m •TANDAFyj JV\ED UTHOF^S 2£ COLLEGE OF AGRICULTURE DAVIS, CALIFORNIA MAY THE MICROSCOPE AND ITS REVELATIONS BY WILLIAM B. CARPENTER, C.B. M.D. LL.D. F.R.S. F.G.S. F.L.S. CORRESPONDING MEMBER OP THE INSTITUTE OF PRANCE, AND OP THE AMERICAN PHILOSOPHICAL SOCIETY, ETC., ETC. SIXTH EDITION ILLUSTRATED BY TWENTY-SIX PLATES AND FIVE HUNDRED WOOD ENGRAVINGS VOLUME II. NEW YORK WILLIAM WOOD & COMPANY 56 & 58 LAFAYETTE PLACE 1883 UNIVERSITY OF CALIFORNIA LIBRARY COLLEGE OF AGRICULTURE TABLE OF COHTEISTS. CHAPTER X. MICROSCOPIC FORMS OF ANIMAL LIFE: — PROTOZOA. PAGE Protozoa, 1 Monerozoa, 2 Rhizopoda, 7 Reticularia, .... 7 PAGE Heliozoa, 11 Lobosa, ..... 14 Coccoliths and Coccospheres . .19 Gregarinida, ..... 21 CHAPTER XL ANIMALCULES:— INFUSORIA AND ROTIFERA. INFUSORIA, 24 I Infusoria continued: — Flagellata 26 [ Ciliata, 41 Cilio-flagellata, . . ,87 ROTIFERA, 53 Suctoria, 39 i Tardigrada, . . . .62 CHAPTER XII. FORAMINIFERA AND POLYCYSTINA. FORAMINIFERA, . . . .64 Porcellanea, .... 70 Arenacea, . . . .77 Vitrea, 85 Eozoon Canadense, . . . 101 RADIOLARIA, 109 Discida, 112 Polycystina, . . . .113 Acanthometrina, . . .113 Colloza, . . . ., . 115 1 CHAPTER XIII. SPONGES AND ZOOPHYTES. SPONGES, 117 ZOOPHYTES, 122 Hydrozoa, 123 Zoophytes continued: — Acalephse, .... 132 Actinozoa, . . . 134 Ctenophora, . . . .137 Production of Medusoids, . 126 CHAPTER XIV. ECHINODERMATA. Structure of Skeleton, . . . 140 | Echinoderm-Larvae, . . .150 CHAPTER XV. POLYZOA AND TUNICATA. POLYZOA, 157 | TUNICATA, 163 CHAPTER XVI. MOLLUSCOUS ANIMALS GENERALLY. Structure of Shells, . . .171 Palate of Gasteropods, . . .180 Development of Mollusks, . . 183 Ciliary motion on Gills, . . 189 Organs of Sense of Mullusks, . 190 Chromatophores of Cephalopods, . 191 IV TABLE OF CONTENTS. CHAPTER XVII. ANNULOSA OR WORMS. ENTOZOA, TURBELLARIA, PAGE 193 194 ANNELIDA, . Development of Annelids, PAGE , 195 197 CHAPTER XVIII. CRUSTACEA. PYCNOOONIDA, . . . .205 ENTOMOSTRACA, . 207 SUCTORIA, 212 ClRRHIPEDA, . MALACOSTRACA, Metamorphosis of Decapods, 213 214 215 CHAPTER XIX. INSECTS AND ARACHINDA. Number and variety of Objects af- forded by Insects, . . . 21 8 Structure of Integument, . .219 Scales and Hairs, .... 220 Eyes, . . . . . .229 Antennae, 232 Mouth, 233 Circulation of the Blood, . . 237 Respiratory Apparatus, . . . 238 Feet, 243 Stings and Ovipositors, . Eggs, . Agamic Reproduction, . Embryonic Development, . 245 . 246 . 246 . 248 Acarida, ..... 248 Parts of Spiders, . 250 CHAPTER XX. VERTEBRATED ANIMALS. Elementary Tissues, . . . 252 Cells and Fibres, . . .253 Bone, 255 Teeth, . . . . .258 Scales of Fish, . . . .261 Hairs, 263 Featkers, 266 Hoofs. Horns, etc., . . . 267 Blood, 267 White and Yellow Fibres, . 271 Skin, Mucous and Serous Mem- branes, ..... 274 Epidermis, Pigment-Cells, . Epithelium, Fat, . . Cartilage, .... Glands Muscle, .... Nerve, .... Circulation of the Blood, Injected Preparations, . Vessels of Respiratory Organs, 275 275 276 277 278 279 281 284 286 292 299 CHAPTER XXI. APPLICATION OF THE MICROSCOPE TO GEOLOGY. Fossilized Wood, Coal, . . .302 Fossil Foraminif era; Chalk, . . 304 Organic Materials of Lime-stones, 308 Fossil Bones, Teeth, etc., . .310 Inorganic materials of Rocks, .312 Nachet's Mineralogical Microscope, 315 CHAPTER XXII. INORGANIC OR MINERAL KINGDOM. — POLARIZATION. Mineral Objects, Crystallization of Salts, Molecular Coalescence, 318 319 323 Organic Structures suitable for Polariscope, .... 323 Micro-Chemistry, .... 326 APPENDIX. An- " Numerical Aperture" and gular Aperture, " .... 327 Watson's New Model Microscopes, 331 Swift's "Wale" Model Students' Microscope, Nachet's Objective-carrier, 332 333 THE MICROSCOPE. CHAPTER X. MICROSCOPIC FORMS OF ANIMAL LIFE -.-PROTOZOA. 391. PASSING-OX, now, to the Animal Kingdom, we begin by direct- ing our attention of those minute and simple forms, which correspond in the Animal series with the Protophyta in the Vegetable (Chap, vi.); and this is the more desirable, since the formation of a distinct group to which the name of PROTOZOA (first proposed by Siebold) may be appro- priately given, is one of the most interesting results of Microscopic inquiry. This group, which must be placed at the very base of the Animal scale, beneath the great Sub-Kingdoms marked-out by Cuvier, is character- ized by the extreme simplicity that prevails in the structure of the beings composing it; the lowest of them being single protoplasmic particles or ' jelly specks;' whilst even among the highest, however numerous their units may be, the^e are (as among protophytes, § 227) mere repetitions of one another, each capable of maintaining an independent existence. In this there is a very curious and significant parallelism to the earliest embryonic stage of higher Animals. For the fertilized germ of any one of these first shapes itself as a single cell; and then, by repeated binary subdivisions, develops itself into a morula or ' mulberry-mass ' of cells (Fig. 403), corresponding to the ' multicellular ' organisms met with among the higher Protozoa (Fig. 350). There is, so far, in neither case, any sign of that ' differentiation ' of organs which is characteristic of the higher Animals; but whilst, in the Protozoon, each cell is not merely similar to its fellows, but is independent of them, the morula, in such as go on to a higher stage, becomes the subject of a series of developmen- tal changes, tending to the production of a single whole, whose parts are mutually-dependent. The first of these changes is its conversion into a gastrula or primitive stomach, whose wall is formed of a double mem- brane,— the outer lamella, or ectoderm, being derived directly from the external cell-layer of the morula, whilst the inner, or endoderm, is formed by the * invagination ' of that layer into the space left void by the disso- lution of the central cells of the ' morula.'1 This gastrula- stage, as we 1 It has not yet been certainly ascertained that the endoderm is formed by invagination in all cases ; but as several of the supposed exceptions have dis- appeared under the light of fuller investigation, it seems probable that the re- mainder will be found conformable to the general rule. 1 2 THE MICROSCOPE AND ITS REVELATIONS. shall see hereafter (§ 513), remains permanent in the great group of Ccelenterata; though the endoderm and ectoderm are separated from each other in its higher forms by the development of generative and other organs between them. But in all Classes above the Coelenterates, the primitive stomach has only a transitory existence, being superseded by the permanent structures that have their origin in its walls. — Thus the whole Animal Kingdom may be divided, in the first place, into the PROTOZOA, which are either single cells, or aggregates of similar cells corresponding to the worwZa-stage of higher types; and the METAZOA, in which the morula takes-on the condition of an individualized organism, the life of every part of which contributes to the general life of the whole. 392. The lowest of the Protozoa, however, like the simplest Proto- phytes, do not even attain the rank of a true cell, — understanding by that designation a definite protoplasmic unit, limited by a cell-wall, and con- taining a ' nucleus.7 For they consist of particles of protoplasm, termed ('cytodes' or 'plastids') of indefinite extent, which have neither cell- wall nor nucleus, but which yet take-in and digest food, convert it into the material of their own bodies, cast out the indigestible portions, and reproduce their kind, with the regularity and completeness that we have been accustomed to regard as characteristic of higher Animals. Between some of these Monerozoa (as they have been designated by Prof. Haeckel, who first drew attention to them and the Myxomycetes (§ 222) the Chla- midomyxis (§ 324) already described, no definite line of division can be drawn; the only justification for the separation here adopted 'being that the affinities of the former seem to be rather with the lowest forms of Vegetation, whilst the whole life-history of the types now to be described and the connected gradation by which they ;pass into undoubted Khizo- pods, leave no doubt of their claim to a place in the Animal Kingdom. MONEROZOA. \ 393. A characteristic example of this lowest Protozoic type is pre- sented by the Protomyxa aurantiaca (Fig. 279), a marine ' Moner ' of an orange-red color, found by Professor Haeckel upon dead shells of Spirula near the Canary Islands. In its active state is has the stellar form shown at F; its arborescent extensions dividing and inosculating so as to form a constantly changing network of protoplasmic threads, along which stream in all directions orange-red granules obviously belong- ing to the body itself, together with foreign organisms (b c)— such as marine Diatoms, Eadiolarians, and Infusoria, — which, having been en- trapped in the pseudopodial network, are carried by the protoplasmic stream into the central mass, where the nutrient matter of their bodies is extracted, the hard skeletons being cast out. Neither nucleus nor contractile vesicle is to be discerned; but numerous floating and incon- stant vacuoles (a) are dispersed through the substance of the body. — After a time, the currents become slower; the ramified extensions are gradually drawn inwards; and, after ejecting any indigestible parti- cles it may still include, the body takes the form of an orange-red sphere, round which a cyst soon forms itself, as shown at A. After a period of quiescence, the protoplasmic substance retreats from the interior of the cyst, and breaks up into a number of small spheres (B), which, at first inactive, soon begin to move within the cyst, and change their shape to that of a pear with the small end drawn out to a point. The cyst then MICROSCOPIC FORMS OF ANIMAL LIFE. 3 bursts, and the red pear-shaped bodies issue forth into the water (c), moving freely about by the vibrations oiflagella formed by the drawing- out of their small ends, — just as do the flagellated zoospores of proto- phytes (§ 231). These bodies, being without trace of either nucleus, contractile vesicle, or cell-wall, are to be accounted as particles of simple homogeneous protoplasm, to which the designation plastidules has been appropriately given. After about a day the motions cease; the flagella are drawn in, and the plastidules take the form and lead the life of Amoeba (§ 403), putting forth inconstant pseudopodial processes, and engulfing nutrient particles in their substance (D). Two or more of these amoebif orm bodies unite to form a ' plasmodium ' (as in the Myxo- FIG. 279. Protomyxaaurantiaca:—*., encysted statospore; B, incipient formation of swarm spores, shown at c escaping from the cyst, at D swimming freely by their flagellate appendages, and at K creep- ing in the amoeboid condition; F, fully -developed reticulate organism, showing numerous vacuoles, a, and captured prey, 6, c. mycetes, § 222); its pseudopodial extensions send out branches which inosculate to form a network; and the body grows, by the ingestion of nutriment, to the size of the original. — In this cycle of change there seems no intervention of a generative act, the coalescence of the amrebi- f orm plastidules having none of the characters of a true 'conjugation.' But it is by no means improbable that after a long course of multiplica- tion by successive subdivisions, a sexual act of some kind may intervene. 394. Another very interesting 'moneric' type is, the Vampyrella; of which one form (Fig. 280, B) has long been known in its encysted con- 4: THE MICROSCOPE AND ITS REVELATIONS. dition as a minute brick-red sphere attached to the filaments of the Con- jugate Spirogyra: whilst another (Fig. 281, a, a) similarly attaches itself to the branches of Gomphonema (§ 294). The wails of the cysts are com- posed of two membranes; of which the interior gives the characteristic reaction of cellulose, whilst the softer external layer is nitrogenous. After remaining some time in the quiescent condition, the encysted pro- toplasm breaks up into two or four ( tetraspores ' (Fig. 281, b, d)', these escape by openings in the cyst (Fig. 280, c); and soon take the spherical form, emitting very slender pseudopodial filaments (Figs. 280, D, 281, B) like those of an Actinoplirys, but possessing neither nucleus nor contrac- tile vesicle. In this condition they show great activity; moving about in search of the special nutriment they require, drawing themselves out in strings and fine filaments which tear asunder and again unite to send off branches and form fine fan-like expansions, and these occasionally contract- Fig. 280. Vampyrella spirogyrce, as seen at A sucking-out contents of Spirogyra-cell ; at B in encysted condition, the cyst a inclosing granular protoplasm b; at c, division of contents of cyst into tetra- spores, of whicn one is escaping in the amoeboid condition, to develop itself into the adult form shown at D. ing again into minute spheres. When the V. spirogyrm is watched in water containing some filaments of Spirogyra, it may be seen to wander until it meets one of these filaments, to which, if it be healthy and loaded with chlorophyll, it attaches itself. It soon begins to perforate the wall of the filament; and when the interior of this has been reached, its endo- plasm, carrying with it the chlorophyll-granules it includes, passes slowly into the body of the Vampyrella. In this manner, cell after cell is emptied of its contents; and the plunderer, satiated with food, resumes its quiescent spherical form to digest it. The chlorophyll granules which it has ingested become diffused through the body, but gradually cease to be distinguishable, the protoplasmic mass assuming a brick-red color. MICROSCOPIC FORMS OF ANIMAL LIFE. 5 The first layer it exudes to form its cyst is the outer or nitrogenous in- yestment, within which the cellulose layer is afterwards formed, — The V. gomphonematis in like manner creeps over the stems and branches of the Gomphonema (Fig. 281, e), adapting itself to the form of its support; and as soon as it has reached one of the terminal siliceous cells of the Dia- tom, it extends itself over it so as completely to envelop the cell in a thin layer of protoplasm. From thp surface of this, a number of fine pseud o- podia radiate into the surrounding water (/); whilst another portion of the protoplasm finds its way between the two siliceous valves into the interior, Fig. 281. ampyrella gomphonematis: — A, colony of Gomphonema attacked by Vampyrellce; a, encysted state; 6, 6. cysts with contents breaking-up into tetraspores, d, d, seen escaping at e; at/is shown a Vampyrella sucking-out contents of Gomphonema-cetls, the emptied frustules of which, g, h, are cast forth.— B, isolated Vampyrella, creeping about by its extended pseudopodia. and appropriates its contents. The valves, when emptied, break off from their support, and are cast out of the body of the Vampyrella, which soon proceeds to another Gomphonema-cell and plunders it in the same manner. After thus ingesting the nutriment furnished by several cells, and ac- quiring its full size, it passes, like V. spirogyrce, into the encysted con- dition, to recommence — after a period of quiescence — the same cycle of change. 395. Intermediate between the foregoing and the ' reticularian ' THE MICROSCOPE AND ITS REVELATIONS. FIG. 282. Rhizopods to be presently described, is another simple Protozoon discovered in ponds in Germany by M-M. Claparede and Lachmann, and named by them Lielerkuhnia Wageneri. 1 The whole substance of the body of this animal and its pseudopodial extensions (Fig. 282) is composed of a homo- geneous, semifluid, granular protoplasm; the particles of which, when the animal is in a state of activity, are continually performing a circula- tory movement, which may be likened to the rotation of the particles in the protoplasmic network within the cell of a Tradescantia (§ 355). It is a marked peculiarity of the pseudopodial extension of this type, that it does not take place by radiation from all parts of the body indifferently; but that it proceeds entirely from a sort of trunk that soon divides into branches, which, again, speedily multiply by further subdivision, until at last a multitude of finer and yet finer threads are spun-out, by whose continual inosculations a com- plicated network is produced, which may be likened to an animated Spider's web. The entire absence of anything like a membranous envelope is clearly evidenced by the readiness with which the subdivi- sion and the coalescence of the pseudo- podia alike take place. Any small ali- mentary particles that may come into con- tact with the glutinous surface of the pseudopodia, are retained in adhesion by it, and speedily partake of the general movement going-on in their substance. This movement takes place in two prin- cipal directions; from the body towards the extremities of the pseudopodia, and from these extremities back to to the body again. In the larger branches a double current may be seen, two streams passing at the same time in opposite direc- tions; but in the finest filaments the current is single, and a granule may be seen to move in one of them to its very extremity, and then to return, per- haps meeting and carrying back with it a granule that was seen advancing in the opposite direction. Even in the broader processes, granules are sometimes observed to come to a stand, to oscillate for a time, and then to take a retrograde course, as if they had been entangled in the opposing current, — just as often is to be seen in Cham. When .a granule arrives at a point where a filament bifurcates, it is often arrested for a time, until drawn into one or the other current; and when carried across one of LieberTcuhnia Wageneri. 1 "Etudes sur les Infusoires et les Rhizopods;" Geneva, 1850-1861. The beauti- ful figure of Lieberkuhnia, given by M. Claparede, has been reproduced by the Author in Plate 1 of his ' Introduction to the Study of the Foraminifera.' — A Rhizo- pod of the same type has been discovered by Mr. Siddall (of Chester) in Sea- water from the North and South Coasts of Wales, which he regards as especially identi- cal with L. Wageneri ("Quart. Microsc. Journ.," N. S., Vol. xx., p. 144), but which the Author (who has great confidence in the accuracy of the excellent observers by whom the latter was described) must regard as differentiated from it (1) by the existence of a pellucid flexible investment (foreshadowing the ' test ' of Gromia), having a definite orifice bordered by four infolded lips, through which the sarco- dic trunk issues forth ; and (2) by the presence of a number of highly refractive, short, rod-like spicules set at various angles on the external surface. MICROSCOPIC FORMS OF ANIMAL LIFE. 7 the bridge-like connections into a different band, it not unfrequently meets a current proceeding in the opposite direction, and is thus carried back to the bod/ without having proceeded very far from it. The pseu- dopodial network along which this 'cyclosis' takes place, is continually undergoing changes in its own arrangement; new filaments being put forth in different directions, sometimes from its margin, sometimes from the midst of its ramifications, whilst others are retracted. Not unfre- quently it happens that to a spot where two or more filaments have met, there is an afflux of the protoplasmic substance that causes it to accumu- late there as a sort of secondary centre, from which a new radiation of filamentous processes takes place. Occasionally the pseudopodia are en- tirely retracted, and all activity ceases; so that the body presents the ap- pearance of an inert lump. But if watched sufficiently long, its activity is resumed; so that it may be presumed to have been previously satiated with food, which is undergoing digestion during its stationary period. No encysting process has been noticed in Lieberkuhnia; and the manner in which this type reproduces itself is at present entirely unknown. As the marine type of it occurs on our own coasts, the fresh- water type may very likely be found in our ponds; and either may be recommended as a most worthy object of careful study. RHIZOPODA. 396. "We now arrive at the group of Rhizopods, or ' root-footed ' animals, first established by Dujardin for the reception of the Amoeba (§ 403) and its allies, which had been included by Prof. Ehrenberg among his Infusory Animalcules, but which Dujardin separated from them as being mere particles of sarcode (protoplasm), having neither the definite body-wall nor the special mouth of the true Infusoria, but put- ting forth extensions of their sarcodic substance, which he termed pseudopodia (or false feet), serving alike as instruments of locomotion, and as prehensile organs for obtaining food. According to Dujardin's definition of this group, the Monerozoa already described would be included in it; but it seems on various grounds desirable to limit the term Rhizopoda to those Protozoa in which the presence of a nucleus, the differentiation of an ectosarc (or firmer superficial layer of proto- plasm) from the semi-fluid endosarc, together with the more definite form and restricted size, indicate a distinct approach to the condition of true cells. — Many different schemes for the classification of the Rhizopods have been proposed; but none of them can be regarded as entirely satis- factory, our knowledge of the Reproductive processes, and of other important parts of the life-history of these creatures, being still extremely imperfect. And as some parts of the scheme proposed by the Author twenty years ago,1 based on the characters of the pseud opodial extensions, have been accepted by more recent systematists, he thinks it best still to adhere to it, as seeming to him to be on the whole most natural. i. In the First division, Reticularia, the pseudopodia freely ramify and inosculate, so as to form a network, exactly as in Lieberkuhnia; from which they are distinguished by the possession of a nucleus, and by the investment of their sarcodic bodies in a firm envelope. This is most commonly either a calcareous shell of very definite shape, or a test built up of sand-grains or other minute particles more or less firmly united by 1 "Natural History Review," 1861, p. 456; and "Introduction to the Study of the Foraminifera" (1862), Chap. n. 8 THE MICROSCOPE AND ITS REVELATIONS. a calcareous cement exuded from the sarcodic body. These testaceous forms, which are exclusively marine, constitute the group of Foramini- fera; whose special interest to the microscopist entitles it to separate consideration (Chap. xn.). And it is only for convenience, that two Reticularia which inhabit fresh water also, and the envelopes of whose bodies are usually membranous, are here separated from the Foraminif era (to which they properly belong) for description as types of the group. The Reticularia have little locomotive power, and only seem to exercise it to find a suitable situation for their attachment; the capture of their food being effected by their pseudopodial network. ii. The Second division, Heliozoa * consists of the Rhizopods whose pseudopodia extend themselves as straight radiating rods, having little or no tendency to subdivide or ramify, though they are still sufficiently soft and homogeneous (at least in the lower types, § 399), to coalesce when they come into contact with each other. These have usually (probably always) a contractile vesicle as well as a nucleus; and the higher forms of them are characterized by the inclosure of peculiar yellow corpuscles (whose import is unknown) in the substance of their endosarc. By far the larger number of this group also have skeletons of Mineral matter, which are always siliceous; and these are sometimes perforated casings of great regularity of form, as in the marine Poly- cystina ; sometimes internal frameworks of marvellous symmetry, as in tne marine Radiolaria. These two groups, also, will be reserved for special notice (Chap, xii.); the simple Heliozoa which are among the commonest inhabitants of fresh water, furnishing the best illustrations of the essential characters of the type. They seem for the most part to have but little locomotive power, capturing their prey by their extended pseudopodia. in. The Third group, Lolosa, contains the Rhizopods which most nearly approach the condition of true Cells, in the differentiation of their almost membranous ectosarc and their almost liquid endosarc, and in the non-coalescence of their pseudopodial extensions, which, instead of being either thread-like or rod-like, are lobate, that is, irregular projections of the body, including both ectosarc and endosarc, which are continually undergoing change both in form and number. The Lobosa are com- paratively active in their habits, moving freely about in search of food, which is still received into the substance of their bodies through any part of their surface, — unless this is inclosed in envelopes, such as are formed by many of them, either by exudation from the surface of their bodies of some material (probably chitinous) which hardens into a membrane, or by aggregating and uniting grains of sand or other small solid particles, which they build up into ' tests/ A large proportion of them are inhabi- tants of fresh water, and some are even found in damp earth. 397. Reticularia. — This type is very characteristically represented by the genus Gromia (Fig. 283); some of whose species are marine, and are found, like ordinary Foraminif era, among tufts of Corallines, Algae, etc. ; whilst others inhabit fresh water, adhering to Confervse and other Plants of running streams. It was in this type that the presence of a nucleus formerly supposed to be wanting in Reticularia generally, was first estab- 1 To this group the Author formerly extended the name Radiolaria given by Miiller to one section of it; but he now thinks it preferable to employ the general term Heliozoa given to it by Herjwig and Lesser, restricting the term Radiolaria to the group to which it was originally applied. MICROSCOPIC FORMS OF ANIMAL LIFE. 9 Fio.283. lished by Dr. Wallich. The sarcode-body of this animal is incased in an egg-shaped, brownish-yellow, chitinous envelope, which may attain a dia- meter of from l-12th to l-10th of an inch, looking to the naked eye so like the egg of a Zoophyte or the seed of an aquatic Plant, that its real nature would not be suspected as long as it remains quiescent. The * test ' has a single round orifice, from which, when the Animal is in a state of activity, the sarcodic substance streams forth, speedily giving off ramifying extensions, which, by further ramification and inosculation, form a network like that of Lieberkiihnia. But the sarcode also extends itself so as to form a continuous layer over the whole exterior of the ' test;' and from any part of this layer fresh pseudopodia may be given off. By the alternate extension and contrac- tion of these, minute Protophytes and Protozoa are entrapped and drawn into the interior of the test, where their nutritive material is ex- tracted and assimilated; and if the ' test ' (as happens in some species) be sufficiently transparent, the indiges tible hard parts (such as the siliceous valves of Diatoms, shown in Fig. 283) may be distinguished in the midst of the sarcodic substance. By the same agency, the Grown' a some- times creeps up the sides of a glass vessel. In the intervals of quies- cence, on the other hand, the whole sarcodic body, except a film that serves for the attachment of the test, is withdrawn into its interior. 398. Another example of the Re- ticularian group la afforded by the curious little Microgromia socialis (Fig. 284), first discovered by Mr. Archer, and further investigated with great care by Hertwig;1 which has the curious habit of uniting with neighboring individuals, by the fus- ion of the pseudopodia, into a com- mon * colony;' the individuals some- times remaining at a distance from one another as at A, but sometimes aggregating themselves into compact masses as at B. The nearly globular thin calcareous shell is prolonged into a short neck having a circular orifice, from which the sarcode-body extends itself, giving off very slender pseudo- podia which radiate in all directions. A distinct nucleus can be seen in the deepest part of the cavity; while a contractile vesicle lies imbedded in the sarcodic substance nearer the mouth. Multiplication by duplicative subdivision has been distinctly observed in this type; but with a peculiar Gromia oviformis, with its pseudopodia extended. 1 'Ueber Microgromia;' in " Archiv fur Mikr. Anat.," Bd. x., Supplement. 10 THE MICROSCOPE AND ITS REVELATIONS. departure from the usual method. A transverse constriction divides the body into two halves — as shown in two individuals of colony A, — each half possessing its own nucleus and contractile vesicle; the posterior seg- ment, which at first lies free at the bottom of the cell, then presses for- wards towards its orifice, as shown at c, and finally, by amceboid move- ments, escapes from it, sometimes stretching itself out like a worm (as seen at D), sometimes contracting itself into a globe, and sometimes FIG. 284. Microgromiasocialis:— A, colony of individuals in extended state, some of them undergoing trans- verse fission; B, colony of individuals (some of them separated from the principal mass) in com pact state; c, D, formation and escape of swarm-spore, seen free at E. spreading itself out irregularly over the pseudopodia of the colony. But it finally gathers itself together and takes an oval form; and either develops a pair of flagella, and forsakes the colony as a free swimming Monad (§ 416), or assumes the form of an Actinophrys, moving about by three or four pointed pseudopodia, — probably in each case coming after a time to rest, excreting a shell, and laying the foundation of a new colony. There is reason to think that a multiplication by longitudinal fis- MICROSCOPIC FOKMS OF ANIMAL LIFE. 11 sion also takes place, in which the escaping segment and the one left be- hind in the old shell remain attached by their pseudopodia, and the former develops a new shell without undergoing any change of condition. 399. Heliozoa. — The Actinophrys sol, sometimes termed the ' sun-ani- malcule' (Fig. 285), is one of the commonest examples of this group; being often met-with in lakes, ponds, and streams, amongst Confervas and other aquatic plants, as a whitish-gray spherical particle distinguish- able by the naked eye, from which (when it is brought under a suffi- cient magnifying power) a number of very pellucid, slender, pointed rods are seen to radiate. The central portion of the body is composed of homogeneous sarcode, inclosing a distinct nucleus with a large nucle- olus (as in Fig. 287, N) ; but the peripheral part has a f vesicular ' aspect, as in the type next to be described (Fig. 286). This appearance is due to the number of ' vacuoles ' filled with a watery fluid, which are included in the sarcodic substance, and which may be artificially made either to coalesce into larger ones, or to subdivide into smaller. A ' contractile vesicle/ pulsating rhythmically with considerable regularity, is always to be distinguished, either in the midst of the sarcode-body, or (more com- monly) at or near its surface; and it sometimes projects considerably from this, in the form of a flattened sacculus with a delicate membra- nous wall, as shown at o. The cavity of this sacculus is not closed ex- ternally, but communicates with the surrounding medium; not, however, by any distinct and permanent orifice, the membraniform wall giving way when the vesicle contracts, and then closing-over again. This al- ternating action seems to serve a respiratory purpose, the water thus taken-in and expelled being distributed through a system of channels and vacuoles excavated in the substance of the body; some of the vacuoles which are nearest the surface being observed to undergo distention when the vesicle contracts, and to empty themselves gradually as it re-fills. The body of this animal is nearly motionless, but it is supplied with nourishment by the instrumentality of its pseudopodia; its food being derived not merely from Vegetable particles, but from various small Animals, some of them (as the young of Entomostraca) possessing great activity as well as a comparatively high organization. When one of these happens to come into contact with one of the pseudopodia (which have firm axis-filaments clothed with a granular sarcode), this usually retains it by adhesion, but the mode in which the particle thus taken captive is introduced into the body, differs according to circumstances. If the prey is large and vigorous enough to struggle to escape from its entanglement, it may usually be observed that the neighboring pseudo- podia bend over and apply themselves to it, so as to assist in holding it cap- tive, and that it is slowly drawn by their joint retraction toward the body of its captor. Any small particle not capable of offering active re- sistance, on the other hand, may be seen after a little time to glide towards the central body along the edge of the pseu'dopodium, without any visi- ble movement of the latter, much in the same manner as in Gromia. When in either of these modes the food has been brought to the surface of the body, this sends over it on either side a prolongation of its own sar- code-substance; and thus a marked prominence is formed (Fig. 285 c), which gradually subsides as the food is drawn more completely into the interior. The struggles of the larger Animals, and the ciliary action of Infusoria and Rotifera, may sometimes be observed to continue even after they have been thus received into the body; but these movements at last cease, and the process of digestion begins. The alimentary sub- 12 THE MICROSCOPE AND ITS REVELATIONS. stance is received into one of the vacuoles of the endosarc (Fig. 287, F), where it lies in the first instance surrounded by liquid; and its nutri- tive portion is gradually converted into an undistinguishable gelatinous mass, which becomes incorporated with the material of the sarcode-body, as may be seen by the general diffusion of any coloring particles it may contain. Several vacuoles may FIG. 285. be thus occupied at one time by alimentary particles; frequently four to eight are thus distin- guishable, and occasionally ten or twelve ; Ehrenberg, in one instance, counted as many as six- teen, which he describedas mul- tiple stomachs. Whilst the di- gestive process, which usually occupies some hours, is going on, a kind of slow circulation takes place in the entire mass of the endosarc with its included va- cuoles. If, as often happens, the body taken-in as food possesses some hard indigestible portion (as the shell of an Entomostra- can or Rotifer), this, after the Acti^kn^in different states:-,, in itsordin- dige,Sti°n °f £» S°ft ****>.* arysun-likeform, with a prominent contractile vesi- gradually pushed towards the cle o ; B, in the act of division or of conjugation, with Hnrfape» nr,H ;c fV»cmna ovfrnrlorl two contractile vesicles o, o ; c, in the act of feeding; f lace, anc D, in the act of discharging faecal (?) matters, a and b. by a process exactly the Converse of that by which it was drawn in. If the particle be large, it usually escapes at once by an opening which (like the mouth) extemporizes itself for the occasion (D); but, if small, it sometimes glides along a pseudopodium from its base to its point, and escapes from its extremity. 400. The ordinary mode of Reproduction in Actinovhrys seems to be by binary subdivision: its spherical body showing an "annular constric- tion, which gradually deepens so as to separate its two halves by a sort of hour-glass contraction; and the connecting band becoming more and more slender, until the two halves are completely separated. This pro- cess of fission, which may be completed within half an hour from its commencement, seems to take place first in the contractile vesicle; for each segment very early shows itself to be provided with its own (B, 0, o), and the two vesicles are commonly removed to a considerable distance from one another. The segments thus divided are not always equal, and sometimes their difference in size is very considerable. A junction of two individuals, on the other hand, has been seen to take place in Actino- phrys, and has been supposed to correspond to the ' conjugation ' of Pro- tophytes; it is very doubtful, however, whether this junction really in- volves a complete fusion of the substance of the bodies which take part in it; and there is not sufficient evidence that it has any true generative character. Certain it is that such a junction or ' zygosis ' may take place, not between two only, but between several individuals at once, their number being recognized by that of their contractile vesicles; and that, after remaining thus united for several hours, they may separate again without having undergone any discoverable change. MICROSCOPIC FORMS OF ANIMAL LIFE. 13 401. Under the generic name Actinoplirys was formerly ranked the larger but less common Heliozoon now distinguished as Actinosphmrium Eichornii (Fig. 286); one important difference consisting in the struc- ture of the radiating pseudopodia, each of which has here a firm axis-fila- ment or 'spine/ which passing through the superficial zone, rests on the surface of the central sphere, as shown at a a, Fig. 287. This axis is clothed with a layer of soft sarcode derived from the superficial or cor- tical zone of the body. Several nuclei (n, n) are usually to be seen em- bedded in the protoplasmic mass. — The general life-history of this type corresponds with that of the preceding; but its mode of reproduction presents some marked peculiarities. The binary segmentation is pre- ceded by a withdrawal of the pseudopodia, even their clearly-defined axis becoming indistinct and finallydisappearing; the body becomes enveloped, FIG. 286. ActinosphcBrium Eichornii:— m, endosarc ; r, ectosarc ; c, c, contractile vacuoles. by a clear gelatinous exudation, which forms a kind of cyst; and within this the process of binary subdvision is repeatedly performed, until the original single mass is replaced by a sort of morula (§ 391), each spherule of which shows the distinction between the central and cortical regions, the former including a single nucleus, whilst the latter is strengthened by siliceous deposit into a firm investment. After remaining in this state during the winter, the young Actinosplicericv come forth in the spring without this siliceous investment; and gradually grow into the likenesses of their parent. 402. A large number of new and curious fresh-water forms of this type have been recently brought under notice; of which the Clathrulina elegans (Fig. 288) may be specially mentioned as presenting an obvious transition to the Polycystine type (§ 504). This has been found in various 14: THE MICROSCOPE AND ITS REVELATIONS. parts of the Continent, and also (by Mr. Archer1) in Wales and Ireland; occurring chiefly in dark ponds shaded by trees and containing decaying leaves. Its soft sarcode body is incased by a siliceous capsule of spheri- cal form, regularly perforated with oval apertures, and supported on a long silicified peduncle. The body itself, and the pseudopodia which it puts forth through the apertures of the capsule, seem closely to corre- spond with those of ActinopJirys. — Eeproduction here takes place not only by binary fission, but by the formation of 'swarm spores.' In the first mode, one of the two segments remains in possession of the silice- ous capsule, whilst the other finds its way out through one of the aper- tures, lives for some hours in a free condition as an Actinophrys, and ultimately produces the capsule and stem characteristic of its type. In the second mode, numerous small rounded sarcode-masses, each possess- ing a nucleus, are produced within the capsule, in what manner cannot FIG. 287. Marginal portion of Actinosphcerium Eichornii, as seen in optical section under a higher mag- nifying powder:— m, endosarc; r, ectosarc; a, a, a, pseudopodia; n, n, nuclei with nucleoli ; /, ingested food-mass. be clearly made-out; and every one of these is enveloped in a firm envel- ope, set round with short spines, probably siliceous. These cysts remain for months within the common capsule; and when the time arrives for their further development, the sarcode-corpuscles slip out of their cysts, and escape through the orifices of the capsule as flagellated Monads of oval form (Fig. 288, B,) each having a nucleus, n, near the base of the flagella, and two contractile vesicles near its opposite end. After swarm- ing for some hours in this condition, they change to the free Actinophrys form, and finally acquire the siliceous capsule and stem of the Clathru- lina. 403. Lobosa. — No example of the Rhizopod type is more common in 1 See his Memoir on Fresh-water Radiolaria in "Quart. Journ. of Micros. Sci.," N.S., Vol. ix. (1869), p. 230. MICROSCOPIC FORMS OF ANIMAL LIFE. 15 streams and ponds, vegetable infusions, etc., than the Amoeba (Fig. 289); a creature which cannot he described by its form, for this is as changeable as that of the fabled Proteus, but may yet be definitely characterized by peculiarities that separate it from the two groups already described. The distinction between ' ecto- Fio. 288. , , . , , . , sarc and • endosarc is here clearly marked, so that the body approaches much more closely in its characters to an ordinary 'cell' composed of cell- wall and cell-contents. It is through the ' endosarc7 alone, EIST, that those colored and granular particles are dif- fused, on which the hue and opacity of the body depend ; its central portion seems to have an almost watery consistence, the granular particles being seen to move quite freely upon one another with every change in the shape of the body; but its superficial portion is more viscid, and graduates insensibly into the firmer substance of the 'ectosarc.' The ectosarc, EC, which is perfectly pellucid, forms an almost membranous investment to the endosarc; still it is not possessed of such tenacity as to oppose a solution of its continuity at any point, for the introduction of alimen- tary particles, or for the ex- trusion of effete matters; and thus there is no evidence, in Clathrulina eleqans :— A, complete organism ; B, swarm- A vn ^tin n n c\ i fc i m m &{\ i n f o spore, showing nucleus, n, and Ltwo contractile vesicles AmCBOtt ana ItS immediate near its opposite end. allies, of the existence of any more definite orifice, either oral or anal, than exists in other Ehizopods. The more advanc- ed differentiation of the ecto- sarc from the endosarc of Amoeba, is made evident by the effects of re-agents. If an Amoeba radio sa be treated with a dilute alkaline solution, the granular and molecular endo- sarc shrinks together and re- treats towards the centre, leaving the radiating exten- sions of the ectosarc in the condition of csecal tubes, of Diagrammatic representation of Amoeba protcus;— E c, which the Walls are not Soluble ectosarc; EN, endosarc; cv, contractile vesicle; N, nuc- ±41 j- leus; p.pseudopodia; VIL, vmous tuft, at the ordinary temperature,, Fio. 289. EC VIL 16 THE MICROSCOPE AND ITS REVELATIONS. either in acetic or mineral acids, or in dilute alkaline solutions; thus agreeing with the envelope noticed by Cohn as possessed by Paramecium and other ciliated Infusoria, and with the containing membrane of ordi- nary animal cells. A 'nucleus/ N, is always distinctly visible in Amoeba, adherent to the inner portion of the ectosarc, and projecting from this into the cavity occupied by the endosarc; when most perfectly seen, it presents the aspect of a clear flattened vesicle surrounding a solid and usually spherical nucleolus; it is readily soluble in alkalies, and first ex- pands and then dissolves when treated with acetic or sulphuric acid of moderate strength; but when treated with dilute acid it is rendered darker and more distinct, in consequence of the precipitation of a finely granular substance in the clear vesicular space that surrounds the nucle- olus. A ' contractile vesicle,' cv, seems also to be uniformly present, though it does not usually make itself so conspicuous by its external prominence as it does in Actinoplirys ; and the neighboring part of the body is often prolonged into a set of villous processes VIL, the presence of which has been thought by some to mark a specific distinction, but which seems too variable and transitory to be so regarded. 404. The pseudopodia, which are not so much appendages, as lobate extensions of the body itself, are few in number, short, broad, and rounded; and their outlines present a sharpness which indicates that the substance of which their exterior is composed possesses considerable tena- city. No movement of granules can be seen to take place along the sur- face of the pseudopodia: and when two of these organs come into contact, they scarcely show any disposition even to mutual cohesion, still less to fusion of their substance. Sometimes the protrusion seems to be formed by the ectosarc alone, but more commonly the endosarc also extends into it, and an active current of granules may be seen to pass from what was previously the centre of the body into the protruded portion, when the lat- ter is undergoing rapid elongation; whilst a light current may set towards the centre of the body from some other protrusion which is being with- drawn into it. It is in this manner that an Amoeba moves from place to place; a protrusion like the finger of a glove being first formed, into which the substance of the body itself is gradually transferred; and another protrusion being put forth, either in the same or in some different direction, so soon as this transference has been accomplished, or even before it is complete. The kind of progression thus executed by an Amoeba is described by most observers as a ' rolling ' movement, this being certainly the aspect which it commonly seems to present; but it is main- tained by M.M. Claparede and Lachmann that the appearance of rolling is an optical illusion, for that the nucleus and contractile vesicle always- maintain the same position relatively to the rest of the body, and that ' creeping* would be a truer description of their mode of progression. It is in the course of this movement from place to place, that the Amoeba encounters particles which are fitted to afford it nourishment: and it appears to receive such particles into its interior through any part of the ectosarc, whether of the body itself or of any of its lobose expansions; in- soluble particles which resist the digestive process being got rid of in the like primitive fashion. 405. It may often be seen that portions of the sarcode-body of an Amceba, detached from the rest, can maintain an independent existence;, and it is probable that such separation of fragments is an ordinary mode of increase in this group. When a pseudopodial lobe has been put-forth to a considerable length, and has become enlarged and fixed at its extrem- MICROSCOPIC FORMS OF ANIMAL LIFE. 17 ity, the subsequent contraction of the connecting portion, instead of either drawing the body towards the fixed point, or retracting the lobe into the body, causes the connecting band to thin-away until it separates; and the detached portion speedily shoots out pseudopodial processes of its own, and comports itself in all respects as an independent Amoeba. Multipli- cation also takes place by regular binary subdivision. And an issue of ( swarm-spores,' which swim about for a time like Infusoria, has been witnessed by a competent observer.1 In the A. terricola discovered by G-reef in earth and dry sand, this process is seen to commence in the nu- cleus, which breaks-up into rounded corpuscles that diffuse themselves through the substance of the endosarc. The creature then ceases to take- FIG. 290. Pelomyxa palustris:— ^, as it appears when in amoeboid motion:— B, portion more highly magni- fied; showing a, a, the hyaline ectosarc; 6, one of the vacuoles of the endosarc; c, rod-like bodies scattered through the endosarc; d, protruded extension of ectosarc, with endosarc passing into it; et e, nuclei; /,/, globular hyaline bodies. in food : its motions become less active, and its functions seem to be entirely confined to the nurture of the germs, which finally make their way out, and soon attain the size and aspect of their parent. — No sexual act has been certainly recognized as part of the life-history of Amoeba, the union of two or more individuals, which may be occasionally witnessed, having more the character of the 'zygosis' of Actinophrys (§ 400). 406. A sarcodic organism discovered by Greef, and named by him Pelomyxa palustris (Fig. 290), which spreads over the bottom of stagnant ponds in the condition of slimy masses of indefinite form, exhibits a 29 1 Prof. A. M. Edwards (U. S.) in " Monthly Microsc. Journ.," Vol. viii. (1872), p» 18 THE MICROSCOPE AND ITS REVELATIONS. further advance upon the Amoeban type. The substance of its body ex- hibits a very clear differentiation between the homogeneous hyaline ecto- sarc (B, a, d), and the contained endosarc, which contains such a multi- tude of spherical vacuoles, #, as to have a ' vesicular ' or frothy aspect. When it feeds upon the decomposing vegetable matter at the bottom of the pools it inhabits, its body acquires a blackish hue; but in other sit- uations it may be colorless. Besides the vacuoles, there are seen in the endosarc a great number of nucleus-like bodies, e, e, and also many hya- line globular brilliant bodies, /, /, which are regarded by Greef as germs or swarm-spores, developed from nucleoli set free within the general cav- ity of the body by the bursting of the nuclei. This creature, during the active period of its life, moves like an Amoeba, either by general undula- tions of its surface, or by special pseudopodial extensions d. After a time, however, its movements cease, and it looks as if dead; but by the giving- way of its ectosarc, a multitude of minute amcebiform bodies break forth, each having its nucleus and contractile vesicle. These at first live as Amoeba, but afterwards pass into a resting state, assuming a spherical or oval shape, and then put-forth flagella, by which they swim actively for a time, — probably then settling-down to develop themselves into the parental form. Fia. 291. Testaceous forms of Amoeban Rhizopods:— A, Difflugia proteiformis; B, Difflugia oblonga\ c, Arcella acuminata; D, Arcella dentata. 407. The Amoeban like the Actinophryan type shows itself in the testa- ceous as well as in the naked form; the commonest examples of this being known under the names .Arcella and Difflugia. The body of the former is inclosed in a ( test ' composed of a horny membrane, apparently resem- bling in constitution the chitine which gives solidity to the integuments of Insects: it is usually discoidal (Fig. 291, c, D) with one face flat and the other arched, the aperture being in the centre of the flat side; and its surface is often, marked with a minute and regular pattern. The test of Difflugia, on the other hand, is more or less pitcher-shaped (A, B), and is chiefly made up of minute particles of gravel, shell, etc., cemented to- gether. In each of these genera, these sarcode-body resembles that of Amc&ba in every essential particular; the contrast being very marked be- tween its large, distinct lobose extensions, and the ramifying and inoscu- lating pseudopodia of Gromia (Fig. 283). In each case a detached portion of the sarcodic body will put forth pseudopodia of its own type; and the separation of a bud or gemmule put forth from the mouth of the test seems to be an ordinary mode of propagation among the Amoebans thus inclosed. In Arcella it has been observed that the pseudopodia of two or more individuals unite by bridges of protoplasm, and afterwards separate; but it seems doubtful whether this is a true generative ' conjugation,' or a mere 'zygosis.' It has been observed by Butschli, however, that after the separation of three individuals which had been thus united, the sar- MICROSCOPIC FORMS OF ANIMAL LIFE. 19 FIG. 292. codio body of one of them had withdrawn itself for a considerable space from the wall of the test, and that in the liquid which filled the inter- val a number of Vibrio-like bodies (spermatozoids?) swarmed; while numerous disk-shaped masses of protoplasm lay on the surface of the body. After some time these showed lively amoeboid movements, creep- ing about between the body of the parent and the wall of the test, and ultimately escaping through its orifice. Each of them contained a nu- cleus and contractile vesicle, and moved by means of blunt pseudopodia; and it seems probable that they were embryoes which would in time form the characteristic ArceUa-test. 408. Many testaceous Amcebans have been recently discovered, which form tests of remarkable regularity and sometimes of singular beauty; and it is difficult to determine, in many cases, whether the minute plates of which they are composed have been formed by exudation from their own bodies, or have been picked up from the surface over which the animals crawl.1 There can be no doubt of this kind, however, in regard to the Quadrula symmetrica represented in Fig. 292, whose sarcode-body is en- cased in a pear-shaped test of glassy transparence, made up of a great number of square plates which touch each other by their edges. The sarcode body does not usually fill the test; the intervening space being occupied by a clear liquid, and traversed by bands of proto- plasm. In the posterior part of the body is seen a large clear spherical nucleus, with a distinct dark nutleolus; and in front of this are contractile vesicles, usually two in number. 409. Coccoliths and CoccospJieres. — This would seem the most appro- priate place for the description of certain peculiar little bodies found very extensively diffused over the deep-sea bottom, especially abound- ing in the Globigerina-mud (§ 480), which may be considered as Chalk in process of formation. It was in the specimens of this mud brought up by the ' Cyclops ' soundings in 1857, that Prof. Huxley first found the Coccoliths (Fig. 293, 1, 2) which Dr. Wallich in 1860 found aggregated in the spherical masses which he designated as ' coccospheres ' (3 ). Regarding the gelatin- ous matrix in which they were imbedded as a new type of the Monerozoa described by Haeckel, having the condition of an indefinitely extended plasmodiwri) Prof. Huxley proposed to designate it by the name Bathy- bius, indicative of its habitat in the depths of the sea; and this idea was 1 See especially the recent admirable work of Prof. Leidy on the Freshwater Rhizopods of the United States (1880).— It is to be regretted that its able Author's time and opportunities did not permit him to follow-out the life-histories of the many interesting forms which he has described and figured. Quadrula symmetrica, with extended pseudo- podia. 20 THE MICROSCOPE AND ITS REVELATIONS. accepted by Haeckel, whose representation of a living specimen of Bathy- bius, with imbedded coccoliths, is given in Fig. 293, 4. The observations made in the ' Challenger' Expedition, however, have not confirmed this view; the supposed BathyMus being a gelatinous precipitate, consisting of sulphate of lime, slowly deposited in water to which strong spirit has been added. Whatever be their nature, Coccoliths and Coccospheres are bodies of great interest; since their occurrence in Chalk and in very early Limestones (§ 699) is an additional link in the evidence of the similarity of the conditions under which they were formed, to those at present pre- vailing on the sea-bed of the Atlantic and others oceans. — Two distinct types are recognizable among the Coccoliths, which Prof. Huxley has designated respectively discoliths and cyatholiths. The former are round or oval disks, having a thick strongly-refracting rim and a thinner internal portion, the greater part of which is occupied by a slightly-opaque, cloud- like patch lying round a central corpuscle (Fig. 293, 5). In general, the ' discoliths ' are slightly convex on one side, slightly concave on the other, and the rim is raised into a prominent ridge on the more convex side; FIG. 293. Coccoliths and Coccospheres:— 1, 2, 7, Cyatholiths seen obliquely;— 3, Coceosphere, with im- bedded cyatholiths;— 4, Coccoliths imbedded in supposed protoplasmic expansion ;— 5, Discolith seem in front view;— -6, Cyatholith seem in front view, showing' (1) central corpuscle, (-2) granular zone, (3) transparent outer zone;— 8, 9, Discoliths seen edgeways. so that when viewed edgewise, they present the appearances shown in figs. 8, 9. Their length is ordinarily between l-4000th and l-50QOth of an inch; but it ranges from l-2700th to 1-11, 000th. The largest are com- monly free; but the smallest are generally found imbedded among heaps of granular particles, of which some are probably discoliths in an early stage of development. — The ' cyatholiths, ' also, when full grown, have an oval contour ; though they are often circular when immature. They are convex on one face and flat or concave on the other; and when left to themselves, they lie on one or other of these two faces. In either of these aspects, they seem to be composed of two concentric zones (fig. 6, 2, 3) surrounding an oval thick-wall central corpuscle (1), in the centre of which is a clear space sometimes divided into two. The zone (2) immediately surround- ing the central corpuscle is usually more or less distinctly granular, and sometimes has an almost bead-like margin. The narrower outer zone (3) MICROSCOPIC FORMS OF ANIMAL LIFE. 21 is generally clear, transparent, and structureless; but sometimes shows radiating striae. When viewed sideways or obliquely, however, the ' cyatho- liths ' are found to have a form somewhat resembling that of a shirt-stud (figs. 1, 2, 7). Each consists of a lower plate, shaped like a deep saucer or watch-glass; of a smaller upper plate, which is sometimes flat, some- times more or less concavo-convex; of the oval, thick-wall, flattened cor- puscle, which connects these two plates together at their centres; and of an intermediate granular substance, which more or less completely fills up the interval between the two plates. The length of these cyatholiths ranges from about l-1600th to l-8000th of an inch, those of l-3000th of an inch and under being always circular. — It appears from the action of dilute acids upon the Coccoliths, that they must mainly consist of calcareous matter, as they readily dissolve, leaving scarcely a trace behind. When the cyatholiths are treated with very weak acetic acid, the central corpuscle rapidly loses its strongly refracting character; and there remains an ex- tremely delicate, finely-granular membranous framework. When treated with iodine, they are stained, but not very strongly; the intermediate sub- stance being the most affected. Both discoliths and cyatholiths are com- pletely destroyed by strong hot solutions of caustic potass or soda. — The Coccospheres (Fig. 3) are made up by the aggregation of bodies resembling ' cyatholiths ' of the largest size in all but the absence of the granular zone; they sometimes attain a diameter of l-760th of an inch. — What is their relation to the Coccoliths, and under what conditions these bodies are formed, are questions whereon no positive judgment can be at pres- ent given. (See § 710.) GrREGARI^IDA. 410. A very curious animal parasite is often to be met with in the intestinal canal of Earthworms, Insects, etc., and sometimes in that of higher animals, the simplicity of whose structure requires that it should be ranked among the Protozoa. Each individual Gregarina (Fig. 294, A) essentially consists of a large single cell, usually more or less ovate in form, and sometimes attaining the extraordinary length of two-thirds of an inch.1 A sort of beak or proboscis frequently projects from one extrem- ity; and in some instances this is furnished with a circular row of hook- lets, closely resembling that which is seen on the head of Taenia. There is here a much more complete differentiation between the cell-membrane and its contents, than exists either in Actinoplirys or in Amceba; and in this respect we must look upon Gregarina as representing a decided ad- vance in organization. Being nourished upon the juices already prepared for it by the digestive operations of the animal which it infests, it has no need of any such apparatus for the introduction of solid particles into the interior of its body, as is provided in the < pseudopodia' of the Ehizo- pods and in the oral cilia of the Infusoria. Within the cavity of the cell, whose contents are usually milk-white and minutely granular, there is generally seen a pellucid nucleus; and when, as often happens, the cell undergoes duplicative subdivision, the process commences in a constric- tion and cleavage of this nucleus. The membrane and its contents, except the nucleus, are soluble in acetic acid. Cilia have been detected both upon the outer and the inner surface; but these would seem destined, not so much to give motion to the body, as to renew the stratum of fluid 1 See Prof. Ed. VanBeneden on Gregarina gigantia, in " Quart. Journ. Microsc Sci.," N. S., Vol. x. (1870), p. 51, and Vol. xi., p. 242. 22 THE MICROSCOPE AND ITS REVELATIONS. in contact with it; for such change of place as the animal does exhibit, is effected by the contractions and extensions of the body generally, as in Amoeba (§ 403). An 'encysting process/ very much resembling that of the lower Protophytes, is occasionally observed to take place in Gre- garincB9 and seems to be preparatory to their multiplication. Whatever the original form of the body may be, it becomes globular, ceases to move, and becomes invested by a structureless 'cyst/ within which the substance of the body undergoes a singular change. The nucleus dis- appears; and the sarcodic mass breaks up into a series of globular parti- cles, which gradually resolve themselves (as shown at B, c) into forms very like those of Naviculce. These ' pseudo-navicellse ' are set-free, in time, by the bursting of the capsule that incloses them; and they develop themselves into a new generation of Gregarinae, first passing through an Amoeba-like stage. — A sort of 'conjugation' has been seen to take place between two individuals, whose bodies, coming into contact with each FIG. 294. Gregarina of the Earthworm: — A, in its ordinary aspect; B, in its encysted condition; c, D, show- ing division of its contents into pseudo-navicellae; E, F, free pseudo-navicellae ; o, H, free amoeboids produced from them. other by corresponding points, first become more globular in shape, and are then encysted by the formation of a capsule around them both; the partition-walls between their cavities disappear; and the substance of the two bodies becomes completely fused together. But as the product of this 'zygosis' is the same as that of the ordinary encysting process, there seems no sufficient reason for regarding it, like the ' conjugation ' of Protophytes, as a true Generative act. Prof. Haeckel's Memoirs on Jtfonera and the Gastrcea Theory will be found in the successive Nos. of the " Jenaische Zeitschrift" beginning with 1868; and in a collected form, in the two parts of his " Biologische Studien." The first of his Memoirs on Monera is translated in " Quart. Journ. Microsc. Sci.," N.S., Vol. ix. (1869); and the first of his Papers on the Gastrcea Theory in Vol. xiv. (1874) MICROSCOPIC FORMS OF ANIMAL LIFE. 23 of the same Journal. See also the valuable series of papers on the Freshwater Rhizopods by Mr. Wm. Archer, in the current series of the " Quart. Journ. Microc. Sci. ;" the important Memoirs of Hertwig and Lesser in the " Archiv f iir Mikr. Anat." (especially the Suppl. Heft to Bd. x., 1874), and the Presidential Addresses of Prof. Allman to the Linnasan Society for 1876 and 1877 (in Nos. 69 and 71 of its Journal) on " Recent Researches on some of the more simple Sarcode-Organisms," of which the Author has freely availed himself. 24 THE MICROSCOPE AND ITS REVELATIONS. CHAPTER XL ANIMALCULES.— INFUSORIA AND ROTIFERA. 411. NOTHING can be more vague or scientifically inappropriate than the title Animalcules; since it only expresses the small dimensions of the beings to which it is applied, and does not indicate any of their character- istic peculiarities. In the infancy of Microscopic knowledge, it was natural to associate together all those creatures which could only be discerned at all under a high magnifying power, and whose internal structure could not be clearly made out with the instruments then in use; and thus the most heterogeneous assemblage of Plants, Zoophytes, minute Crustaceans, larvae of Worms, Mollusks, etc., came to be aggregated with the true Animalcules under this head. The Class was being gradually limited by the removal of all such forms as could be referred to others; but still very little was known of the real nature of those that remained in it, until the study was taken up by Prof. Ehrenberg, with the advantage of instru- ments which had derived new and vastly improved capabilities from the application of the principle of Achromatism. One of the first and most important results of his study, and that which has most firmly maintained its ground, notwithstanding the overthrow of Prof. Ehrenberg's doctrines on other points, was )he separation of the entire assemblage into two dis- tinct groups, having scarcely any feature in common except their minute size; one being of very low, and the other of comparatively high organiza- tion. On the lower group he conferred the designation of Polygastrica (many-stomached), in consequence of having been led to form an idea of their organization which the united voices of the most trustworthy ob- servers now pronounces to be erroneous; and as the retention of this term must tend to perpetuate the error, it is well to fall back on the name In- fusoria, or Infusory Animalcules, which simply expresses their almost universal prevalence in infusions of organic matter. To the higher group, Prof. Ehrenberg's name Rotifera or Rotatoria is on the whole very appro- priate, as significant of that peculiar arrangement of their cilia upon the anterior parts of their bodies, which, in some of their most common forms, gives the appearance (when the cilia are in action) of wheels in revolution; the group, however, includes many members in which the ciliated lobes are so formed as not to bear the least resemblance to wheels. In their general organization, these * Wheel-animalcules ' must certainly be considered as members of the Articulated division of the Animal King- dom; and they seem to constitute a Class in that lower portion of it, to which the designation Worms is now commonly given. — Notwithstanding the wide zoological separation between these two kinds of Animalcules, it seems most suitable to the plan of the present work to treat of them in connection with one another; since the Microscopist continually finds them associated together, and studies them under similar conditions. MICROSCOPIC FORMS OF ANIMAL LIFE. 25 SECTION I. — INFUSORIA, 412. This term, as now limited by the separation of the Rhizopoda on the one hand, and of the Rotifera on the other, is applied to a far smaller range of forms than was included by Prof. Ehrenberg under the name of ' polygastric ' animalcules. For a large section of these, includ- ing the DesmidiacecB, Diatomacece, Volvocinecv, and many other Proto- phytes, have been transferred, by general (though not universal) consent, to the Vegetable kingdom. And it is not impossible that many of the reputed Infusoria may be but larval forms of higher organisms, instead of being themselves complete animals. Still an extensive group remains, of which no other account can at present be given, than that the beings of which it is composed go through the whole of their lives, so far as we are acquainted with them, in a grade of existence which is essentially Protozoic (§ 391); each individual apparently consisting of but a single cell, though its parts are often so highly differentiated, as to represent (only, however, by way of analogy} the *f organs' of the higher animals after which they are usually named. 413. Among the ciliate Infusoria, which form not only by far the largest, but also the most characteristic division of the group, there is probably none which has not a mouth, or permanent orifice for the intro- duction of food, which is driven towards it by ciliary currents; while a dis- tinct anal orifice, for the ejection of the indigestible residue, is also gen- erally present. The mouth is often furnished with a dental armature; and leads to an wsophageal canal, down which the food passes into the digestive cavity. This cavity is still occupied, however, as in Khizopods (403), by the enaosarc of the cell; but instead of lying in mere vacuoles formed in the midst of this, the food-particles are usually aggregated, during their passage down the oesophagus, into minute pellets, each of which receives a special investment of firm protoplasm, constituting it a digestive vesicle (Fig. 299); and these go through a sort of circulation within the cell-cavity. 414. The * contractile vesicles ' again, attain a much higher develop- ment in this group, and are sometimes in a connection with a network of canals chanelled-out in the ' ectosarc ' while their rhythmical action resembles that of the circulatory and respiratory apparatuses of higher animals. There is ample evidence, also, of the presence of a specially contractile modification of the protoplasmic substance, having the action (though not the structure) of muscular fibre; and the manner in which the movements of the active free-swimming Infusoria are directed, so as to avoid obstacles and find-out passages, seems to indicate that another portion of their protoplasmic substance must have to a certain degreee the special endowments which characterize the nervous systems of higher animals. Altogether, it may be said that in the Ciliate Infusoria the Life of the Single Cell finds its highest expression.* 1 The doctrine of the unicellular nature of the Infusoria has been a subject of keen controversy among Zoologists, from the time when it was first definitely put forward by Von Siebold(" Lehrbuch der vergleich. Anat.," Berlin, 1845) in opposition to the then paramount doctrine of Ehrenberg as to the complexity of their organization, which had as yet been called in question only by Dujardin "Hist. Nat. des Infusoires," Paris, 1841). Of late, however, there has been a decided convergence of opinion in the direction above indicated; which has been brought about in great degree by the contrast between the Protozoic simplicity of the reproductive and developmental processes in Infusoria, and the com- 2b THE MICROSCOPE AND ITS REVELATIONS. 415. Before proceeding to the description of the ciliate Infusoria, however, it will be of advantage to notice two smaller groups — the flagellate and the suctorial — which, on account of the peculiarities of their struc- ture and actions, are now ranked as distinct, and of whose 'unicellular' character there can be no reasonable doubt, since they are for the most part ' closed ' cells, scarcely distinguishable morphologically from those of Protophytes. 416. FLAGELLATA. — Our knowledge of this tribe has been greatly augmented in recent years, not only by the discovery of a great variety of new forms, but still more by the careful study of the life history of several among them. The Monads, properly so called,1 which are the smallest animals at present known, are its simplest representatives; but it also includes organisms of much greater complexity; and some of its composite forms have a very remarkable relation to Sponges (§ 508). The monas lens, long familiar to Microscopists as occurring in stagnant waters and infusions of decomposing organic matter, is a spheroidal particle of protoplasm, from l-2000th to l-5200th of an inch in diameter, inclosed in a delicate hyaline investment or 'ectosarc' and moving freely through the water by the lashing action of its slender flagellum, whose length is from three to five times the diameter of the body. Within the body may be seen a variable number of vacuoles; aud these are occasionally occu- pied by particles distinguishable by their color, which have been intro- duced as food. These seem to enter the body, not by any definite mouth (or permanent opening in the ectosarc), but through an aperture that forms itself in some part of the oral region near the base of the flagellum. In the smallest Monadince, neither nucleus nor contractile vesicle is dis- tinguishable; but in larger forms a nucleus can be clearly seen. The life-history of several simple Monadince, presenting themselves in infu- sions of decaying animal matter (a cod's head being found the most pro- ductive material), has been studied with admirable perseverance and thoroughness by Messrs. Dallinger and Drysdale, of whose important observations a general summary will now be given.2 417. The Monad-form most recently and completely studied by Mr. Dallinger — with all the advantages derived from trained experience, and under objectives of the highest quality and greatest magnifying power — is the Dallingeria Drysdali (Kent) represented in Plate xiii. Its normal shape, as seen in fig. 1, is a long oval, slightly constricted in the middle, and having a kind of pointed neck (a), from which proceds a flagellum about half as long again as the body. From the shoulder-like projections behind this (b, c) arise two other long and fine flagella, which are directed backwards. The sarcode body is clear, and apparently structureless, with minute vacuoles distributed through it; and in its hinder part a nucleus (d] is distinguishable. The extreme length of the body is seldom more than the 1-4, 000th of an inch, and is often less. This Monad swims with plexity of the like processes as seen even in the lowest of the Metazoa (§ 391) which has been specially and forcibly insisted on by Haeckel (" Zur Morphologic der Infusorien," Jenaische Zeitschr., Bd. vh., 1873). — An excellent summary of the whole discussion was given by Prof. Allman, in his Presidential Address to the Linnaean Society in 1875. 1 The Family monadina of Ehrenberg and Dujardin consists of an aggregate of forms now known to be of very dissimilar nature, many of them belonging to the Vegetable Kingdom. 2 See their successsive Papers in the " Monthly Microsc. Journ.," Vol. x. (1873). pp. 53, 245; Vol. xi. (1874), pp. 7, 69, 97; Vol. xii. (1874), p. 261; and Vol. xiii. (1875), p. 185;— and " Proceed. Roy. Soc.," Vol. xxvii. (1878), p. 332. MICS08COPIC FORMS OF ANIMAL LIFE. PLATE XIII. ./;•>:- ' ^t: LIFE-HISTORY OF FLAGELLATE INFUSORIUM Fig. 1. Normal form, showing three flagella, a, 6, c, and nucleus d. 2. Anterior flagellum, a, 6, double ; c, nucleus. 3. Fission commencing in nucleus c, and in anterior portion of body, a. 4. Fission more advanced, and showing itself also in posterior portion of body, a. 5. Fission still more advanced, both in nucleus, a, 6, and in body. 6. 7. Fission proceeding to completion. 8. Change to amoeboid condition, with single flagellum and granular band a. 9. Conjugation of this with free-swimming form. 10. 11. Stages of progressive fusion, terminating in production of still sac, 12 which afterwards opens and pours out spores, as at 13, 14, the progressive growth of which is shown in figs. 15-21. 28 THE MICROSCOPE AND ITS REVELATIONS. great rapidity; its movements, which are graceful and varied, being pro- duced by the action of the flagella, which can not only impel it in any direction, but can suddenly reverse its course or check it altogether. But besides tbis free-swimming movement, a very curious ' springing ' action is performed by this Monad when the decomposing organic matter of the infusion is breaking up, the process of disintegration being apparently assisted by it. The two posterior flagella anchor themselves and coil into a spiral, and the body then darts forwards and upwards, until the anchored flagella straighten out again, when the body falls forward to its horizon- tal position, to be again drawn back by the spiral coiling of the anchored flagella. This Monad multiplies by longitudinal fission; the first stage of which is the splitting of the anterior flagellum into two (fig. 2, a, b), and a movement of the nucleus (c) towards the centre. In the course of from thirty to sixty seconds the fission extends down the neck fig. 3, «; aline of division is also seen at the posterior end (c), and the nucleus (b) shows an incipient cleavage. In a few seconds the cleavage-line runs through the whole length of the body, the separation being widest posteriorly (fig. 4, a); and in from one to four minutes the cleavage becomes almost complete (fig. 5), the posterior part of the body, with the two halves (a and b) of the original nucleus, being now quite disconnected, though the anterior parts are still held together by a transverse band of sarcode, as seen in fig. 6. This soon narrows and elongates, as shown in fig. 7; and at last it gives way, setting the two bodies entirely free. The whole process of fission, from first to last, is completed in from four to seven minutes; and being repeated at intervals of a few minutes, this mode of multiplication produces a rapid increase in the number of the Monads. 