dorsally on the thorax. B, emergence of an adult Aédes from its pupal skin. C, a predaceous robber fly, Aprocera, feeding upon the juices of a leafhopper. Note the small, club-shaped haltere, or balancer, attached to the meta- thorax of the fly; this is the char- acteristic condition of the hind wings in dipterans. (From E. S. Ross, Insects Close Up, copyright 1953 by University of California Press, reprinted by permission.) THE PHYLA ARTHROPODA AND ONYCHOPHORA for the most part by the eminent Austrian biologist Karl von Frisch and since confirmed by many other experimenters. ' Ants appear even more specialized in their social life than other hymenop- terans, since there are no existing examples of solitary ants, and since the workers of most ant colonies are specialized into two or more castes which carry on special activities. Ant colonies, particularly in some of the tropical species, may be very large, as judged both from the size of the nest and from the number of individual members. ‘The essential features of social organiza- tion are illustrated by many of our native forms. In a typical example, the males, which are winged throughout their brief existence, participate in the mating flight with the females and then die. ‘These females, the queens, then pull off their wings, establish nests, lay eggs, and tend the young until workers develop to carry on these labors. When the mating season approaches, winged males and females are produced and leave the nests in the swarms that are seen at certain seasons of the year. Ants, bees, and wasps among the Hymenoptera, as well as termites (order Isoptera, p. 458), are of particular interest because of their social life and their behavior. From the standpoint of social organization, insects represent the only group in which anything approaching the complexity of human society exists. From the standpoint of animal behavior, these insects exhibit amazing instinctive reactions, the result of inherited reflexes so fixed in their nature that they have become recognizable characteristics of the species, just as reliable as any morphological characteristics. Order Diptera (two wings)—true flies, such as the housefly and the mosquito. The mouth parts of the dipterans are typically suctorial, although they have been modified in various ways, as for lapping and for piercing and sucking. ‘There is an anterior pair of membranous, functional wings; the posterior wings are much reduced to form club-shaped halteres or balancers (Fig. 15.37), which vibrate in flight and have been demonstrated to serve as gyroscopic stabilizers. The life cycle is holometabolous, with larvae com- monly known as maggots. The common housefly, Musca domestica, is perhaps the most familiar of all insects; and, since it is recognized as a carrier of disease-producing micro- organisms, its life cycle is widely known. ‘The adult has mouth parts adapted for lapping fluids; in feeding, it generally extrudes saliva onto the food and then ingests the saliva with dissolved food substances. ‘The fly feeds upon almost any kind of organic matter that may be available. ‘The extent to which houseflies may become distributors of bacteria can be shown by allow- ing a single individual to crawl across a sterile plate of nutrient gelatin and noting the colonies of bacteria which later appear along its trail. Eggs are laid in various kinds of decomposing organic matter. Larvae, or maggots, hatch in about 6 hours and grow through three larval instars, lasting a total "For a fuller and entirely delightful account of these and other interesting phenomena among honeybees, see Karl von Frisch, Bees: their Vision, Chemical Senses, and Language, copy- right 1950 by Cornell University Press. 475 GENERAL ZOOLOGY of 5 or 6 days, before pupation. The pupal case, or puparium, is formed by the hardened and darkened exoskeleton of the last-stage larva. ‘The adult emerges from this case about 5 days after pupation. ‘Thus, a generation may be developed every 12 days under favorable conditions; and, since every female may lay about 100 eggs, the potential rate of multiplication in a single season is very great. Many of the adults die with the onset of winter, but some hibernate in protected places. ‘These overwintering individuals then become active again in early spring and lay the eggs from which the flies of another season arise. Mosquitoes have assumed great importance since it was discovered, about 1895, that certain mosquitoes carry the protozoan parasite causing malaria, and later, that the virus causing yellow fever is also transmitted by mosqui- toes. Adult females, whose mouth parts are capable of piercing the skin and sucking the blood of man, are responsible for the transfer of these parasites, since the mouth parts of the male cannot penetrate the human skin. Ap- parently the malaria parasites of importance in human malaria are carried only by mosquitoes of the genus Anopheles, and the virus of yellow fever is carried only by members of the genus Aédes. ‘The most common mosquitoes of temperate latitudes belong to the genus Culex, which does not serve as a host for either of these parasites. Culicine mosquitoes lay eggs fastened to- gether in little rafts which float upon the surface of fresh waters. The larvae hatching from these eggs are the wrigglers commonly seen in standing water, where they hang suspended from the surface film by tubes at the posterior ends of their bodies; air reaches their tracheae through these tubes (Fig. 15.37). The larval stage of Culex lasts from 1 to 4 weeks, depending on the species and to some extent on temperature and the abundance of the food supply. ‘The food during this period consists of minute organisms captured and ingested by the mandibulate mouth parts of the larva. With their third molt the larvae change into active but non-feeding pupae, in which the head and thoracic regions are enlarged and the wings and antennae of the adult can be seen, as in the chrysalis of a butterfly. ‘These pupae, like the larvae, must secure air from the surface, but the air tubes of the pupa are located on the dorsal side of the thorax. After 2 to 5 days the pupal skin is shed, and the adult emerges (Fig. 15.37). Both larvae and pupae are killed when the water in which they live is covered with a film of oil, for this film prevents their access to the atmospheric air which they require. Other methods of control involve drainage of swamps and other bodies of stagnant water, and emptying even such smaller breeding places as tin cans and rain barrels. A great many insects of economic, medical, and veterinary importance to man belong to the order Diptera. ‘The tsetse fly of Africa, which transmits the trypanosomes of sleeping sickness; stable flies, horn flies, botflies, warble flies, horseflies, and screwworm flies which variously parasitize, feed upon, or otherwise annoy cattle and horses; the Hessian fly, which destroys standing crops of grain; all these are but a few of the insects with which man must constantly contend. 476 THE PHYLA ARTHROPODA AND ONYCHOPHORA Importance of Insects to Man. Although a few insects, such as the silkworm and the honeybee, are specifically useful, a vast array of insect species are detrimental to man. Again, some insects have important roles in the economy of nature, in the pollination of flowers, as food for many animals, and as destroyers of other insects harmful to livestock and crops. More often, however, the insect is destructive of the plant and animal life most necessary toman. Estimates made for the United States in recent years place the total insect damage to crops, domestic animals, and stored products at millions of dollars annually. In the past man had to reckon more with his fellow mam- mals; in the present and for the future the insects, above all other forms of life, strive with man for the control of his environment. Finally, many insects are dangerous to man as pests of his body and his habitations and, most im- portant of all, as hosts and carriers of the microorganisms of various diseases of humans. With increasing knowledge of habits, life cycles, and ecological relationships of economically important insects, it has become possible to institute various measures for their control or eradication. Natural factors of insect control, such as bacterial and parasitic diseases affecting insects themselves, have been advantageous. Mechanical methods of picking the pests from plants and destroying them, as well as cutting and burning infested plants, are of considerable value. The most widespread technique of insect control is by means of chemicals. These are sometimes used to repel insects or to attract them to poisoned food and into traps; but most commonly they are employed as insecticides. Arsenic, sulfur, and fluorine compounds are among the oldest and most common insect poisons, along with a variety of soaps, oils, nicotine compounds, and an in- creasing number of very effective synthetic organic products. Among these last is DDT (dichloro diphenyl trichloroethane) made famous by its extensive use in World War II. Many ingenious devices have been developed for the application of these insecticides to growing plants. One of the most effective is the use of airplanes and helicopters to spray or dust crops and woodlands on a large scale, or to reach areas not easily accessible by other means. Insecticides exert their effects on insects in a variety of ways, notably as “‘stomach poisons” when ingested with food, as in the case of arsenic com- pounds, or as contact agents by clogging the spiracles or tracheal tubes, as oil sprays do. ‘The action of DDT and some of the newer organic insecticides is somewhat different: DD'T has been shown to operate as a nerve poison, blocking the action of a specific enzyme which normally inhibits constant ex- citation along nerve pathways. Under the influence of DDT, a susceptible organism is thrown into violent and continuous muscular spasms which end with the death of the animal. Not all insect species of economic importance are susceptible to DDT, however; and of those which are, some populations rapidly develop strains that are markedly resistant to concentrations of the poison ordinarily fatal to the species. In the same manner, populations of scale insects have developed in California which are resistant to the fatal 477 GENERAL ZOOLOGY effects of one of the most potent poisons known to man, hydrocyanic acid gas. These are examples of the general adaptability of living organisms to en- vironmental changes and may be regarded as the result of a process of artificial selection. According to this interpretation, certain individuals, by the chance operation of random mutations, are capable of withstanding ordinarily lethal concentrations of the poison. Continued subjection to the insecticide rigorously “‘selects” for survival those individuals which are most resistant, until eventually the breeding population is composed entirely, or preponderantly, of resistant stocks. THE CLASSES CHILOPODA AND DIPLOPODA Members of these two arthropod classes show certain superficial similarities: they are tracheate animals with mandibulate mouth parts, with long, slender bodies composed of a large but indefinite number of segments, and with numerous paired, jointed appendages. ‘These similarities have led to their classification within a single group, the class Myriapoda, in older systems. However, closer study reveals many significant differences between centipedes and millipedes which indicate a more remote relationship. Centipedes are somewhat flattened dorsoventrally; the body is composed of a definite head—bearing antennae, ocelli, and the mouth parts—and a many- segmented trunk (Fig. 15.38). Each of the trunk segments, except the last two, bears a pair of walking legs; those of the first segment are modified into conspicuous poison fangs, sometimes referred to as maxillipeds. “The gonads are dorsal to the gut, and, as in insects, the genital openings are at the posterior end of the body. Centipedes are predatory carnivores, feeding on small animals captured and killed with the aid of the poison fangs. Millipedes are cylindrical and worm-like. As in centipedes, a head is present, bearing antennae and groups of ocelli. ‘The trunk, however, may be divided into four anterior thoracic segments and a large number of abdominal segments. ‘Those of the abdominal region are fused in pairs, so that each apparent division seems to bear two pairs of walking legs (Fig. 15.38). The gonads lie ventral to the gut, and the genital openings are anterior, on one of the thoracic segments. Millipedes are retiring, her- bivorous animals which when disturbed usually roll up into a tight spiral and emit a disagreeable odor from segmental scent glands. Diplopods, and some chilopods, are characterized by anamorphic growth: the young individual does not have the full adult number of somites, but at each molt as it grows to maturity new segments are added in a posterior zone of growth, just anterior to the segment bearing the anus. As we have seen, this type of growth is characteristic of annelids, as well as of the most primitive insects (Protura). Other features reminiscent of annelids are the long, worm-like body, the relatively undifferentiated nature of the somites, and the presence of paired appendages on most of the segments. More ad- vanced characteristics include the development of a definite head with specific 478 THE PHYLA ARTHROPODA AND ONYCHOPHORA eH FY yy * af oe fee i ‘| a, ; $s Pimp. Fig. 15.38. A, Chilopoda: a large tropical centipede. Note the poison fangs attached to the posterior end of the head; each contains in its base a gland, the toxic secretion of which passes through a duct to a minute pore near the tip of the fang. 8B, Diplopoda: a millipede, Spzrobolus. There are two pairs of appendages for each division of the body, with the exception of a few just behind the head. Each division represents the product of fusion between two adjacent somites. (A, photograph courtesy New York Zoological Society; B, photograph by Charles W. Schwartz.) mouth parts and sense organs, the tracheal respiratory system with segmental spiracles, and the exoskeleton which must be molted to permit growth. Al- together, chilopods and diplopods present an interesting mixture of annelid- like and insect-like features, in addition to adaptive characteristics peculiar to their kinds. ‘This has led to the supposition that their ancestors developed from an evolutionary stock of terrestrial mandibulate arthropods rather early in its history, and that they demonstrate the capacity of this stock to evolve features which were later perfected in the Insecta. 479 GENERAL ZOOLOGY Fig. 15.39. Representative Chelicerata. A and B, class Eurypterida: restoration of a generalized fossil eurypterid, dorsal and ventral views. C;) class Arachnida: a scorpion, Chactas. D, class Pycno- gonida: a sea spider, Nymphon. (Redrawn from R. E. Snod- grass, Textbook of Arthropod Anatomy, copyright 1952 by Cornell University __ Press, printed by permission.) THE SUBPHYLUM CHELICERATA In addition to the extinct eurypterids, chelicerates include horseshoe crabs, pycnogonids, and arachnids (Fig. 15.39); within this last group are placed such familiar forms as spiders, scorpions, ticks, and mites. In all chelicerates the body is divided into an anterior prosoma and a posterior opisthosoma; these regions are comparable, but not equivalent, to the cephalothorax and ab- domen of crustaceans. One of the major unifying features of chelicerates 1s their possession of six pairs of appendages, borne on the prosoma and developed in a consistent pattern. ‘The anteriormost of these are the pincer- like chelicerae, from which the name of the group is derived. ‘The second pair, the so-called pedipalps, are variously specialized: in the horseshoe 480 THE PHYLA ARTHROPODA AND ONYCHOPHORA Prosoma Eye Fig. 15.40. Class Xipho- surida: Limulus polyphemus, the horseshoe crab of the Atlantic coast of North America. (Redrawn from R. E. Snod- grass, Textbook of Arthropod Anatomy, copyright 1952 by Cornell University Press, printed by permission.) Opisthosoma Tail crab they are unmodified walking legs; in scorpions they form the conspicuous great chelae used in capturing prey; in male spiders they are often very com- plex structures adapted for the transfer of spermatozoa to the female at mating. ‘The remaining four pairs of appendages are relatively undifferen- tiated walking legs. Only the horseshoe crab, Limulus, exceptional in many respects, bears segmental appendages on the opisthosoma (Fig. 15.40). Most modern chelicerates are small, terrestrial animals. Limulus is an exception to this generalization also; it is a marine form, often reaching 2 feet in length. Fossil rernains of forms ancestral to Limulus show that this genus has remained almost unchanged since Devonian times, some 350,000,000 years ago. In correlation with its marine habitat, Lzmulus bears on the ventral side of the opisthosoma many leaf-like gills which function in oxygena- tion of the blood. ‘These delicate gills are protected by plate-like expansions of the opisthosomatic appendages, used also for swimming. Among terrestrial arachnids, scorpions and most spiders have developed internal cavities termed book lungs, containing many thin, hollow plates between which the blood Sternum Spiracle Fig. 15.41. Book lung of a scorpion. ‘The hollow lamellae, filled with air, communicate with the cavity of the atrium; blood circulates between the individual lamellae. (Redrawn from R. E. Snodgrass, Textbook of Arthropod Anatomy, copyright 1952 by Cornell University Press, printed by permission. ) 481 GENERAL ZOOLOGY Fig. 15.42. Class Arachnida. A, a tarantula. B, a scorpion, Uroctonus mordax, which has cap- tured a Jerusalem cricket, Steno- pelmatus longispina. Note the use of the powerful chelate pedipalps in grasping the insect, which has been stung by the poison claw at the tip of the scorpion’s tail. (A, photograph courtesy General Biological Supply House, Inc.; B, photograph by E. S. Ross.) circulates (Fig. 15.41). The individual ‘“‘pages” of the book lung contain air spaces, and the collective air spaces of each lung communicate with the outside through a slit-like spiracular opening. Some spiders possess, in addition to or in place of book lungs, tracheal systems analogous to those of insects, with segmentally arranged spiracles Relatively few spiders, among them the “black widow,” Latrodectes mactans, are dangerous to man. By far the majority of spiders are beneficial through their destruction of insects (Fig. 15.42). Of even greater economic importance are the arachnids called ticks and mites, many of which are parasitic on man and his domestic animals, and a few of which transmit disease-producing microorganisms from host to host. 482 THE PHYLA ARTHROPODA AND ONYCHOPHORA The Phylum Onychophora The onychophores (‘‘claw bearers’), sometimes considered a class of the phylum Arthropoda, are a small but interesting group of animals represented in the modern fauna by only about a dozen genera. Of these, the genus Peripatus is best known. ‘This ‘walking worm” is a predaceous animal, feeding upon small insects; it is restricted to moist terrestrial habitats in subtropical regions. Pertpatus presents a curious mixture of characteristics, some reminiscent of annelids and others resembling those of arthropods. ‘The body is worm-like, with numerous pairs of appendages arising at intervals (Fig. 15.43). Unlike those of arthropods, these appendages are not jointed but are fleshy, lobe-like outgrowths of the body wall containing groups of muscles and bearing hooked claws at their distal ends. The head consists of a simple cephalic lobe bearing antennae, simple eyes, and a mandibulate mouth. The body of the adult is not segmented, although many structures lie in what appears to be a segmental arrangement. In embryonic develop- ment a series of paired coelomic pouches form, but the body cavity of the adult is a hemocoel; remnants of the coelom are found only in the cavities of the gonads and of the excretory organs. ‘These organs are a series of paired nephridia, closed internally by reduced coelomic end sacs and opening externally at pores on the bases of the legs. “The nephridial tubules are ciliated like those of annelids; cilia are altogether lacking in all arthropods. The body wall of Peripatus is covered by a thin, flexible cuticle rather than a hardened exoskeleton. Although a tracheal system is present as in many arthropods, in onychophores the tracheae arise in tufts from the inner ends of spiracular openings which are scattered over the surface of the body, not restricted to segmental intervals. Taken as a group, onychophores may be considered as persistent, little- changed representatives of an ancient stock of animals which evolved from the annelid-arthropod stem independently of the groups that later gave rise to je MARE E jae ee a Fig. 15.43. Phylum Onychophora: general features of Peripatus capensis, natural size. Note the serially repeated, lobe-like, clawed appendages of the trunk, which are characteristic of animals in this phylum. (Redrawn from A. Sedgwick, 1888, Studies from the Morphological Labora- tory in the Unwersity of Cambridge, vol. 4.) 483 GENERAL ZOOLOGY true arthropods. Fossil remains that are unmistakably onychophoran are known from the middle Cambrian period, and the external features of Peripatus show remarkably few alterations when compared with those of the ancient forms. Peripatus should not be regarded as a ‘“‘missing link”? between annelids and insects (cf. p. 630). Pertpatus itself and onychophores generally are in their own way highly specialized evolutionary end forms, and such end forms cannot logically be considered as links between different phyla. In addition, it would be surprising indeed if insects, the most advanced class of modern arthropods, could be traced directly to annelid-like ancestors. The Arthropod Body Plan and Its Evolution The fundamental similarities in general organization between annelids and the arthropod-onychophore groups indicate that a common ancestry lies be- hind all these types. If we look for factors significant in determining the different evolutionary directions taken by annelids and arthropods, leading to the marked differences between their modern representatives, one factor stands out pre-eminently—the development of the hardened exoskeleton in the arthropod stem. We may assume that somewhere in the evolutionary history of arthropods, after the divergence of the ancient onychophores but antedating the appearance of chelicerates and mandibulates, the soft cuticle characteristic of the worm-like ancestor began to be replaced by a more rigid, unyielding, protective coat. Correlated with this development, several other changes must have occurred in the ancient groups. The problem of growth was eventually solved, in a manner of speaking, by perfection of the complex process of molting, with its implication of an endocrine system to integrate the activities of all parts of the body involved. ‘The locomotion characteris- tic of the annelids, either by serpentine undulations or by reciprocal changes in the length and diameter of the body, could no longer be effected; the rigidly encased body could no longer bend or change its shape, except as flexible, intersegmental joints could be by chance developed and perpetuated through natural selection. Correlated with this event must have been changes in the musculature, the formerly extensive sheets of circular and longitudinal muscles being replaced by specifically acting groups of flexors and extensors, operating to bend one part of the body in relation to another. Also involved in this change in locomotion was the development of segmental appendages, with definite joints and muscle groups for their manipulation. With the development of the undifferentiated, generalized series of jointed appendages, capable of specific actions, it became possible for regional specialization to begin. Certain of the anterior segments, we may imagine, became incor- porated into the primitive head, and their appendages were modified until they formed sense organs and mouth parts. Other segmental appendages in the long series must have become variously specialized as swimming paddles, chelipeds, and walking legs. Along with regional specialization, 484 THE PHYLA ARTHROPODA AND ONYCHOPHORA and the consequent partition of functions, must have occurred reduction in the number of somites; this would have been of survival value in that it led to the formation of a more compact and efficient body. It apparently involved coalescence of segments and their ganglia, as well as eventual elimination of anamorphic growth with its constant addition of posterior segments. Similarly widespread changes occurred internally. Coelomic cavities suffered reduction and were replaced by the large blood spaces of the hemocoel. Segmental excretory organs associated with coelomic pouches became more and more restricted. In the crustacean stem these are now represented by a short and variable series of glandular structures, such as the green glands of the Malacostraca and the maxillary glands of more primitive forms. In chelicerates the coxal glands form a similar series of excretory organs, believed to represent modified nephridia. All the foregoing changes were characteristic of aquatic arthropods and set the stage for the rise of the first terrestrial arthropods. In this important epoch, possession of an exoskeleton generally impervious to the passage of water, and of walking legs capable of supporting the weight of the body, took on a new significance. ‘Through many thousands of years, random mutations and the selective action of the terrestrial environment brought about exten- sive internal modifications, notably in the systems concerned with excretion and respiration. Excretion in fresh-water arthropods involves a considerable expenditure of water, easily replaced from the surrounding medium. In terrestrial arthropods the necessity of water conservation made advantageous the suppression of the nephridial type of excretory organ and the perfection of Malpighian tubules. ‘These organs eliminate excreta in a relatively dry state, and in the hind-gut the wastes are subjected to still further dehydra- tion, the extracted water being returned to the blood. Onychophores, like earthworms, have retained nephridia and are restricted to moist terrestrial environments. Blood gills, the characteristic organs of gas exchange in aquatic forms, are too wasteful of water to be serviceable on land. Except in some terrestrial crustaceans, such as sow bugs, which cannot survive in really dry conditions, terrestrial arthropods have developed tracheal respiratory sys- tems or book lungs. Both of these systems could develop through natural selec- tion, having as their chief selective advantage the fact that they operate with very little loss of water by evaporation. ‘The spiders and insects that have returned to life in fresh water, either as adults or as immature forms, have not re- gained the blood gills of their remote ancestors. They either carry with them films or bubbles of atmospheric air, as do many beetles and _ spiders; breathe at the surface, like mosquito larvae and pupae; or, like the naiads of the Odonata and other orders, develop tracheal gills. To all of the adaptively advantageous modifications inherited from earlier terrestrial arthropods, insects have added yet another—the power of flight. Together with their complex, innate behavior patterns, this may be regarded as a major factor in the success of this group, as judged from their vast numbers and infinite diversity. 