418. Such fission does not, however, continue indefinitely; for certain individuals undergo a peculiar change, which shows itself first in the absorption of the two lateral flagella and the great development of the nucleus, and afterwards in the formation of a transverse granular band across the middle of the body (fig. 8, a). One of these altered forms swimming into a group in the ' springing ' state, within a few seconds firmly attaches itself to one of them, which at once unachors itself, and the two swim freely and vigorously about, as shown in fig. 9, generally for from thirty-five to forty-five minutes. Gradually, however, a i fusion' of the two bodies and of their respective nuclei takes place, the two trail- ing flagella of the ' springing ' form being drawn-in (fig. 10); and in a short time longer the two anterior flagella also disappear, and all trace of the separate bodies is lost, the nuclei vanish, and the resultant is an irregular amoeboid mass (fig. 11), which gradually acquires the smooth, distended, and ' stiir condition represented in fig. 12. This a cyst filled with reproductive particles of such extraordinary minuteness, that, when emitted from the ends of the cyst (fig. 13) after the lapse of four or five hours, they can only be distinguished under an amplification of 5,000 diameters, with perfect central illumination through an aperture in the diaphragm of from l-80th to the l-100th of an inch in diameter. Yet these particles, when continuously watched, are soon observed to enlarge and to undergo elongation (figs. 15-17); and within two hours after their emission from the sac, the anterior flagellum, and afterwards the two lateral flagella (fig. 18) can be distinguished. Slight movements then commence; the neck-like protrusion shows itself (fig. 19, a, b), and in about half an hour more the regular swimming action begins. About four hours after the escape of its germ from the sac, the Monad acquires MICROSCOPIC FORMS OF ANIMAL LIFE. 29 its characteristic form (fig. 20), though still only one-half the length of its parent; but this it attains (passing through the stage shown in fig. 21) in another hour, and the process of multiplication by fission, as already described, commences very soon afterwards. — There can be no reasonable doubt that the * conjugation ' of two individuals, followed by the trans- formation of their fused bodies into a sac filled with reproductive germs, is to be regarded (as in protophytes) in the light of a true generative pro- cess; and it is interesting to observe the indication of sexual distinction here marked by the different states of the two conjugating individuals. — There is every reason to believe that the entire life-cycle of this Monad has thus been elucidated; and it will now be sufficient to notice the principal diversities observed by Messrs. Dallinger and Drysdale in the life-cycles of the other Monadine forms which they have studied. 419. Their simple uniflagellate Monad (Nonas Dallingeri, Kent), having an ovate form with a long diameter never exceeding l-4000th of an inch, and advancing slowly with a straight, uniform motion like that of Monas termo, differs from the preceding in its mode of multiplication; for this takes place, not by duplicative fission, but by the breaking-up of the sarcodic substance (as in the production of f swarm spores' by Proto- phytes) into from thirty to sixty segments, which, at first lying closely packed together, make their escape as free-swimming Monads, each pro- vided with its flagellum. Conjugation, in this type, occurs between the ordinary forms and certain individuals distinguished between their some- what larger size, and by the granular aspect of their sarcode towards the flagellate end; and there is reason to think that the latter have never un- dergone the segmentation by which the former have been multiplied. The smaller are absorbed, as it were, into the larger; and the latter passes after a time into the encysted state, corresponding in its subsequent his- tory with the preceding type. — The bi-flagellate or 'acorn' Monad of the same observers (identified by Kent with the Polytoma uvella of Ehren- berg) presents some remarkable peculiarities in its mode of reproduction. Its binary fission extends only to the protoplasmic substance of its body, leaving its envelope entire; and by a repetition of the process, as many as 10 segments, each attaining the likeness of the parent, are seen thus inclosed, their flagella protruding through the general investment. This compound state being supposed by Ehrenberg to be the normal one, he named it accordingly. But the parent-cyst soon bursts, and sets free the contained ' macro-spores,' which swim about freely, and soon attain the size of the parent. Again, the posterior part of the body of certain individuals shows an accumulation of granular protoplasm, giving to that region a roughened acorn-cup-like aspect; the bursting of the projection, while the creature is actively swimming through the water, sets free a multitude of shapeless granular fragments, within each of which a minute bacterium-like corpuscle is developed; and this, on its release, acquires in a few hours the size and form of the original monad. This process seems analogous to the development of ' micro-spores ' among Protophytes, by the direct breaking-up of the protoplasm. It is, like the previous process, non-sexual or gonidial; the true generative process consisting here, as in the preceding cases, in the ( conjugation ' of two in- dividuals, with the usual results. 420. A Cercomonas (C. typicus, Kent), characterized by the posses- sion of a flagellum at each end, was found to multiply, during eight days (and nights) of continuous observation, by transverse duplicative subdivi- sion alone. But certain individuals then exhibited a remarkable change, 30 THE MICROSCOPE AND ITS REVELATIONS. becoming amoeboid and less active; and when two of these came into con- tact, they underwent a complete fusion, the product of which was a globular cyst, with a very definite investment, filled with reproductive germs. — The ( springing Monad' of the same observers (Heteromita ros- trata, Kent) is of a long ovate form, with an average length of about l-3000th of an inch. From its narrower extremity a sort of beak arises, from which proceeds a fine flagellum about half as long again as the body; and at a little distance behind this, another and longer flagellum arises, with which the Monad anchors itself to the covering-glass, constantly springing backwards and forwards by its recurrent coil and uncoil. A nucleus shows itself near the rounded posterior end of the body. This Monad multiplies by longitudinal fission, commencing at the beaked end, and completed in six or seven minutes; and the process may be repeated continuously for many days. Among enormous numbers, there are a few distinguishable from the others by a slight excess of size, and by the power to swim freely; these become ' still ' — for a time amoeboid — then round; a small cone of sarcode pushes out, dividing and increasing into another pair of flagella; the disk splits, each part becomes possessed of a nuclear body, and two well-formed free-swimming Monads are set free. These conjugate with individuals of the ordinary form which have just undergone fission, the nuclei of the two approximating to each other; a complete fusion of sarcode and nuclei takes place; the body, at first motile, comes to rest, assumes a triangular form, and loses its flagella; it then becomes clear and distended, and emits its contained reproductive granules at the angles. — The ' hooked Monad' (Heteromita uncinata, Kent) is another bi-flagellate form, usually ovate with one end pointed, and from l-3000th to l-4000th of an inch in length; being distinguished from the preceding by the peculiar character of its flagella, of whicn the one that projects forward is not more than half the length of the body, and is permanently hooked, while the other, whose length is aoout twice that of the body, is directed backwards, flowing in graceful curves. Its motion consists of a succession of springs or jerks rapidly following each other, which seems produced by the action of the hooked flagellum. Multiplication takes place by transverse fission, and continues uninter- ruptedly for several days. A difference then becomes perceptible between larger and smaller individuals; the former being further distinguished by the presence of what seems to be a contractile vesicle in the anterior part of the body. Conjugation occurs between one of the larger and one of the smaller forms, the latter being, as it were, absorbed into the body of the larger; and the resulting product is a spherical cyst, which soon begins to exhibit a cleavage-process in its interior. This continues until the whole of its sarcodic substance is subdivided into minute oval particles, which are set free by the rupture of the cyst, and of which each is usually fur- nished with a single flagellum, by whose lashing movement it swims freely. These germs speedily attain the size and form of the parent, and then begin to multiply by transverse fission — thus completing the ' gene- tic ' cycle. 421. The ' calycine Monad ' of the same observers (Tetramitus rostra- tus, Perty), has a length of from l-900th to l-1000th of an inch, and a compressed body tapering backwards to a point. Its four flagella (which constitute its generic distinction) arise nearly together from the flattened front of the body; and its swimming movement is a graceful gliding. Xear the base of the flagella is a pair of contractile vesicles; and further behind is a large nucleus. Multiplication takes place by longitudinal MICROSCOPIC FOKMS OF ANIMAL LIFE. 31 fission, which is preceded by a change to a semi-amoeboid state. This gives place to a more regular pear-like form, the four flagella issuing from the large end; and the fission commences at their base, two pairs being separated by the cleavage-plane. The nucleus also undergoes cleavage, and its two halves are carried apart by the backward extension of the cleavage. The two half-bodies at last remain connected only by their hinder prolongations, which speedily give way, and set them free. Each, however, has, as yet, only twofiagella; but these speedily fix themselves by their free extremities, undergo a rapid vibratory movement, and in the course of about two minutes split themselves from end to end. A still more complete change* into the amoeboid condition, in which the creature not only moves, but also feeds, like an Amoeba (devouring all the living and dead Bacteria in its neighborhood), occurs previously to ' conjugation;' and this takes place between two of the amoeboid forms, which begin to blend into one another almost immediately upon coming into contact. The conjugated bodies, however, swim freely about for a time, the two sets of flagella apparently acting in concert. But by the end of about eighteen hours, the fusion of the bodies and nuclei is complete, the fla- gella are retracted, and a spherical distended sac is then formed, which, in a few hours more, without any violent splitting or breaking up, sets free innumerable masses of reproductive particles. These, under a mag- nifying power of 2,500 diameters, can be just recognized as oval granules, which rapidly develop themselves into the likeness of their parents, and in their return multiply by duplicative fission, — thus completing the ' genetic ' cycle. 422. One of the most important researches thus ably prosecuted by Messrs. Dallinger and Drysdale, has reference to the Temperatures re- spectively endurable by the adult or developed forms of these Monads, and by their reproductive germs. A large number of experiments upon the several *forms now described, indubitably led to the conclusion that all the adult forms, as well as all those which had reached a stage of development in which they can be distinguished from the reproductive granules, are utterly destroyed by a temperature of 150° Fahr. But, on the other hand, the reproductive granules emitted from the cysts that originate in ' conjugation ' were found capable of sustaining a fluid heat of 220°, and a dry heat of about 30° more, — those of the Cercomonad surviving exposure to a dry heat of 300° Fahr. This is a fact of the high- est interest in its bearing on the question of ' spontaneous generation ' or Abiogenesis; since it shows (1) that germs capable of surviving desic- cation may be everywhere diffused through the air, and may, on account of their extreme minuteness (as they certainly do not exceed 1-200, 000th of an inch in diameter), altogether escape the most careful scrutiny and the most thorough cleansing processes; while (2) their extraordinary power of resisting heat will prevent these germs from, being killed either by boiling, or by dry-heating up to even 300° Fahr.1 423. The structural resemblance of these simple Flagellate Infusoria to the ' Monads ' of Volvox and its allies (§ 237), is so close that no other than physiological reasons can be assigned for separating them. Whilst the VolvocinecB grow and multiply under conditions which seem to jus- tify our regarding them as members of the Vegetable Kingdom (§ 220), 1 Descriptions of the special apparatus used by Messrs. Dallinger and Drysdale in their researches will be found in "Monthly Micr. Journ.," Vol. xi. (1874), p. 97; ibid., Vol. xv. (1876), p. 165; and "Proceed. Roy. Soc.," Vol. xxrii. (1878), p. 343. 32 THE MICROSCOPE AND ITS REVELATIONS. Fig. 295. the 'flagellated ' agree with the * ciliated' Infusoria in ordinarily drawing their nutriment from organic compounds; and it seems clear that, although unpossessed of a mouth, they can introduce solid food-particles into the interior of their bodies. It is, however, not a little remarkable that (according to the statement of Messrs. Dallinger and Drysdale)1 these Flagellata — like Bacteria and other forms referred to the group of Fungi — can be cultivated in Cohn's ' nutritive fluid ' (§ 303, note), which consists only of tartrate of ammonia and mineral salts, without any al- buminous matter. 424. A large series of more complex forms of Flagellate Infusoria has been recently brought to our knowledge by the researches of the late Prof. James-Clark (U. S.),2 followed by those of Stein and Saville Kent. In some of these, a sort of collar-like extension of what appears to be the sarcodic ectosarc, proceeds from the anterior extremity of the body (Fig. 295, cl), forming a kind of funnel, from the bottom of which the flagellum arises; and by its vibrations a current is produced within the funnel, which brings down food-particles to the ' oral disk ' that surrounds its origin, where the ectosarc seems softer than that which envelops the rest of the body. Towards the base of the collar, a nucleus (n) is seen; while, near the posterior termination of the body, is a single or double contractile vesicle cv. The body is attached by a pedicle proceeding from its posterior extremity, which also seems to be a prolongation of the ectosarc. — These Ani- malcules multiply by longitudinal fission; and this, in some cases (as in the genus Monosiga], proceeds to the extent of a complete separa- tion of the two bodies, which henceforth, as in the ordinary Monadina, live quite inde- pendently of each other. But in other forms, as Codosiga, the fission does not extend through the pedicel ; and the twin bodies be- ing thus held together at their bases, and themselves undergoing duplicative fission, clusters are produced which spring from com- mon pedicels (Fig. 296). And by the exten- sion of the division down the pedicels, them- selves, composite arborescent fabrics, like those of Zoophytes, are produced. 425. In an another group, a structureless and very transparent horny calyx, closely re- sembling in miniature the polype-cell of a Campanularia (Plate xx.), forms itself around the Dody of the Monad, which can retract itself into the bottom of it. And in the genus Salpingceca both calyx and collar are present. In some forms of this group, mul- tiplication seems to take place, not by fis- sion, but by gemmation; and, as among Hydroia Polypes, the gemma may either detach themselves and live inde- pendently, or may remain in connection with their parent-stocks, form- 1 " Monthly Microscopical Journal," Vol. xiii. (1875\ p. 190. 2 See his Memoirs in "Ann. Nat. Hist.," Ser. 3, Vol. xviii. (1866); ibid., Sen 4, Vol. i. (1868); Vol. vii. (1871); and Vol. ix. (1872). Single zooid of Codosiga umbel- lata:—cl, collar; n, nucleus; cv, double contractile vesicle. MICROSCOPIC FOKMS OF ANIMAL LIFE. 33 ing composite fabrics, in some of which the calyces follow one another in linear series, whilst in others they take on a ramifying arrangement. While some of these composite organisms are sedentary, others, as Dino- bryon, are free-swimming. 426. Two solitary Flagellate forms, Anthopliysa and Anisonema, may be specially noticed as presenting several interesting points of resem- blance to the peculiar type next to be described; the most noticeable being the presence of a distinct mouth, and the possession of two different motor organs — one a comparatively stout and stiff bristle of uniform diameter throughout, which moves by occasional jerks; and the other a very delicate tapering flagellum, which is in constant vibratory motion. If, as appears from the recent observations of Biitschli, the well-known Astasia — of which one species has a blood-red color, and sometimes multiplies to such an extent as to tinge with it the water of the ponds it inhabits — has Fig. 296. osiga umbellata .-—colony-stock, springing from single pedicel tripartitely branched. a true mouth for the reception of its food, it must be regarded as an Animal, and separated from the Euglena (with which it has been gener- ally associated), the latter being pretty certainly a Plant belonging to the same group as Volvox. 1 427. There can be no longer any doubt that the well-known Nocti- luca miliaris — to which is attributable the diffused luminosity that fre- quently presents itself in British seas — is to* be regarded as a gigantic type of the ' unicellular ' Flagellata. This animal, which is of spheroidal form, and has an average diameter of about 160th of an inch, is just large enough to be discerned by the naked eye when the water in which it may be swimming is contained in a glass jar held up to the light; and its tail- like appendage, whose length about equals its own diameter, and which serves as an instrument of locomotion, may be discerned with a hand- 1 See the Memoir by Prof. Biitschli, in "Zeitschrift f. Wissensch. Zool.." Bd. xxx.; of which an abridgment (with Plate) is given in " Quart. Journ. Microsc. Sci.," Vol. xix. (1879), p. 63. 3 34: THE MICROSCOPE AND ITS REVELATIONS. magnifier. The form of Noctiluca is nearly that of a sphere, so com- pressed that while on one aspect (Fig. 297, A) its outline, when projected on a plane, is nearly circular, it is irregularly oval in the aspect (B) at right angles to this. Along one side of this body is a meridional groove, resembling that of a peach; and this leads'at one end into a deep depres- sion of the surface, a, termed the atrium, from the shallower commence- ment of which the tentacle, d* originates, whilst it deepens down at the base of the tentacle to the mouth, e. Along the opposite meridian there extends a slightly elevated ridge, c, which commences with the ap- pearance of a bifurcation at the end of the atrium farthest from the ten- tacle; this is of a firmer consistence than the rest of the body, and has somewhat the appearance of a rod imbedded in its walls. The mouth opens into a short oesophagus, which leads directly down to the great central protoplasmic mass; on the side of this canal farthest from the Fig. 297. , . jht angles to it:— a. entrance to atrium; 6, atrium: c, superficial ridge; d, tentacle: e, mouth leading to oesophagus within which are seem the flagellum springing from its base, and the tooth-like process proJ3Cting into it from above; /, broad process from the central protoplasmic mass, proceeding to superficial ridge; '' , #, are represented as thus elongated in preparation for another subdivision, whilst the ovary «, «, has begun, as it were, to unroll itself, and to break up into fragments which are connected by the tube m. It is in this condition that the object of the conjugation appears to be effected, by the passage of the seminal capsules of each individual, previously to their complete matura- tion, into the body of the other. In Fig. 4 is shown the condition of a Paramedum ten hours after the conclusion of the conjugation; the ovary has here completely broken up into separate granular masses, of 52 THE MICROSCOPE AND ITS REVELATIONS. which some a, a, remain unchanged, whilst others, o, o, o, o, either two, four, or eight in number, are converted into ovules that appear to be fertilized by the escape of the spermatozoa from the seminal capsules, these being now seen in process of withering. Finally, in fig. 5, which represents a Paramecium three days after the completion of the conjuga- tion, are seen four complete ova, 0, o, o, o, within the connecting tube m, m ; whilst the seminal capsules have now altogether disappeared. In figs. 13-18 are seen the successive stages of the development of the ovule, which seems at first (fig. 13) to consist of a germ-cell having within it a secondary cell containing minute granules, which is to become the ' vitelline vesicle.' This secondary cell augments in size, and becomes more and more opaque from the increase of its granular contents (figs. 14, 15, 16), forming the 'vitellus' or yolk; in the midst of which is seen the clear ( germinal vesicle/ which shows on its wall, as the ovule approaches maturity, the ' germinal spot' (fig. 17). The germinal vesicle is subsequently concealed (fig. 18) by the increase in the quantity and opacity of the vitelline granules. The fertilized ova seem to be ex- pelled by the gradual shortening of the tube that contains them; and this shortening also brings together the scattered fragments of the-gran- ular substance of the original ovarium, so as to form a mass resembling that shown in fig. 1, a, by the evolution of which, after the same fashion, another brood of ova may be produced. 443. Now there can be no doubt as to the occurrence of ' conjuga- tion ' among Ciliated Infusoria; and this not only in the free-swimming, but also in the attached forms, as Stentor (Plate xiv., fig. 21). In Vorticella, according to several recent observers, what has been regarded as gemmiparous multiplication — the putting-f orth of a bud from the base of the body — is really the conjugation of a small individual in the free- swimming stage with a fully -developed fixed individual, with whose body its own becomes fused. But it is doubtful whether such conjuga- tion has any reference to the encysting process. According to Biitschli and Engelmann, the conjugating process results in the breaking up of the nucleus and (so-called) nucleolus of the conjugating individuals; these individuals separate again, and after the expulsion of the broken- up nuclear structures, the characteristic nucleus and nucleolus are reformed. The same excellent observers adduce strong grounds for distrusting Balbiani's assignment of sexual characters to the nucleus and nucleolus. For although a striation may be observed on the surface of the latter, no one has witnessed its subdivision into spermatozoidal filaments. And if embryos are really produced at the expense of the nucleus, what Balbiani described as sexual ova are really non-sexual gemmules, each consisting (like the zoospore of Protophytes) of a seg- ment of the nucleus surrounded by an envelope of protoplasm. — There is still much uncertainty in regard to the embryonic forms of Ciliate Infu- soria; some eminent observers asserting that the 'gemmule ' in the first instance, besides forming a cilia-wreath, puts forth suctorial appendages (Plate xiv., fig. 19, A, B, c), by means of which it imbibes nourishment until the formation of its mouth permits it to obtain its supplies in the ordinary way; whilst others maintain these acinetiform bodies to be parasites, which even imbed themselves in the substance of the Infusoria they infest.1 1 There can be no doubt that Stein was wrong in his original doctrine that the fully-developed Acinetina are only transition-stages in the development of Vorti- cellina and other Ciliated Infusoria. But the balance of evidence seems to the MICROSCOPIC FORMS OF ANIMAL LIFE. 53 444. It is obvious that no Classification of Infusoria can be of any permanent value, until it shall have been ascertained by the study of their entire life-history, what are to be accounted really distinct forms. And the differences between them, consisting chiefly in the shape of their bodies, the disposition of their cilia, the possession of other locomotive appendages, the position of the mouth, the presence of a distinct anal orifice, and the like, are matters of such trivial importance as compared with those leading features of their structure and physiology on which we have been dwelling, that it does not seem desirable to attempt in this place to give any detailed account of them. The life-history of the ciliate Infusoria is a subject pre-eminently worthy of the attention of Mitroscopists, who can scarcely be better employed than in tracing out the sequence of its phenomena, with the same care and assiduity as have been displayed by Messrs. Dallinger and Drysdale in the study of the Monadina. — " In .pursuing our researches," say these excellent observers, " we have become practically convinced of what we have theoretically assumed — the absolute necessity for prolonged and patient observation of the same forms. Two observers, independently of each other, examining the same Monad, if their inquiries were not sufficiently prolonged, might, with the utmost truthfulness of interpretation, assert opposite modes of development. Competent optical means, careful interpretation, close observation, and time, are alone capable of solving the problem." SECTION II. — ROTIFER A, OR WHEEL- ANIMALCULES. 445. We now coine to that higher group of Animalcules, which, in point of complexity of organization, is as far removed from the preced- ing, as Mosses are from the simplest Protophytes; the only point of real resemblance between the two groups, in fact, being the minuteness of size which is common to both, and which was long the obstacle to the recognition of the comparatively elevated character of the Rotifera, as it still is to the precise determination of certain points of their structure. Some of the Wheel- Animalcules are inhabitants of salt water only; but by far the larger proportion are found in collections of fresh water, and rather in such as are free from actively decomposing matter, than in those which contain organic substances in a putrescent state. Hence when they present themselves in Vegetable infusions, it is usually after that offensive condition which is favorable to the development of many of the Infusoria has passed-away ; and they are consequently to be looked-for after the disappearance of many successions (it may be) of Animalcules of inferior organization. Rotifera are more abundantly developed in liquids which have been long and freely exposed to the open air, than in such as have been kept under shelter; certain kinds, for example, are to be met with in the little pools left after rain in the hollows of the lead with which the tops of houses are partly covered ; and they are occasionally found in enormous numbers in cisterns which are not beneath roofs or otherwise covered over.1 They are not, how- ever, absolutely confined to collections of liquid : for there are a few species which can maintain their existence in damp earth ; the common Rotifer is occasionally found in the interior of the leaf-celte of Sphagnum (§ 339); and at least two species QiNotommata also are known to be para- writer to be in favor of his later statement, that the bodies figured in PI. xrv., fig. 19, are really Infusorian embryos, and not parasitic Acinetae. 1 See a remarkable instance of this in vol. i., p. 232, note. 54 THE MICROSCOPE AND ITS REVELATIONS. si tic, the one in the large cells of Vauclieria (§ 219), and another in the sphere ofVolvox (§ 236). — The Wheel-like organs from which the class derives its designation, are most characteristically seen in the common Rotifer (Fig. 310), where they consist of two disk-like lobes or projections of the body, whose margins are fringed with long cilia; and it is the uninterrupted succession of strokes given by these cilia, each row of which nearly returns (as it were) into itself, that gives rise by an optical illusion to the notion of ' wheels.' This arrangement, however, is by no means universal; in fact, it obtains in only a small proportion of the group ; and by far the more general plan is that seen in Fig. 309, in FIG. 309. FIG. 310. Rotifer vulgaris, as seen at A with the wheels drawn-in, and at B with the wheels expanded : — a, mouth; b, eye-spots; c, wheels; d, calcar (an- tenna?); e, jaws and teeth; /, alimentary canal; Brachionus pala. g, glandular ( ?) mass enclosing it; h, longitudinal muscles; i, i, tubes of water-vascular system; fc, young animal ; I, cloaca. which the cilia form one continuous line across the body, being disposed upon the sinuous edges of certain lobes or projections which are borne upon its anterior portion. Some of the chief departures from this plan will be noticed hereafter (§453). 446. The great transparence of the Eotifera permits their general structure to be easily recognized. They have usually an elongated form, similai on the two sides; but this rarely exhibits any traces of segmental division. The body is covered with a double envelope, both layers of which are extremely thin and flexible in some species, whilst in others the outer one seems to possess a horny consistence. In the former case the whole integument is drawn together in a wrinkled manner when the MICROSCOPIC FORMS OF ANIMAL LIFE. 55 body is shortened ; in some of the latter the sheath has the form of a polype-cell, and the body lies loosely in it, the inner layer of the integu- ment being separated from the outer by a considerable space (Fig. 312); whilst in others the envelope or lorfca is tightly fitted to the body, and strongly resembles the horny casing of an Insect or the shell of a Crab, except that it is not jointed, and does not extend over the head and tail, which can be projected from the openings at its extremities, or com- pletely drawn within it for protection (Fig. 313). In those Kotifera in which the flexibility of the body is not interfered with by the consolida- tion of the external integument, we usually find it capable of great varia- tion in shape, the elongated form being occasionally exchanged for an almost globular one, as is seen especially when the animals are suffering from deficiency of water; whilst by alternating movements of contraction and extension, they can make their way over solid surfaces, after the manner of a Worm or a Leech, with considerable activity, — some even of the loncated species being rendered capable of this kind of progression by the contractility of the head and tail. All these, too, can swim readily through the water by the action of their cilia ; and there are some species which are limited to the latter mode of progression. The greater number have an organ of attachment at the posterior extremity of the body, which is usually prolonged into a tail, by which they can affix themselves to any solid object; and this is their ordinary position, when keeping their ' wheels' in action for a supply of food or of water; they have no difficulty, however, in letting-go their hold and moving through the water in search of a new attachment, and may therefore be considered as perfectly free. The sessile species, in their adult stage, on the other hand, remain attached by the posterior extremity to the spot on which they have at first fixed themselves ; and their cilia are conse- quently employed for no other purpose than that of creating currents in the surrounding water. 447. In considering the internal structure of Rotifera, we shall take as its type the arrangement which it presents in the Rotifer vulgaris (Fig. 310); and specify the principal variations exhibited elsewhere. The body of this animal, when fully extended, possesses greater length in proportion to its diameter than that of most others of its class; and the tail is composed of three joints or segments, which are capable of being drawn up, one within another, like the sliding tubes of a telescope, each having a pair of prongs or -points at its extremity. Within the ex- ternal integument of the body are seen a set of longitudinal muscular bands (Ji), which serve to draw the two extremities towards each other; and these are crossed by a set of transverse annular bands, which also are probably muscular, and serve to diminish the diameter of the body, and thus to increase its length. Between the wheels is a prominence bearing two red spots (b), and having the mouth (a) at its extremity; these red spots differ altogether from those common in Infusoria and Protophyta, each having a minute highly-refracting spherical lens set in red pigment, and being clearly a rudimentary eye; and the prominence that bears them may be considered, therefore, as a true head, notwithstanding that it is not clearly distinguishable from the body. This head also bears upon its under surface a projecting spur-like organ (d), which was thought by Prof. Ehrenberg to be a siphon for the admission of water to the cavity of the body for the purpose of respiration; this, however, is certainly not the case, the 'spur' being imperf orate at its extremity; and there seems much more probability in the idea of Dujardin, that it represents the 56 THE MICROSCOPE AND ITS REVELATIONS. FIG. 311. antenncB or palpi of higher Articulata, the single organ being replaced in many Rotifera by a pair, of which each is furnished at its extremity with a brush-like tuft of hairs that can be retracted into the tube. The esophagus, which is narrow in the Rotifer, but is dilated into a crop in Stephanoceros (Fig. 312) and in some other genera, leads to the masti- cating apparatus (Fig. 310, e), which in these animals is placed far behind the mouth, and in close proximity to the stomach. — The Masti- cating apparatus has been made the subject of attentive study by Mr. P. H. Grosse; who has given an elaborate account of the various types of form which it presents in the several subdivisions of the group. 1 The following description of one of the more complicated will serve our present purpose. The various movable parts are included in a muscular bulb, termed the mastax (Fig. 311, a), which intervenes between the buccal funnel (m) and the oesophagus (p). The mastax includes a pair of organs, which, from the resemblance of their action to that of hammers working on an anvil, may be called mallei, and a third, still more complex, termed the incus. Each malleus consists of two prin- cipal parts placed nearly at right angles to each other, the manu- Irium (c), and the uncus (e) ; these are articulated to one another by a sort of hinge-joint. The former, as its name imports, serves the purpose in some degree of a han- dle; and it is the latter which is the instrument for crushing and dividing the food. This is done by means of the finger-like pro- cesses with which it is furnished at the edge where it meets its fel- low; these being five or six in number, set parallel to each other like the teeth of a comb. The in- cus also consists of distinct arti- culated portions, namely two stout rami (a) resting on what seems a slender footstalk (h) termed the fulcrum ; when viewed laterally, however, the fulcrum is seen to be a thin plate, having the rami so jointed to one edge of it that they can open and close like a pair of shears. The uncus of each malleus falls into the concavity of its respective ramus, and is connected with it by a stout triangular muscle (i), which is seen passing from the hollow of the ramus to the under surface of the uncus. It is difficult to say with cer- tainty what is the substance of which these firm structures are composed; it is not affected by solution of potass, but is instantly dissolved without effervescence by the mineral acids and by acetic acid. Besides the mus- cles already described, a thick band (/) embraces the upper and outer angle of the articulation of the malleus; and is inserted in the adjacent wall of the mastax; and a semi-crescentic band (k) is inserted by its broad end into the inferior and basal part of the uncus, and by its slender end into the middle of the inner side of the manubrium; the former of these Masticating Apparatus of Euchlanis deflexa: —a, Mastax; c, manubrium, and e, uncus, of Malleus; g, rami, and h, fulcrum, of Incus; i, mus- cle connecting ramus and uncus; j, mucles pass- ing from malleus to mastax: fc, muscle connecting uncus and manubrium; m, buccal funnel; n, saliv- ary glands; p, oesophagus. " Philosophical Transactions," 1856, p. 419. MICROSCOPIC FORMS OF ANIMAL LIFE. 57 may be considered as an extensor, and the latter as a flexor, of the mal- leus. By these and other muscles which cannot be so clearly distin- guished, the two tinci are made to approach and recede by a perpendicular motion on the hinge-joint, so that their opposing faces come into contact, and their teeth bruise down the particles of food; but at the same time they are carried apart and approximated laterally by the movement of the free extremities of the manubria. The rami of the incus also open and shut with the working of the mallei: and by the conjoint action of the whole, the food is effectually comminuted in its passage downwards.1 448. The Alimentary Canal, which lies loose in the ' general cavity of the body/ is sometimes a simple tube, passing without enlargement or con- striction from the masticating apparatus to the anal orifice at the posterior part of the body; whilst in other instances there is a marked distinction between the stomach and intestinal tube, the former being a large globu- lar dilatation immediately below the jaws, whilst the latter is cylindrical and comparatively small. The alimentary canal of Rotifer (Fig. 310) most resembles the first of these types, but presents a dilatation (I) close to the anal orifice, which may be considered as a cloaca; that of BracM- onus (Fig. 309) is rather formed upon the second. Connected with the alimentary canal are various glandular appendages, more or less devel- oped; sometimes clustering round its walls as a mass of separate follicles, which seems to be the condition of the glandular investment (g) of the alimentary canal in Rotifer ; in other cases having the form of caecal tubuli. Some of these open into the stomach close to the termination of the oesophagus, and have been supposed to be salivary or pancreatic in their character, whilst others, which discharge their secretion into the intestinal tube, have been regarded, and probably with correctness, as the rudiment of a liver. — In the genus Asplanchna (Gosse), there is a wide departure from the ordinary Rotifer type; as the species belonging to it have neither intestine nor anus. The stomach consists of a large bag at the end of the gullet, about which, when the animals are quiet, the ovary is bent in a horseshoe form. The indigestible matters are ejected through the mouth. The curious absence of any digestive apparatus in the males of this group, will be presently noticed (§ 450). 2 449. There does not appear to be any special Circulating apparatus in these animals; but the fluid which is contained in the perivisceral cavity is probably to be regarded as nutritive in its character; and its aeration is provided for by a peculiar apparatus, which seems to be a rudimentary iorm of the ' water- vascular system/ that attains a high development in the class of Worms. On either side of the body there is usually to be observed a long flexuous tube (Fig. 309), which extends from a contrac- tile vessel common to both and opening into the cloaca (Fig. 310, i, i), towards the anterior region of the body, where it frequently subdivides into branches, one of which may arch over towards its opposite sides, and inosculate with a corresponding branch from its tube. Attached to each of these tubes are a number of peculiar organs (usually from two to eight on each side), in which a trembling movement is seen, very like that of a flickering flame; these appear to be pear-shaped sacs, attached by hollow stalks to the main tube, and each having a flagelliform cilium in 1 See also the description of the mastax of Melicerta ringens and Conochilus by Mr. Bedwell in " Journ. of Roy. Micr. Soc.," Vol. i. (1878), p. 176. 2 See Brightwell in "Ann. Nat. His.," Ser. 2, Vol. ii. (1848), p. 153; Dalrymple in "Philos. Trans." (1849), p. 339; and Gosse in "Ann. Nat. Hist.," Ser. 2, Vols. iii. (1848), p. 518; vi. (1850), p. 18; and viii. (1851), p. 198. 58 THE MICROSCOPE AND ITS KEVELATIOISTS. its interior, that is attached by one extremity to the interior of the sac, and vibrates with a quick undulatory motion in its cavity; and there can be little doubt that their function is to keep up a constant movement in the contents of the aquiferous tubes, whereby fresh water may be contin- ually introduced from without for the aeration of the fluids of the body. l The Nervous system is represented by only a single ganglionic body (sometimes bilobed, however), which lies at one side of the oesophagus, in near proximity to the eye-spots, the spur-like organ, and the ciliated pit, and has also, in some Rotifers, an auditory vesicle attached to it. No nerve-trunks proceeding to the muscular bands have as yet been cer- tainly distinguished. 450. The Keproduction of the Rotifera has not yet been completely elucidated. Although they were affirmed by Prof. Ehrenberg to be herm- aphrodite, yet the existence of distinct sexes has been detected in so many genera (for the most part by Mr. Gossea), that it may fairly be presumed to be the general fact. The male is inferior in size to the fe- male; and sometimes differs so much in organization, that it would not be recognized as belonging to the same species, if the copulative act had not been witnessed. In all the cases yet known, as in the Asplanclma of which the separate male was the first discovered, there is an absolute and universal atrophy of the digestive system; neither mastax, jaws, oesopha- gus, stomach, nor intestines being discoverable in any male; no other organs, in fact, being fully developed, than those of generation. The male would appear, therefore, quite unfit to obtain aliment for itself; and its existence is probably a very brief one, being continued only so long as the store of nutriment supplied by the egg remains unexhausted. In the remarkable six-limbed Rotifer discovered by Dr. Hudson, and named by him Pedalion mira, the virgin female was found to lay female eggs during the greater part of the year, while male eggs, which are not found in the same individuals, " are half the size of the female ones, and are carried in clusters of often a score at a time." The males are very small in comparison with the females, and are very short-lived, sometimes dying within an hour. In Rotifer, however, as in a large proportion of the group, no males have yet been discovered, probably because they are produced only at certain times. The female organ consists of a single ovarian sac, which frequently occupies a large part of the cavity of the body, and opens at its lower end by a narrow orifice into the cloaca. — Although the number of eggs in these animals is so small, yet the rapid- ity with which the whole process of their development and maturation is accomplished, renders the multiplication of the race very rapid. The egg of the Hi/datina is extruded from the cloaca within a few hours after the first rudiment of it is visible; and within twelve hours more the shell bursts, and the young animal comes forth. Three or four eggs being deposited at once, it was calculated by Prof. Ehrenberg that nearly seventeen millions may be produced within twenty-four days from a sin- gle individual. In Rotifer and several other genera, the development of the embryo takes-place whilst the egg is yet retained within the body of the parent (Fig. 310, k), and the young are extruded alive; whilst in 1 See Prof. Huxley's account of these organs in his description of Lacinularia socialis, "Transact, of Microsc. Soc.," N.S., Vol. i. (1853), p. 1. 2 " Philosophical Transactions," 1853, p. 313. Seealso Dr. Hudson in ;' Monthly Microsc. Journ.," Vol. xiii. (1875), p. 45. 3 " Monthly Microsc. Journ.," Vol. viii. (1872), p. 209; and " Quart. Journ. Mic. Sci.," Vol. xii. (1872), p. 333. MICROSCOPIC FORMS OF ANIMAL LIFE. 59 some other instances the eggs, after their extrusion, remain attached to the posterior extremity of the body (Fig. 309), until the young are set free. The transparence of the egg-membrane, and also of the tissues, of the parent Rotifer, allows the process of development to be watched, even when the egg is retained within the body; and it is curious to ob- serve, at a very early period, not merely the red eye spot of the embryo, but also a distinct ciliary movement. " In general it would seem that whether the rupture of the egg-membrane takes- place before or after the egg has left the body, the germinal mass within it is developed at once into the form of the young animal, which usually resembles that of its parent; 110 preliminary metamorphosis being gone through, nor any parts developed which are not to be permanent. In Floscularia ornata, however, the young leave the eggs in the shape of little maggots, from one end of which a tuft of cilia soon appears. The form changes in a few hours, the ciliated end becoming lobed, and the body rounded. The foot is developed later. 1 — In the curious Notommata Werneckii, which is found parasitic in the reproductive capsules of Vauclieria (§ 249), the young animal has the general organization of the free-swimming Eotifers, and leads a similarly active life; but when its eggs are becoming mature, it finds its way into one of these capsules and there undergoes a remark- able deformation, its characteristic organs disappearing, and its body be- coming a large egg-sac, which seems to be nourished by absorption. 9 451. Even in those species which usually hatch their eggs within their bodies, a different set of Ova is occasionally developed, which are furnished with a thick glutinous investment; these, which are extruded entire, and are laid one upon another, so as at last to form masses of considerable size in proportion to the bulk of the animals, seem not to be destined to come so early to maturity, but very probably remain dormant during the whole winter season, so as to produce a new brood in the spring. These ' winter-eggs ' are inferred by Prof. Huxley, from the history of their development, to be really gemmce produced by a non- sexual operation; while the bodies ordinarily known as ova, he considers to be true generative products. Prof. Cchn, however, states that he has ascertained, by direct experiment upon those species in which the sexes are distinct, that the bodies commonly termed ' ova' (Figs. 309, 310) are really internal gemmce, since they are reproduced, through many succes- sions, without any sexual process, just like the external gemmae of Hydra (§ 515), or the internal gemmae of Entomostraca (§ 609) and Aphides (§ 643); whilst the ' winter-eggs,' are only produced as the result of a true generative act.3 By M. Balbiani, however, it is affirmed (loc. cit.} that the 'winter-eggs,' like the ordinary eggs, are produced non-sexually; so that it would seem as if the intervention of the true generative act is only occasionally required for the continued propagation of these inter- esting creatures. 452. Certain Rotifera, among them the common Wheel-Animalcule,, are remarkable for their tenacity of life, even when reduced to such a state of dryness that they will break in pieces when touched with the point of a needle (as the Author Jias himself ascertained); for they can be kept in this condition for any length of time, and will yet revive very speedily upon being moistened. Taking advantage of this fact, some- 1 See Mr. Slack's " Marvels of Pond Life," 3d Edit., p. 54. 2 See Balbiani in " Journ. Roy. Microsc. Soc.," Vol. ii. (1879), p. 530. 3 See his Memoir, ' Ueber die Fortpflanzung der Raderthiere,' in " Sie-ho Kolliker's Zeitschrift," 1855. 60 THE MICROSCOPE AND ITS REVELATIONS. Microscopists are in the habit of keeping by them stocks of desiccated Eotifers, which can be distributed in the condition of dry dusty powder. The desiccating process has been carried yet farther with the tribe of Tar- digrada (§ 453, iv.); individuals of which have been kept in a vacuum for thirty days, with sulphuric acid and chloride of calcium, and yet have not lost their capability of revivification. These facts, taken in connec- tion with the extraordinary rate of increase mentioned in the preceding paragraph, remove all difficulty in accounting for the extent of the dif u- sion of these animals, and for their occurrence in incalculable numbers in situations where, a few days previously, none were known to exist. For their entire bodies may be wafted in a dry state by the atmosphere from place to place; and their return to a state of active life, after a desicca- tion of unlimited duration, may take place whenever they meet with the requisite conditions — moisture, warmth, and food. It is probable that the Ova are capable of sustaining treatment even more severe than the fully developed Animals can bear; and that the race is frequently continued by them when the latter have perished. — It is not requisite to suppose, however, that in any of the foregoing cases the desiccation is com- plete; for it appears that Wheel- Animalcules, in drying, exude a glutinous matter that forms a sort of impervious casing, which may keep-in the re- maining fluid. 1 When acted on by heat as well as by drought, Rotifers and Tardigrades lose their vitality; yet the former have survived a grad- ual heating up to 200° Fahr. 453. The principles on which the various forms that belong to this •Class should be systematically arranged, have not yet been satisfactorily determined. By Prof. Ehrenberg, the disposition of the ciliated lobes or wheel-organs, and the inclosure or non-inclosure of the body in a lorica or case, were taken as the basis of his classification; but as his ideas on both these points are inconsistent with the actual facts of organization, the arrangement founded upon them cannot be received. Another division of the class has been propounded by M. Dujardin, which is based on the several modes of life of the most characteristic forms. And in a third, more recently put forth by Prof. Leydig, the general configuration of the body, with the presence, absence, and conformation of the foot (or tail) are made to furnish the characters of the subordinate groups. Either of the two latter is certainly more natural than the first, as bringing together for the most part the forms which most agree in general orga- nization, and separating those which differ; and we shall adopt that of M. Dujardin as most suitable to our present purpose. I. The first group includes those that habitually live attached by the foot, which is prolonged into a pedicle; and it includes two families, the Floscularians and the Melicertians, the members of which are commonly found attached to the stems and leaves of aquatic plants, by a long pedi- cle or foot-stalk, bearing a somewhat bell-shaped body. In one of the most beautiful species, the Stephanoceros Eicliornii (Fig. 312), this body has five long tentacles, beset with tufts of cilia, whilst the body is inclosed in a gelatinous cylindrical cell. At first sight, the tenacles of this Roti- ler may seem to resemble those of the Polyzoa\ but, if there are carefully illuminated, the filaments which beset them will be found to be much larger, to be arranged differently, and to exhibit only an occasional motion, not at all resembling the regular rhythmical vibrations of the 1 See Davis in " Monthly Micros. Journ ," Vol. ix. (1863), p. 207; also Slack, at p. 241 of same volume. MICKOSCOPIC FORMS OF ANIMAL LIFE. 61 FIG. 312. cilia of Polyzoa.1 In fact, they seem rather to deserve the designation of setcB (bristles); for "their action is spasmodic, it creates no vortex, and it is only by actual contact with these setce that floating particles are whipped within the area inclosed by the lobes, where by the same whip- ping action they are twitched from point to point irregularly downwards, until they come within the range of a vortex that is due, not to any action of the setce, but to a range of minute cilia in the funnel ."2 A careful comparison of Stephanoceros with other forms, shows that its tentacles are only extensions of the ciliated lobes which are common to all the members of these families; and the cylindrical ' cell ' which en- velops the body is formed by the gelatinous secretion from its surface, thrown-off in rings, the indications of which often remain as a series of constrictions. In respect of the length of the filaments projecting from its lobes, and the breadth of these expansions, Flos- cularia is still more aberrant. — The body of Melicerta is protected by a most curious cylindrical tube, com- posed of little rounded pellets agglutinated together; this is obviously an artificial construction, and the process by which it is built may be watched by any Microscopist who is fortunate enough to capture it.3 Beneath a projection on its head, there is observed a small disk-like organ, in which, when the ' wheels ' are at work, a movement is seen very much resembling that of a revolving ventilator. Towards this disk the greater proportion of the solid particles that may be drawn from the surrounding liquid into the vortex of the wheel-organs, are driven by their ciliary movement, • a small part only being taken into the alimentary canal; and there they accumulate until the aggrega- tion (probably cemented by a glutinous secretion fur- nished by the organ itself) acquires the size and form of one of the globular pellets of the case; the time ordinarily required being about three minutes. The head of the animal then bends itself down, the pellet- disk is applied to the edge of the tube, the newly- formed pellet is attached there, and, the head being lifted into its former position, the formation of a new pellet at once commences. — Another curious example of this family is presented by the Conochilus volvox; which is found in spherical clusters composed of a considerable number of individuals adherent by their tails, their bodies being arranged in a radiating manner, and the intervals between them being filled up by a gelatinous substance. There is not, however, any such organic connec- 1 In ordinary drawings, the filaments of the Stephanoceros are represented as short bristles; this is an error arising from bad instruments or defective illumina- tion. It requires considerable skill to show these filaments, or those of the FJos- CMlaria, in their true length; but the beauty of the object is geatly increased when this is accomplished. '2 See Mr. C. Cubitt's ' Observations on the Economy of Stephanoceros,' in ' Monthly Microsc. Journ.," Vol. iii. (1870), p. 242. aSee Gosse ' On the architectual instincts of Melicerta ringens,' in ' Trans, of Microsc. Soc.," Vol. iii. (1852), p. 58; also Bedwell in '• Monthly Microsc. Journ.,'" Vol. xvi. (1877), p. 214; and Hudson in " Journ. Roy. Microsc. Soc.," Vol. ii. (1879), p. 1. Stephanoceros Eich- ornii. 62 THE MICROSCOPE AND ITS REVELATIONS. tion between them as exists in the Ophrydium (§ 440) ; and the uniting substance seems to be nothing else than the clear slimy secretion which probably all Rotifera exude from the surface of their bodies. It is into this that the eggs are extruded; and as they are hatched in it, the young produced from them remain to form part of the cluster; but, as its num- bers increase, the cluster breaks up into two or more, which in their turn enlarge and then subdivide, so that a pond to whose bottom the ( winter eggs ' of the year before have subsided, becomes alive with them in the early summer of the following year. ' — The Lacinularia socialis, in like manner, forms transparent gelatinous-looking globular clusters, about l-5th of an inch in diameter, which attach themselves to the leaves of aquatic plants. II. The next of M. Dujardin's primary groups (ranged by him, how- ever, as the third) consists of the ordinary Rotifer and its allies, which pass their lives in a state of alternation between the conditions of those attached by a pedicle, of those which habitually swim freely through the water, and of those wh ich creep or crawl over hard surfaces. — As these have already been fully described, it is not requisite to dwell longer upon them. in. The next group consists of those Eotifera which seldom or never attach themselves by the foot, but habitually swim freely through the water; and putting aside the peculiar aberrent form Albertia which has only been found as a parasite in the intestines of Worms, it may be divided into families, the Brachionians and the Furcularians. The for- mer are for the most part distinguished by the short, broad, and flattened form of the body (Figs. 309, 313); which is, moreover, inclosed in a sort of cuirass formed by the consolidation of the external integument. This cuirass is often very beautifully marked on its surface, and may be pro- longed into extensions of various forms, which are sometimes of very considerable length. The latter (corresponding almost exactly with the Hydatinece of Prof. Ehrenberg) derived their name from the bifurcation of the foot into a sort of two-bladed forceps; their bodies are ovoidal or cylindrical, and are inclosed in a flexible integument, which is often seen to wrinkle itself into longitudinal and transverse folds at equidistant lines. To this family belongs the Hydatina senta, one of the largest of the Rotifera, which was employed by Prof. Ehrenberg as the chief sub- ject of his examination of the internal structure of this group; as does also the Asplanchna, the curious condition of whose digestive apparatus has been already noticed (§ 448). iv. The fourth of M. Dujardin's primary orders consists of the very curious tribe, first carefully investigated by M. Doyere, to which the name of Tardigrada has been given, on account of the slowness of their creeping movement. It seems now clear, however, that they have no near relationship to the true Rotifera; corresponding to them only in their minute size and simple structure. They are found in the same lo- calities with the Rotifers, and, like them, can be revivified after desicca- tion (§ 452): but they have a vermiform body, divided transversely into five segments, of which one constitutes the head, whilst each of the others bears a pair of little fleshy protuberances, furnished with four curved hooks, and much resembling the pro-legs of a caterpillar. The head is entirely unpossessed of ciliated lobes; and the" mouth, situated at the end of a sort of beak furnished with two longitudinal stylets, leads, through 1 See Davis in " Monthly Microsc. Journ.," Vol. xvi. (1876), p. 1. MICROSCOPIC FORMS OF ANIMAL LIFE. 63 a muscular pharynx, into a wide alimentary canal, which gradually nar- rows to the anus. There are no special organs of circulation or respira- tion, but the nervous system is much more developed than in the Roti- fera; a cerebral mass, bearing two eyes, giving origin to two longitudinal cords, on which are seated pairs of ganglia in connec- tion with the members, as in Articulated animals erally. Their nearest ai FIG. 313. Noteus quadricomis: A, dorsal view; B, side view. jen- ini- ties seem with the lowest forms of the Arachnida. 454. Notwithstanding that all the best-informed Zoologists are now agreed in ranking the true Rotifera among Articulated animals, yet there is still a consider- able discordance of opinion as to the precise part of that series in which they should stand. Prof. Leydig, who has devoted much attention to the study of the class, regards them as most allied to the Crustacea, and terms them ( Cilio-crustaceans; ' and the curious Entomos- tracan-looking Pedalion of Dr. Hudson might seem a link with that group.1 Prof. Huxley, on the other hand, has argued that they are more connected with the Annelida, through the resemblance which they bear to the early larval forms of that class (§ 595); while in their single bilobed nerve-ganglion and water-vascular system, they seem allied to Planaria (§ 593). 2 1 See Prof. E. Ray Lankester's 'Remarks on Pedalion^ in "Quart. Journ. Microsc. Sci.," Vol. xii. (1878), p. 338. 2 The following Treatises and Memoirs (in addition to those already referred to) contain valuable information in regard to the life-history of Animalcules and their principal forms: — Ehrenberg, "Die Infusionsthierchen, ' Berlin, 1838; Du- jardin, "Histoire Naturelle des Zoophytes Infusoires," Paris, 1841; Pritchard, " History of Infusoria," 4th Ed., London, 1861 (a comprehensive repertory of in- formation); Stein, " Der Organismus der Infusionsthiere," Leipzig, Erste Abthei- lung, 1859, Zweite Abtheilung, 1867, Dritte Abtheilung, Halfte i., 1878: Saville Kent's "Manual of the Infusoria," 1880-1; and Prof. Biitschli's Pro tozoa (1880, 1881) in the new edition of " Bronn's Thierreich."— For the RHIZOPODA and IN- FUSORIA specially, see Claparede and Lachmann, " Etudes sur les Infusoires et les Rhizopodes," Geneva, 1858-1861; Cohn, in " Siebold and Kolliker's Zeitschrift," 1851-4, and 1857; Lieberkuhn, in "Miiller's Archiv," 1856, and " Ann. of Nat. Hist.," 3d Ser., Vol. xviii., 1856; Engellmann, " Zur Naturgeschichte der Infusions- Thiere" (1862); and Prof. Butschli's " Studien fiber die Conjugation der Infuso- rien," etc., 1876. — For the ROTIFERA specially, see Leydig, in " Siebold and Kolli- ker's Zeitschrift," Bd. vi., 1854; Gosse on Melicerta ringens, in " Quart. Journ. of Microsc. Science," Vol. i. (1853), p. 1; Huxley on Lacinularia socialis in "Trans- act, of Microsc. Soc.," Ser. 2, Vol. i. (1853), p. 1; and Cohn, in " Siebold and Kolli- ker's Zeitschrift," Bde. vii., ix. (1856, 1858). Mr. Slack's " Marvels of Pond Life " (2d Edit., London, 1871) contains many interesting observations on the habits of Infusoria and Rotifera. 64: THE MICROSCOPE AKD ITS REVELATIONS. CHAPTER XII. FORAMINIFERA AND RADIOLARIA. 455. RETUKNING now to the lowest or Rhizopod type of Animal life (Chap, x)., we have to direct our attention to two very remarkable series of forms, almost exclusively Marine, under which that type manifests itself; all of them distinguished by skeletons so consolidated by Mineral deposit, as to retain their form and intimate structure long after the Animals to which they belonged have ceased to live, even for those un- defined periods in which they have been imbedded as Fossils in strata of various geological ages. In the first of these groups, the Foraminifera, the skeleton usually consists of a calcareous many-chambered Shell, which closely invests the sarcode-body, and which, in a large proportion of the group, is perforated with numerous minute apertures; this shell, how- ever, is sometimes replaced by a ' test/ formed of minute grains of sand cemented together; and there are a few cases (§ 397) in which the Ani- rnal has no other protection than a membranous envelope. — In the sec- ond group, the Radiolaria, the skeleton is always siliceous ; and may be either composed of disconnected spicules, or may consist of a symme- trical open framework, or may have the form of a shell perforated by numerous apertures, which more or less completely incloses the body. — The Foraminifera probably take, and always have taken, the largest share of any Animal group in the maintenance of the solid carcareous portion of the Earth's crust; by separating from its solution in Ocean- water the Carbonate of Lime continually brought down by rivers from the land. The Radiolaria do the same, though in far less measure, for the Silex. And both extract from Sea-water the organic matter univer- sally diffused through it, converting it into a form that serves for the nutrition of higher Marine animals. SECTION I. — EOKAMINIFERA. 456. The animals of this group belong to that Reticularian form of the Rhizopod type (§ 397), in which, — with a differentiation between the containing and the contained sarcodic substance which is involved in the formation of a definite. investment, — a distinct nucleus (sometimes sin- gle, in other cases multiple) is probably always present.1 The Shells of 1 The absence of a nucleus was long supposed to be a characteristic of the ani- mal of the Foraminifera ; and its presence in Gromia (first detected by Dr. Wal- lich) was regarded as differentiating that type from the Foraminifera proper. But the researches of Hertwig and Lesser having established its presence in sev- eral true Foraminifera, and the Author's own observations on other forms having confirmed theirs, its general presence may be fairly assumed, until contradicted by more extended observation. FORAMINIFERA AND RADIOLARIA. 65 PLATE XV: VARIOUS FORMS OF FORAMINIFERA (Original). Fig. 1. Cornuspira. 2. Spiroloculina. 3. Triloculina. 4. Biloculina. 5. Peneroplis. 6. Orbiculina (cyclical form). < . Orbiculina (young) H. Orbiculina (spiral form). 9. Lagena. 10. Nodosaria. Fig. 11. Cristellaria. 12. Globigerina. 13. Polymorphina. 14. Textularia. 15. Discorbina. 16. Polystomella. 17. Planorbulina. 18. Rotalia. 19. Nonionina. 66 THE MICROSCOPE AND ITS REVELATIONS. Foraminifera are, for the most part, polytlialamous or many-chambered (Plate xv.); often so strongly resembling those of Nautilus, Spirula, and other Cephalopod Mollusks, that it is not surprising that the older Naturalists, to whom the structure of these animals was entirely un- known, ranked them under that Class. But independently of the entire difference in the character of the animal bodies by which the two kinds of shells are formed, there is a most important distinction between them in regard to the relation of the animal to the shell. For whilst, in the chambered shells of the Nautilus and other Cephalopods, the animal is a single individual tenanting only the last formed chamber, and with- drawing itself from each chamber in succession, as it adds to this another and larger one, the animal of a nautiloid Foraminifer has a composite body, consisting of a number (sometimes very large) of ' segments/ each repeating the rest, which continues to increase by gemmation or budding from the last-formed segment. And thus each of the chambers, how- ever numerous they may be, is not only formed, but continues to be oc- cupied, by its own segment; which is connected with the segments of earlier and later formation by a continuous ' stolon ' (or creeping stem), that passes through apertures in the septa or partitions dividing tlie chambers. — From what we know of the semi-fluid condition of the sar- code-body in the Reticularian type (§ 397), there can be little doubt that there is an incessant circulatory change in the actual substance of each segment; so that the material taken-in as food by the segment nearest the surface or margin, is speedily diffused through the entire mass. The relation between these ' polytlialamous ' forms, therefore, and the mono- thalamous or single-chambered, — of which we have already had an exam- ple in Gromia (§ 397), and of which others will be presently described, — is simply that whereas any buds produced by the latter detach them- selves to form separate individuals, those put forth by the former remain in continuity with the parent stock and with each other, so as to form a ' composite' Animal and a ' poly thalamous' Shell. 457. According to the plan on which the gemmation takes place, will be the configuration of the shelly structure produced by the segmented body. Thus, if the bud should be put forth from the aperture of a La- yena (Plate xv., fig. 9) in the direction of the axis of its body, and a second shell should be formed around this bud in continuity with the first, and this process should be successionally repeated, a straight rod- like shell would be produced (fig. 10), whose multiple chambers commu- nicate with each other by the openings that originally constituted their mouths; the mouth of the last-formed chamber, being the only aperture through which the sarcode-body, thus composed of a number of segments connected by a peduncle or ' stolon ' of the same material, could now project itself or draw-in its food. The successive segments may be all of the same size, or nearly so, in which case the entire rod will approach the cylindrical form, or will resemble a line of beads; but it often hap- pens that each segment is somewhat larger than the preceding (fig. 11), so that the composite shell has a conical form, the apex of the cone being the original segment, and its base the one last formed. — The method of growth now described is common to a large number of Fora- minifera, chiefly belonging to the genus Nodosarina; but even in that genus we have every gradation between the rectilineal (fig. 10), and the spiral mode of growth (fig. 11); whilst in the genus Peneroplis (fig. 5) it is not at all uncommon for shells which commence in a spiral to ex- change this in a more advanced stage for the rectilineal. When the FORAMINIFERA AND RADIOLARIA. 67 successive segments are added in a spiral direction, the character of the spire will depend in great degree upon the enlargement or non-enlargement of the successively-formed chambers; for sometimes it opens out very rapidly, every whorl being considerably broader than that which it sur- rounds, in consequence of the great excess of the size of each segment over that of its predecessor, as in Peneroplis ; but more commonly there is so little difference between the successive segments, after the spire has made two or three turns, that the breadth of each whorl scarcely exceeds that of its predecessor, as is well seen in the section of the Rotalia rep- resented in Fig. 330. An intermediate condition is presented by such a Rotalia as is shown in Fig. 314, which may be taken as a characteristic type of a very large and important group of Foraminifera, whose general features will be presently described. Again, a spiral may be either ' nau- FIG. 314. Rotalia ornata, with its pseudopodia extended. tiloid ' or ' turbinoid': the former designation being applied to that form in which the successive convolutions all lie in one plane (as they do in the Nautilus), so that the shell is ( equilateral ' or similar on its two sides; whilst the latter is used to mark that form in which the spire passes obliquely round an axis, so that the shell becomes ' inequilateral/ hav- ing a more or less conical form, like that of a Snail or a Periwinkle, the first-formed chamber being at the apex. Of the former we have charac- teristic examples in Polystomelia (Plate xv., fig. 16) and Nonionina (fig. 19); whilst of the latter we find a typical representation in Rotalia Baccarii (fig. 18). Farther, we find among the shells whose increase takes place upon the spiral plan, a very marked difference as to the de- gree in which the earlier convolutions are invested and concealed by the latter. In the great Rotaline group, whose characteristic form is a tur- 68 THE MICROSCOPE AND ITS REVELATIONS. binoid spiral, all the convolutions are usually visible, at least on one side (figs. 15, 17, 18), but among the Nautiloid tribes it more frequently hap- pens that the last-formed whorl incloses the preceding to such an extent that they are scarcely, or not all, visible externally, as is the case in Cris- tellaria (fig. 11), Polystomella (fig. 16), and Nonionina (fig. 19).— The turbinoid spire may coil so rapidly round an elongated axis, that the number of chambers in each turn is very small; thus in Globigerina (fig. 12) there are usually only four; and in Valvulina the regular number is only three. Thus we are led to the Userial arrangement of the cham- bers which is characteristic of the Textularian group (fig. 14); in which we find the chambers arranged in two rows, each chamber communicat- ing with that above and that below it on the opposite side, without any direct communication with the chamber of its own side, as will be understood by reference to Fig. 328, A, which shows a ' cast ' of the sar- code body of the animal. On the other hand, we find in the nautiloid spire a tendency to pass (by a curious transitional form to be presently described, § 464) into the cyclical mode of growth; in which the original segment, instead of budding-forth on one side only, develops gemmce all round, so that a ring of small chambers (or chamberlets) is formed around the primordial chamber, and this in its turn surrounds itself after the like fashion with another ring; and by successive repetitions of the same process the shell co'mes to have form of a disc made up of a great num- ber of concentric rings, as we see in Orbitolites (Fig. 316) and in Cyclo- clypeus (Plate xvi., fig. 1). 458. These and other differences in the plan of growth were made by M. D'Orbigny the foundation of his Classification of this group, which, though at one time generally accepted, has now been abandoned by most of those who have occupied themselves in the study of the Foraminifera. For it has come to be generally admitted that ' plan of growth ' is a character of very subordinate importance among the Foraminifera, so that any classification which is primarily based upon it must necessarily be altogether unnatural; those characters being of primary importance which have an immediate and direct relation to the Physiological condi- tion of the Animal, and are thus indicative of the real affinities of the several groups which they serve to distinguish. The most important of these characters will now be noticed.1 459. Two very distinct types of Shell-structure prevail among ordi- nary Foraminifera — namely, the porcellanous and the hyaline or vitreous. The shell of the former, when viewed by reflected light, presents an opaque- white aspect which bears a strong resemblance to porcelain; but when thin natural or artificial laminae of it are viewed by transmitted light, the opacity gives place to a rich brown or amber color, which in a few instances is tinged with crimson. No structure of any description can be detected in this kind of shell -substance, which is apparently homogeneous throughout. Although the shells of this i porcellanous ' type often present the appearance of being perforated with foramina, yet this appearance is illusory, being due to a mere ' pitting' of the external surface, which, though often very deep, never extends through the whole thickness of the shell. Some kind of inequality of that surface, indeed, is extremely common in the shells of the ' porcellanous ' Fora- 1 This subject will be found amply discussed in the Author's " Introduction to the Study of the Foraminifera," published by the Ray Society; to which work he would refer such of his readers as may desire more detailed information in regard to it. FORAMINIFERA AND BADIOLAKIA. 69 minifera; one of the most frequent forms of it being a regular alterna- tion of ridges and furrows, such as is occasionally seen in Miliola (Plate xv., fig. 3), but which is an almost constant characteristic of Peneroplis (fig. 5). But no difference of texture accompanies either this or any other kind of inequality of surface; the raised and depressed portions being alike homogeneous. — In the shells of the vitreous or hyaline type, on the other hand, the proper shell- substance has an almost glassy trans- parence, which is shown by it alike in thin natural lamellae, and in arti- ficially prepared specimens of such as are thicker and older. It is usually colorless, even when (as in the case with many Rotalince) the substance of the animal is deeply colored; but in certain aberrant Rota- lines the shell is commonly, like the animal body, of a rich crimson hue. All the shells of this type are beset more or less closely with tubular perfo- rations, which pass directly, and (in general) without any subdivision, from one surface to the other. These tubuli are in some instances suffi- ciently coarse for their orifices to be distinguished with a low magnifying power, as i punctations ' on the surface of the shell, as is shown in Fig. 314; whilst in other cases they are so minute as only to be discernible in thin sections seen by transmitted light under a higher magnifying power, as is shown in Figs." 335, 336. When they are very numerous and closely set, the shell derives from their presence that kind of opacity which is characteristic of all minutely-tubular textures, whose tubuli are occupied either by air or by any substance having a refractive power different from that of the inter tubular substance, however perfect may be the transpar- ence of the latter. The straightness, parallelism, and isolation of these tubuli are well seen in vertical sections of the thick shells of the largest examples of the group, such as Nummulina (Fig. 335). It often hap- pens, however, that certain parts of the shell are left unchannelled by these tubuli; and such are readily distinguished, even under a low mag- nifying power, by the readiness with which they allow transmitted light to pass through them, and by the peculiar vitreous lustre they exhibit when light is thrown obliquely on their surface. In shells formed upon this type, we frequently find that the surface presents either bands or spots which are so distinguished; the non-tubular bands usually marking the position of the septa, and being sometimes raised into ridges, though in other instances they are either level or somewhat depressed; whilst the non-tubular spots may occur on any part of the surface, and are most commonly raised into tubercles, which sometimes attain a size and num- ber that give a very distinctive aspect to the shells that bear them. 460. Between the comparatively coarse perforations which are com- mon in the Rotaline type, and the minute tubuli which are characteris- tic of the Nummuline, there is such a continuous gradation as indicates that their mode of formation, and probably their uses, are essentially the same. In the former, it has been demonstrated by actual observation that they allow the passage of pseudopodial extensions of the sarcode- body through every part of the external wall of the chambers occupied by it (Fig. 314); and there is nothing to oppose the idea that they answer the same purpose in the latter, since, minute as they are, their diameter is not too small to enable them to be traversed by the finest of the threads into which the branching pseudopodia of Foraminifera are known to subdivide themselves. Moreover, the close approximation of the tubuli in the most finely-perforated Nummulines, makes their col- lective area fully equal to that of the larger but more scattered pores of the most coarsely-perforated Rotalines. Hence it is obvious that the 70 THE MICROSCOPE AND ITS REVELATIONS. tabulation or non-tubulation of Foraminiferal shells is the key to a very important Physiological difference between the Animal inhabitants of the two kinds respectively; for whilst every segment of the sarcode-body in the former case gives off pseudopodia, which pass at once into the sur- rounding medium, and contribute by their action to the nutrition of the segment from which they proceed, these pseudopodia are limited in the latter case to i\\Q final segment, issuing forth only through the aperture of the last chamber, so that all the nutrient material which they draw-in must be first received into the last segment, and be transmitted thence from one segment to another until it reaches the earliest. With this dif- ference in the physiological condition of the Animal of these two types, is usually associated a further Tiery important difference in the conforma- tion of the Shell — viz., that whilst the aperture of communication be- tween the chambers, and between the last chamber and the exterior, is. usually very small in the ' vitreous' shells, serving merely to give passage to a slender stolon or thread of sarcode from which the succeeding seg- ment may be budded-off, it is much wider in the ' porcellanous ' shells, so as to give passage to a ' stolon' that may not only bud-off new seg- ments, but may serve as the medium for transmitting nutrient material from the outer to the inner chambers. 461. Between the highest types of the Porcellanous and the Vitreous series respectively, which frequently bear a close resemblance to each other in form, there are certain other well-marked differences in struc- ture, which clearly indicate their essential dissimilarity. Thus, for ex- ample, if we compare OrUtolites (Fig. 316) with Cycloclypeus (Plate xvi., fig. 1), we recognize the same plan of growth in each, the chamberlets being arranged in concentric rings around the primordial chamber; and to a superficial observer there would appear little difference between them. But a minuter examination shows that not only is the texture of the shell ' porcellanous ' and non-tubular in Orbitolites, whilst it is * vit- reous ' and minutely tubular in Oycloclypeus; but that the partitions be- tween the chamberlets are single in the former, whilst they are double in the latter, each segment of the sarcode-body having its own proper shelly investment. Moreover, between these double partitions an additional deposit of calcareous substance is very commonly found, constituting what may be termed the intermediate skeleton; and this is traversed by a peculiar system of inosculating canals, which pass around the chamber- lets in interspaces left between the two laminse of their partitions, and which seem to convey through its substance extensions of the sarcode- body whose segments occupy the chamberlets. We occasionally find this ' intermediate skeleton ' extending itself into peculiar outgroioths, which have no direct relation to the chambered shell; of this we have a very curious example in Calcarina (Plate xvi., fig. 3); and it is in these the^ we find the ' canal-system ' attaining its greatest development. Its most regular distribution, however, is seen in Polystomella and in Operculina; and an account of it will be given in the description of those types. 462. PORCELLANEA. — Commencing, now, with the Porcellanous se- ries, we shall briefly notice some of its most important forms, which are so related to each other as to constitute but the one family Miliolida. Its simplest type is presented by the Cornuspira (Plate xv., fig. 1) of our own coasts, found attached to Sea-weeds and Zoophytes; this is a minute spiral shell, of which the interior forms a continuous tube not divided into chambers; the latter portion of the spire is often very much flat- tened-out, as in PeneropHs (fig. 5), so that the form of the mouth is FORAMINIFERA AND RAUIOLARIA. 71 changed from a circle to a long narrow slit. — Among the commonest of the Foraminifera, and abounding near the shores of almost every sea, are some forms of the Milioline type, so named from the resemblance of some of their minute fossilized forms (of which enormous beds of lime- stone in the neighborhood of Paris are almost entirely composed) to mil- let-seeds. The peculiar mode of growth by which these are characterized, will be best understood by examining in the first instance the form which has been designated as Spirolpculina (Plate xv., fig. 2). This shell is a spiral, elongated in the direction of one of its diameters, and having in each turn a contraction at either end of that diameter, which partially divides each convolution into two chambers; the separation between the consecutive .chambers is made more complete by a peculiar projection from the inner side of the cavity, known as the 'tongue' or 'valve,' which may be considered as an imperfect septum; of this a characteristic example is shown in the upper part of fig. 4. Now it is a very general habit in the Milioline type, for the chambers of the later convolutions to extend themselves over those of the 'earlier, so as to conceal them more or less completely; and this they very commonly do somewhat unequally, so that more of the earlier chambers are visible on one side than on the other. Miliolce thus modified (fig. 3) have received the names of Quin- queloculina and Triloculina according to the number of chambers visible externally; but the extreme inconstancy which is found -to mark such distinctions, when the comparison of specimens has been sufficiently ex- tended, entirely destroys their value as differential characters. Some- times the earlier convolutions are so completely concealed by the later, that only the two chambers of the last turn are visible externally; and in this type, which has been designated Biloculin*, there is often such an increase in the breadth of the chambers as altogether changes the usual proportions of the shell, which has almost the shape of an egg when so placed that either the last or the penultimate chamber faces the observer (Plate xv., fig. 4). It is very common in Milioline shells for the exter- nal surface to present a ' pitting/ more or less deep, a ridge-and-furrow arrangement (fig. 3), or a honeycomb division; and these diversities have been used for the characterization of species. Not only, however, may every intermediate gradation be met-with between the most strongly marked forms, but it is not at all uncommon to find the surface smooth on some parts, whilst other parts of the surface in the same shell are deeply pitted or strongly ribbed or honeycombed; so that here again the inconstancy of these differences deprives them of all value as distinctive characters. ^ 463. Reverting again to the primitive type presented in the simple spiral of Cornuspira, we find the most complete development of it in Peneroplis (Plate xv., fig. 5), a very beautiful form, which, although very rare on our own coasts, is one of the commonest of all Foraminifera in the shore-sands and shallow water dredgings of the warmer regions of every part of the globe. This is a nautiloid shell, of which the spire flattens itself out as it advances in growth; it is marked externally by a series of transverse bands, which indicate the position of the internal septa that divide the cavity into chambers; and these chambers commu- nicate with each other by numerous minute pores traversing each of the septa, and giving passage to threads of sarcode that connect the seg- ments of the body. At a is shown the ( septal plane ' closing in the last-formed chamber, with its single row of pores through which the pseudopodial filaments extend themselves into the surrounding medium. 72 THE MICROSCOPE AND ITS REVELATIONS. The surface of the shell, which has a peculiarly ' porcellanous ' aspect, is marked by closely-set strice that cross the spaces between the successive septal bands; these markings, however, do not indicate internal divisions, and are due to a surface-furrowing of the shelly walls of the chambers. This type passes into two very curious modifications; one having a spire which, instead of flattening itself out, remains turgid like that of a Nautilus, having only a single aperture, which sends out fissured exten- sions that subdivide like the branches of a tree, suggesting the name of Dendritina ; the other having its spire continued in a rectilineal direc- tion, so that the shell takes the form of a crosier, this being distin- guished by the name of Spirolina. A careful examination of inter- mediate forms, however, has made it evident that these modifications, though ranked as of generic value byM. D'Orbigny, are merely varietal; a continuous gradation being found to exist from the elongated septal plane of Peneroplis, with its single row of isolated pores, to the arrow- shaped, oval, or even circular septal plane of Dendritina, with all its pores fused together (so to speak) into one dendritic aperture; and a like gradation being presented between the ordinary and the 'spiroline' forms into which both Peneroplis and Dendritina tend to elongate themselves. 464. From the ordinary nautiloid multilocular spiral, we now pass to a more complex and highly-developed form, which is restricted to tropical regions, but is there very abundant — that, namely, which has received the designation Orbiculina (Plate xv., figs. 6, 7, 8). The relation of this to the preceding will be best understood by an examina- tion of its earlier stage of growth, represented in fig. 7; for here we see that the shell resembles that of Peneroplis in its general form, but that its principal chambers are divided by 'secondary septa' passing at right angles to the primary, into ' chamberlets ' occupied by sub-segments of the sarcode-body. Each of these secondary septa is perforated by an aperture, so that a continuous gallery is formed, through which (as in Fig. 316) there passes a stolon that unites together all the sub-segments of each row. The chamberlets of successive rows alternate with one another in position; and the pores of the principal septa are so disposed, that each chamberlet of any row normally communicates with two chamberlets in each of the adjacent rows. The later turns of the spire very commonly grow completely over the earlier, and thus the central portion or l umbilicus ' comes to be protuberant, whilst the growing edge is thin. The spire also opens out at its'growing margin, which tends to encircle the first-formed portion, and thus gives rise to the peculiar shape represented in fig. 8, which is the common aduncal type of this organ- ism. But sometimes, even at an early age, the growing margin extends so far round on each side, that its two extremities meet on the opposite side of the original spire, which is thus completely inclosed by it; and its subsequent growth is no longer spiral but cyclical, a succession of concentric rings being added, one around the other, as shown in fig. 6. This change is extremely curious, as demonstrating the intimate relation- ship between the spiral and the cyclical plans of growth, which at first sight appear essentially distinct. In all but the youngest examples of Orbiculina, the septal plane presents more than a single row of pores, the number of rows increasing in the thickest specimens to six or eight. This increase is associated with a change in the form of the sub-segments of sarcode from little blocks to columns, and with a greater complexity in the general arrangement, such as will be more fully described here- FORAMINIFERA AND RADIOLARIA. 73 after in Orbitolites (§ 466). The largest existing examples of this type are far surpassed in size by those which make up a considerable part of a Tertiary Limestone on the Malabar coact of India, whose diameter reaches 7 or 8 lines. 465. A very curious modification of the same general plan is shown in Alveolina, a genus of which the largest existing forms (Fig. 315) are commonly about one- third of an inch long, while far larger specimens FIG. 315. FIG. 316. Alveolina Quoit :— a, a, septal plane, showing multiple pores. are found in the Tertiary Limestones of Scinde. Here the spire turns round a very elongated axis, so that the shell has almost the form of a cylinder drawn to a point at each extremity. Its surface shows a series of longitudinal lines which mark the principal septa; and the bands that intervene between these are marked transversely by lines which show the subdivision of the principal chambers into ' chamberlets.' The chamber- lets of each row are con- nected with each other, as in the preceding type, by a continuous gallery; and they communicate with those of the next row by a series of multi- ple pores in the principal septa, such as constitute the external orifices of the last-formed series, seen on its septal plane at a, a. 466. The highest de- velopment of that cycli- cal plan of growth which we have seen to be some- times taken-on by Orbi- rmlirm i« frmnrl in LUlina,, IS L Simple disk of Orbitolites complanatus, laid open to show its interior structure :-a, central chamber; 6, circumambient cham- a type Which, ber, surrounded by concentric zones of chamberlets connected long known as a very with each other by armular and radiatins passage*,. abundant fossil in the earlier Tertiaries of the Paris basin, has lately proved to be scarcely less abundant in certain parts of the existing Ocean. The largest recent speci- mens of it, sometimes attaining the size of a shilling, have hitherto been obtained only from the coast of New Holland, the Fijian reefs, and various other parts of the Polynesian Archipelago; but disks of comparatively mi- nute size and simpler organization are to be found in almost all Foraminif- eral sands and dredgings from the shores of the warmer regions of the globe, being especially abundant in those of some of the Philippine Islands, of the Bed Sea, of the Mediterranean, and especially of the ^Egean. When 74 THE MICROSCOPE AND ITS REVELATIONS. such disks are subjected to microscopic examination, they are found (if uninjured by abrasion) to present the structure represented in Fig. 316; where we see on the surface (by incident light) a number of rounded ele- vations, arranged in concentric zones around a sort of nucleus (which has been laid-open in the figure to show its internal structure); whilst at the margin we observe a row of rounded projections, with a single aperture or pore in each of the intervening depressions. In very thin disks the structure may often be brought into view by mounting them in Canada balsam and transmitting light through them; but in those which are too opaque to be thus seen-through, it is sufficient to rub-down one of the surfaces upon a stone, and then to mount the specimen in balsam. Each of the superficial elevations will then be found to be the roof or cover of an ovate cavity or i chamberlet,' which communicates by means of a lateral passage with the chamberlet on either side of it in the same ring; so that each circular zone of chamberlets might be described as a continu- ous annular passage, dilated into cavities at intervals. On the other hand, each zone communicates with the zones that are internal and external to it, by means of passages in a radiating direction; these passages run, however, not from the chamberlets of the inner zone to tnose of the outer, but from the connecting passages of the former to the chamberlets of the latter; so that the chamberlets of each zone alternate in position with those of the zones internal and external to it. The radial passages from the outermost annulus make their way at once to the margin, where they terminate, forming the ' pores ' which (as already mentioned) are to be seen on its exterior. The central nucleus, when rendered sufficiently transparent by the means just adverted-to, is found to consist of a 'pri- mordial chamber' (a), usually somewhat pear-shaped, that communicates by a narrow passage with a much larger ' circumambient chamber' (b), which nearly surrounds it, and which sends off a variable number of ra- diating passages towards the chamberlets of the first zone, which forms a complete ring around the circumambient chamber. 1 467. The idea of the nature of the living occupant of these cavities which might be suggested by the foregoing account of their arrangement, is fully borne-out by the results of the examination of the sarcode-body, which may be obtained by the maceration in dilute acid (so as to remove the shelly investment) of specimens of Orbitolite that have been gathered fresh and preserved in spirit. For this body is found to be composed (Fig. 317) of a multitude of segments of sarcode, presenting not the least trace of higher organization in any part, and connected together by * stolons' of the like substance. The ' primordial' pear-shaped segment, a, is seen to have budded-off its 'circumambient' segment, Z», by a narrow footstalk or stolon; and this circumambient segment, after passing almost entirely round the primordial, has budded-off three stolons, which swell into new sub-segments from which the first ring is formed. Scarcely any two specimens are precisely alike as to the mode in which the first ring 1 Although the above may be considered the typical form of the Orbitolite, yet, in a very large proportion of specimens, the first few zones are not complete circles, the early growth having taken place from one side only; and there is a very beautiful variety in which this one-sidedness of increase imparts a distinctly spiral character to the early growth, which soon, however, gives place to the cyclical. — In the Orbitolites tenuissimus (Fig. 318) brought u-p from depths of 1,500 fathoms or more, the ' nucleus' is formed by three or four turns of a spiral closely resembling that of a Cornuspira (§ 462), with an interruption at every half -turn as in Spiroloculina; the growth afterwards becoming purely concentric. FORAMLNIFERA AND RADIOLARIA. 75 FIG. 317. originates from the ' circumambient segments;' for sometimes a score or more of radial passages extend themselves from every part of the margin of the latter (and this, as corresponding with the plan of growth after- wards followed) is probably the typical arrangement) ; whilst in other cases (as in the example before us) the number of these primary offsets is extremely small. Each zone is seen to consist of an assemblage of ovate sub-segments, whose height (which could not be shown in the figure) corresponds with the thickness of the disk ; these sub-segments, which are all exactly similar and equal to one another, are connec- ted by annular stolons; and each zone is connected with that on its exterior by radial extensions of those stolons passing-off between the sub-segments. 468. The radial extensions of the outermost zone issue-forth as pseudopodia from the marginal pores, searcbing-for and drawing- in alimentary materials in the mannner formerly described (§ 397); the whole of the soft body, which has no communication whatever with the exterior save through these marginal pores, be- ing nourished by the transmission of the products of digestion from zone to zone, through similar bands OI protoplasmic Substance, cumambient segment, giving off peduncles, in which In all cases in which the growth ££^*ZSSSS?£rt^ of the disk takes place with nor- mal regularity, it is probable that a complete circular zone is added at once. Thus we find this simple type of organization giving origin to fabrics of by no means microscopic dimensions, in which, how- ever, there is no other differentiation of parts than that concerned in the formation of the shell ; every segment and every stolon (with the exception of the two forming the ( nucleus ') being, so far as can be ascertained, a precise repetition of every other, and the segments of the nucleus differ- ing from the rest in nothing else than their form. The equality of the endowments of the segments is shown by the fact — of which accident has repeatedly furnished proof — that a small portion of a disk, entirely sepa- rated from the remainder, will not only continue to live, but will so in- crease as to form a new disk (Fig. 318); the want of the ' nucleus' not appearing to be of the slightest consequence, from the time that active life is established in the outer zones. 469. One of the most curious features in the history of this type is its capacity for developing itself into a form which, whilst fundamentally the same as that previously described, is very much more complex. In all the larger specimens of OrbitoUte, we observe that the mar- ginal pores, instead of constituting but a single row, form many rows one above another, and besides this, the chamberlets of the two surfaces, instead of being rounded or ovate in form, are usually oblong and straight-sided, their long diameters lying in a radial direction, 76 THE MICROSCOPE AND ITS REVELATIONS. like those of the cyclical type of OMculina (Plate xv., £g. 6). When a vertical section is made through such a disk, it is found that these oblong chambers constitute two superficial layers, between which are interposed columnar chambers of a rounded form; and these last are connected to- Fio. 318. FIG. 319. Disk of Orbitolites tenuissimus, formed round fragment of previous disk. gether by a complex series of passages, the arrangement of which will be best understood from the examination of a part of the sarcode-body that occupies them (Fig. 319). For the oblong superficial chambers are occu- pied by sub-segments of sarcode, c c, d dy lying side by side, so as to form part of an annulus, but each of them disconnected from its neighbors, and communicating only by a double footstalk with the two annular ' stolons/ a a', I Z>' which obvious- ly correspond with the single stolon of ' simple ' type (Fig. 317). These indirect- ly connect together not merely all the superficial chamberlets of each zone, but also the columnar sub-segments of the intermediate layer; for these columns (e e, e' e') terminate above and below in the annular stolons, sometimes passing direct- ly from one to the other, but sometimes going out of their direct course to coalesce with another column. The columns of the successive zones (two sets of which are shown in the figure) communicate with Portion of Animal of Complex type each other by threads of sarcode, in such or Orbitohtes complanatus: — a a', o o', ,-• •*, / ,-1 • , , \ , the upper and lower rings of two con- a manner that (as in the simple type) each trie centric zones; c c, the upper layer of superficial sub-segments, and d d, the lower layer, connected with the annular bands of both zones; e e and e' e', ver- tical sub-segments of the two zones. column is thus brought into connection with two columns of the zone next inte- rior, to which it alternates in position. FORAMINIFERA AND RADIOLARIA. 7T Similar threads, passing off from the outermost zone, through the mul- tiple ranges of marginal pores, would doubtless act as pseudopodia. 470. Now this plan of growth is so different from that previously described, that there would at first seem ample ground for separating the simple and the complex types as distinct species. But the test furnished by the examination of a large number of specimens, which ought never to be passed-by when it can possibly be appealed to, furnishes these very singular results : — 1st. That the two forms must be considered as speci- fically identical ; since there is not only a gradational passage from one to the other, but they are often combined in the same individual, the inner and first-formed portion of a large disk frequently presenting the simple type, whilst the outer and later-formed part has developed itself upon the complex: — 2d. That although the last- mentioned circumstance would naturally suggest that the change from the one plan to another may be simply a feature of advancing age, yet this cannot be the case ; since, although the complex sometimes evolves itself even from the very first (the 'nucleus,' though resembling that of the simple form, sending out two or more tiers of radiating threads), more frequently the simple prevails for an indefinite number of zones, and then changes itself in the course of a few zones into the complex. — No department of Natural History could furnish more striking instances than are afforded by the different forms presented by the Foraminiferal types now described, of the wide range of variation that may occur within the limits of one and the same species ; and the Microscopist needs to be specially put on his guard as to this point, in respect to the lower types of Animal as to those of Vegetable life, since the determination of form seems to be far less precise among such than it is in the higher types. 471. In what manner the reproduction of Orbtiolites is accomplished, we can as yet do little more than guess; but from appearances sometimes presented by the sarcode-body, it seems reasonable to infer that gemmules, corresponding with the zoospores of Protophytes (§ 244), are occasionally formed by the breaking-up of the sarcode into globular masses; and that these, escaping through the marginal pores, are sent forth to develop themselves into new fabrics. Of the mode wherein that sexual operation is. performed, however, in which alone true Generation consists, nothing whatever is known. 472. ARENACEA. — In certain forms of the preceding family, and especially in the genus Miliola, we not unfrequently find the shells encrusted with particles of sand, which are imbedded in the proper shell- substance. This incrustation, however, must be looked on as (so to speak) accidental ; since we find shells that are in every other respect of the same type, altogether free from it. A similar accidental incrustation presents itself among certain ' vitreous ' and perforate shells; but there, too, it is on usually a basis of true shell, and the sandy incrustation is often entirely absent. There is, however, a group of Foraminifera in which the true shell is constantly and entirely replaced by a sandy enve- lope, which is distinguished as a 'test;' the arenaceous particles being- held together only by a cement exuded by the animal. It is not a little curious that the forms of these arenaceous ' tests ' should represent those of many different types among both the ' porcellanous' and the ' vitreous ' series; whilst yet they graduate into one another in such a manner, as to indicate that all the members of this ' arenaceous7 group are closely related to each other, so as to form a series of their own. And it is further remarkable, that while the Deep-sea dredgings recently carried 78 THE MICROSCOPE AND ITS REVELATIONS. down to depths of from 1,000 to 2,500 fathoms, have brought up few forms of either l porcellanous ' or i vitreous ' Foraminifera that were not previously known, they have added greatly to our knowledge of the 'arenaceous' types, the number and variety of which far exceed all previous conception. These have not yet been systematically described; but the following notice of a few of the more remarkable, will give some idea of the interest attaching to this portion of the new Fauna which has been brought to light by Deep-sea exploration. 473. In the midst of the sandy mud which formed the bottom where the warm area of the ' Globigerina-mud ' (§ 480) abutted on that over which a glacial stream flowed, there were found a number of little pellets, varying in size from a large pin's head to that of a large pea, formed of an aggregation of sand-grains, minute Foraminifers, etc., held together by a tenacious protoplasmic substance. On tearing these open, the whole interior was found to have the same composition; and no trace of any structural arrangement could be discovered in their mass. Hence they might be supposed to be mere accidental agglomerations, were it not for their conformity to the 'monerozoic' type previously described (§ 393); for just as a simple ' moner,' by a differentiation of its homogeneous sar- code, becomes an Amceba, so would one of these uniform blendings of sand and sarcode, by a separation of its two components, — the sand form- ing the investing ' test,' and the sarcode occupying its interior, — become the arenaceous Astrorliiza. This type, which abounds on the sea-bed in certain localities, presents remarkable variations of form : being sometimes globular, sometimes stellate, sometimes cervicorn. But the same general arrangement prevails throughout; the cavity being occupied by a dark-- green sarcode, whilst the 'test' is composed of loosely aggregated sand- grains not held together by any recognizable cement, and has no definite orifice, so that the pseudopodia must issue from interstices between the sand-grains, which spaces are probably occupied during life with living protoplasm, that continues to hold together the sand-grains after death. These are by no means microscopic forms; the ' stellate ' varieties ranging to 0.3 or even 0.4 inch in diameter, and the ' cervicorn ' to nearly 0.5 inch in length.1 474. The purely Arenaceous Foraminifera are arranged by Mr. H. B. Brady3 (by whom they have been specially studied) under two Families: the first of which, Astrorliizida, includes with the preceding a number of coarse sandy forms, usually of considerable size, and essentially monothalamous, though sometimes imperfectly chambered by constric- tions at intervals. Some of the more interesting examples of this family will now be noticed; beginning with the Saccamina (Sars), which is a remarkably regular type, composed of coarse sand-grains firmly cemented together in a globular form, so as to form a wall nearly smooth on the outer, though rough on the inner surface, with a projecting neck sur- rounding a circular mouth (Fig. 319,* a, b, c9). This type, which occurs in extraordinary abundance in certain localities (as the entrance of the Christiania-f jord), is of peculiar interest from the fact that it has been discovered in a fossil state by Mr. H. B. Brady, in a clay seam between two layers of Carboniferous Limestone. Its size is that of very minute seeds. — In striking contrast to the preceding is another single-chambered 1 See the description and figures of this type given by the Author in "Quart. Journ. Microsc. Sci.," Vol. xvi (1876), p. 221. 2 See his " Notes " in " Quart Journ. of Microsc. Soc.,"N.S., Vol. xix. (1879), p. 20 ; and Vol. xxi. (1881), p. 31. FORAMINIFERA AND RADIOLARIA. 79 type, distinguished by the whiteness of its 'test,' to which the Author has given the name of Pilulina, from its resemblance to a homoeopathic * globule' (Fig. 319,* d, e). The form of this is a very regular sphere; and its orifice, instead of being circular and surrounded by a neck, is a slit or fissure with slightly raised lips, and having a somewhat S-shaped curvature. It is by the structure of its 'test,' however, that it is espe- cially distinguished; for this is composed of the finest ends of sponge- spicules, very regularly 'laid' so as to form a kind of felt, through the substance of which very fine sand-grains are dispersed. This 'felt' is somewhat flexible, and its components do not seem to be united by any kind of cement, as it is not affected by being boiled in strong nitric acid; its tenacity, therefore, seems entirely due to the wonderful manner in which the separate siliceous fibres are ' laid.' — It is not a little curious that Fia. 319* Arenaceous Foraminifera:— a, Saccamina spherica; 6, the same laid open; e, portion of the test enlarged to show its component sand-grains:— d, Pilulina Jeffrey sii: e, portion of the test enlarged showing the arrangement of the sponge-spicules. these two forms should present themselves in the same dredging; and that there should be no perceptible difference in the character of their sarcode-bodies, which, as in the preceding case, have a dark-green hue. — The Marsipella elongata (Fig. 320, d), on the other hand, is somewhat fusiform in shape, and has its two extremities elongated into tubes, with a circular orifice at the end of each. The materials of the < tests ' differ remarkably according to the nature of the bottom whereon they live. When they come up with ' Globigerina mud,' in which sponge-spicules abound, whilst sand-grains are scarce, they are almost entirely made up of the former, which are ( laid ' in a sort of lattice- work, the interspaces of which are filled up by fine sand-grains; but when they are brought up from a bottom on which sand predominates, the larger part of the ' test ' is made up of sand -grains and minnte Foraminifera, with here and there a sponge-spicule (Fig. 320, d, f). In each case, however, the tubular extensions (one of which sometimes forms a sort of proboscis, e, nearly equalling the body itself in length) are entirely made up of eponge-spic- 80 THE MICROSCOPE AND ITS KEVELATIONS. ules laid side by side with extraordinary regularity. — The genus Rhab- dammina (Sars) resembles Saccamina in the structure of its ' test,' which is composed of sand-grains very firmly cemented together; but the grains are of smaller size, and they are so disposed as to present a smooth sur- face internally, though the exterior is rough. What is most remark- able about this, is the geometrical regularity of its form, which is typically triradiate (Fig. 321, c), the rays diverging at equal angles from the central cavity, and each being a tube (cl) with an orifice at its extremity. Not unfrequently, however, it is quadri-radiate, the rays diverging at right angles; and occasionally a fifth ray presents itself, its radiation, however, being on a different plane. The three rays are normally of equal length; but one of them is sometimes shorter than the other two; and when this is the case, the angle between the long rays increases at the expense of the other two, so that the long rays lie more nearly in a straight line. Sometimes the place of the third ray is indicated only by a litttle knob: and then the two long rays have very nearly the same direction. FIG. 320. Arenaceous Foraminifera:— a, 6, upper and lower aspects of Halophragmium globigerini forme; c, Hormosina globulifern; d, Marsipella elongata; e, terminal portion, and /, middle portion of the same, enlarged; g, Thurammina papillata; h, portion of its inner surface enlarged. We are thus led to forms in which there is no vestige of a third ray, but merely a single straight tube, with an orifice at each end; and the length of this, which often exceeds half an inch, taken in connection with the abundance in which it presents itself in dredgings in which the triradiate forms are rare, seems to preclude the idea that these long single rods are broken rays of the latter. — It is undoubtedly in this group that we are to place the genus Halipliysema; which, from constructing its ' test ' en- tirely of sponge-spicules, and even including these in its pseudopodial ex- pansions, has been ranked as a Sponge, although observation of it in its living state leaves no doubt whatever of its Khizopodal character.1 1 See Saville Kent in "Ann. of Nat. Hist.," Ser. 5, Vol. ii. (1878); Prof. R. Lankester in "Quart. Journ. Microsc. Sci.," Vol. xix. (1868), p. 476; and Prof. Mobius's " Foraminifera von Mauritius." FORAMINIFERA AND RADIOLARIA. 81 475. Lituolida. — The type of this family, which is named after it, is a large, sandy, many-chambered fossil form occurring in the Chalk, to which the name Lituola was given by Lamarck, from its resemblance in shape to a crozier. A great variety of recent forms, mostly obtained by deep-sea dredging, are now included in it; as bearing a more or less close resemblance to it and to each other in their chambered structure, and in the arrangement of the sand-grains of which their tests are formed. — These grains are, for the most part, finer than those of which the tests of the preceding family are constructed, and are set (so to speak) more artistically; and a considerable quantity of a cement exuded by the ani- mal is employed in uniting them. This is often mixed up with sandy particles of extreme fineness, to form a sort of ' plaster ' with which the exterior of the test is smoothed off, so as to present quite a polished sur- face.— It is remarkable that the cement contains a considerable quantity of oxide of iron, which imparts a ferruginous hue to the * tests ' in whicn it is largely employed. The forms of the Lituoline ' tests ' often simulate in a very curious way those of the simpler types of the Vitreous series. FIG. 321. Arenaceous Foraminifera:— a, 6, Exterior and sectional views of Reophax rudis; c, Bhabdam- mina abyssorum ; d, cross section of one of its arms; e, Reophax scorpiurus; f, Hormosina Car- penteri. Thus, the long, spirally coiled undivided sandy tube of Ammodiscus is the isomorph of Spirillina (§ 479). In the genus Halophragmium (Fig. 320 a, b), we have a singular imitation of the Globiyerine type; and in TJiurammina papillata (Fig. 320, are often so marked, as to forbid the idea that the latter are solely derived from the former.1 For not only are there several specific types Fig. 326. upon original uharnber-wall, which is raised into ridges with tubuli between them, and includes found in each, which do not present themselves in the other, but, as a rule, the shells of the types common to both are larger and thicker in the latter than they are in the former. This evidence strongly supports the conclusion originally drawn by the Author from his own examination of the Globigerina-ooze, that the shells forming its surface-layer must live on the bottom, being incapable of floating in consequence of their weight; and that if they have passed the earlier part of their lives in the upper waters, they drop down as soon as the calcareous deposit continu- ally exuding from the body of each animal, instead of being employed in the formation of new chambers, is applied to the thickening of those 1 " Quart. Journ. Microsc. Sci., " Vol. xix. (1879), p. 295. 88 THE MICROSCOPE AND ITS REVELATIONS. previously formed. — That many types of Foraminifera pass their whole lives at depths of at least 2000 fathoms, is proved, in regard to those forming Calcareous shells, by their attachment to stones, corals, etc. ; and in the case of the Arenaceous types, by the fact that they can only pro- cure on the bottom the sand of which their ' tests ' are made up. 481. A very remarkable type has Fis- 327. recently been discovered, adherent to shells and corals brought from tropical seas, to which the name Carpenter ia has been given; this may be regarded as a highly devel- oped form of Globigerina, its first- formed portion having all the es- sential characters of that genus. It grows attached by the apex of its spire; and its later chambers increase rapidly in size, and are piled on the earlier in such a man- ner as to form a depressed cone with an irregular spreading base. The essential character of Globi- gerina — the separate orifice of each of its chambers — is here retained with a curious modification; for the central vestibule, into which they all open, forms a sort of vent whose orifice is at the apex of the cone, ™d is : sometimes prolonged into a tube that proceeds from it; and the external wall of this cone is so mark- ed-out by septal bands, that it comes to bear a strong resemblance to a mi- nute Balanus (acorn-shell), for which this type was at first mistaken. The principal chambers are partly divided into chamberlets by incomplete partitions, as we shall find them to be in Eozoon (§ 494). The presence of sponge-spicules in large quantity in the chambers of many of the best- preserved examples of this type, was for some time a source of perplexity; but this is now explained by the interesting observations made by Prof. Mobius1 on a large branching and spreading form of Carpenteria, which he recently met-with on a reef near Mauritius, and to which he has given the name of C. raphidodendron. For the pseudopodia of this Rhizopod have the habit, like those of Haliphysema (§ 474), of taking into them- selves sponge-spicules, which they draw into the chambers, so that they become incorporated with the sarcode-body. 482. A less aberrant modification of the Globigerine type, however, is presented in the two great series which may be designated (after the leading forms in each) as the Textularian and the Rotalian. For not- withstanding the marked difference in their respective plans of growth, the characters of the individual chambers are the same; their walls being coarsely-porous, and their apertures being oval, semi-oval, or crescent- shaped, sometimes merely fissured. In Textularia (Plate xv., fig. 14) the chambers are arranged biserially along a straight axis, the position of those on the two sides of it being alternate, and each chamber opening Globigerina, as captured by tow-net, floating at or near surf ace. 1 See his " Foraminifera von Mauritius," Plates v., vi. FORAMINIFERA AND RADIOLARIA. 89 into those above and below it on the opposite side by a narrow fissure; as is well shown in such ' internal casts ' (Fig. 328, A) as exhibit the forms and connections of the segments of sarcode by which the chambers were occupied during life. In the genus Bulimina the chambers are so arranged as to form a spire like that of a Bulimus, and the aperture is a curved fissure whose direction is nearly transverse to that of the fissure of Textularia; but in this, as in the preceding type, there is an extraordi- nary variety in the disposition of the chambers. In both, moreover, the shell is often covered by a sandy incrustation, so that its perforations are completely hidden, and can only be made visible by the removal of the adherent crust. And so many cases are now known, in which the shell of TextularincB is entirely replaced by a sandy test, that some Systematists prefer to range this group among the A renacea. 483. In the Rotalian series, the chambers are disposed in a turbinoid spire, opening one into another by an aperture situated on the lower and inner side of the spire, as shown in Plate xv., fig. 18; the forms and con- nections of the segments of their sarcode-bodies being shown in such ' internal casts' as are represented in Fig. ^328, B. One of the lowest and simplest forms of this type is that very common one now distin- FIG. 328. FIG. 329. Internal siliceous Casts, representing the forms of the segments of the animals, of A, Textularia, B, Rotalia. Tinoporus baculatus. guished as Discorbina, of which a characteristic example is represented in Plate xv., fig. 15. The early form of Planorbulina is a rotaline spire, very much resembling that of Discorbina; but this afterwards gives place to a cyclical plan of growth (fig. 17); and in those most developed forms of this type which occur in warmer seas, the earlier chambers are completely overgrown by the latter, which are often piled-up in an irregular ' acervuline ' manner, spreading over the surfaces of shells, or clustering round the stems of zoophytes. — In the genus Tinoporus there is a more regular growth of this kind, the chambers being piled succes- sively on the two sides of the original median plane, and those of adja- cent piles communicating with each other obliquely (like those of Textu- laria) by large apertures, whilst they communicate with those directly above and below by the ordinary pores of the shell. The simple or smooth form of this genus presents great diversities of shape, with great constancy, in its internal structure; being sometimes spherical, some- times resembling a minute sugar-loaf, and sometimes being irregularly flattened-out. A peculiar form of this type (Fig. 329), in which the 90 THE MICROSCOPE AND ITS REVELATIONS. walls of the piles are thickened at their meeting-angles into solid columns that appear on the surface as tubercles, and are sometimes prolonged into spinous outgrowths that radiate from the central mass, is of very common occurrence in shore-sands and shallow-water dredgings on some parts of the Australian coast and among the Polynesian islands. — To the simple form of this genus we are probably to refer a large part of the fossils of the Cretaceous and early Tertiary period, that have been de- scribed under the name Orbitolina, some of which attain a very large size. Globular Orbitolince, which appear to have been artificially perforated and strung as beads, are not unfrequently found associated with the " flint-implements " of gravel-beds. — Another very curious modification of the Eotaline type is presented by Polytrema, which so much resem- bles a Zoophyte as to have been taken for a minute Millepore; but which is made up of an aggregation of ' globigerine ' chambers communicating with each other like those of Tinoporus, and differs from that genus in nothing else than its erect and usually branching manner of growth, and the freer communication between its chambers. This, again, is of special interest in relation 4o Eozoon; showing that an indefinite zoo- phytic mode of growth is perfectly compatible with truly Foraminiferal structure. 484. In Rotalia, properly so called, we find a marked advance towards the highest type of Foraminiferal structure; the partitions that divide the chambers being composed of two laminae, and spaces being left between them which give passage to a system of canals, whose general distribution is shown in Fig. 330. The proper walls of the chambers, moreover, are thickened by an extraneous deposit or * intermediate skel- eton,' which sometimes forms radiating outgrowths; but this peculiar- ity of conformation is carried much further in the genus Calcarina, which has been so designated from its resemblance to a spur-rowel (Plate xvi., fig. 3). The solid club-shaped appendages with which the shell is provided, entirely belonging to the ' intermediate skeleton ' Z>, which is quite independent of the chambered structure a; and this body is nourished by a set of canals containing prolongations of the sarcode- body, which not only furrow the surface of these appendages, but are seen to traverse their interior when this is laid open by section, as shown at c. In no other recent Foraminifer does the ' canal system' attain a like development; and its distribution in this minute shell, which has been made out by careful microscopic study, affords a val- uable clue to its meaning in the gigantic fossil organism Eozoon Cana- dense (§ 494). The resemblance which Calcarina bears to the radiate forms of Tinoporus (Fig. 329), which are often found with them in the same dredgings, is frequently extremely striking; and in their early growth the two can scarcely be distinguished, since both commence in a ' rotaline ' spire with radiating appendages; but whilst the successive chambers of Calcarina continue to be added on the same plane, those of Tinoporus are heaped-up in less regular piles. 485. Certain beds of Carboniferous Limestone in Russia are entirely made up, like the more modern Nummulitic Limestone (§ 489), of an aggregation of the remains of a peculiar type of Foraminifera, to which the name Fusulina (indicative of its fusiform or spindle-shape) has been given (Fig. 331). In general aspect and plan of growth it so much re- sembles Alveolina, that its relationship to that type would scarcely be questioned by the superficial observer. But when its mouth is examined, it is found to consist of a single slit in the middle of the lip; and the FORAMLNIFERA AND RADIOLARIA. 91 FIG. 330. interior, instead of being minutely divided into cnamberlets, is found to consist of a regular series of simple chambers; while from each of these proceeds a pair of elongated extensions, which correspond to the 'alar prolongations' of other spirally growing Foraminifera (§ 486), but which, instead of wrapping round the preceding whorls, are prolonged in the direction of the axis of the spire, those of each whorl projecting be- yond those of the preceding, so that the shell is elongated with every in- crease in its diameter. Thus it ap- pears that in its general plan of growth, Fusulina bears much the same relation to a symmetrical Eo- taline or Nummuline shell, that AlveoUna bears to Orbiculina ; and this view of its affinities is fully con- firmed by the Author's microscopic examination of the structure of its shell. For although the Fusulina- Limestone of Russia has undergone a degree of metamorphism, which so far obscures the tubularity of its component shells, as to prevent him from confidently affirming it, yet the appearances he could distin- guish were decidedly in its favor. And having since received speci- mens from the Upper Coal Mea- sures of Iowa, U. S., which are in a much more perfect state of pre- servation, he is able to state with certainty, not only that Fusulina is tubular, but that its tubulation is of the large coarse nature that marks its affinity rather to the Rotaline than to the Nummuline series. — This type is of peculiar interest, as having long been regarded as the oldest form of Foraminifera which was known to have occurred in sufficient abundance to form Eocks by the aggregation of its individuals. It will be presently shown, however, that in point both of antiquity and of importance, it is far surpassed by another (§ 493). 486. Nummulinida. — All the most elaborately constructed, and the greater part of the largest, of the ' vitreous ' Foraminifera belong to the group of which the well-known Nummulite may be taken as the repre- sentative. Various plans of growth prevail in the family; but its distin- guishing characters consist in the completeness of the wall that surrounds each segment of the body (the septa being double instead of single as elsewhere), the density and fine porosity of the shell-substance, and the presence of an ' intermediate skeleton,' with a 'canal-system' for its nutrition. It is true that these characters are also exhibited in the high- est of the Eotaline series (§ 484), whilst they are deficient in the genus Amphistegina, which connects the Nummuline series with the Eotaline; but the occurrence of such modifications in their border-forms is common to other truly Natural groups. With the exception of Amphistegiria, all the genera* of this family are symmetrical in form; the spire being nauti- loid in such as follow that plan of growth, whilst in those which follow the cyclical plan there is a constant equality on the two sides of the median plane: but in Amphistegina there is a reversion to the rotalian Section of Rotalia Schroetteriana near its base and parallel to it; showing <*, a, the radiat- ing interseptal canals; b, their Internal bifurca- tions; c, a transverse branch; d, tubular wall of the chambers. yiJ THE MICROSCOPE AND ITS REVELATIONS. type in tlie turbinoid form of its spire, as in the characters already speci- fied, although, its general conformity to the Nummuline type is such as to leave no reasonable doubt as to its title to be placed in this familv. Notwithstanding the want of symmetry of its spire, its accords with Operculina and Nummulina in having its chambers extended by ' alar prolongations ' over each surface of the previous whorl; but on the under side these prolongations are almost entirely cut off from the principal chambers, and are so displaced as apparently to alternate with them in position; so that M. D'Orbignv, supposing them to constitute a distinct series of chambers, described its plan of growth as a biserial spiral, and made this the character of a separate Order.1 487. The existing Nummulinida are almost entirely restricted to tropi- cal climates; but a beautiful little form, the Polystomella crispa (Plate xv., fig. 16), the representative of a genus that presents the most regular and complete development of the ( canal system ' anywhere to be met with, is Section of Fusulina-IAinesione. common on our own coasts. The peculiar surface-marking shown in the figure consists in a strongly marked ridge and furrow plication of the shelly wall of each segment along its posterior margin; the furrows being sometimes so deep as to resemble fissures opening into the cavity of the chamber beneath. No such openings, however, exist; the only com- munication which the sarcode-body of any segment has with the exterior, being either through the fine tubuli of its shelly walls, or through the TOW of pores that are seen in front view along the inner margin of the septal plane, collectively representing a fissured aperture divided by minute bridges of shell. The meaning the plication of the shelly wall comes to be understood, when we examine the conformation of the seg- ments of the sarcode-body, which may be seen in the common Polysto- mella crispa by dissolving away the shell of fresh specimens by the action of dilute acid, but which may be better studied in such internal casts (Fig. 332) of the sarcode-body and canal-system of the large P. craticulata of the Australian coast, as may sometimes be obtained by the same means from 1 For an account of this curious modification of the Nummuline plan of growth, the real nature of which was first elucidated by Messrs. Parker and Rupert Jones, see the Author's ' Introduction to the Study of the Foraminifera (published by the Ray Society). FORAMINIFERA AND KAUIOLARIA. 9S dead shells which have undergone infiltration with ferruginous silicates. * Here we see that the segments of the sarcode-body are smooth along their anterior edge b, I1, but that along their posterior edge, a, they are prolonged backwards into a set of 'retral processes ;' and these processes lie under the ridges of the shell, whilst the shelly wall dips down into the spaces between them, so as to form the furrows seen on the surface. The connections of the segments by stolons, c, c\ passing through the pores at the inner margin of each septum, are also admirably displayed in such * casts.' But what they serve most beautifully to demonstrate is tho canal-system, of which the distribution is here most remarkably complete and symmetrical. At d, d,1 d,* are seen three turns of a spiral canal which passes along one end of all the segments of the like number of convolutions, whilst a corresponding canal is found on the side which in the figure is undermost; these two spires are connected by a set of meridional canals, e, e1, e*, which pass down between the two layers of the septa that divide the segments; whilst from each of these there passes off towards the surface a set of pairs of diverging branches,/,/1, FIG. 332. Internal Cast of Polystomella craticulata :— a, retral processes, proceeding from the posterior margin of one of the segments; 6, &', smooth anterior margin of the same segment; c, ci, stolons connecting successive segments, and uniting themselves with the diverging branches of the meridional canals; d, d\ d\ three turns of one of the spiral canals; e, e1, e\ three of the meridi- onal canals; /, /', A their diverging branches. /2, which open upon the surface along the two sides of each septal band, the external openings of those on its anterior margin being in the furrows between the retral processes of the next segment. These canals appear to be occupied in the living state by prolongations of the sarcode-body; and the diverging branches of those of each convolution unite themselves, when this is inclosed by another convolution, with the stolon-processes 1 It was by Prof. Ehrenberg that the existence of such ' casts ' in the Green Sands of various Geological periods (from the Silurian to the Tertiary) was first pointed out, in his Memoir * Ueber den Griinsand und seine Einlauterung des organischen Lebens,' in " Abhandlungen den Konigl. Akad.der Wissenschaften," Berlin, 1855. It was soon afterwards shown by the late Prof. Bailey ("Quart. Journ. Microsc. Sci.," Vol. v., 1857, p. 83) that the like infiltration occasionally takes place in recent Foraminifera, enabling similar 'casts' to be obtained from them by the solution of their shells in dilute acid; the Author, as well as Messrs. Parker and Rupert Jones, soon afterwards obtained most beautiful and complete internal casts from recent Foramimfera brought from various localities; and a large collection of green sands yielding similar casts was made in the * Chal- lenger.' 94 THE MICROSCOPE AND ITS REVELATIONS. connecting the successive segments of the latter, as seen at cl. There can be little doubt that this remarkable development of the canal- system has reference to the unusual amount of shell-substance which is deposited as an ' intermediate skeleton ' upon the layer that forms the proper walls of the chambers, and which fills up with a solid ' boss 'what would other- wise be the depression at the umbilicus of the spire. The substance of this 'boss' is traversed by a set of straight canals, which pass directly from the spiral canal beneath, towards the external surface, where they open in little pits, as is shown in PI. xv., fig. 16; the umbilical boss in P. crispa, however, being much smaller in proportion than it is in P. craticulata. There is a group of Foraminifera to which the term Nonionina is properly applicable, that is probably to be considered as a sub-genus of Polystomella; agreeing with it in its general conformation, and especially in the distribution of its canal system; but differing in its aperture, which is here a single fissure at the inner edge of the septal plane (Plate xv., fig. 19), and in the absence of the ' retral processes' of the segments of the sarcode-body, the external walls of the chambers being smooth. This form constitutes a transition to the ordinary Nummuline type, of which Polystomella is a more aberrant modification. 488. The Nummuline type is most characteristically represented at the present time by the genus Operculina; which is so intimately united to the true Nummulite by intermediate forms, that it is not easy to sep- arate the two, notwithstanding that their typical examples are widely dissimilar. The former genus (Plate xvi., fig. 2) is represented on our own coast by very small and feeble forms; but it attains a much higher development in Tropical seas, where its diameter sometimes reaches l-4th of an inch. The shell is a flattened nautiloid spire, the breadth of whose earlier convolutions increases in a regular progression, but of which the last convolution (in full-grown speciirfens) usually flattens itself out like that of Peneroplis, so as to be very much broader than the preceding. The external walls of the chambers, arching over the spaces between the septa, are seen at #, #; and these are bounded at the outer edge of each convolution by a peculiar band «, termed the ( marginal cord.' This cord, instead of being perforated by minute tubuli like those which pass from the inner to the outer surface of the chamber-walls without division or inosculation (Fig. 335), is traversed by a system of compara- tively large inosculating passages seen in cross section at a' ' ; and these form part of the canal-system to be presently described. The principal cavities of the chambers are seen at c, c; while the ' alar prolongations ' of those cavities over the surface of the preceding whorl are shown at c', c'. The chambers are separated by the septa, d, d, d, formed of two laminae of shell, one belonging to each chamber, and having spaces between them in which lie the * interseptal canals,' whose general distri- bution is seen in the septa marked e, e, and whose smaller branches are seen irregularly divided in the septa d', d', whilst in the septum d" one of the principal trunks is laid open through its whole length. At the approach of each septum to the marginal cord of the preceding, is seen the narrow fissure which constitutes the principal aperture of communica- tion between the chambers; in most of the septa, however, there are also some isolated pores (to which the lines point that radiate from e, e) varying both in number and position. The interseptal canals of each septum take their departure at its inner extremity from a pair of spiral canals, of which one passes along each side of the marginal cord; and they communicate at their outer extremity with the canal-system of the FORAMINIFERA AND RADIOLARIA. 95 PLATE XVI. Fio. 1. FIG. 2 VARIOUS FORMS OF FORAMINIFERA (Original). Fig. 1. Cycloclypeus, showing external surface, and vertical and horizontal sections. 2. Operculina, laid open to show its internal structure : a, marginal core., seen in cross section at a' ; b, 6, external walls of the chambers; c, c, cavities of the chambers; c' c'. their alar prolonga- tions; d, d, septa, divided at d' d' and at d", so as to lay open the interseptal canals, the general distribution of which is seen in the septa e, e ; the lines radiating from e, e, point to the secondary pores; g, g, non-tubular columns. 3. Calcarina, laid open to show its internal structure :— a, chambered portion ; 6, intermediate skeleton; c, one of the radiating prolongations proceeding from it, with extensions of the canal- 96 THE MICROSCOPE AND ITS REVELATIONS. ' marginal cord/ as shown in Fig. 337. The external walls of the chambers are composed of the same finely-tubular shell-substance that forms them in the Nummulite; but, as in that genus, not only are the septa themselves composed of vitreous non-tubular substance, but that which lies over them, continuing them to the surface of the shell, has the same character; showing itself externally in the form sometimes of continuous ridges, sometimes of rows of tubercles, which mark the posi- tion of the septa beneath. These non-tubular plates or columns are often traversed by branches of the canal-system, as seen at g, q. Similar columns of non-tubular substance, of which the summits show them- selves as tubercles on the surface, are not unfrequently seen between the septal bands, giving a variation to the surface-marking, which, taken in conjunction with variations in general conformation, might be fairly held sufficient to characterize distinct species, were it not that on a com- parison of a great number of specimens, these variations are found to be so gradational, that no distinct line of demarcation can be drawn between the individuals which present them. FIG. 333. A, piece of Nummulitic Limestone from Pyrenees, showing Nummulites laid open by fracture through median plane; B, vertical section of Nummulite; c. Orbitoides. 489. The Genus Nummulina, of which the fossil forms are commonly known as Nummulites, though represented at the present time by small and comparatively infrequent examples, was formerly developed to a vast extent; the Nummulitic Limestone chiefly made-up by the aggregation of its remains (the material of which the Pryamids are built) forming a band, often 1,800 miles in breadth and frequently of enormous thick- ness, that may be traced from the Atlantic shores of Europe and Africa, through Western Asia to Northern India and China, and likewise over vast areas of North America (Fig, 333). The diameter of a large pro- portion of fossil Nummulites ranges between half an inch and an inch; but there are some whose diameter does not exceed l-16th of an inch, whilst others attain the gigantic diameter of 4£ inches. Their typical form is that of a double-convex lens; but sometimes it much more nearly approaches the globular shape, whilst in other cases it is very much flat- tened; and great differences exist in this respect among individuals of what must be accounted one and the same species. Although there are some Nummulites which closely approximate Operculincz in their mode of growth, yet the typical forms of this genus present certain well-marked distinctive peculiarities. Each convolution is so completely invested by that which succeeds it, and the external wall or spiral lamina of the new convolution is so completely separated from that of the convolution it FOKAMINIFERA AND RADIOLAKIA. 97 incloses by the ' alar prolongations ' of its own chambers (the peculiar arrangement of which will be presently described), that the spire is scarcely if at all visible on the external surface. It is brought into view, however, by splitting the Nummulite through the median plane, which may often be accomplished simply by striking it on one edge with a hammer, the. opposite edge being placed on a firm support; or, if this method should not succeed, by heating it in the flame of a spirit-lamp, and then throwing it into cold water or striking it edgeways. Nummu- lites usually show many more turns, and a more gradual rate of increase in the breadth of the spire, than Foraminifera generally; this will be apparent from an examination of the vertical section shown in Fig. 334, which is taken from one of the commonest and most characteristic fossil examples of the genus, and which shows no fewer than ten convolutions in a fragment that does not nearly extend to the centre of the spire. This section also shows the complete inclosure of the older convolutions by the newer, and the interposition of the alar prolongations of the chambers between the suc- cessive layers of the spiral FlQ- 334< lamina. These prolonga- tions are variously arranged in different examples of the genus: thus in some, as N. distant, they keep their own , separate course, all tending'1 radially towards the centre; in others, as N. Icevigata, their partitions inosculate with each other, so as to di- _^^^ Vide the Space intervening Vertical section of portion of Nummulina Icevigata;- between each lavei* and the °' mar^in of external whorl; &, one of the outer row of . J i chambers; c, c, whorl invested by a; d. one of the chambers next into an UTegUlar n et- of the fourth whorl from the margin; e, e'. marginal por- wnrlc •nrp<5PnHno- in vprfioal tions of tne inclosed whorls; /, investing portions of outer W01K, pit ing 11 Vemcaiwhorl. ^ ^ spaces left between the investing portion of Section the appearance.showil successive whorls; h, h, sections of the partitions dividing in Fig. 334; whilst in N. gar- these" ansensis they are broken up into a number of chamberlets, having little or no direct communication with each other. 490. Notwithstanding thab the inner chambers are thus so deeply buried in the mass of investing whorls, yet there is evidence that the segments of sarcode which they contained were not cut off from commu- nication with the exterior, but that they may have retained their vitality to the last. The shell itself is almost everywhere minutely porous, being penetrated by parallel tubuli which pass directly from one surface to the other. These tubes are shown, as divided lengthways by a ver- tical section, in Fig. 335, a, a ; whilst the appearance they present when cut across in a horizontal section is shown in Fig. 336, the transparent shell-substance a, a, «, being closely dotted with minute punctations which mark their orifices. In that portion of the shell, however, which forms the margin of each whorl (Fig. 335, b, V), the tubes are larger, and diverge from each other at greater intervals; and it is shown by horizontal sections that they communicate freely with each other later- ally, so as to form a network such as is seen at b, b, Fig. 337. At cer- tain other points, d, d, d (Fig. 335), the shell-substance is not perfo- rated by tubes, but is peculiarly dense in its texture, forming solid pil- lars, which seem to strengthen the other parts; and in Nummulites whose 7 98 THE MICROSCOPE AND ITS REVELATIONS. surfaces have been much exposed to attrition, it commonly happens that the pillars of the superficial layer, being harder than the ordinary shell- substance, and being consequently less worn down, are left as promi- nences, the presence of which has often been accounted (but erroneously) as a specific character. The successive chambers of the same whorl communicate with each other by a passage left between the inner edge of the partition that sepa- Fl°- 335- rates them, and the ' mar- ginal cord ' of the preced- ing whorl; this passage is sometimes a single large broad aperture, but is more commonly formed by the more or less complete co- alescence of several separ- ate perforations, as is seen in Fig. 334, b. There is also, as in Operculina, a variable number of isolated Portion of a thin Section of Nummulina lavigata, taken in Pores in most °f tne Septa, the direction of the preceding, highly magnified to show the forming a SCCOndaiT means minute structure of the shell:— a, a. portions of the ordinary . ,- r , shell-substance traversed^ by parallel tubuli; 6, 6, portions 01 Communication between the chambers. — The Ca- nal-system of Nummulina seems to be distributed upon essentially the same plan as in Operculina; its passages, however, are usually more or less obscured by fossilizing material. A careful exami- nation will generally disclose traces of them in the middle of the parti - forming the marginal cord, traversed by diverging and larger tubuli; c, one of the chambers laid open; d, d. d, pillars of solid substance not perforated by tubuli. FIG. 336. FIG. 337. Portion of Horizontal Section of Nummulite, showing the structure of the walls and of the septa of the chambers: — a, a. a, portion of the wall covering three chambers, the puncta- tions of which are the orifices of tubuli ; &, 6, septa between these chambers, containing canals which send out late- ral branches, c, c, entering the cham- bers by larger orifices, one of which is seen at d. Internal cast of two of the chambers, a, a, of Nummulina striata, with the network of Canals, b, 6, in the marginal cord, communicating with canals pass- ing between the chambers. tions that divide the chambers (Fig: 336, b, b), while from these may be seen to proceed the lateral branches (c, c), which, after burrowing (so to speak) in the walls of the chambers, enter them by large orifices (d). These ' interseptal ' canals, and their communication with the inosculat- FORAMINIFERA AND RADIOLARIA. 99 ing system of passages excavated in the marginal cord, are extremely well seen in the 'internal cast' represented in Fig. 337. 491. A very interesting modification of the Nummuline type is pre- sented in the genus Heterostegina (Fig. 338), which bears a very strong resemblance to Orbiculinam its plan of growth, whilst in every other respect it is essentially different. If the principal chambers of an Oper- culina were divided into chamberlets by secondary partitions in a direc- tion transverse to that of the principal septa, it would be converted into a Heterostegina ; just as a Peneroplis would be converted by the like subdi- vision into an OrUculina (§ 464). Moreover, we see in Heterostegina, as in Orbiculina, a great tendency to the opening-out of the spire with the advance of age; so that the apertural margin extends round a large part of the shell,, which thus tends to become discoidal. And it is not a little FIG. 338. FIG. 339. He terostegina. Section of Orbitoides Fortisii, paral- lel to the surface; traversing at a. a, the superficial layer, and at 6, b, the median laj'er. curious that we have in this series another form, Cydodypeus, which bears exactly the same relation to Heterostegina, that Orbitolites does to Orbi- culina; in being constructed upon the cydical plan from the commence- ment, its chamberlets being arranged in rings around a central chamber (Plate XYI., fig. 1). This remarkable genus, at present only known by specimens dredged up from considerable depths off the coast of Borneo, is the largest of existing Foraminifera; some specimens of its discs in the British Museum having a diameter of 2£ inches. Notwithstanding the difference of its plan of growth, it so precisely accords with the Num- muline type in every character which essentially distinguishes the genus, that there cannot be a doubt of the intimacy of their relationship. It will be seen from the examination of that portion of the figure which shows Cydodypeus in vertical section, that the solid layers of shell by which the chambered portion is inclosed are so much thicker, and con- sist of so many more lamellae, in the central portion of the disk, than 100 THE MICROSCOPE AND ITS REVELATIONS. they do nearer its edge, that new lamellaB must be progressively added to the surfaces of the disk, concurrently with the addition of new rings of chamberlets to its margin. These lamellae, however, are closely applied one to the other, without any intervening spaces; and they are all tra- versed by columns of non-tubular substance, which spring from the sep- tal bands, and gradually increase in diameter with their approach to the surface, from which they project in the central portion of the disk as glistening tubercles. 492. The Nummulitic Limestone of certain localities (as the South- west of France, North-eastern India, etc. ) contains a vast abundance of discoidal bodies termed Orbitoides (Fig. 333, c), which are so similar to Kummulites as to have been taken for them, but which bear a much closer resemblance to Cyclo- F*0- 34°- clypeus. These are only known a in the fossil state; and their structure can only be ascer- tained by the examination of sections thin enough to be translucent. When one of these disks (which vary in size, in different species, from that of a four-penny piece to that of half-a-crown) is rubbed- down so as to display its inter - nal organization, two different kinds of structure are usually seen in it; one being com- posed of chamberlets of very definite form, quadrangular in some species, Portions of the Section of Orbitoides fortisii shown in Fig. 339, more highly magnified;— a superficial lay- er; 6, median layer. ' FIG. 341. Vertical Sections of Orbitoides Fortisii, showing the large central chamber at a, and the median layer surrounding it, covered above and below by the superficial layer. circular in others, arranged with a general but not constant regularity in concentric circles (Figs. 339, 340, #, &); the other, less transparent, being Flo 342 formed of minuter chamberlets which have no such constancy of form, but which might almost be taken for the pieces of a dissected map (a, a). In the upper and lower walls of these last, minute punetuations may be observed, which seem to be the orifices of the connecting tubes whereby they are per- forated. The relations of these two kinds of structure to each other are made evident by the examination of a vertical section (Fig. Internal Cast of portion of median 341) : which sllOWS that the portion b, FigS. plane of Orbitoides Fortisii, showing oo9, d40, forms the median plane, its con- *a,_a',a', af a", six cham >f centrjc circles of chamberlets being arrang- ed round a large central chamber, as in Cy- doclypeus; whilst the chamberlets of the portion a are irregularly superposed one upon the other, so as to form sev- communications; and at b 6, b' b' b b", portions of three annular canals. FORAMIN1FERA AND KADIOLARIA. 