485 CHAPTER 16 SPINY-SKINNED The Phylum Echinodermata The Echinodermata are non-segmented, triploblastic forms showing a five- part radial symmetry masking a fundamental bilaterality. Coelomic cavities are extensive, forming in the embryo as outpocketings of the primitive diges- tive tract. “There is an endoskeleton composed of calcareous plates or spicules. A unique characteristic of echinoderms is the presence of a system of fluid- filled internal ducts, the so-called water-vascular system. In most echino- derms the blood-vascular system is so reduced as to be non-functional and very inconspicuous, and there are no traces of nephridial excretory organs. The phylum may be divided into the subphylum Pelmatozoa, containing primitively stalked or attached forms, and the subphylum_ Eleutherozoa, including free-moving, unattached echinoderms. ‘The Pelmatozoa, once very numerous, are now represented by only one modern class, the class Crinoidea, sea lilies and feather stars. Several other pelmatozoan classes, among them the Cystoidea and Blastoidea, contain extinct stalked forms known only as fossils (Fig. 16.1). The Eleutherozoa contain the remaining four classes of echinoderms: the class Asteroidea, sea stars or starfishes; the class Ophiuroidea, brittle stars, serpent stars, and basket stars; the class Echinoidea, sea urchins and sand dollars; and the class Holothuroidea, sea cucumbers. Echinoderms are typically slow-moving, bottom-dwelling animals. ‘They are exclusively marine, and there is no evidence from the fossil record that members of the phylum have ever become established in fresh water. Because of their radial symmetry, the echinoderms were at one time classed with the coelenterates as ‘‘zoophytes.”’ Further study, however, showed that echinoderms are animals with an extensive coelom, which coelenterates lack, and that even their radial symmetry differs markedly from the coelenterate type. A significant feature of the life cycle of echinoderms is the occurrence of a segmented, bilaterally symmetrical larva, which develops by a com- plicated metamorphosis into the non-metameric, radially symmetrical adult. The structure of the adult presents many puzzling characteristics when compared with such familar types as mollusks, annelids, or chordates. If we interpret the evolution of echinoderms from the events in the life cycle of a 486 45 ae: Ky ANIMALS: = —N 4 = a = . = S SPL, < Fig. 16.1. Extinct stalked echinoderms. A, reconstruction of a cystoid, Plewrocystites, of Ordovician age. 8, Blastoidocrinus, an Ordovician blastoid, reconstructed from fossil remains. C, fossil crinoids in a slab of Devonian limestone. For orientation in geological periods, see Figure 20.1. (A, redrawn after F. A. Bather, 1900, in Lankester’s Treatise; B, redrawn from R. C. Moore, C. G. Lalicker, and A. G. Fischer, Invertebrate Fossils; copyright 1952 by McGraw- Hill Book Co., Inc., printed by permission; C, photograph courtesy Ward’s Natural Science Establishment, Inc.) j GENERAL ZOOLOGY “e%<. et e. 6 a -@. : typical representative such as the starfish, however, their morphological peculiarities become intelligible. The indications are that the remote ancestors of echinoderms were free-swimming, bilaterally symmetrical forms with segmentally arranged coelomic pouches. At some time long antedating the beginning of our fossil record, the ancestral forms apparently took up an attached way of life, and the secondary radial symmetry developed through some millions of years in connection with this fixed existence. The anatomical peculiarities of modern echinoderms, even of those that are no longer attached forms, are evidently related to these ancient adaptive changes. Embryological and serological evidence indicates that echinoderms are more closely related to the chordate evolutionary stem than to any other large group of animals (see Fig. 7.3, p. 219). From an economic standpoint, echinoderms are unimportant, except for starfishes, which sometimes destroy whole beds of shellfish. One of the few echinoderms used as food by man is a sea cucumber, gathered, dried, and sold as béche-de-mer, or trepang. Neither do echinoderms serve as food for many other animals; codfishes and related forms feed on starfishes, and the dugong, an aquatic mammal, eats sea cucumbers. On the whole, however, the spiny surfaces and the preponderance of skeletal material in the bodies of echinoderms have helped them avoid destruction by predatory enemies. 488 THE PHYLUM ECHINODERMATA Fig. 16.2. External features of a starfish, Asterias forbest. A, general view of the sur- face of the body, showing the prominent spines, the finger-like papulae or dermal branchiae, and the rosettes of small white pedicellariae surrounding the spines. B, small region of the body, highly magnified, showing these features in greater detail. Note the jaws of the pedicellariae and the distension of the papulae with circulating coelomic fluid. (Photographs by Bassett Maguire, Jr.) In this chapter we shall review the structure and activities of various echinoderms from general and comparative points of view. The Class Asteroidea The Starfish: Structure and Activities. The commonest starfishes of the Atlantic coast of North America are species of the genus Asterias; A. forbesi occurs in shallow waters south of Cape Cod and is replaced in deeper waters and north of the Cape by A. vulgaris. In general, these animals are very similar. Asterias forbes: is found upon rocky and shelly bottoms, from high- tide mark to a depth of about 125 feet; in these zones the bivalve mollusks constituting the greater part of its food are most abundant. ‘These starfishes sometimes occur on sandy or muddy bottoms and may crawl up wharf piles in search of food; on rocky shores they are often found in pools left by the receding tide. The body consists of a central disk and five radiating arms or rays (Fig. 16.2). The upper or aboral surface is covered with short spines, which are skeletal structures; the surface between the spines bears thin-walled, finger- like projections called papulae or dermal branchiae, functioning in respiratory 489 GENERAL ZOOLOGY exchange and in excretion. Clustered at the bases of the spines and among the papulae are minute, pincer-like structures, the pedicellariae, which func- tion in keeping the surface free of foreign matter. In such a sluggish, slow- moving animal as the starfish, this is a very important function indeed. In addition to finely divided particles of silt and detritus, the ocean is full of minute larvae of such forms as sponges, coelenterates, encrusting ectoprocts, and barnacles, seeking surfaces on which to settle and produce their character- istic growths. Starfishes are never encumbered by such growths, probably be- cause of the activities of pedicellariae in removing them and keeping the papulae free of obstructions. At one side of the disk, between the bases of two of the arms, lies a porous plate, the madreporite or sieve plate; through its minute openings the internal water-vascular system communicates with the exterior. For convenience in reference, the two arms between which the madreporite lies are termed the bivium; the other three are the trivium. On the oral surface the mouth occupies the center of the disk, surrounded by an oral membrane or peristome. Radiat- ing from the mouth along the arms are five ambulacral grooves, from which project numerous locomotor organs called tube feet. At the outer end of each ambulacral groove lies a small, reddish eye spot below a short sensory tentacle. The starfish creeps slowly along the bottom by coordinated stepping move- ments of its tube feet. Although it seems inflexible, and its arms may be broken off by rough handling, the animal can bend and twist in a great variety of ways. For example, when a starfish is turned upside-down, it rights itself by twisting the arms until some of the tube feet become attached to the bottom. Using these attached points for traction and for reference, the animal slowly turns itself back to the normal position. Changes in shape and attitude are brought about by the action of muscles which interconnect the calcareous plates of the body wall. Normal locomotion, however, is effected primarily by the tube feet, which in Asferias terminate in suckers and can be firmly attached to a hard substrate. Under certain conditions these suckers seem to be necessary, as when the animal clings to a wharf pile or stone or walks up the glass wall of an aquarium. But the animal can walk perfectly well without attaching its suckers, over a loose, sandy bottom or upon a greased surface, and the tube feet of many species of starfishes lack suckers altogether. The mechanism by which the tube feet operate will be explained in connection with the ambulacral system of which they are a part. There is nothing like a head or an anterior end in the starfish; the animal can travel in any direction, and no part appears to assume the lead more frequently than others. Once the animal has started in a particular direction, however, the tube feet of all the arms step in the direction taken by those of the leading arm. ‘The starfish thus progresses steadily until it encounters some- thing to change the course of locomotion. A comparable coordination is shown in the righting reaction, when all the arms and their tube feet move in an integrated fashion as soon as a firm hold has been secured by some of the tube feet of one of the arms. 490 THE PHYLUM ECHINODERMATA Structures and Functions Related to Metabolism and Responsiveness. The food of the starfish consists principally of mollusks, such as clams, oysters, scallops, and snails. In feeding, the animal assumes a characteristic “humped” position over the prey; the more proximal tube feet are attached by their suckers to the outer surface of the shell, and the more distal ones commonly appear to secure a hold upon the substrate. In some manner still imperfectly understood, the starfish Soon causes the valves of the mollusk to gape open. An older theory, without any demonstrable basis, held that the starfish secretes some noxious substance which finds its way between the valves, paralyzing the adductor muscles of the bivalve. Experimental work has shown that by powerful, sustained contraction of its muscles (probably those connecting the ambulacral ossicles), the starfish can exert enough force through its firmly attached tube feet to pull the valves apart, at least a few millimeters. It is clear that such a small gape can be produced, without damaging the adductor muscles of the clam, by the application of a degree of force which the starfish is capable of bringing to bear on the valves. When even a small opening has been produced, the stomach of the starfish is everted through the mouth and inserted into the mantle cavity of the bivalve. It has been established by experiment that the folds of the everted stomach can pass through a surprisingly small orifice. Having penetrated the outer defenses of the clam, the starfish releases into the mantle cavity digestive enzymes so powerful that they gradually bring about the complete disintegration of the soft parts of the mollusk. The products of this extracellular digestion are con- ducted by powerful flagellary currents into the digestive cavity of the starfish. After completing its meal of the body of the mollusk, the starfish with- draws its stomach, closes its mouth, and crawls away, leaving behind only the empty shell of its prey. By this peculiar method of feeding, Asterias forbes: and A. vulgaris destroy large numbers of shellfish. One investigator reported that during a 6-day period, a single starfish devoured 56 clams, some as long as an arm of the starfish itself. Starfishes also feed on dead or injured animals other than shellfish and even attack other starfishes and sea urchins. In the absence of food, a starfish can survive starvation for several months. The central portion of the digestive system occupies the short oral-aboral axis of the starfish (Fig. 16.3). The mouth opens into the cardiac stomach, the folded walls of which take up much of the cavity of the disk. Above the cardiac stomach, and communicating broadly with it, lies the small, flattened pyloric stomach, which leads upward into the short intestine. The intestine opens to the exterior at the anus, which is somewhat eccentrically placed on the aboral surface of the disk. The intestine bears a pair of branched, tubular organs, the rectal caeca, which lie between the roof of the pyloric stomach and the aboral body wall. These may be considered as homologous with the “water lungs” of holothurians (pp. 504-505), but their functions are unknown. In addition to these central parts, the digestive tract also includes branched diverticula which extend into the cavities of the arms. These are the five 49] GENERAL ZOOLOGY -Ambulacral ossicles Gonads Rectal caeca Pyloric duct Cardiac stomach Madreporite Fig. 16.3. A starfish, Asterias, partially dissected from the aboral surface, to show relation- ships of various internal structures. (Redrawn from W. R. Coe, 1912, Geological and Natural History Survey, State of Connecticut, Bulletin 19.) pairs of pyloric caeca; one pair lies in each arm, and a single duct leads from each pair into the pyloric stomach. ‘The pyloric caeca consist of series of sac-like evaginations, extending along a central, tube-like canal. The walls of the sacs contain numerous glandular cells which are the apparent source of powerful digestive enzymes. Interspersed among the glandular elements are other cells, which function in the absorption of products of digestion and in the storage of reserves. All these cells together constitute a single-layered epithelium, like the lining of the intestine in a vertebrate. In the pyloric caeca, however, the cells bear long flagella which maintain a steady circula- tion of fluids within the cavities of the organs. Specifically directed currents sweep digestive juices toward the cardiac stomach, where the enzymes are 492 THE PHYLUM ECHINODERMATA principally active, and carry the products of digestion into the caeca for absorption. The coelomic fluid contained in the extensive coelomic body cavities (Fig. 16.4) performs the functions of a circulatory system in the starfish. This colorless fluid contains many phagocytic, amoeboid cells and, very much like the hemocoelic blood of an arthropod, bathes the tissues of the body. In the starfish, however, there are no functional blood vessels, and there is no pulsatile heart. Circulation of the coelomic fluid is brought about by the action of flagella borne on the cells of the peritoneal lining. ‘This single- layered epithelium covers both the inner surface of the body wall and the outer surfaces of all the organs. It is composed of cuboidal or flattened cells, each of which bears a single long flagellum. ‘These flagella together maintain specific currents which prevent stagnation of the fluid and facilitate exchange of materials between the fluid and the tissues. Nutrients are passed into the coelomic fluid by the cells of the pyloric caeca, for transport to all other parts of the body; oxygen diffuses into the fluid through the thin-walled, finger-like papulae of the body wall and is carried in simple physical solu- tion. Carbon dioxide and soluble nitrogenous excreta are transferred to the Spine i “e Pyloric caecum _- Papula Genital _— pore ~ Gonad Ampulla Lateral i pics Radial canal ‘f Interossicular 3 muscle Radial perihaemal : canal : ! : "—~Pedicellaria ey nerve cord Podium yi INS “— Sucker Longitudinal muscle layer Fig. 16.4. Diagrammatic cross section of an arm of Asterias. Note that the genital pores, included in this drawing to show general relationships, actually lie very near the proximal end of the arm, in the angle between adjacent arms. (Redrawn, after H. C. Chadwick, from W. M. Reid in F. A. Brown, Jr., et al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission.) 493 GENERAL ZOOLOGY external sea water through the papulae, which thus function also as excretory organs. In general, it is clear that the circulating coelomic fluid performs the major functions which in most other animals are subserved by the cir- culating blood. ‘This is particularly interesting, in view of the fact that, aside from small amounts of proteins and nutrients dissolved in it, the coelomic fluid is almost identical with sea water in its composition. In fact, certain organs of the starfish, notably isolated pyloric caeca, will survive for several days in cool, aerated sea water. The nervous system of the starfish is basically organized on the radial plan typical of other parts of the body. Its chief components are a circumoral nerve ring, surrounding the peristomial membrane; a series of five radial nerve cords, one in each arm, extending from the nerve ring; and a generally distributed subepidermal nerve plexus with connections into the radial cords. In addition to the conspicuous eye spots and sensory tentacles at the tip of each arm, receptors are scattered throughout the epidermal layer. ‘The nerve ring and radial cords, superficially located, consist of thickened and specialized areas of the epidermal layer (Fig. 16.4). They contain many neurons, arranged in sensory, association, and motor tracts. Aside from the circular and radial nerve cords, there are no true nerves in the starfish; nerve cell bodies are not restricted to the cords, and nerve fibers pass out from the cords more or less individually. The receptors that occur in the epidermis send afferent branches into the plexus layer, from which motor fibers run directly to muscles of the papulae, pedicellariae, and spines. Localized reflex activities of these structures are thus possible without the intervention of the “central” nervous system. More generalized responses involve afferent fiber tracts from the plexus into the radial cords, and complex efferent or motor pathways in- volving series of neurons that course from the cords to specific muscles of the body wall and tube feet. At the central end of each radial cord, the nerve ring contains a ‘“‘motor center,” a large group of nerve cell bodies which appear to be responsible for coordinating the activities of the tube feet in all the arms. In locomotion, the assumption by one arm of the “lead” position seems to involve a temporary dominance by the motor center of the leading arm over all the other motor centers. ‘This condition is transitory, however, and the “‘lead” passes to other arms and their centers in turn. The wall of the digestive tract contains a conspicuous nerve plexus layer, which undoubtedly has connections with visceral receptors and also with the muscle layers of the gut wall. The functions of this part of the nervous system have never been analyz7d. Although the nervous system of the star- fish presents many peculiarities, both structural and functional, the operation of this sytem is apparently fundamentally comparable with that of the nervous systems of other metazoans. A unique feature of the anatomy of all echinoderms is the water-vascular system, or ambulacral system (Fig. 16.5). Through the projecting tube feet, the ambulacral system of the starfish functions chiefly in locomotion and in adherence to the substrate, although it may contribute significantly to the 494 THE PHYLUM ECHINODERMATA Madreporite Fig. 16.5. Diagram of the am- bulacral system of Asterzas. (Re- drawn from W. R. Coe, 1912, Geological and Natural History Survey, State of Connecticut, Bulletin 19.) Lateral canal Tube foot process of respiratory exchange also. The madreporite, lying on the aboral surface of the disk, contains many small openings into a tube, the stone canal, which passes orally and joins a circular ring canal around the mouth. From the ring canal a radial canal extends to the tip of each arm, passing just above the radial nerve cord. At frequent intervals the radial canal gives rise to short lateral canals, each of which terminates in a tube foot. At its inner end each tube foot bears a muscular bulb, the ampulla; the stalk of the tube foot projects through the body wall, passing between the closely set skeletal plates. The system as a whole contains a fluid which, like the coelomic fluid, is practically identical with sea water. Circulation through the system is maintained by the action of flagella on the lining epithelium. The arrangement is such that the fluid contents of the system may pass freely into each tube foot through the lateral canal, and within the tube foot may flow back and forth between the stalk and the ampulla. Backflow from the tube foot into the lateral canal is prevented by a valve-like structure. The wall of the stalk contains longitudinal muscle fibers, and the terminal disk or sucker is provided with muscle fibers whose contraction raises the central portion of the disk to provide a vacuum for attachment to the substrate. The longitudinal muscles are basally attached in a radial fashion to the adjacent skeletal plates of the body wall. These so-called ‘“‘postural muscles” pro- vide for the pointing of the tube foot in any direction and thus allow directed locomotion. The complex interactions of the musculature are reflexly co- ordinated: contraction of the muscles of the ampulla forces fluid into the stalk, causing it to extend in a direction determined by the state of the postural muscles; the sucker is then placed on the substrate and attached by contraction of its special muscles. As the body moves forward, the sucker is released, and contraction of the longitudinal muscles of the stalk forces fluid back into the ampulla and brings about retraction of the stalk. ‘The cycle 495 GENERAL ZOOLOGY Fig. 16.6. Generalized, sche- matic life cycle of an asteroid echinoderm. ‘The gastrula (A) transforms through inter- mediate stages into a bipin- naria larva (8), which metamorphoses into a young starfish (C). (Adapted from H. Barraclough Fell, 1948, Biological Reviews, vol. 23, printed by permission of the Cambridge University Press.) is then repeated through a new stepping sequence. ‘The integration of all these activities involves the coordinating functions of the central nervous system. The skeletal system is of mesodermal origin and lies beneath the epidermis of the body wall. It is thus an endoskeleton, showing similarities to the skeleton of a vertebrate, rather than a cuticular exoskeleton of the type more common among invertebrate animals. ‘The plasticity of the body, as shown in righting and other activities, is accounted for by the fact that the skeleton of the starfish is composed of many small plates, bound together by connective tissue and muscle fibers. The Reproductive System, Reproduction, and Development. Starfishes are dioecious. ‘The reproductive system consists of five pairs of gonads, either ovaries or testes, one pair lying in the coelomic cavity of each arm, lateral to the pyloric caeca. Each gonad is continuous with a short stalk which forms its duct and attaches it to the body wall. The ducts open to the outside at genital pores located deep in the angles between the bases of adjacent arms. Gametes are discharged into the sea, where fertilization occurs. ‘The eggs, and thus the zygotes, contain very little yolk, and cleavage is total and equal. Development proceeds rapidly through blastula and gastrula stages, the gastrula soon transforming into a bilateral, ciliated larva, the bipinnaria (Fig. 16.6). This larva has a complete digestive tract and feeds on uni- cellular algae as it swims about near the surface. Its free life may last for several weeks. Finally, the larva sinks to the bottom, becomes temporarily attached, and undergoes a radical metamorphosis into a tiny starfish. In the course of this metamorphosis, the organization of the larva changes completely. The left side of the larva becomes the oral surface of the adult; the old openings of the digestive tract disappear, to be replaced by a new 496 THE PHYLUM ECHINODERMATA mouth and anus in shifted locations. ‘The paired coelomic sacs of the larva are transformed into specific adult structures: one of the pouches establishes an external connection, the future madreporite, and develops into the water- vascular system; other pouches give rise to various parts of the spacious perivisceral coelom of the adult. It is perhaps significant, in interpreting the evolution of echinoderms, that the change from bilaterality to radial symmetry in the starfish occurs during a temporarily attached phase of the life cycle. Regeneration. Starfishes are often found in nature with one or more arms smaller than the rest. ‘The small arms are in the process of regeneration. Under experimental conditions as many as four arms may be removed with- out causing the death of the animal, and all four can eventually be replaced by regeneration. When all five arms are removed, regeneration can still occur if the individual is fed after the formation of the new arms has begun. Although an isolated arm of Asterias can survive for several weeks, it will eventually die, since it cannot restore the disk and other arms. In some other starfishes, however, even a single isolated arm has the capacity to regenerate into a complete individual. Under rough or damaging treatment a starfish may shed an injured or restrained arm by a process termed autotomy. A break of this kind always occurs at the base of the arm, where the body cavity is restricted as it passes from disk to arm. ‘The resultant opening in the side of the disk is immediately closed by the contraction of the adjacent body-wall musculature, and regenerative changes then begin. Other Asteroidea. In all the members of the class Asteroidea, the body is stellate, whether the arms are long and slender, as in Henricia, or short and broad, as in Oreaster. In some species more than five arms are present, as in Solaster. In the leather stars, Dermasterias, spines are lacking, and the body is covered by a smooth skin concealing the underlying reduced skeleton. ‘There are few members of the class so modified that they are not immediately recognizable as asteroids. The Class Ophiuroidea Members of this class, the so-called serpent stars, brittle stars, and basket stars, are recognizable by the relatively large and conspicuous disk and the slender, mobile arms (Fig. 16.7). In basket stars the arms branch repeatedly and terminate in many small, flexible tendrils. In ophiuroids the organs of the digestive tract do not extend into the arms; the volume of the skeleton is relatively much greater than in the asteroids, and the arms are largely occupied by jointed skeletal units sometimes called “‘vertebrae.”’ Brittle stars are so named because of the fragility of their arms, which lash about actively and break off very easily. In spite of their peculiarities, the ophiuroids are clearly echinoderms. ‘The life cycle (Fig. 16.8) includes a bilateral, ciliated 497 GENERAL ZOOLOGY Fig. 16.7. Class Ophiuroidea. A brittle star, Ophiopholis aculeata; note the characteristic sharp distinction between the bulbous central disk and the slender, flexible arms. (Photograph by George Lower.) Fig. 16.8. Generalized, schematic life cycle of an ophiuroid echinoderm. The gastrula’ (A) transforms through an indifferent pluteus stage into a fully developed ophiopluteus larva (B), which undergoes metamorphosis in- to a young brittle — star. (Adapted from H. B. Fell, 1948, Biological Reviews, vol. 23, printed by permission of the Cambridge University Press. ) (A) mn 498 THE PHYLUM ECHINODERMATA Fig. 16.10. Generalized, sche- matic life cycle of an echi- noid echinoderm. Through an indifferent pluteus stage the gastrula (A) transforms into a fully developed echi- nopluteus (8); after a period of larval life this meta- morphoses into a young sea urchin (C). (Adapted from H. B. Fell, 1948, Biological Reviews, vol. 23, printed by permission of the Cambridge University Press.) radial appendages of the digestive system as in the starfish. Aside from these special features, however, the internal organization is similar to that of the asteroids: the ambulacral and reproductive systems, as well as the nervous system, are generally comparable in arrangement to those of the starfish, although the circulatory system is better developed. In Arbacia the sexes are separate; the five gonads are radially arranged, and each sheds its sexual products into the sea water through one of the pores located on the genital plates. Fertilization is thus external, and development proceeds rapidly to a bilateral, ciliated larval stage, the echinopluteus (Fig. 16.10). This larva is comparable to the bipinnaria of the starfish, although in the presence of several pairs of long arms, each supported by an internal spine, it resembles even more closely the ophiopluteus of the ophiuroids. After several weeks of pelagic life, feeding on diatoms and microscopic algae, the pluteus sinks to the bottom and undergoes metamorphosis into a miniature sea urchin. Other Echinoidea. In Arbacia the body is circular in its lateral outline, and the 20 rows of plates are arranged in a regular pattern. ‘This regularity of skeletal arrangement has not always been characteristic of sea urchins, as shown by some fossil forms in which the plates are irregularly arranged. Among the modern echinoids the shape and symmetry of the body are highly variable (Fig. 16.11). In Clypeaster the mouth is in the center of the oral surface, but the anus lies in an interambulacral area on the lateral margin of the somewhat flattened body; thus the animal is actually bilaterally sym- metrical. In the sand dollar, Echinarachnius parma, the organization is like that of the clypeasters, although the flattening of the body is more extreme. In the type represented by Spatangus, the mouth has shifted peripherally, or 501 GENERAL ZOOLOGY ABORAL ORAL Mouth aor ORAL Fig. 16.11. Cleaned tests of some echinoids which approach bilateral symmetry in some external features. A and B, a sand dollar, Clypeaster, in which both mouth and anus lie on the “median plane” of the oral surface. C, a heart urchin, Spatangus, with the mouth displaced “anteriorly” and the anus at the opposite end of the body. D and £, a sand dollar, Echinarachnius, with the mouth centrally located and the anus at the “posterior” edge of the flattened body. These modifications may be interpreted in relation to the habits of the animals, which either inhabit burrows or live just under the surface of sandy bottoms. 502 THE PHYLUM ECHINODERMATA in an “anterior” direction, along the ambulacral area opposite the anus. The fossil record and the life cycles of these bilateral urchins indicate that they have descended from ancestors which were circular and radial like Arbacia. ‘The existence of fossil forms with irregularly arranged plates sug- gests, in turn, that the Arbacia type, with 20 rows regularly arranged, arose from ancestors without this skeletal regularity. The Class Holothuroidea The Sea Cucumber. Thyone briareus, a sea cucumber common along the Atlantic coast from Cape Cod southward, is an example of the holothurian type of echinoderm (Fig. 16.12). Fundamentally, it is radially symmetrical, but the characteristic elongation of the body between oral and aboral ends, and certain other specializations, give it a bilateral and often worm-like appearance. The texture of the body is very different from that found in the starfish and sea urchin: the expanded Thyone is soft, like a bladder partly distended with fluid, and there is no skeleton except minute calcareous spicules embedded in the body wall and a few larger plates in the oral region. At one end is the mouth, surrounded by ten branched tentacles, and at the other is the anus. ‘The tube feet are not in distinct rows but lie scattered all over the body, although internally they are connected with five Fig. 16.12. A sea cucumber, Thyone briareus. In this individual the feeding tentacles are well extended; note also the sand grains adhering to the tube feet, scattered irregularly over the surface of the body. (Photograph by George Lower.) 503 GENERAL ZOOLOGY PBS cesta Tentacles 7 Ring canal Retractor muscles of ~ peristome SQ 0 SPAY Fig. 16.13. Internal anatomy S of a sea cucumber, Thyone Intestine briareus. One of the ‘“‘water lungs,” or respiratory trees, has been removed, and the mesenteries supporting the di- gestive tract are not shown. aA MCUs Sad ca aiden mY Soro ss Note that the madreporite opens into the perivisceral coelom, not externally. (Re- drawn from W. M. Reid in F. A. Brown, Jr., e¢ al., Selected Invertebrate Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission.) radial ambulacral canals as in other echinoderms. ‘The animal usually les on one side, and this “‘ventral”’ surface has larger and more numerous tube feet than the “dorsal” or uppermost side. In addition, the distance from mouth to anus is greater along the ventral midline than along the dorsal; and to this extent 7hyone presents a bilateral appearance, with dorsoventral as well as anteroposterior differentiation. Thyone lives partly embedded in the ooze of muddy bottoms just below low-tide level, with its anterior and posterior ends exposed. ‘The anterior end is directed diagonally upward, and the tentacles are free to move over the surface of the surrounding mud. If the animal is disturbed, the tentacles are withdrawn by inversion of the entire oral end of the body; excess water is discharged from the anus, and the body becomes tense and turgid. In feed- ing, the tentacles are moved about until well covered with silt from the bottom and are then thrust, one at a time, deep within the mouth and re- lieved of their burden as they are again withdrawn. The animal “breathes” sea water, pumping it in and out through the anus. In locomotion, the organism moves by extending its tube feet, attaching their terminal suckers, 504 THE PHYLUM ECHINODERMATA and dragging itself along. By this means it can even walk up the glass side of an aquarium. The digestive system begins as a small, muscular pharynx, surrounded by the tough, ring-like structure to which the tentacles are attached, and which can be retracted by the contraction of five stout muscles running to the body wall (Fig. 16.13). The pharynx is followed by a short esophagus, a small muscular stomach, and a long, looped intestine. ‘The intestine traverses the length of the coelomic cavity three times, supported along part of its course by dorsal and ventral mesenteries containing blood vessels. At its posterior end, the intestine enlarges to form the cloaca, which opens to the exterior at the anus. ‘The cloaca bears a pair of branching, tubular structures, the “water lungs,” which are filled with water drawn through the anus and pumped into them by cloacal contractions. ‘Through the walls of these tubules respiratory exchange occurs between the water and the coelomic fluid, and the water is periodically expelled by contraction of the body wall. Thyone shows peculiarities in other organ systems. ‘The ambulacral system consists of the same parts found in the starfish and the sea urchin, arranged somewhat differently. ‘The ampullae of the tube feet are scattered over the internal surface of the body wall but connect with five radial ambulacral canals. The stone canal springs from the ring canal about the pharynx and ends in an internal madreporite, which hangs free in the coelomic cavity and has no external openings. A so-called haemal system, very rudimentary in other echinoderms, is well developed in holothurians. In Thyone it is par- ticularly conspicuous in connection with the digestive tract, to which branches of the haemal system run in the mesenteries. Although there is no heart or other propulsive organ, the haemal system contains a fluid which probably Fig. 16.14. Generalized, sche- matic life cycle of a holothuroid echinoderm. ‘Through _inter- mediate stages essentially sim- ilar to those of an asteroid, the gastrula (A) transforms into an auricularia larva (8); this later metamorphoses into a young sea cucumber (C). (Adapted from H. B. Fell, 1948, Biological Re- views, vol. 23, printed by per- mission of the Cambridge University Press.) 505 GENERAL ZOOLOGY Me N A Qe UH) Ws t NY a~ C SG ES i is 7 AMT . NMS y ROY OM Sty d iy y - Fig. 16.15. Class Crinoidea. A feather star, Antedon rosaceus. A, general appearance of the animal. B, oral view of the central disk, showing the am- bulacral grooves converging on the mouth. The anus is elevated on a conical struc- ture. (Redrawn from H. C. Chadwick, 1907, Liverpool Marine Biological Committee Memoirs, no. 15.) Ambulacral grooves circulates and serves some of the functions of a blood-vascular system. As in other echinoderms, however, the coelomic fluid is most important in this respect. The reproductive system in holothurians is peculiar in that there is only one gonad, the duct of which opens externally in the dorsal interambu- lacral area between two of the tentacles. The animals are dioecious; gametes are shed into the sea, fertilization is external, and as in other echinoderms there is a bilateral, ciliated larval stage, called in this instance the auricularia (Fig. 16.14). If its lack of a skeleton is disregarded, a sea cucumber is like a sea urchin with its body elongated in the axis of radial symmetry, which extends from mouth to anus. Correlated with this elongation are the anteroposterior and dorsoventral differentiations and the appearance of a superficial bilateral symmetry. 506 THE PHYLUM ECHINODERMATA Other Holothuroidea. Thyone and Cucumaria, a species with five distinct rows of tube feet, represent a type of sea cucumber in which the body form is not highly specialized by extreme modifications and loss of parts. In contrast, Leptosynapta, a burrowing type, lacks tube feet altogether; its echino- derm characteristics are revealed, however, by the presence of minute skeletal plates and five radial muscle bands, visible through the translucent body wall. One of the more highly modified holothurian types is represented by Psolus chitinoides, a species common on the North Pacific coast. In this form the dorsal surface is protected by scale-like skeletal plates, and the ventral surface resembles the creeping foot of a mollusk, except for the presence of three rows of tube feet. The Class Crinoidea Members of the class Crinoidea have branched arms and are often attached to the bottom by a stalk, which joins the aboral surface of the disk. This gives the animal a flower-like appearance and is responsible for the common name, “‘sea lily.””. Even the type represented by Antedon, the so-called feather star, which has no stalk and is not permanently attached, bears a tuft of aboral filaments by which it commonly clings to the substrate (Fig. 16.15). In all crinoids, the oral surface is uppermost; the mouth occupies the center of the oral surface of the disk, surrounded by five highly branched arms. The anus also lies on the oral surface, near the mouth, at the tip of an anal papilla. Ciliated ambulacral grooves radiate from the mouth, traversing the Fig. 16.16. Generalized, sche- matic life cycle of a crinoid, (C) based on Antedon. ‘The free- swimming larva (A) is relatively undifferentiated; it attaches and transforms into a_ pentacrinoid stage (B), which later meta- morphoses into a free-swimming feather star (C). (Adapted from H. B. Fell, 1948, Buological Re- views, vol. 23, printed by per- mission of the Cambridge University Press.) (B) 507 GENERAL ZOOLOGY disk and continuing along the oral surfaces of the arms and their branches (Fig. 16.15). Currents maintained in these grooves converge upon the mouth and carry into it the microscopic particulate’ matter which constitutes the food of the crinoids. Tube feet are present, but they lack ampullae and terminal suckers; they have tactile functions and serve also in respiratory exchange. As in holothurians, the water-vascular system of the adult crinoid has no external madreporite; there are multiple ““water tubes” or stone canals which furnish communications between the ambulacral ring canal and _ the coelomic cavity. The life cycle of the crinoids is best known for the feather star (Fig. 16.16). It includes a barrel-shaped, ciliated larva which, though simplified, is com- parable with the larvae of other echinoderms. After a free-swimming period, this larva becomes attached and undergoes metamorphosis into a juvenile form with a jointed stalk like that of the sea lilies. Eventually, the connection between the stalk and the disk is broken, and the animal becomes free-moving. This stalked phase in the life cycle of Antedon indicates that feather stars have evolved from ancestors attached throughout their adult life. The fossil record shows that stalked crinoids have had an extremely long evolutionary history, dating from the end of the Cambrian period. Different crinoid groups have flourished during successive geological eras; most of them were stalked forms, and by far the majority have become extinct. The modern crinoid fauna is dominated by the free-moving feather star type. The Echinoderm Body Plan When compared with the organization of other eucoelomate metazoans, the body plan of a typical echinoderm appears in some respects highly aberrant. Examination of details reveals, however, that there are more similarities than differences between echinoderms and other eucoelomate animals. As _ is typical of all, echinoderms are triploblastic and have a well-developed coelom. Moreover, the arrangement of tissues in the body wall and in the wall of the gut in an echinoderm is fundamentally like the arrangement in such other metazoans as annelids and vertebrates. In each of these forms the body wall is covered externally by an epidermis and internally by a peritoneum, and between these two layers are muscular and connective tissues. The wall of the digestive tract in each type consists of a covering peritoneum, a region of muscular and connective tissues, and a lining of columnar epithelium. Furthermore, these tissues arise from similar germ layers in the embryonic development of these different forms, in ways which differ only slightly among the three. Considering these and other general correspondences, it is clear that the major dissimilarities stem from the fact that, unlike all other eucoe- lomates, echinoderms are organized on a radially symmetrical plan. If, in addition, the bilateral larva and its probable evolutionary sig- nificance are considered, the body of an adult echinoderm may be interpreted 508 THE PHYLUM ECHINODERMATA as an extreme modification of the bilateral-triploblastic-eucoelomate plan which appears in all the more highly developed phyla—a modification which involved the remodelling of typical bilateral organization along the lines of radial symmetry. ‘The larvae (bipinnaria, pluteus, auricularia, etc.) char- acteristic of the various classes show many similarities, and it has been suggested that they may all have arisen from a hypothetical ancestral larval form called the dipleurula. But how does it happen that an animal which is radially symmetrical as an adult has a bilateral larva? ‘Two answers may be considered. Either this larval stage has developed secondarily and represents what happened to be produced in the adjustments of larval life during the long evolutionary history of the echinoderms; or, more probably, it occurs in the development of echinoderms because it represents a stage in their ancestry, like the fish-like stages in the ontogeny of a higher vertebrate. If we regard a dipleurula-like animal as representing a bilaterally symmetrical ancestor of echinoderms, we may suppose that this ancestor became attached and acquired a five-part radial symmetry in correlation with a sessile way of life. The oldest known echinoderm fossils represent attached forms, and the fixed habit has been retained by modern crinoid sea lilies. The ancestors of asteroids, ophiuroids, holothurians, and echinoids, on the other hand, no longer attached forms, have become variously modified for free life. But even the free-moving asteroids and feather stars show a temporary attached phase at the time in the life cycle when the bilaterality of the larva is replaced by the radial symmetry of the adult. Finally, it should be noted that in some holothurians and sea urchins, the secondary radial symmetry has begun to be replaced by a new bilaterality, which is unrelated to the bilateral symmetry of their larvae. 