101 eral layers which are most numerous towards the centre of the disk, and thin-away gradually towards its margin. The disposition and connec- tions of the chamberlets of the median layer in Orbitoides seem to corre- spond very closely with those which have been already described as pre- vailing in Cyclodypeus; the most satisfactory indications to this effect being furnished by the siliceous ' internal casts ' to be met with in cer- tain Green Sands, which afford a model of the sarco-de-body of the ani- mal. In such a fragment (Fig. 342) we recognize the chamberlets of three successive zones, a, a', a", each of which seems normally to com- municate by one or two passages with the chamberlets of the zone inter- nal and external to its own; whilst between the chamberlets of the same .zone there seems to be no direct connection. They are brought into rela- Fro. 343. Vertical Section of Eozoon Canadense> showing alternation of Calcareous (light) and Serpentin- •ous (dark) lamellae. tion, however, by means of annular canals, which seem to represent the spiral canals of the Nummulite, and of which the ' internal casts ' are seen at b b, V b', V9 I". 493. A most remarkable Fossil, referable to the Foraminiferal type, has been recently discovered in strata much older than the very earliest that were previously known to contain Organic remains; and the deter- mination of its real character may be regarded as one of the most inter- esting results of Microscopic research. This fossil, which has received the name Eozoon Canadense (Fig. 343), is found in beds of Serpentine Limestone that occur near the base of the Laurentian formation1 of 1 This Laurentian formation was first identified as a regular series of stratified rocks, underlying the equivalents not merely of the Silurian, but also of the Upper and Lower Cambrian systems of this country, by Sir William Logan, the former able Director of the Geological Survey of Canada. 102 THE MICROSCOPE AND ITS REVELATIONS. Canada, which has its parallel in Europe in the * fundamental gneiss ' of Bohemia and Bavaria, and in the very earliest stratified rocks of Scandi- navia and Scotland. These beds are found in many parts to contain masses of considerable size, but usually of indeterminate form, disposed after the manner of an ancient Coral Reef, and consisting of alternating layers — frequently numbering from 50 to 100 — of Carbonate of Lime and Serpentine (silicate 'of magnesia). The regularity of this alternation, and the fact that it presents itself also between other Calcareous and Siliceous minerals, having led to a suspicion that it had its origin in Organic structure, thin sections of well-preserved specimens were sub- mitted to microscopic examination by Dr. Dawson of Montreal, who at once recognized its Foraminiferal nature:1 the calcareous layers present- ing the characteristic appearances of true shell, so disposed as to form an irregularly chambered structure, and frequently traversed by systems of ramifying canals corresponding to those of Calcarina (§ 484); whilst the serpentinous or other siliceous layers were regarded by him as having been formed by the infiltration of silicates in solution into the cavities originally occupied by the sarcode-body of the animal, — a process of whose occurrence at various Geological periods, and also at the present time, abundant evidence has already been adduced. Having himself taken up the investigation (at the instance of Sir William Logan), the Author was not only able to confirm Dr. Dawson's conclusions, but to- adduce new and important evidence in support of them.3 Although this determination has been called in question, on the ground that some resemblance to the supposed organic structure of Eozoon is presented by bodies of purely Mineral origin,3 yet, as it has been accepted not only by most of those whose knowledge of Foraminiferal structure gives weight to their judgment (among whom the late Prof. Max Schulze may be specially named), but also by Geologists who have specially studied the Micro-mineralogical structure of the older Metamorphic rocks,4 the Author feels justified in here describing Eozoon as he believes it to have existed when it originally extended itself as an animal growth over vast areas of the sea-bottom in the Laurentian epoch. 1 This recognition was due, as Dr. Dawson has explicitly stated in his original Memoir (" Quarterly Journal of the Geological Society," Vol. xxi., p. 54), to his acquaintance not merely with the Author's previous researches on the mi- nute structure of the Foraminifera, but with the special characters presented by thin sections of Calcarina which had been transmitted to him by the Author, Dr. D. has given an excellent account of the Geological and Mineralogical rela- tions of Eozoon, as well of its Organic structure, in a small book entitled " The Dawn of Life." 2 For a fuller account of the results of the Author's own study of Eozodn, and of the basis on which the above reconstruction is founded, see his Papers, in " Quart. Journ. of Geol. Soc.," Vol. xxi., p. 59, and Vol. xxii., p. 219, and in the " Intellectual Observer," Vol. vii. (1865), p. 278; and his ' Further Researches,' in "Ann. of Nat. Hist.," June, 1874. 3 See the Memoirs of Profs. King and Rowney, in " Quart. Journ. of GeoL Soc.," Vol. xxii., p. 185; and " Ann. of Nat. Hist.," May, 1874. 4 Among these the Author is permitted to mention Prof. Geikie, of Edinburgh, who has thus studied the older rocks of Scotland, and Prof. Bonney, of Cam- bridge and London, who has made a like study of the Cornish and other Serpen- tines. By both these eminent authorities he is assured that they have met with no purely Mineral structure in the least resembling Eozoon, either in its regular alternation of Calcareous and Serpentinous lamellae, or in the dendritic exten- sions of the latter into the former; and while they accept as entirely satisfactory the doctrine of its Organic origin maintained by the Author, they find them- selves unable to conceive of any Inorganic agency by which such a structure could have been produced. FORAMINIFERA AND RADIOLARIA. 103 494. Whilst essentially belonging to the Nummuline group, in virtue of the fine tribulation of the shelly layers forming the ' proper wall ' of its chambers, Eozoon is related to various types of recent Foraminifera in its other characters. For in its indeterminate zoophytic mode of growth, it agrees with Polytrema (§ 483); in the incomplete separation of its chambers, it has its parallel in Carpenteria (§ 481); whilst in the high development of its 'intermediate skeleton' and of the 'canal-sys- tem ' by which this is formed and nourished, it finds its nearest represen- tative in Calcarina (§ 484). Its calcareous layers were so superposed, one upon another, as to include between them a succession of ' storeys ' of chambers (Plate xvn., fig. 1, A1, A1, Aa, A2); the chambers of each 'storey' usually opening one into another, as at a, a, like apartments en suite; but being occasionally divided by conrolete septa, as at I. 5. These septa are traversed by passages of communication between the F™- 344- chambers which they separate; resembling those which, in existing types, are occupied by stolons connecting together the segments of the sarcode- body. Each layer of shell * consists of two finely-tubulat- ed or ' nummuline ' lamellae, B, B, which form the boun- daries of the chambers be- neath and above, serving (so to speak) as the ceiling of the former, and as the^oor of the latter; and of an intervening deposit of homogeneous shell- substance c, c, which consti- tutes the ' intermediate ske- leton. ' The tubuli of this Vert.cal Sect.on of a n one Calcareous 6 nummuline layer (Fig. 344) lamellae of Eozoon Canadense:-^ a, Nummuline layer, nvo nQnnllir -filler! nr» ffta in perforated by parallel tubuli, which show & flexure along aie usually n j^as mfhe Une a, y; beneath this is seen the intennediate the N UmmullteS OI the num- skeleton, e, c, traversed by the large canals, 6, 6, and by mulitip limestone') by min- fflSSl^8^ PlanCS' ^ich extend also into the Num- eral infiltration, so as in transparent sections to present a fibrous appearance; but it fortunately happens that through their haying in some cases escaped infiltration, the tabulation is as distinct as it is even in recent Kummuline shells (Fig. 344), bearing a singular resemblance in its occasional waviness to that of the Crab's claw (§ 613). The thickness of this interposed layer varies con- siderably in different parts of the same mass; being in general greatest near its base, and progressively diminishing towards its upper surface. The 'intermediate skeleton' is occasionally traversed by large passages (D), which seem to establish a connection between the successive layers of chambers; and it is penetrated by arborescent systems of canals (E, E), which are often distributed both so extensively and so minutely through its substance, as to leave very little of it without a branch. These canals take their origin, not directly from the chambers, but from irregular lacuncB or interspaces between the outside of the proper chamber-walls and the 'intermediate skeleton,' exactly as in Calcarina (§ 484); the ex- tensions of the sarcode-body which occupied them having apparently 104 THE MICROSCOPE AND ITS KEVELATIONS. PLATE XVII, FIG. 1. STRUCTURK OP KOZOON CANADKNSE (Original). Fig. 1. Portion of its calcareous Shell, as it would appear if the Serpentine Jthat fills its cham- bers were dissolved away :— A1, A1, chambers of lower story, opening into each other at a, a, but occasionally separated by a septum 6, 6 ; A2, A2, chambers of upper story ; B, u, proper walls of the chambers, formed of a finely-tubular or nummuline substance; c, c, intermediate skeleton, occa- sionally traversed by large stolon-passages, D, connecting the chambers of different stories, and penetrated by the arborescent systems of canals E, E, E. 2. Decalcified portion showing the Serpentinous internal cast of the chambers, canals, and tubuli of the original; presenting an exact model of the animal substance which originally filled them. FORAMINIFERA AND RADIOLARIA. 105 been formed by the coalescence of the pseudopodial filaments that passed through the tubulated lamellae. 495. In the fossilized condition in which Eozoon is most commonly found, not only the cavities of the chambers, but the canal-systems to their smallest ramifications, are filled up by the siliceous infiltration which has taken the place of the original sarcode-body, as in the cases already cited (§ 487 note)\ and thus when a piece of this fossil is sub- jected to the action of dilute acid, by which its calcareous portion is dissolved-away, we obtain an internal cast of its chambers and canal- system (Plate xvii., fig. 2), which, though altogether dissimilar in ar- rangement, is essentially analogous in character to the ' internal casts' represented in Figs. 328, 332. This cast presents us, therefore, with a model in hard Serpentine of the soft sarcode-body which originally occu- pied the chambers, and extended itself into the ramifying canals, of the calcareous shell; and, like that of Polystomella (§ 487), it affords an even more satisfactory elucidation of the relations of these parts, than we could have gained from the study of the living organism. We see that each of the layers of serpentine, forming the lower part of such a specimen, is made up of a number of coherent segments, which have only undergone a partial separation; these appear to have extended themselves horizontally without any definite limit; but have here and there developed new segments in a vertical direction, so as to give origin to new layers. In the spaces between these successive layers, which were originally occupied by the calcareous shell, we see the ' internal casts' of the branching canal-system; which give us the exact models of the extensions of the sarcode-body that originally passed into them. — But this is not all. In specimens in which the nummuline layer consti- tuting the 'proper wall' of the chambers was originally well preserved, and in which the decalcifying process has been carefully managed (so as not, by too rapid an evolution of carbonic acid gas, to disturb the ar- rangement of the serpentinous residuum), that layer is represented by a thin white film covering the exposed surfaces of the segments; the super- ficial aspect of which, as well as its sectional view, are shown in fig. 2. And when this layer is examined with a sufficient magnifying power, it is found to consist of extremely minute needle-like fibres of Serpentine, which sometimes stand upright, parallel, and almost in contact with each other, like the fibres of asbestos (so that the film which they form has been termed the 'asbestiform layer'), but which are frequently grouped in converging brush-like bundles, so as to be very close to each other in certain spots at the surface of the film, whilst widely separated in others. Now these fibres, which are less than 1-10, 000th of an inch in diameter, are the ' internal casts ' of the tubuli of tho Nummuline layer (a precise parallel to them being presented in the ' internal cast ' of a recent Am- phistegina in the Author's possession); and their arrangement presents all the varieties which have been mentioned (§ 488) as existing in the shells of Operculina. — Thus these delicate and beautiful siliceous fibres represent those pseudopodial threads of sarcode, which originally tra- versed the minutely-tubular walls of the chambers; and a precise model of the most ancient animal of which we have any knowledge, notwith- standing the extreme softness and tenuity of its substance, is thus pre- sented to us, with a completeness that is scarcely even approached in any later fossil. 496. In the upper part of the ' decalcified ' ^specimen shown in Plate xvii., fig. 2, it is to be observed that the segments are confusedly heaped 106 THE MICROSCOPE AND ITS REVELATIONS. together, instead of being regularly arranged in layers; the lamellated mode of growth having given place to the acervuline. This change is by no means uncommon among Foraininifera; an irregular piling-together of the chambers being frequently met-with in the later growth of types, whose earlier increase takes place upon some much more definite plan. After what fashion the earliest development of Eozoon took place, we have at present no knowledge whatever; but in a young specimen which has been recently discovered, it is obvious that each successive ' storey' of chambers was limited by the closing-in of the shelly layer at its edges, so as to give to the entire fabric a definite form closely resembling that of a straightened Peneroplis (Plate xv., fig. 5). Thus it is obvious that the chief peculiarity of Eozoon lay in its capacity for indefinite extension ; so that the product of a single germ might attain a size comparable to that of a massive Coral. — Now this, it will be observed, is simply due to the fact that its increase by gemmation takes place continuously ; the new segments successively budded-off remaining in connection with the original stock, instead of detaching themselves from it as in Foramini- fera generally. Thus the little GloMgerina forms a shell of which the number of chambers does not usually seem to increase beyond sixteen, any additional segments detaching themselves so as to form separate shells; but by the repetition of this multiplication, the sea-bottom of large areas of the Atlantic Ocean at the present time has come to be cov- ered with accumulations of Gloligerinm, which, if fossilized, would form beds of Limestone not less massive than those which have had their origin in the growth of Eozoon. — The difference between the two modes of increase may be compared to the difference between a Plant and a Tree. For in the Plant the individual organism never attains any con- siderable size, its extension by gemmation being limited; though the aggregation of individuals produced by the detachment of its buds (as in a Potato field) may give rise to a mass of vegetation as great as that formed in the largest Tree by the continuous putting forth of new buds. 497. It has been hitherto only in the Laurentian Serpentine-Lime- stone of Canada, that Eozoon has presented itself in such a state of pres- ervation as fully to justify the assumption of its Organic nature. But from the greater or less resemblance which is presented to this by Ser- pentine-Limestones occurring in various localities, among strata that seem the Geological equivalents of the Canadian Laurentians, it seems a justifi- able conclusion that this type was very generally diffused in the earlier ages of the Earth's history; and that it had a large (and probably the chief) share in the production of the most ancient Calcareous strata, separat- ing Carbonate of Lime from its solution in Ocean-water, in the same manner as do the Polypes by whose growth Coral-reefs and islands are being upraised at the present time. An elaborate work, " Der Bau des Eozoon Canadense" (1878) has been re- cently published by Prof. Mobius of Kiel, in which the structure of Eozoon is compared with that of various types of Foraminifera, and, as it differs from that of every one of them, is affirmed not to be organic at all, but purely Min- eral. Upon this the Author would remark, that if the validity of this mode of reasoning be admitted, any Fossil whose structure does not correspond with that of some existing type, is to be similarly rejected. Thus, the Stromatopora of Silurian and Devonian rocks, which some Palaeontologists regard as a Coral, others as Polyzoary, others as a Calcareous Sponge, and others as Foraminifer, would not be a fossil at all, because it differs from every known living form. Yet the suggestion that it is of Mineral origin would be scouted as absurd by every Palaeontologist. Agai*, it is urged by Prof. Mobius that as the supposed canal-system of Eozoon has not the constancy and regularity of distribution which FORAMLNTFERA AND RADIOLARIA. 10T it presents in existing Foraminifera, it must be accounted a Mineral infiltration. To this the Author would reply: — (1) That a prolonged and careful study of this 'canal-system,' in a great variety of modes, with an amount of material at his disposal many times greater than Prof. Mobius could command, has satisfied him that in well-preserved specimens the canal-system, so far from being vague and indefinite, has a very regular plan of distribution; — (2) That this plan does not differ more from the arrangements characteristic of the several types of existing Foraminifera, than those differ from each other,. its general conformity to them being such as to satisfy Prof. Max Schultze (one of the ablest Foraminiferalists of his time) of its Foraminiferal character; — and (3) that not only does the distri- bution of the canal-system of Eozoon differ in certain essential features from every form of Mineral infiltration hitherto brought to light, but that canal-sys- tems in no respect differing from each other in distribution are occupied by dif- ferent minerals, — a fact which seems conclusively to point to their pre-existence in the Calcareous layers, and the subsequent penetration of these minerals into the passages previous occupied by sarcode, — precisely as has happened in those * internal casts ' of existing Foraminifera (§ 497) which Prof. Mobius altogether ignores. The argument for the Foraminiferal nature of Eozoon is essentially a cumula- tive one, resting on a number of independent probabilities, no one of which, taken separately, has the cogency of & proof ; yet the accordance of them all with that hypothesis has an almost demonstrative value, no other hypothesis accounting at once for the whole assemblage of facts. — As it is the Author's intention to set forth this in the best and completest form he can devise, at the earliest possible period, he would beg for a suspension of judgment on the part of those who have credited Prof. Mobius with having completely settled the question; the small amount of evidence contained in his Memoir bearing no comparison to that of an opposite bearing of which the Author is in possession. 498. Collection and Selection of Foraminifera. — Many of the Forami- nifera attach themselves in the living state to Sea -weeds, Zoophytes, etc. ;. and they should, therefore, be carefully looked-for on such bodies, espe- cially when it is desired to observe their internal organization and their habits of life. They are often to be collected in much larger numbers, however, from the sand or mud dredged-up from the sea-bottom, or even from that taken from between the tide-marks. In a paper containing some valuable hints on this subject,1 Mr. Legg mentions that, in walking over the Small-mouth Sand, which is situated on the north-side of Port- land Bay, he observed the sand to be distinctly marked with white ridges, many yards in length, running parallel with the edge of the water; and upon examining portions of these, he found Foraminifera in considerable abundance. One of the most fertile sources of supply that our own coasts afford, is the ooze of the Oyster-beds, in which large- numbers of living specimens will be found; the variety of specific forms, however, is usually not very great. In separating these bodies from the particles of sand, mud, etc., with which they are mixed, various methods may be adopted, in order to shorten the tedious labor of picking them out, one by one, under the Simple Microscope; and the choice to be made among these will mainly depend upon the condition of the Foram- inifera, the importance (or otherwise) of obtaining them alive, and the nature of the substances with which they are mingled. — -"Thus, if it be desired to obtain living specimens from the oyster-ooze, for the examina- tion of their soft parts, or for preservation in an Aquarium, much time will be saved by stirring the mud (which should be taken from the surface only of the deposit) in a jar with water, and then allowing it to- stand for a few moments; for the finer particles will remain diffused though the liquid, while the coarser will subside; and as the Forami- nifera (in the present case) will be among the heavier, they will be found 1 " Transaction of Microscopical Society," 2d Series, Vol. ii. (1854), p. 19. 108 THE MICROSCOPE AND ITS REVELATIONS. at the bottom of the vessel with comparatively little extraneous matter, :after this operation has been repeated two or three times. It would always be well to examine the first deposit let fall by the water that has been poured-away; as this may contain the smaller and lighter forms of Foraminifera. — But supposing that it be only desired to obtain the dead shells from a mass of sand brought-up by the dredge, a very different method should be adopted; The whole mass should be exposed for some hours to the heat of an oven, and be turned-over several times, until it is found to have been thoroughly dried throughout; and then, after being allowed to cool, it should be stirred in a large vessel of water. The chambers of their shells being now occupied by air alone (for the bodies of such as were alive will have shrunk-up almost to nothing), the Foraminifera will be the lightest portion of the mass; and they will be found floating on the water, while the particles of sand, etc., subside. Another method, devised by Mr. Legg, consists in taking ad- vantage of the relative sizes of different kinds of Foraminifera and of the substances that accompany them. This, which is especially applica- ble to the sand and rubbish obtainable from Sponges (which may be got in large quantity from the sponge-merchants), consists in sifting the whole aggregate through successive sieves of wire-gauze, commencing with one of 10 wires to the inch, which will separate large extraneous par- ticles, and proceeding to those of 20, 40, 70, and 100 wires to the inch, each (especially that of 70) retaining a much larger proportion of Foram- iniferal shells than of the accompanying particles; so that a large portion of the extraneous matter being thus got rid of, the final selection becomes •comparatively easy. — Certain forms of Foraminifera are found attached to Shells, especially bivalves (such as the Chamacece) with foliated sur- faces; and a careful examination of those of tropical seas, when brought home ' in the rough,' is almost sure to yield most valuable results. — The final selection of specimens for mounting should always be made under some appropriate form of Single Microscope (§§ 43-48); a fine camel- hair pencil, with the point wetted between the lips, being the instrument which may be most conveniently and safely employed, even for the most delicate specimens. In mounting Foraminifera as Microscopic objects, the method to be adopted must entirely depend upon whether they are to be viewed by transmitted or by reflected light. In the former case it^ should be mounted in Canada balsam (§ 210); the various precautions to prevent the retention of air-bubbles, which have been already described, being carefully observed. In the latter no plan is so simple, easy, and •effectual, as the attaching them with a little gum to wooden slides (Fig. 124). They should be fixed in various positions, so as to present all the •different aspects of the shell, particular care being taken that its mouth is clearly displayed; and this may often be most readily managed by at- taching*the specimens sideways to the wall of the circular depression of the slide. Or -the specimens may be attached to disks fitted for being held in Morris's Disk-holder (Fig. 95); whilst for the examination of specimens in every variety of position, Mr. R. Beck's Disk-holder (Fig. 94) will be found extremely convenient. Where, as will often happen, the several individuals differ considerably from one another, special care should be taken to arrange them in series illustrative of their range of variation and of the mutual connections of even the most diverse forms. — For the display of the internal structure of Foraminifera, it will often be necessary to make extremely thin sections, in the manner already de- scribed (§§ 192-194); and much time will be saved by attaching a nunv FORAMINIFERA AND RADIOLARIA. ber of specimens to tlie glass slide at once, and by grinding them down together (§ 192, note). For the preparation of sections, however, of the extreme thinness that is often required, those which have been thus reduced should be transferred to separate slides, and finished-off each one by itself. KADIOLAHIA. 499. It has been shown that one series of forms belonging to the Rhizopod type is characterized by the radiating arrangement of their rod- like pseudopodia (§ 399), suggesting the designation Heliozoa or ' sun-ani- malcules;' and that even among those fresh- water forms that do not depart widely from the common Actinophrys (Fig. 285), there are some whose- bodies are inclosed in a complete siliceous skeleton. Now just as the Flo. 345. Fossil Radiolaria from Barbadoes. — a, Podocyrtis mitra; 6, Rhabdolithus sceptrum; c, Lych- nocanium falciferum; d, Eucyrtidium tubulus; e, Flustrellaconcentrica;/, Lychnocaniumlucerna; » dcmrlav r*mfiinlpc (T?\cr bell Wltn Blendei tentacles IJJ Ig. constricted Polype-body; 6, its original cir- cle of tentacles; c, its secondary circle of tentacles: d, proboscis of most advanced 359, C, a), assumes a cylindrical form, * Polype-bud from side of and elongates itself considerably; a constriction or indentation is then seen around it, just below the ring which encircles the mouth and gives origin to the tentacles; and similar constrictions are soon repeated round the lower parts of the cylinder, so as to give to the whole body somewhat the appearance of a rouleau of coins (Fig. 359, A); a sort of fleshy bulb, a, SPONGES AND ZOOPHYTES. 133 FlG 360 somewhat of the form of the original polype, being still left at the at- tached extremity. The number of circles is indefinite, and all are not formed at once, new constrictions appearing below, after the upper por^ tions have been detached; as many as 30 or even 40 have thus been pro- duced in one specimen. The constrictions then gradually deepen, so as to divide the cylinder into a pile of saucer-like bodies; the division being most complete above, and the upper disks usually pre- senting some increase in diameter; and whilst this is taking place, the edges of the disks become divided into lobes (B), each lobe soon presenting the cleft with the supposed rudimentary eye at the bottom of it, which is to be plainly seen in the detached Medusas Development of Chrysaora from Hydra tuba: — A, de- tached individual viewed sideways, and enlarged, showing the proboscis a, and b the bifid lobes: B, individual seen from above, showing the bifid lobes of the margin, and the quadrilateral mouth; c, one of the bifid lobes still more en- (?) at the bottom of as seen swimming in (Fig. 360, c). Up to this period, the tentacles of the original polype surmount the highest of the disks; but before the detachment of the topmost disk, this circle disappears, and a new one is developed at the summit of the Inilb which remains at the base of the pile (c, c). At last the topmost and largest disk begins to ex- hibit a Sort Of Convulsive farged showjng the rudimentaiy eye (5 Struggle; it becomes detach- the cleft; D, group of young Medusae, a; ed, and SWimS freely away; the water, of the natural size. and the same series of changes takes-place from above downwards, until the whole pile of disks is detached and converted into free-swimming Medusae. But the original polypoid body still remains, and may return to its original polype-like mode of gemmation (D, e) ; becoming the pro- genitor of a new colony, every member of which may in its turn bud-off a pile of Medusa-disks. 524. The bodies thus detached have all the essential characters of the adult MeduscB. Each consists of an umbrella-like disk, divided at its edge into a variable number of lobes, usually eight; and of a stomach, which occupies a considerable proportion of the disk, and projects down- wards in the form of a proboscis, in the centre of which is the quadrangu- lar mouth (Fig. 360, A, B). As the animal advances towards maturity, the intervals between the segments of the border of the disk gradually fill-up, so that the divisions are obliterated; tubular prolongations of the stomach extend themselves over the disk; and from its border there sprout forth tendril-like filaments, which hang down like a fringe around its margin. From the four angles of the mouth, which, even in the youngest detached animal, admits of being greatly extended and pro- truded, prolongations are put forth, which form the four large tentacles of the adult. The young Medusas are very voracious, and grow rapidly, so as to attain a very large size. The Cyanece and Chrysaorce, which are 134: THE MICROSCOPE AND ITS REVELATIONS. common all round our coasts, often have a diameter of from 6 to 15 inches; while the Rhizostoma sometimes reaches a diameter of from two to three feet. The quantity of solid matter, however, which their fabrics contain is extremely small. It is not until adult age has been attained, that the generative organs make their appearance, in four chambers dis- posed around the stomach, which are occupied by plaited membranous ribands containing sperm-cells in the male and ova in the female, and the embryoes evolved from the latter, when they have been fertilized by the agency of the former, repeat the extraordinary cycle of phenomena which has been now described, developing themselves in the first instance into Hydroid Polypes, from which Medusoids are subsequently budded-off. 525. This cycle of phenomena is one of those to which the term 'alternation of generations' was applied by Steenstrup,1 who brought together under this designation a number of cases in which generation A does not produce a form resembling itself, but a different form, B; whilst generation B gives origin to a form which does not resemble itself, but returns to the form A, from which B itself sprang. It was early pointed out, however, by the Author,2 that the term ' alternation of generations ' does not appropriately represent the facts either of this case, or of any of the other cases grouped under the same category: the real fact being that the two organisms, A and B, constitute two stages in the life-history of one generation; and the production of one form from the other being in only one instance by a truly generative or sexual act, whilst in the other it is by a process of gemmation or budding. Thus the Medusce of both orders (the ' naked-eyed ' and the ' covered-eyed ' of Forbes) are detached flower-buds, so to speak, of the Hydroid Zoophytes which bud them off; the Zoophytic phase of life being the most conspicuous in such Thecata as Campanularida and Sertularida, whose Medusa-buds are of small size and simple conformation, and not unfrequently do not detach themselves as independent organisms; whilst the Medusan phase of life is the most conspicuous in the ordinary Acalephs, their Zoophytic stage being passed in such obscurity as only to be detected by careful research. — The Author's views on this subject, which were at first strongly contested by Prof. E. Forbes, and other eminent Zoologists, have now come to be generally adopted. 526. ACTINOZOA. — Of this group, the common Sea- Anemonies may be taken as types; constituting, with their allies, the order Zoantharia, or Helianthoid polypes, which have numerous tentacles disposed in several rows. Next to them come the Alcyonaria, consisting of those whose polypes, having only six or eight broad short tentacles, present a star- like aspect when expanded; as is the case with various composite Sponge- like bodies, unpossessed of any hard skeleton, which inhabit our own shores, and also with the Ked Coral and the TuMpora of warmer seas, which have a stony skeleton that is internal in the first case and external in the second, as also with the Sea-pens, and the Gorgonice or Sea-fans. A third order, Rugosa, consists of fossil Corals, whose stony polyparies are intermediate in character between those of the two preceding. And lastly, the Ctenophora, free swimming gelatinous animals, many of which are beautiful objects for the Microscope, are by most Zoologists ranked with the Actinozoa. 1 See his Treatise on " The Alternation of Generations," published by the Ray Society. 2" Brit, and For. Med.-Chir. Review," Vol. i. (1848), p. 192, et seq. SPONGES AND ZOOPHYTES. 135 527. Of the Zoantharia, the common Actinia or ' sea anemone ' may be taken as the type; the individual polypites of all the composite fabrics included in the group being constructed upon the same model. In by far the larger proportion of these Zoophytes, the bases of the polypites, as well as the soft flesh that connects together the members of aggregate masses, are consolidated by calcareous deposit into stony Corals; and the surfaces of these are beset with ' cells,' usually of a nearly circular form, each having numerous vertical plates or lamellcs radiating from its centre towards its circumference, which are formed by the consolidation of the lower portions of the radiating partitions that divide the space interven- ing between the stomach and the general integument of the animal into separate chambers. This arrangement is seen on a large scale in the Fungia or ' mushroom-coral ' of tropical seas, which is the stony base of a solitary Anemone-like animal; on a far smaller scale, it is seen in the little Caryophyllia, a like solitary Anemone of our own coasts, which is scarcely distinguishable from an Actinia by any other character than the presence of this disk, and also on the surface of many of those stony corals known as ' madrepores;' whilst in some of these the individual polype-cells are so small, that the lamellated arrangement can only be made-out when they are considerably magnified. Portions of the surface of such Corals, or sections taken at a small depth, are very beautiful objects for low powers, the former being viewed by reflected, and the latter by transmitted light. And thin sections of various fossil Corals of this group are very striking objects for the lower powers of the Oxy- hydrogen Microscope. 528. The chief point of interest to the Microscopist, however, in the structure of these animals, lies in the extraordinary abundance and high development of those ' filiferous capsules,' or ' thread-cells,' the presence of which on the tentacles of the Hydroid polypes has been already noticed (§ 514), and which are also to be found, sometimes sparingly, sometimes very abundantly, in the tentacles surrounding the mouth of the Medusae, as well as on other parts of their bodies. If a tentacle of any of the Sea-anemonies so abundant on our coasts (the smaller and more transparent kinds being selected in preference) be cut-off, and be subjected to gentle pressure between the two glasses of the Aquatic-box or the Compressorium, multitudes of little dart-like organs will be seen to project themselves from its surface near its tip; and if the pressure be gradually augmented, many additional darts will every moment come into view. Not only do these organs present different forms in different species, but even in one and the same individual very strongly marked diversities are shown, of which a few examples are given in Fig. 361. At A, B, c, D, is shown the appearance of the 'filiferous capsules,' whilst as yet the thread lies coiled-up in their interior; and at E, F, G, H, are seen a few of the most striking forms which they exhibit when the thread or dart has started-f orth. These thread-cells are found not merely in the tentacles and other parts of the external integument of Actinozoa, but also in the long filaments which lie in coils within the chambers that surround the stomach, in contact with the sexual organs which are attached to the lamellae dividing the chambers. The latter sometimes contain ' sperm-cells ' and sometimes ova, the two sexes being here divided, not united in the same individual. — What can be the office of the filiferous filaments thus contained in the interior of the body, it is difficult to guess-at. They are often found to protrude from rents in the external tegument, when any violence has been used in detaching the 136 THE MICROSCOPE AND ITS REVELATIONS. animal from its base; and when there is no external rupture, they are often forced through the wall of the stomach into its cavity, and may be seen hanging out of the mouth. The largest of these capsules, in their unprotected state, are about l-300th of an inch in length; while the thread or dart, in Corynactis Allmanni, when fully extended, is not less than l-8th of an inch, or thirty-seven times the length of its capsule.1 FIG. 361. FIG. 362. Spicules of Alcyonium and Gorgonia^ FIG. 363. Flliferous Capsules of Actinozoaf— A, B, Corynactis Allmanni; c, E, p, Caryophyllia Smithii ; D, G, Actinia crassicornis ; H, Ac- tinia Candida. A, Spicules of Oorgania guttata. B, Spicules of Muricia elongata. 529. Of the Alcyonaria a characteristic example is found in the Alcyonium digitatum of our coasts; a lobed sponge-like mass, covered with a tough skin; which is commonly known under the name of ' dead- man's toes/ or by the more elegant name of ( mermaids ' fingers.' When 1 See Mr. Gosse's " Naturalist's Rambles on the Devonshire Coast," and Prof. Mobius 'Ueber den Bau, etc., der Nesselkapseln einiger Polypen und Quailen,' in " Abhandl. Naturw. Vereins zu Hamburg," Band v., 1866. SPONGES AND ZOOPHYTES. 137 a specimen of this is first torn from the rock to which it has attached itself, it contracts into an unshapely mass, whose surface presents nothing but a series of slight depressions arranged with a certain regularity. But after being immersed for a little time in a jar of sea-water, the mass swells-out again, and from every one of these depressions an eight-armed polype is protruded, "which resembles a flower of exquisite beauty and perfect symmetry. In specimens recently taken, each of the petal-like tentacula is seen with a hand-glass to be furnished with a row of 'deli- cately-slender pinncB or filaments, fringing each margin, and arching onwards; and with a higher power, these pinnae are seen to be roughened throughout their whole length, with numerous prickly rings. After a day's captivity, however, the petals shrink up into short, thick, unshapely masses, rudely notched at their edges" (Gosse). When a mass of this sort is cut-into it is found to be channelled-out somewhat like a Sponge, by ramifying canals; the vents of which open into the stomachal cavities of the polypes, which are thus brought into free communication with each other, — a character that especially distinguishes this Order. A movement of fluid is kept-up within these canals (as may be distinctly seen through their transparent bodies) by means of cilia lining the inter- nal surfaces of the polypes; but no cilia can be discerned on their external surfaces. The tissue of this spongy polypidom is strengthened through- out, like that of Sponges (§ 510), with mineral spicules (always, however, calcareous), which are remarkable for the elegance of their forms; these are disposed with great regularity around the bases of the polypes, and even extend part of their length upwards on their bodies. In the Gor- tjonia, or sea-fan, whilst the central part of the polypidom is consolidated into a horny axis, the soft flesh which clothes this axis is so full of tuberculated spicules, especially in its outer layer, that, when this dries-up they form a thick yellowish or reddish incrustation upon the horney stem; this crust is, however, so friable, that it may be easily rubbed down between the fingers, and when examined with the Microscope, it is found to consists of spicules of different shapes and sizes, more or less resembling those shown in Figs. 362, 303, sometimes colorless, but some- times of a beautiful crimson, yellow, or purple. These spicules are best seen by Black-ground illumination, especially when viewed by the Bino- cular Microscope. They are, of course, to be separated from the animal substance in the same manner as the calcareous spicules of Sponges (§ 512); and they should be mounted, like them, in Canada balsam. The spicules always possess an organic basis; as is proved by the fact, that when their lime is dissolved by dilute acid, a gelatinous-looking residuum is left, which preserves the form of the spicule. 530. The Ctenophora, or ' comb-bearers/ are so named from the comb-like arrangement of the rows of tiny * paddles/ by the movement of which the bodies of these animals are propelled. A very beautiful and not uncommon representative of this order is furnished by the Cy- dippe pileus (Fig. 364), very commonly known as the Beroe, which designation, however, properly appertains to another animal (Fig. 365) of the same grade of organization. The body of Cydippe Is a nearly- globular mass of soft jelly, usually about 3-8ths of an inch in diameter; and it may be observed, even with the naked eye, to be marked by eight bright bands, which proceed from pole to pole like meridian lines. These bands are seen with the Microscope to be formed of rows of flat- tened filaments, far larger than ordinary cilia, but lashing the water in the same manner; they sometimes act quite independently of one 138 THE MICROSCOPE AND ITS REVELATIONS. another, so as to give to the body every variety of motion, but sometimes work altogether. If the sun -light should fall upon them when they are in activity, they display very beautiful iridescent colors. In addition to these ' paddles,' the Cydippe is furnished with a pair of long tendril-like filaments, arising from the bottom of a pair of cavities in the posterior part of the body, and furnished with lateral branches (A); within these cavities they may lie doubled-up, so as not to be visible externally; and when they are ejected, which often happens quite suddenly, the main filaments first come-forth, and the lateral tendrils subsequently uncoil themselves, to be drawn-in again and packed-up within the cavities with almost equal suddenness. The mouth of the animal, situated at one of the poles, leads first to a quadrifid cavity bounded by four folds which seem to represent the oral proboscis of the ordinary Medusae (Fig. 359); and this leads to the true stomach, which passes towards the opposite pole, near to which it bifurcates, its branches passing towards the polar FIG. 364. FIG. 365. Cydippe pileus, with its tentacles extended. JBeroe Forskalii, showing the tubular prolongations of the stomach. surface on either side of a little body which has every appearance of being a nervous ganglion, and which is surmounted externally by a fringe-like apparatus that seems essentially to consist of sensory ten- tacles.1 From the cavity of the stomach, tubular prolongations pass-off beneath the ciliated bands, very much as in the true JBeroe (B); these may easily be injected with colored liquids, by the introduction of the extremity of a fine-pointed glass-syringe (Fig. 106) into the mouth. The liveliness of this little creature, which may sometimes be collected 1 It is commonly stated that the two branches of the alimentary canal open on the surface by two pores situated in the hollow of the fringe, one on either side of the nervous ganglion. The Author, however, has not been able to satisfy him- self of the existence of such excretory pores in the ordinary Cydippe or Beroe, although he has repeatedly injected their whole alimentary canal and its exten- sions, and has attentively watched the currents produced by ciliary action in the interior of the bifurcating prolongations, which currents alwaysappear to him to return as from csecal extremities. He is himself inclined to believe that this ar- rangement has reference solely to the nutrition of the nervous ganglion and ten- tacular apparatus, which lies imbedded (so to speak) in the bifurcation of the alimentary canal, so as to be able to draw its supply of nutriment direct from that cavity. SPONGES AND ZOOPHYTES. 139 in large quantities at once by the Stick-net, renders it a most beautiful subject for observation when due scope is given to its movements; but for the sake of Microscopic examination, it is of course necessary to con- fine these. — Various species of true Beroe, some of them even attaining the size of a small lemon, are occasionally to be met with on our coasts; in all of which the movements of the body are effected by the like agency of paddles arranged in meridional bands. These are splendidly luminous in the dark, and the luminosity is retained even by fragments of their bodies, being augmented by agitation of the water containing them. — All the Ctenophora are reproduced from eggs, and are already quite ad- vanced in their development by the time they are hatched. Long before they escape, indeed, they swim about with great activity within the walls x)f their diminutive prison; their rows of locomotive paddles early attaining a large size, although the long flexile tentacles of Cydippe are then only short stumpy protuberances. Through the embryonic forms of the two groups, Prof. Alex. Agassiz considers the Ctenophora as re- lated to EcJiinodermata. Those who may desire to acquire a more systematic and detailed acquaintance with the Zoophyte-group, may be especially referred to the following Treatises and Memoirs, in addition to those already cited, and to the various recent syste- matic Treatises on Zoology:— Dr. Johnston's "History of British Zoophytes," Prof. Milne-Ed wards's " Recherches sur les Polypes," and his " Histoire des Co- rallaires" (in the ' Suites a Buffon'), Paris, 1857, Prof. Van Beneden ' Sur'les Tubu- laires,' and « Sur lesCampanulaires.' in " Mem. de FAcad. Roy. de Bruxelles," Tom. xvii., and his " Recherches sur 1'Hist. Nat. des Polypes qui frequentent les C6tes deBelgique," Op. cit., Tom. xxxvi., Sir J. G. Daly ell's "Rare and Remarkable Animals of Scotland," Vol. i., Trembley's "Mem. pour servir a Thistoire d'un genre de Polype d'Eau douce," M. Hollard's ' Monographic du Genre Actinia,' in <; Ann. des Sci. Nat." Ser. 3, Tom. xv., Prof. Max Schultze, 'On the Male Repro- ductive Organs of Campanularia geniculata,' in " Quart. Journ. of Microsc. Sci.," Vol. iii. (1855), p. 59, Prof. Agassiz's beautiful Monograph on American Medusae, forming the third volume of his "Contributions to the Natural History of the United States of America," Mr. Hincks's " British Hydro-id Zoophytes," Prof. Allman's admirable Memoirs on Cordylophora and Myriothela in the Philos. Transact, for 1853 and 1875, Prof. J. R. Greene's "Manual of the Sub-Kingdom Coelenterata," which contains a Bibliography very complete to the date of its publication, and the articles ' Actinozoa,' ' Ctenophora,' and 'Hydrozoa,' in the Supplement to the Natural History Division of the " English Cyclopaedia." The Ctenophora are specially treated of in Vol. iii. of Prof. Agassiz's " Contributions to the Natural History of the United States." See also Prof. Alex. Agassiz " Sea- side Studies in Natural History," and his " Illustrated Catalogue of the Museum of Comparative Anatomy at Harvard College," Prof. James-Clark in "American Journal of Science," Ser. 2, Vol. xxxv., p. 348, Dr. D. Macdonald in "Transact. Roy. Soc. Edinb.," Vol. xxiii., p. 515, Mr. H. N. Moseley 'On the Structure of a species of Millepora,' in "Philos. Trans.," 1877, p. 117, and 'On the Structure of the Stylasteridce,' Ibid., 1878, p. 425; and on the 'Acalephce,' Prof. Haeckel's " Bei- trage zur Naturgeschichte der Hydromedusen," the masterly work of the brothers Hertwig, " Das Nervensystem und die Sinnesorgane der Medusen," 1878, and the Memoir of Prof. Schafer ' On the Nervous System of Aurelia aurita,' in " Philos. Trans.," 1878, p. 563. 14-0 THE MICROSCOPE AND ITS KEVELATIONS. CHAPTER XIV. ECHINODERMATA. 531. As we ascend the scale of Animal life, we meet with such a rapid advance in complexity of structure, that it is no longer possible to acquaint one's-self with any organism by Microscopic examination of it as a whole; and the dissection or analysis which becomes necessary, in order that each separate part may be studied in detail, belongs rather to the Comparative Anatomist than to the ordinary Microscopist. This is especially the case with the Echinus (Sea-Urchin), Asterias (Star-fish), and other members of the class Echinodermata; even a general account of whose complex organization would be quite foreign to the purpose of this work. Yet there are certain parts of their structure which furnish Microscopic objects of such beauty and interest that they cannot by any means be passed by; while the study of their Embryonic forms, which can be prosecuted by any Sea-side observer, brings into view an order of facts of the highest scientific interest. 532. It is in the structure of that Calcareous Skeleton which proba- bly exists under some form in every member of this class, that the ordi- nary Microscopist finds most to interest him. This attains its highest development in the Echinida; in which it forms a box-like shell or ' test,' composed of numerous polygonal plates jointed to each other with great exactness, and beset on its external surface with ' spines/ which may have the form of prickles of no great length, or may be stout club- shaped bodies, or, again, may be very long and slender rods. The inti- mate structure of the shell is everywhere the same; for it is composed of a network, which consists of Carbonate of Lime with a very small quan- tity of animal matter as a basis, and which extends in every direction (i.e., in thickness as well as in length and breadth), its areolce or inter- spaces freely communicating with each other (Figs, 366, 367). These 'areolae/ and the solid structure which surrounds them, may bear an ex- tremely variable proportion one to the other; so that in two masses of equal size, the one or the other may greatly predominate; and the tex- ture may have either a remarkable lightness and porosity, if the network be a very open one like that of Fig. 366, or may possess a considerable degree of compactness, if the solid portion be strengthened. Generally speaking, the different layers of this network, which are connected to- f ether by pillars that pass from one to the other in a direction perpen- icular to their plane, are so arranged that the perforations in one shall correspond to the intermediate solid structure in the next; and their transparence is such that when we are examining a section thin enough to contain only two or three such layers, it is easy, by properly focussing the Microscope, to bring either one of them into distinct view. From this very simple but very beautiful arrangement, it comes to pass that the plates of which the entire ' test ' is made-up possess a very consider- ECHINODERMATA. 141 able degree of strength, notwithstanding that their porousness is such, that if a portion of a fractured edge, or any other part from which the investing membrane has been removed, be laid upon fluid of almost any description, this will be rapidly sucked-up into its substance. — A very beautiful example of the same kind of calcareous skeleton, having a more regular conformation, is furnished by the disk or ' rosette ' which is con- tained in the tip of every one of the tubular suckers put forth by the FIG. 367. Section of Shell of Echinus, showing the calcareous network of which it is composed:— a a, portions of a deeper layer. Transverse Section of central portion of Spine of Acrocladia, showing its more open network. living Echinus from the ( ambulacral pores ' that are seen in the rows of smaller plates interposed between the larger spine-bearing plates of its box-like shell. If the entire disk be cut-off, and be mounted when dry in Canada balsam, the calcareous rosette may be seen sufficiently well; but its beautiful structure is better made-out when the animal membrane that incloses it has been got-rid of by boiling in a solution of caustic potass; and the appearance of one of the five segments of which it is composed, when thus prepared, is shown in Fig. 368. 533. The most beautiful display f, * • , . -, , -, -,•*• rlG. 000. this reticulated structure how- ever, is shown in the structure of the 'spines' of Echinus, Cidaris, etc. ; in which it is combined with solid ribs or pillars, disposed in such a manner as to increase the strength of these organs; a regular and ela- borate pattern being formed by their intermixture, which shows considerable variety in different species. — When we make a thin transverse section (Plate n., fig. 1) of almost any spine belonging to the genus Echinus (the small spines of our British species, however, being exceptional in this respect) or its immediate allies, we see it to be made up of a number of concentric layers, arranged in a manner that strongly reminds us of the concentric rings of an Exogenous tree (Fig. 254). The number of these layers is extremely variable; depending not merely upon the age One of the segments of the calcareous skeleton of an Ambulacral Disk of Echinus. 14:2 THE MICROSCOPE AND ITS REVELATIONS. of the spine, but (as will presently appear) upon the part of its length from which the section happens to be taken. The centre is usually occu- pied by a very open network (Fig. 367); and this is bounded by a row of transparent spaces (like those at a, a', b b' , c c,' etc., Fig. 369), which on a cursory inspection might be supposed to be void, but are found on examination to be the sections of solid ribs or pillars, which run in the direction of the length of the spines, and form the exterior of every layer. Their solidity becomes very obvious, when we either examine a section of a spine whose substance is pervaded (as often happens) with a coloring matter of some depth, or when we look at a very thin section by black- ground illumination. Around the innermost circle of these solid pillars there is another layer of the calcareous network, which again is surrounded by another circle of solid pillars; and this arrangement may be repeated many times, as shown in Fig. 369, the outermost row of pillars forming the projecting ribs that are commonly to be distinguished on the surface of the spine. Around the cup-shaped base of the spine is a membrane which is continuous with that covering the surface of the shell, and serves not merely to hold-down the cup upon the tubercle over which it works, but also by its contractility to move the spine in any required direction. FIG. 369. 9 Portion of transverse section of Spine of Acrocladia mammillata. This membrane is probably continued onwards over the whole surface of the spine, although it cannot be clearly traced to any distance from the base, and the new formations may be presumed to take place in its sub- stance. Each new formation completely ensheathes the old; not merely surrounding the part previously formed, but also projecting considerably beyond it; and thus it happens that the number of layers shown in a trans- verse section will depend in part upon the place of that section. For if it cross near the base, it will traverse every one of the successive layers from the very commencement; whilst if it cross near the apex, it will traverse only the single layer of the last growth, notwithstanding that, in the club-shaped spines, this terminal portion may be of considerably larger diameter than the basal; and in any intermediate part of the spine, so many layers will be traversed, as have been formed since the spine first attained that length. The basal portion of the spine is enveloped in a reticulation of a very close texture, without concentric layers; form- ing the cup or socket which works over the tubercle of the shell. 534. Their combination of elegance of pattern with richness of coloring, renders well-prepared specimens of these Spines among the most beautiful objects that the Microscopist can anywhere meet with. ECHLNODERMATA. 143 The large spines of the various species of the genus Acrocladia furnish sections most remarkable for size and elaborateness, as well as for depth of color (in which last point, however, the deep purple spines of Echinus lividus are pre-eminent); but for exquisite neatness of pattern, there are no spines that can approach those of Echinometra heteropora (Plate II., fig. 1) and E. lucunter. The spines of Heliocidaris variolaris are also remarkable for their beauty. — No succession of concentric layers is seen in the spines of the British Echini, probably because (according to the opinion of the late Sir J. G. Dalyell) these spines are cast-off and renewed every year; each new formation thus going to make an entire spine, instead of making an addition to that previously existing. — Most curious indications are sometimes afforded by sections of Echinus-spines, of an extraordinary power of reparation inherent in these bodies. For irregularities are often seen in the transverse sections, which can be accounted for in no other way than by supposing the spines to have received an injury when the irregular part was at the exterior, and to have had its loss of substance supplied by the growth of new tissue, over which the subsequent layers have been formed as usual. And sometimes FIG. 370. Spines of Spatangus. a peculiar ring may oe seen upon the surface of a spine, which indicates the place of a complete fracture; all beyond it being a new growth, whose unconformableness to the older or basal portion is clearly shown by a longitudinal section.1 — The spines of Cidaris present a marked departure from the plan of structure exhibited in Echinus; for not only are they destitute of concentric layers, but the calcareous network which forms their principal substance is encased in a solid calcareous sheath perforated with tubules, which seems to take the place of the separate pillars of the Echini. This is usually found to close in the spine at its tip also; and thus it would appear that the entire spine must be formed at once, since no addition could be made either to its length or to its diameter, save on the outside of the sheath, where it is never to be found. The sheath itself often rises up in prominent points or ridges on the surface of these spines; thus giving them a character by which they may be distinguished from those of Echini. — The slender, almost filamentary spines of Spatangus (Fig. 370), and the innumerable minute 'See the Author's description of such Reparations in the "Monthly Micro- scopical Journal," Vol. iii. (1870), p. 225. , 14:4: THE MICROSCOPE AND ITS REVELATIONS. hair-like processes attached to the shell of Clypeaster, are composed of the like regularly-reticulated substance; and these are very beautiful objects for the lower powers of the Microscope, when laid upon a black ground and examined by reflected light without any further preparation. — It is interesting also to find that the same structure presents itself in the curious Pedicellarica (forceps-like bodies mounted on long stalks), which are found on the surface of many Echinida, and the nature of which was formerly a source of much perplexity to Naturalists, some having maintained that they are parasites, whilst others considered them as proper appendages of the Echinus itself. The complete conformity which exists between the structure of their skeleton and that of the animal to which they are attached, removes all doubt of their being truly appendages to it, as observation of their actions in the living state would indicate. 535. Another example of . the same structure is found in the peculiar framework of plates which surrounds the interior of the oral orifice of the shell, and which includes the five teeth that may often be seen projecting externally through that orifice; the whole forming what is known as the ' lantern of Aristotle.' The texture of the plates or jaws resembles that of the shell in every respect, save that the network is more open; but that of the teeth differs from it so widely, as to have been likened to that of the bone and dentine of Vertebrate animals. The careful investigations of Mr. James Salter,1 however, have fully demonstrated that the appearances which have suggested this compari- son are to be otherwise explained; the plan of structure of the tooth being essentially the same as that of the shell, although greatly modified in its working-out. The complete tooth has somewhat the form of thai of the front tooth of a Eodent; save that its concave side is strengthened by a projecting ' keel,' so that a transverse section of the tooth presents the form of a j_. This keel is composed of cylindrical rods of carbonate of lime, having club-shaped extremities lying obliquely to the axis of the tooth (Fig. 371, A, d)'j these rods do not adhere very firmly together, so that it is difficult to keep them in their places in making sections of the part. The convex surface of the tooth (c, c, c) is covered with a firmer layer, which has received the name of 'enamel; ' this is composed of shorter rods, also obliquely arranged, but having a much more inti- mate mutual adhesion than we find among the rods of the keel. The principal part of the substance of the tooth (A, b) is made-up of what may be called the ' primary plates; ' these are triangular plates of calcareous shell-substance, arranged in two series (as shown at B), and constituting a sort of framework with which the other parts to be presently described become connected. These plates may be seen by examining the growing base of an adult tooth that has been preserved with its attached soft parts in alcohol, or (which is preferable) by exam- ining the base of the tooth of a fresh specimen, the minuter the better. The lengthening of the tooth below, as it is worn-away above, is mainly affected by the successive addition of new ' primary plates.' To the outer edge of the primary plates, at some little distance from the base, we find attached a set of lappet-like appendages, which are formed of similar plates of calcareous shell-substance, and are denominated by Mr. Salter ( secondary plates.' Another set of appendages termed 'flabelli- 'See his Memoir ' On the Structure and Growth of the Tooth of Echinus,' in. " Philos. Transact." for 1861, p. 387. ECHINODERMATA. 145 form processes' is added at some little distance from the growing base; these consist of elaborate reticulations of calcareous fibres, ending in fan-shaped extremities. And at a point still further from the base, we find the different components of the tooth connected together by ' solder- ing particles/ which are minute calcareous disks interposed between the previously-formed structures; and it is by the increased development of this connective substance, that the intervening spaces are narrowed into the semblance of tubuli like those of bone or dentine. Thus a vertical section of the tooth comes to present an appearance very like that of the bone of a Vertebrate animal, with its lucunae, canaliculi, and lamellae; but in a transverse section the body of the tooth bears a stronger resemblance to dentine; whilst the keel and enamel-layer more resemble an oblique section of Pinna than any other form of shell-structure. FIG. 371. Structure of the Tooth of Echinus:— A, vertical section, showing the form of the apex of the tooth as produced by wear, and retained by the relative hardness of its elementary parts; a, the clear condensed axis; b, the body formed of plates; c, the so-called enamel; d, the kee.1:— B, com- mencing growth of the tooth, as seen at its base, showing its two systems of plates; the dark appearance in the central portion of the upper part is produced by the incipient reticulations of the flabelliform processes:— c, transverse section of the tooth, showing at a the ridge of the keel, at 6 its lateral portion, resembling the shell in texture; at c, c, the enamel. 536. The calcareous plates which form the less compact skeletons of the Asteriada (' star-fish' and their allies), and of the Ophiurida ( ( sand-stars ' and ' brittle-stars '), have the same texture as those of the shell of Echinus. And this presents itself, too, in the spines or prickles of their surface, when these (as in the great Goniaster equestris) are large enough to be furnished with a calcareous framework, and are not mere projections of the horny integument. An example of this kind, furnished by the Astropliyton (better known as the Euryale), is repre- sented in Fig. 372. The spines with which the arms of the species of Ophiocoma ('brittle-star') are beset, are often remarkable for their beauty of conformation; those of 0. rosula, one of the most common kinds, might serve (as Prof. E. Forbes justly remarked), in point of lightness and beauty, as models for the spire of a cathedral. These are seen to the greatest advantage when mounted in Canada balsam, and viewed by the Binocular Microscope with black-ground illumination. — It; is interesting to remark that the minute tooth of Ophiocoma 10 146 THE MICROSCOPE AND ITS REVELATIONS. clearly exhibits, with scarcely any preparation, that gradational trans- ition between the ordinary reticular structure of the shell and the peculiar substance of the tooth, which, in the adult tooth of the Echinus, can only be traced bv making sections of it near its base. The tooth of Ophiocoma may be mounted in balsam as a transparent object, with scarcely any grinding down; and it is then seen that the basal portion of the tooth is formed upon the open re- no. 372. ticular plan characteristic of the ' shell/ whilst this is so modified in the older portion by subsequent addition, that the upper part of the tooth has a bone-like character. 537. The calcareous skeleton is very highly developed in the Crinoidea; their stems and branches being made-up of a calcareous network closely resem- bling that of the shell of the Echinus. This is extremely well seen, not only in Calcareous plate and claw of Astrophyton the 1'CCent PentdCrinUS Caput MeduSCB, (Euryaie). ft somewhat rare animal of West Indian seas, but also in a large proportion of the fossil Crinoids, whose remains are so abundant in many of the older Geological formations; for notwithstanding that these bodies have been penetrated in the act of fossilization by a Mineral infiltration, which seems to have substituted itself for the original fabric (a regularly-crys- talline cleavage being commonly found to exist in the fossil stems of Encrinites, etc., as in the fossil spines of Echinida), yet their organic structure is often most perfectly preserved.1 In the circular stems of Encrinites, the texture of the calcareous network is uniform, or nearly so, throughout; but in the pentangular Pentacrini, a certain figure or pattern is formed by variations of texture in different parts of the trans- verse section.2 538. The minute structure of the Shells, Spines, and other solid parts of the skeleton of Echinodermata can only be displayed by thin sections made upon the general plan already described (§§ 192-195). But their peculiar texture requires that certain precautions should be taken; in the first place, in order to prevent the section from breaking whilst being reduced so the desirable thickness; and in the second, to prevent the interspaces of the network from being clogged by the particles abraded in the reducing process. — A section of the Shell, Spine, or other portion of the skeleton should first be cut with a fine saw, and be rubbed on a flat file until it is about as thin as ordinary card, after which it should be smoothed on one side by friction with water on a Water-of-Ayr stone. It should then, after careful washing, be dried, first on white blotting-paper, afterwards by exposure for some time to a gentle heat, so 1 The calcareous skeleton even of living Echinoderms has a crystalline aggre- gation, as is very obvious in the more solid spines of Echinometrce, etc. ; for it is difficult, in sawing these across, to avoid their tendency to cleavage in the oblique plane of calcite. And the Author is informed by Mr. Sorby, that the calcareous deposit which fills up the areolae of the fossilized skeleton has always the same crystalline system with the skeleton itself, as is shown not merely by the uni- formity of their cleavage, but by their similar action on Polarized light. 2 See Figs. 74-76 of the Author's Memoir on " Shell Structure" in the Report of the British Association for 1847. ECHINODERMATA. 147 that no water may be retained in the interstices of the network, which would oppose the complete penetration of the Canada balsam. Next, it is to be attached to a glass-slip by balsam hardened in the usual manner; but particular care should be taken, first, that the balsam be brought to exactly the right degree of hardness, and second, that there be enough not merely to attach the specimen of the glass, but also to saturate its substance throughout. The right degree of hardness is that at which the balsam can be with difficulty indented by the thumb-nail; if it be made harder than this, it is apt to chip-off the glass in grinding, so that the specimen also breaks away; and if it be softer, it holds the abraded particles, so that the openings of the network become clogged with them. If, when rubbed-down nearly to the required thinness, the section appears to be uniform and satisfactory throughout, the reduction may be com- pleted without displacing it?; but if (as often happens) some inequality in thickness should be observable, or some minute air bubbles should show themselves between the glass and the under surface, it is desirable to loosen the specimen by the application of just enough heat to melt the balsam (special care being taken to avoid the production of fresh air- bubbles), and to turn it over so as to attach the side last polished to the glass, taking care to remove or to break with the needle-point any air- bubbles that there may be in the balsam covering the part of the glass on which it is laid. The surface now brought uppermost is then to be very carefully ground down; special care being taken to keep its thickness uni- form through every part (which may even be better judged-of by the touch than by the eye), and to carry the reducing process far enough, without carrying it too far. Until practice shall have enabled the operator to judge of this by passing his finger over the specimen, he must have continual recourse to the Microscope during the latter stages of his work; and he should bear constantly in mind, that, as the specimen will become much more transparent when mounted in balsam and covered with glass, than it is when the ground surface is exposed, he need not carry his reducing process so far as to produce at once the entire transparence he aims at, the attempt to accomplish which would involve the risk of the destruction of the specimen. In ' mounting' the specimen, liquid bal- sam should be employed, and only a very gentle heat (not sufficient to produce air-bubbles, or to loossen the specimen from the glass) should be applied; and if, after it has been mounted, the section should be found too thick, it will be easy to remove the glass cover and to reduce it further, care being taken to harden to the proper degree the balsam which has been newly laid-on. 539. If a number of sections are to be prepared at once (which it is often useful to do for the sake of economy of time, or in order to com- pare sections taken from different parts of the same spine), this may be most readily accomplished by laying them down, when cut-off by the saw, without any preliminary preparation save the blowing of the cal- careous dust from their surfaces, upon a thick slip of glass well covered with hardened balsam; a large proportion of its surface may thus be occupied by the sections attached to it, the chief precaution required being that all the sections come into equally close contact with it. Their surfaces may then be brought to an exact level, by rubbing them down, first upon a flat piece of grit (which is very suitable for the rough grind- ing of such sections), and then upon a large Water-of-Ayr stone whose surface is ' true. ' When this level has been attained, the ground surface is to be well washed and dried, and some balsam previously hardened is 14:8 THE MICROSCOPE AND ITS REVELATIONS. to be spread over it, so as to be sucked-in by the sections, a moderate heat being at the same time applied to the glass slide; and when this has been increased sufficiently to loosen the sections without overheating the balsam, the sections are to be turned-over, one by one, so that the ground surfaces are now to be attached to the glass slip, special care being taken to press them all into close contact with it. They are then to be very carefully rubbed-down, until they are nearly reduced to the required thinness; and if, on examining them from time to time, their thinness should be found to be uniform throughout, the reduction of the entire set may be completed at once; and when it has been carried sufficiently far, the sections, loosened by warmth, are to be taken-up on a camel-hair brush 'dipped in turpentine, and transferred to separate slips of glass whereon some liquid balsam has been previously laid, in which they are to be mounted in the usual manner. It more frequently happens, however, that, notwithstanding every care, the sections, when ground in a number together, are not of uniform thickness, owing to some of them being underlaid by a thicker stratum of balsam than others; and it is then necessary to transfer them to separate slips before the reducing process is completed, attaching them with hardened balsam, and finishing each section separately. 540. A very curious internal skeleton, formed of detached plates or spicules, is found in many members of this class; often forming an in- FIQ. 373. Calcareous plates in skin of Holothuria. vestment like a coat of mail to some of the viscera, especially the ovaries. The forms of these plates and spicules are generally so diverse, even in closely-allied species, as to afford very good differential characters. — This subject is one that has been as yet but very little studied, Mr. Stewart being the only Microscopist who has given much attention to it;1 but it is well worthy of much more extended research. 541. It now remains for us to notice the curious and often very beautiful structures, which represent, in the order Holothurida, the solid calcareous skeleton of the others already noticed. All the animals belonging to this Order are distinguished by the flexibility and absence of firmness of their envelopes; and excepting in the case of certain species which have a set of calcareous plates, supporting teeth, disposed around the mouth, very much as in the Echinida, we do not find among them any representation that is apparent to the unassisted eye, of that skeleton which constitutes so distinctive a feature of the class generally. But a microscopic examination of their integument at once brings to view the existence of great numbers of minute isolated plates, every one 1 bee his Memoir in the " Linnsean Transactions," Vol. xxv., p. 365. ECHINODERMATA. 149 of them presenting the characteristic reticulated structure, which are set with greater or less closeness in the substance of the skin. Various forms of the plates which thus present themselves in Holothuria are shown in Fig. 373; and at A is seen an oblique view of the kind marked a, more highly magnified, showing the very peculiar manner wherein one parfc is superposed on the other, which is not at all brought into view when it is merely seen through in the ordinary manner. — In the Synapta, one of the long-bodied forms of this order, which abounds in the Adriatic Sea, and of which two species (the S. digitata and 8. in- hcerens) occasionally occur upon our own coasts,1 the calcareous plates of the integument have the regular form shown at A, Fig. 374 ; and each FIG. 374. Calcareous Skeleton of Synapta:— A., plate imbedded in Skin; B, the same, with its anchor-like spine attached; c , anchor- like spine separated. of these carries the curious anchor-like appendage, c, which is articu- lated to it by the notched piece at the foot, in the manner shown (in side view) at B. The anchor-like appendages project from the surface of the skin, and may be considered as representing the spines of Echinida. — Nearly allied to the Synapta is the Chirodota, the integument of which is entirely destitute of ' anchors/ but is furnished with very remarkable wheel-like plates; those represented in Fig. 375 are found in the skin of Chirodota violacea, a species inhabiting the Mediterranean. These ' wheels ' are FIG. 375. objects of singular beauty and delicacy, being especially remarkable for the very minute notching (scarcely to be discerned in the figures without the aid of a magni- fying-glass) which is traceable around the inner margin of their ' tires ' — There can be scarcely any reasonable doubt that every member of this Order has some kind of calcareous skeleton, disposed in a man- ner conformable to the examples now cited ; and it would be very valuable to determine how far the marked peculiar- ities by which they are respectively distinguished, are characteristic of genera and species. The plates may be obtained separately by the usual method of treating the skin with a solution of potass; and they should be mounted in Canada balsam. But their position in the skin can only be ascertained by making sections of the integument, both vertical and parallel to its surface; and these sections, when dry, are most advan- tageously mounted in the same medium, by which their transparence is 1 See Woodward in " Proceedings of Zoological Society," July 13, 1858. Wheel-like plates from Skin of Chiro- dota violacea. 150 THE MICROSCOPE AND ITS REVELATIONS. greatly increased. All the objects of this class are most beautifully dis- played by the Black-ground illumination; and their solid forms are seen with increased effect under the Binocular. The Black-ground illumina- tion applied to very thin sections of Echinus spines brings out some effects of marvellous beauty; and even in these the solid form of the net- work connecting the pillars is better seen with the Binocular than it can be with the ordinary Microscope.1 542. EcUinoderm-LarvcB. — We have now to notice that most remark- able set of objects furnished to the Microscopic inquirer by the larval states of this class; for our knowledge of which we are chiefly indebted to the painstaking and widely-extended investigations of Prof. J. Miiller. All that our limits permit is a notice of two of the most curious forms of | these larvae, by way of sample of the wonderful phenomena which his i researches brought to light, and to which the attention of Microscopists I who have the opportunity of studying them should be the more assid- uously directed; as even the most delicate of these organisms have been found capable of such perfect preservation, as to admit of being studied, when mounted as preparations, even better than when alive (§ 545, a). The peculiar feature by which the early history of the Echinoderms generally seems to be distinguished is this, — that the embryonic mass of cells is converted, not into a larva which subsequently attains the adult form by a process of metamorphosis, but into a peculiar e zooid ' or pseudembryo, which seems to exist for no other purpose than to give origin to the Echinoderm by a kind of internal gemmation, and to carry it to a distance by its active locomotive powers, so as to prevent the spots inhabited by the respective species from being overcrowded by the accu- mulation of their progeny.1 The larval zooids are formed upon a type quite different from that which characterizes the adults; for instead of a radial symmetry, they exhibit a bilateral, the two sides being precisely alike, and each having a ciiliated fringe along the greater part or the whole of its length. The two fringes are united by a superior and an inferior transverse ciliated band : and between these two the mouth of the zooid is always situated. Further, although the adult Star-fish and Sand-stars have usually neither intestinal tube nor anal orifice, their larval zooids, like those of other Echinoderms, always possess both. The external forms of these larvae, however, vary in a most remarkable degree, owing to the unequal evolution of their different parts ; and there is also a considerable diversity in the several Orders, as to the pro- portion of the fabric of the larva which enters into the composition of the adult form. In the fully developed Star-fish and Sea-urchin, the only part retained is a portion of the stomach and intestine, which is pinched-off, so to speak, from that of the larval zooid. 543. One of the most remarkable forms of Echinoderm-larvae is that which has received the name of Bipinnaria (Fig. 376), from the symme- trical arrangement of its natatory organs. The mouth («), which opens in the middle of a transverse furrow, leads through an oesophagus a' to a large stomach, around which the body of a Star-fish is developing itself; and on one side of this mouth are observed the intestinal tube and anus (b). On either side of the anterior portion of the body are six or more 1 It may be here pointed out that the reticulated appearance is sometimes de- ceptive; what seems to be solid network being in many instances a hollow net- work of passages channelled-out in a solid calcareous substance. Between these two conditions, in which the relation be ween the solid frame- work and the inter- vening space is completely reversed, there is every intermediate gradation. ECHINODERMATA. 151 FIG. 376. narrow fin-like appendages, which are fringed with cilia; and the poste- rior part of the body is prolonged into a sort of pedicle, bilobed towards its extremity, which also is covered with cilia. The organization of this larva seems completed, and its movements through the water become very active, before the mass at its anterior extremity presents anything of the aspect of the Star-fish; in this respect corresponding with the movements of the pluteus of the Echinida (§ 545). The temporary mouth of the larva does not remain as the permanent mouth of the Star-fish; for the ossophagus of the latter enters on what is to become the dorsal side of its body, and the true mouth is subsequently formed by the thinning-away of the in- tegument on its ventral surface. The young Star-fish is separated from the Bi- pinnarian larva by the forcible contrac- tions of the connecting stalk, as soon as the calcareous consolidation of its integ- ument has taken-place and its true mouth has been formed, but long before it has attained the adult condition; and as its ulterior development has not hitherto been observed in any instance, it is not yet known what are the species in which this mode of evolution prevails. The lar- val zooid continues active for several days after its detachment; and it is pos- sible, though perhaps scarcely probable, that it may develop another Asteroid by a repetition of this process of gemmation. 544. In the Bipinnaria, as in other larval zooids of the Asteriada, there is no internal calcareous frame- work; such a frame- work, however, is found in the larvae of the EcMnida and Ophiurida, Of Bipinnaria asterigera, or Larva of i_ . •• 1 1 » JT a • TV oivrv • Star-fish: — a, mouth; a', oesophagus; o, Which the form delineated in Fig. 377 IS intestinal tube and anal orifice ;c, furrow an ovaTYiiVlp TVio pmV»rvrk iaanpc fvrun T>IP in which the mouth is situated; d d', bi- an example. ,ne em Dryo issues iiom tne lobed pedimcle; lf 2,3, 4, 5, e, 7, ciliated ovum as soon as it has attained, by repeat- arms, ed 'segmentation' of the yolk (§ 581), the condition of the ' mulberry-mass;' and the superficial cells of this are covered with cilia, by whose agency it swims freely through the water. So rapid are the early processes of development, that no more than from twelve to twenty-four hours intervene between fecundation and the emersion of the embryo; the division into two, four, or even eight seg- ments taking-place within three hours after impregnation. Within a few hours after its emersion, the embryo changes from the spherical into a sub-pyramidal form with a flattened base; and in the centre of this base is a depression, which gradually deepens, so as to form a mouth that communicates with a cavity in the interior of the body, which is sur- rounded by a portion of the yolk- mass that has returned to the liquid granular state. Subsequently a short intestinal tube is found, with an anal orifice opening on one side of the body. The pyramid is at first tri- angular, but it afterwards becomes quadrangular; and the angles are greatly prolonged round the mouth (or base), whilst the apex of the pyramid is sometimes much extended in the opposite direction, but is sometimes rounded off into a kind of dome (Fig. 377, A). All parts of 152 THE MICROSCOPE AND ITS REVELATIONS. this curious body, and especially its most projecting portions, are strength- ened by a frame-work of thread-like calcareous rods (e). In this condi- tion the embryo swims freely through the water, being propelled by the action of the cilia, which clothe the four angels of the pyramid and its projecting arms, and which are sometimes thickly set upon two or four projecting lobes (/); and it has received the designation of pluteus. The mouth is usually surrounded by a sort of proboscis, the angles of which are prolonged into four slender processes (g, g, g, g), shorter than the four outer legs, but furnished with a similar calcareous frame-work. 545. The first indication of the production of the young Echinus from its ' pluteus/ is given by the formation of a circular disk (Fig. 377, A, c), on one side of the central stomach (#); and this disk soon presents five prominent tubercles (B), which subsequently become elongated into tubular cirrhi. The disk gradually extends itself over the stomach, and between its cirrhi the rudiments of spines are seen to protrude (c); these, with the cirrhi, increase in length, so as to project against the envelope FIG. 377. Embryonic development of Echinus:— A, Pluteuslarva at the time of the first appearance of the disk; a, mouth in che midst of the four-pronged proboscis; 6, stomach; c, Echinoid disk; d, d. d, d, four arms of the pluteus-body ; e, calcareous framework; /, ciliated lobes; g, g, g, g, ciliated processes of the proboscis; — B, Disk with the first indication of the cirrhi: — c, Disk with the origin of the spines between the cirrhi;— D, more advanced disk, with the cirrhi, g, and spines, x, project- ing considerably from the surface. (N.B — In B, c, and D, the Pluteus is not represented, its parts having undergone no change, save in becoming relatively smaller.) of the pluteus, and to push themselves through it; whilst, at the same time, the original angular appendages of the pluteus diminish in size, the ciliary movement becomes less active, being superseded by the action of the cirrhi and spines, and the mouth of the pluteus closes-up. By the time that the disk has grown over half of the gastric sphere, very little of the pluteus remains, except some of the slender calcareous rods; and the number of cirrhi and spines rapidly increases. The calcareous frame- work of the shell at first consists, like that of the Star-fishes, of a series of isolated networks developed between the cirrhi; and upon these rest the first-formed spines (D). But they gradually become more consoli- dated, and extend themselves over the granular mass, so as to form the series of plates constituting the shell. The mouth of the Echinus (which ECHINODERMATA. 153 is altogether distinct from that of the pluteus) is formed at that side of the granular mass over which the shell is last extended; and the first in- dication of it consists in the appearance of the five calcareous concretions, which are the summits of the five portions of the frame-work of jaws and teeth that surround it. All traces of the original pluteus are now lost"; and the larva, which now presents the general aspect of an Echinoid animal, gradual augments in size, multiplies the number of its plates, cirrhi, and spines, evolves itself into its particular generic and specific type, and undergoes various changes of internal structure, tending to the development of the complete organism. a. An excellent summary of the developmental history of the several Echino- derm-types, with references to the principal Memoirs which treat of it, will be found in Chap. xx. of Mr. Balfour's "Comparative Embryology." — In collecting the free-swimming larvae of Echinodermata, the Stick-net should be carefully employed in the manner already described (§ 217); and the search for them is of course most likely to be successful in those localities in which the adult forms of the respective species abound, and on warm calm days, in which they seem to come to the surface in the greatest numbers. The following mode of preparing and mounting them has been kindly communicated to the Author by Mr. Percy Sladen: — "For killing and preserving Echinoderm zooids, I have come to prefer either Osmic acid or the Picro-sulphuric mixture of Kleinenberg (§ 199, e) of one- third strength. The latter, of course, destroys all calcareous structures, but the soft parts are preserved in a wonderful manner. If the diluted Kleinenberg's mixture is used, let the zooids remain in it for one or two hours; then wash them thoroughly in 70 per cent Spirit until all trace of acid is removed; then stain; then again wash in 70 per cent Spirit, transfer them to 90 per cent Spirit for some hours, and lastly to absolute Alcohol. Transfer them from this to Oil of Cloves; and finally mount in Canada balsam in the usual manner. — If Osmic acid be used, place three or four of the living zooids in a watch-glass of sea- water, and add a drop of the 1 per cent solution. They should not remain even in this weak solution for more than a minute; and should then be thoroughly washed in a superabundance of 35 per cent Spirit, to prevent the deposit of crystals of salt consequent on the action of the osmic acid. Then transfer the specimens to 70 per cent Spirit; and proceed as in the other case. 546 . One of the most interesting to the Microscopist of all Echino- dermata is the Antedon l (more generally known as Comatula), or 'feather-star' (Fig. 378), which is the commonest existing representative of the great fossil series Crinoidea, or ' lily star,' that were among the most abundant types of this class in the earlier epochs of the world's his- tory. Like these, the young of Antedon is attached by a stalk to a fixed base, as shown in Fig. 379; but when it has arrived at a certain stage of development, it drops off from this like a fruit from its stalk; and the animal is thenceforth free to move through the ocean-water it inhabits. It can swim with considerable activity; but it exerts this power chiefly to gain a suitable place for attaching itself by means of the jointed prehensile cirrhi put forth from the under side of the central disk (Fig. 378); so that, notwithstanding its locomotive power, it is nearly as sta- tionary in its free adult condition, as it is in its earlier Pentacrinoid stage. The pentacrinoid larva 2 — first discovered by Mr. J. V. Thomp- 1 The Author has found himself obliged, by the accepted rules of Zoological nomenclature, to adopt the designation Antedon, instead of the much better known and very appropriate name given to this type of Lamarck. See his ' Researches on the Structure, Physiology, and Development of Antedon rosaceus,' Part. I., in " Philos. Transact.," 1866, p. 671. 2 The Pentacrinoid larvae of Antedon have been found abundantly (attached to Sea-weeds and Zoophytes) at Millport on the Clyde, and in Lamlash Bay, Arran; in Kirkwall Bay, Orkney; in Lough Strangford, near Belfast, and in the Bay of Cork; and at Ilfracombe, and in Salcombe Bay, Devon. 154 THE MICROSCOPE AND ITS REVELATIONS. son of Cork, in 1823, but originally supposed by him to be a perma- nently attached Crinoid — forms a most beautiful object for the lower powers of the Microscope, when well preserved in fluid, and viewed by a strong incident light (Plate xxi., fig. 3); and a series of specimens in different stages of development, shows most curious modifications in the form and arangement of the various component pieces of its calcareous skeleton. In its earliest stage (Fig. 379, A), the body is inclosed in a calyx composed of two circles of plates; namely, five basals t forming a sort of pyramid whose apex points downward, and is attached to the highest joint of the stem; and five orals superposed on these, forming when closed a like pyramid whose apex points upwards, but usually separating to give passage to the tentacles, of which a circlet surrounds the mouth. In this condition there is no rudiment of arms. In the more advanced stage shown at B, the arms have begun to make their FIG. 378. FIG. 379. Antedon (Comatula) or Feather-star, seen from its under side. Crinoid Larva of Antedon:— A, B, c, successive stages of development. appearance; and the skeleton, when carefully examined, is found to con- sist of the following pieces, as shown in Plate xxi., fig. 1: — b, b, the cir- clet of basals supported on the top of the stem: rl the circlet of first radials, now interposed between the basals and the orals, and alternating with both; between two of these is interposed the single anal plate, a; whilst they support the second and the third radials (r2, r3), from the latter of which the bifurcating arms spring; finally, between the second radials we see the five orals, lifted from the basals on which they origi- nally rested, by the interposition of the first radials. In the more ad- vanced stage shown in Fig. 379, c, and on a larger scale in Plate xxi., figs. 2, 3, we find the highest joint of the stem beginning to enlarge, to form the centro-dorsal plate (fig. 2, c d], from which are beginning to spring the dorsal cirrhi (c i r), that serve to anchor the animal when it drops from the stem; this supports the basals (b), on which rest the ECHINODERMATA, 155 PLATE XXI PENTACRINOID LARVA OF ANTEDON (Original). Fig. 1. Skeleton of early Pentacrinoid, under Black-ground illumination, showing its component plates:— 6, 6, basals, articulated below to the highest point of the stem; r1, r1, first radials, between two of which is seen the single anal plate, a; r-, second radials; r3, third radials, giving off the bifurcating arms at their summit; o, o, orals. 2, 3. Back and front views of a more advanced Pentacrinoid, as seen by incident light, one of the pair of arms being cut away in Fig. 3, in order to bring the mouth and its surrounding parts into view :— 6,6, basals ; r1, r8, r3, first, second, and third radials; a, anal, now carried upwards by the projection of the vent v: o, o, orals; cir, dorsal cirrhi, developed from the highest joint of the stem. 156 THE MICROSCOPE AND ITS REVELATIONS. first radials (r1); whilst the anal plate (a) is now lifted nearly to the level of the second radials (r2), by the development of the anal funnel or vent (v) to which it is attached. The oral plates are not at first appa- rent, as they no longer occupy their first position; but on being carefully looked-for, they are found still to form a circlet around the mouth (fig. 3, 0, 0), not having undergone any increase in size, whilst the visceral disk and the calyx in which it is lodged have greatly extended. These oral plates finally disappear by absorption; while the fiasals are at first con- cealed by the great enlargement of the centro-dorsal (which finally ex- tends so far as to conceal the first radials also); and at last undergo metamorphosis into a beautiful 'rosette/ which lies between the cavity of the centro-dorsal and that of the calyx. — In. common with other mem- bers of its Class, the Antedon is represented in its earliest phase of develop- ment by a free-swimming ' larval zooid' or pseudembryo, which was first observed by Busch, and has been since carefully studied by Pro. Wyville Thomson ! and Goette .2 This zooid has an elongated egg-like form, and is furnished with transverse bands of cilia, and with a mouth and anus of its own. After a time, however, rudiments of the calcareous plates form- ing the stem and calyx begin to show themselves in its interior; a disk is then formed at the posterior extremity, by which it attaches itself to a Sea- weed (very commonly Laminaria), Zoophyte, or Polyzoary; the calyx, containing the true stomach, with its central mouth surrounded by ten- tacles, is gradually evolved; and the sarcodic substance of the pseudem- bryo, by which this calyx and the rudimentary stem were originally in- vested, gradually shrinks, until the young Pentacrinoid presents itself in its charateristic form and proportions.3 1 ' On the Development of Antedon rosaceus ' in " Philos. Transact." for 1865, p. 513. 2"Archiv f. Mikrosk. Anat.," Bd. xii., p. 583. 3 The general results of the Author's own later studies of this most interesting type (the key to the life-history of the entire Geological succession^ of Crinoidea) are embodied in a notice communicated to the for 1876, p. 211, and in a sul cently made to our knowled^ der Crinoideen' (Leipzig, 1877), forming part of his " Morphologische Studien an Echinoderinen," is the most important. POLYZOA AND TUNIC ATA 157 CHAPTER XV. POLYZOA AND TUNICATA. 547. AT the lower extremity of the great series of Molluscous animals, we find two very remarkable groups, whose mode of life has much in common with Zoophytes, whilst their type of structure is conformable in essential particulars to that of the true Mollusks. These animals are for the most part microscopic in their dimensions; and as some members of both these groups are found on almost every coast, and are most interest- ing objects for anatomical examination, as well as for observation in the living state, a brief general account of them will be here appropriate. 548. POLYZOA. — The group which is known under this name to British naturalists (corresponding with that which by Continental Zoolo- gists is designated Bryozoa) was formerly ranked as an order of Zoophytes; and it has been entirely by Microscopic study that its comparatively high organization has been ascertained. — The animals of the Polyozoa, in con- sequence of their universal tendency to multiplication by gemmation, are seldom or never found solitary, but form clusters or colonies of various kinds, and as each is inclosed in either a hornv or a calcareous sheath or * cell/ a composite structure is formed, closely corresponding with the 'polypidom' of a Zoophyte, which has been appropriately designated the polyzoary. The individual cells of the polyzoary are sometimes only con- nected with each other by their common relation to a creeping stem or stolon, as in Laguncula (Plate xxn.); but more frequently they bud-forth directly, one from another, and extend themselves in different directions over plane surfaces, as is the case with Flustrce, Lepralice, etc. (Fig. 380); whilst not unfrequently the polyzoary develops itself into an arborescent structure (Fig. 381), which may even present somewhat of the density and massiveness of the Stony Corals. Each individual, designated as a polypide or polype-like animal, is composed externally of a sort of sac, of which the outer or tegumentary layer is either simply membranous, or is horny, or in some instances calcified, so as to form the cell; this investing sac is lined by a more delicate membrane, which closes its orifice, and which then becomes continuous with the wall of the alimentary canal; this lies freely in the visceral sac, floating (as it were) in the liquid which it contains. 549. The principal features in the structure of this group will be best understood from the examination of a characteristic example, such as the Laguncula repens; which is shown in the state of expansion at A, Plate xxn., and in the state of contraction at B and c. The mouth is sur- rounded by a circle of tubular tentacles, which are clothed by vibratile cilia; these tentacles, in the species we are considering, vary from ten to twelve in number, but in some other instances they are more numerous. 158 THE MICROSCOPE AND ITS REVELATIONS. FIG. 380. By the ciliary investment of the tentacles, the Polyzoa are at once dis- tinguishable from those Hydroid polypes to which they bear a superficial resemblance, and with which they were at one time confounded; and accordingly, whilst still ranked among Zoophytes, they were characterized as ciliobrachiate. The tentacula are seated upon an annular disk, which is termed the lophophore, and which forms the roof of the visceral or peri- gastric cavity; and this cavity extends itself into the interior of the tenta- cula, through perforations in the lophophore, as is shown at D, Plate xxii., representing a portion of the tentacular circle on a larger scale, a a being the tentacula, b b their internal canals, c the muscles of the tentacula, d the lophophore, and e its retractile muscles. The mouth situated in the centre of the lopho- phore, as shown at A, leads to a funnel-shaped cavity or pharynx, Z>, which is separated from the oeso- phagus, d, by a valve at c; and this oesophagus opens into the stomach e, which occupies a considerable part of the visceral cavity. (In the Bow- erlankia and some other Polyzoa, a muscular stomach or gizzard for the trituration of the food inter- venes between the oesophagus and the true digestive stomach). The walls of the stomach, h, have con- siderable thickness; and they are beset with minute follicles, which Cells of Lepralice.—K, L. Hyndmanni; B, L. figu- seem to have the character of a lans; c, L. verruca. rudimentary liver. This, however, is more obvious in some other mem- bers of the group. The stomach is lined, especially at its upper part, with vibratile cilia, as seen at c, g; and by the action of these the food is kept in a state of constant agitation during the digestive process. From the upper part of the stomach, which is (as it were) doubled upon itself, the intestine i opens, by a pyloric orifice, /, which is furnished with a regular valve; within the intestine are seen at Tc particles of excre- inentitious matter, which are discharged by the anal orifice at I. No special circulating apparatus here exists; but the liquid which fills the cavity that surrounds the viscera contains the nutritive matter which has been prepared by the digestive operation, and which has transuded through the walls of the alimentary canal; a few corpuscles of irregular size are seen to float in it. The visceral sacs of the different polypides put forth from the same stem appear to communicate with each other. No other respiratory organs exist than the tentacula; into whose cavity the nutritive fluid is probably sent from the perivisceral cavity, for aera- tion by the current of water that is continually flowing over them. 550. The production of gemmce or buds may take place either from the bodies of the polypides themselves, which is what always happens when the cells are in mutual apposition; or from the connecting stem or 'stolon,' where the cells are distinct one from the other as in Laguncula. In the latter case there is first seen a bud-like protuberance of the horny POLYZOA AND TUNIC ATA. 159 PLATE XXII. STRUCTURE OF LAGUNCULA REPENS (after Van Beneden). A, Polypide expanded; B, polypide retracted; c, another view of the same, with the visceral apparatus in outline, that the manner in which it is doubled on itself, with the tentacular crown and muscular system, maybe more distinctly seen:— a, a, tentacula ; 6, pharynx ; c, pharyngeal valve ; d, oesophagus; e, stomach ; /, its pyloric orifice ; g, cilia on its inner surface ; h, biliary fol- licles lodged in its wall ; i, intestine; fc, particles of excrementitious matter; I, anal orifice; m, tes- tis; n, ovary; o, ova lying loose in the peri-visceral cavity ; p, outlet for their discharge; q, sperma- tozoa in the perivisceral cavity ; r, «, t, u, v, w, x, muscles. p, Portion of the Lophophore more enlarged:— a, a, t3ntacula; 6, 6, their internal canals; c, their muscles; d, lophopnore; e^its retractor muscles. 160 THE MICROSCOPE AND ITS REVELATIONS. external integument, into which the soft membranous lining prolongs itself; the cavity thus formed, however, is not to become (as in Hydra and its allies) the stomach of the new zooid; but it constitutes the cham- ber surrounding the digestive viscera, which organs have their origin in a thickening of the lining membrane, that projects from one side of the cavity into its interior, and gradually shapes itself into the alimentary canal with its tentacular appendages. Of the production of gemmae from the polypides themselves, the best examples are furnished by the FlustrcB and their allies. From a single cell of the Flustra, five such buds may be sent-off, which develop themselves into new polypides around it; and these, in their turn, produce buds from their unattached margins, so as rapidly to augment the number of cells. To this extension there seems no definite limit; and it often happens that the cells in the central portion of the leaf like expansion of a Flustra are devoid of con- tents and have lost their vitality, whilst the edges are in a state of active growth. — Independently of their propagation by gemmation, the Polyzoa have a true sexual generation; the sexes, however, being usually, if not invariably, united in the same polypides. The sperm-cells are developed in a glandular body, the testis m, which lies beneath the base of the stomach; when mature they rupture, and set free the spermatozoa q q, swim freely in the liquid of the visceral cavity. The ova, on the other hand, are formed in an ovarium n, which is lodged in the membrane lining the tegumentary sheath near its outlet; the ova, having escaped from this into the visceral cavity, as at 0, are fertilized by the spermato- zoa which they there meet with ; and are finally discharged by an outlet at j9, beneath the tentacular circle. 551. These creatures possess a considerable number of muscles, by which their bodies may be projected from their sheaths, or drawn within them; of these muscles, r, s, t, u, v, w, x, the direction and points of attach- ment sufficiently indicate the uses; they are for the most part retractors, serving to draw-in and double up the body, to fold-together the circle of tentacula, and to close the aperture of the sheath, when the animal has been completely withdrawn into its interior. The projection and expansion of the animal, on the contrary, appear to be chiefly accom- plished by a general pressure upon the sheath, which will tend to force- out all that can be expelled from it. The tentacles themselves are furnished with distinct muscular fibres, by which their separate move- ments seem to be produced. At the base of the tentacular circle, just above the anal orifice, is a small body (seen at A, a), which is a nervous ganglion; as yet no branches have been distinctly seen to be connected with it in this species; but its character is less doubtful in some other Polyzoa. — Besides the independent movements of the individual poly- pides, other movements may be observed, which are performed by so many of them simultaneously, as to indicate the existence of some con- necting agency; and such connecting agency, it is affirmed by Dr. Fritz, Miiller,1 is furnished by what he terms a ( colonial-nervous system.' In a Serialaria having a branching polyzoary that spreads itself on sea-weeds over a space of three or four inches, he states that a nervous ganglion may be distinguished at the origin of each branch, and another ganglion at the origin of each polypide-bud; all these ganglia being connected together, not merely by principal trunks, but also by plexuses of nerve- >See his Memoir in " Wiegmann's Archiv," 1860, p. 311; translated in "Quart. Journ. of Microsc. Science," New Ser., Vol. i. (1861), p. 300. POLYZOA AND TUNIC AT A. 161 fibres, which may be distinctly made-out with the aid of Chromic acid in the cylindrical joints of the polyzoary. His views, however, have not been universally accepted; some observers still maintaining that what he regards as nerve-fibres are only connective tissue. 552. Of all the Polyzoa of our own coasts, the Flustrm or ' sea-*mats ' are the most common; these present flat expanded surfaces, resembling in form those of many sea-weeds (for which they are often mistaken), but exhibiting, when viewed with even a low magnifying power, a most beautiful network, which at once indicates their real character. The cells are arranged on both sides; and it was calculated by Dr. Grant, that as a single square inch of an ordinary Flustra contains 1800 such cells, and as -an average specimen presents about 10 square inches of surface, it will consist of no fewer than 18,000 polypides. The want of trans- parence in the cell-wall, however, and the infrequency with which the animal projects its body far beyond the mouth of the cell, render the Polyzoa of this genus less favorable subjects for microscopic examination than are those of the Bowerbankia, a Polyzoon with a trailing stem and separated cells like those of Laguncula, which is very commonly found clustering around the base of masses of Flustrae. It was in this that many of the details of the organization of the interesting group we are considering were first studied by Dr. A. Farre, who discovered it in 1837, and subjected it to a far more minute examination than any Poly- zoon had previously received;1 and it is one of the best adapted of all the marine forms yet known, for the display of the beauties and wonders of this type of organization. — The Halodactylus (formerly called Alcyo- nidium), however, is one of the most remarkable of all the marine forms for the comparatively large size of the tentacular crowns; these, when expanded, being very distinctly visible to the naked eye, and presenting a spectacle of the greatest beauty when viewed under a sufficient magni- fying power. The polyzoary of this genus has a spongy aspect and texture, very much resembling that of certain Alcyonian Zoophytes (§ 529), for which it might readily be mistaken when its contained animals are all withdrawn into their cells; when these are expanded, however, the aspect of the two is altogether different, as the minute plumose tufts which then issue from the surface of the Halodactylus, making it look as if it were covered with the most delicate downy film, are in striking contrast with the larger, solid-looking polypes of Alcyo- nium. The opacity of the polyzoary of the Halodactylus renders it quite unsuitable for the examination of anything more than the tentacu- lar crown and the oesophagus which it surmounts; the stomach and the remainder of the visceral apparatus being always retained within the cell. It furnishes, however, a most beautiful object for the Binocular Microscope, when mounted with all its polypides expanded, in the man- ner described in § 521. — Several of the fresh-water Polyzoa are pecu- liarly interesting subjects for Microscopic examination; alike on account of the remarkable distinctness with which the various parts of their organization may be seen, and the very beautiful manner in which their ciliated tentacula are arranged upon a deeply-crescentic or horseshoe- shaped lophophore. By this peculiarity the fresh-water Polyzoa are separated as a distinct sub-class from the marine; the former being designated as Hippocrepia (horseshoe-like), while the latter are termed 1 See his Memoir ' On the Minute Structure of some of the higher forms of Polypi,1 in the " Philosophical Transactions" for 1837, p. 387. 11 162 THE MICROSCOPE AND ITS REVELATIONS. Infundibulata (funnel-like). The cells of the Hippocrepia are for the most part lodged in a sort of gelatinous substratum, which spreads over the leaves of aquatic plants, sometimes forming masses of considerable size; but in the very curious and beautiful Cristatella, the polyzoary is unat- tached, so as to be capable of moving freely through the water. ' 553. The Infundibulata or Marine Polyzoa, constituting by far the most numerous division of the class, are divided into four Orders, as fol- lows:— i. Cheilostomata, in which the mouth of the cell is sub-terminal, or not quite at its extremity (Fig. 380), is somewhat crescentic in form, and is furnished with a movable (generally membranous) lip, which closes it when the animal retreats. This includes a large part of the species that most abound on our own coasts, notwithstanding their wide differences in form and habit. Thus the polyzoaries of some (as F lustra) are horny and flexible, whilst those of others (as EscJiara and Retepora) are so penetrated with calcareous matter as to be quite rigid; some grow as independent plant-like structures (as Bugula and Gemellaria), whilst others, having a like arborescent form, creep over the surfaces of rocks or stones (as Hippothoa}; and others, again, have their cells in close apposition, and form crusts which possess no definite figure (as is the case with Lepralia and Membranipora}. — n. The second order, Cyclostomata, consists of those Polyzoa which have the mouth at the termination of tubular calcareous cells, without any movable appendage or lip (Fig. 381). This includes a comparatively small number of genera, of which Crisia and Tubulipora contain the largest proportion of the species that occur on our own coasts. — in. The distinguishing character of the third order, Ctenosomata, is derived from the presence of a comb-like circular fringe of bristles, connected by a delicate mem- brane, around the mouth of the cell, when the animal is projected from it; this fringe being drawn in when the animal is retracted. The Polyzoaries of this group are very various in character, the cells being sometimes horny and separate (as in Laguncula and Bowerbankia), sometimes fleshy and coalescent (as in Halodactylus). — iv. In the fourth order, Pedicellinece, which includes only a single genus, Pedicellina, the lophophore is produced upwards on the back of the tentacles, uniting them at their base in a sort of muscular calyx, and giving to the animal when expanded somewhat the form of an inverted bell, like that of Vorticella (Fig. 305). — As the Polyzoa altogether resemble Hydroid Zoophytes in their habits, and are found in the same localities, it is not requisite to add anything to what has already been said (§ 521), respect- ing the collection, examination, and mounting of this very interesting class of objects.2 554. A large proportion of the Polyzoa of the first Order are furnished with very peculiar motile appendages, which are of two kinds, avicularia and vibracula. The avicular-ia or * bird's-head processes/ so named from the striking resemblance they present to the head and jaws of a bird (Fig. 381, B), are generally ' sessile ' upon the angles or 1 See Prof. Allman's beautiful " Monograph on the British Fresh-water Poly- zoa," published by the Ray Society, 1857. 2 For a more detailed account of the Structure and Classification of the Marine Polyzoa, see Prof. Van Beneden's ' Recherches sur les Bryozoaires de la Cote d'Ostende,' in " Mem. de 1'Acad. Roy. de Bruxelles," torn, xvii.; Mr. G. Busk's " Catalogue of the Marine Polyzoa in the Collection of the British Museum;" Mr. Hincks's " British Marine Polyzoa," 1880; and Nitsthe, ' Beitrage zur Kenntniss der Bryozoen, in Zeitschrift f. wiss. Zool.," Bde. xx., xxi., xxiv. POLYZOA AND TtJNICATA. 163 FIG. 381. margins of the cells, that is, are attached at once to them without the intervention of a stalk, as at A, being either ' projecting ' or 'immersed;' but in the genera Bugula and Bicellaria, where they are present at all, they are ' pedunculate/ or mounted on footstalks (B). Under one form or the other, they are wanting in but few of the genera belonging to this order; and their presence or absence furnishes valuable char- acters for the discrimination of species. Each avicularium has two ( mandibles/ of which one is fixed, like the upper jaw of a bird, the other movable, like its lower jaw; the latter is opened and closed by two sets of muscles which are seen in the interior of the 'head;' and between them is a peculiar body, furnished with a pencil of bristles, which is prob- ably a tactile organ, being brought forwards when the mouth is open, so that the bristles project beyond it, and being drawn back when the mandible Closes. The avicularia A> Portion of cellularia ciUata, enlarged; B, one .keep Up a Continual Snapping aC-of the 'bird's-head' processes of Bugula avicularia, tion during the life of the Po1yZo-S°'S1h01§1he1rymagnmed'and8eenlntheaotof 8ra8p' ary ; and they may often be observed to lay hold of minute Worms or other bodies, sometimes even closing upon the beaks of adjacent organs of the same kind, as shown at B. In the pedunculate forms, besides the snapping action, there is a continual rhythmical nodding of the head upon the stalk; and few spectacles are more curious than a portion of the polyzoary of Bugula avicularia (a very common British species) in a state of active vitality, when viewed under a power sufficiently low to allow a number of these bodies to be in sight at once. It is still very doubtful what is their precise function in the economy of the animal; whether it is to retain within the reach of the ciliary current, bodies that may serve as food; or whether it is, like the PedicellariaB of Echini (§ 534), to remove extraneous particles that may be in contact with the surface of the polyzoary. The latter would seem to be the function of the vibracula, which are long bristle-shaped organs (Fig. 380, A), each one springing at its base out of a sort of cup that contains muscles by which it is kept in almost constant motion, sweep- ing slowly and carefully over the surface of the polyzoary, and removing what might be injurious to the delicate inhabitants of the cells when their tentacles are protruded. Out of 191 species of Cheilostomatous Polyzoa described by Mr. Busk, no fewer than 126 are furnished either with Avicularia, or with Vibracula, or with both these organs.1 555. TU^ICATA. — The Tunicated Mollusca are so named from the in- 1 See Mr. G-. Busk's « Remarks on the Structure and Function of the Avicula- rian and Vibracular Organs of Polyzoa,' in " Transact, of Microsc. Soc.," Ser. 2, Vol. ii. (1854), p. 26. 164 THE MICROSCOPE AND ITS REVELATIONS. closure of their bodies in a ' tunic/ which is sometimes leathery or even cartilaginous in its texture, and which very commonly includes calcare- ous spicules, whose forms are often very beautiful. They present a strong resemblance to the Polyzoa, not merely in their general plan of conforma- tion, but also in their tendency to produce composite structures by gem- mation; they are differentiated from them, however, by the absence of the ciliated tentacles which form so conspicuous a feature in the external aspect of the Polyzoa, by the presence of a distinct circulating apparatus, and by their peculiar respiratory apparatus, which may be regarded as a dilatation of their pharynx. In their habits, too, they are for the most part very inactive, exhibiting scarcely anything comparable to those rapid movements of expansion and retraction which it is so interesting to watch among the Polyzoa; whilst, with the exception of the SalpidoB and other floating species which are chiefly found in seas warmer than those that surround our coast, and the curious Appendicularia to be presently no- ticed (§ 560), they are rooted to one spot during all but the earliest period of their lives. The larger forms of the Ascidian group, which constitutes the bulk of the class, are always solitary; either not propagating by gem- mation at all, or, if this process does take place, the gemmae being de- tached before they have advanced far in their development. — Although of special importance to the Comparative Anatomist and the Zoologist, this group does not afford much to interest the ordinary Microscopist, except in the peculiar actions of its respiratory and circulatory apparatus. In common with the composite forms of the group, the solitary Ascidians have a large branchial sac, with fissured walls, resembling that shown in Figs. 382 and 384; into this sac water is admitted by the oral orifice, and a large proportion of it is caused to pass through the fissures, by the agency of the cilia with which they are fringed, into a surrounding chamber, whence it is expelled through the anal orifice. This action may be distinctly watched through the external walls in the smaller and more transparent species; and not even the ciliary action of the tentacles of the Polyzoa affords a more beautiful spectacle. It is peculiarly re- markable in one species that occurs on our own coasts, the Ascidid pa- rallelogramma,1 in which the wall of the branchial sac is divided into a, number of areolae, each of them shaped into a shallow funnel; and round one of these funnels each branchial fissure makes two or three turns of a spiral. When the cilia of all these spiral fissures are in active movement at once, the effect is most singular. — Another most remarka- ble phenomenon presented throughout the group, and well seen in the solitary Ascidian just referred -to, is the alternation in the direction of the Circulation. The heart, which lies at the bottom of the branchial sac, is composed of two chambers imperfectly divided from each other; one of these is connected with the principal trunk leading to the body, and the other with that leading to the branchial sac. At one time it will be seen that the blood flows from the respiratory apparatus to the cavity of the heart in which its trunk terminates, which then contracts so as to drive it into the other cavity, which in its turn contracts and propels it • through the systemic trunk to the body at large; but after this course has been maintained for a time, the heart ceases to pulsate for a moment or two, and the course is reversed, the blood flowing into the heart from the body generally, and being propelled to the branchial sac. After this 1 See Alder in " Ann. of Nat. Hist.," 3d Ser., Vol. xi. (1863), p. 157; and Han- cock in " Journ. of Linn. Soc.," Vol. ix., p. 333. POLYZOA AND TUNICATA. 165 reversed course has continued for some time, another pause occurs, and the first course is resumed. The length of time intervening between the changes does not seem by any means constant. It is usually stated at from half-a-minute to two minutes in the composite forms; but in the solitary Ascidia parallelogramma (a species very common in Lamlash Bay, Arran), the Author has repeatedly observed an interval of from five to fifteen minutes, and in some instances he has FIG. 382. seen the circulation go-on for half-an-hour, or even longer, without change, — always, how- ever, reversing at last. 556. The Compound Ascidians are very commonly found ad- herent to Sea- weeds, Zoophytes, and stones between the t i d e- marks; and they pre- sent objects of great interest to the Micro- scopist, since the small size and transparence of their bodies when they are detached from the mass in which they are imbedded, not only enables their structure to be clearly discerned without dissection, but allows many of their living actions to be watched. Of these we have a characteristic example in Amoroucium proliferum; of which the form of the composite mass and the anatomy of a single individual are displayed in Fig. 382. Its clusters appear almost completely inanimate, exhibiting no very obvious movements when irritated; but if they be placed when fresh in sea-water, a slight pouting of the orifices will soon be perceptible, and a constant and energetic series of currents will be found to enter by one set and to be ejected by the other, indicating that all the machinery of active life is going-on within these apathetic bodies. In the tribe of Polyclinians to which this genus belongs, the body is elongated, and may be divided into three regions, the thorax (A) which is chiefly occupied by the respiratory sac, the abdo- men (B) which contains the digestive apparatus, and the post-abdomen (c) in which the heart and generative organs are lodged. At the summit of the thorax is seen the oral orifice c, which leads to the branchial sac c\ this is perforated by an immense number of slits, which allow part of the water to pass into the space between the branchial sac and the muscular mantle, where it is especially collected in the thoracic sinus/. At k is Compound mass of Amoroucium, proliferum with the anatomy of a single zooid:— A, thorax; B, abdomen; c, post-abdomen; — c, oral orifice; e, branchial sac; /, thoracic sinus: i, anal orifice ; i', projection overhanging it; j, nervous ganglion; fc, oeosphagus; t, stomach surrounded by biliary tu- buli; m, intestine; n, termination of intestine in cloaca; o, heart; o', peri- cardium; p, ovarium; p', egg ready to escape; q, testis; r, spermatic canal; r', termination. cloaca. of this canal in the 166 THE MICROSCOPE AND ITS REVELATIONS. seen the oesophagus, which is continuous with the lower part of the pha- ryngeal cavity; this leads to the stomach /, which is surrounded by biliary follicles; and from this passes-off the intestine m, which terminates at n in the cloaca, or common vent. A current of water is continually drawn- in through the mouth by the action of the cilia of the branchial sac and of the alimentary canal; a part of this current passes through the fissures of the branchial sac into the thoracic sinus, and thence into the cloaca; whilst another portion, entering the stomach by an aperture at the bottom of the pharyngeal sac, passes through the alimentary canal, giving up any nutritive materials it may contain, and carrying away with it any excre"- mentitious matter to be discharged; and this having met the respiratory current in the cloaca, the two mingled currents pass forth together by the anal orifice i. The long post-abdomen is principally occupied by the large ovarium, p, which contain ova in various stages of development. These, when matured and set-free, find their way into the cloaca; where two large ova are seen (one marked p', and the other immediately below it) waiting for expulsion. In this position they receive the fertilizing influence from the testis, q, which discharges its products by the long spermatic canal, r, that opens into the cloaca, r'. At the very bottom of the post-abdomen we find the heart, o, inclosed in its pericardium, 0'.— In the group we are now considering, a number of such animals are im- bedded together in a sort of gelatinous mass, and covered with an integu- ment common to them all; the composition of this gelatinous substance is remarkable as including Cellulose, which generally ranks as a Vegetable product. The mode in which new individuals are developed in this mass, is by the extension of stolons or creeping stems from the bases of those previously existing; and from each of these stolons several buds may be put-forth, every one of which may evolve itself into the likeness of the stock from which it proceeded, and may in its turn increase and multiply after the same fashion. A communication between the circulating sys- tems of the different individuals is kept-up, through their connecting stems, during the whole of life; and thus their relationship to each other is somewhat like that of the several polypes on the polypidom of a Cam- panularia (§519). 557. In the family of Didemnians the post-abdomen is absent, the heart and generative apparatus being placed by the side of the intestine in the abdominal portion of the body. The zooids are frequently ar- ranged in star-shaped clusters, their anal orifices being all directed to- wards a common vent which occupies the centre. — This shortening is still more remarkable, however, in the family of Boctryllians, whose beautiful stellate gelatinous incrustations are extremely common upon Sea-weeds and submerged rocks (Fig. 383). The anatomy of these animals is very similar to that of the Amoroudum already described; with this exception, that the body exhibits no distinction of cavities, all the organs being brought together in one, which must be considered as thoracic. In this respect there is an evident approximation towards the solitary species.1 558. This approximation is still closer, however, in the ' social 9 Ascidians, or Clavellinidce; in which the general plan of structure is nearly the same, but the zooids are simply connected by their stolons 1 For more special information respecting the Compound Ascidians, see espe- cially the admirable Monograph of Prof. Milne-Edwards on that group; Mr. Lister's Memoir ' On the Structure and Functions of Tubular and Cellular Polypi, and of Ascidiae,' in the " Philos. Transact.," 1834; and the Art. Tunicata, by Prof. T. Rupert Jones, in the " Cyclopaedia of Anatomy and Physiology." POLYZOA AND TUNIC ATA. 167 FIG. 383. (Fig. 384), instead of being included in a common investment; so that their relation to each other is very nearly the same as that of the poly- pides of Laguncula (§ 549), the chief difference being that a regular circulation takes place through the stolon in the one case, such as has no existence in the other. A better opportunity of studying the living actions of the Ascidians can scarcely be found, than that which is afforded by the genus Perophora, first discovered by Mr, Lister; which occurs not unfrequently on the south coast of England and in the Irish Sea, living attached to Sea-weeds, and looking like an assemblage of minute globules of jelly, dotted with orange and brown, and link- ed by a silvery winding thread. The isolation of the body of each zooid from that of its fellows, and the ex- treme transparence of its tunics, not only enable the movements of the fluid with- in the body to be distinctly discerned, but also allow the action of the cilia that bor- der the slits of the respiratory sac to be clearly made out. This sac is perforated with four rows of narrow oval , through which a portion of the water that enters its oral ) escapes into the space between the sac and the mantle, and is FIG. 3S4. Botryllus violaceus: — A, cluster on the surface of a Fucus:— B, portion of the same enlarged. opemn orifice ^A, Group of Perophora (enlarged), growing from a common stalk:— B, single Perophora; a, test; 6, inner sac; c, branchial sac, attached to the inner sac along the line c' c' ; e e, finger-like processes projecting inwards; /, cavity between test and internal coat; /', anal orifice or funnel; a, oral ori- fice; gr', oral tentacula; h, downward stream of food; h', oesophagus- i, stomach; fc, vent; I, ovary (?) ; n, vessels connecting the circulation in the body with that in the stalk. thus discharged immediately by the annal funnel (/). "Whatever little 168 THE MICROSCOPE AND ITS REVELATIONS. particles, animate or inanimate, the current of water brings, flow into the sac, unless stopped at its entrance by the tentacles (#'), which do not appear fastidious. The particles which are admitted usually lodge some- where on the sides of the sac, and then travel horizontally until they arrive at that part of it down which the current proceeds to the entrance of the stomach (i), which is situated at the bottom of the sac. Minute animals are often swallowed alive, and have been observed darting about in the cavity for some days, without any apparent injury either to them- selves or to the creature which incloses them. In general, however, particles which are unsuited for reception into the stomach are rejected by the sudden contraction of the mantle (or muscular tunic), the vent being at the same time closed, so that they are forced out by a powerful current through the oral orifice. — The curious alternation of the circula- tion that is characteristic of the Class generally (§ .555), may be particu- larly well studied in Perophora. The creeping-stalk (Fig. 384) that connects the individuals of any group, contains two distinct canals, which send off branches into each peduncle. One of these branches terminates in the heart, which is nothing more than a contractile dilata- tion of the principal trunk; this trunk subdivides into vessels (or rather sinuses, which are mere channels not having proper walls of their own), of which some ramify over the respiratory sac, branching off at each of the passages between the oval slits, whilst others .are first distributed to the stomach and intestine, and to the soft surface of the mantle. All these reunite so as to form a trunk, which passes to the peduncle and con- stitutes the returning branch. Although the circulation in the different bodies is brought into connection by the common stem, yet that of each is independent of the rest, continuing when, the current through its own footstalk is interrupted by a ligature; and the stream which returns from the branchial sac and the viscera is then poured into the posterior part of the heart, instead of entering the peduncle. 559. The development of the Ascidians, the early stages ot which are observable whilst the ova are still within the cloaca of the parent, pre- sents some phenomena of much interest to the Microscopist. After the ordinary repeated segmentation of the yolk, whereby a ' mulberry mass ' is produced (§ 581), a sort of ring is seen encircling its central portion; but this soon shows itself as a tapering tail-like prolongation from one side of the yolk, which gradually becomes more and more detached from it, save at the part from which it springs. Either whilst the egg is still within the cloaca, or soon after it has escaped from the vent, its envelope bursts, and the larva escapes; and in this condition it presents very much the appearance of a tadpole, the tail being straightened out, and pro- pelling the body freely through the water by its lateral strokes. The centre of the body is occupied by a mass of liquid yolk; and this is con- tinued into the interior of three prolongations which extend themselves from the opposite extremity, each terminating in a sort of sucker. After swimming about for some hours with an active wriggling movement, the larva attaches itself to some solid body by means of one of these suckers; if disturbed from its position, it at first swims about as before; but it soon, completely loses its activity, and becomes permanently attached; and important changes manifest themselves in its interior. The pro- longations of the central yolk-substance into the anterior processes and tail are gradually drawn back, so that the whole of it is concentrated into one mass; and the tail, now consisting only of the gelatinous envelope, is either detached entire from the body by the contraction of the connect- POLYZOA AND TUNICATA. 169 ing portion, or withers, and is thrown off gradually in shreds. The shaping of the internal organs out of the yolk-mass takes place very rapidly, so that by the end of the second day of the sedentary state the outlines of the branchial sac and of the stomach and intestine may be traced; no external orifices, however, being as yet visible. The pulsation of the heart is first seen on the third day, and the formation of the branchial and anal orifices takes-place on the fourth; after which the ciliary currents are immediately established through the branchial sac and alimentary canal. — The embryonic development of other Ascidians, solitary as well as composite, takes-place on a plan essentially the same as the foregoing, a free tadpole-like larva being always produced in the first instance. ' 560. This larval condition is represented in a very curious adult free- swimming form, termed Appendicularia, which is frequently to be taken with the Tow-net on our own coasts. The animal has an oval or flask- like body, which in large specimens attains the length of one-fifth of an inch, but which is often not more than one-fourth or one-fifth of that size. It is furnished with a tail-like appendage three or four times its own length, broad, flattened, and rounded at its extremity; and by the powerful vibrations of this appendage it is propelled rapidly through the water. The structure of the body differs greatly from that of the Asci- dians, its plan being much simpler; in particular, the pharyngeal sac is entirely destitute of ciliated branchial fissures opening into a surrounding cavity; but two canals, one on either side of the entrance to the stomach, are prolonged from it to the external surface; and by the action of the long cilia with which these are furnished, in conjunction with the cilia of the branchial sac, a current of water is maintained through its cavity. From the observations of Prof. Huxley, however, it appears that the direction of this current is by no means constant; since, although it usually enters by the mouth and passes out by the ciliated canals, it sometimes enters by the latter and passes out by the former. The caudal appendage has a central axis, above and below which is a riband-like layer of muscular fibres; a nervous cord, studded at intervals with minute ganglia, may be traced along its whole length. — By Mertens, one of the early observers of this animal, it was said to be furnished with a peculiar gelatinous envelope or H*aus (house), very easily detached from the body, and capable of being re-formed after having been lost. Notwithstanding the great numbers of specimens which have been studied by Miiller, Huxley, Leuckart, and Gegenbaur, neither of these excellent observers has met with this appendage; but it has been since seen by Prof. Allman, who describes it as an egg-shaped gelatinous mass, in which the body is imbedded, the tail alone being free; whilst from either side of the central plane there radiates a kind of double fan, which seems to be 1 The study of the development of Ascidians has derived a new interest and importance from the discovery made by Kowalevsky in 1857, that their free- swimming larvae present a most striking parallelism to Vertebrate embryoes, in exhibiting the beginnings of a spinal marrow and a spinal column; thus bridging over the gulf that was supposed to separate them from Invertebrata, and (when taken in connection with the curious Ascidian affinities of Amphyoocus, the low- est Vertebrate at present known) affording strong reason to believe in the deriva- tion of the Vertebrate and Tunicate types from a common original. See his Memoir « Entwickelungsgeschichte der einfachen Ascidien,' in " Mem. St. Petersb. Acad. Sci.," Tom. x., 1867, and the abstract of it in "Quart. Journ. Microsc. Sci.," Vol. x., N.S. (1870), p. 59; also Prof, Haeckel's " History of Creation," Vol. ii., pp. 152, 200. 170 THE MICROSCOPE AND ITS REVELATIONS. formed by a semicircular membranous lamina folded upon itself. It is surmised by Prof. Allman, with much probability, that this curious appendage is * nidamental/ having reference to the development and protection of the young; but on this point further observations are much needed; and any Microscopist, who may meet with Appendicularia furnished with its ' house,' should do all he can to determine its struc- ture and its relations to the body of the animal. ' 1 For details in respect to the structure of Appendicularia, see Huxley, in "Philos. Transact. "for 1851, and in "Quart. Journ. of Microsc. Science," Vol. iv. (1856), p. 181; also Allman in the same Journal, Vol. vii. (1859), p. 86; Gegen- baur in Siebold and Kolliker's "Zeitschrift," Bd. vi. (1855), p. 406; Leuckart's " Zoologische Untersuchungen," Heft ii., 1854; and Fol's • Etudes sur les Appen- diculaires' in " Archiv. Zool. Experim.," Tom. i. (1872), p. 57.— For the Tunicata generally, see Prof. T. Rupert Jones, in Vol. iv. of the " Cyclop, of Anatomy and Physiology;" Mr. Alder's 'Observations on the British Tunicata,' in "Ann. of Xat. Hist.," Ser. 4, Vol. xi. (1863), p. 153; and Mr. Hancock's Memoir 'On the Anatomy and Physiology of the Tunicata,' in the "Journal of the Linngean Society," Vol. ix., p. 309. MOLLUSCOUS ANIMALS GENERALLY. 171 CHAPTER XVI. MOLLUSCOUS ANIMALS GENERALLY. 561. THE various forms of ' Shell-fish/ with tReir 'naked' or shel- less allies, furnish a great abundance of objects of interest to the Micro- scopist; of which, however, the greater part may be grouped under three heads: — namely, (I) the structure of the shell, which is most interesting in the COSCHIFERA and BRACHIOPODA, in both of which classes the shells are ( bivalve/ while the animals differ from each other essentially in gen- eral plan of structure; (2) the structure of the tongue or palate of the GASTEROPODA, most of which have 'univalve' shells, others, however, being 'naked;' (3) the developmental history of the embryo, for the study of which certain of the Gasteropods present the greatest facilities. — These three subjects, therefore, will be first treated of systematically; and a few miscellaneous facts of interest will be subjoined. 562. Shells of Mollusca. — These investments were formerly regarded as mere inorganic exudations, composed of calcareous particles, cemented together by animal glue; Microscopic examination, however, has shown that they possess a definite structure, and that this structure presents certain very remarkable variations in some of the groups of which the Molluscous series is composed. — We shall first describe that which may be regarded as the characteristic structure of the ordinary Bivalves; tak- ing as a type the group of Margaritacece, which includes the Avicula or 'pearl-oyster' and its allies, the common Pinna ranking amongst the latter. In all these shells we readily distinguish the existence of two dis- tinct layers; an external, of a brownish-yellow color; and an internal, which has a pearly or ' nacreous ' aspect, and is commonly of a lighter hue. 563. The structure of the outer layer may be conveniently studied in the shell of Pinna, in which it commonly projects beyond the inner, and there often forms laminas sufficiently thin and transparent to exhibit its general characters without any artificial reduction. If a small portion of such a lamina be examined with a low magnifying power by trans- mitted light, each of its surfaces will present very much the appearance of a honeycomb; whilst its broken edge exhibits an aspect which is evi- dently fibrous to the eye, but which, when examined under the Microscope with reflected light, resembles that of an assemblage of segments of basaltic columns (Fig. 488, P). This outer layer is thus seen to be com- posed of a vast number of prisms, having a tolerably uniform size, and usually presenting an approach to the hexagonal shape. These are arranged perpendicularly (or nearly so) to the surface of the lamina of the shell; so that its thickness is formed by their length, and its two surfaces by their extremities. A more satisfactory view of these prisms is obtained by grinding-down a lamina until it possesses a high degree of 172 THE MICROSCOPE AND ITS REVELATIONS. transparence; the prisms being then seen (Fig. 385) to be themselves composed of a very homogeneous substance, but to be separated by definite and strongly marked lines of division. When such a lamina is submitted to the action of dilute acid, so as to dissolve-away the carbonate of lime, a tolerably firm and consistent membrane is left, which exhibits the prismatic structure just as perfectly as did the original shell (Fig. 386); its hexagonal divisions bearing a strong resemblance to the walls of the cells of the pith or bark of a Plant. By making a section of the shell perpen- dicularly to its surface, we obtain a view of the prisms cut in the direction of their length (Fig. 387); and they are frequently seen to be marked by FIG. 385. FIG. 386. Section of Shell of Pinna, take.ii transversely to the directions of its prisms. Membranous basis of the same. delicate transverse striae (Fig. 388), closely resembling those observable on the prisms of the enamel of teeth, to which this kind of shell struc- ture may be considered as bearing a, very close resemblance, except as regards the mineralizing ingredient. If a similar section be decalcified by dilute acid, the membranous residuum will exhibit the same resem- FIG. 387. FIG. 388 Section of the shell of Pinna, in the direction of its prisms. Oblique Section of Prismatic Shell-substance. blance to the walls of prismatic cells viewed longitudinally, and will be seen to be more or less regularly marked by the transverse striae just alluded to. It sometimes happens in recent, but still more commonly in fossil shells, that the decay of the animal membrane leaves the contained prisms without any connecting medium; as they are then quite isolated, MOLLUSCOUS ANIMALS GENERALLY. 173 they can be readily detached one from another; and each one may be observed to be marked by the like striations, which, when a sufficiently high magnifying power is used, are seen to be minute grooves, apparently resulting from a thickening of the intermediate wall in those situations. These appearances seem best accounted-for, by supposing that each is lengthened by successive additions at its base, the lines of junction of which correspond with the transverse striation; and this view corresponds well with the fact, that the shell-membrane not unfrequently shows a tendency to split into thin laminae along the lines of striation; whilst we occasionally meet with an excessively thin natural lamina lying between the thicker prismatic layers, with one of which it would have probably coa- lesced, but for some accidental cause which preserved its distinctness. That the prisms are not formed in their entire length at once, but that they are progressively lengthened and consolidated at their lower extremi- ties, would appear aL=o from the fact that where the shell presents a deep color (as in Pinna nigrina), this color is usually disposed in distinct strata, the outer portion of each layer being the part most deeply tinged, whilst the inner extremities of the prisms are almost colorless. 564. This ' prismatic' arrangement of the carbonate of lime in the shells of Pinna and its allies, has been long familiar to Conchologists, and regarded by them as the result of crystallization. When it was first more minutely investigated by Mr. Bowerbank1 and the Author,3 and was shown to be connected with a similar arrangement in the membran- ous residuum left after the decalcification of the shell-substance by acid, Microscopists generally3 agreed to regard it as a ' calcified epidermis :' the long prismatic cells being supposed to be formed by the coalescence of the epidermic cells in piles, and giving their shape to the deposit of carbonate of lime formed within them. The progress of inquiry, how- ever, has led to an important modification of this interpretation; the Author being now disposed to agree with Prof. Huxley4 in the belief that the entire thickness of the shell is formed as an excretion from the surface of the epidermis^ and that the horny layer which in ordinary shells forms their external envelope or ' periostracum,' 5 being here thrown out at the same time with the calcifying material, is converted into the likeness of a cellular membrane by the pressure of the prisms that are formed by crystallization at regular distances in the midst of it. — The peculiar conditions under which calcareous concretions form themselves in an organic matrix, have been carefully studied by Mr. Eainey and Dr. W. M. Ord; of whose researches some account will be given hereafter (§ 711). 565. The internal layer of the shells of the MargaritacecB and some other families has a ' nacreous' or iridescent lustre, which, depends (as Sir D. Brewster has shown 8) upon the striation of its surface with a 1 ' On the Structure of the Shells of Molluscous and Conchiferous Animals,' in " Transact, of Microsc. Society," 1st Ser. (1844), Vol. i., p. 123. 2 ' On the Microscopic Structure of Shells,1 in " Reports of British Association " for 1844 and 1847. 3 See Mr. Quekett's " Histological Catalogue of the College of Surgeons' Mu- seum," and his " Lectures on Histology," Vol. ii. 4 See his article ' Tegumentary Organs,' in "Cyclopaedia of Anatomy and Phy- siology," Supplementary Volume, pp. 489-492. 5 The Periostracum is the yellowish-brown membrane covering the surface of many shells, which is often (but erroneously) termed their epidermis. 6 "Philosophical Transactions," 1814, p. 397.— The late Mr. Barton (of the Mint) succeeded in producing an artificial iridescence on metallic buttons, by 174 THE MICROSCOPE AND ITS REVELATIONS. Fig. 389. series of grooved lines, which usually run nearly parallel to each other (Fig. 389). As these lines are not obliterated by any amount of polish- ing, it is obvious that their presence depends upon something peculiar in the texture of this substance, and not upon any mere superficial arrange- ment. When a piece of the nacre (commonly known as ' mother-of- pearl') of the Avicula or ' pearl-oyster ' is carefully examined, it becomes evident that the lines are produced by the cropping-out of laminae of shell situated more or less obliquely to the plane of the surface. The greater the dip of these laminae, the closer will their edges be; whilst the less the angle which they make with the surface, the wider will be the interval between the lines. When the section passes for any distance in the plane of a lamina, no lines will present themselves on that space. And thus the appearance of a section of nacre is such as to have been aptly compared by Sir J. Her- schel to the surface of a smoothed dealboard, in which the woody layers are cut per- pendicularly to their surface in one part, and nearly in their plane in another. Sir D. Brew- ster (loc. cit. ) appears to have supposed that nacre consists of a multitude of layers of carbon- ate of lime alternating with animal membrane; and that the presence of the grooved lines on the most highly-polished surface is due to the wearing away of the edges of the ani- mal laminae, whilst those of the hard calcareous laminae stand out. If each line upon the nacreous surface, however, indicates a distinct layer of shell-substance, a very thin section of ( mother-of-pearl ' ought to contain many hundred laminae, in accordance with the number of lines upon its surface; these being fre- quently no more than l-7500th of an inch apart. But when the nacre is treated with dilute acid so as to dissolve its calcareous portion, no such repetition of membranous layers is to be found; on the contrary, if the piece of nacre be the product of one act of shell-formation, there is but a single layer of membrane. This layer, however, is found to present a more or less folded or plaited arrangement; and the lineation of the nacreous surface may perhaps be thus accounted for. — A similar arrange- ment is found in pearls; which are rounded concretions projecting from the inner surface of the shell of Avicula, and possessing a nacreous structure corresponding to that of ' mother-of-pearl.' Such concretions are found in many other shells, especially the fresh-water mussels, Unio and Anodon; but these are usually less remarkable for their pearly lustre; and, when formed at the edge of the valves, they may be partly or even entirely made-up of the prismatic substance of the external layer, and may be consequently altogether destitute of the pearly character. drawing closely-approximating lines with a diamond-point upon the surface of the steel die by which they were struck. Section of nacreous lining of Shell of Avicula marga- ritacea (Pearl-oyster). MOLLUSCOUS ANIMALS GENERALLY. 175 FIG. 300. 566. In all the genera of the Margaritacece, we find the external layer of the shell prismatic, and of considerable thickness; the internal layer being nacreous. But it is only in the shells of a few families of Bivalves, that the combination of organic with mineral components is seen in the same distinct form; and these families are for the most part nearly allied to Pinna. In the Unionid® (or e fresh- water mussels '), nearly the whole thickness of the shell is made-up of the internal or ' nacreous ' layer; but a uniform stratum of prismatic substance is always found between the nacre and the periostracum, really constituting the inner layer of the latter, the outer being simply horny. — In the Ostracece (or oyster tribe) also, the greater part of the thickness of the shell is composed of a ' sub- nacreous' substance (§ 568) representing the inner layer of the shells of Margaritaceae, its successively-formed laminae, however, having very lit- tle adhesion to each other; and every one of these laminae is bordered at its free edge by a layer of the prismatic substance, distinguished by its brownish-yellow color. In these and some other cases, a distinct membranous residuum is left after the decalcification of the prismatic layer by dilute acid; and this is most tenacious and substantial, where (as in the Margaritacece) there is no proper periostracum. Generally speaking, a thin prismatic layer may be detected upon the external sur- face of Bivalve shells, where this has been protected by a periostracum, or has been prevented in any other manner from undergoing abrasion; thus it is found pretty generally in Chama, Trigonia, and Solen, and oc- casionally in Anomia and Pecten. 