509 amen 0 |) THE VITAL FUNCTIONS AND ORGAN SYSTEMS OF INVERTEBRATES The vital functions and organ systems of vertebrate animals have been dis- cussed in the early chapters of this book, and in subsequent chapters com- parisons have been drawn between the organ systems of vertebrates and the structures serving similar functions in invertebrates. Let us now consider the functional systems of invertebrates in more general terms; such a survey will illustrate the kind of diversity and unity that is everywhere apparent when the world of animal life is carefully studied. Often the plan of an organ system is remarkably similar throughout a considerable number of phyla. But in other phyla the varied structural relationships clearly indicate that the systems in these phyla are not homologous, although non-homologous parts may be functionally analogous. It should be recalled that the simpler and more primitive types of invertebrate animals do not possess organ systems, if we define these as groups of organs which, taken as a whole, perform some common function. Indeed, the simpler animals may even lack organs, and the simplest types of metazoans have not evolved tissues. Nevertheless, one important generalization emerges from comparative studies of invertebrates, and this will be the theme of the present chapter. All animals, of whatever grade of complexity, are (figuratively speaking) faced with certain basic problems of existence; and different groups of animals have solved these common problems in a variety of ways, utilizing the structural possibilities available to them within their particular levels of organization. For example, the metabolism of all animals imposes certain general requirements: all animals must have sources of energy to maintain their metabolic reactions. In simple, plant-like protozoans, energy-rich compounds are manufactured by a photosynthetic process; in more typical animals, metabolic requirements are met by the many activities involved in the capture, ingestion, digestion, absorption, circulation, and assimilation of energy-rich compounds which 510 Buccal or Pharynx Esophagus Stomach Intestine oral cavity Rectum Salivary gland Digestive gland (paired) (paired) Fig. 17.1. Schematic diagram of the gen- eralized digestive system in invertebrates. serve as food. ‘These reactions tend to become more elaborate, or to be served by more elaborate structural specializations, as the size and complexity of animals increase. Holozoic animals ingest many kinds of food, and their di- gestive mechanisms are correspondingly adapted, both structurally and functionally, to their individual necessities. And yet, in all animals that have digestive tracts, the digestive systems show obvious similarities; in fact, the digestive system has much the same fundamental structure in all animals that have both mouth and anus. In contrast, the reproductive systems of the different phyla show great diversity of structure, although there are remark- able functional parallels. Thus, to return to our figure of speech, the problem of developing an efficient reproductive system has been solved in a great variety of ways. The comparative review to be undertaken in the present chapter furnishes data for consideration by the student of evolution, although little reference will be made to this aspect of the subject. It should become evident, how- ever, that homologous structures may be modified for diverse functions, and that structures of varied embryonic origins may be adapted to common func- tional purposes. Systems Related to Metabolism Digestive Systems. Members of the phylum Protozoa perform all of their vital activities at the unicellular level, and their structural specializations for various functions are often referred to as organelles. In the more complex protozoans, such as the paramecium, there. is a “‘system” of organelles specialized for various aspects of nutrition. ‘The cytostome or cell mouth, the 511 GENERAL ZOOLOGY gullet, and the anal spot where egestion occurs are permanent features of the cell, along with certain modifications of the oral ciliation used in feeding. The food vacuoles, on the other hand, are not permanent but transitory. The amoeba possesses only temporary structures, the food vacuoles and_ the pseudopodia. ‘The available evidence indicates that the particles of food contained in food vacuoles are subjected to enzymatic hydrolysis, or digestion, and that the functions of the food vacuoles are thus those of “temporary stomachs.” In the Mesozoa and Porifera there is no enteron, or digestive cavity; mesozoans absorb food in solution through the cells of the outer layer, and sponges ingest particulate matter through the activities of individual collar cells lining the water channels. In the Coelenterata, Ctenophora, and Platyhelminthes, the enteron is a cavity, usually with only one external open- ing. If we define a digestive tract as a continuous tube through which food moves from mouth to anus, these groups have not evolved true digestive tracts. In the other phyla of invertebrates there is, typically, a complete enteron with both mouth and anus, and the term digestive tract is applicable. It is interesting to note that in some protozoans a tract-like course is followed by the food vacuoles as they traverse the cytoplasm of the single-celled body. The intracellular digestion which occurs in protozoans and in the digestive cells of many metazoans presents another parallel between unicellular and multicellular animals. As we pass on to the invertebrates with complete digestive tracts, with both mouth and anus, it is clear that a fundamental plan is characteristic of all. Once the tubular digestive tract had been evolved in its basic aspects, it was apparently preserved in all subsequent groups through the selective ad- vantages it presents. Comparisons may be made by means of a generalized diagram such as Figure 17.1. This is a schematic representation of an in- vertebrate digestive system with all the important parts that might be found in a single species. Certain invertebrates have all these parts, but in most species some are missing whereas others are highly developed. ‘There is probably no organ system of animals which demonstrates more clearly than the digestive system the relation between structure and function. Digestive systems are adapted to many different kinds of food, therefore to many ways of feeding and the related requirements of digestion. Parts that are necessary for ingestion and digestion in some species may be unnecessary in others, and so may be absent or reduced, even in species within the same phylum. For example, compare the digestive system of the clamworm, a predaceous, free- swimming annelid (pp. 398-399), with that of the earthworm, a compara- tively sedentary, herbivorous annelid (pp. 405-406). ‘The digestive systems of forms described in preceding chapters may be effectively reviewed by comparisons with the schematic figure. With respect to functions, the tubular digestive tract shows regional specializations related to various aspects of the digestive process. ‘The mouth parts, oral cavity, and pharynx are concerned chiefly with ingestion; 512 STRUCTURE AND FUNCTION IN INVERTEBRATES the esophagus is a passageway; the stomach is the place where digestion and absorption begin; the intestine is the region where these processes are com- pleted; and the terminal region called the rectum forms feces for egestion and may have special ancillary functions such as reabsorption of water. Salivary glands secrete digestive enzymes or merely lubricating fluids. A digestive gland, paired or single, opening into the region between stomach and intes- tine, commonly supplies the principal digestive juices. In certain inverte- brates, as in the crayfish and starfish, such glandular diverticula are also the chief sites of digestion and absorption. Unicellular glands which secrete lubricants or enzymes may also occur throughout the lining of the digestive tract. As in the vertebrate, absorption involves the passage of the products of digestion through the lining of the tract into any blood or lymph spaces in its wall, or directly into the fluid of the body cavity. The parallel with structure and function in the vertebrates is obvious, and it is clear that the same fundamental mechanisms operate in the digestive systems of all animals possessing a complete digestive tract with mouth and anus. Circulatory Systems. The circulatory systems of invertebrates are very diverse. In the protozoans cyclotic movements of the cytoplasm often trans- port food vacuoles and absorbed nutrients throughout the cytoplasm, perform- ing a function which corresponds to that of the circulatory systems of metazoans. Another analogous but fundamentally dissimilar mechanism is found in sponges, where currents of water passing through the body furnish a transportation system for food and for excreta. In Coelenterata and Ctenophora, the need for a vascular transport system is obviated by the close relationship existing between the digestive cavity and all other parts of the body. No parts are far removed from the cavity itself, or from its branching extensions in some forms. By muscular contractions, or by flagellary currents in the enteron, food and products of digestion are circulated through the entire body, and the enteron is therefore sometimes called a gastrovascular cavity. Among bilateral forms Platyhelminthes present a similar relationship: there is no anus, and the highly branched enteron, when present, commonly extends throughout the body. A circulatory function is often attributed to the lympth-like fluid which fills the interstices of the mesenchymal mass sur- rounding the organs. In Nemertinea, which have a complete digestive tract, there are blood vessels, although they form a very simple vascular system without a localized propulsive mechanism and without a definitely directed circulation (pp. 340-343). The more efficient blood vascular systems of higher invertebrates are char- acteristically provided with some mechanism which propels the blood along a definite circulatory pathway. The simplest of these involves peristaltic con- tractions of the principal blood vessels. ‘This may be illustrated specifically by the circulatory system of an annelid, such as the earthworm (pp. 406- 407); here the larger vessels are contractile and so drive the blood through the body in a definite course. ‘The so-called “hearts” of the earthworm are merely enlarged vessels connecting the dorsal and ventral parts of the system. 513 GENERAL ZOOLOGY Polyp | B B i ne ud Growing tip Stolon A Stolon Blood system Mouth Anus “Hearts” Capillaries Fig. 17.2. Schematic diagrams of circulatory mechanisms in invertebrates. A, circulation within a coelenteron, extending to all parts of the body (Coelenterata). 6, blood vessels without pulsatile walls, the blood being driven back and forth by movements of the body (Nemertinea). C, blood vessels extensively developed, the larger ones being pulsatile and maintaining a definite circulatory course of the blood (Annelida). The true heart, which is found in all well-developed circulatory systems, probably arose in evolution through localization and specialization of the con- tractile functions of a major blood vessel. Among well-developed invertebrate circulatory systems, two major types can be distinguished. In one, called the closed system, the blood is en- closed within vessels that are continuous through a circuit of heart, arteries, capillaries, and veins. Such a system is found in many annelids, and in cephalopod mollusks such as the squid (see Fig. 13.19, p. 391). More com- mon among invertebrates is the second type, the so-called open system, ex- emplified in pelecypod mollusks (see Fig. 13.3, p. 374). In these animals blood flows from the heart through arteries to all parts of the body; leaving the smallest branches of the arteries, it passes not into capillaries but into intercellular spaces of the mesh-like mesenchymal tissue, collectively called sinuses. From these sinuses blood enters the smallest branches of the veins and so is returned to the heart. Arthropods such as the crayfish have a similarly open system, in which the sinuses form large perivisceral cavities constituting the hemocoel. Such 514 STRUCTURE AND FUNCTION IN INVERTEBRATES a system, unlike that of pelecypod mollusks, lacks veins; arteries extend from the heart to all parts, and from these blood percolates through the tissues into the hemocoel. Return to the heart involves certain tubular specializations of the hemocoel, plus the pericardial sinus. In insects the system is even simpler, because there are no arteries, although the anterior extension of the heart may be termed an aorta. A unique feature of the arthropod system is the manner in which the pericardial chamber has become a part of the hemocoel; blood enters the heart from this cavity through small openings in the heart wall termed ostia. These admit blood to the heart during the diastolic phase of its beat, and the backflow of blood during systole is pre- vented by the action of valve-like flaps covering the ostia. In pelecypod mollusks the pericardial cavity is not involved in the circulatory function; present in the ventricle wall are ostia through which blood enters from a pair of auricles or atria which are a part of the venous system. In some tracheate or air-breathing arthropods the circulation through the hemocoel is aided by dorsal and ventral perforated membranes, termed diaphragms, Mouth Anus C Fig. 17.3. Invertebrate circulatory mechanisms, continued. A, the function of transport served by fluid in the pseudocoel, circulated by movements of the body (Aschelminthes). B, an “open” circulatory system in which a pulsatile ventricle and distributing vessels con- duct blood to the tissues, where it flows into sinuses; blood is returned to the ventricle through vein-like channels and a pair of auricles or atria (pelecypod mollusks). ©, an open system with a tubular heart bearing lateral ostia through which blood enters the heart; blood flows anteriorly into the intercellular spaces of the hemocoel, eventually returning to the heart (Insecta). SHS GENERAL ZOOLOGY which apparently function in directing the flow of blood through the body spaces. It will be recalled that in these tracheate forms the blood has no oxygen-carrying capacity, this function being performed by the branching tracheal tubules. Examples of well-developed and representative circulatory systems among the invertebrates are thus found in the Annelida, the Mollusca, and the Arthropoda; the fundamental functional requirements are met in these groups by related though dissimilar structural specializations. The effective cir- culatory mechanism of most echinoderms is unique, involving the coelomic fluid which is kept in motion by the flagellated peritoneum. Among echino- derms ‘only holothurians and echinoids have a blood-vascular system, the haemal system, retaining a residual circulatory function. In other phyla of invertebrates circulatory systems are absent or but slightly developed. Except for certain invertebrates, such as the tracheate arthropods, general functions of the circulatory system are similar wherever such a system exists, whether in vertebrates or in invertebrates. The circulating fluid brings nutrients, oxygen, and in many instances hormones, to the tissues and carries away for disposal the gaseous and nitrogenous waste products of metabolism. Various circulatory mechanisms are shown schematically in Figures 17:2 and 17.3. Respiratory Systems. ‘Typical animals must continuously exchange oxygen and carbon dioxide with their environments. Special organs to serve this function are not commonly found among invertebrates in which gas exchange can be carried out over a large part of the body surface, as in the- hydra and the earthworm. Generally speaking, development of such special structures occurs concomitantly with increase in size and thickness of the body. This generalization follows the well-known fact that large bodies have much less surface area per unit of volume than smaller bodies. The flattened, leaf-like body form in the relatively large platyhelminth worms may be interpreted as an adaptation that compensates for the lack of special respiratory organs by decreasing the thickness of the body. Among inverte- brates that have developed organs for respiration, the apparently diverse mechanisms can be classified into a few general types, schematically repre- sented in Figure 17.4. These structures are without exception related to the environmental situation in which the animal lives, although they are often conditioned by the ancestral history of the animal. Primarily aquatic species are commonly provided with blood gills, which are outgrowths of the surface of the body with a particularly copious blood supply. ‘These may be external gills, as in the branchial filaments and parapodia of polychaete annelids (pp. 398-402); or, as in pelecypod mollusks and the larger, crus- tacea, they may be enclosed within a cavity opening at the surface. Many echinoderms have dermal branchiae or other thin-walled outpocketings of the body wall through which respiratory exchange occurs between coe- lomic fluid and external sea water. Holothurians, again, are unusual in ¢ their possession of the ‘‘water lungs,” which are functionally analogous 516 STRUCTURE AND FUNCTION IN INVERTEBRATES but certainly not homologous with the respiratory organs of other echino- derms. Only rarely do terrestrial animals. possess blood gills, and those that do, like terrestrial isopod crustaceans, are strictly limited to very moist environments. Adaptation to the typically dry conditions of life on land has involved the development of either lungs of various kinds or of tracheal systems. The lungs of pulmonate gastropod mollusks are modifications of the mantle cavity and its lining; and the ‘‘book lungs” of spiders suggest the plate- like gills of some aquatic chelicerates, withdrawn within a protected cavity. The tracheal systems of terrestrial arthropods such as insects consist of air tubes through which atmospheric air is brought into the body. It may be recalled that readaptation to an aquatic environment has occurred among both pulmonate gastropods and tracheate arthropods, but never has there been a return to the blood-gill system. Adult forms continue to use at- mospheric air, but aquatic immature stages of insects often develop tracheal, gills. The functional relationships are similar in all these adaptations; a thin layer of cells separates the blood or body fluid of the animal from the external water or air that is the source of oxygen for the animal. ‘The site at which oxygen is gained by the blood is also the site at which carbon dioxide is eliminated from the body, whether it be over the general body surface, at gills, in lungs, or in tracheae. ‘Thus it is clear that there is no discontinuity in these functions betwen invertebrates and vertebrates, which also possess gills or lungs. As previously indicated the processes of cellular metabolism are also fundamentally similar throughout the animal kingdom; oxygen is utilized within the cells in enzymatically controlled, sequential reactions involved in the transformations of energy which are the basis of all metabolism (see pp. 35-38). ‘There are special cases of animals, notably intestinal para- sites, inhabiting environments in which free oxygen is unavailable or present in very low concentration. ‘The metabolism of these forms chiefly involves only the anaerobic phase. External gills — Body surface Body surface External gills Tracheal system mS LF DS J A CET EBLD2 997 20,% ‘A BS Sons a) Paine = Teo ae Gr Mouth ao i ais Anus Lung Spiracles Respiratory tree (“water lung”) Fig. 17.4. Schematic diagram representing a composite of many mechanisms of gaseous ex- change found among various invertebrate animals. SI7/ GENERAL ZOOLOGY Multiple Malpighian External gills Body surface flames tubules (VD & Pes . i /] oe Mouth SS eee Single terminal flame Nephridium Nephridiostome Fig. 17.5. Schematic diagram representing a composite of many excretory mechanisms in invertebrate animals. Excretory Systems. As a result of metabolic processes, the cells of all animals are constantly producing waste substances. One of the chief by- products of cellular oxidation is carbon dioxide, and the metabolism of pro- teins and amino acids results in the formation of various nitrogenous wastes as by-products. Carbon dioxide and nitrogenous wastes are generally of no further use to the animal, and continued accumulation of these sub- stances would poison the body. ‘Therefore, one of the problems of animal existence is the elimination of these wastes. As we have seen, important functions of the circulatory system involve the transport of excreta from the tissues to specialized sites of elimination. Carbon dioxide is commonly lost to the external environment in the respiratory process, but the removal of nitrogenous wastes poses special problems which have been solved in different animal groups in diverse ways. Like the respiratory mechanisms, these excretory devices are markedly influenced by environmental factors, conditioned by the evolutionary history of the animal and its level of organization. Various excretory mechanisms of invertebrates are shown schematically in Figure 17.5. Microscopic and moderately small animals, such as Protozoa, Porifera, Coelenterata, and Ctenophora, have no specialized excretory organs. In these forms the area of body surface exposed to the external environment is very extensive, and excreta are eliminated by diffusion. In some protozoans we have seen how contractile vacuoles may be involved in the excretory processes, although their primary function is to maintain the water balance of the organism. In some of the simpler metazoans, such as the radiate phyla, aggregations of cells have been described in which crystalline deposits of nitrogenous substances are stored. ‘These cells may perform an excretory function in isolating complex nitrogenous wastes, but it has been demon- strated that the principal excretory product in all these animals is ammonia, which is highly soluble and easily eliminated by diffusion from small bodies. Most bilateral forms have developed organs of one type or another which are Classically interpreted as excretory. Among these are protonephridia, or 518 STRUCTURE AND FUNCTION IN INVERTEBRATES flame-bulb systems; nephridia (including metanephridia), which generally possess ciliated coelomic funnels; and Malpighian tubules. _Protonephridial systems are commonest among acoelomate and pseudocoelomate Bilateria; they occur in practically all Platyhelminthes, in many Aschelminthes, in Entoprocta, and others; but they are also found in eucoelomate forms, in the larval stages of many mollusks, in annelids, and even among the invertebrate chordates such as Branchiostoma. With such a wide phylogenetic distribution, protonephridial systems exhibit great variability. The chief unifying feature is the presence of flame bulbs, but these may consist of individual cells with single tufts of cilia, groups of cells with numerous ciliary “‘flames,” or single or grouped cells each bearing a single, long flagellum beating in a tube; this last type is generally termed a solenocyte (Fig. 17.6). A primary excretory function has never been conclusively demonstrated for protonephridial sys- tems. ‘The evidence indicates rather that they are involved in maintenance of the water balance of the organism. However, like protozoan contractile vacuoles, it is likely that in eliminating excess water they also flush out wastes in solution. Nephridial or metanephridial systems are characteristic of many types of eucoelomate animals. Morphologically, the systems vary considerably from one phylum to another, but they usually exhibit as a common feature ducts with funnels opening into the coelomic cavity or its derivatives. In pelecypod mollusks, for example, there is a single pair of these organs, and the funnels open into the pericardial coelom. In such annelids as the clamworm and the common earthworm, there is a pair of nephridia with individual ducts and external nephridiopores for each of the segmental coelomic cavities (Fig. 17.5). In chelicerate and crustacean arthropods, where the coelom has been largely replaced by a hemocoel, the excretory organs (coxal glands and green glands, respectively) are interpreted as remnants of a primitive series of paired nephridial organs; the internal end sac of the crustacean green gland is thought to be a vestige of the ancestral coelomic cavity (see Fig. 15.6, p. 433). ‘The assignment of excretory functions to nephridial systems rests on firm experimental evidence, although details of the processes involved are unknown for many types. In mollusks and annelids the excretory organs are associated not only with the coelomic fluid but also with specific blood vessels. The functions of these organs, in those annelids in which they have been especially studied, appear to involve an initial passage of wastes from the coelomic fluid, followed by selective reabsorption of salts into the blood in the specific vessels associated with the tubular parts of the organs. In crustaceans the functions of the green glands and the nature of the urine, whether concentrated or dilute, appear to be correlated with the environment of the animal. In the fresh-water crayfish, for example, with well-developed tubules, the salt content of the urine is less than that of the blood; the tubules appear to extract salts from the urine, returning them to the blood. In the lobster, however, the urine has the same salt concentration as the blood; it is not modified in passing through the tubular parts, which are very 519 GENERAL ZOOLOGY External opening Flagellum Flame cell Nephridiostome Lumen Septum Fig. 17.6. Some excretory mechanisms in in- vertebrates. A, protonephridial system of Pedicellina, an entoproct (pp. 356-357). B, portion of a solenocytic protonephridial unit of the polychaete annelid Phyllodoce, shown in sec- tion. C, nephridiostome of a nephridium in another polychaete, Trypanosyllis; note the ar- rangement of cilia in the open funnel and prox- imal parts of the tubule (cf. nephridium of Lumbnius, Fig. 14.9, p. 408). (A, from C. Cori in W. Kikenthal and T. Krumbach, 1933, Hand- buch der Zoologie; B and C, from C. G. Rogers, Textbook of Comparative Physiology, copyright 1938 by McGraw-Hill Book Co., Inc., reprinted by permission. ) much reduced. In an apparently entirely different environmental situation, but one governed by the same need to conserve water as that facing marine animals, some species of earthworms adapted to dry terrestrial conditions have been found with nephridial systems unlike those of the common forms. In these specialized annelids, the nephridia open not on the surface of the body but into the digestive tract, where active reabsorption of water occurs through the intestinal wall into the blood. Malpighian tubules, developed in most terrestrial arthropods, apparently represent an analogous adaptation for water conservation. Nitrogenous 520 STRUCTURE AND FUNCTION IN INVERTEBRATES wastes are collected from the hemocoelic blood into the tubules, where they are concentrated by reabsorption of water and passed into the hind-gut (see Fig. 15.19, p. 450). Here further amounts of water are extracted, and the residue is finally eliminated from the body as a relatively dry, crystalline mass. Still another mechanism for the removal of wastes from the body fluids is widely distributed among invertebrates, appearing in many forms as the only excretory pathway other than simple diffusion, and in others as an accessory to one of the systems just discussed. ‘This involves the activities of numerous fixed or amoeboid cells, of mesodermal origin, which accumulate wastes from the blood or coelomic fluid and store them as granules or spherules within theircytoplasm. Thisstorage may be temporary or permanent. In echinoderms amoeboid cells of the coelomic fluid accumulate wastes and engulf foreign particles, and the cells containing these accumulations are eliminated from the body by passing through the walls of the dermal branchiae or “water lungs” into the sea water. Cells in other locations appear to. store isolated wastes throughout the life of the echinoderm. In insects the cells of the fat body and other tissues function similarly in the accumulation of wastes; this is particularly notable during the pupal stage, when elimination of nitrogenous excreta from the body is impossible. The activities of all these waste-storing cells, known generally as athrocytes, help prevent wastes from reaching toxic concentrations in the body fluids. From the functional standpoint it is not necessary that the wastes be completely eliminated from the body; they can be as effectively isolated by sequestration in certain cells where they are held in ‘“‘dead storage.” Broadly comparative studies make it clear that the nephridial excretory systems of eucoelomate invertebrates have developed primarily in connection with the coelomic cavities, and it is probable that their original functions involved the elimination of nitrogenous wastes dissolved in the coelomic fluid. In many of these forms a copious blood supply to the nephridia has been established through the vascular system, indicating a possible shift of the pathway of excretion from the coelomic fluid to the cirulating blood. S—> ; RAAAAAUAAAAARAAAAATS CLIX LLLLLL LL Zooid cups CLELLLLL LNT LLL. S909 vex C Fig. 18.2. Phylum Hemichordata. A, general features of Rhabdo- pleura; B, Cephalodiscus dodecalophus, ventral view. CC, reconstruction of what is interpreted as a floating community of colonies of the Ordovician graptolite Diplograptus, viewed from above. (A and B, redrawn from C. Dawydoff in P.