567. In many other instances, how- ever, nothing like a cellular struc- ture can be distinctly seen in the deli- cate membrane left after decalcifica- tion; and in such cases the animal basis bears but a very small propor- tion to the calcareous substance, and the shell is usually extremely hard. This hardness appears to depend upon the mineral arrangement of the car- bonate of lime; for whilst in the prismatic and ordinary nacreous layer this has the crystalline condition of calcite, it can be shown in the hard shell of PTiolas to have the arrange- ment of arragonite; the difference be- tween the two being made evident by Polarized light. A very curious appearance is presented by a section of the large hinge-tooth of My a arenaria (Fig. 390), in which the carbonate of lime seems to be de- posited in nodules that possess a crystalline structure resembling that of the mineral termed Wavellite. Approaches to this curious arrangement are seen in many other shells. 568. There are several Bivalve shells which almost entirely consist of what may be termed a sub-nacreous substance; their polished surfaces being marked by lines, but these lines being destitute of that regularity of arrangement which is necessary to produce the iridescent lustre. This is the case, for example, with most of the Pectinidce (or scallop tribe), also with some of the Mytilacece (or mussel tribe), and with the common Oyster. In the internal layer of by far the greater number of Bivalve Section of hinge-tooth, of Mya arenaria. 176 THE MICROSCOPE AND ITS REVELATIONS. shells, however, there is not the least approach to the nacreous aspect; nor is there anything that can be described as definite structure; 1 and the residuum left after its decalcification is usually a structureless 'base- ment-membrane. ' 569. The ordinary account of the mode of growth of the shells of Bivalve Mollusca,— that they are progressively enlarged by the deposition of new laminae, each of which is in contact with the internal surface of the preceding, and extends beyond it, — does not express the whole truth; for it takes no account of the fact that most shells are composed of two layers of very different texture, and does not specify whether loth these layers are thus formed by the entire surface of the ' mantle ' whenever the shell has to be extended, or whether only one is produced. An ex- amination of Fig. 391 will clearly show the mode in which the operation is effected. This figure represents a section of one of the valves of Unio I'll * ) /> T * fl J 1 view the two substances of which the shell is composed; traversing the FIG. 391. Vertical section of the lip of one of the valves of the shell of Unto:— a, 6, c, successive forma- tions of the outer prismatic layer; a', b', c', the same of the inner nacreous layer. outer or prismatic layer in the direction of the length of its prisms, and passing through the nacreous lining in such a manner as to bring into view its numerous laminae, separated by the lines a a', I V , c c', etc. These lines evidently indicate the successive formations of this layer; and it may be easily shown by tracing them towards the hinge on the one side and towards the margin on the other, that at every enlargement of the shell its whole interior is lined by a new nacreous lamina in imme- diate contact with that which preceded it. The number of such laminae, therefore, in the oldest part of the shell, indicates the number of enlarge- ments which it has undergone. The outer or prismatic layer of the growing shell, on the other hand, is only formed where the new structure projects beyond the margin of the old; and thus we do not find one layer of it overlapping another, except at the lines of junction of two distinct formations. When the shell has attained its full dimensions, however, new laminae of both layers still continue to be added, and thus the lip becomes thickened by successive formations of prismatic structure, each being ap- plied to the inner surface of the preceding, instead of to its free margin. — A like arrangement may be well seen in the Oyster; with this differ- 1 For an explanation of the real nature of what was formerly described by the Author as ' tubular ' Shell-substance, see § 816. MOLLUSCOUS ANIMALS GENERALLY. 177 ence, that the successive layers have but a comparatively slight adhesion to each other. 570. The shells of Tertibratula, however, and of most other Brachio- pods, are distinguished by peculiarities of structure which differentiate them from all others. When thin sections of them are microscopically examined, they exhibit the appearance of long flattened prisms (Fig. 392, A, b), which are arranged with such obliquity that FIG. 392, their rounded extremities A B crop-out upon the inner surface of the shell in an imbricated (tile-like) man- ner (a). All true Terebra- tulidce, both recent and fossil, exhibit another very remarkable peculiarity; namely, the perforation of the shell by a large number of canals, which generally pass nearly perpendicularly from one surface to the other (as is shown vertical sections, TTirr 3Q3\ anrl forTninnfp A» Internal surface (a), and oblique section (6), of Shell *lg. 6V6), ana terminate Qf Terebratlda (Waldheimia) ausfralis- B, external sur- mternally by open orifices face of the same. (Fig. 392, A), whilst exter- nally they are covered by the FIG. 393. periostracum (B). Their dia- meter is greatest towards the external surface, where they sometimes expand sud- denly, so as to become trum- pet-shaped; and it is usually narrowed rather suddenly, when, as sometimes hap- pens, a new internal layer is formed as a lining to the preceding (Fig. 393, A,dd). Hence the diameter of these canals, as shown in different transverse sections of one and the same shell, will vary according to the part of Vertical Sectiong of ghell of Terebratula Its tniCKlieSS Which the Sec- heimia) australis; showing at A the canals opening by firm harmpna fr> trnvprap large trumpet- shaped orifices on the outer surface, and nappens tO tl avei Se. — contracting at d, d, into narrow tubes; and showing at B a 1 The Shells Of different Species bifurcation of the canals. of perforated Brachiopods, however, present very striking diversities in the size and closeness of their canals, as shown by sections taken in corresponding parts; three examples of this kind are given for the sake of comparison in Figs. 394-396. These canals are occupied in the living state by tubular prolongations of the mantle, whose interior is filled with a fluid containing minute cells and granules, which, from its corresponding in appearance with the fluid contained in the great sinuses of the mantle, may perhaps be considered to be the animal's blood. Of their special function in the economy of ITS THE MICROSCOPE AND ITS REVELATIONS the animal, it is difficult to form any probable idea; but it is interesting to remark (in connection with the hypothseis of a relationship between Brachiopods and Polyzoa) that they seem to have their parallel in exten- sions of the peri-visceral cavity of many species of Plustra, Escliara, Lepralia, etc., into passages excavated in the walls of the cells of the polyzoary. 571. In the Family Rhynchonellidw, which is represented by only two recent species (the Rh. psittacea and Rh. nigr icons, both formerly rank- ing as Terebratulae), but which contains a very large proportion of fossil Brachiopods, these canals are almost entirely absent; so that the uniform- ity of their presence in the Terebratulidae, and their general absence in the Rhynchonellidae, supplies a character of great value in the discrimi- nation of the fossil shells belonging to these two groups respectively. Great caution is necessary, however, in applying this test; mere surface- markings cannot be relied-on; and no statement on this point is worthy of reliance, which is not based on a Microscopic examination of thin sec- tions of the shell. — In the Families Spiriferidce and Strophomenidm, on the other hand, some species possess the perforations, whilst others are desti- Fio. 394. FIG. 395. FIG. 396. FIG. 394. Horizontal section of Shell of Terebratula bullata (fossil, Oolite). FIG. 395. Ditto . of Meaerlia lima (fossil, Chalk). FIG. 396. Ditto . . of Spiriferina rostrata (Triassic). tute of them; so that their presence or absence there serves only to mark- out subordinate groups. This, however, is what holds-good in regard to characters of almost every description, in other departments of Natural History; a character which is of fundamental importance from its close relation to the general plan of organization in one group, being, from its want of constancy, of far less account in another. * 572. There is not by any means the same amount of diversity in the structure of the Shell in the class of Gasteropods; a certain typical plan of construction being common to by far the greater number of them. The small proportion of animal matter contained in most of these shells, is a very marked feature in their character; and it serves to render other 1 For a particular account of the Author's researches on this group, see his Memoir on the subject, forming part of the introduction of Mr. Davidson's " Monograph of the British Fossil Brachiopoda," published by the Palaeontogra- phical Society. — A very remarkable example of the importance of the presence or absence of the perforations, in distinguishing shells whose internal structure shows them to be generically different, whilst from their external conformation they would be supposed to be not only generically but specifically identical, will be found in the " Annals of Natural History," Ser. 3, Vol. xx. (1867), p. 68. MOLLUSCOUS 'ANIMALS GENERALLY. 179 features indistinct, since the residuum left after the removal of the cal- careous matter is usually so imperfect, as to give no clue whatever to the explanation of the appearances shown by sections. Nevertheless, the structure of these shells is by no means homogeneous, but always exhibits indications, more or less clear, of a definite arrangement. The ' porcel- lanous' shells are composed of three layers, all presenting the same kind of structure, but each differing from the others in the mode in which this is disposed. For each layer is made-up of an assemblange of thin laminae placed side-by-side, which separate one from another, apparently in the planes of rhomboidal cleavage, when the shell is fractured; and as was first pointed out by Mr. Bowerbank, each of these laminae consists of a series of elongated spicules (considered by him as prismatic cells filled with carbonate of lime) lying side-by-side in close apposition; and, these series are disposed alternately in contrary directions, so as to inter- sect each other nearly at right angles, though still lying in parallel planes. The direction of the planes is different, however, in the three layers of the shell, bearing the same relation to each other as have those three sides of a cube which meet each other at the same angle; and by this arrangement, which is better seen in the fractured edge of the Cyprc&a or any similar shell, than in thin sections, the strength of the shell is greatly augmented. — A similar arrangement, obviously answering the same purpose, has been shown by Mr. Tomes to exist in the enamel of the teeth of Rodentia. 573. The principal departures from this plan of structure are seen in Patella, Chiton, Haliotis, Turbo and its allies, and in the ' naked Gasteropods, many of which last, both terrestrial and marine, have some rudiment of a shell. Thus in the common Slug, Limax rufus, a thin oval plate of calcareous texture is found imbeddedln the shield-like fold of the mantle covering the fore-part of its back; and if this be examined in an early stage of its growth, it is found to consist of an aggregation of minute calcareous nodules, generally somewhat hexagonal in form, and sometimes quite transparent, whilst in other instances it presents an appearance closely resembling that delineated in Fig. 390. — In the epidermis of the mantle of some species of Doris, on the other hand, we find long calcareous spicules, generally lying in parallel directions, but not in con- tact with each other, giving firmness to the whole of its dorsal portion; and these are sometimes covered with small tubercles, like the spicules of Gorgbnia (Fig. 363). They may be separated from the soft tissue in which they are imbedded, by means of caustic potash; and when treated with dilute acid, whereby the calcareous matter is dissolved-away, an organic basis is left, retaining in some degree the form of the original spicule. This basis cannot be said to be a true cell; but it seems to be rather a cell in the earliest stage of its formation, being an isolated parti- cle of sarcode without wall or cavity; and the close correspondence between the appearance presented by thin sections of various Univalve shells, and the forms of the spicules of Doris, seems to justify the con- clusion that even the most compact shells of this group are constructed out of the like elements, in a state of closer aggregation and more definite arrangement, with the occasional occurrence of a layer of more spheroidal bodies of the same kind, like those forming the rudimentary shell of Limax. 574. The structure of Shells generally is best examined by making sections in different planes as nearly parallel as may be possible to the surfaces of the shell, and other sections at right angles to these: the 180 THE MICROSCOPE AND ITS REVELATIONS. former may be designated as horizontal, the latter as vertical. Nothing need here be added to the full directions for making such Sections, which have already been given (§§ 192-194). Many of them are beau- tiful and interesting objects for the Polariscope. — Much valuable infor- mation may also be derived from the examination of the surfaces presented by fracture. The membranous residua left after the decalcifi- cation of the shell by dilute acid, may be mounted in weak spirit or in Goadby's solution. 575. The animals composing the class of Cephalopoda (cuttle-fish and nautilus tribe) are for the most part unpossessed of shells; and the struc- ture of the few that we meet-with in the genera Nautilus, Argonauta (' paper-nautilus'), and Spirula, does not present any peculiarities that need here detain us. The rudimentary shell or sepiostaire of the com- mon Cuttle-fish, however, which is frequently spoken-of as the ' cuttle- fish bone,' exhibits a very beautiful and remarkable structure, such as causes sections of it to be very interesting Microscopic objects. The outer shelly portion of this body consists of horny layers, alternating with calcified layers, in which last may be seen a hexagonal arrangement somewhat corresponding with that in Fig. 390. The soft friable sub- stance that occupies the hollow of this boat-shaped shell, is formed of a number of delicate calcareous plates, running across it from one side to the other in parallel directions, but separated by intervals several times wider than the thickness of the plates; and these intervals are in great part filled-up by what appear to be fibres or slender pillars, passing from one plate or floor to another. A more careful examination shows, however, that instead of a large number of detached pillars, there ex- ists a comparatively small number of very thin sinuous laminae, which pass from one surface to the other, winding and doubling upon themselves, so that each lamina occupies a considerable space. Their precise ar- rangement is best seen by examining the parallel plates, after the sinuous laminae have been detached from them; the lines of junction being dis- tinctly indicated upon these. By this arrangement each layer -is most effectually supported by those with which it is connected above and below; and the sinuosity of the thin intervening laminae, answering ex- actly the same purpose as the ' corrugation ' given to iron plates for the sake of diminishing their flexibility, adds greatly to the strength of this curious texture; which is at the same time lightened by the large amount of open space between the parallel plates, that intervenes among the sinuosities of the laminae. The best method for examining this struc- ture; is to make sections of it with a sharp knife in various directions, taking care that the sections are no thicker than is requisite for holding- together; and these may be mounted on a Black Ground as opaque ob- jects, or in Canada balsam as transparent objects, under which last aspect they furnish very beautiful objects for the Polariscope. 576. Palate of Gasteropod Mollusks. — The organ which is sometimes referred to under this designation, and sometimes as the ' tongue,' is one of a very singular nature; and cannot be likened to either the tongue or the palate of higher animals. For it is a tube that passes backward and downwards beneath the mouth, closed at its hinder end, whilst in front it opens obliquely upon the floor of the mouth, being (as it were) slit-up and spread-out so as to form a nearly flat surface. On the interior of the tube, as well as on the flat expansion of it, we find numerous trans- verse rows of minute teeth, which are set upon flattened plates; each principal tooth sometimes having a basal plate of its own, whilst in MOLLUSCOUS ANIMALS GENERALLY. 181 other instances one plate carries several teeth. — Of the former arrange- ment we have an example in the palate of many terrestrial Gasteropods, such as the snail (Helix) and Slug (Limax), in which the number of plates in each row is very considerable (Figs. 397, 398), amounting to 180 in the large garden Slug (Limax maximus); whilst the latter prevails in many marine Gasteropods, such as the common Whelk (Buccinum unda- tum), the palate of which has only three plates in each row, one bearing the small central teeth, and the two others the large lateral teeth (Fig. FIG. 397. FIG. 398. Portion of the left half of the Palate of Helix hortensis ,' the rows of teeth near the edge separated from each other to show their form. Palate of Zonites cellaring 401). The length of the palatal tube, and the number of rows of teeth, vary greatly in different species. Generally speaking, the tube of the terrestrial Gasteropods is short, and is contained entirely within the nearly globular head; but the rows of teeth being closely set together are usually very numerous, there being frequently more than 100, and in FIG. 399. FIG 400 Palate of Trochus zizyphinus. Palate of Doris tuberculata. some species as many as 160 or 170; so that the total numer of teeth may mount-up, as in Helix pomatia, to 21,000, and in Limax maximus, to 26,800. The transverse rows are usually more or less curved, as shown in Fig. 398, whilst the longitudinal rows are quite straight: and 182 THE MICROSCOPE AND ITS REVELA.TION8. the curvature takes its departure on each side from a central longitudi- nal row, the teeth of which are symmetrical, whilst those of the lateral .portions of each transverse row present a modification of that symmetry, the prominences on the inner side of each tooth being suppressed, whilst those on the outer side are increased; this modification being observed to augment in degree, as we pass from the central line towards the edges. 577. The palatal tube of the marine Gasteropods is generally longer, and its teeth larger; and in many instances it extends far beyond the head, which may, indeed, contain but a small part of it. Thus in the common Limpet (Patella), we find the principal part of the tube to lie f olded-up, but perfectly free, in the abdominal cavity, between the in- testine and the muscular foot; and in some species its length is twice or even three times as great as that of the entire animal. In a large pro- portion of cases, these palates exhibit a very marked separati6n between the central and the lateral portions (Figs. 399, 401); the teeth of the central band being frequently small and smooth at their edges, whilst those of the lateral are large and serrated. The palate of Trochus zizy- phinus, represented in Fig. 399, is one of the most beautiful examples of this form; not only the large teeth of the lateral bands, but the deli- cate leaf -like teeth of the central portion, haying their edges minutely serrated. A yet more complex type, however, is found in the palate of Haliotis ; in which there is a central band of teeth having nearly straight edges instead of points: then, on each side, a lateral band consisting of large teeth shaped like those of the Shark; and beyond this, again, another lateral band on either side, composed of several rows of smaller teeth. — Very curious differences also present themselves among the dif- ferent species of the same genus. Thus in Doris pilosa, the central band is almost entirely wanting, and each lateral band is formed of a single row of very large hooked teeth, set obliquely like those of the lateral band in Fig. 399; whilst in Doris tuber culata, the central band is the part most developed, and contains a number of rows of conical teeth, standing almost perpendicularly, like those of a harrow (Fig. 400). 578. Many other varieties might be described, did space permit; but we must be content with adding, that the form and arrangement of the teeth of these ' palates' afford characters of great value in classification, as was first pointed out by Prof. Loven (of Stockholm) in 1847, and has been since very strongly urged by Dr. J. E. Gray, who considers that the structure of these organs is one of the best guides to the natural affinities of the species, genera, and families of this group, since any im- portant alteration in the form or position of the teeth must be accom- panied by some corresponding peculiarity in the habits and food of the animal.1 Hence a systematic examination and delineation of the struc- ture and arrangement of these organs, by the aid of the Microscope and Camera Lucida, would be of the greatest service to this department of Natural History. The short thick tube of Limax and other terrestrial Gasteropods, appears adapted for the trituration of the food previously to its passing into the oesophagus; for in these animals we find the roof of the mouth furnished with a large strong horny plate, against which the flat end of the tongue can work. On the other hand, the flattened portion of the palate of Buccinum (whelk) and its allies is used by these animals as a file, with which they bore holes through the shells of the 1 " Annals of Natural History," Ser. 2, Vol. x. (1852), p. 413. MOLLUSCOUS ANIMALS GENERALLY. 183 Mollusks that serve as their prey; this they are enabled to effect by evert- ing that part of the probosis-shaped mouth whose floor is formed by the flattened part of the tube, which is thus brought to the exterior, and. by giving a kind of sawing-motion to the organ by means of the alter- nate action of two pairs of muscles, — a protractor, and a retractor,— which put-forth and draw-back a pair of cartilages whereon the tongue is supported, and also elevate and depress its teeth. Of the use of the long blind tubular part of the palate in these Gasteropods, however, scarcely any probable guess can be made; unless it be a sort of * cavity of reserve/ from which a new toothed surface may be continually sup- plied as the old one is worn- away, somewhat as the front teeth of the Ro- dents are constantly being regenerated from the surface of the pulps which occupy their hollow conical bases, as fast as they are rubbed-down at their edges. 579. The preparation of these Palates for the Microscope can, of course, be only accomplished by carefully dissecting them from their attachments within the head; and it will be also necessary to remove the membrane that forms the sheath of the tube, when this is thick enough to interfere with its transparence. The tube itself should be slit up with a pair of fine scissors through its entire length; and should be so opened out, that its expanded surface may be a continuation of that which forms the floor of the mouth. The mode of mounting it will de- pend upon the manner in which it is to be viewed. For the ordinary purposes of Microscopic examination, no method is so good as mounting in fluid; either weak Spirit or Goadby's solution answering very well. But many of these palates, especially those of the marine Gasteropods, be- come most beautiful objects for the Polariscope when they are mounted in Canada balsam; the form and arrangement of the teeth being very strongly brought-out by it (Fig. 401), and a gorgeous play of colors being exhibited when a selenite plate is placed behind the object, and the analyzing prism is made to rotate.1 580. Development of Mollusks. — Leaving to the scientific Embryologist the large field of study that lies open to him in this direction,2 the ordi- nary Microscopist will find much to interest him in the observation of cer- tain special phenomena of which a general account will be here given. At- tached to the gills of fresh-water Mussels ( Unio and Anodon) there are often found minute bodies, which, when first observed, were described as parasites, under the name of Olocliidia, but are now known to be their own progeny in an early phase of development. When a Fish is near, they are expelled from between the valves of their parent, and attach themselves in a peculiar manner to its fins and gills (Fig. 402, A). In this stage of the existence of the young Anodon, its valves are provided with curious barbed or serrated hooks (D, £), and are continually snap- ping together (so as to remind the observer of the avicularia of Polyzoa, § 554), until they have inserted their hooks into the skin of the Fish, which seems so to retain the barbs as to prevent the reopening of the valves. In this stage of its existence no internal organ is definitely formed, except the strong 'adductor muscle* (c, a) which draws the valves together, and the long, slender, byssus-filament (B, «, D) which makes its appearance while the embryo is still within the egg membrane, 1 For additional details on the organization of the Palate and Teeth of the Gasteropod Mollusks, see Mr. W. Thomson, in "Cyclop, of Anat. and Physiol. '" Vol. iv., pp. 1142, 1143; and in "Ann. of Nat. Hist.," Ser. 2 Vol. vii., p. 86. 8 See Balfour's " Comparative Embryology," Chap. ix. 184: THE MICROSCOPE AND ITS REVELATIONS. lying coiled-up between the lateral lobes. The hollow of each valve is filled with a soft granular-looking msss, in which are to be distinguished what are perhaps the rudiments of the branchiae and of oral tentacles; but their nature can only be certainly determined by further observation, which is rendered difficult by the opacity of the valves. By keeping a supply of Fish, however, with these embryoes attached, the entire history of the development of the fresh-water Mussel may be worked out.1 581. In certain members of the Class Gasteropods, the history of em- bryonic development presents numerous phenomena of great interest. The eggs (save among the terrestrial species) are usually deposited in aggregate masses, each inclosed in a common protective envelope or nidamentum. The nature of this envelope, however, varies greatly : thus, in the common Limnceus stagnalis or t water-snail ' of our ponds FIG. 401. FIG. 402. Palate of Buccinum undatum as seen under Polarized Light. Parasitic Larva (Glochidium) of Anodon:— A, glochidia at- tached to the tail of a Stickleback ; B, side view of glochidium still inclosed in the egg-membrane, showing the hooks of its valves and the byssus-filamenta,1 c, giochidium with its valves widely opened, showing the adductor-muscle a; D, side view of glochidium; with the valves opened to show the origin of the byssus-filament and the three pairs of tentacular ( ?) or- gans, the barbed hooks 6, and the muscular or membran- ous folds c, c, connected with them. and ditches, it is nothing else than a mass of soft jelly about the size of a sixpence, in which from 50 to 60 eggs are imbedded, and which is at- tached to the leaves or stems of aquatic plants; in the Buccinum unda- tum, or common Whelk, it is a membranous case, connected with a con- siderable number of similar cases by short stalks, so as to form large globular masses which may often be picked-up on our shores especially between April and June ; in the Purpura lapillus, or ' rock-whelk/ it is a little flask-shaped capsule, having a firm horny wall, which is attached by a short stem to the surface of rocks between the tide-marks, great numbers being often found standing erect side by side; whilst in the • l See the Rev. W. Houghton ' On the Parasitic Nature of the Fry of the Ano- donta cygnea,' in "Quart. Journ. of Microsc. Sci.," N.S., Vol. ii. (1861), p. 162; and Balfour, op. cit., pp. 220-223. MOLLUSCOUS ANIMALS GENERALLY. 185 Nudilranchiate order generally (consisting of the Doris, Eolis, and other 'sea-slugs') it forms a long tube with a membranous wall, in which im- mense numbers of eggs (even half a million or more) are packed closely together in the midst of a jelly-like substance, this tube being disposed in coils of various forms, which are usually attached to the Sea- weeds or Zoophytes. — The course of development, in the first and last of these instances, may be readily observed from the very earliest period down to that of the emersion of the embryo; owing to the extreme transparence FIG. 403. Embryonic development of Doris bilamellata:—A, Ovum, consisting of enveloping membrane a and yolk 6 ; B, c. D, E, F, successive stages of segmentation of yolk; o, first marking-out of the shape of the embryo ; H, embryo on the 8th day; i, the same on the 9th day; K, the same on the 12th day, seen on the leftside at L; M, still more advanced embryo, seen at N as retracted within its shell:— a, superficial layer of yolk-segments coalescing to give origin to the shell; c, c, ciliated lobes; d, foot; gr, hard plate or operculum attached to it; h, stomach; i, intestine; m, n, masses (glandular ?) at the sides of the oesophagus; o, heart (?); s, retractor muscle (?;; t, situation of funnel; v, membrane enveloping the body; x, auditory vesicles; y, mouth. of the nidamentum and of the egg-membranes themselves. The first change which will be noticed by the ordinary observer, is the ' segmenta- tion ' of the yolk-mass, which divides itself (after the manner of a cell undergoing binary subdivision) into two parts, each of these two into 186 THE MICROSCOPE AND ITS REVELATIONS. two others, and so on until a morula or mulberry-like mass of minute yolk-segments is produced (Fig. 403, A-F), which is converted by 'in- vagination' into a 'gastrula' (§391), whose form is shown at G. This 'gastrula' soon begins to exhibit a very curious alternating rotation within the egg, two or three turns being made in one direction, and the same number in a reverse direction : this movement is due to the cilia fringing a sort of fold of the ectoderm termed the velum, which after- wards usually gives origin to a pair of large ciliated lobes (H-L, c) resem- bling those of Kotifers. The velum is so little developed in Limnceus, however, that its existence has been commonly overlooked until recog- nized by Prof. Kay Lankester/ who also has been able to distinguish its fringe of minute cilia. This, however, has only a transitory existence; and the later rotation of the embryo, which presents a very curious spectacle when a number of ova are viewed at once under a low magnifying power, is due to the action of the cilia fringing the head and foot. 582. A separation is usually seen at an early period, between the anterior or 'cephalic' portion, and the posterior or 'visceral' portion, of the embryonic mass; and the development of the former advances with the greater activity. One of the first changes which is seen in it, con- sists in its extension into a sort of fin-like membrane on either side, the edges of which are fringed with long cilia (Fig. 403, H-L, c), whose move- ments may be clearly distinguished whilst the embryo is still shut-up within the egg; at a very early period may also be discerned the 'auditory vesicles ' (K, x) or rudimentary organs of hearing (§ 587), which scarcely attain any higher development in these creatures during the whole of life; and from the immediate neighborhood of these is put-forth a pro- jection, which is afterwards to be evolved into the 'foot' or muscular disk of the animal. While these organs are making their appearance, the shell is being formed on the surface of the posterior portion, appear- ing first as a thin covering over its hinder part, and gradually extending itself until it becomes large enough to inclose the embryo completely, when this contracts itself. The ciliated lobes are best seen in the embryoes of Nudibranchs; and the fact of the universal presence of a shell in the embryoes of that group is of peculiar interest, as it is destined to be cast- off very soon after they enter upon active life. These embryoes may be seen to move-about as freely as the narrowness of their prison permits, for some time previous to their emersion; and when set free by the rup- ture of the egg-cases, they swim forth with great activity by the action of their ciliated lobes, — these, like the 'wheels' of Kotifera, serving also to bring food to the mouth, which is at that time unprovided with the reducing apparatus subsequently found in it. The same is true of the embryo of Lymnceus, save that its swimming movements are less active, in consequence of the non -development of the ciliated lobes; and the currents produced by the cilia that fringe the head and the orifice of the respiratory sac, seem to have reference chiefly to the provision of sup- plies of food, and of aerated water for respiration. The disappearance of the cilia has been observed by Mr. Hogg to be coincident with the development of the teeth to a degree sufficient to enable the young water- snail to crop its vegetable food; and he has further ascertained that if the growing animal be kept in fresh water alone for some time, without vege- 1 See his valuable * Observations on the Development of Limnceus stagnalus, and on the other stages of other Mollusca,' in " Quart. ^Journ. Microsc. Science," Oct. 1874. See also Lereboullet, ' Recherches sur le Developpement du Limnee,' in " Ann. des Sci. Nat. Zool.," 4ieme Ser., Tom. xviii., p. 47. MOLLUSCOUS ANIMALS GENERALLY. 187 table matter of any kind, the gastric teeth are very imperfectly developed, and the cilia are still retained.1 583. A very curious modification of the ordinary plan of development is presented in the Purpura lapillus; and it is probable that something of the same kind exists also in Buctinwn, as well as in other Gasteropods of the same extensive Order (Pectinilranchiata). — Each of the capsules already described (§ 581) contains from 500 to 600 egg-like bodies (Fig. 404, A), imbedded in a viscid gelatinous substance; but only from 12 to 30 embroyes usually attain complete development; and it is obvious from the large comparative size which these attain (Fig. 405, B), that each of them must include an amount of substance equal to that of a great num- ber of the bodies originally found within the capsule. The explanation of this fact (long since noticed by Dr. J. E. Gray, in regard to Buccinum) seems to be as follows: — Of those 500 or 600 egg-like bodies, only a small part are fertile ova, the remainder being unfertilized eggs, the yolk- FlG.404. FIG. 405. Early stages of Embryonic Development of Purpura lapillus: — A, egg-like spherule; B, c, E, P, G, successive stages of segmenta- tion of yolk-spherules; D, H, i, j, K, succes- sive stages of development of early em- bryoes. Later stages of embryonic Development of Pur- pura lapillus: — A, conglomerate mass of vitelline segments, to which were attached the embryoes, a, 6, c, d, e: — B, full-size embryo, in more advanced stage of development. material of which serves for the nutrition of the embryoes in the later stages of their intra-capsular life. The distinction between them mani- fests itself at a very early period, even in the first segmentation; for while the latter divide into two equal hemispheres (Fig. 404, B), the fertilized ova divide into a larger and a smaller segment (D); in the cleft between these are seen the minute * directive vesicles,' which appear to be always double or even triple, although from being seen ' end on,' only one may be visible; and near these is generally to be seen a clear space in each segment. The difference is still more strongly marked in the subsequent divisions; for whilst the cleavage of the infertile eggs goes-on irregularly, so as to divide each into from 14 to 20 segments, having no definiteness of arrangement (c, E, F, G), that of the fertile ova takes place in such a manner as to mark-out the distinction already alluded-to between the 1 See " Transact, of Microsc. Soc.," 2d Ser., Vol. ii. (1854), p. 188 THE MICROSCOPE AND ITS REVELATIONS. ' cephalic ' and the ' visceral ' portions of the mass (H) ; and the evolution of the former into distinct organs very speedily commences. In the first instance, a narrow transparent border is seen around the whole embryonic mass, which is broader at the cephalic portion (i); next, this border is fringed with short cilia, and the cephalic extension into two lobes begins to show itself; and then between the lobes a large mouth is formed, open- ing through a short, wide oesophagus, the interior of which is ciliated, into the visceral cavity, occupied as yet only by the yolk-particles origi- nally belonging to the ovum (K). 584. Whilst these developmental changes are taking place in the em- bryo, the whole aggregate of segments formed by the yolk-cleavage of the infertile eggs coalesces into one mass, as shown at A, Fig. 405; and the embryoes are often, in the first instance, so completely buried within this, as only to be discoverable by tearing its portions asunder: but some of them may commonly be found upon its exterior; and those contained in one capsule very commonly exhibit the different stages of development represented in Fig. 404, H-K. After a short time, however, ib becomes apparent that the most advanced embryoes are beginning to swalloiv the yolk-segments of the conglomerate mass; and capsules will not unfre- quently be met-with, in which embryoes of various sizes, as a, 1), c, d, e (Fig. 405, A), are projecting from its surface, their difference of size not being accompanied by advance in development, but merely depending upon the amount of this * supplemental ' yolk which the embryoes have respectively gulped-down. For during the time in which they are engaged in appropriating this additional supply of nutriment, although they in- crease in size, yet they scarcely exhibit any other change; so that the large embryo, Fig. 405, e, is not apparently more advanced as regards the for- mation of its organs, than the small embryo, Fig. 404, K. So soon as this operation has been completed, however, and the embryo has attained its full bulk, the evolution of its organs takes-place very rapidly; the cili- ated lobes are much more highly developed, being extended in a long sinuous margin, so as almost to remind the observer of the ' wheels' of Rotifera (§ 445), and being furnished with very long cilia (Fig. 405, B); the auditory vesicles, the tentacula, the eyes, and the foot, successively make their appearance; a curious rhythmically-contractile vesicle is seen, just beneath the edge of the shell in the region of the neck, which may, perhaps, serve as a temporary heart; a little later, the real heart may be seen pulsating beneath the dorsal part of the shell; and the mass of yolk- segments of which the body is made-up, gradually shapes itself into the various organs of digestion, respiration, etc., during the evolution of which (and while they are as yet far from complete) the capsule thins- away at its summit, and the embiyoes make their escape from it.1 585. It happens not unfrequently that one of the embryoes which a capsule contains does not acquire its ' supplemental ' yolk in the manner now described, and can only proceed in its development as far as its ori- ginal yolk will afford it material; and thus, at the time when the other embryoes have attained their full size and maturity, a strange-looking 1 The Author thinks it worth while to mention the method which he has found most convenient for examining the contents of the egg-capsules of Purpura; as he believes that it may be advantageously adopted in many other cases. This consists in cutting off the two ends of the capsule (taking care not to cut far into its cavity), and in then forcing a jet of water through it, by inserting the end of a fine-pointed syringe (§ 127) into one of the orifices thus made, so as to drive the contents of the capsule before it through the other. These should be received into a shallow cell, and tirst examined under the Simple Microscope. MOLLUSCOUS ANIMALS GENERALLY. 189 creature, consisting of two large ciliated lobes with scarcely the rudiment of a body, may be seen in active motion among them. This may happen, indeed, not only to one but to several embryoes within the same capsule, especially if their number should be considerable; for it sometimes appears as if there were not food enough for all, so that whilst some attain their full dimensions and complete development, others remain of unusually small size, without being deficient in any of their organs, and others again are more or less completely abortive, — the supply of supplemental yolk which they have obtained having been too small for the develop- ment of their viscera, although it may have afforded what was needed for that of the ciliated lobes, eyes, tentacles, auditory vesicles, and even the foot, — or, on the other hand, no additional supply whatever having been acquired by them, so that their development ha?s been arrested at a still earlier stage. — These phenomena are of so remarkable a character, that they furnish an abundant source of interest to any Microscopist who may happen to be spending the months of August and September in a locality in which the Purpura abounds; since, by opening a sufficient number of capsules, no difficulty need be experienced in arriving at all the facts which have been noticed in this brief summary.1 It is much to be desired that such Microscopists as possess the requisite opportunity, would apply themselves to the study of the corresponding history in other Pectini- branchiate Gasteropods, with a view of determining how far the plan now described prevails through the Order. And now that these Mollusks have been brought not only to live, but to breed, in artificial aquaria, it may be anticipated that a great addition to our knowledge of this part of their life-history will ere long be made. 586. Ciliary Motions on Gills. — There is no object that is better suited to exhibit the general phenomena of Ciliary motion (§ 4o5), than a portion of the gill of some bivalve Mollusk. The Oyster will answer the purpose sufficiently well; but the cilia are much larger on the gills of the Mussel,* as they are also on those of the Anodon or common ' fresh- water mussel' of our ponds and streams. Nothing more is necessary than to detach a small portion of one of the riband-like bands, which will be seen running parallel with the edge of each of the valves when the shell is opened; and to place this, with a little of the liquor contained within the shell, upon a slip of glass, — taking care to spread it out sufficiently with needles to separate the bars of which it is composed, since it is on the edges of these, and round their knobbed extremities, that the ciliary movement presents itself, — and then covering it with a thin-glass disk. Or it will be convenient to place the object in the Aquatic-box (§ 122), which will enable the observer to subject it to any degree of pressure that 1 Fuller details on this subject will be found in the Author's account of his researches, in "Transactions of the Microscopical Society," 2d Ser., Vol. iii. (1855), p. 17. His account of the process was called in question by MM. Koren anjd Danielssen, who had previously given an entirely different version of it, but was fully confirmed by the observations of Dr. Dyster; see "Ann. of Nat. Hist." 2d Ser., Vol. xx. (1857), p. 16. The independent observations of M. Clapa- rede on the development of Neritina fluviatilis (Mullens "Archiv," 1857, p. 109, and abstract in "Ann. of Nat.*Hist.," 2d Ser., Vol. xx. (1857), p. 196, showed the mode of development in that species to be the same in all essential particulars as that of Purpura. The subject has again been recently studied with great minute- ness by Selenka, " Niederlandisches Archiv fur Zoologie," Bd. i., July, 1862 2 This Shell-fish may be obtained, not merely at the sea-side, but likewise at the shops of the fishmongers who supply the humbler classes, even in midland towns. 190 THE MICROSCOPE AND IT8 REVELATIONS. he may find convenient. A magnifying power of about 120 diameters is amply sufficient to afford a general view of this spectacle; but a much greater amplification is needed to bring into view the peculiar mode in which the stroke of each cilium is made. Few spectacles are more striking to the unprepared mind, than the exhibition of such wonderful activity as will then become apparent, in a body which to all ordinary ob- servation is so inert. This activity serves a double purpose; for it not only drives a continual current of water over the surface of the gills them- selves, so as to effect the aeration of the blood, but also directs a portion of this current (as in the Timicata, § 555) to the mouth, so as to supply the digestive apparatus with the aliment afforded by the Diatomacece, Infusoria, etc., which it carries-in with it. " 587. Organs of Sense of Mollusks. — Some of the minuter and more rudimentary forms of the special organs of sight, hearing, and touch, which the Molluscous series presents, are very interesting objects of Mi- croscopic examination. Thus, just within the margin of each valve of Pecten, we see (when we observe the animal in its living state, under water) a row of minute circular points of great brilliancy, each surrounded by a dark ring; these are the eyes, with which this creature is provided, and by which its peculiarly-active movements are directed. Each of them, when their structure is carefully examined, is found to be protected by a sclerotic coat with a transparent cornea in front; and to possess a colored iris (having a pupil) that is continuous with a layer of pigment lining the sclerotic, a crystalline lens and vitreous body, and a retinal expansion pro- ceeding from an optic nerve which passes to each eye from the trunk that runs along the margin of the mantle.1 — Eyes of still higher organization are borne upon the head of most Gasteropod Mollusks, generally at the base of one of the pairs of tentacles, but sometimes, as in the Snail and slug, at the points of these organs. In the latter case, the tentacles are furnished with a very peculiar provision for the protection of the eyes; for when the extremity of either of them is touched, it is drawn-back into the basal part of the organ, much as the finger of a glove may be pushed- back into the palm. The retraction of the tentacle is accomplished by a strong muscular band, which arises within the head, and proceeds to the extremity of the tentacles; whilst its protrusion is eifected by the agency of the circular bands with which the tubular wall of the tentacle is itself furnished, the inverted portion being (as it were) squeezed-out by the contraction of the lower part in which it has been drawn back. The structure of the eyes, and the curious provision just described, may easily be examined by snipping-off one of the eye-bearing tentacles with a pair of scissors. — None but the Cephalopod Mollusks have distinct organs of hearing; but rudiments of such organs may be found in most Gasteropods (Fig. 403, K, x), attached to some part of the nervous collar that sur- rounds the oesophagus; and even in many Bivalves, in connection with the nervous ganglion imbedded in the base of the foot. These ' auditory vesicles/ as they are termed, are minute sacculi, each of which contains a fluid, wherein are suspended a number of minute calcareous particles (named otoliths or ear-stones), which are kept in a state of continual movement by the action of cilia lining the vesicles. This " wonderful spec- tacle," as it was truly designated by its discoverer Siebold, may be brought into view without any dissection, by submitting the head of any small 1 See Mr. S. J. Hickson on 'The Eye of Pecten,' in " Quart. Journ. Microsc. Sci.," Vol. xx., N.S. (1880), p. 443. MOLLUSCOUS ANIMALS GENERALLY. 191 and not very thick-skinned Gasteropod, or the young of the larger forms, to gentle compression under the Microscope, and transmitting a strong light through it. The very early appearance of the auditory vesicles in the embryo Gasteropod has been already alluded-to (§ 582). — Those who have the opportunity of examining young specimens of the common Pecten, will find it extremely interesting to watch the action of the very delicate tentacles which they have the power of putting-forth from the margin of their mantle, the animal being confined in a shallow cell, or in the zoophyte-trough; and if the observer should be fortunate enough to obtain a specimen so young that the valves are quite transparent, he will find the spectacle presented by the ciliary movement of the gills, as well as the active play of the foot (of which the adult can make no such use), to be worthy of more than a cursory glance. 588. Chromatopliores of Cephalopods. — Almost any species of Cuttle- fish (Sepia) or Squid (Loligo) will afford the opportunity of examining the very curious provision which their skin contains for changing its hue. This consists in the presence of numerous large ( pigment-cells/ contain- ing coloring-matter of various tints; the prevailing color, however, being that of the fluid of the ink-bag. These pigment-cells may present very different forms, being sometimes nearly globular, whilst at other times they are flattened and extended into radiating prolongations; and, by the peculiar contractility with which they are endowed, they can pass from, one to the other of these conditions, so as to spread their colored con- tents over a comparatively-large surface, or to limit them within a com- paratively small area. Very commonly there are different layers of these pigment-cells, their contents having different hues in each layer and thus a great variety of coloration may be given, by the alteration in the form of the cells of which one or another layer is made-up. It is curious that the changes in the hue of the skin appear to be influenced, as in the case of the Chameleon, by the color of the surface with which it may be in proximity. The alternate contractions and extensions of these pigment-cells or chromatophores may be easily observed in a piece of skin detached from the living animal and viewed as a transparent ob- ject; since they will continue for some time, if the skin be placed in sea- water. And they may also be well seen in the embryo cuttle-fish, which will sometimes be found in a state of sufficient advancement in the grape- like eggs of these animals attached to Sea- weeds, Zoophytes, etc. — The eggs of the small cuttle-fish termed the Sepiola, which is very common on our southern coasts, are imbedded, like those of the Doris, in gelati- nous masses, which are attached to Sea-weeds, Zoophytes, etc. ; and their embryoes, when near maturity, are extremely beautiful and interesting objects, being sufficiently transparent to allow the action of the heart to be distinguished, as well as to show most advantageously the changes incessantly occurring in the form and hue of the * chromatophores.7 192 THE MICROSCOPE AND ITS REVELATIONS. CHAPTEK XVII. ANNULOSA, OR WORMS. 589. UNDER the general designation of ' Annulose ' animals, or Worms, may be grouped-together all that lower portion of the great Artwulated Sub-kingdom, in which the division of the body into longitudinally- arranged segments is not distinctly marked-out, and there is an absence of those ' articulated ' or jointed limbs that constitute so distinct a feature of Insects and their allies. This group includes the classes of Entozoa or Intestinal Worms, Rotifera or wheel-animalcules, Turbellaria, and Annelida; each of which furnishes many objects for Microscopic exami- nation, that are of the highest scientific interest. As our business, how- ever, is less with the professed Physiologist, than with the general inquirer into the minute wonders and beauties of Nature, we shall pass over these classes (the Rotifera having been already treated-of in detail, Chap. xi. ) with only a notice of such points as are likely to be specially deserving the attention of observers of the latter order. 590. ENTOZOA. — This class consists almost entirely of animals of a very peculiar plan of organization, which are parasitic within the bodies of other animals, and which obtain their nutriment by the absorption of the juices of these, — thus bearing a striking analogy to the parasitic Fungi (§§ 312-316). The most remarkable feature in their structure consists in the entire absence or the extremely low development of their nutritive sys- tem, and the extraordinary development of their reproductive apparatus. Thus, in the common Tcenia (' tape -worm'), which may be taken as the type of the Cestoid group, there is neither mouth nor stomach, the so-called 'head* being merely an organ for attachment, whilst the seg- ments of the 'body' contain repetitions of a complex generative appa- ratus, the male and female sexual organs being so united in each as to en- able it to fertilize and bring to maturity its own very numerous eggs; and the chief connection between these segments is established by two pairs of longitudinal canals, which, though regarded by some as represent- ing a digestive apparatus, and by others as a circulating system, appear really to represent the ' water- vascular system/ whose simplest condi- tion has been noticed in the wheel-animalcule (§ M9). — Few among the recent results of Microscopic inquiry have been more curious, than the elucidation of the real nature of the bodies formerly denominated Cystic Entozoa, which had been previously ranked as a distinct group. These are not found, like the preceding, in the cavity of the alimentary canal of the animals they infest; but always occur in the substance of solid organs, such as the glands, muscles, etc. They present themselves to the eye as bags or vesicles of various sizes, sometimes occurring singly, some- ANNULOSA, OK WORMS. 193 times in groups; but upon careful examination each vesicle is found to bear upon some part a 'head' furnished with booklets and suckers; and this may be either single, as in Cysticercus (the entozoon whose presence gives to pork what is known as the ' measly ' disorder), or multiple, as in Cmnurus, which is developed in the brain, chiefly of sheep, giving rise to the disorder known as ' the staggers.' Now in none of these Cystic forms has any generative apparatus ever been discovered, and hence they are obviously to be considered as imperfect animals. The close resem- blance between the ' heads' of certain Cysticerci and that of certain Tcenice first suggested that the two might be different states of the same animal; and experiments made by those who have devoted themselves to the work- ing-out of this curious subject have led to the assured conclusion, that the Cystic Entozoa are nothing else than Cestoid Worms, whose develop- ment has been modified by the peculiarity of their position, — the large bag being formed by a sort of dropsical accumulation of fluid when the young are evolved in the midst of solid tissues, whilst the very same bodies, conveyed into the alimentary canal of some carnivorous animal which has fed upon the flesh infested with them, begin to bud-forth the generative segments, the long succession of which, united end-to-end, gives to the entire series a Worm-like aspect. 591. The .higher forms of Entozoa, belonging to the Nematoid or thread-like Order, — of which the common Ascaris may be taken as a type, one species of it (the A. lumbricoides, or * round worm') being a common parasite in the small intestine of man, while another (the A. ver- micularis, or thread-worm ') is found rather in the lower bowel, — approach more closely to the ordinary type of conformation of Worms; having a distinct alimentary canal, which commences with a mouth at the anterior extremity of the body, and which terminates by an anal orifice near the other extremity; and also possessing a regular arrangement of circular and longitudinal muscular fibres, by which the body can be shortened, elongated, or bent in any direction. The smaller species of Ascaris, by some or other of which almost every Vertebrated animal is infested, are so transparent that every part of their internal organization may be made- out, especially with the assistance of the Compressor (§ 125) without any dissection; and the study of the structure and actions of their Generative apparatus has yielded many very interesting results, especially in regard to the first formation of the ova, the mode of their fertilization, and the history of their subsequent development. — Some of the Worms belong- ing to this group are not parasitic in the bodies of other animals, but live in the midst of dead or decomposing Vegetable matter. The Gordius or 'hair worm,' which is peculiar in not having any perceptible anal orifice, Beems to be properly a parasite in the intestines of water-insects; but it is frequently found in large knot-like masses (whence its name) in the water or mud of the pools inhabited by such insects, and may apparently be developed in these situations. The Anguillulce are little eel-like worms of which one species, A. fluviatilis, is very often found in fresh water amongst Desmidiece, Confer VCB, etc., also in wet moss and moist earth, and sometimes also in the alimentary canals of snails, frogs, fishes, insects, and larger worms; whilst another species, A. tritici, is met-with in the ears of Wheat affected with the blight termed the 'cockle;' another, the A. glutinis, is found in sour paste; and another, the A. aceti, was often found in stale vinegar, until the more complete removal of mucilage and . the addition of sulphuric acid, in the course of the manufacture, ren- dered this liquid a less favorable ' habitat ' for these little creatures. A 13 194: THE MICROSCOPE AND ITS REVELATIONS. writhing mass of any of these species of f eels/ is one of the most curious spectacles which the Microscopist can exhibit to the unscientific observer; and the capability which they all possess (in common with Rotifers and Tardigrades, § 452), of revival after desiccation, at however remote an interval, enables him to command the spectacle at any time. A grain of wheat within which these worms (often erroneously called Vibriones) are being developed, gradually assumes the appearance of a black pepper- corn; and if it be divided in two, the interior will be found almost complete filled with a dense white cottony mass, occupying the place of the flour, and leaving merely a small place for a little glutinous matter. The cottony substance seems to the eye to consist of bundles of fine fibres closely packed-together; but on taking-out a small portion, and putting it under the Microscpe with a little water under a thin glass-cover, it will be found after a short time (if not immediately) to be a wriggling mass of life, the apparent fibres being really Anguillulw, or ' eels ' of the Micro- scopist. If the seeds be soaked in water for a couple of hours before they are laid open, the eels will be found in a state of activity from the first; their movements, however, are by no means so energetic as those of the A. glutinis or 'paste-eel.' This last frequently makes its appearance spontaneously in the midst of paste that is turning sour; but the best means of securing a supply for any occasion, consists in allowing a portion of any mass of paste in which they may present themselves to dry up, and then, laying this by so long as it may not be wanted, to introduce it into a mass of fresh paste, which if it be kept warm and moist, will be found after a few days to swarm with these curious little creatures. 592. Besides the foregoing Orders of Entozoa, the Trematode group must be named; of which the Distoma hepaticum or 'fluke,' found in the livers of Sheep affected with the 'rot,' is atypical example. Into the details of the structure of this animal, which has the general form of a sole, there is no occasion for us here to enter; it is remarkable, how- ever, for the branching form of its digestive cavity, which extends throughout almost the entire body, very much as in Planariae (Fig. 406) ; and also for the curious phenomena of its development, several distinct forms being passed through between one sexual generation and another. These have been especially studied in the Distoma, which infests the Lymnc&us; the ova of which are not developed into the likeness of their parents, but into minute worm-like bodies, which seem to be little else than masses of cells inclosed in a contractile integument, no formed organs being found in them ; these cells, in their turn, are developed into independent zooids, which escape from their containing cyst in the con- dition of free ciliated Animalcules; in this condition they remain for some time, and then imbed themselves in the mucus that covers the tail of the Mollusk, in which they undergo a gradual development into true Distomata; and having thus acquired their perfect form, they penetrate the soft integument, and take-up their habitation in the interior of the body. Thus a considerable number of Distomata may be produced from a single ovum, by a process of cell -multiplication in an early stage of its development. In some instances the free ciliated larva possesses dis- tinct eyes; although these organ are wanting in the fully developed Dis- toma, the peculiar f habitat ' of which would render them useless. 593. TURBELLARIA. — This group of animals, which is distinguished by the presence of cilia over the entire surface of the body, seems inter- mediate in some respects between the ' trematode ' Entozoa and the Leech- tribe among Annelida. It deserves special notice here, chiefly on account ANNULOSA, OK WORMS. 195 FIG. 406. of the frequency with which the worms of the Planarian tribe present themselves among collections both of marine and of fresh-water animals (particular species inhabiting either locality), and on account of the curi- ous organization which many of these possess. Most of the members of this tribe have elongated flattened bodies, and move by a sort of gliding or crawling action over the surfaces of aquatic Plants and Animals. Some of the smaller kind are sufficiently transparent to allow of their internal structure being seen by transmitted light, especially when they are slightly compressed; and the accompanying figure (Fig. 406) displays the general conforma- tion of their principal organs, as thus shown. The body has the flattened sole- like shape of the Trematode Entozoa; its mouth, which Avhich is situated at a consi- derable distance from the anterior ex- tremity of the body, is surrounded by a circular sucker that is applied to the living surface from which the animal draws its nutriment; and the buccal cavity (b) opens into a short oesophagus* (c), which leads at once to the cavity of the stomach. In the true PlanaricB the mouth is furnished with a sort of long funnel-shaped proboscis; and this, even when detached from the body, continues to swallow anything presented to it. The cavity of the stomach does not give origin to any intestinal tube, nor is it Erovided with any second orifice; but a irge number of ramifying canals are pro- longed from it, which carry its contents into every part of the body. This seems to render unnecessary any system of vessels for the circulation of nutritive fluid ; and the two principal trunks, with connecting and ramifying branches, which may be observed in them, are probably to be re- garded in the light of a water-vascular system, the function of which is essentially respiratory. Both sets of sexual organs are combined in the same individuals; tllOUgn the COngreSS OI tWO, each impreg- ach; e, ramifications of gastric canals; nnHno- DTP nva nf HIP nihpv cppma fn hp /< cephalic ganglia and their nervous .61, S( ms IO 06 fiiaments; g, g, testes;/i, vesicula semi- generally necessary. The ovaria, as in the naiis; *, male genital canal; fc, k ovi- TT< i. -i Ji-\ L £ ducts; I, dilatation at their point of -hntozoa, extend through a large part Of junction; m, female genital orifice. the body, their ramifications proceed- ing from the two oviducts (k, &), which have a dilatation (I) at their point of junction. — There is still much obscurity about the history of the embryonic development of these animals; as the accounts given of it by different observers by no means harmonize with each other.1 — The Planariae, however, do not multiply by eggs alone; for they occa- sionally undergo spontaneous fission in a transverse direction, each seg- ment becoming a perfect animal; and an artificial division into two or even Structure of Polycelis levigatus (a Planarian worm).— a, Mouth surround- 1 See Balfour's "Comparative Embryology," Vol. i., pp. 159-162. 196 THE MICROSCOPE AND ITS REVELATIONS. more parts may be practised with a like result. In fact, the power of the Planariae to reproduce portions which have been removed, seems but little inferior to that of the Hydra (§ 515); a circumstance which is pecu- liarly remarkable, when the much higher character of their organization is borne in mind. They possess a distinct pair of nervous ganglia (f, /), from w'hich branches proceed to various parts of the body; and in the neighborhood of these are usually to be observed a number (varying from 2 to 40) of ocelli or rudimentary eyes, each having its refracting body or crystalline lens, its pigment-layer, its nerve bulb, and its cornea-like bulg- ing of the skin. The integument of many of these animals is furnished with ' thread-cells ' or ' filiferous capsules,' very much resembling those of Zoophytes (§ 528). 594. ANNELIDS. — This Class includes all the higher kinds of Worm- like animals, the greater part of which are marine, though there are several species which inhabit fresh water, and some which live on land. The body in this class is usually very long, and nearly always presents a well-marked segrneiital division, the segments being for the most part similar and equal to each other, except at the two extremities; but in the lower forms, such as the Leech and its allies, the segmental division is very indistinctly seen, on account of the general softness of the integu- ment. A large proportion of the marine Annelids have special respiratory appendages, into which the fluids of the body are sent for aeration; and these are situated upon the head (Fig, 407), in those species which (like the Serpula, Terebella, Sabellaria, etc.) have their bodies inclosed by tubes, either formed of a shelly substance produced from their own surface, or built up by the agglutination of grains of sand, fragments of shell, etc.; whilst they are distributed along the two sides of the body in such as swim freely through the water, or crawl over the surfaces of rocks, as is the case with the Nereidce, or simply bury themselves in the sand, as the Arenicola or ' lob-worm.' In these respiratory appendages the circula- tion of the fluids may be distinctly seen by Microscopic examination; and these fluids are of two kinds, — first, a colorless fluid, containing numerous cell-like corpuscles, which can be seen in the smaller and more transparent species to occupy the space that intervenes between the outer surface of the alimentary canal and the inner wall of the body, and to pass from this into canals which often ramify extensively in the respira-- tory organs, but are never furnished with a returning series of passages, — and second, a fluid which is usually red, contains few floating parti- cles, and is inclosed in a system of proper vessels that communicates with a central propelling organ, and not only carries away the fluid away from this, but also brings it back again. In Terebella we find a distinct provision for the aeration of both fluids; for the first is transmitted to the tendril-like tentacles which surround the mouth (Fig. 407, b, b), whilst the second circulates through the beautiful aborescent gill-tufts (k, k), situated just behind the head. The former are covered with cilia, the action of which continually renews the stratum of water in contact with them, whilst the latter are destitute of these organs; and this seems to be the general fact as to the several appendages to which these two fluids are respectively sent for aeration, the nature of their distribution varying .greatly in the different members of the class. The red fluid is commonly considered as blood, and the tubes through which it circulates as blood-vessels; but the Author has elsewhere given his reasons1 for coinciding in the opinion of Prof. Huxley, that the colorless 1 See his " Principles of Comparative Physiology," 4th Edit., §§ 218, 219, 292. ANNULOSA, OR WORMS. 19T FIG. 407. \ corpusculated fluid which moves in the peri-visceral cavity of the body and in its extensions, is that which really represents the blood of other Articulated animals; and that the system of vessels carrying the red fluid is to be likened on the one hand to the ' water- vascular system ' of the inferior Worms, and on the other to the tracheal apparatus of Insects (§ 634). — In the observation of the beauti- ful spectacle presented by the respiratory circulation of the various kinds of Anne- lids which swarm on most of our shores, and in the examination of what is going on in the interior of their bodies (where this is rendered, possible by their trans- parence), the Microscopist will find a most fertile source of interesting occupation; and he may easily, with care and patience, make many valuable additions to our pre- sent stock of knowledge on these points. There are many of these marine Annelids, in which the appendages of various kinds put forth from the sides of their bodies fur- nish very beautiful microscopic objects; as do also the different forms of teeth, jaws, etc., with which the mouth is commonly armed in the free or non-tubicolar species, these being eminently carnivorous. 595. The early history of the Develop- ment of Annelids, too, is extremely curi- ous; for they come forth from the egg in a condition very little more advanced than the ciliated gemmules of Polypes, consist- ing of a globular mass of untransformed cells, certain parts of whose surface are covered with cilia; in a few hours, how- ever, this embryonic mass elongates, and the indications of a segmental division be- come apparent, the head being (as it were) marked off in front, whilst behind this is a large segment thickly covered with cilia, then a narrower and non-ciliated segment, and lastly the caudal or tail-segment, which is furnished with cilia. A little later, a new segment is seen to be interposed in front of the caudal; and the dark internal Circulating Apparatus of Terebella conchilega:—a, labial ring; 6, 6, ten- tacles ; c, first segment or the trunk ; d, skin of the back; e, pharynx; /. intestine; g, longitudinal muscles of the inferior surface of the body; h, glandular organ (liver ?) ; i, organs of generation; j, feet; fc, fc, branchiae; I, dorsal vessel acting as a respira- tory heart; m, dorso-intestinal vessel ; n, venous sinus surrounding oesopha- gus; «/, inferior intestinal vessel; o, o, ventral trunk; p, lateral vascular branches. granular mass shapes itself into the out- line of an alimentary canal. 1 The number of segments progressively increases by the interposition of new ones between the caudal and its preceding segments; the various-internal organs become more and more distinct, eye-spots make their appearance, little bristly appendages are 1 A most curious transformation once occurred within the Author's experience in the larva of an Annelid, which was furnished with a broad collar or disk fringed with very long cilia, and showed merely an appearance of segmentation in its hinder part; for in the course of a few minutes, during which it was not under observation, this larva assumed the ordinary form of a marine Worm three 198 THE MICROSCOPE AND ITS REVELATIONS. put forth from the segments, and the animal gradually assumes the like- ness of its parent; a few days being passed by the tubicolar kinds, how- ever, in the actively moving condition, before they settle down to the formation of a tube.1 596. To carry out any systematic observations on the embryonic development of Annelids, the eggs should be searched for in the situa- tions which these animals haunt; but in places where Annelids abound, free-swimming larvae are often to be obtained at the same time and in the same manner as small Medusae (§ 522); and there is probably no part of our coasts, off which some very curious forms may not be met with. The following may be specially mentioned as departing widely from the ordinary type, and as in themselves extremely beautiful objects. — The Actinotrocha (Fig. 408) bears a strong resemblance in many particulars to the ' bipinnarian ' larva of a Star-fish (§ 543), having an elongated body, with a series of ciliated tentacles (d) symmetrically arranged; these tentacles, however, proceed from a sort of disk which somewhat resembles the 'lophophore5 of certain Polyzoa (§ 549). The mouth (e) is concealed by a broad but pointed hood or ' epistome ' (a), which some- times close down upon the tentacular disk, but is sometimes raised and extended forwards. The nearly cylindrical body terminates abruptly at the other extremity, where the anal orifice of the intestine (b) is sur- rounded by a circlet of very large cilia. This animal swims with great activity, sometimes by the tentacular cilia, sometimes by the anal circlet, sometimes by both combined; and besides its movement of progression, it frequently doubles itself together, so as to bring the anal extremity and the epistome almost in contact. It is so transparent that the whole of its alimentary canal may be as distinctly seen as that of Laguncula (§ 549); and, as in that Polyzoon, the alimentary masses often to be seen within the stomach (c) are kept in a continual whirling movement by the agency of cilia with which its walls are clothed. This very interest- ing creature was for a long time a puzzle to Zoologists; since, although there could be little doubt of its being a larval form, there was no clue to the nature of the adult produced from it, until this was discovered by Krohn in 1858 to be a Gephyrean W°rm-2 An even more extraordinary departure from the ordinary type is presented by the larva which has received the name Pilidiuin (Fig. 409); its shape being that of a helmet, the plume of which is replaced by a single long bristle-like appendage that is in continual motion, its point moving round and round in a circle. This curious organism, first noticed by Miiller, has been since ascertained to be the larva of the well-known Nemertes, a Turbellarian or four times its previous length, and the ciliated disk entirely disappeared. An accident unfortunately prevented the more minute examination of this Worm, which the Author would have otherwise made; but he may state that he is cer- tain that there was no fallacy as to the fact above stated; this larva having been placed by itself in a cell, on purpose that it might be carefully studied, and having been only laid aside for a short time whilst other selections were being made from the same gathering of the Tow-net. 1 For further information on this subject, see Balfour's " Comparative Embry- ology," Chap, xii., and the Memoirs there cited. 24Ueber Pilidium und Actinotrocha' in " Mutter's Archiv," 1858, p. 293. For more recent observations upon this interesting creature, see Balfour's ' ' Com- parative Embryology," Vol. i., pp. 299-302, and a paper on 'The Origin and Sig- nificance of the Metamorphosis of Actinotrocha,' by Mr. E. B, Wilson (of Baltimore), in " Quart, Journ, Microsc, Sci." April, 1881. ANNULOSA, OR WORMS. 199 worm of enormous length, which is commonly found entwining itself among the roots of Algse.1 597. Among the animals captured by the Tow-net, the marine Zoolo- gist will be not unlikely to meet with an Annelid which, although by no means Microscopic in its dimensions, is an admirable subject for Micro- scopic observation, owing to the extreme transparence of its entire body, which is such as to render it difficult to be distinguished when swimming in a glass jar, except by a very favorable light. This is the Tomopteris, so named from the division of the lateral portions of its body into a suc- cession of wing-like segments (Plate xxin., B), each of them carrying at its extremity a pair of pinnules, by the movements of which it is rapidly FIG. 408. FIG. 409. Actinotrocha branchiata:—a, Epi- Pilidium gyrans:—A, young, showing at a the ali- stome or hood; 6, anus; c, stomach; d, mentary canal, and at b the rudiment of the Ne- cuiated tentacles; e, mouth. mertid; B, more advanced stage of the same; c, newly-freed Nemertid. propelled through the water. The full-grown animal, which measures nearly an inch in length, has first a curious pair of ' frontal horns ' pro- jecting laterally from the head, so as to give the animal the appearance of a ' hammer-headed ' Shark; behind these there is a pair of very long antennae, in each of which we distinguish a rigid bristle-like stem or seta, inclosed in a soft sheath, and moved at its base by a set of muscles con- tained within the lateral protuberances at the head . Behind these are 1 See especially Leuckart and Pagenstecher's ' Untersuchungen fiber niedere Seethiere,' in Muller's " Archiv," 1853, p. 569, and Balfour, op. cit., p. 165. The Author has frequently met with Pilidium in Lamlash Bav. 200 THE MICROSCOPE AND ITS REVELATIONS. PLATE XXIIL STRUCTURE AND DEVELOPMENT OP TOMOPTERIS ONISCIFORMIS (Original). A. Portion of caudal prolongations, containing the spermatic sacs, a, a. B. Adult Male specimen. c. Hinder part of adult Female specimen, more enlarged, showing ova lying freely in the perivis ceral cavity and its caudal prolongation. D. Ciliated canal, commencing externally in the larger and smaller rosette-like disks, a, 6. K. One of the pinnulated segments, showing the position of the ciliated canal, c, and its rosette- like disks, a, 6; showing also the incipient development of the ova, d, at the extremity of the seg- ment. F. Cephalic Ganglion, with its pair of auditory (?) vesicles, a, a, and its two ocelli, 6, 6. o. Very young Tomopteris, showing at a, a the larval antennae; 6, 6, the incipient long antennae of the adult; c, a, e, /, four pairs of succeeding pinnulated segment, followed by the bifid tail. ANNULOSA, OR WORMS. 201 about sixteen pairs of the ordinary pinnnlated segments, of which the hinder ones are much smaller than those in front, gradually lessening in size until they become almost rudimentary; and where these cease, the body is continued onwards into a tail-like prolongation, the length of which varies greatly according as it is contracted or extended. This pro- longation, however, bears four or five pairs of very minute appendages, and the intestine is continued to its very extremity; so that it is really to be regarded as a continuation of the body. In the head we find, between the origins of the antennas, a ganglionic mass, the component cells of which may be clearly distinguished under a sufficient magnifying power, as shown at F; seated upon this arc two pigment-spots (b, b), each bear- ing a double pellucid lens-like body, which are obviously rudimentary eyes: whilst imbedded in its anterior portion are two peculiar nucleated vesicles, #, «, which are probably the rudiments of some other sensory organs. On the under side of the head is situated the mouth, which, like that of many other Annelids, is furnished with a sort of proboscis that can be either projected or drawn-in; a short oesophagus leads to an elongated stomach, which, when distended with fluid, occupies the whole cavity of the central portion of the body, as shown in fig. B, but which is sometimes so empty and contracted as to be like a mere cord, as shown in fig. c. In the caudal appendage, however, it is always narrowed into an intestinal canal; this, when the appendage is in extended state as at c, is nearly straight; but when the appendage is contracted, as seen at B, it is thrown into convolutions. The perivisceral cavity is occupied by fluid in which some minute corpuscles may be distinguished; and these are kept in motion by cilia which clothe some parts of the outer surface of the alimentary canal and line some part of the wall of the body. No other more special apparatus, either for the circulation or for the aeration of the nutrient fluid, exists in this curious Worm; unless we are to regard as subservient to the respiratory function the ciliated canal which may be observed in each of the lateral appendages except the five anterior pairs. This canal commences by two orifices at the base of the segment, as shovm at fig. E, Z>, and on a larger scale at fig. D; each of these orifices (D, «, b) is surrounded by a sort of rosette; and the rosette of the larger one (a) is furnished with radiating ciliated ridges. The two branches incline towards each other, and unite into a single canal, that runs along for some distance in tho wall of the body, and then terminates in the perivisceral cavity; and the direction of the motion of the cilia which line it, is from without inwards. 598. The Reproduction and Developmental history of this Annelid present many points of great interest. The sexes appear to be distinct, ova being found in some individuals, and spermatozoa in others. The development of the ova commences in certain ' germ-cells ' situated within the extremities of the pinnulated segments, where they project inwards from the wall of the body; these, when set free, float in the fluid of the perivisceral cavity, and multiply themselves by self-division; and it is only after their number has thus been considerably augmented, that they begin to increase in size and to assume the characteristic appearance of ova. In this stage they usually fill the perivisceral cavity not only of the body, but of its caudal extension, as shown at c; and they escape from it through transverse fissures which form in the outer wall of the body, at the third and fourth segments. The male reproductive organs, on the other hand, are limited to the caudal prolongation, where the sperm-cells are developed within the pinnulated appendages, as the germ-cells of the 202 THE MICROSCOPE AND ITS KEVELATIONS. female are within the appendages of the body. Instead of being set free,, however, into the peri visceral cavity, they are retained within a saccular envelope forming a testis (A, a, a) which fills up the whole cavity of each appendage; and within this the spermatozoa may be observed, when ma- ture, in active movement. They make their escape externally by a pas- sage that seems to communicate with the smaller of the two just-men- tioned rosettes; but they also appear to escape into the peri visceral cavity by an aperture that forms itself when the spermatozoa are mature. Whether the ova are fertilized while yet within the body of the female, by the entrance of spermatozoa through the ciliated canals, or after they have made their escape from it, has not yet been ascertained. — Of the earliest stages of embryonic development nothing whatever is yet known; but it has been ascertained that the animal passes through a larval form, which differs from the adult not merely in the number of the segments of the body (which successively augment by additions at the posterior extremity), but also in that of the antennae. At G is repre- sented the earliest" larva hitherto met-with, enlarged as much as ten times in proportion to the adult at B; and here we see that the head is destitute of the frontal horns, but carries a pair of setigerous antennae, a, a, be- hind which there are five pairs of bifid appendages, #, c, d, e, f, in the first of which, Z>, one of the pinnules is furnished with a seta. 'In more advanced larvae having eight or ten segments, this is developed into a second pair of antennae resembling the first; and the animal in this stage has been described as a distinct species, T. quadricornis. At a more advanced age, however, the second pair attains the enormous develop- ment shown at B; and the first or larval antennae disappear, the setigerous portions separating at a sort of joint (G, a, a), whilst the basal projec- tions are absorbed into the general wall of the body. — This beautiful creature has been met-with on so many parts of our coast, that it cannot be considered at all uncommon; and the Microscopist can scarcely have a more pleasing object for study.1 Its elegant form, its crystal clear- ness, and its sprightly, graceful movements render it attractive even to the unscientific observer; whilst it is of special interest to the Physiolo- gist, as one of the simplest examples yet known of the Annelid type. 599. To one phenomenon of the greatest interest, presented by vari- ous small Marine Annelids, the attention of the Microscopist should be specially directed; this is their luminosity, which is 'not a steady glow like that of the Glow-worm or Fire-fly, but a series of vivid scintillations (strongly resembling those produced by an electric discharge through a tube spotted with tin-foil), that pass along a considerable number of seg- ments, lasting for an instant only, but capable of being repeatedly ex- cited by any irritation applied to the body of the animal. These scin- tillations may be discerned under the Microscope, even in separate segments, when they are subjected to the irritation of a needle-point or to a gentle pressure; and it has been ascertained by the careful observa- tions of M. de Quatrefages, that they are given out by the muscular fibres in the act of contraction.2 600. Among the fresh-water Annelids, those most interesting to the Microscopist are the worms of the Nais tribe, which are common in our 1 Seethe Memoirs of the Author anjl M. Claparede in Vol. xxii. of the " Lin- nsean Transactions," and the authorities there referred to; also a recent Memoir by Dr. F. Vejdovsky in " Zeitschrift f. wiss. Zool.," Bd. xxxi., 1880. 5 See his Memoirs on the Annelida of LaManche, in " Ann. des Sci. Nat.," Ser. 2, Zool., Tom. xix., and Ser. 3, Zool., Tom. xiv. ANNULOSA, OK WORMS. 203 rivers and ponds, living chiefly amidst the mud at the bottom, and espe- cially among the roots of aquatic plants. Being blood-red in color, they give to the surface of the mud, when they protrude themselves from it in large numbers and keep the protruded portion of their bodies in con- stant undulation, a very peculiar appearance; but if disturbed, they with- draw themselves suddenly and completely. These Worms, from the extreme transparency of their bodies, present peculiar facilities for Mi- croscopic examination, and especially for the study of the internal circu- lation of the red liquid commonly considered as blood. There are here no external respiratory organs; and the thinness of the general integu- ment appears to supply all needful facility for the aeration of the fluids. One large vascular trunk (dorsal) may be seen lying above the intestinal canal, and another (ventral) beneath it; and each of these enters a con- tractile dilatation, or heart-like organ, situated just behind the head. The fluid moves forwards in the dorsal trunk as far as the heart, which it enters and dilates; and when this contracts, it propels the fluid partly to the head, and partly to the ventral heart, which is distended by it. The ventral heart, contracting in its turn, sends the blood backwards along the ventral trunk to the tail, whence it passes towards the head as before. In this circulation, the stream branches- oif from each of the principal trunks into numerous vessels proceeding to different parts of the body, which then return into the other trunk; and there is a peculiar set of vascular coils, hanging down in the perivisceral cavity that con- tains the corpusculated liquid representing the true blood, which seems specially destined to convey to it the aerating influence received by the red fluid in its circuit, thus acting (so to speak) like internal gills. — The Naiad-worms have been observed to undergo spontaneous division during the summer months; a new head and its organs being formed for the posterior segment behind the line of constriction, before its separation from the anterior. It has . been generally believed that each segment continues to live as a complete worm; but it is asserted by Dr. T. Wil- liams that from the time when the division occurs, neither half takes-in any more food, and that the two segments only retain vitality enough to enable them to be (as it were) the f nurses ' of the eggs which both in- clude.— In the Leech tribe, the dental apparatus with which the mouth is furnished, is one of the most curious among their points of minute structure; and the common ' medicinal 9 Leech affords one of the most interesting examples of it. What is commonly termed the ' bite ? of the leech, is really a saw-cut, or rather a combination of three saw-cuts, radi- ating from a common centre. If the mouth of a leech be examined with a hand-magnifier, or even with the naked eye, it will be seen to be a tri- angular aperture in the midst of a sucking disk; and on turning back the lips of that aperture, three little white ridges are brought into view. Each of these is the convex edge of a horny semicircle, which is bor- dered by a row of eighty or ninety minute hard and sharp teeth; whilst the straight border of the semicircle is imbedded in the muscular sub- stance of the disk, by the action of which it is made to move back- wards and forwards in a saw-like manner, so that the teeth are enabled to cut into the skin to which the suctorial disk has affixed itself. l 1 Among the more recent sources of information as to the Anatomy and Phy- siology of the Annelids, the following may be specially mentioned: — The " His- toire Naturelle des Anneles Marins et d'Eau douce " of M. de Quatrefages, forming part of the " Suites a Buff on; " the successive admirable Monographs of the late Prof. Ed, Claparede, "Recherches Anatomiques sur les Annehdes, Turbellaries, 204 THE MICROSCOPE AND ITS REVELATIONS. Opalines, et Gregarines, observes dans les Hebrides " (Geneva, 1861); "Recherches Anatomiques sur les Oligochetes " (Geneva, 1862); " Beobachtungen iiber Anato- niie und Entwickelungsgeschichte Wirbelloser Thiere an der Ktiste von Norman- die" (Leipzig, 1863); and "Les Annelides Chetopodes du Golfe de Naples" (Ge- neva, 1868-70); the Monograph of Dr. Ehler's, " Die Borstenwiirmer (Annelida Chsetopoda)," 1864-8; and lastly, Dr. Macintosh's "Monograph of the British An- nelids," now in course of publication by the Ray Society. CRUSTACEA. 205- CHAPTER XVIII. CRUSTACEA. 601. PASSING from the lower division of the Articulated series to that of Arthropods, in which the body is furnished with distinctly articu- lated or jointed limbs, we come first to the Class of Crustacea, which in- cludes (when used in its most comprehensive sense) all those animals belonging to this group, which are fitted for aquatic respiration. It thus comprehends a very extensive range of forms; for although we are accus- tomed to think of the Crab, Lobster, Cray-fish, and other well-known species of the order Decapoda (ten-footed) as its typical examples, yet all these belong to the highest of its many orders; and among the lower are many of a far simpler structure, and not a few which would not be recognized as belonging to the class at all, were it not for the information derived from the study of their development as to their real nature, which is far more apparent in their early than it is in their adult condition. Many of the inferior kinds of Crustacea are so minute and transparent, that their whole structure may be made-out by the aid of the Microscope- without any preparation; this is the case, indeed, with nearly the whole group of Entomostraca (§ 603), and with the larval forms even of the- Crab and its allies (§ 614); and we shall give our first attention to these, afterwards noticing such points in the structure of the larger kinds as are likely to be of general interest. 602. A curious example of the reduction of an elevated type to a very simple form is presented by the group of Pycnogonida, some of the members of which may be found by attentive search in almost every locality where sea-weeds abound; it being their habit to crawl (or rather to sprawl) over the surfaces of these, and probably to imbibe as food the gelatinous substance with which they are -invested.1 The general form of their bodies (Fig. 410) usually reminds us of that of some of the long- legged Crabs; the abdomen being almost or altogether deficient, whilst the head is very small, and fused (as it were) into the thorax; so that the last-named region, with the members attached to it, constitutes nearly the whole bulk of the animal. The head is extended in front into a probosis-like projection, at the extremity of which is the narrow orifice of the mouth; which seems to be furnished with vibratile cilia, that serve to draw into it the semi-fluid aliment. Instead of being furnished (as in the higher Crustaceans) with two pairs of antennas and numerous pairs of 'feet-jaws/ it has but a single pair of either; it also bears four minute ocelli, or rudimentary eyes, set at a little distance from each other on a 1 It is remarkable that very large forms of this group, sometimes extending to more than twelve inches across, have been brought up from great depths of the sea. 206 THE MICROSCOPE AND ITS REVELATIONS. Fia. 410. sort of tubercle. From the thorax proceed four pairs of legs, each com- posed of several joints, and terminated by a hooked claw; and by these members the animal drags itself slowly along, instead of walking actively upon them like a crab. The mouth leads to a very narrow O3sophagus (a), which passes back to the central stomach (#) situated in the midst of the thorax, from the hinder end of which a narrow intestine (c) passes-off, to terminate at the posterior extremity of the body. From the central stomach five pairs of caecal prolongations radiate; one pair (d) entering the feet-jaws, the other four (e, e) penetrating the legs, and passing along them as far as the last joint but one; and those extensions are covered with a layer of brownish-yellow granules, which are prob- ably to be regarded as a dif- fused and rudimentary con- dition of the liver. The stomach and its caecal pro- longations are continually executing peristaltic move- ments of a very curious kind; for they contract and dilate with an irregular al- ternation, so that a flux and reflux of their contents is constantly taking place between the central portion and its radiating extensions, and between one of these extensions and another. The perivisceral space be- tween the widely-extended stomach and the walls of the body and limbs is oc- cupied by a transparent liquid, in which are seen floating a number of minute transparent corpuscles of irregular size; and this fluid, which represents the blood, is kept in continual motion, not only by the general movements of the ani- mal, but also by the actions of the digestive apparatus; since, whenever the caecum of any one the legs undergoes dilatation, a part of the circumambient liquid will be pressed-out from the cavity of that limb, either into the thorax, or into some other limb whose stomach is contracting. The fluid must obtain its aeration through the general surface of the body, as there are no special organs of respiration. The nervous system consists of a single ganglion in the head (formed by the coalescence of a pair), and of another in the thorax (formed by the coalescence of four pairs), with which the cephalic ganglion is connected in the usual mode, namely, by two nerv- ous cords which diverge from each other to embrace the oesophagus.— In the study of the very curious phenomena exhibited by the digestive apparatus, as well as of the various points of internal conformation which have been described, the Achromatic Condenser will be found use- Ammothea pycnogonoides:— a, narrow oesophagus; &• stomach; c, intestine; d, digestive caeca of the feet-jaws? «, «, digestive caeca of the legs. CRUSTACEA. 207 ful, even with the 1 inch, 2-3d inch, or % inch Objective; for the imper- fect transparence of the bodies of these animals renders it of importance to drive a large quantity of light through them, and to give to this light such a quality as shall sharply define the internal organs.1 603. ENTOMOSTRACA. — This group of Crustaceans, nearly all the ex- isting members of which are of such minute size as to be only just visible to the naked eye, is distinguished by the inclosure of the entire body within a horny or shelly casing; which sometimes closely resemble a bi- valve shell in form and in the mode of junction of its parts, whilst in other instances it is formed of only a single piece, like the hard envelope of certain Rotifera (§ 453, in.). The segments into which the body is divided, are frequently very numerous, and are for the most part similar to each other; but there is a marked difference in regard to the append- ages which they bear, and to the mode in which these minister to the lo- comotion of the animals. For in the Lophyropoda, or 'bristly-footed' tribe, the number of legs is small, not exceeding five pairs, and their function is limited to locomotion, the respiratory organs being attached to the parts in the neighborhood of the mouth; whilst in the Branchio- poda, or 'gill-footed* tribe, the same members (known as 'fin-feet') serve both for locomotion and for respiration, and the number of these is commonly large, being in Apus not less than sixty pairs. The character of their movements differ accordingly; for whilst all the members of the first-named tribe dart through the water in a succession of jerks, so as to have acquired the common name of ' water-fleas,' those among the latter which possess a great number of ' fin-feet,' swim with an easy gliding move- ment, sometimes on their back alone (as in the case with Branchipus), and sometimes with equal facility on the back, belly, or sides (as is done by Artemia salina, the 'brine shrimp'). — Some of the most common forms of both tribes will now be briefly noticed. 604. The tribe of Lophyropoda is divided into two Orders; of which the first, Ostracoda, is distinguished by the complete inclosure of the body in a bivalve shell, by the small number of legs, and by the absence of an external ovary. One of the best known examples is the little Cypris, which is a common inhabitant of pools and streams: this may be recog- nized by its possession of two pairs of antennae, the first having numerous joints with a pencil-like tuft of filaments, and projecting forwards from the front of the head, whilst the second has more the shape of legs, and is directed downwards; and by the limitation of its legs to two pairs, of which the posterior does not make its appearance outside the shell, being bent upwards to give support to the ovaries. The valves are generally opened widely enough to allow the greater part of both pairs of antennae and of the front pair of legs to pass-out between them; but when the ani- mals are alarmed, they draw these members within the shell, and close the valves firmly. They are very lively creatures, being almost constantly seen in motion, either swimming by the united action of their foot-like antennae and legs, or walking upon plants and other solid bodies floating in the water. — Nearly allied to the preceding is the Cy there, whose body is furnished with three pairs of legs, all projecting out of the shell, and whose superior antennae are destitute of the filamentous brush; this genus 1 Certain points of resemblance borne by Pycnogonida to Spiders, makes the careful study of their development a matter of special interest and importance; as there is some reason to regard them rather as Arachnida adapted to a marine habitat, than as Crustacea. — See Balfour's " Comparative Embryology," pp. 448, 449, and the authorities there referred to. 208 THE MICROSCOPE AND ITS REVELATIONS. FIG. 411. is almost entirely marine, and some species of it may almost invariably be met-with in little pools among the rocks between the tide-marks, creep- ing about (but not swimming) amongst Confervas and Corallines. — There is abundant evidence of the former existence of Crustacea of this group, of larger size than any now existing, to an enormous extent; for in cer- tain fresh-water strata, both of the Secondary and Tertiary series, we find layers, sometimes of great extent and thickness, which are almost entirely composed of the fossilized shells of Cyprides; whilst in certain parts of the Chalk, which was a marine deposit, the remains of bivalve shells re- sembling those of Cythere present themselves in such abundance as to form a considerable part of its substance. 605. In the order Copepoda, there is a jointed shell forming a kind of buckler or carapace that almost entirely incloses the head and thorax, an opening being left beneath, through which the members pro- ject; and there are five pairs of legs, mostly adapted for swimming, the fifth pair, however, being rudi- mentary in the genus Cyclops, the commonest example of the group. This genus receives its name from possessing only a single eye, or rather a single cluster of ocelli; which character, however, it has in common with the two genera already named, as well as with Daphnia (§ 606), and with many other En- tomostraca. It contains numerous species, some of which belong to fresh- water, whilst others are ma- rine. The Fresh- water species often abound in the muddiest and most stagnant pools, as well as in the clearest springs; the ordinary water with which London is supplied fre- quently contains large numbers of them. Of the marine species, some are to be found in the localities in which the Cythere is most abun- mose setae of tail;— B, tail, with external egg- dailt, whilst OUiei'S inhabit the Open SSt^'ySdt!' °' successive sta*es of develop- ocean, and must be collected by the Tow-net. The body of the Cyclops is soft and gelatinous, and it is composed of two distinct parts, a thorax (Fig. 411, a) and an abdomen (£), of which the latter, being compara- tively slender, is commonly considered as a tail, though traversed by the intestine which terminates near its extremity. The head, which coa- lesces with the thorax, bears one very large pair of antennae (c), possess- ing numerous articulations and furnished with bristly appendages, and another small pair (c?); it is also furnished with a pair of mandibles or true jaws, and with two pairs of 'feet-jaws,' of which the hinder pair is the longer and more abundantly supplied with bristles. The legs (e) are all beset with plumose tufts, as is also the tail (/,/) which is borne at the extremity of the abdomen. On either side of the abdomen of the female, there is often to be seen an egg-capsule or external ovarium (B); CRUSTACEA. 209 within which the ova, after being fertilized, undergo the earlier stages of their development. — The Cyclops is a very active creature, and strikes the water in swimming, not merely with its legs and tail, but also with its antennae. The rapidly-repeated movements of its feet-jaws serve to create a whirlpool in the surrounding water, by which minute animals of various kinds, and even its own young, are brought to its mouth to be devoured. 606. The tribe of Branchiopoda also is divided into two Orders, of which the Cladocera. present the nearest approach to the preceding, hav- ing a bivalve carapace, no more than from four to six pairs of legs, two pairs of antennae, of which one is large and branched and adapted for swim- ming, and a single eye. The commonest form of this is the Daphniapulex, sometimes called the 'arborescent water-flea,' from the branching form of its antennae. It is very abundant in many ponds and ditches, coming to the surface in the mornings and evenings and in cloudy weather, but seeking the depths of the water during the heat of the day. It swims by taking short springs; and feeds on minute particles of vegetable sub- stances, not, however, rejecting animal matter when offered. Some of the peculiar phenomena of its reproduction will be presently described (§ 609). 607. The other order, Phyllopoda, includes those Branchiopoda whose body is divided into a great number of segments, nearly all of which are furnished with leaf-like members, or * fin-feet.' The two Families which this order includes, however, differ considerably in their conformation; for in that of which the genera Apus and Nebalia are rep- resentatives, the body is inclosed in a shell, either shield-like or bivalve, and the feet are generally very numerous; whilst in that in which contains Branchipus and Artemia, the body is entirely unprotected, and the number of pairs of feet does not exceed eleven. The Apus cancriformis, which is an animal of comparatively large size, its entire length being about 2% inches, is an inhabitant of stagnant waters; but although occa- sionally very abundant in particular pools or ditches, it is not to be met- with nearly so commonly as the Entomostraca already noticed. It is recognized by its large oval carapace, which covers the head and body like a shield; by the nearly cylindrical form of its body, which is composed of thirty articulations; and by the multiplication of its legs, which amount to about sixty pairs. The number of joints in these and in the other ap- pendages is so great, that in a single individual they may be safely esti- mated at not less than two millions. These organs, however, are for the most part small; and the instruments chiefly used by the animal for loco- motion are the first pair of feet, which are very much elongated (bearing such a resemblance to the principal antennae of other Entomostraca, as to be commonly ranked in the same light), and are distinguished as rami or oars. With these they can swim freely in any position; but when the rami are at rest and the animal floats idly on the water, its fin-feet may be seen in incessant motion, causing a sort of whirlpool in the water, and bringing to the mouth the minute animals (chiefly the smaller Entomos- traca inhabiting the same localities) that serve for its food. — The Bran- chipus stagnalis has a slender, cylindriform, and very transparent body of nearly an inch in length, furnished with eleven pairs of fin-feet, but is destitute of any protecting envelope; its head is furnished with a pair of very curious prehensile organs (which are really modified antennae), whence it has received the name of CheirocepJialus; but these are not used by it for the seizure of prey, the food of this animal being vegetable, and their function is to clasp the female in the act of copulation. The 14 210 THE MICROSCOPE AND ITS REVELATIONS. Brancliipus or Cheirocephalus is certainly the most beautiful and elegant of all the Entomostraca, being rendered extremely attractive to the view by " the uninterrupted undulatory wavy motion of its graceful branchial feet, slightly tinged as they are with a light reddish hue, the brilliant mixture of transparent bluish-green and bright red of its prehensile antennae, and its bright red tail with the plumose setae springing from it;" unfortunately, however, it is a comparatively rare animal in this country. — The Artemia salina or ' brine shrimp ' is an animal of very similar organization, and almost equally beautiful in its appearance and movements, but of smaller size, its body being about half an inch in length. Its ' habitat ' is very peculiar; for it is only found in the salt- pans or brine-pits in which sea- water is undergoing concentration as at Lymington); and in these situations it is sometimes so abundant as to communicate a red tinge to the liquid. 608. Some of the most interesting points in the history of the Entromostraca lie in the peculiar mode in which their generative func- tion is performed, and their tenacity of life when desiccated, in which last respect they correspond with many Rotifers (§ 452). By this pro- vision they escape being completely exterminated, as they might other- wise soon be, by the drying-up of the pools, ditches, and other small collections of water which constitute their usual ' habitats.' It does not appear, however, that the adult Animals can bear a complete desiccation, although they will preserve their vitality in mud that holds the smallest quantity of moisture; but their eggs are more tenacious of life, and there is ample evidence that these will become fertile 011 being moistened, after having remained for a long time in the condition of fine dust. Most Entomostraca, too, are killed by severe cold, and thus the whole race of adults perishes every winter; but their eggs seem unaffected by the lowest temperature, and thus continue the species, which would be otherwise exterminated. — Again, we frequently meet in this group with that agamic reproduction, which we have seen to prevail so extensively among the lower radiata and Mollusca. In many species there is a double mode of multiplication, the sexual and the non-sexual. The former takes-place at certain seasons only; the males (which are often so differ- ent in conformation from the females, that they would not be supposed to belong to the same species, if they were not seen in actual congress) disappearing entirely at other times. The latter, on the other hand, continues at all periods of the year, so long as warmth and food are sup- plied; and is repeated many times (as in the Hydra) so as to give origin to as many successive 'broods.' Further, a single act of impregnation serves to fertilize not merely the ova which are then mature or nearly so, but all those subsequently produced by the same female, which are deposited at considerable intervals. In these two modes, the multiplica- tion of these little creatures is carried-on with great rapidity, the young animal speedily coming to maturity and beginning to propagate; so that according to the computation of J urine, founded upon data ascertained by actual observation, a single fertilized female of the common Cyclops quadricornis may be the .progenitor in one year of 4,442,189,120 young. 609. The eggs of some Entomostraca are deposited freely in the water, or are carefully attached in clusters to aquatic Plants; but they are more frequently carried for some time by the parent in special recep- tacles developed from the posterior part of the body; and in many cases they are retained there until the young are ready to come-forth, so that these animals may be said to be ovo-viviparous. In Daphnia, the eggs CRUSTACEA. 211 are received into a large cavity between the back of the animal and its shell, and there the young undergo almost their whole development, so as to come-forth in a form nearly resembling that of their parent. Soon after their birth, a moult or exuviation of the shell takes-place; and the egg-coverings are cast-off with it. In a very short time afterwards, another brood of eggs is seen in the cavity, and the same process is repeated, the shell being again exuviated after the young have been brought to maturity. At certain times, however, the Dap/Mia may be seen with a dark opaque substance within the back of the shell, which has been called the ephippium, from its resemblance to a saddle. This, when carefully examined, is found to be of dense texture, and to be com- posed of a mass of hexagonal cells; and it contains two oval bodies, each consisting of an ovum covered with a horny casing, enveloped in a capsule which opens like a bivalve shell. From the observations of Sir J. Lub- bock,1 it appears that the ephippium is really only an altered portion of the carapace; its outer valve being a part of the outer layer of the epider- mis, and its inner valve the corresponding part of the inner layer. The development of the ephippial eggs takes-place at the posterior part of the ovaries, and is accompanied by the formation of a greenish-brown mass of granules; and form this situation the eggs pass into the receptacle formed by the new carapace, where they become included between the two layers of the ephippium. This is cast-off, in process of time, with the rest of the skin, from which, however, it soon becomes detached; and it con- tinues to envelop the eggs, generally floating on the surface of the water until they are hatched with the returning warmth of spring. This curious provision obviously affords protection to the eggs which are to endure the severity of winter cold; and an approach to it may be seen in the re- markable firmness of the envelopes of the ' winter eggs ' of some Eotifera (§ 451). There seems a strong probability, from the observations of Sir J. Lubbock, that the * ephippial ' eggs are true sexual products, since males are to be found at the time when the ephippia are developed; whilst it is certain that the ordinary eggs can be produced'non-sexually, and that the young which spring from them can multiply the race in like manner. The young produced from the ephippial eggs seem to have the same power of continuing the race by non-sexual reproduction, as the young developed under ordinary circumstances. 6 LO. In most Entomostraca, the young at the time of their emersion from the egg differ considerably from the parent, especially in having only the thoracic portion of the body as yet evolved, and in possessing but a small number of locomotive appendages (see Fig. 411, C-G); the visual organs, too, are frequently wanting at first. The process of devel- opment, however, takes place with great rapidity; the animal at each successive moult (which process is very commonly repeated at intervals of a day or two) presenting some new parts, and becoming more and more like its parent, which it very early resembles in its power of multi- plication, the female laying eggs before she has attained her own full size. Even when the Entomostraca have attained their full growth, they continue to exuviate their shell at short intervals during the whole of life; and this repeated moulting seem to prevent the animal from being injured, or its movements obstructed, by the over-growth of Darasitic Animalcules and Confervae; weak and sickly individuals being frequently 1 An account of the two methods of Reproduction in Daphnia, and of the structure of the Ephippium,' in " Philosophical Transactions," 1857, p. 79. 212 THE MICROSCOPE AND ITS REVELATIONS . seen to be so covered with such parasites, that their motion and life are soon arrested, apparently because they have not strength to cast-off and renew their envelopes. The process of development appears to depend in some degree upon the influence of light, being retarded when the ani- mals are secluded from it; but its rate is still more influenced by heat; and this appears also to be the chief agent that regulates the time which elapses between the moultings of the adult, these, in Daphnia, taking- place at intervals of two days in warm summer weather, whilst several days intervene between them when the weather is colder. The cast shell carries with it the sheaths not only of the limbs and plumes, but of the most delicate hairs and setae which are attached to them. If the animal have previously sustained tne loss of a limb, it is generally renewed at the next moult, as in higher Crustacea.1 611. Closely connected with the Entomostracous group is the tribe of suctorial Crustacea; which for the most part live as parasites upon the exterior of other animals (especially Fish), whose juices they imbibe by means of the peculiar proboscis-like organ which takes in them the place of the jaws of other Crustaceans; whilst other appendages, representing the feet-jaws, are furnished with hooks, by which these parasites attach themselves to the animals from whose juices they derive their nutriment. Many of the suctorial Crustacea bear a strong resemblance, even in their adult condition, to certain Entomostraca; but more commonly it is between the earlier forms of the two groups that the resemblance is the closest, most of the suctoria undergoing such extraordinary changes in their progress towards the adult condition, that, if their complete forms were alone attended-to, they might be excluded from the class altogether, as has (in fact) been done by many Zoologists. — Among those Suctorial Crustacea which present the nearest approach to the ordinary Entomos- tracous type, may be specially mentioned the Argulus foliaceus, which attaches itself to the surfaces of the bodies of fresh-water Fish, and is commonly known^under the name of the 'fish louse.' This animal has its body covered with a large firm oval shield, which does not extend, however, over the posterior part of the abdomen. The mouth is armed with a pair of styliform mandibles; and on each side of the proboscis there is a large short cylindrical appendage, terminated by a curious sort of sucking-disk, with another pair of longer jointed members, terminated by prehensile hooks. These two pairs of appendages, which are probably to be considered as representing the feet-jaws, are followed by four pairs of legs, which, like those of the Branchiopods, are chiefly adapted for swimming; and the tail, also, is a kind of swimmeret. This little animal can leave the fish upon which it feeds, and then swims freely in the water, usually in a straight line, but frequently and suddenly changing its direction, and sometimes turning over and over several times in suc- cession. The stomach is remarkable for the large caecal prolongations which it sends out on either side, immediately beneath the shell; for these subdivide and ramify in such a manner, that they are distributed almost as minutely as the caecal prolongations of the stomach of the Planaria (Fig. 406). The proper alimentary canal, however, is con- tinued backwards from the central cavity of the stomach, as an intestinal tube, which terminates in an anal orifice at the extremity of the abdo- men.— A far more marked departure from the typical form of the class 1 For a systematic and detailed account of this group, see Dr. Baird's " Natu- ral History of the British Entomostraca," published by the Eay Society. CRUSTACEA. 213 is shown in the Lernaa, which is found attached to the gills of Fishes. This creature has a long suctorial proboscis; a short thorax, to which is attached a single pair of legs, which meet at their extremities, where they bear a sucker which helps to give attachment to the parasite; a large abdomen; and a pair of pendent egg-sacs. In its adult condition it buries its anterior portion in the soft tissue of the animal it infests, and appears to have little or no power of changing its place. But the young? when they come forth from the egg, are as active as the young of Cyclops (Fig. 411, c, D), which they much resemble; and only attain the adult form after a series of metamorphoses, in which they cast-off their loco- motive members and eyes. It is curious that the original form is retained with comparatively slight change by the males, which increase but little in size, and are so unlike the females that no one would suppose the two to belong to the same family, much less to the same species, but for the Microscopic study of their development.1 612. From the parasitic Suctorial Crustacea, the transition is not really so abrupt as it might at first sight appear to the group of Cirrhi- peda, consisting of the Barnacles and their allies: for these, like many of the Suctoria, arc fixed to one spot during the adult portion of their lives, but come into the world in a condition that bears a strong resemblance to the early state of many of the true Crustacea. The departure from the ordinary Crustacean type in the adults, is, in fact, so great that it is not surprising that Zoologists in general should have ranked them in a distinct Class; their superficial resemblance to the Mollusca, indeed, having caused most systematists to place them in that series, until due weight was given to those structural features which mark their ( articu- lated ' character. We must limit ourselves, in our notice of this group, to that very remarkable part of their history, the Microscopic study of which has contributed most essentially to the elucidation of their real nature. The observations of Mr. J. V. Thompson,2 with the extensions and rectifications which they have subsequently received from others (especially Mr. Spence Bate3 and Mr. Darwin4) show that there is no essential difference between the early forms of the sessile (Balanidae or 'acorn-shells ') and of the pedunculated Cirrhipeds (Lepadidaeor ' barna- cles'); for both are active little animals (Fig. 412, A), possessing three pairs of legs, and a pair of compound eyes, and having the body covered with an expanded carapace, like that of many Entomostracous Crusta- ceans, so as in no essential particular to differ from the larva of Cyclops (Fig. 411, c). After going through a series of Metamorphoses, one stage of which is represented in Fig. 412, B, c, these larvas come to pre- sent a form, D, which reminds us strongly of that of Daphnia; the body being inclosed in a shell composed of two valves, which are united along the back, whilst they are free along their lower margin, where they sepa- rate for the protrusion of a large and strong anterior pair of prehensile limbs provided with an adhesive sucker and hooks, and of six pairs of posterior legs adapted for swimming. This Bivalve shell, with the 1 As the group of Suctorial Crustacea is rather interesting to the professed Naturalist than to the amateur Microscopist, even an outline view of it would be unsuitable to the present work; and the Author would refer such of his readers as may desire to study it, to the excellent Treatise by Dr. Baird already referred to. 1 "Zoological Researches," No. iv., 1830, and Philos. Transact., 1835, p. 355. 3 ' On the Development of the Cirripedia,' in " Ann. of Nat. Hist.," Ser. 2, Vol. viii. (1851), p. 324. 4 " Monograph of the Sub- Class Cirripedia," published by the Ray Society. 214 THE MICROSCOPE AND ITS REVELATIONS. members of both kinds, is subsequently thrown-off; the animal then attaches itself by its head, a portion of which, in the Barnacle, be- comes excessively elongated into the ' peduncle' of attachment, whilst in Balanus it expands into a broad disk of adhesion; the first thoracic segment sends backwards a prolongation which arches over the rest of the body so as completely to inclose it, and of which the exerior layer is consolidated into the 'multi valve' shell; whilst from the other thoracic segments are evolved the six pairs of cirrhi, from whose peculiar char- acter the name of the group is derived. These are long, slender, many, jointed, tendril-like appendages, fringed with delicate filaments covered with cilia, whose action serves both to bring food to the mouth, and to maintain aerating currents in the water. The Balani are peculiarly inter- esting objects in the Aquarium, on account of the pumping action of their beautiful feathery appendages, which may be watched through a Tank- FIG. 412. Development of Balanus balanoides;—*., earliest form; B, larva after second moult; c, side view of the same; D, stage immediately preceding the loss of activity; a, stomach (?); b, nucleus of future attachment (?). Microscope; and their cast skins, often collected by the Tow-net, are well worth mounting. 613. MALACOSTRACA. — The chief points of interest to the Micro- scopist in the more highly organized forms of Crustacea, are furnished by the structure of the shell, and. by the phenomena of metamorphosis, both which may be best studied in the commonest kinds. — The Shell of the Decapods in its most complete form consists of three strata — namely, 1, a horny structureless layer covering the exterior; 2, an areolated stratum; and 3, a laminated tubular substance. The innermost and even the middle layers, however, may be altogether wanting; thus, in the PJiyllosomcB or i glass-crabs,' the envelope is formed by the trans- parent horny layer alone; and in many of the small crabs belonging to the genus Portuna, the whole substance of the carapace beneath the horny investment presents the areolated structure. It is in the large thick-shelled Crabs that WQ find the three layers most differentiated. CRUSTACEA. 215 Thus, in the common Cancer pagurvs, we may easily separate the struc- tureless horny covering after a short maceration in dilute acid; the areolated layer, in which the pigmentary matter of the colored parts of the shell is chiefly contained, may be easily brought into view by grind- ing away from the inner side as flat a piece as can be selected, having first cemented the outer surface to the glass slide, and by examining this with a magnifying power of 250 diameters, driving a strong light through it with the Achromatic Condenser; whilst the tubular structure of the thick inner layer may be readily demonstrated, by means of sections parallel and perpendicular to its surface. This structure, which resem- bles that of dentine (§ 655), save that the tubuli do not branch, but remain of the same size through their whole course, may be particularly well seen in the black extremity of the claw, which (apparently from some peculiarity in the molecular arrangement of its mineral particles) is much denser than the rest of the shell; the former having almost the semi-transparence of ivory, whilst the latter has a chalky opacity. In a transverse section of the claw, the tubuli may be seen to radiate from the central cavity towards the surface, so as very strongly to resemble their arrangement in a tooth; and the resemblance is still further in- creased by the presence, at tolerably regular intervals, of minute sinuosi- ties corresponding with the laminations of the shell, which seem, like the 'secondary curvatures' of the dentinal tubuli, to indicate successive stages in the calcification of the animal basis. In thin sections of the areolated layer it may be seen that the apparent walls of the areolas are merely translucent spaces from which the tubuli are absent, their orifices being abundant in the intervening spaces.1 The tubular layer rises up through the pigmentary layer of the Crab's shell in little papillary eleva- tions, which seem to be concretionary nodules; and it is from the deficiency of the pigmentary layer at these parts, that the colored portion of the shell derives its minutely-speckled appearance. — Many departures from this type are presented by the different species of Decapods; thus, in the Prawns, there are large stellate pigment-spots (resembling those of Frogs, Fig. 465, c), the colors of which are often in remarkable con- formity with those of the bottom of the rock-pools frequented by these creatures; whilst in the Shrimps there is seldom any distinct trace of the areolated layer, and the calcareous portion of the skeleton is disposed in the form of concentric rings, which seem to be the result of the con- cretionary aggregation of the calcifying deposit (§ 713). 614. It is a very curious circumstance, that a strongly-marked differ- ence exists between Crustaceans that are otherwise very closely allied, in regard to the degree of change to which their young are subject in their progress towards the adult condition. For whilst the common Crab, Lobster, Spiny Lobster, Prawn, and Shrimp undergo a regular meta- morphosis, the young of the Cray -fish and some Land-crabs come forth from the egg in a form which corresponds in all essential particulars with that of their parents. Generally speaking, a strong resemblance exists among the young of all the species of Decapods which undergo a 1 The Author is now quite satisfied of the correctness of the interpretation put by Prof. Huxley (see his Article, ' Tegumentary Organs,' in the " Cyclop, of Anat. and Phys.," Vol. v., p. 487) and by Prof. W. C. Williamson (' On some Histological Features in the Shells of Crustacea,' in " Quart. Journ. of Microsc. Science," Vol. viii., 1860, p. 38), upon the appearances which he formerly described ("Reports of British Association" for 1847, p. 128) as indicating a cellu- lar structure in this layer. 216 THE MICROSCOPE AND ITS REVELATIONS. metamorphosis, whether they are afterwards to belong to the macrourons (long-tailed) or to the brachyourous (short-tailed) division of the group; and the forms of these larvae are so peculiar, and so entirely different from any of those into which they are ultimately to be developed, that they were considered as belonging to a distinct genus, Zoea, until their real nature was first ascertained by Mr. J. V. Thompson. Thus, in the earliest state of Carcinus mmnas (small edible Crab), we see the head and thorax, which form the principal bulk of the body, included within a large carapace or shield (Fig. 413, A) furnished with a long projecting spine, beneath which the fin-feet are put forth: whilst the abdominal segments, narrowed and prolonged, carry at the end a flattened tail-fin, by the strokes of which upon the water, the propulsion of the animal is chiefly effected. Its condition is hence comparable, in almost all essen- tial particulars, to that of Cyclops (§ 605). In the case of the Lobster, Prawn, and other f macrourous ' species, the metamorphosis chiefly con- sists in the separation of the locomotive and respiratory organs; true legs being developed from the thoracic segments for the former, and true gills (concealed within a special chamber formed by an extension of the cara- pace beneath the body) for the latter; while the abdominal segments Metamorphosis of Carcinus Mcenas :— A, first or Zoea stage; B, second or Megalopa stage; c, third stage, in which it begins to assume the adult form; D, perfect form. increase in size, and become furnished with appendages (false feet) of their own. In the Crabs, or ' brachyourous ' species, on the other hand, the alteration is much greater; for besides the change first noticed in the thoracic members and respiratory organs, the thoracic region becomes much more developed at the expense of the abdominal, as seen at B, in which stage the larva is remarkable for the large size of its eyes, and hence received the name of Megalopa when it was supposed to be a dis- tinct type. In the next stage, c, we find the abdominal portion reduced to an almost rudimentary condition, and bent under the body; the thoracic limbs are more completely adapted for walking, save the first pair, which are developed into chela or pincers; and the little creature entirely loses the active swimming habits which it originally possessed, and takes on the mode of life peculiar to the adult. 1 615. In collecting minute Crustacea, the King-net should be used for the fresh-water species, and the Tow-net for the marine. In localities favorable for the latter, the same ' gathering ' will often contain multi- 1 On the Metamorphosis of Crustacea and Cirripedia, see especially the recent " Untersuchungen liber Crustaceen" nf Prof. Glaus; Vienna, 1876. CEUSTACEA. 217 tildes of various species of Entomostraca, accompanied, perhaps, by the larvae of higher Crustacea, Echinoderm larvae, Annelid larvae, and the smaller Medusae. The water containing these should be put into a large glass jar, freely exposed to the light; and, after a little practice, the eye will become so far habituated to the general appearance and modes of movement of these different forms of animal life, as to be able to distin- guish them one from the other. In selecting any specimen for Micro- scopic examination, the Dipping-tube (§ 126) will be found invaluable. The collector will frequently find Megalopa larvae, recognizable by the brightness of their two black eye-spots, on the surface of floating leaves of Zoster a. — The study of the Metamorphosis will be best prosecuted, how- ever, by obtaining the fertilized eggs which are carried about by the females, and watching the history of their products. — For preserving specimens, whether of Entomostraca, or of larvae of the 'higher Crustacea, the Author would recommend Glycerine- jelly as the best medium. 218 THE MICROSCOPE AND ITS REVELATIONS. CHAPTER XIX. INSECTS AND ARACHNIDA. 616. THERE is no Class in the whole Animal Kingdom which affords to the Microscopist such a wonderful variety of interesting objects, and such facilities for obtaining an almost endless succession of novelties, as that of Insects. For, in the first place, the number of different kinds that may be brought together (at the proper time) with extremely little trouble, far surpasses that which any other group of animals can supply to the most painstaking collector; then again, each specimen will afford, to him who knows how to employ his materials, a considerable number of Microscopic objects of very different kinds; and thirdly, although some of these objects require much care and dexterity in their preparation, a large proportion may be got out, examined, and mounted, with very little skill or trouble. Take, for example, the common House-fly: — its eyes may be easily mounted, one as a transparent, the other as an opaque object (§ 626); its attennce, although not such beautiful objects as those of many other Diptera, are still well worth examination (§ 628); its tongue or 'proboscis' (§ 629) is a peculiarly interesting object, though requiring some care in its preparation ; its spiracles, which may be easily cut off from the sides of its body, have a very curious structure (§ 635); its ali- mentary canal affords a very good example of the minute distribution of the trachece (§ 634); its wing, examined in a living specimen newly come forth from the pupa state, exhibits the circulation of the blood in the 'nervules' (§ 633), and when dead shows a most beautiful play of iri- descent colors, and a remarkable areolation of surface, when examined by light reflected from its surface at a particular angle (§ 638); its foot has a very peculiar conformation, which is doubtless connected with its singular power of walking over smooth surfaces in direct opposition to the force of gravity, and on the action of which additional light has lately been thrown (§ 640); while the structure and physiology of its sexual apparatus, with the history of its development and metamorphoses, would of itself suffice to occupy the whole time of an observer who should desire thoroughly to work it out, not only for months but for years.1 Hence, in treating of this department in such a work as the present, the Author labors under the embarras des richesses; for to enter into such a description of the parts of the structure of Insects most interesting to the Microscopist, as should be at all comparable in fulness with the accounts which it has been thought desirable to give of other Classes, would swell out the volume to an inconvenient bulk; and no course seems open, but to limit the treatment of the subject to a notice of the 1 See Mr. Lowne's valuable Treatise on " The Anatomy and Physiology of the Blow-fly," 1870. INSECTS AND ARACHNID A. 219 kinds of objects which are likely to prove most generally interesting, with a few illustrations that may serve to make the descriptions more clear, and with an enumeration of some of the sources whence a variety of specimens of each class may be most readily obtained. And this limitation is the less to be regretted, since there already exist in our language numerous elementary treatises on Entomology, wherein the general structure of Insects is fully explained, and the conformation of their minute parts as seen with the Microscope is adequately illustrated. 617. A considerable number of the smaller Insects — especially those belonging to the Orders Coleoptera (Beetles), Neuroptera (Dragon-fly, May-fly, etc)., Hymenoptera (Bee, Wasp, etc.), and Diptera (two-winged Flies)— may be mounted entire as opaque objects for low magnifying powers; care being taken to spread out their legs, wings, etc., so as adequately to display them, which may be accomplished, even after they have dried in other positions, by softening them by stepping them in hot water, or, where this is objectionable, by exposing them to steam. Full directions on this point, applicable to small and large Insects alike, will be found in all Text-books of Entomology. There are some, however, whose translucence allows them to be viewed as transparent objects; and these are either to be mounted in Canada balsam or in Deane's medium, Glycerine-jelly, or Farrant's gum, according to the degree in which the horny opacity of their integument requires the assistance of the balsam to facilitate the transmission of light through it, or the softness and deli- cacy of their textures render an aqueous medium more desirable. Thus, an ordinary Flea or Bug will best be mounted in balsam; but the various parasites of the Louse kind, with some or other of which almost every kind of animal is affected, should be set-up in some of the l media.' Some of the aquatic larvae of the Diptera and Neuroptera, which are so transparent that their whole internal organization can be made-out with- out dissection, are very beautiful and interesting objects when examined in the living state, especially because they allow the Circulation of the blood and the action of the dorsal vessel to be discerned (§ 63#). Among these, there is none preferable to the larva of the Ephemera marginata (Day-fly), which is distinguished by the possession of a number of beauti- ful appendages on its body and tail, and is, moreover, an extremely com- mon inhabitant of our ponds and streams. This insect passes two or even three years in its larval state, and during this time it repeatedly throws-off its skin; the cast skin, when perfect, is an object of extreme beauty, since, as it formed a complete sheath to the various appendages of the body and tail, it continues to exhibit their outlines with the ut- most delicacy; and by keeping these larvae in an Aquarium, and by mounting tiie entire series of their cast skins, a record is preserved of the successive changes they undergo. Much care is necessary, however, to extend them upon slides, in consequence of their extreme fragility; and the best plan is to place the slip of glass under the skin whilst it is floating on water, and to lift the object out upon the slide. — Thin sections of Insects, Caterpillars, etc., which bring the internal parts into .view in their normal relations, maybe cut with the Microtome (§ 184), by first soaking the body (as suggested by Dr. Halifax) in thick gum-mucilage, which passes into its substance, and gives support to its tissues, and then inclosing it in a casing of melted paraffin, made to fit the cavity of the Secti on-instrument. 618. Structure of the Integument. — In treating of those separate parts of the organization of Insects which furnish the most interesting objects 5ii() rills MK'UosroCK AND ITS ItKV KI.A'I'IONS. of Microscopic :;l.iidy, we in, TV most appropriate!! commence with lidc-miicul Mini its appendages (scales, hairs, etc). The hody :i.nd their mem- bers are closely invested by :t hardened skin, \\lncli ;ic!s as I heir skeleton, and all'ords points of a 1 1 achmenf to I he muscles hy which their sr\cral parts are moved; being soft and llexible, however, at the joints. This skin is usually more or less horny in its texture, and is consolidated hy the animal substance termed ('Inline, as well as, in some cases, l>v a sma I <|iianl ily of mineral matter. II, is in the ('<>/<•<>/>/<'/•<( that it- attains its ••n-aicsl, development ; I he * dermo-skeleton ' of many licet les being so linn as not only to confer upon them an c\l raordinary power of passive resist- ance, 1ml, also to enahle them to put forth enormous force hy the action of I he powerful muscles which arc attached to it. It ina\ he staled as ;i ".encral ride, that. I he outer layer of I his dermo-skeleton is alwavs cellular, hiking the place of an epidermis; and that, (lit1 cells are straight-sided and closely tilled togct her, so as to he polygonal (usually hexagonal) in form. Of this' wo ha\e a- very good example in the s/t/icr/icitd layers (Kig. 427, H) of the thin horny lamella' or hlades which constitute the terminal portion of the antenna of the ('<><• &•<•//< r/cr (l<'ig. -lv<»); this lavcr being easily distinguished from Llie inlermediate portion (A) of the lamina by careful focussing. In many Heel les, the hexagonal areolalion of the surface is distinguishable when the light is relleefed from it at a particu- lar jingle, even when not discernihle in transparent sect ions. The integu- ment of I he common lint .\nl exhibits (lie hexagonal cellular arrangement \ery distinctly throughout; and tho broad Hal. expansion of the lei;' of I he ( 'ni/>ro (' sand-wasp ') all'ords anol her beau! ifnl example of a dist inct 1\- cellular slrnclnre in the outer la\er of the integument. The inner layer, however, which eonslilnles 1 he principal part of the thickness of 'the horny easing of the IVcl le-t.ril»e, seldom exhihits any distinct organi/ji- tion; though it may ht> usuallv separaleil into several lamella', which are sometimes traversed by tubes that pass into them from the inner surface, and extend towards I he outer wit bout reaching it. ('•I1.'. Tt-/><'>i(t(ti/t's, ha\ing either the form of broad Hat Scales, or that, of Hairs more or less approaching (he cylindrical shape, or some form intermediate belwiHMi tho two. — The fii-it/i/ investment is most complete nmonjj the l.c/nilofttrra (Hntterily and Moth tribe); the distinguishing character of the insects of this order being derived from the presence of a regular layer of scales upon each side ot (heir l:irge membranous wings. ll is to the peculiar coloration of the scales that the various hues and figures are due, by which these wings are so commonlv distinguished; all the scales of one patch (for example) being green, those of another red, and soon: for the subjacent membrane remains perfectly transparent and colorless, when (he scales have been brushed oil' from its surface. Kach scale seems to he composed of two or more membranous lamella', often with an intervening deposit of pigment, on which, especially in I.cpidoptera, their color depends. Certain scales, however, especially in the Invlle- tribe, have a metallic' lustre, and exhibit, brilliant colors that varv with the mode in which the light- glances from them; and this ' iridescence,' which is specially noteworthy in the scales of the Curruln) ini/writilift ('diamond-beetle'), seems to be a purely optical clTeet, depending either ^like the prismatic hues of a soap-bubble) on the extreme thinness of the membranous lamella*, or (like those of 'mother-of-pearl.' ij f>tif>) Oil a lineation of surface produced by their corrugation. Kach scale is INSKCTS AM) AKACHNIMA. furnished at one end with a sort of handle «»r * pedicle ' (Figa. 414, 415), b\ which it is lilted inio ;i ininule socket ;ill:iched to the surface of the insect; and on the wind's of I .efiidofileni these sockets are so arranged that the scales lie in \/i/nin/t', is met with :miong the /'ieridir. one of the principal families of (he hiurnal Lepidoplera. The * plumules ' :i re not Hat, but cylin- drical or bellows-shaped, and are hollow; they are at (ached to tllO wing h\ a hull), :it the cud of a- thin elastic peduncle thai ditl'ers in length in dilTcrcnl species, a.nd proceeds from the liroadcr, not, from the narrower end of the scale; whilst, (he free extremity usually tapers oil', and cud: in a, kind of brush, (.hough sometimes i( is broad and has its cdi'e fringed with minute lilanients. rrhcso 'plumules/ which are peculiar to the males, are found on the upper surface of the win^s, partly between and partly under (he ordinary scales. They seem to he represented amon^ the /. //r. 'The ncculiar markin-s exhibited by many of these Scales, very early allracicd Mie al.tenl ion of <>plici:,,ns onga^ed in I he application of Achromatism to (he Microscope (i< !.">); for, as ( he cleai'iiess and strength wilh which (hey could be shown, were found to depend on the decree (o which the angular aperture of lin Objective could ho opened without sac rilice of perfect correction for .spherical and chroma! ic. aJterrat ion, HlUJh scales proved very serviceable as ' (cs(,s.' The Author can well remember the time when those of Mnr/t/in inrtu'/dH.t ( Ki^. -Ill), (he ordiiiai-y and ' hat t ledore ' scales of the j^i/i/o/tn/io/ us tin/its (l^i^'s. -ll.r), -ll'i), and the scales of the /,{'/H'XHHI- Mtirlntriint ( V\^. -Mi), which are now only used for lest in:; Objectives of /tun or -medium power (£ I.V.>, I., II.), were the recognized tests for objectives of ///V///. power; while (ho exhibition of alternating lighl, and da,i'k bands on a, l*oiatom-tesls greatly promoted the enlargement of angular aperture, yet the Author has Mm authority of the ablest, constructors of high-power Objectives in (bis county for stating, (hat (hey still regard the Tod lira-scale as the best lest for definition, and consequently for that nf ion of ipialities which is most required in Objectives (o be used for l-iiological investigations of tlio greatest dillieulty ($ l.r»S, YiAa As the real structure of this scale, of which the ' notes of admiration, or the 'exclamation-markings' constitute the optical expression, has been a mailer of much controversy, the question requires special consideration; 'See Mr Watson's Memoirs 'On the Scales of Battledore Butterflies,' in " Monthly Microseo|)ical Journal." Vol. ii., pp. T.\, :U-I. •'The Author is assured that it is hy no means an uncommon experience, on liist putting together tin Ohjeetivo of wiclo aperture, to lind it capahle of resol\ ing a dilliciilt Diatom, whilst, when tested on a Podura-xcale, it utterly fails, Oil ueeount of its imperfect ' deli nil ion." 222 THE MICROSCOPE AND ITS REVELATIONS. and in discussing it, regard should be had to what we are taught by the study of the larger and more strongly marked forms of Insect-scales, as to what scales are.— That they are in reality flattened cells, analogous to the Epidermic cells of higher animals (§ 671), can scarcely be doubted by any Physiologist. Their ordinary flattening is simply the result of their drying up; and the exception presented by the * plumules' and 1 battledore' scales (Fig. 416), which have the two surfaces separated by a considerable cavity, helps to prove the rule. It is perfectly clear in some of these, that the membranous wall of the cell is strengthened by longitudinal ribs, which diverge from the peduncle; as is particularly well seen in the plumules of two West African butterflies, Pieris Aga- tliina and Pieris Phloris, in which the plumules are as much as l-300th of an inch in length (large enough to be studied under the Binocular Microscope), and are of cylindrical form, save that they are drawn in as FIG. 414. FIG. 415. FIG. 416. Scale of Morpho Menelaus. Scales of Pelyommatus argus (Azure- Battledore Scale of blue);— a, battledore-scale; b, inerfer- Polyommatus argus ence striae. (Azure-blue). if by a cord at about one-half or one-third of their length, the ribs curving inwards to this constriction.1 In ordinary scales we find similar ribs, sometimes running parallel to each other, or nearly so (Figs. 414, 415), and occasionally connected by distinct cross-bars (Fig. 418), but sometimes diverging from the ' quill; ' and where, as in Lepisma (Fig. 417), the ribs are parallel on one surface and divergent on the other, a very curious set of appearances is presented by their optical intersection, which throws considerable light on the meaning of the Podura-msn±- ings. 621. The easier test-scales are furnished by the order Lepidoptera (Butterflies and Moths); and among the most beautiful of these, both for color and for regularity of marking, are those of the Morpho Mene- laus (Fig. 414). These are of a rich blue tint, and exhibit strong longi- 1 See Watson, loc. cit., p. 75. INSECTS AND AKACHNIDA. 223 tndinal striae, which seem due to ribbed elevations of one of the superfi- cial layers. There is also an appearance of transverse striation, which cannot be seen at all with an inferior objective, but becomes very decided with a good objective of medium focus; and this is found, when sub- mitted to the test of a high power and good illumination, to depend upon the presence of transverse thickenings or corrugations (Fig. 414), probably on the internal surface of one of the membranes.— The large scales of the Polyommatus argus (' azure-blue ' butterfly) resemble those of the Menelaus in form and structure, but are more delicately marked (Fig. 415)., Their ribs are more nearly parallel than those of the Menelaus scale, and do not show the same transverse striation. When one of these scales lies partly over another, the effect of the optical inter- section of the two sets of ribs at an oblique angle is to produce a set of interrupted striations (#), very much resembling those of the Podura- scale. The same Butterfly furnishes smaller scales, which are commonly termed the t battledore ' scales, from their resemblance in form to that object (Fig. 415, a). These scales, which occur in the males of several genera of the family Lyccenidm, and present a considerable variety of shape,1 are marked by narrow longitudinal ribbings, which at intervals seem to expand into rounded or oval elevations that give to the scales a dotted appearance (Fig. 416); at the lower part of the scale, however, these dots are wanting. Dr. Anthony describes and figures them as elevated bodies, somewhat resembling dumb-bells or shirt-studs, ranged along the ribs, and standing out from the general surface.2 Other good observers, however, whilst recognizing the stud-like bodies described by Dr. Anthony, regard them as not projecting from the external surface of the scale, but as interposed between its two lamellae;3 and this view seems to the author to be more conformable than Dr. Anthony's to gen- eral probability. 622. The more difficult 'test-scales' are furnished by little wingless insects ranked together by Latreille in the order Thysanura, but now separated by Sir John Lubbock,4 on account of important differences in internal structure, into the two groups Collembola and true Tliysanura. Of the former of these, the LepiamidcB constitute the typical family; and the scale of the common Lepisma saccharina, or ( sugar-louse,5 very early attracted the attention of Microscopists on account of its beautiful shell- like sculpture. "When viewed under a low magnifying power, it presents a beautiful ' watered silk ' appearance, which, with higher amplification, is found to depend (as Mr. E. Beck first pointed out)' upon the intersec- tion of two sets of striae, representing the different structural arrange- ments of its two superficial membranes. One of its surfaces (since ascer- tained by Mr. Joseph Beck7 to be the under or attached surface of the 1 See Watson, loo. cit. 2 ' The Markings on the Battledore Scales of some of the Lepidoptera,' in •" Monthly Microsc. Journal," Vol. vii. (1872), pp. 1, 250. 3 See " Proceedings of the Microscopical Society," op. cit., p. 278. 4 See his "Monograph of tha Collembola and Thysanura," published by the Ray Society, 3872. 6 This insect may be found in most old houses, frequenting damp warm cup- boards, and especially such as contain sweets; it may be readily caught in a small pill-box, which should have a few pin-holes in the lid; and if a drop of chloro- form be put over the holes, the inmate will soon become insensible, and may be then turned out upon a piece of clean paper, and some of its scales transferred to -a slip of glass by simply pressing this gently on its body. 6 "The Achromatic Microscope," p. 50. 7 See his Appendix to Sir John Lubbock's " Monograph." 224 THE MICROSCOPE AND ITS REVELATIONS. scale) is raised, either by corrugation or thickening, into a series of strongly- marked longitudinal ribs, which run nearly parallel from one end of the scale to the other, and are particularly distinct at its margins and at its free extremity; whilst the other surface (the free or outer, according to Mr. J. Beck) presents a set of less definite corrugations, radiating from the pedicle, where they are strongest, towards the sides and free extremity of the scale, and therefore crossing the parallel ribs at angles more or less acute (Fig. 417). It was further pointed out by Mr. K. Beck, that the intersection of these two sets of corrugations at differ- ent angles produces most curious effects upon the appearances which optically represent them. For where the diverging ribs cross the longi- tudinal ribs very obliquely, as they do near the free extremity of the FIG. 417. FIG. 418. Scale of Lepisma, saccharina. [Scale of Machilis polypoda. scale, the longitudinal ribs seem broken up into a series of ' exclamation- markings,' like those of the Podura; but where the crossing is transverse or nearly so, as at the sides of the scale, an appearance is presented as of successions of large bright beads. The conclusion drawn by the Messrs. Beck, that these interrupted appearances are "produced by two sets of uninterrupted lines on different surfaces," has been confirmed by the careful investigations of Mr. Morehouse.1 The minute beaded structure observed by Dr. Koyston-Pigott2 alike in the ribs and in the intervening spaces, may now be pretty certainly regarded as an optical effect of dif- fraction (§ 156). In the scale of a type nearly allied to Lepisma, the Machilis polypoda, the very distinct ribbing (Fig. 418) is produced by the 1 "Monthly Microsc. Journal," Vol. xi. (1874), p. 13, and Vol. xviii. (1877), p. 81. 