-P. Grassé, Ed., Traité de zoologie, 1948, vol. 11, A after Delage and Hérouard, B after McIntosh; C, redrawn, after Ruedemann, from Y. Delage and E. Hérouard, Traité de zoologie concréte, 1901, vol. 2.) 544 THE PHYLA HEMICHORDATA AND CHORDATA from which by successive radiations the ancestors of the modern enterocoelous groups arose. The hemichords would seem to have evolved from this an- cestral line after the divergence of the ancient echinoderms and chaetognaths but before the rise of the true chordates. ‘These evolutionary changes must have antedated the beginning of our fossil record and undoubtedly involved small forms lacking hard parts. The Phylum Chordata THE ACRANIATA The Urochordata. These organisms constitute one of the three sub- phyla into which the phylum Chordata is divided. ‘They are invertebrates, in that they lack vertebrae, but their characteristics are unmistakably those of the chordates. “The subphylum Urochordata includes the classes Ascidiacea, Larvacea, and Thaliacea, all of which are marine animals. Of these, the Ascidiacea, or sea squirts, are the best known; the Larvacea are an unusual type; and the Thaliacea are the most highly specialized. The Ascidiacea. ‘The important features of the ascidians are the specializa- tions of the adult for an attached mode of life and the apparent evolution of the group from free-moving ancestors. On the basis of superficial and func- tional characteristics, ascidians were long classified as mollusks; it was not until their life cycles became known, in 1866, that the chordate nature of the early developmental stages was recognized. ‘This establishment of relationship through the study of developmental stages is a celebrated event in the history of embryology. The sea squirt, Molgula manhattensis, is one of the species of tunicates most abundant along the Atlantic coast of North America. Molgula is found at- tached to various submerged objects in shallow water, but it is most easily collected from the piling under wharves, where it is commonly associated with such other ascidians as Styela and Ciona (Fig. 18.3). An expanded Molgula appears as a globular mass attached at one end and having two tubular processes, the siphons, extending from the other. If an undisturbed specimen is observed, a gentle current of water may be seen entering the longer of these siphons and flowing outward from the shorter one. As in pelecypod mollusks (p. 372), the entire economy of the ascidian depends on these currents of water, which, again as in the mollusk, are maintained by internal ciliary activity. From the stream entering the incurrent siphon, the animal obtains its food by straining out minute organisms and particles of detritus; and the blood is aerated as the water passes through the modified gill slits. The feces and reproductive products are carried outward by the water as it flows through the atrium and the excurrent siphon. In any typical ascidian the entire body is covered by a membrane, thick in Molgula and Styela but thinner in Ciona, termed the test or tunic. This 545 GENERAL ZOOLOGY Fig. 18.3. Subphylum Urochordata: simple ascidians. A, Ciona intestinalis, general external appearance. 8, internal anatomy of a sea squirt, Molgula. (A, photograph by George Lower; B, photograph of a model, courtesy American Museum of Natural History.) is firmly attached to the body only in the region of the siphons and can be removed without disturbing the internal parts of the animal. ‘The tunic functions as a tough, elastic shell, although it contains the cells by which it is secreted, as well as blood spaces through which nutrients reach these cells. ‘The principal constituent of the test is an organic compound which has been named tunicin; this is very similar to cellulose, a compound com- mon in plants but rarely produced by animal tissues. Removal of the test exposes the true outer surface of the body. Most of this surface is the so- called mantle, which encloses an extensive cavity, the atrium. ‘The excurrent siphon is essentially a specialization of the mantle enclosing the median portion of the atrial cavity, from which lateral portions extend on each side beneath the mantle. Since the atrium is formed by the outgrowth of double flaps from the outer surface, it is lined by epidermal cells. “The mantle con- sists of inner and outer epidermal layers, between which lie muscle fibers, connective tissue, and blood sinuses. To understand the processes involved in feeding and in respiration, it is necessary to understand the structural relationships between the digestive tract and the atrium. ‘The opening of the incurrent siphon may be called the mouth, and the cavity within this siphon, the oral cavity. A circlet of tenta- cles marks the beginning of the pharynx, or branchial sac, which is relatively large and specialized for food collection and aeration of the blood. ‘The re- 546 THE PHYLA HEMICHORDATA AND CHORDATA lationships between the siphons, pharynx, esophagus, stomach, intestine, and anus are apparent in Figure 18.3. A digestive gland is also present, connected with the stomach by a duct. Water passes into the mouth, enters the bran- chial sac, and passes through the many small openings in its wall into the atrial cavity on either side, and thence to the excurrent siphon, from which it is discharged. ‘The openings in the branchial sac thus function in the manner of the less numerous openings called gill slits in other chordates. As in hemichords, food is obtained by straining the nutrient material from water received through the mouth and discharged through lateral openings in the pharynx. In the tunicate the food particles are caught in mucus as the water leaves the pharynx and are conveyed by ciliary currents along a specialized path to the esophagus; at the same time the oxygen dissolved in this water diffuses into the blood within the vessels of the pharynx. The circulatory system consists of a tubular heart, lying along the outer curvature of the stomach, with tubules extending from one end directly to the pharynx and mantle, and from the other end to the stomach and nearby organs and thence to the pharynx. ‘There are no true blood vessels, the blood circulating through extensive tubular cavities which lack an endothelial lining. The blood contains several kinds of free cells, some of which contain pigments which function in the transport of oxygen. A unique feature of the circulatory system is the periodic reversal of the heart beat; after the peristaltic waves of contraction have swept across the heart in one direction for a time, they cease, shortly to be resumed in the reverse direction. ‘This brings about a corresponding reversal in the flow of the blood. In the adult ascidian there is no cavity that can be called a coelom, unless the pericardium and the cavities within the excretory and reproductive organs can be so designated. Excretory functions are assumed for a mass of cells, without a duct, lying near the intestine; it has been shown to contain uric acid. The nervous system consists of a single elongated ganglion, embedded in the mantle between the two siphons, and of sensory and motor nerves extending from each end of the ganglion to the siphons and other parts of the body. A glandular mass beneath the ganglion has been compared to the hypophysis of vertebrates. Most tunicates are monoecious. ‘The reproductive system includes an ovary and a testis, lying against the intestine, with ducts opening into the atrial cavity near the anus. In most of the solitary ascidians, such as Molgula, the gametes are discharged into the atrium, and fertilization occurs in this cavity or in the external water, where development takes place. ‘The stages of cleavage and early differentiation are comparable with those of Branchiostoma (see Fig. 5.14). Embryonic development culminates in the appearance of a larva, the so-called tadpole, which possesses a dorsal, tubular nerve cord, a notochord, and gill slits (Fig. 18.4). Later, this larva becomes attached by suckers at its anterior end and undergoes a complicated metamorphosis, dur- ing which its more conspicuous chordate characteristics are lost or modified. 547 Notochord Remnant of notochord Notochord Ww. Anus Nerve cord Stomach @ Heart es Remnant of notochord Endostyle Nerve cord Heart Endostyle Anus Stomach Atrium Ei Fig. 18.4. Successive stages in the metamorphosis of an ascidian, from the attachment of the tadpole larva to the establishment of the adult form. Note particularly the degeneration of the tail and notochord, the extreme reduction of the nervous system, and the change in orienta- tion of the mouth and digestive tract. (Redrawn, after various authors, from Y. Delage and E. Hérouard, Trailé de zoologie concréte, 1898, vol. 8.) 548 THE PHYLA HEMICHORDATA AND CHORDATA Fig. 18.5. Compound ascidians. 4, colonies of Botryllus schlossen; the white stripes radiate from the common, central excurrent opening to the incurrent open- ing of each individual. B, diagrammatic vertical section through a colony, show- ing the relation of the individuals to the entire group. (A, photograph by George Lower; B, after O. Seeliger, from W. Stempell, 1926, 588 oe 3 Sige Bi: ake ge THE PHYLA HEMICHORDATA AND CHORDATA placental mammals, although they are not ancestral to these mammals. Study of mammalian and reptilian embryos shows that the embryonic mem- branes of the mammal so closely resemble those of the reptile and bird that they must have been derived from the reptilian source. ‘There is no yolk in the egg of a placental mammal, but a yolk sac forms during development (p. 162). Fig. 18.38. Representative Cetacea. A, hump-backed whale, Megaptera nodosa; this is a “whalebone whale,” feeding with the aid of plates of baleen suspended from the upper jaw. B, sperm whale, Physeter, one of the toothed whales, pur- suing the squid which form the greater share of its food. The structural adapta- tions to marine life in these mammals may be compared with those of fishes, ancient marine reptiles, and modern marine Carnivora such as seals and sea lions. (A, photograph of a model; B, photograph of a painting by F. L. Jacques; both courtesy American Museum of Natural History.) 589 ok sat te Sim { — a 4 si y : . PR fe oe mm Pe ate a <> peed -o - ‘eo oo se ne as eet oe ot ont ae rt F sigal a er 4 ; : ae sal bag, t pie wate ge REDS aS soe Fig. 18.39. Representative Rodentia. A, beaver, Castor canadensis, and a tree showing the work of the animal’s chisel-like incisor teeth. B, the house mouse, Mus musculus. C, eastern chipmunk, Tamas stratus. (Photographs courtesy New York Zoological Society.) 590 THE PHYLA HEMICHORDATA AND CHORDATA Thus, mammals of small size were present as an insignificant fraction of the population during the millions of years that witnessed the rise and dominance of dinosaurs and other ruling reptiles and the rise of birds. ‘The fossil record is incomplete; but if there had been many larger species of mammals, some would certainly have been preserved as fossils and found before this. Only toward the close of the Mesozoic, which marks the end of the Age of Reptiles, did the diversification of the mammals begin. In the Age of Mammals, when this class of warm-blooded vertebrates became dominant upon the land, they seem to have replaced reptiles by an expansion into territory that was being relinquished. ‘The mammals were the more efficient of the two; they had warm blood, which enabled them to range at will, and greater possibilities of locomotion, to say nothing of wits. What seems to have happened is not that the more efficient type drove out the less efficient but that the reptiles declined for some unknown reason, and so the land again became free for new occupants. ‘The mammals were at hand and became the dominant land forms of the Tertiary, or Age of Mammals, as the early amphibians and reptiles had become dominant upon the older land surfaces. The early mammals were small insect eaters, but diversification into the principal mammalian types was rapid, and with increase in size of the in- dividuals in many lines the great mammalian fauna of the ‘Tertiary came into being. Like the fishes, amphibians, and reptiles, the mammals had their day; they were a waning race even before many recent forms were confronted with ultimate extinction through the activities of Homo sapiens. Representative types of existing mammals are shown in Figures 18.30-18.39. Along with that of other mammals, the human line of descent begins with the insect eaters of the trees at the close of the Age of Reptiles. Surviving offshoots that mark the path of this evolution are the existing lemurs, tarsiers, monkeys, and great apes; this descent is confirmed by what is known from fossils. Only in the late Tertiary or earliest Pleistocene, it seems, did our ancestors descend from the trees; binocular vision and important quali- ties of hand and brain, along with the beginnings of an upright posture, were’ established before man’s forebears came to earth. ‘The later phases of human evolution are outlined in Chapter 20. Speaking of our ancestors in the early Tertiary, W. D. Matthew describes their appearance as intermediate between that of a lemur and a mongoose. ‘They were animals “rather catholic in their tastes, living among and partly in the trees, with a sharp nose, bright eyes and a shrewd little brain behind them, looking out, if you will, from a perch among the branches, upon a world that was to be singularly kind to them and their descendants.” Summary The phylum Chordata includes certain invertebrate animals along with the familiar vertebrates. The species representing the lowly members of the ‘et GENERAL ZOOLOGY phylum are few in number and superficially unlike their numerous vertebrate relatives. Yet they have the gill slits, notochord, and nervous system of the chordate. Since vertebrates appear as fishes in the Ordovician, it is con- cluded that these first vertebrates of the fossil record must have been pre- ceded by fairly complex ancestors in the Cambrian, in which representatives of all the other major phyla are found. ‘This means that a common ancestor for all chordates would be sought in the Age of Invertebrates. No fossils representing such an ancestor are available, and it is unlikely that any will ever be found. ‘The origin of vertebrates from some invertebrate source was the subject of much speculation, based on the data of comparative anatomy and embryology, when such theorizing was the vogue in the last decades of the nineteenth century; nothing that proved convincing was ever made of the matter, and it is unlikely that anything ever will be. The animal life of the Cambrian is remote, but it was preceded by millions of years from which we have virtually no fossils and during which the Cambrian types were evolved. In studying evolutionary history within the subphylum Vertebrata, we have access to the fossil record and also to “surviving fossils,’ such as the egg-laying mammals, whose structure and development can be fully ex- amined. Reconstructing the past from the data available, we conclude that the earliest vertebrates were small, heavily armored, bottom-dwelling, fish- like animals, living in fresh water rather than the salt water from which their remote ancestors presumably came. From such early fishes came others that made a beginning of air-breathing while still in fresh water, and some of these invaded the ocean. Late in this Age of Fishes came the first land vertebrates, the Amphibia, descended from the air-breathing fishes known as lobe fins. In the Age of Amphibians, when the great coal measures were being laid down, these vertebrates were the dominant forms upon the marshy land surface. Reptiles arose from early amphibians and succeeded them in the Age of Reptiles. Both birds and mammals appeared as early offshoots from reptilian lines; the mammals remained small and insignificant animals until the reptiles began to decline. In the Age of Mammals the members of this class were able to range more widely than any of their predecessors, because of their effective locomotion, their warm-bloodedness, and their mode of development within the parent. As the mammals declined toward the end of the ‘Tertiary, the human stock became differentiated from other Primates; the present is sometimes called the Age of Man. O92 CHAPTER spe ECOLOGY: Environmental Relationships The characteristics of an animal, and the range of activities of which it is capable, are largely determined by its genes; these represent the material of inheritance, or heredity. It is clear, however, that although an animal comes into being with a specific hereditary endowment, the expression of its inherent characteristics is conditioned at all levels by intrinsic and extrinsic factors which collectively may be termed its environment. What an animal is, there- fore, depends on the complex interaction of hereditary and environmental factors. ‘The study of heredity, discussed at length in Chapter 6, forms the subject matter of the field of genetics; the environmental relationships of organisms are the concern of the field of ecology, using the term in its broadest sense. At the molecular, cellular, and organismal levels, various aspects of the internal environment of the cells and tissues of the individual are studied by the biochemist, the cytologist, and the physiologist; the ecol- ogist deals with the interrelationship between the organism and its external environment. The external environment consists of every factor in the habitat which in any way, directly or indirectly, affects the organism. Animals are dependent on the external environment in a great many ways. Among other things, the environment furnishes a substrate for support and locomotion, a medium for gaseous exchange, and food materials for the growth, maintenance, and repair of the living organism. A great many environments, tremendously varied, have proved themselves capable of supplying these needs to various groups of animals: oceans, lakes, ponds, and streams; hot and cold springs; arctic, temperate, and tropical land masses; dung heaps, fallen logs, the bodies of plants and other animals; and so on. Each of these environments may be thought of as a composite of what we call physico- chemical factors, such as temperature, oxygen, light, and others; and biotic 594 Temperature Duration of Percentage (°C.) Pupal Stage of Flies (Days) Emerging Fig. 19.1. Effects of different constant temperatures on the pupal stage of the fruit fly, Drosophila melanogaster. ‘The figures are means of values ob- Apparent tained separately for mal - P yuel aigle gag 26 3.9 100 { optimum female flies in the original ex- periments. (Adapted from D. Ludwig and R. M. Cable, 1933, Physiological Zoology, vol. 6.) Apparent 32 3.4 | optimum 78 factors, those for which all the organisms living together in the environment are responsible. Every individual inhabiting a particular environmental situation exerts effects on its surroundings and on other organisms and thus constitutes a factor in their environment. Ecology has been defined as the study of the action of the environment on the organism, and the reaction of the organism on the environment. In seeking to elucidate the complex inter- relationships implicit in such a definition, students of ecology have developed the field into a broad and very active area of investigation. The environmental relationships of organisms have interested observers since the beginnings of biological science; the work of early naturalists and systematists, and much of the field of “natural history” in later years, formed the foundation of the modern science of ecology. It has been said, indeed, that ecology is “‘scientific natural history,” which has developed from the older observational science by the application of experimental methods and rigorous quantitative treatment to recognized problems. Modern ecology makes use of the techniques of chemistry, physics, analytical and statistical mathematics, population genetics, and other highly specialized fields, in addition to the more general methods of biology. ‘The importance of ecological studies cannot be overemphasized. Like all scientific work, they increase our knowledge of the world about us. In addition, these studies make it possible to plan intelligently the proper utilization and conservation of natural resources, and to predict the probable consequences of current practices in the many areas in which human activities come into contact, and often interfere, with the environment. It is a common observation, and it is evident from much that has been said in previous chapters, that animals are well adjusted to the environments in which they occur. It might better be said that animals are found in en- 595 GENERAL ZOOLOGY vironments in which the prevailing conditions make it possible for them to survive and to reproduce. ‘The maintenance of this harmony between organism and environment, as well as the clearly adaptive nature of the differences between related organisms living in different environments, indicates that adaptation to environmental conditions must have played a major role in the evolution of animals. ‘The evaluation of this role of the environment as a conditioning or guiding factor in evolution has been of great significance in the development of theories of evolution, to be discussed in Chapter 20. In the present chapter we shall consider some of the general principles that have emerged from studies of the environmental relationships of existing animals—principles illustrating the significance of environmental factors in determining the survival and distribution of animals. The Physicochemical Environment The physicochemical environment is a composite of a great many physical and chemical factors, any one of which may be of primary importance in determining the suitability or unsuitability of a particular environment for a specific type of organism. Although any organism is always exposed to many of these factors simultaneously, it is instructive, and indeed often necessary, to isolate the effects of a few of them individually in order to understand how they limit the activities and distribution of living things. Temperature. It is probable that temperature affects animals more conspicuously, and in more different ways, than any other environmental factor. ‘Temperatures vary widely, in different geographical locations, at different depths or altitudes, and even in the same localities at different times of the day or seasons of the year. Changes in temperature exert marked effects on the metabolic rates and activities of animals, as well as on their processes of growth and reproduction. The metabolic processes of animals are fundamentally chemical reactions, and the rates at which these reactions proceed are determined by the tempera- ture of the living system. ‘The majority of animals are dependent for the maintenance of their body temperatures on heat from the external environ- ment (i.e., are ectothermous or “‘cold-blooded”’); therefore, the rates of their metabolic reactions are determined by the temperature of the environ- ment. With lowered temperature there is a progressive decrease in metabolic rate, to a point at which dormancy ensues and metabolism is barely detect- able. At still lower temperatures, below 0°C., the fluids of the body eventually freeze and the animal usually dies. There is great variability between different species of ectothermous animals in the degree of resistance to low temperatures. Some insects, for example, living usually at more normal temperatures, can remain in a cold-induced dormancy for long periods and can even withstand freezing. Others die after a few hours or days of exposure to intense cold. On the other hand, there are many species 596 ENVIRONMENTAL RELATIONSHIPS of cold-blooded animals which become active and thrive only at temperatures much too low to sustain metabolism in other forms. With rising temperatures, ectothermous animals become more active; their metabolic rates increase, and they “‘live faster.” At a certain point, how- ever, again varying between different species, continued increase in tempera- ture begins to affect the animal adversely. Eventually, a heat-induced dormancy supervenes, followed very soon by the death of the animal. ‘The maximum temperatures that even the most heat-resistant species can with- stand appear to lie between 48 and 52°C.; most animals are killed by considerably lower temperatures. ‘The primary effect of heat is a derange- ment of the delicately balanced physical state of cellular constituents, prob- ably through minute changes in proteins and lipids. For every species of ectothermous animal, there is thus a specific range of temperature within which the organism remains active and capable of carry- ing on its vital functions. Within this range, between the maximum and minimum tolerable temperatures, there is a narrower zone at which the animal operates with greatest efficiency; this is its optimum temperature. Some difficulty is often experienced in precisely defining the optimum temperature; it may vary between stages in the life cycle, and, as indicated in Figure 19.1, it may differ slightly depending on the criteria used. From the standpoint of the most rapid development, the optimum temperature for the pupal stage of Drosophila ranges from 27.5 to 33°C.; however, this temperature range is evidently slightly above the optimum judged by the number of flies successfully completing the pupal stage. From this latter standpoint, the optimum temperature range extends from 22.5 to 27.5°C., but here development proceeds somewhat more slowly. The adverse effects of temperatures even slightly above the optimum, however defined, are evident from the data presented. For a given species the temperature range may be extensive, or it may be restricted. Again, the range may cover only a few degrees near zero; it may lie, for example, between 20 and 30°C.; or it may be much nearer, or even beyond, the temperature at which most other animals are killed by heat. For any species the temperature range and the optimum temperature can be determined only by observation and experimentation. We may draw the general conclusion, however, that animals with restricted temperature ranges will be found in nature in the relatively few environments where their temperature tolerances are never exceeded; but species with broader temperature ranges are likely to be much more widely distributed. Thus, temperature acts as a very significant factor in determining the survival and distribution of ectothermous animals. By various special activities and characteristics, many kinds of ectothermous animals are able to maintain fairly constant body temperatures through a wide range of external temperature conditions. Notable among these are such insects as wasps and honeybees and such reptiles as lizards. This primitive temperature regulation is of limited significance, however, and 597 GENERAL ZOOLOGY can usually be maintained only for short periods of time. ‘The capacity to regulate body temperature by the use of heat derived from metabolic activities, more or less independently of temperature in the external environment, is developed to its highest point in the vertebrate classes Aves and Mammalia, which are said to be endothermous or ‘“‘warm- blooded.” In these forms the body temperature is maintained at a gen- erally favorable level for the species, despite changes in the environmental temperature. ‘The original development of endothermy by the ancestors of modern birds and mammals was undoubtedly correlated with the perfection of hair and feathers as insulating materials, but more deep-seated physiologi- cal mechanisms are also involved. Mammals, for example, rapidly adapt to changing temperatures by complex compensatory reactions involving the breathing mechanism, heart and circulation, skin, and endocrine control of metabolism, integrated by the central nervous system (see p. 125). Long- term anatomical and physiological adaptations are common in mammals habitually exposed to low or to high temperatures. Mammals of the arctic regions have thick fur and heavy, continuous blankets of subcutaneous adipose tissue, and heat loss through radiation is often minimized by reduction of such appendages as external ears. In contrast, mammals of the warmer temperate zones, and particularly of the tropics, generally have less dense fur, lack continuous subcutaneous fat deposits, and may have large, fan-like ears which function effectively to enhance cooling through radiation (Fig. 19.2). In any environment marked by unfavorable extremes of temperature, endo- thermous animals frequently display adaptive behavior patterns, involving hibernation, estivation, migration, retreat to shelter, and so on, which enable them to avoid excessive exposure. The capacity for endothermous temperature regulation makes possible, for birds and mammals, existence under a wider range of external temperature conditions than is true of any other vertebrate group. Although mammalian and avian species are structurally and functionally adapted to life between customary maxima and minima of temperature, they are not so seriously or so rapidly affected as most amphibians or reptiles would be by external temper- ature changes. ‘The compensatory reactions of endothermous forms are not without limits and are not perfect; therefore, even mammals and birds are to a considerable extent dependent on favorable environmental temperatures. In man, for example, the temperature of the extremities is often several degrees below the deep body temperature; it is well known that humans may freeze to death, or under other circumstances may suffer heat exhaustion or sunstroke. The indirect effects of temperature acting through other physical factors may be as significant to animals as the more direct effects on metabolism and other vital functions. Notable in this connection is the fact that the solu- bility of oxygen in water decreases markedly with increasing temperature. The exclusion of an aquatic species from waters above a certain temper- ature might actually be an effect of oxygen deficiency, rather than a direct 598 ENVIRONMENTAL RELATIONSHIPS temperature effect. Similarly, the ability of the atmosphere to hold water vapor in suspension changes as the temperature changes. In general, a warm atmosphere is less easily saturated with water vapor than a cold one; that is, at a given relative humidity, expressed as a percentage of complete saturation, a warm atmosphere holds more moisture than a colder one. ‘Thus, an ap- parent direct effect of temperature on the survival of a terrestrial animal might actually reflect the action of temperature in altering the atmospheric humidity. It is clear, therefore, that experiments to test the effects of temper- ature on organisms must be planned, and the results interpreted, with atten- tion to the indirect effects of temperature acting through other environmental factors. Oxygen. The availability of oxygen is of obvious significance in de- termining the survival and distribution of animals. In an overwhelming majority of species cellular metabolism and energy release depend on a series of reactions, the ultimate step of which involves the combination of carbon with oxygen to form carbon dioxide. In the absence of adequate amounts of free oxygen, the aerobic phase of cellular metabolism is impossible. The external environment is the source of oxygen for the animal, and we have previously noted (pp. 516-517) the variety of structural and functional adaptations perfected in animals for the abstraction of oxygen from the environment, in the process of gas exchange. Only a relatively few species of animals, notably saprozoic free-living protozoans and a number of intestinal parasites among the Metazoa, can live by anaerobic metabolism in the absence of free oxygen. Metabolism in these forms is a process com- parable with the anaerobic phase of metabolism in aerobic organisms. Many species appear to be “‘obligate anaerobes” which cannot survive in the presence of free oxygen; others utilize such small amounts of oxygen as may be present but are not dependent on it. Some natural waters, such as the bottom layers of deep lakes, and waters rich in decaying organic matter, contain no free oxygen and are thus suitable only for anaerobic organisms. In other aquatic situations the percentage of Fig. 19.2. Comparison of the heads of foxes from different climatic regions. A, arctic fox, Canis lagopus; PULL es B, red fox, Canis vulpes, of { { 4 temperate regions; C, desert N 4 fox, Canis zerda. (Redrawn K bea 2 4 from R. Hesse, W. C. Allee, SS a a Z, and K. P. Schmidt, Ecological POS Animal Geography, second edi- A i B : Cc tion, copyright 1951 by John Wiley and Sons, Inc., printed by permission.) 599 GENERAL ZOOLOGY dissolved oxygen varies, being affected by temperature, degree of agitation, presence of photosynthetic plants, salt content, and other factors. As with temperature, different species of aquatic animals are commonly adapted to specific ranges of oxygen concentration, between tolerable maxima and minima, and are limited to environments where these favorable conditions obtain. Under normal circumstances, disregarding anaerobic forms, a species is more likely to be limited in its distribution by lack of oxygen than by an excess of oxygen. Air-breathing animals, whether terrestrial or aquatic, are rarely subjected to significant variations in the amount of free oxygen for breathing. ‘The oxygen content of atmospheric air is remarkably constant over the surface of the earth at all altitudes normally supporting animal life. ‘There may be slight temporary or local variations—for example, near active volcanoes or in extensive industrial areas—but air normally contains about 21 per cent oxygen. ‘This percentage does not change appreciably with increasing alti- tude, yet it is common knowledge that man, for example, cannot survive for any length of time under normal conditions at altitudes much above 20,000 feet. ‘The explanation of this seeming paradox demonstrates the effect of barometric pressure on the availability of atmospheric oxygen. ‘The respira- tory exchange mechanisms of man and other mammals are physiologically adjusted to extract oxygen from air at barometric pressures characteristic of sea level, about 760 mm of mercury. With increasing altitude, barometric pressure steadily decreases, with a consequent decrease in the density of the air. At 18,000 feet, for example, barometric pressure is approximately half the sea-level value; here a given volume of air contains only half as many molecules as the same volume at sea level. Although 21 per cent of these are oxygen molecules, as at sea level, it is obvious that the absolute amount of oxygen has been reduced by one-half. Under such conditions, and increas- ingly at higher altitudes, the metabolic requirements for oxygen exceed the available supply; exhaustion, and in extreme cases unconsciousness and death, results. In high-altitude mountaineering, and in some military aircraft, auxiliary supplies of oxygen are commonly used. In commercial aircraft designed for operation at high altitudes, the air in the cabin is artificially maintained at approximately sea-level pressure. It has been demonstrated experimentally that many terrestrial inverte- brates, such as insects, can survive for long periods under conditions of reduced barometric pressure equivalent to altitudes of as much as 15 miles above sea level. It is questionable whether in these experiments the upper limits of survival are imposed by oxygen lack, by desiccation, or by some more direct effect of reduced pressure itself. There are clear indications, however, that the absence of insects from high mountain ranges is not dictated by oxygen deficiency; the experiments demonstrate that insects are tolerant of more extreme oxygen deficiencies than any found naturally on the surface of the earth. The temperature conditions at high altitudes are probably the limiting factor in this case; temperature decreases with increas- 600 ENVIRONMENTAL RELATIONSHIPS ing altitude at an average rate of 6°C. for every 3280 feet, to a minimum of —55°C. near 36,000 feet. Water and Salts. Water, containing many salts and other substances in solution, is the most abundant compound in the bodies of organisms. ‘The ability of an animal to maintain a favorable water balance between its internal and external environments is of great significance in determining its survival. ‘The interchange of water between the internal and external media involves its passage through living membranes, and such movements of water obey the physical laws of diffusion and osmosis. ‘Therefore, in aquatic organisms, the maintenance of a proper water balance depends to a great extent on the relative concentrations of materials in solution in the body fluids and in the surrounding water; and the question of water as an environ- mental factor cannot be considered apart from that of the materials which the water holds in solution. We may think of a concentrated aqueous solution of salts as containing relatively less water than a more dilute solu- tion. Hence, the ocean is a relatively drier environment than fresh water, and marine and fresh-water animals face very different problems in their water relations with the environment. Sea water is of rather constant com- position, its salinity averaging about 35 parts per 1000. ‘The body fluids of many marine animals, notably vertebrates, are much more watery than sea water, and adaptation to survival in a marine environment must involve perfection of mechanisms to imbibe water and eliminate salts, and to counter- act the tendency of the body to lose water to the environment. Fresh waters vary widely in salinity; they are never free of salt, but in general animals inhabiting fresh water must be adjusted to a range of salt concentration far lower than that of their body fluids. The tendency here, which must be constantly counteracted, is for water to enter the body and dilute its fluids. In addition to their effects on osmotic relations, the salts in the external medium are the source of specific ions necessary for the survival of aquatic organisms. ‘The vital functions of organisms require that certain substances be available in minimum concentrations, to maintain the chemical constitu- tion of the body fluids. Aside from its unfavorable osmotic effects, distilled water is unsuitable as a medium for aquatic life because it lacks the necessary materials in solution. Specific examples of individual requirements are fur- nished by crustaceans and mollusks, which are excluded from waters deficient in calcium carbonate; without this material the organisms are unable to secrete and maintain their exoskeletons. Aquatic organisms are commonly adjusted to life within a specific range of salt concentration, and thus to particular ranges of water conditions. Adaptations of common occurrence involve modifications of the body wall to minimize general gain or loss of water, and special activities of the gills, excretory organs, and intestinal epithelium to conserve or eliminate water and salts. In simpler animals contractile vacuoles and_ protonephridial systems come into play. Usually the limitations of these adaptive mecha- nisms restrict the animal to either fresh or salt waters, but there are, of 601 GENERAL ZOOLOGY course, wide variations in the tolerances of different species to changes in the medium. ‘These differences are most striking when we compare the adaptability of animals inhabiting tidal waters, such as the estuaries of rivers, with that of related marine and fresh-water forms in localities where they are not subjected to periodic tidal fluctuations in salinity. ‘The broadly distributed estuarine species are tolerant of large variations in salt concen- tration. ‘The essentially fresh-water forms at the upper end of the estuary are very often excluded from more brackish waters by increases in salinity of as little as one or two parts per thousand. At the other extreme, truly marine species are excluded by their inability to regulate their water balance in the face of periodic dilution of estuarine waters by fresh water from upstream. The most striking and best-known examples of unusual adaptability are furnished by certain migratory forms, such as salmon and eels. For example, young salmon hatch in the headwaters of certain rivers, where the salt concen- tration is very low. As juveniles, they migrate to the ocean and then pass several years as marine fish. At maturity, they again enter the rivers of their origin and make their way to the headwaters for breeding. Eels exhibit essentially the reverse pattern, hatching in the sea, spending their maturing years in rivers, and returning to the ocean for breeding. ‘The exceptional physiological adjustments involved in maintaining the water balance in the face of such extreme variations in external conditions must be far-reaching indeed. For terrestrial organisms, the availability of water is of the utmost signifi- cance as an environmental factor. By definition, terrestrial animals are more or less independent of water as an environmental medium; nevertheless, the maintenance of a favorable water balance is no less important for these than for aquatic forms. There are great variations in the tolerance of different species to differences in the availability of water. At one extreme are such forms as the isopod crustaceans, earthworms, onychophores, and amphibians, which depend, at least in part, on a moist skin for respiration. ‘These may be considered as only imperfectly adapted to terrestrial life; they require very moist environments, and as we have seen, most amphibia and some insects depend on water as an environmental medium for their developmental stages. At the other extreme are found the many species of insects, mammals, and reptiles, in particular, that are adapted to life under arid desert conditions. Between these extremes lie the great majority of terrestrial animals, with widely varying degrees of adaptation to abundance or scarcity of available moisture. The water relations of terrestrial animals involve balancing gain against loss. Water may be obtained by drinking, by eating succulent food, by utilizing “‘water of metabolism” yielded in the metabolic breakdown of even dry foods, or by absorption from a moist environment. Water is lost chiefly by evaporation, through the skin generally or in the organs of respiratory exchange, or by its use as a vehicle for the elimination of egesta and excreta. Successful adaptation to dry conditions is correlated with extreme economy 602 ENVIRONMENTAL RELATIONSHIPS of water, with emphasis on prevention of evaporation, and on reclamation and reutilization of water involved in the process of excretion. Mammals, in which evaporation of water from the skin is an important aspect of temper- ature regulation, must make good this water loss by commensurate intake of water. In terms of the availability of water, as with other environmental factors, each species of terrestrial animal exhibits a specific range of tolerance, beyond which it cannot survive. ‘This is clearly illustrated by experiments with developmental stages of various insects which deposit their eggs on surfaces exposed to air. During the embryonic period within the eggshell, atmos- pheric humidity largely determines gain or loss of water by the insect through the shell and its membranes. Similar series of eggs, maintained at a constant temperature but under different conditions of atmospheric humidity, show very definitely the range of tolerable humidities, and in some cases a narrower zone of optimum humidity, for embryonic development. In some species development and hatching occur only in an atmosphere of 100 per cent relative humidity, saturated with water vapor. In other species, as illustrated by Figure 19.3, such a high humidity may be slightly less favorable than lower ones. ‘The eggs of some species are so well protected against evapora- tion that development proceeds normally even in a perfectly dry atmosphere. In such conditions young larvae often die of desiccation as soon as they have ruptured the eggshell at hatching. Similarly, larvae may be unable to survive in saturated atmospheres after completing their embryonic development. In the experiment illustrated by Figure 19.3, more larvae developed than were able to survive after hatching, at all degrees of humidity, but particularly at [a] Per cent hatching WM Additional per cent developed, failing to hatch Fig. 19.3. Effects of different rela- tive humidities, at a constant tem- perature, on the development and hatching of eggs of a moth, Telea polyphemus. (From D. Ludwig and J. M. Anderson, 1942, Ecology, vol. 23, reprinted by permission.) Per cent Per cent relative humidity 603 GENERAL ZOOLOGY those above and below the optimum. ‘This illustrates the general principle that the water relations of a species may not be the same at different phases of the life cycle; humidities suitable for embryonic development may be too low or too high for larval survival. Projecting these experimental results into conditions in nature, we may conclude that a species of insect will occur only in environments where the humidity is within the limits of the range permitting development and survival of all stages, including the most sus- ceptible, in its life cycle. It should be borne in mind that, as pointed out earlier, there is a direct relationship between atmospheric humidity and temperature. Changes in temperature affect the drying power of air, as markedly as they determine the solubility of oxygen in water. In comparing the results of humidity experi- ments at different temperatures, it is often difficult to separate the direct effects of temperature from those of temperature acting through humidity. General Considerations. We might analyze the limiting effects of a great many additional physical factors, such as acidity or alkalinity of the medium, light and other radiations, movements or currents in the medium, and soon. But the examples just discussed should suffice to demonstrate the general effectiveness of physical environmental factors. We may then proceed to draw several broad conclusions, with the understanding that they are supported by a great body of ecological data. m For each species of animal, every physical factor in the environment imposes its own specific and peculiar limits to survival and distribution, and these limits may differ for different stages in the life cycle of the individual animal. m A particular species can exist only in environments in which none of its tolerable limits of survival is exceeded, including those of its most susceptible stages. em For any specific environmental factor, the limits of a given species may be close together (narrow range of tolerance), or they may be widely separated (broad tolerance). em It follows that a species will be most stringently restricted in its distribu- tion and survival by the single factor for which it has the narrowest range of tolerance; this is the factor of most significance in determining the existence of the species in any potential habitat. pm A species with no very narrow tolerances will be found almost universally distributed in the general type of habitat to which it is adapted, but even a related species with one or a few specific narrow tolerances will be less broadly distributed. Animals are seldom found in habitats where one or more physical factors approach the limits for survival of the species. Exceptional are animals, incapable of migration, making a last stand for survival in an environment undergoing relatively rapid changes in a direction unfavorable to the species. More commonly, animals occur under conditions which approach the opti- 604 ENVIRONMENTAL RELATIONSHIPS mum. Here they function with greatest efficiency, reproducing and develop- ing most rapidly, and expending the least energy in counteracting the effects of unfavorable environmental tendencies. In relation to physicochemical factors, the evident harmony between the animal and its environment results from avoidance by the animal of markedly unfavorable conditions, acclimatiza- tion of the individual to prevailing conditions within its limits of tolerance, and gradual adaptation of the species, by selection, through changes in its tolerances. The Biotic Environment Every environmental situation capable of supporting life, whether it be a sand dune, a fresh-water pond, a marine tide pool, or the intestine of a frog, contains a characteristic population of different kinds of organisms. All these organisms are adapted to the prevailing physical conditions of the habitat, and collectively they form what is termed a community. ‘The fundamental character of the community is determined by the nature of the habitat, that is, by its physical features. Superimposed upon these are the biotic factors, which bind the members of the community together in a complex fabric of action and reaction. In a typical community careful investigation of the animals alone may reveal thousands of individuals, of dozens of species, representing several different phyla. Considering the plant species in addition (and even in animal ecology, the plants may not be ignored), the immense difficulty of establishing clearly the interrelationships within such a com- munity is apparent. ‘The complexity of typical communities, and the num- bers of organisms involved, have indeed been obstacles to rapid progress in this aspect of ecology. Yet from the many excellent studies which have been made there emerge several principles, apparently of universal applicability to problems of the interrelationships between organisms within communities. We shall discuss some of these principles in the paragraphs that follow. Food Relations. Only plants of various kinds, and the relatively few species of photosynthetic green protozoans, are capable of utilizing directly the radiant energy of sunlight. All other organisms depend on foods for their energy; that is, on the energy-rich organic compounds contained in the bodies of other organisms. Foods constitute the most important single requirement of animals, and it is not surprising to find that the basic relationship between organisms in a community involves their food requirements and food supplies. As indicated in Figure 19.4, the members of the community are intercon- nected by definite food chains, each animal feeding upon the kind next below it in the chain and itself serving as food for the type next above it. Thus, the fundamental relationships between animals are those of predator and prey. A particular kind of animal may have a place in more than one food chain, and hence the chains intermesh to involve the entire population of a community in what are sometimes termed food webs. 605 GENERAL ZOOLOGY Mussels ae Snails Bullheads Small aquatic insects bacteria, ———— decaying Crayfishes == ——\_ vegetation Large aquatic | \ insects Black bass NJ adults Small oe oa| Pickerel Black bass ee eee Fig. 19.4. Schematic diagram of predator-prey relationships in a pond community. The arrows in each case lead from prey or food to predator or feeder; they may be interpreted as meaning “eaten by.” (Adapted from V. E. Shelford, Animal Communities in Temperate America, copyright 1913 by Chicago Geographical Society, printed by permission.) In analyzing one of the food chains in the pond community represented in Figure 19.4, we find that the microscopic green plants called algae serve as food for protozoans; these, and algae, are eaten by small aquatic insects, which in turn support a population of larger aquatic insects. The large insects are eaten by fish called bullheads, many of which are preyed upon by black bass. Adult black bass, if large enough, may be the terminal link in this chain, but smaller bass are important items in the diet of the larger pickerel. At almost any stage in this sequence, it is possible to follow alter- nate pathways, all of which, however, lead eventually to the pickerel. Implicit in these intricate interrelationships are several interesting gener- alities. In the first place, plants form the broad base of the food relations of the community, because they alone are competent to entrap solar energy in the synthesis of organic compounds; plants, then, serve as the initial repositories of stored energy. In every community the plants support a population of herbivorous animals which transform plant materials into animal flesh. In our example, the protozoans, the small crustaceans, mussels, snails, and small insects represent the herbivorous links in the several food chains. These, in turn, serve as food for various carnivorous animals, which thus 606 ENVIRONMENTAL RELATIONSHIPS obtain stored solar energy very indirectly. In any food chain, there may be two or three, but seldom more, successive carnivorous links, until at last the chain terminates in a large carnivore which does not serve as prey for any other member of the community. ‘These largest carnivores, while living, support only their indigenous populations of ecto- and endoparasites, although they may be preyed upon by transient carnivores, such as birds or bears, from other communities. If the pickerel die in the pond, their bodies are decayed by microorganisms, and their substance re-enters the food cycle as inorganic compounds required for the growth of aquatic plants. Let us now consider size of food as it relates to the food chain. Note that in our example the successive links are: algae—protozoans—small insects— large insects—small fishes—large fishes. ‘That is, there is a stepwise increase in the size of food from the beginning to the end of the chain. ‘The large fishes do not feed on protozoans, or normally even on small insects; it is virtually impossible for them to capture enough of these tiny animals to sat- isfy their food requirements. On the other hand, neither do the small insects prey on the large fishes, which are beyond their ability to capture or ingest. Thus, every carnivore is restricted by its own limitations of size and strength to kinds of food small enough for it to manage; but there are also lower limits to the size of food which it can profitably handle. In short, the size of an animal largely determines its place in the complex predator-prey sequence called a food chain. Related to the food chains, and also to the size of food, are certain consider- ations of numbers of animals. Every food chain represents what has been termed a “pyramid of numbers’; at every upward step in the chain, there is an increase in the size of organisms but a corresponding decrease in their numbers. In our pond, very large numbers of herbivores are required for the support of a smaller number of insects; these, in turn, suffice to maintain a few hundreds of small fishes, which furnish only enough food for a few dozen large fishes. In such an environment as that of the pond, limitations of space, sunlight, and available chemicals impose finite restrictions on the amount of plant growth that can occur. ‘These restrictions are transmitted stepwise up- ward through the food chains, with the result that the maximum growth of plants will support, at the third or fourth remove, only a small number of the largest carnivores. But why should this be so? The really significant factors underlying these numerical relationships appear to be considerations of mass and energy. As we have seen, the basis of food relations is the requirement of energy; but at every step in the food chain, about 80 to 90 per cent of the available energy is dissipated. For example, the flesh of the small fishes, upon which the large fishes prey, contains only about 10 to 20 per cent of the energy present in the bodies of the many insects supporting the small fishes. This accounts for the relatively small numbers of large carnivores in the community; in view of the inefficiency of energy transfer along the food chain, there is not enough energy left at the end of the chain to maintain more than a few terminal members. 607 GENERAL ZOOLOGY This also offers an explanation for the fact that there are seldom more than two or three successive carnivorous links in a food chain. The relationships just discussed in terms of the pond community exist, in a generally comparable way, in any natural community. In all, there are plants, herbivores, and varying numbers of carnivorous animals forming inter- locking food chains. Among ecologists, each of these levels of activity is spoken of as a niche; and the niche occupied by a particular species in a com- munity conveys an idea of its relationships with other members of the com- munity. ‘The types of animals occupying the various niches differ, of course, in different environments, but the parts they play are always comparable. For example, the very important niche of the chief herbivore in the com- munity is filled in aquatic habitats by small crustaceans; in woodland and grassland communities, various small rodents, such as mice and _ rabbits, serve this function; in the arctic tundra, the principal herbivore is the lemming. In any community the maintenance of the whole superstructure of the food chains depends directly on the activities of the herbivores, and the activities of these animals are often referred to as the ‘‘key industries” of the various communities. Exploring these relationships a bit farther, we find that in any environ- ment there are usually a certain number of ecologically important niches to be filled. If through some catastrophe all the occupants of a particular niche in a community become extinct, the niche will eventually be occupied by some other type of animal. ‘The new occupant may be an immigrant from another community, whose spread has been favored by the availability of the unoccupied niche. Alternatively, over a longer period of time, the vacant niche may be filled by the evolutionary appearance of a new form of life, adapted to the functions of the particular niche through natural selection from some ancestral type. In the fauna of oceanic islands, and of larger but similarly isolated land masses such as Australia, there are many examples of the formation of entire communities of animals through “‘adaptive radiation” from a generalized common ancestor. For instance, until the introduction of placental mammals into Australia and New Zealand, relatively late in historic time, all the mammalian occupants of various niches there were marsupials, with the minor exception of a few prototheres such as the platypus. ‘The marsupials had evidently descended from a primitive, generalized ancestor of the opossum type, adapted in a bewildering variety of ways as herbivores and carnivores, with habits and requirements suited to the functional demands of the environment and the community (Fig. 19.5). The complex interrelationships of the community are not necessarily in- variable, however. ‘There are many instances, as with ruminant mammals, in which herbivores are relatively large animals, and above these the food chain may consist of only a single carnivorous member. Also, many animals, by the development of special feeding adaptations or techniques, have found it possible to by-pass food chains. ‘The whalebone whales, among the largest animals of the earth, feed almost exclusively upon microscopic crustaceans. 608 ENVIRONMENTAL RELATIONSHIPS ye Marsupial “mouse” care Marsupial “mole” Bandicoot Fig. 19.5. Adaptive radiation among recent marsupial mammals of the Australian region. (From E. H. Colbert, Evolution of the Vertebrates, copyright 1955 by John Wiley and Sons, Inc., reprinted by permission. ) b) Having developed very efficient filters of ‘baleen,’ gesting the tiny herbivorous copepods in sufficient numbers to maintain life. Various carnivores are able, by hunting in packs, to exhaust and pull down large animals which would be beyond the abilities of the predators as in- dividuals. Man, of course, by his inventiveness and ingenuity, obtains access to any desirable food, from the tiny seeds of grain crops to the flesh of the largest animals. Animal Populations, Competition, and the ‘‘Balance of Nature.”’ Theoretical considerations indicate that under ideal conditions the “innate capacity for increase”’ of a species, its reproductive potential, is unlimited. That is, populations are theoretically capable of increasing by geometrical progression from generation to generation. “The numbers of animals which would be produced in even a few generations by such rates of increase are almost beyond belief. Elephants are relatively slow breeders, but a single pair of elephants would have at the end of 750 years nearly 19 million de- scendants, if all the individuals lived 90 years and each female gave birth to 6 young. More astronomical figures can be calculated for more rapidly breeding forms. An insect, the cabbage aphid, can produce 12 generations of offspring between the end of March and the middle of August. Each parthenogenetic female produces an average of 41 young. During a single they are capable of in- 609 GENERAL ZOOLOGY 80 70 or Oo Total population > oO 20 10 Units of time Fig. 19.6. Comparison of curves describing population growth. Homo heidelbergensis. (Europe Pithecanthropus (China) Pithecanthropus (Java) 900,000 B.C. 1,000,000 B.C. Australopithecus 641 GENERAL ZOOLOGY During the interglacial periods the climate was warm; there were extensive forests, and the fauna of Europe included the mastodon, the mammoth, the woolly rhinoceros, and the saber-toothed tiger. Man came to Europe as a puny competitor of these mighty animals but succeeded in developing the cunning and prowess necessary for survival. An interesting series of skeletal remains, and much more numerous collections of artifacts and cultural relics, enable us to reconstruct the physical characteristics and to some extent the lives of these primitive men. A large jawbone, found near Heidelberg, represents the oldest known human type in Europe. It has been assigned to the species Homo heidel- bergensis; 1t may be noted that this is the first of the progenitors of modern man considered sufficiently advanced to be placed, with modern man, in the genus Homo. During subsequent ages, Europe was peopled by a race which either descended from men of the Heidelberg type or arrived in a later wave of migration from elsewhere. ‘These constitute what has been called a generalized Neanderthaloid stock, because they show in some degree the special characteristics of Neanderthal man (Homo neanderthalensis), a species named from remains related to a much later geologic period. The gen- eralized Neanderthaloids ranged widely over Europe; their skeletal and cultural remains have been discovered in Spain, France, Central Europe, the Crimea, Palestine, and other regions. ‘They were a hunting people, and groups of them occupied caves in certain areas for extended periods of time. The skeletons show that these men were powerfully built, with a rather 642 THE EVOLUTION OF ANIMAL LIFE stooping posture and short, muscular arms and legs. ‘The characteristics of the skulls indicate somewhat ape-like facial features, with beetling brow, prognathous mouth, and receding forehead and chin. ‘The paleo- anthropological evidence is still too scanty to permit a clear statement of the antiquity of the generalized Neanderthaloids, or of their relationship to modern man. However, some authorities incline to the belief, at least provisionally, that they date from the Middle Pleistocene in Europe. ‘There appears to be nothing in the characteristics of the generalized Neanderthaloids to argue strongly against including them, with modern man, in the species Homo sapiens. ‘Vhe true or classical Homo neanderthalensis appears, on the other hand, to represent a relatively small and isolated group which originated from this generalized stock just prior to the onset of the fourth Pleis- tocene glaciation, about 125,000 B.c., and became extinct before its close at about 15,000 B.c. The earliest undoubted members of our own species, known as the men of Cro-Magnon, supplanted the earlier Neanderthaloids in Europe in _post- glacial times. They were tall, erect, and evidently fine physical specimens and were probably relatively intelligent. The caves which they occupied for long periods, or which they may have used for ceremonial purposes, show a variety of surprisingly artistic and realistic paintings and carvings of men and animals (Fig. 20.10). From these cave paintings alone it would be possible to draw up lists of the larger animals contemporaneous with Cro- Magnon man in Europe; many of these animals have since become extinct or Fig. 20.10. Specimens of cave art from the cavern of Les Combarelles, France. Discovered in 1901, these surprisingly realistic carvings and paintings date from late in the Old Stone Age, about 20-25,000 B.c. In addition to the horses (of which three kinds can be recognized in the cave), such other forms as cave bears, reindeer, ibexes, and mammoths are shown, all of which are now extinct or no longer occur in this part of Europe. (Redrawn from H. Breuil, 1926, Natural History, vol. 26, printed by permission. ) 643 GENERAL ZOOLOGY no longer occur in Europe. Cro-Magnon man was either displaced or absorbed by the later progenitors of modern Europeans. The history of mankind in America, though much longer than was once supposed, covers a comparatively brief span of time. The earliest human immigrants apparently reached North America from Asia by way of a land connection at Bering Strait. They arrived prior to the last Pleistocene glaciation; there is an abundant record of long human occupation of some of the Aleutian Islands and in the Point Barrow region of Alaska. Various discoveries in different parts of the American continents indicate that men spread southward and eastward from the ancient Bering gateway, and that they were well established in both North and South America by 25,000 B.c. Stone arrow or lance points of a distinctive type, fashioned by a race of hunters named Folsom man, have been found widely scattered in the United States (Fig. 20.11). Many of these points, discovered as far east as New York and Pennsylvania, were probably carried eastward by later peoples; their distribution does not necessarily coincide with the area occupied by Folsom man. However, in undisturbed sites in Colorado and New Mexico, the Folsom points, associated with the bones of extinct mammals, give evidence of the presence of these people at a period ranging from 10,000 to 12,000 years ago. Folsom man was not the earliest human inhabitant of the southwest; deposits in the floor of the Sandia Cave, in New Mexico, have yielded the distinctive stone weapons of a culture dating from _ possibly 20,000 B.c. “These are overlain by barren deposits, and these, in turn, by layers containing Folsom artifacts. The barren layers are believed to represent long periods when the cave was unoccupied, probably during the period of climatic upheaval associated with the retreat of the last continental glacier in this area. Another nomad-hunter culture apparently existed in the southwest at about the same period as that represented by the early Sandia remains, and may have been much older. ‘This is known as the Clovis culture, again recognized by distinctive stone weapons; materials from New Mexico date from 12,000 to 16,000 years ago, while similar Clovis remains from a campsite recently unearthed in North Texas have been declared older than 37,000 years. If this dating is accurate, the Clovis group represents the oldest known North American men. Although many campsites, artifacts, and burned animal bones from early American cultures have been found, there have been no discoveries of actual human skeletal remains of the earliest Americans. It is unlikely, however, that in their physical characteristics these people differed markedly from the later American Indians; Homo sapiens must have been established in North- east Asia much earlier than the time of the earliest Bering land-bridge migra- tions. ‘There is abundant evidence that the American Indians are remote descendants from an Asiatic stock, either through the Clovis, Sandia, and Folsom people or from successive later waves of migration. Although the fossil record of mankind is extremely fragmentary, enough physical and cultural remains are available to make it abundantly clear 644 Fig. 20.11. Stone artifacts from Europe and North America. 4, face and profile views of a hand ax typical of the Acheulian cul- tural epoch of Europe, centering about the second interglacial stage of the Pleistocene; note that the stone is finished on one side only. B, chipped stone implement of the Mousterian culture, spanning the third interglacial stage; such im- plements were used by Neanderthal man. C, “laurel leaf’ point of the Solutrean culture which flourished in parts of Europe during the fourth or Wurm glaciation. ‘The shaded figures with B and C show the stones in cross section, for com- parison of thickness. D, two Fol- som points from North America, similar in method of manufacture to the finely finished Solutrean points from Europe, which were thinned and shaped by flaking. (A, B, and C, after J. Andrée, from L. F. Zotz, 1951, Altsteinzeit- kunde Mitteleuropas; D, photograph courtesy American Museum _ of Natural History.) N } ay 645 GENERAL ZOOLOGY that man has undergone an evolutionary process comparable with those of other forms of life. Theories Concerning Organic Evolution Organic evolution as an historic fact is attested by the evidence outlined in the preceding section. We may now examine the more important theories concerning the factors that have conditoned organic evolution. Most notable among these are Lamarck’s Theory of the Inheritance of Acquired Character- istics and Darwin’s Theory of Natural Selection. We shall also discuss briefly conclusions from recent studies in genetics and ecology. Any compre- hensive theory must take into account both internal and external factors. Among internal factors are the phenomena of heredity, variation, reproduc- tion, and development; external factors include all environmental conditions that affect individuals and populations. Lamarck’s Theory of the Inheritance of Acquired Characteris- tics: Historical. The works of Lamarck (1744-1829), written principally during the first quarter of the nineteenth century, postulated that evolu- tionary changes are conditioned by the inheritance of characters acquired during the life of the individual. Lamarck built this theory upon the earlier works of another Frenchman, Buffon (1707-1788), who had previously stated the concept of evolution as opposed to special creation. Many of Lamarck’s statements, examples of which will be given later, appear fanciful in the light of modern knowledge. His essential claim that characters acquired by an individual during its lifetime are inherited by its offspring, and thus produce evolutionary changes, has never been substantiated. Nevertheless, the pub- lication of Lamarck’s theory served to focus attention on the subject of organic evolution. Explanation of Lamarck’s Theory. In its modern form the Lamarckian theory states that during the life of an individual new characters can be ac- quired by use or disuse of parts, and also by the direct effects of the environ- ment on somatic cells. It is a familiar fact that the use of muscles increases their development; and the old adage “‘practice makes perfect” finds many illustrations in the refinement of neuromuscular coordination through pro- longed repetition of actions. Conversely, disuse leads to deficiency or even complete loss of function, as illustrated by the fanatic of India who holds a limb in one position until it cannot be moved, and by many other examples. Such effects of use and disuse, and many effects of the environment on the individual, are known technically as acquired characters. ‘There can be no doubt that they occur; the Lamarckian theory holds, however, that such effects constitute heritable variations and thus condition evolution. The process supposed to occur in nature may be illustrated by citing some of Lamarck’s own examples, in somewhat modified terms. If swift-footed animals, such as deer, have acquired their fleetness by running from their 646 THE EVOLUTION OF ANIMAL LIFE enemies, it follows that each generation has been forced to exert itself to the utmost, like an athlete always in training for a race, and that the effects of such training in each generation have been passed on to the next. ‘Thus, fleetness has gradually increased up to limits determined by the nature of the organism. Similarly, the fleetness of the pursuing wolves may have been increased generation after generation. Animals living in cold climates, where the environment stimulates a heavier growth of hair or the formation of more fat beneath the skin, are believed to transmit these characters by heredity; their descendants at length reach the state seen in arctic forms. Many other examples of this line of reasoning could be cited, such as the degeneration of the eyes in cave-dwelling animals, the increase in neck length in giraffes, and so on. Lamarck also believed that the animal in some way ‘“‘willed” or determined the course of its evolution. Present Status and Critique. If it could be shown that the effects of use and disuse and the direct effects of environment upon the individual are actually inherited, there would be little criticism of Lamarck’s theory. Many attempts have been made to obtain specific evidence, but none of the alleged examples has held up under subsequent investigation. Experiments involving the destruction of parts, such as the amputation of tails in mice during many generations, and experiments in the functional stimulation of various parts, and in the effects of changed environment, have given negative results. ‘The organism may develop new characters in a new environment, but when it is returned to the original environment, the alterations do not persist. In general, it may be said that experimentation has failed to sup- port the Lamarckian theory; it appears that characters acquired by the individual during its lifetime, in the manner postulated by Lamarck, are not heritable. A theoretical objection may be raised to the entire idea of the inheritance of such acquired characters. A new individual develops not from its parents’ somatic cells but from their germ cells; and germ cells are in most cases set apart at an early stage in development and are little influenced by what hap- pens to the somatic cells in the normal activities of the animal. The Lamarckian scheme would require that a change in somatic cells of a part of the body be transmitted to the germ cells, in such a way as to affect whatever it is in the germinal material that conditions the development of this specific part. ‘To use a homely if old-fashioned illustration, a blacksmith’s son inherits his arms not from his father’s arms but through his father’s and mother’s germ cells; and it is the germ cells that must be changed before any modification can be inherited. ‘The facts of genetics, embryology, and physiology give virtually no theoretical support to the Lamarckian doctrines and thus con- firm the negative results of experimentation. Darwin’s Theory of Natural Selection: Historical. The teachings of Lamarck regarding evolution attained considerable popularity during the early nineteenth century but were apparently overthrown by Cuvier (1769-1832), the greatest zoologist of his day, who opposed the concept 647 GENERAL ZOOLOGY of evolution. In 1830, a year after Lamarck’s death, a debate was held in the French Academy in which Saint-Hilaire (1772-1844) upheld the Lamarckian doctrines against Cuvier. Despite his opposition to the idea of evolution, Cuvier had been forced to admit the differences between animals of the past and those of the present, differences which could not exist if animals had been originally created in their present form and had _ not changed. ‘Therefore, Cuvier had espoused the Doctrine of Cataclysms, which assumed not one but a series of creations, each followed by a cataclysm that destroyed all life. By supposing that each successive creation was on a higher level than the preceding, it was possible to explain the succession of types appearing in the fossil record. But the work of the geologists, culminat- ing in Lyell’s Principles of Geology (1830), showed that there was no evidence for cataclysms. ‘The period between 1830 and 1859, during which Darwin was engaged in the studies summarized in his book, The Orgin of Species, was one of relative quiescence for the evolutionary theory. ‘There was much popular interest in the subject, however, as shown by the large sales of Robert Chambers’ book, The Vestiges of Creation (1844). Cuvier won his debate with Saint-Hilaire, but in 1830 the case was already settled in favor of evolution, as subsequent developments showed. Charles Darwin (1809-1882) began his studies 20 years before the publica- tion of his famous volume. ‘The fact that he was interested in determining whether species originated by creation or transmutation (that is, evolution) shows that the question was then under discussion. ‘The idea seemed new in 1859 only because the evidence was so ably presented by Darwin and so rapidly accepted by scientists and others. From our present point of view, it is difficult to understand why biologists failed to recognize at an earlier date the evidence for organic evolution—evidence that had been steadily accumulating since the work of Buffon (1749), and that was sufficient to justify acceptance of the concept 20 years before 1859. Nevertheless, Darwin deserves his fame because it was he who brought about the acceptance of the evolutionary doctrine. His Origin of Species was a masterful summary and extension of the evidence for organic evolution as an historic fact. Its publication marked the beginning of a new epoch in human thought, as well as in biological science. Explanation of Darwin’s Theory. In addition to bringing together and ex- tending the evidence for the reality of organic evolution, Darwin proposed as a major factor in the origin of species, and hence in evolution, what he called natural selection. The principles of natural selection were inde- pendently recognized by Alfred Russel Wallace in 1858. As conceived by Wallace and Darwin, these principles may be summarized as follows. Organisms possess an innate capacity for unlimited increase in numbers, but under normal conditions populations remain approximately stationary. The limitations on increase reflect a struggle for existence on the part of the organism. In every population, random heritable variations occur in the characteristics of organisms; these are not imposed or evoked by any action 648 THE EVOLUTION OF ANIMAL LIFE of the environment or of the organism but appear spontaneously and in all directions. By chance, some of the variations will be advantageous in the struggle for existence, and others will be unrelated to survival or will be detrimental. Natural selection involves the action of the environment in selecting for survival those forms which, by chance, are best adapted to environmental conditions, and in eliminating those less well adapted. ‘This results in the survival of the fittest, in terms of any specific environment. If the characteristics of the environment change, a new process of selection begins and a new group of organisms is selected, with modifications in adapta- tion to the changed environment. In The Ongin of Species, Darwin cites example after example of observations supporting these principles. In his accounts of the environmental relation- ships of organisms, Darwin anticipates many of the important generalizations that have arisen from modern ecological studies. Much of our discussion in Chapter 19 states in specialized terminology the principles and conclusions that Darwin adduced in support of his theory of evolution. Consider, for ex- ample, Darwin’s concept of the “‘struggle for existence.” Since the capacity for reproduction is restricted by checks upon increase, relatively few of the individuals that begin life in any generation will reach maturity. Each in- dividual, therefore, must engage in a fight for survival. As Darwin conceived it, this struggle for existence is seldom an actual conflict, although this may be involved when animals fight with one another for mates or for food. He thought that the struggle would be most acute between individuals of the same species, since these compete for the same conditions of life; or between different species using the same food, as when insects devour the food of graz- ing mammals. It is important to bear in mind that Darwin used the term struggle in a metaphorical sense. In the vast majority of cases there is nothing that can be called a struggle in the sense of actual conflict. Metaphorically, however, it can be said that the trees of a forest, competing for soil nutrients and for light, “struggle” to exist or ‘‘fight” for life. Darwin concluded that such a struggle, in one or more of its aspects, is ever-recurring for all organisms, although it is intermittent and may not act for considerable periods in the life of any individual. The elements of Darwin’s struggle for existence are implicit in the broad modern concept of competition (pp. 609— 614). The modern views of variation and heredity have been presented in Chapter 6. Heredity has been defined as the tendency of individuals to resemble their ancestors and relatives, and variation as the tendency of individuals related by descent to differ in various ways. ‘The two are intimately connected as different expressions of the reproductive and develop- mental processes. Darwin observed that the members of species varied, and he believed that many of these variations, small though they might be in many instances, were inherited. He was interested in heredity and variation as such and studied them intensively; but so far as they concerned natural selection, it was not necessary to explain them. His argument was: given 649 GENERAL ZOOLOGY heritable variations and the reproductive capacities of organisms, a struggle for existence and natural selection inevitably follow. Among inherited variations of many sorts, some will be of value to the in- dividual in its struggle for existence; that is, some will have survival value. According to Darwin, if the members of a species of plant varied in their ability to resist frost, those that were sufficiently resistant would survive temperatures that would be fatal to the great majority. Inheritance of the variation by the next generation would follow, and such a process of selection, repeated through many generations, would produce a population better fitted to meet this particular condition of existence. ‘Thus, evolution might occur by modification of this feature of the organism in a manner to suit a changing environment or to enable the species to extend its range northward. In a similar manner, heritable differences in resistance to a disease would produce a more resistant race; if wits were more important than strength, selection would develop a more cunning type. If concealment were of survival value, coloration and other features that tend to make the individual resemble its surroundings would be at a premium and therefore selected. Darwin called the process by which useful variations were sorted out natural selection, be- cause it resembles the artificial selection practiced by breeders of animals and plants in picking individuals that please the fancies or necessities of man. Herbert Spencer called the process survival of the fittest, because the individuals best fitted to the conditions of existence were the survivors in the struggle for existence. In terms of genetics, the rate and extent of any evolu- tion thus directed by selection depend on the occurrence of heritable vari- ations that can be acted on by selection. Minor fluctuations in the expression of a characteristic are of no importance, since they are not inherited. In our ecological discussions we considered, for the most part, short-term environmental changes and their effects on the populations of particular habitats. Yet great changes also occur over very long periods of time, as when continents are made and unmade by geologic evolution, or when pro- found climatic changes occur, such as the advent of an Ice Age or the change from forest to desert conditions. ‘These are less important in the day-to-day activities of animals in a community than environmental changes which may seem insignificant in comparison. ‘The introduction or destruction of a plant upon which various animals feed may produce far-reaching changes in the environmental conditions of a given species. New enemies entering a district may bring new standards of selection; new parasites or pathogenic micro- organisms may put a premium on qualities that have not hitherto been selected. In the interplay of forces it is possible that conditions, and therefore selection, may remain stable for long periods, or that selection may suddenly take new directions. Changes of many sorts are conceivable within the limits of the selection pressure and the heritable characters avail- able for selection. In terms of natural selection, the environment may be compared to a sieve that selects individuals presented to it but does not determine their nature. As long as the sieve remains unchanged, it allows 650 THE EVOLUTION OF ANIMAL LIFE the same kind of individuals to pass its meshes—to survive. But the sieve may change and may then select new kinds of individuals for survival. Evolution now in one direction, now in another, is therefore perfectly possible. It should be reiterated that the theory of natural selection, as set forth by Darwin, does not attempt to explain the nature of variation and heredity. Selection is viewed as a directive rather than a creative factor in evolution. Also, selection cannot be thought to control or direct the evolution of non- adaptive or non-useful characters, unless these are linked in heredity with characters that are adaptive. Present-Day Concepts of the Mechanism of Organic Evolution The major outlines of Darwin’s theory of natural selection stand as a recognized major contribution in the history of evolutionary thought. The soundness of Darwin’s conclusions, resting on innumerable observations patiently fitted together into a logical pattern of interpretation, is the more impressive when it is realized that they were drawn in complete ignorance of the basic mechanisms of heredity, and without the significant data yielded by modern experimental work in the environmental relations of animals and plants. In the years between 1859 and 1900, no biological topic was more widely discussed than that of natural selection; the idea of the struggle for existence was particularly questioned. Although numerous biologists busied themselves collecting information that might be brought to bear on the evolu- tion question, there appear to have been virtually no persistent investigations by experimental methods to establish the validity of Darwin’s ideas, and his critics did not present any strong evidence against the theory. With the gradual rise of the science of genetics after 1900 and the later application of its principles to studies of population phenomena, and with the develop- ment of ecology as an experimental science that has demonstrated the com- petitive basis and the reality of the struggle for existence, Darwin’s ideas have taken their place as the keystone of present-day evolutionary theory. A major contribution from the field of genetics has been the understanding that mutations constitute the “random heritable variations’ postulated by Darwin, and that the establishment of new sets of inherited characteristics in a population is the basis of evolutionary change. Although the production of mutations is understood in a general way, and the general selective action of the environment can be appreciated, it is difficult to determine precisely when or where a new species is established, in a continuously changing group of organisms. An answer to this question depends, of course, on how we de- fine a species. One particularly dynamic view of this important concept holds that a species is the stage, in a process of continued evolutionary divergence, at which the members of formerly freely interbreeding populations have 651 GENERAL ZOOLOGY changed to such an extent that they no longer successfully interbreed. One of the most significant factors conditioning the establishment of new genetic combinations, and thus contributing to the formation of species, involves various forms of isolation. Free interbreeding tends to promote uniformity in a population. ‘There- fore, some degree of isolation is necessary for new combinations of character- istics to become established and to form steps in evolutionary change. From the standpoint of genetics, the conditions in nature that effect such isolation are many and varied. For example, the production of hybrid zygotes may be prevented by ecological, seasonal, or geographical factors operating in such ways that adult members of different populations never encounter each other. This effectively prevents interbreeding and the consequent exchange of heritable characteristics between the groups. A_ classical example of ecological isolation is furnished by snails of the family Achatinellidae, found in the Hawaiian Islands during the 1850’s by John 'T. Gulick (1832-1923). Snails of this family live in trees. Since they cannot travel any distance over a land surface devoid of shade or moisture, their distribution is restricted. Along the sides of the principal mountains on the island of Oahu there are small valleys in which these snails find suitable habitats. But the snails can- not easily cross the ridges between adjacent valleys, or the crest of the mountain; neither can they move out upon the plain below. A population that becomes established in any valley tends to remain isolated from those in other valleys as long as similar climatic and topographical conditions prevail. At the time of Gulick’s original collections, he found that almost every valley had its particular species or subspecies, differing in size, color, and shape of shell. More recent collectors have reported a species that seems to have been restricted to a single tree, sufficiently isolated to prevent migra- tion and contact with other groups. It is difficult to regard differences of the kind shown by these snails as useful or adaptive, and thus as having a selective value in the environments observed; the environment in all the valleys appears to be essentially uni- form. ‘The varied characteristics of these populations of snails may be inter- preted as non-adaptive and hence as having no significance in the process of natural selection. ‘Their appearance may best be accounted for on the basis of the isolation of the breeding populations. Under such conditions of isola- tion, each population evolves independently of the others, and because of the somewhat different inviduals originally present or subsequently appearing at random in the different groups, all are likely to evolve in different direc- tions. Genetically speaking, each population consisted originally of in- dividuals with certain combinations of genes, no two populations being identical. ‘The mere sequence of generations produced, by random assort- ment and recombination, increasingly varied gene complexes in the different populations, and mutations continued to occur at random in each of the groups. From these possibilities, in accordance with the laws of probability, different types, and hence evolutionary changes, resulted. Even from popula- 652 THE EVOLUTION OF ANIMAL LIFE tions made up originally of genetically homogeneous and freely interbreeding individuals, separate small groups maintained in isolation for long periods of time would undoubtedly in the long run develop into distinct species, just as these snails of Oahu have done. There are many other examples of the effectiveness of such geographical or ecological isolation. Sometimes, isolating mechanisms operate even when adult forms occur together. These mechanisms may be psychological, as when for any reason males and females of the two forms fail to show sexual interest in each other; they may be mechanical, involving a physical incom- patibility which prevents copulation; or they may be physiological, involving failure of spermatozoa to reach the ova to effect fertilization. In still other forms, hybrid offspring are inviable, dying at some stage of development be- fore they reach sexual maturity. Hybrids that survive to the stage at which reproduction occurs in the parent species are commonly incapable of pro- ducing viable gametes. Mechanisms of isolation, particularly those involving reproductive functions, are undoubtedly related to the establishment of incipient species by the formation of subspecies or lesser groups that breed successfully only among themselves; at later stages they are important in maintaining the boundary lines between established species. This origin of incipient species must be recognized as the next stage in evolution after the appearance of heritable variations. It is possible to understand, even without an extended explanation of the genetic principles involved, that the appearance of heritable variations (changed gene com- plexes) and their perpetuation, first in small groups of individuals and then in larger groups which eventually become species, are the first steps in evolution. ‘Thus, one species may arise from a pre-existing species, and a third from the second, and so on; and great evolutionary changes may occur in the course of time. Since the origin of life on earth, there has certainly been enough time to permit the operation of such mechanisms of evolution. In the last analysis, as Darwin clearly recognized, accounting for the origin of species is the key to an understanding of the evolutionary process. It has been argued that this view of evolutionary mechanisms and of the importance of natural selection is inadequate, because it fails to explain many lines of evidence that seem to indicate that mutations and subsequent evolutionary changes do not occur at random and in all directions. ‘The evidence often cited involves evolutionary sequences in which changes all seem directed toward some pre-determined end form, and in which it is pos- sible to detect steady trends, such as increase in size and the consistent development of special characteristics which appear to be non-adaptive or without obvious selective value to the organism. From the standpoint of genetics, it is argued that the occurrence of mutations is not a chance phenomenon, because, for example, the same mutations occur repeatedly in populations of organisms. All this evidence has been taken to indicate the existence of what has been termed “‘plan and purpose”’ in evolution. It must be borne in mind, however, that living systems involve specifically 653 GENERAL ZOOLOGY organized and delicately balanced functional mechanisms, about which the structural features are organized. Such systems obviously cannot be changed in completely random ways, or with any large deviations from the normal, without disrupting their fundamental conditions of equilibrium. The char- acteristics of organisms are thus bounded at all stages by rather finite limits, and any changes beyond these limits are detrimental to the organism. ‘To this extent, then, heritable variations or mutations cannot occur at random. For any particular kind of organism, the number of directions in which change can proceed, consonant with continued life, is limited; and only such changes as may proceed in these directions can persist and give rise to new evolutionary stocks. Changes in some directions may, under certain environ- mental conditions, be of survival value to the organism and thus may be selected for preservation. Changes in other directions may appear to be non- adaptive, or to have no demonstrable survival value; these may persist because they are “neutral,” because selective processes do not operate to eliminate them, or because they have become genetically linked with selected characteristics. “This may account for the persistence, through long evolutionary sequences, of such apparently non-adaptive characteristics as overgrown spines and large, complicated shells among some of the inverte- brate groups. Changes which are adaptive, and thus selected, in some stages of the history of a group may, under changed conditions, become neutral; or they may actually become detrimental and so contribute to the eventual ex- tinction of the race. The evolution of non-adaptive characteristics, and the evidence that evolution is not a completely random process, may be ex- plained by some such reasonable considerations. Summary The history of organisms, as indicated by the data of biology and other sciences, has involved gradual processes of change from some _ primitive form or forms of life. How and when these primitive forms originated on our planet are matters of speculation. ‘The place of origin was probably in the primordial seas, and the time must have been some period after these waters had cooled sufficiently to permit living systems to exist. Evidence drawn from a wide range of observations and experiments demonstrates the reality of the evolutionary changes that have produced existing forms from this primitive ancestry. The facts of distribution, both geologic and geographic, and the facts of anatomy, embryology, and physiology can be most reasonably ex- plained in accordance with the theory of organic evolution. ‘The strength of this evidence is in its extent and diversity. Any other explanation of the data is a violation of common sense as well as of scientific reasoning. Much of the evidence is indirect and circumstantial, but more direct and experi- mental evidence, based on studies in plant and animal breeding, genetics, and ecology, confirms the conclusion that the only reasonable explanation for a 654 THE EVOLUTION OF ANIMAL LIFE vast array of facts is provided by the theory of evolution. Mankind is not exempt; the evidence clearly indicates that man originated as an evolutionary offshoot from the mammalian stock that produced the higher apes, and that man’s more remote ancestry is in common with that of all other vertebrates. With the historic fact of organic evolution thus established, and its general course indicated, the factors that have conditioned evolution remain to be explained. A full explanation of these factors has not been accomplished, but it is evident that variation and heredity are the beginning of evolutionary modification; that isolation of incipient stages in small groups is important; and that natural selection has directed evolution along pathways which fitted the organism to its environment. ‘These have certainly been among the factors conditioning organic evolution. No case has been established for the Lamarckian theory; the principal aspects of Darwin’s theory of natural selection, interpreted in the light of modern knowledge of genetics and ecology, serve as a basis of a reasonable theory describing the probable mechanisms of evolutionary change. As a fundamental generalization of biological science, only the Cell Theory can rank with the Theory of Organic Evolution. Developing somewhat earlier in the history of biology, the growth of the Cell Theory was correlated with great advances in our understanding of biological science and in its practical application. Attaining prominence and widespread acceptance some years later, the Theory of Organic Evolution has had a similarly uni- versal impact on biological thought and interpretation, and its importance continues to grow with the accumulation of new evidence from many areas of biological science. 655 ae os ish ot hit i ee Want ey ery a ! ots “ai oe SRP, itary jay la ‘Ait YE COM = Ra iy Peieei 7 De i) htiwW that ~ : Nee bition: aren > lider ated py bia gti dit o Wayens eae o.