2 "Monthly Microsc. Journ.," Vol. ix. (1873), p. 63. INSECTS AND ARACHNID A. 225 corrugation of the under membranous lamina alone; the upper or exposed lamina being smooth, with the exception of slight undulations near the pedicle; and the cross-markings being due to structure between the super- posed membranes, probably a deposit on the interior surface of one or both of them.1 623. Although the Poduridce and Lepismidcv now rank as distinct Families, yet they approximate sufficiently in general organization, as well as in habits, to justify the expectation that their scales would be framed upon the same plan. The Podurida are found amidst the saw- dust of wine-cellars, in garden tool-houses, or near decaying wood; and derive their popular name of ' spring-tails ' from the possession by many of them of a curious caudal appendage, by which they can leap like fleas. This is particularly well developed in the species now designated Lepido- cyrtus curvicollis, which furnishes what are ordinarily known as ( Podura '- FIG. 419. FIG. 420. Test-scales of Lepidocyrtus curvicollis:— A, large, strongly-marked scale: B, small scale, more faintly marked. Ordinary scale of Lepidocyrtus curvicollis. scales. "When full-grown and unrubbed," says Sir John Lubbock, "this species is very beautiful, and reflects the most gorgeous metallic tints." Its scales are of different sizes and of different degrees of strength of marking (Fig. 419, A, B), and are therefore by no means of uniform value as tests. The general appearance of their surface, under a power not sufficient to resolve their markings, ia that of watered silk, — light and dark bands passing across it with wavy irregularity; but a well-cor- rected Objective of very moderate angular aperture now suffices to resolve every dark band into a row of distinct 'exclamation marks' (Plate II., fig. 2). If, however, they are illuminated by oblique light from above (the scales being placed under the objective without any cover, so as to avoid the loss of light by reflection from its surface), the appearances presented are those shown in fig. 4 when the markings are at right angles 1 See Mr. Joseph Beck, in Sir J. Lubbock's " Monograph," p. 253. 15 226 THE MICROSCOPE AND ITS REVELATIONS. to the direction of the light, and in fig. 5 when they lie in the same direc- tion as the light with their narrow ends pointing to it. When this last direction is reversed, the light from the points is so slight, that the scales appear to have lost their markings altogether. If moisture should insin- uate itself between the scale and the covering-glass, the markings disap- pear entirely, as shown in fig. 3; and this is true also of the scale of Lepisma. A certain longitudinal continuity may be traced between the ' exclamation-marks ' in the ordinary test-scale; but this is much more apparent in other scales from the same species (Fig. 420), as well as in the scales of various allied types, which were carefully studied by the late Mr. R. Beck.1 In certain other types, indeed, the scales have very dis- tinct longitudinal parallel. ribs, sometimes with regularly disposed cross- bars; these ribs, being confined to one surface only (that which is in con- tact with the body), are not subject to any such interference with their optical continuity as has been shown to occur in Lepisma; but more or less distinct indications of radiating corrugations often present them- selves. The appearance of the interrupted ' exclamation-marks ' Mr. J. Beck (op. cit., p. 254) considers to be due "to irregular corrugations of the outer surface of the under membrane, to slight undulations on the outer surface of the upper membrane, and to structure between the super- posed membranes." It has been recently stated by Mr. Joseph Beck, that the scales of a Lepidopterous insect belonging to the genus Mormo, which under a low power present the watered-silk appearance seen in the Podura-scale, under a l-5th show the ' exclamation ' markings, whilst under a l-10th they exhibit distinct ribs from pedicle to apex; thus show- ing in one scale how the appearances run from one scale into the other.2 On the other hand, we are assured by Dr. Royston-Pigott, not only that what a lens most perfectly corrected for spherical aberration ought to show, is a minute beaded structure, alike in the ' exclamation-markings ' and in the spaces between them; but that the markings whose perfect definition had been previously considered the aim of all constructors of high-power Objectives, are altogether illusory, these markings representing nothing else than the manner in which the rouleaux of beads lie with reference to one another.3 The Author has fully satisfied himself bv his own study, under an oil-immersion l-25th of Messrs. Powell and Lealand, of a Podura-scale illuminated by the < immersion paraboloid ' (which gives a view of it entirely different than any that can be obtained either by transmitted or reflected light), that the 'exclamation-markings' are — as maintained by the Messrs. Beck — the optical expression of a corrugated or ribbed arrangement of the lower membrane of the scale, slightly modified by the internal structure of the upper membrane, and probably also (as confirmed by Mr. Wenham) by a structure interposed between the two membranes.4 And this conclusion is borne out, in opposition to the doctrine of Dr. Royston Pigott, by two unrivalled Photographs taken of the Podura-scale by Col. Dr. Woodward. One of these, taken with a magnifying power of 3200 diameters, central monochromatic light, im- mersion l-16th, and amplifier, shows the ' exclamation-marks ' better 1 "Trans. of.Microsc. Soc.," N.S., Vol. x. (1862), p. 83. See also Mr. Joseph Beck, in the Appendix to Sir John Lubbock's " Monograph," and in "Monthly Microsc. Journ.," Vol. iv., p. 253. 12 "Journ. Roy. Microsc. Soc.," Vol. ii. (1879), p. 810. 3 See his paper ' On High Power Definition,' in " Monthly Microscopical Jour- nal," Vol. ii. (1869), p. 295, and several subsequent papers. 4 " Monthly Journ. of Microsc. Sci.," Vol. xi. (1874), p. 75. INSECTS AND ARACHNIDA. 227 than any photographic representation previously obtained; and it is clear that Dr. Woodward regards this as the truest view. " Immediately after- wards," he says, "with the same optical combination and magnifying power, without any change in the cover-correction, by simply rendering the illuminating pencil oblique, and slightly withdrawing the objective from its first focal position, I obtained a negative which displays the ' bead-like ' or varicose appearance of the ribbing more satisfactorily than I had previously been able to do."1 The beaded appearance shown in this photograph, a copy of a portion of which is given in Fig. 421, cor- responds so entirely with that which Dr. Woodward afterwards found to be producible in the scale of the Gnat by a like alteration in the illumina- tion (§ 156), that the Author feels fully justified in adhering to his ori- ginal opinion that it does not represent real structure, but is an optical •effect of diffraction.2 624. The Hairs of many Insects, and still more of their larvae, are very interesting objects for the microscope, on account of their branched or tufted conformation: this being particularly remarkable in those FIG. 421. Portion of a Podura-scale, from a Photograph by Col. Dr. Woodward. with which the common hairy Caterpillars are so abundantly beset. Some of these afford very good tests for the perfect correction of Objec- tives. Thus the hair of the Bee is pretty sure to exhibit strong prismatic colors, if the Chromatic aberration should not have been exactly neutral- ized; and that of the larva of a Dermestes (commonly but erroneously termed the ( bacon-beetle ') was once thought a very good test of defining power, and is still useful for this purpose. It has a cylindrical shaft (Fig. 422, B) with closely-set whorls of spiny protuberances, four or five 1 "Monthly MicroscopicalJournal," Vol. v., p. 246. 2 The successive Volumes of the "Monthly Microscopical Journal," from the 2d (in which Dr. Royston-Piggott's views were first promulgated) to the present date, teem with Papers on this subject from Mr. Jos. Beck, Mr. Mclntire, Dr. Maddox, Dr. Royston-Pigott, Mr. Wenham, and Col. Dr. Woodward; which, with a Paper by Mr. Slack in " The Student," Vol. v., p. 49, and a Paper by Mr. Morehouse, giving the results of his examination of the scales of Lepisma and Podura as opaque objects, under very high immersion objectives, with Beck's Vertical Illuminator ("Monthly Microsc. Journ.," Vol. xviii., 1877, p. 31), should be consulted by such as wish to follow out the inquiry. 228 THE MICROSCOPE AND ITS REVELATIONS. FIG. 422. in each whorl; the highest of these whorls is composed of mere knobby spines: and the hair is surmounted by a curious circle of six or seven large filaments, attached by their pointed ends to its shaft, whilst at their free extremities they dilate into knobs. An approach to this structure is seen in the hairs of certain Myriapods (centipedes, gaily- worms, etc.), of which an example is shown in Fig. 422, A; and some minute forms of this class are most beautiful objects under the Binocular Microscope, on ac- count of the remarkable structure and regular arrangement of their hairs. 625. In examining the Integument of Insects, and its appendages, parts of the surface maybe viewed either by reflected or transmitted light, according to their degree of transparence and the nature of their cover- ing. The Beetle and Butterfly tribes furnish the greater number of the specimens suitable to be viewed as opaque objects: and nothing is easier than to mount portions of the elytra, of the former (which are usually the most showy parts of their bodies), or of the wings of the latter, in the manner described in § 175. The tribe of Curculionidce, in which the surface of the body is beset with scales hav- ing the most varied and lustrous hues, is distinguished among Coleoptera for the brilliancy of the objects it affords; the most remarkable in this respect being the well-known Curculio wiperialis, or * diamond- beetle' of South America, parts of whose elytra, when properly illuminated and looked-at with a low power, show like clusters of jewels flashing against a dark velvet ground. In many of the British Cur- culionidae, which are smaller and less brilliant, the scales lie at the bottom of little depressions of the surface; and if the elytra of the ' diamond beetle ' be carefully examined, it will be found that each of the clusters of scales which are arranged upon it in rows, seems to rise out of a deep pit which sinks-in by its side. The transition from Scales to Hairs is" ex- tremely well seen by comparing the different parts of the surface of the diamond-beetle with each other. The beauty and brilliancy of many objects of this kind are increased by mounting them in cells in Canada balsam, even though they are to be viewed with reflected light; other objects, however, are ren- B; lair of jKS?£. dercd less attractive by this treatment; and in order to ascertain whether it is likely to improve or to deteriorate the specimen, it is a good plan first to test some other portion of the body having scales of the same kind, by touching it with turpen- tine, and then to mount the part selected as an object, either in balsam or dry, according as the turpentine increases or diminishes the brilliancy of the scales on the spot to which it was applied. Portions of the wings of Lepidoptera are best mounted as opaque objects, without any other preparation then gumming them flat down to the disk of the wooden slide (§ 175); care being taken to avoid disturbing the arrangement of the scales, and to keep the objects, when mounted, as secluded as possible from dust. In selecting such portions, it is well to choose those which have the brightest and the most contrasted colors, exotic butterflies being in this respect usually preferable to British; and before attaching them to slides, care should be taken to ascertain in what position, with INSECTS AND ARACHNIDA. 229 the arrangement of light ordinarily used, they are seen to the best advantage, and to fix them there accordingly. — Whenever portions of the integument of Insects are to be viewed as transparent objects, for the display of their intimate structure, they should be mounted in Canada balsam, after soaking for some time in turpentine; since this substance has a peculiar effect in increasing their translucence. Not only the horny casings of perfect Insects of various orders, but also those of their pupae, are worthy of this kind of study; and objects of great beauty (such as the chrysalis case of the Emperor-moth), as well as of scientific interest, are sure to reward such as may prosecute it with any assiduity. Further information may often be gained by softening such parts in potash, and viewing them in fluid. — The scales of the wings of Lepidop- tera, etc. , are best transferred to the slide, by simply pressing a portion of the wing either upon the slip of glass or upon the cover; if none should adhere, the glass may first be gently breathed-on. Some of them are best seen when examined ' dry,' whilst others are more clear when mounted in fluid; and for the determination of their exact structure, it is well to have FIG. 423. FIG. 424. Head and Compound Eyes of the Bee, showing the ocellites -in situ on one side (A), and displaced on the Other (o) ; a, a, a, stem- mata, b, b, antennae. Section of the Composite Eye of Melolontha vulgar is (Cockchafer): — a, facets of the cornea; 6, trans- parent pyramids surrounded with pigment; c, fibres of the optic nerve; d, trunk of the optic nerve. recourse to both these methods. Hairs, on the other hand, are best mounted in Balsam. 626. Parts of the Head. — The eyes of Insects, situated upon the upper and outer part of the head, are usually very conspicuous organs, and are frequently so large as to touch each other in front (Fig. 423). We find hi their structure a remarkable example of that multiplication of similar parts, which seems to be the predominating ( idea ' in the conformation of Articulated animals; for each of the large protuberant bodies which we designate as an eye, is really a ' compound ' eye, made up of many hundred or even many thousand minute conical ocellites (B). Approaches to this structure are seen in Annelida and Entomastraca; but the number of 'ocellites' thus grouped-together is -usually small. In the higher Crustacea, however, the ' ocellites ' are very numerous; and their com- pound eyes are constructed upon the same general plan as those of In- sects, though their shape and position are often very peculiar (Fig. 491). The individual ocellites are at once recognized, when the ' compound 230 THE MICROSCOPE AND ITS REVELATIONS. eyes ' are examined under even a low magnifying power, by the ' facetted ' appearance of the surface (Fig. 423, A), which is marked-out by very regular divisions either into hexagons or into squares: each facet is the 'corneule ' of a separate ocellite, and has a convexity of its own; hence by counting the facets, we can ascertain the number of ocelli tes in each ' com- pound eye. ' In the two eyes of the common Fly, there are as many as 4,000; in those of the Cabbage Butter-fly there are about 17,000; in the Dragon-fly, 24,000; and in the Mardella Beetle, 25,000. Behind each 'corneule' is a layer of dark pigment, which takes the place and serves the purpose of the 'iris' in the eyes of vertebrate animals; and this is perforated by a central aperture or ' pupil,' through which the rays of light that have traversed the corneule gain access to the interior of the eye. The further structure of these bodies is best examined by vertical sections (Fig. 424); and these show that the shape of each ocellite (b) is conical, or rather pyramidal, the corneule forming its base (a), whilst its apex abuts upon the extremity of a fibre (c) proceed- FIG. 425. ing from the termination of the optic nerve (d). The details of the structure of each ocellite are shown in Fig. 425; in which it is shown that each corneule is a double-convex lens, made up by the junction of two plano-convex lenses, a a and a' a', which have been found by Dr. Hicks to possess different refractive powers; by this arrangement (it seems probable) the aberrations are diminished, as they are by the combina- tion of ' humors ' in the Human eye. That each ' cor- neule ' acts as a distinct lens, may be shown by de- taching the entire assemblage by maceration, and then drying it (flattened out) upon a slip of glass; for when this is placed under the Microscope, if the point of a knife, scissors, or any similar object, be interposed Minute structure of between the mirror and the stage, the image of this Point wil1 be«een, bv a proper adjustment of the focus- . comeuie; a' a', its pos- of the microscope, in every one of the lenses. The ^Ip^ures's^l focus of each « corneule ' has been ascertained by ed by intervening pig- experiment to be equivalent to the length of the pyra- mentd d; bb, pyramids -i i i • -i •, 11 separated by pigment mid behind it; so that the image which it produces ee, b), present a most wonderful variety of conformation in the several tribes of Insects; often differing considerably in the several species of one genus, and even in the two sexes of the same species. Hence the characters which they afford are extremely useful in classification; especially since their structure must almost necessarily be in some way related to the habits and general economy of the creatures to which they belong, although our imperfect acquaintance with their function may prevent us from clearly discerning this relation. Thus among the Coleoptera we find one large family, in- cluding the Glow-worm, Fire-fly, Skip-jack, etc., distinguished by the toothed or serrated form of the antennae, and hence called Serricornes; in another, of which the Burying-beetle is the type, the antennae are ter- minated by a club-shaped enlargement, so that these beetles are termed FIG. 426. FIG. 427. Minute structure of leaf -like expansions of An- tenna of Melnlontha: — A, their internal layer; B, their superficial layer. Antenna of Melolontha (Cockchafer). Clavicornes; in another, again, of which the Hydrophilus or large Water- beetle is an example, the antennae are never longer and are commonly shorter than one of the pairs of palpi, whence the name of Palpicorne* is given to this group; in the very large family that includes the Lucani or Stag-beetles with the Scaralcei, of which the Cockchafer is the com- monest example, the antennae terminate in a set of leaf -like appendages, which are sometimes arranged like a fan or the leaves of an open book (Fig. 426), are sometimes parallel to each other like the teeth of a comb, and sometimes fold one over the other, thence giving the name Lamelli- cornes; whilst another large family is distinguished by the appellation Longicornes, from the great length of the antennae, which are at least as long as the body, and often longer. Among the Lepidoptera, again, the conformation of the antennae, frequently enables us at once to distin- guish the group to which any specimen belongs. As every treatise on Entomology contains figures and descriptions of the principal .types of INSECTS AND AKACHNIDA. 233 conformation of tnese organs, there is no occasion here to dwell upon them longer than to specify such as are most interesting to the Micro- scopist: — Coleoptera, Brachinus, Calathus, Harpalus, Dytiscus, Staphyli- nus, Philonthus, Elater, Lampyris, Silpha, Hydrophilus, Aphodius, Melolontha, Cetonia, Curculio; — Orthoptera, Forficula (Earwig), Blatta {Cockroach) ; — Lepidoptera, Sphinges (Hawk-moth), and Nocturna (Moths) of various kinds, the large ' plumed' antennae of the latter being peculiarly beautiful objects under a low magnifying power; — Diptera, Culicidae (Gnats of various kinds), Tipulidae (Crane-flies and Midges), Tabanus, Eristalis, and Muscidae (Flies of various kinds). All the larger antennae, when not mounted f dry ' as opaque objects, should be put up in Balsam, after being soaked for some time in turpentine; but the small feathery antennae of Gnats and Midges are so liable to distortion when thus mounted, that it is better to set them up in fluid, the head with its pair of antennae being thus preserved together when not too large. — A curious set of organs has been recently discovered in the antennae of many Insects, which have been supposed to constitute collectively an apparatus for Hearing. Each consists of a cavity hollowed out in the norny integument, sometimes nearly spherical, sometimes flask-shaped, and sometimes prolonged into numerous extensions formed by the folding of its lining membrane; the mouth of the cavity seems to be normally closed-in by a continuation of this membrane, though its presence can- not always be satisfactorily determined; whilst to its deepest part a nerve- fibre may be traced. The expanded .lamellae of the antennae of Melolontha present a great display of these cavities, which are indicated in Fig. 427, A, by the small circles that beset almost their entire area; their form, which is very peculiar, can here be only made out by vertical sections; but in many of the smaller antennae, such as those of the Bee, the cavities can be seen sideways without any other trouble than that of bleaching the specimen to render it more transparent.1 629. The next point in the organization of Insects to which the attention of the Microscopist may be directed, is the structure of the mouth. Here, again, we find almost infinite varieties in the details of •conformation; but these may be for the most part reduced to a small number of types or plans, which are characteristic of the different orders of Insects. It is among the CoUoptera, or Beetles, that we find the several parts of which the mouth is composed, in their most distinct form; for although some of these parts are much more highly developed in other Insects, other parts may be so much altered or so little developed as to be scarcely recognizable. The Coleoptera present the typical con- formation of the mandibulate mouth, which is adapted for the prehen- sion and division of solid substances; and this consists of the following parts: — 1, a pair of jaws, termed mandibles, frequently furnished with 1 See the Memoir of Dr. Hicks ' On a new Structure in the Antennae of Insects,' in " Trans, of Linn. Soc.," Vol. xxii., p. 147; and his « Further Remarks,' at p. 383 of the same volume. See also the Memoir of M. Lespes, ' Sur 1'Appareil Auditif des Insectes,' in "Ann. des Sci. Nat.," Ser. 4, Zool., Tom. ix., p. 258; and that of M. Claparede, * Sur les pretendus Organes Auditifs des coleopteres lamelhcornes et autres Insectes,' in "Ann. des. Sci. Nat.," Ser. 4, Zool., Tom. x., p. 236. Dr. Hicks lays great stress on the * bleaching process,' as essential to success in this investi- gation; and he gives the following directions for performing it: — Take of Chlorate of Potass a drachm, and of Water a drachm and a half; mix these in a small wide bottle containing about an ounce; wait five minutes, and then add about a drachm and a half of strong Hydrochloric Acid. Chlorine is thus slowly developed; and the mixture will retain its bleaching power for some time. 234: THE MICROSCOPE AND ITS REVELATIONS. powerful teeth, opening laterally on either side of the mouth, and serv- ing as the chief instruments of manducation; 2, a second pair of jaws, termed maxillce, smaller and weaker than the preceding, beneath which they are placed, and serving to hold the food, and to convey it to the back of the mouth; 3, an upper lip, or Idbrum; 4, a lower lip or labium; 5, one or two pairs of small jointed appendages termed palpi, attached to the maxillae, and hence called maxillary palpi; 6, a pair of labial palpi. The labium is often composed of several distinct parts; its basal portion being distinguished as the mentum or chin, and its anterior portion being sometimes considerably prolonged forwards, so as to form an organ which is properly designated the liqida, but which is more commonly known as the ' tongue/ though not really entitled to that designation, the real tongue being a soft and Drojecting organ which forms the floor of the FIG. 428. Tongue of common Fly:— a, lobes of ligula; 6, portion inclosing the lancets, formed by the- metamorphosis of the maxillae; c, maxillary palpi:— A, portion of one of the pseudotracheae enlarged. mouth, and which is only found as a distinct part in a comparatively small number of Insects, as the Cricket. — This ligula is extremely devel- oped in the Fly kind, in which it forms the chief part of what is com- monly called the 'proboscis' (Fig. 428) ;J and it also forms the ' tongue' of the Bee and its allies (Fig. 429). The ligula of the common FJy presents a curious modification of the ordinary tracheal structure 1 The representation given in the figure is taken from one of the ordinary prep- arations of the Fly's proboscis, which is made by slitting it open, flattening it out, and mounting it in Balsam. For representations of the true relative positions of the different parts of this wonderful organ, and for minute descriptions of them, the reader is referred to Mr. Suffolk's Memoir ' On the Proboscis of the Blow-fly/ in " Monthly Microsc. Journ.," Vol. i., p. 331; and to Mr. Lowne's Treatise on " The Anatomy and Physiology of the Blow-fly," p. 41. INSECTS AND ARACHNID A. 235 (§ 634), the purpose of which is not apparent; for instead of its tra- cheae being kept pervious, after the usual fashion, by the winding of a continuous spiral fibre through their interior, the fibre is broken into rings, and these rings do not surround the whole tube, but are termi- nated by a set of arches that pass from one to another (Fig. 428, A).1 — In the Diptera or two-winged Mies generally, the labrum, maxillae, mandibles, and the internal tongue (where it exists) are converted into- delicate lancet-shaped organs termed setce, which, when closed-together, are received into a hollow on the upper side of the labium (Fig. 428, £), but which are capable of being used to make punctures in the skin of Animals or the epidermis of Plants, whence the juices may be drawn forth by the proboscis. Frequently, however, two or more of these organs may be wanting, so that their number is reduced from six, to four, three, or two. — In the Hymenoptera (Bee and Wasp tribe), the labrum and the mandibles (Fig. 429, I) much resemble those of Mandibulate Insects, and are used for corresponding purposes; the maxillae (c) are greatly elongated, and form, when closed, a tubular sheath for the Ligula or 'tongue,' through which the honey is drawn up; the labial (d) also are greatly developed, FIG. 429. ana fold together, like the maxillae, so as to form an inner sheath for the ' liula ' itself (e) ' tongue;' while the **&. is a long tapering muscular organ, marked by an immense number of short annular divisions, and densely covered over its own length with long hairs (B). It is not tubular, as some have stated, but is solid; when actively employed in taking food, it is extended to a great distance beyond the other parts of the mouth; but when at rest it is closely packed-lip and Concealed between the prolon^eTlabiumrcommolQiy'terme'd'the maxillae. < < The manner," says Mr. New- g^^S^^&S?*" °f th* port, "in which the honey is obtained when the organ is plunged into it at the bottom of a flower, is by ' lapping,' or a constant succession of short and quick extensions and contractions of the organ, which occasion the fluid to accumulate upon it and to ascend along its upper surface, until it reaches the orifice of the tube formed by the approximation of the maxillae above, and of the labial palpi and this part of the ligula below." 630. By the plan of conformation just described, we are led to that 1 According to Dr. Anthony (" Monthly Micros. Journ.," Vol. xi., p. 242), these ' pseudo-tracheae ' are suctorial organs, which can take-in liquid alike at their ex- tremities and through the whole length of the fissure caused by the interruption of the rings; the edges of this fissure being formed by the alternating series of * ear-like appendages,' connected with the terminal ' arches,' the closing-together of which converts the pseudo-trachea into a complete tube. Dr. A. considers each of these ear-like appendages to be a minute sucker, "either for the adhesion of the fleshy tongue, or for the imbibition of fluids, or perhaps for both purposes." —The point is well worthy of further investigation. THE MICROSCOPE AND ITS REVELATIONS. which prevails among the Lepidoptera or Butterfly tribe, and which, being pre-eminently adapted for suction, is termed the hau stellate mouth. In these Insects, the labrum and mandibles are reduced to three minute triangular plates; whilst the maxillae are immensely elongated, and are united together along the median line to form the haustellium or true * proboscis/ which contains a tube formed by the junction of the two grooves that are channelled out along their mutually applied surfaces, and which serves to pump-up the juices of deep cup-shaped flowers, into which the size of their wings prevents these insects from entering. The length of this haustellium varies greatly: thus in such Lepidoptera as take no food in their perfect state, it is a very insignificant organ; in some of the white Hawk-moths, which hover over blossoms without .alighting, it is nearly two inches length; and in most Butterflies and Moths it is about as long as the body itself. This ' haustellium,' which, when not in use, is coiled-up in a spiral beneath the mouth, is an extremely beautiful Microscopic object, owing to the peculiar banded arrangement it exhibits (Fig. 430), which is probably due to the disposition of its mus- cles. In many instances, the two halves may be seen to be locked together FIG. 430. Haustellium (proboscis) of Vanessa. "by a set of hooked teeth, which are inserted into little depressions between the teeth of the opposite side. Each half, moreover, may be ascertained to contain a trachea or air- tube (§ 634); and it is probable, from the observations of Mr. Newport, that the sucking-up of the juices of a flower through the proboscis (which is accomplished with great rapidity) is effected by the agency of the respiratory apparatus. The proboscis of many Butterflies is furnished, for some distance from its extremity, with a double row of small projecting barrel-shaped bodies (shown in Fig. 430), which are surmised by Mr. Newport (whose opinion is confirmed by the kindred inquiries of Dr. Hicks, § 628) to be organs of taste.— Numerous other modifications of the structure of the mouth, existing in the different tribes of Insects, are well worthy of the careful study of the Microscopist; but as detailed descriptions of most of these will be found in every Systematic Treatise on Entomology, the foregoing general ac- count of the principal types must suffice. 631. Parts of the Body. — The conformation of the several divisions of the alimentary canal presents such a multitude of diversities, not only INSECTS AND ARACHNIDA. 237 in different tribes of Insects, but in different states of the same individual, that it would be utterly vain to attempt here to give even a general idea of it; more especially as it is a subject of far less interest to the ordinary Micro- scopist, than to the professed Anatomist. Hence we shall only stop to mention that the ' muscular gizzard ' in which the oesophagus very com- monly terminates, is often lined by several rows of strong horny teeth for the reduction of the food, which furnish very beautiful objects, espe- cially for the Binocular. These are particularly developed among the Grasshoppers, Crickets, and Locusts, the nature of whose food causes them to require powerful instruments of its reduction. 632. The Circulation of Blood may be distinctly watched in many of the more transparent larvae, and may sometimes be observed in the perfect insect. It is kept up, not by an ordinary heart, but by a 'dorsal vessel' (so named from the position it always occupies along the middle of the back), which really consists of a succession of muscular hearts or contractile cavities, one for each segment, opening one into another from behind forwards, so as to form a continuous trunk divided by valvular partitions. In many larvae, however, these partitions are very indistinct; and the walls of the ' dorsal vessel ' are so thin and transparent, that it can with difficulty be made-out, a limitation of the light by the dia- phragm being often necessary. The blood which moves through this trunk, and which is distributed by it to the body, is a transparent and nearly-colorless fluid, carrying with it a number of 'oat shaped' corpus- cles, by the motion of which its flow can be followed. The current enters the ' dorsal vessel ' at its posterior extremity, and is propelled forwards by the contractions of the successive chambers, being prevented from moving in the opposite direction by the valves between the chambers, which only open forwards. Arrived at the anterior extremity of the * dor- sal vessel,' the blood is distributed in three principal channels; a central one, namely, passing to the head, and a lateral one to either side; de- scending so as to approach the lower surface of the body. It is from the- two lateral currents that the secondary streams diverge, which pass into- the legs and wings, and then return back to the main stream; and it i& from these also, that, in the larva of the Ephemera tnarginata (Day-fly), the extreme transparence of which renders it one of the best of all sub- jects for the observation of Insect Circulation, the smaller currents diverge into the gill-like appendages with which the body is furnished (§ 636). The blood-currents seem rather to pass through channels excavated among the tissues, than through vessels with distinct walls; but it is not improb- able that in the perfect Insect the case may be different. In many aqua- tic larvae, especially those of the Culicidce (Gnat tribe), the body is almost entirely occupied by the visceral cavity; and the blood may be seen to- move backwards in the space that surrounds the alimentary canal, which here serves the purpose of the channels usually excavated through the solid tissues, and which freely communicates at each end with the ' dor- sal vessel.' This condition strongly resembles that found in many Anne- lida.1 633. The circulation may be easily seen in the wings of many Insects in their pupa state, especially in those of the Neuroptera (such as Dragon - flies, and Day-flies), which pass this part of their lives under water in a JSee the Memoirs on Corethra plumicornis, by Prof. Rymer Jones, in "Trans- act, of Microsc. Soc.," N.S., Vol. xv. (1867), p. 99; by Prof. E. Ray Lankester, in the " Popular Science Review" for October, 1865; and by Dr. A. Weissmann, in. "Siebold and Kolliker's Zeitschrift," Bd. xvi., p. 45. THE MICROSCOPE AND ITS REVELATIONS. condition of activity; the pupa of Agrion puella, one of the smaller dra- gon-flies, being a particularly favorable subject for such observations. Each of the 'nervures' of the wings contains a ( trachea' or air-tube (§ 634), which branches-off from the trached system of the body; and it is in a space around the trachea that the blood may be seen to move, when the hard framework of the nervure itself is not too opaque. The same may be seen, however, in the wings of pupae of Bees, Butterflies, etc., Avhich remain shut-up motionless in their cases; for this condition of apparent torpor is one of great activity of their nutritive system, — those organs, especially, which are peculiar to the perfect Insect, being then in a state of rapid growth, and having a vigorous circulation of blood through them. In certain insects of nearly every order, a movement of fluid may be seen in the wings for some little time after their last meta- morphosis; but this movement soon ceases, and the wings dry-up. The common Fly is as good a subject for this observation as can be easily found; it must he caught within a few hours or days of its first appear- ance; and the circulation maybe most conveniently brought into view by inclosing it (without water) in the aquatic box, and pressing-down the cover sufficiently to keep the body at rest without doing it any injury. 634. The Respiratory apparatus of Insects affords a very interesting series of Microscopic objects; for, with great uniformity in its general plan there is almost infinite variety in its details. The aeration of the blood in this class is provided-for, not by the transmission of the fluid to any special organ representing the lung of a Vertebrated animal (§ 692) or the gill of a Mollusk (§ 586), but by the introduction of air into every part of the body, through a system of minutely-distributed trachea or air- tubes, which penetrate even the smallest and most delicate organs. Thus, as we have seen, they pass into the haustellium or f proboscis ' of the Butterfly (§ 630), and they are minutely distributed in the elongated Idbium or ' tongue ' of the Fly (Fig. 428). Their general distribution is shown in Fig. 431; where we see two long trunks (f) passing from one •end of the body to the other, and connected with each other by a trans- verse canal in every segment; these trunks communicate on the one hand, by short wide passages, with the ' stigmata,' 'spiracles,' or ' breathing pores' (g}, through which the air enters and is discharged; whilst they give off branches to the different segments, which divide again and again into ramifications of extreme minuteness. They usually communicate also with a pair of air-sacs (fi) which is situated in the thorax; but the size of these (which are only found in the perfect Insect, no trace of them existing in the larvae) varies greatly in different tribes, being usually greatest in those insects which (like the Bee) can sustain the longest and most powerful flight, and least in such as habitually live upon the ground or upon the surface of the water. The structure of the air- tubes reminds us of that of the 'spiral vessels' of Plants, which seem destined (in part at least) to perform a similar office (§ 362); for within the membrane that forms their outer wall, an elastic fibre winds round and round, so as to form a spiral closely resembling in its position and functions the spiral wire-spring of flexible gas-pipes; within this again, however, there is another membranous wall to the air-tubes, so that the spire winds between their innner and outer coats. — When a portion of one of the great trunks with some of the principal branches of the trachea! system has been dis- sected-out, and so pressed in mounting that the sides of the tubes are flattened against each other (as has happened in the specimen represented in Fig. 432), the spire forms two layers which are brought into close INSECTS AND ARACHN1DA. 239 apposition; and a very beautiful appearance, resembling that of watered silk, is produced by the crossing of the two sets of fibres, of which one overlies the other. iThat this appearance, however, is altogether an opti- cal illusion, may be easily demonstrated by carefully following the course of any one of the fibres, which will be found to be perfectly regular. FIG. 431. FIG. 432. Portion of a large Trachea of Dytiscus, with some of its principal branches. FIG, 433. ;- Tracheal system of Nepa (Water-scor- pion):—a, head; 6, first pair of legs; c, first segment of thorax; d, second pair of wings; e, second pair of legs; /, tracheal trunk: gr, one of the stigmata; h, air-sac. Spiracle of Common Fly. 635. The ' stigmata' or 'spiracles' through which the air enters the tracheal system, are generally visible on the exterior of the body of the insect (especially on the abdominal segments) as a series of pores along •each margin of the under surface. In most larvae, nearly every segment is provided with a pair; but in the perfect insect several of them remain, closed, especially in the thoracic region, so that their number is often considerably reduced. The structure of the spiracles varies greatly in re- gard to complexity in different insects; and even where the general plan is the same, the details of conformation are peculiar, so that perhaps in scarcely any two species are they alike. Generally speaking they are fur- nished with some kind of sieve at their entrance, by which particles of 240 THE MICROSCOPE AND ITS REVELATIONS. dust, soot, etc., which would otherwise enter the air-passages, are filtered out; and this sieve may be formed by the interlacement of the branches of minute arborescent growths from the border of the spiracle, as in the common Fly (Fig. 433), or in the Dytiscus; or it may be a membrane perforated with minute holes, and supported upon a framework of bars that is prolonged in like manner from the thickened margin of the aper- ture (Fig. 434), as in the larva of the Melolontha (Cockchafer). Not unfrequently, the centre of the aperture is occupied by an impervious disk, from which radii proceed to its margin, as is well seen in the spira- cle of Tipula, (Crane-fly). — In those aquatic Larvae which breathe air, we often find one of the spiracles of the last segment of the abdomen prolonged into a tube, the mouth of which remains at the surface while the body is immersed; the larvae of the Gnat tribe may frequently be observed in this position. 636. There are many aquatic Larvae, however, which have an entirely- different provision for respiration; being furnished with external leaf -like or brush-like appendages into which the tracheae are prolonged, so that, by absorbing air from the water that FIG- 434. bathes them, they may convey this into the interior of the body. We cannot have a better example of this than is afforded by the larva of the common Ephemera (Day-fly), the body of which is furnished with a set of branchial appendages resem- bling the 'fin-feet' of Branchiopods (§ 603), whilst the three-pronged tail also is fringed with clusters of delicate hairs which appear to minister to the same function. In the larva of the Libellula (Dragon-fly), the extension of the surface __ for aquatic respiration takes place with- spiracie of Larva of Cockchafer. in the termination of the intestine; the lining membrane of which is folded into an immense number of plaits, each containing a minutely ramified system of tracheae; the water, slowly drawn-in through the anus for bathing this surface, is ejected with such violence that the body is impelled in the opposite direction; and the air taken-up by its tracheae- is carried, through the system of the air-tubes of which they form-part, into the remotest organs. This apparatus is a peculiarly interesting object for the Microscope, on account of the extraordinary copiousness of the distribution of the tracheae in the intestinal folds. 637. The main trunks of the tracheal system, with their principal ramifications, may generally be got-out with little difficulty, by laying- open the body of an Insect or Larva under water in a Dissecting-trough (§ 180), and removing the whole visceral mass, taking care to leave as many as possible of the branches which will be seen proceeding to this from the two great longitudinal tracheae, to whose position these branches will serve as a guide. Mr. Quekett recommends the following as the most simple method of obtaining a perfect system of tracheal tubes from a larva: — a small opening having been made in its body, this is to be placed in strong acetic acid, which will soften or decompose all the vis- cera; and the tracheae may then be well- washed with the syringe, and removed from the body with the greatest facility, by cutting away the connections of the main tubes with the spiracles by means of fine pointed INSECTS AND ARACHNIDA. scissors. In order to mount them, they should be floated upon the slide, on which they should then be laid-out in the position best adapted for displaying them. If they are to be mounted in Canada balsam, they should be allowed to dry upon the slide, and should then be treated in the usual way; but their natural appearance is best preserved by mount- ing them in fluid (weak spirit or Goadby's solution), using a shallow cell to prevent pressure. The finer ramifications of the tracheal system may generally be seen particularly well in the membranous wall of the stom- ach or intestine; and this, having been laid-out and dried upon the glass, may be mounted in balsam so as to keep the tracheae full of air (whereby they are much better displayed), if care be taken to use balsam that has been previously thickened, to drop this on the object without liquefying it more than is absolutely necessary, and to heat the slide and the cover (the heat may be advantageously applied directly to the cover, after it has been put-on, by turning-over the slide so that its upper face shall look downward) only to such a degree as to allow the balsam to spread and the cover to be pressed-down. — The spiracles are easily dis- sected-out by means of a pointed knife or a pair of fine scissors; they should be mounted in glycerine-jelly when their texture is soft, and in balsam when the integument is hard and horny. 638. Wings. — These organs are essentially composed of an extension of the external membranous layer of the integument, over a framework formed by prolongations of the inner horny layer, within which prolon- gations tracheae are nearly always to be found, whilst they also include channels through which blood circulates during the growth of the wing and for a short time after is completion (§ 633). This is the simple structure presented to us in the Wings of Neuroptera (Dragon-flies, etc.), Hymenoptera (Bees and Wasps), Diptera (two-winged-Flies, and also of many Homoptera (Cicadas and Aphides); and the principal interest of these wings as Microscopic objects lies in the distribution of their 'veins' or ' nervures ' (for by both names are the ramifications of their skeleton known), and in certain points of accessory structure. The venation of the wings is most beautiful in the smaller Neuroptera; since it is the dis- tinguishing feature of this order that the veins, after subdividing, reunite again, so as to form a close network; whilst in the Hymenoptera and Diptera such reunions are rare, especially towards the margin of the wings, and the areolae are much larger. Although the membrane of which these wings are composed appears perfectly homogeneous when viewed by transmitted light, even with a high magnifying power, yet, when viewed by light reflected obliquely from their surfaces, an appearance of cellular areolation is often discernible; this is well seen in the common Fly, in which each of these areolse has a hair in its centre. In order to make this observation, as well as to bring-out the very beautiful iridescent hues which the wings of many minute Insects (as the Aphides) exhibit when thus viewed, it is convenient to hold the wing in the Stage-forceps for the sake of giving it every variety of inclination; and when that position has been found which best displays its most interesting features, it should be set up as nearly as possible in the same. For this purpose it should be mounted on an opaque slide; but instead of being laid down upon its surface, the wing should be raised a little above it, its ' stalk ' being held in the proper position by a little cone of soft wax, in the apex of which it may be imbedded. — The wings of most Hymenoptera are remarkable for the peculiar apparatus by which those of the same side are connected together, so as to constitute in flight but one large wing; this consists of 16 242 THE MICROSCOPE AND ITS REVELATIONS. a row of curved booklets on the anterior margin of the posterior wing, which lay hold of the thickened and doubled-down posterior edge of the anterior wing. These booklets are sufficiently apparent in the wings of the common Bee, when examined with even alow magnifying power; bat they are seen better in the Wasp, and better still in the Hornet. — The peculiar scaly covering of the wings of the Lepidoptera has already been noticed (§ 619); but it may here be added that the entire wings of many of the smaller and commoner insects of this order, such as the Tineidce or ' clothes-moths,' form, very beautiful opaque objects for low powers; the most beautiful of all being the divided wings of the Fissipe tines or ' plumed moths/ especially those of the genus Pterophorus. 639. There are many Insects, however, in which the Wings are more or less consolidated by the interposition of a layer of horny substance between the two layers of membrane. This plan of structure is most fully carried-out in the Coleoptera (Beetles), whose anterior wings are metamorphosed into elytra or ' wing-cases-/ and it is upon these that the brilliant hues by which the integument of many of these insects is dis- tinguished are most strikingly displayed. In the anterior wings of the ForficulidcB or Ear wig- tribe (which form the connecting link between this order and the Orthoptera), the cellular structure may often be readily distinguished when they are viewed by transmitted light, especially after having been mounted in Canada balsam. The anterior wings of the Orthoptera (Grasshoppers, Crickets, etc.), although not by any means so solidified as those of Coleoptera, contain a good deal of horny matter; they are usually rendered sufficiently transparent, however, by Canada balsam, to be viewed with transmitted light; and many of them are so colored as to be very showy objects (as are also the posterior fan-like wings) for the Electric or Gas-microscope, although their large size, and the absence of any minute structure, prevent them from affording much interest to the ordinary Microscopist. — We must not omit mention, how- ever, the curious Sound-producing apparatus which is possessed by most insects of this order, and especially by the common House-cricket. This consists of the ( tympanum ' or drum, which is a space on each of the upper wings, scarcely crossed by veins, but bounded externally by a large dark vein provided with three or four longitudinal ridges; and of the ' file ' or ' bow/ which is a transverse horny ridge in front of the tym- panum, furnished with numerous teeth: and it is believed that the sound is produced by the rubbing of the two bows across each other, while its intensity is increased by the sound-board action of the tympanum. — The wings of the Fulgoridm (Lantern-flies) have much the same texture with those of Orthoptera, and possess about the same value as Microscopic objects; differing considerably from the purely membranous wings of the Cicadse and Aphides, which are associated with them in the order Homop- tera. In the order Hemiptera, to which belong various kinds of land and water Insects that have a suctorial mouth resembling that of the common bug, the wings of the anterior pair are usually of parch men ty consistence, though membranous near their tips, and are often so richly colored as to become very beautiful objects, when mounted in Balsam and viewed by transmitted light; this is the case especially with the ter- restrial vegetable-feeding kinds, such as the Pentatoma and its allies, some of the tropical forms of which rival the most brilliant of the Beetles. The British species are by no means so interesting; and the aquatic kinds, which, next to the bed-bugs, are the most common, always have a dull brown or almost black hue: even among these last, however, — of which INSECTS AND ARACHNIDA. 243 the Notonecta (water-boatman) and the Nepa (water-scorpion) are well- known examples, — the wings are beautifully variegated by differences in the depth of that hue. The halteres of the Diptera, which are the representatives of the posterior wings, have been shown by Dr. J. B. Hicks to present a very curious structure, which is found also in the elytra of Coleoptera and in many other situations; consisting in a multi- tude of vesicular projections of the superficial membrane, to each of which there proceeds a nervous filament, that comes to it through an aperture in the tegumentary wall on which it is seated. Various con- siderations are stated by Dr. Hicks, which lead him to the belief that this apparatus, when developed in the neighborhood of the spiracles or breathing-pores, essentially ministers to the sense of smell, whilst, when developed upon the palpi and other organs in the neighborhood of the mouth, it ministers to the sense of taste.1 640. Feet. — Although the feet of Insects are formed pretty much on one general plan, yet that plan is subject to considerable modifications, in accordance with the habits of life of different species. The entire limb usually consists of five divisions, namely the coxa or hip, the tro- chanter, the femur or thigh, the tibia or shank, and the tarsus or foot; FIG. 435. and this last part is made up of several successive joints. The typical number of these joints seems to be five; but that number is subject to reduction; and the vast order Cole- optera is subdivided into primary groups, according as the tarsus con- sists of five, four, or three segments. The last joint of the tarsus is usually furnished with a pair of strong hooks or claws (Figs. 435, 436); and these are often serrated (that is, fur- nished with saw-like teeth), especially near the base. The under-surface of the other joints is frequently beset with tufts of hairs, which are arranged in various modes, sometimes Foot of Fly. forming a complete 'sole;' this is especially the case in the family Curculionidce; so that a pair of the feet of the ' diamond-bettle/ mounted so that one shows the upper surface made resplendent by its jewel-like scales, and the other the hairy cushion beneath, is a very interesting object. In many Insects, especially of the Fly kind, the foot is furnished with a pair of membranous expansions termed pulvilli (Fig. 435); and these are beset with numerous hairs, each of which has a minute disk at its extremity. This structure is evidently connected with the power which these Insects possess of walking over smooth surfaces in opposition to the force of gravity; yet there is still considerable uncertainty as to the precise mode in which it ministers to 1 See his Memoir 'On a new Organ in Insects,' in "Journal of Linnsean Society," Vol. i. (1856), p. 136; his ' Further Remarks on the Organs found on the bases of the Halteres and Wings of Insects,' in " Trans, of the Linn. Society," Vol. xxii. , p. 141 ; and his Memoir ' On certain Sensory Organs in Insects, hitherto undescribed,' in "Trans of Linn. Soc.," Vol. xxiii., p. 189. 244: THE MICROSCOPE AND ITS REVELATIONS. FIG. 436. this faculty. Some believe that the disks act as suckers, the Insect being held-up by the pressure of the air against their upper surface, when a vacuum is formed beneath; whilst others maintain that the adhesion is the result of the secretion of a viscid liquid from the under side of the foot. The careful observations of Mr. Hepworth have led him to a con- clusion which seems in harmony with all the facts of the case — namely, that eacn hair is a tube conveying a liquid from a glandular sacculus situated in the tarsus; and that when the disk is applied to a surface, the pouring-forth of this liquid serves to make its adhesion perfect. That this adhesion is not produced by atmospheric pressure alone, is proved by the fact that the feet of Flies continue to hold-on to the interior of an exhausted receiver; whilst, on the other hand, that the feet pour-forth a secreted fluid, is evidenced by the marks left by their attachment on a clean surface of glass. Although, when all the hairs have the strain put upon them equally, the adhesion of their disks suffices to support the insect, yet each row may be detached separately by the gradual raising of the tarsus and pulvilli, as when we remove a piece of adhesive plaster by lifting it from the edge or corner. Flies are often found adherent to window-panes in the autumn, their reduced strength not being sufficient to enable them to detach their tarsi.1 — A similar ap- paratus on a far larger scale, presents itself on the foot of the Dytiscus (Fig. 436, A). The first joints of the tarsus of this insect are widely ex- panded, so as to form a nearly-circular plate: and this is provided with a very remarkable apparatus of suckers, of which one disk (a) is extremely large, and is furnished with strong rad- iating fibres, a second (b) is a smaller one formed on the same plan (a third, of the like kind, being often present), whilst the greater number are comparatively small tubular club-shaped bodies, each having a very delicate membranous sucker at its extremity, as shown on a larger scale at B. These all have essentially the same structure; the large suckers being furnished, like the hairs of the Fly's foot, with secret- ing sacculi, which pour forth fluid through the tubular footstalks that carry the disks, whose adhesion is thus secured; whilst the small suckers form the connecting link between the larger suckers and the hairs of many beetles, especially Curculionidce.* The leg and foot of the Dytiscus, if 1 See Mr. Hepworth's communications to the " Quart. Journ. of Microsc. Science," Vol. ii. (1854;, p. 158, and Vol. iii. (1855), p. 312. See also Mr. Tuffen West's Memoir ' On the Foot of the Fly, 'in " Transact, of Linnaean Society," Vol. xxii., p. 393, and Mr. Lowne's " " A, Foot of Dytiscus, showing its apparatus of suckers; a, b, large suckers; c, ordinary suckers: — B, one of the ordinary suckers more highly magnified. Anatomy of the Blow-fly," p. 19. -called Sucke Monthly Microscopical Journal," Vol. v., p. 267. ., . , 2 See Mr. Lowne ' On the so-called Suckers of Dytiscus and the Pulvilli of Insects,' in " " INSECTS AND ARACHNIDA. 245 mounted without compression, furnish a peculiarly beautiful object for the Binocular Microscope. — The Feet of Caterpillars differ consider- ably from those of perfect Insects. Those of the first three segments, which are afterwards to be replaced by true legs, are furnished with strong horny claws; but each of those of the other segments, which are termed 6 pro-legs/ is composed of a circular series of comparatively slender curved hooklets, by which the Caterpillar is enabled to cling to the minute rough- nesses of the surface of the leaves, etc., on which it feeds. This structure is well seen in the pro-legs of the common Silkworm. 641. Stings and Ovipositors. — The insects of the order Hymenoptera are all distinguished by the prolongation of the last segment of the abdo- men into a peculiar organ, which in one division of the order is a * sting/ and in the other is an ' ovipositor ' or instrument for the deposition of the eggs, which is usually also provided with the means of boring a hole for their reception. The former group consists of the Bees, Wasps, Ants, etc.; the latter of the Saw-flies, Gall-flies, Ichneumon-flies, etc. These two sets of instruments are not so unlike in structure, as they are in func- tion.— The 'sting' is usually formed of a pair of darts, beset with barbed teeth at their points, and furnished at their roots with powerful muscles, whereby they can be caused to project from their sheath, which is a horny case formed by the prolongation of the integument of the last seg- ment, slit into two halves, which separate to allow the protrusion of the of the sting; whilst the peculiar 'venom' of the sting is due to the ejec- tion, by the same muscular action, of a poisonous liquid, from a bag situ- ated near the root of the sting, which passes down a canal excavated between the darts, so as to be inserted into the puncture which they make. The stings of the common Bee, Wasp, and Hornet, may all be made to display this structure without much difficulty in the dissection. —The 'ovipositor' of such insects as deposit their eggs in holes ready- made, or in soft animal or vegetable substances (as is the case with the Ichneumonidce), is simply a long tube, which is inclosed, like the sting, in a cleft sheath. In the Gall-flies ( Cynipidce), the extremity of the ovi- positor has a toothed edge, so as to act as a kind of saw whereby harder substances may be penetrated; and thus an aperture is made in the leaf, stalk, or bud of the plant or tree infested by the particular species, in which the egg is deposited, together with a drop of fluid that has a pecu- liarly irritating effect upon the vegetable tissues, occasioning the produc- tion of the 'galls,' which are new growths that serve not only to protect the larvae, but also to afford them nutriment. The oak is infested by by several species of these Insects, which deposit their eggs in different parts of its fabric; and some of the small 'galls' which are often found upon the surface of oak-leaves, are extremely beautiful objects for the lower powers of the Microscope. In the Tenthredinidto, or 'saw-flies,' and in their allies the Siricidcv, the ovipositor is furnished with a still more powerful apparatus for penetration, by means of which some of these Insects can bore into hard timber. This consists 'of a pair of 'saws' which are not unlike the 'stings' of Bees, etc., but are broader, and toothed for a greater length, and are made to slide along a firm piece that supports each blade, like the 'back' of a carpenter's 'tenon-saw;' they are worked alternately (one being protruded while the other is drawn back) with great rapidity; but, when not in use, they lie in a fissure be- neath a sort of arch formed by the terminal segment of the body. When a slit has been made by the working of the saws, they are withdrawn into this sheath; the ovipositor is then protruded from the end of the abdo- 24:6 .THE MICROSCOPE AND ITS REVELATIONS. men (the body of the insect being curved downwards); and, being guided into the slit by a pair of small hairy feelers, there deposits an egg. * Many other insects, especially of the order Diptera, have very prolonged ovipositors, by means of which they can insert their eggs into the integu- ments of animals, or into other situations in which the larvae will obtain appropriate nutriment. A remarkable example of this is furnished by the Gad-fly (Tabanus), whose ovipositor is composed of several joints, capable of being drawn together or extended like those of a telescope, and is terminated by boring instruments; and the egg being conveyed by its means, not only into but through the integument of the Ox, so as to be imbedded in the tissue beneath, a peculiar kind of inflammation is set-up there, which (as in the analogous case of the gall-fly) forms a nidus appro- priate both to the protection and to the nutrition of the larva. Other insects which deposit their eggs in the ground, such as the Locusts, have their ovipositors so shaped as to answer for digging holes for their recep- tion.— The preparations which serve to display the foregoing parts, are best seen when mounted in Balsam; save in the case of the muscles and poison-apparatus of the sting, which are better preserved in fluid or in glycerine-jelly. 642. The Sexual organs of Insects furnish numerous objects of extreme interest to the Anatomist and Physiologist; but as an account of them would be unsuitable to the present work, a reference to a copious source of information respecting one of their most curious features, and to a list of the species that afford good illustrations, must here suffice. 2- The eggs of many Insects are objects of great beauty, on account of the regularity of their form, and the symmetry of the markings on their sur- face (Fig. 437). The most interesting belong for the most part to the order Lepidoptera; and there are few among these that are not worth examination, some of the commonest (such as those of the Cabbage butter- fly, which are found covering large patches of the leaves of that plant) being as remarkable as any. Those of the Puss- moth ( Cirura vinula), the Privet hawk-moth (Sphinx ligustri), the small Tortoiseshell butterfly ( Vanessa urticce), the Meadow-brown butterfly (Hipparchiajanira), the Brimstone-moth (Rumia cratmgatd), and the Silkworm (Bomlyx mori), may be particularly specified: and from other orders, those of the Cock- roach (Blatta orientalis), Field Cricket (Acheta campestris), Water-scor- pion (Nepa ranatra), Bug ( Cimex lectularius), Cow-dung-fly (Scatophaga ster cor aria), and Blow-fly (Musca vomitorid). In order to preserve these eggs, they should be mounted in fluid in a cell; since they will otherwise dry up and may lose their shape. — They are very good objects for the ' conversion ^of relief ' effected by Nachet's Stereo-pseudoscopic Binocular (§ 38). 643. The remarkable mode of Reproduction that exists among the Aphides must not pass unnoticed here, from its curious connection with 1 The above is the account of the process given by Mr. J. W. Gooch; who has informed the Author that he has repeatedly verified the statement formerly made by him (" Science Gossip," Feb. 1, 1873), that the eggs are deposited, not as origi- nally stated by Reaumur, by means of a tube formed by the coaptation of the saws, but through a separate ovipositor, protruded when the saws have been with- drawn. 2 See the Memoirs of M. Lacaze-Dutheirs, ' Sur 1'armure genitale des Insectes,' in "Ann. des Sci. Nat.," Ser. 3, Zool., Tomes xii., xiv., xvii., xyiii., xix.; and M. Ch. Robin's " Memoire sur les Objets qui peuvent etre conserves en Preparations Microscopiques " (Paris, 1856), which is peculiarly full in the enumeration of the objects of interest afforded by the Class of Insects. INSECTS AND ARACHNID A. 247 the non-sexual reproduction of Entomostraca (§ 609) and Rotifera (§ 451) as also of Hydra (§ 515) and Zoophytes generally; all of which fail spe- cially, most of them exclusively, under the observation of the Microscopist. The Aphides which may be seen in the spring and early summer, and which are commonly but not always wingless, are all of one sex, and give birth to a brood of similar Aphides, which come into the world alive, and before long go through a like process of multiplication. As many as from seven to ten successive broods may thus be produced in the course of a single season; so that from a single Aphis, it has been calcu- lated that no fewer than ten thousand million millions, may be evolved within that period. In the latter part of the year, however, some of these viviparous Aphides attain their full development into males and females; and these perform the true Generative process, whose products are eggs, which, when hatched in the succeeding spring, give origin to a new vivi- parous brood that repeat the curious life-history of their predecessors. It appears from the observations of Prof. Huxley,1 that the broods of viviparous Aphides originate in ova which are not to be distinguished from those deposited by the perfect winged female. Nevertheless, this non-sexual or agamic reproduction must be considered analogous rather to the < gemmation ' of other Animals and Plants, than to their sexual FIG. 437. Eggs of Insects, magnified;— A, PontianapijB, Vanessa urticce;c, Hipparchiatithous; D Aravn- nis Lathonia. ( generation;' for it is favored, like the gemmation of Hydra, by warmth and copious sustenance, so that by appropriate treatment the viviparous reproduction may be caused to continue (as it would seem) indefinitely, without any recurrence to the sexual process. Further, ib seems now certain that this mode of reproduction is not at all peculiar to the Aphides, but that many other Insects ordinarily multiply by ( agamic ' propagation, the production of males and the performance of the true generative act being only occasional phenomena; and the researches of Prof. Siebold have led him to conclude that even in the ordinary economy of the Hive-bee the same double mode of reproduction occurs. The queen, who is the only perfect female in the hive, after impregnation by one of the drones (or males), deposits eggs in the ' royal' cells> which are in due time developed into young queens; others in the drone-cells, which become drones; and others in the ordinary cells, which become workers or neuters. It has long been known that these last are really undeveloped females, which, under certain conditions, might become queens; and it has been observed by bee-keepers that worker-bees, in common with . » 1 ' On the Agamic Reproduction and Morphology of Aphis,' in " Transact, of Linn. Soc.," Vol. xxii., p. 193. 248 THE MICROSCOPE AND ITS REVELATIONS. virgin or unimpregnated queens, occasionally lay eggs, from which eggs none but drones are ever produced. From careful Microscopic examination of the drone eggs laid even by impregnated queens, Siebold drew the conclusion that they have not received the fertilizing influence of the male fluid, which is communicated to the queen-eggs and worker eggs alone; so that the products of sexual generation are always female, the male being developed from these by a process which is essentially one of gemmation.1 644. The Embryonic Development of Insects is a study of peculiar interest, from the fact that it may be considered as divided (at least in such as undergo a ' complete metamorphosis') into two stages that are separated by the whole active life of the larva; that, namely, by which the Larva is produced within the egg, and that by which the Imago of perfect insect is produced within the body of the Pupa. Various circum- stances combine, however, to render the study a very difficult one; so that it is not one to be taken up by the inexperienced Microscopist. The following summary of the process in the common Blow-fly, however, will probably be acceptable. — A gastrula with two membranous lamellae (§ 391) having been evolved in the first instance, the outer lamella very rapidly shapes itself into the form of the larva, and shows a well-marked segmental division. The alimentary canal, in like-manner, shapes itself from the inner lamella; at first being straight and very capacious, includ- ing the whole yolk; but gradually becoming narrow and tortous, as addi- tional layers of cells are developed between the two primitive lamellae, from which the other internal organs are evolved. When the larva comes forth from the egg, it still contains the remains of the yolk; it soon begins, however, to feed voraciously; and in no long period it grows to many thousand times it original weight, without making any essential progress in development, but simply accumulating material for future use. An adequate store of nutriment (analogous to the ' supplemental yolk ' of Purpura, § 584) having thus been laid up within die body of the larva, it resumes (so to speak) its embryonic development, its pas- sage into the pupa state, from which the imago is to come forth, involv- ing a degeneration of all the larval tissues: whilst the tissues and organs of the imago " are re-developed from cells which originate from the dis- integrated parts of the larva, under conditions simiJar ^o those apper- taining to the formation of the embryonic tissues from fche yolk." The development of the segments of the head and body in Insects generally proceeds from the corresponding larval segments; but according to Dr. Weissman, there is a marked exception in the case oj the Diptera and other insects whose larvae are unfurnished with legsv — their head and thorax being newly formed from 'imaginal disks' which adhere to the nerves and tracheae of the anterior extremity of the larva;2 and, strange as this assertion may seem, it has been confirmed by the subsequent in- vestigations of Mr. Lowne. 645. ARACHNTDA. — The general remarks which have been made in regard to Insects, are equally applicable to this Class; which includes, along with the Spiders and Scorpions, the tribe of Acarida, consisting of Mites and Ticks. Many of these are parasitic, and are popularly 1 See Prof. Siebold's Memoir "On true Parthenogenesis in Moths and Bees," translated by W. S. Dallas : London, 1857. 2 See his ' Entwickelung der Dipteren,' in "Kolliker and Siebold's Zeitschrif t, " Bande xiv.-xvi.; and Mr. Lowne's "Anatomy of the Blow-fly," pp. 6-9, 113-121. INSECTS AND ARACHNID A. 249 associated with the wingless parasitic Insects, to which they bear a strong general resemblance, save in having eight legs instead of six. The true 'mites' (Acarince) generally have the legs adapted for walking, and some of them are of active habits. The common cheese-mite) as seen by the naked-eye, is familiar to every one; yet few who have not seen it under a Microscope have any idea of its real conformation and move- ments; and a cluster of them, cut out of the cheese they infest, and placed under a magnifying power sufficiently low to enable a large num- ber to be seen at once, is one of the most amusing objects that can be shown to the young. There are many other species, which closely re- semble the Cheese-mite in structure and habits, but which feed upon different substances; and some of these are extremely destructive. To this group belongs a small species, the Sarcoptes scabiei, whose presence appears to be the occasion of one of the most disgusting diseases of the skin — the itch — and which is hence commonly termed the ' itch-insect.' It is not found in the pustule itself, but in a burrow which passes-off from one side of it, and which is marked by a red line on the surface; and if this burrow be carefully examined, the creature will very com- monly, but not always, be met-with. It is scarcely visible to the naked eye; but when examined under the microscope, it is found to have an oval body, a mouth of conical form, and eight feet, of which the four anterior are terminated by small suckers, whilst the four posterior end in very prolonged bristles. The male is only about half the size of the female. The Ricinice or ' ticks ' are usually destitute of eyes, but have the mouth provided with lancets, that enable them to penetrate more readily the skins of animals whose blood they suck. They are usually of a flattened, round, or oval form; but they often acquire a very large size by suction, and become distended like a blown-bladder. Different species are parasitic upon different animals; and they bury their suckers (which are often furnished with minute recurved hooks) so firmly in the skins of these, that they can hardly be detached without pulling away the skin with them. It 4s probably the young of a species of this group, which is commonly known as the ( harvest-bug,' and which is usually designated as the Acarus autumnalis; this is very common in the autumn upon grass or other herbage, and insinuates itself into the skin at the roots of the hair, producing a painful irritation; like other Acar- ida, it possesses only six legs for some time after its emersion from the egg (the other pair being only acquired after the first moult), so that its resemblance to parasitic Insects becomes still stronger. — It is probable that to this group also belongs the Demodexfolliculorum, a creature which is very commonly found parasitic in the sebaceous follicles of the Human skin, especially in those of the nose. In order to obtain it, pressure should be made upon any one of these that appears enlarged and whitish with a terminal black spot; the matter forced-out will consist principally of the accumulated sebaceous secretion, having the parasites with their eggs and young mingled with it. These are to be separated by the addi- tion of oil, which will probably soften the sebaceous matter sufficiently to set free the animals, which may be then removed with a pointed brush; but if this mode should not be effectual, the fatty matter may be dis- solved-away by digestion in a mixture of alcohol and ether. The pus- tules in the skin of a Dog affected with the ' mange' were found by Mr. Topping to contain a Demodex, which seems only to differ from that of the human sebaceous follicles in its somewhat smaller size; and M. Gruby is said to have given to a dog a disease resembling the mange, if 250 THE MICROSCOPE AND ITS REVELATIONS. not identical with" it, by inoculating it with the Human parasite. — The Acarida are best preserved, as Microscopic objects, by mounting in one or other of the 'media' described in § 206. 646. The number of objects of general interest furnished to the Microscopist by the Spider tribe, is by no means considerable. Their Eyes exhibit a condition intermediate between that of Insects and Crus- taceans, and that of Vertebrata; for they are simple, like the ' stemmata' of the former (§ 626), usually number from six to eight, are sometimes clustered- together in one mass, though sometimes disposed separately; while they present a decided approach in internal structure to the type characteristic of the visual organs of the latter. — The structure of the Mouth is always mandibulate, and is less complicated than that of the 'mandibulate' insects. — The Respiratory apparatus, which, where devel- oped at all among the Acarida, is tracheary like that of Insects, is here constructed upon a very different plan; for the f stigmata ' which are usu- ally four in number on each side, open into a like number of respiratory sacculi, each of which contains a series of leaf-like folds of its lining membrane, upon which the blood is distributed so as to afford a large surface to the air. — In the structure of the limbs, the principal point Fig. 438. Foot, with comb-like claws, of the common Spider (Epeira). worthy of notice is the peculiar appendage with which they usually ter- minate; for the strong claws, with a pair of which the last joint of the Foot is furnished, have their edges cut into comb-like teeth (Fig. 438), which seem to be used by the animal as cleansing-instruments. 647. One of the most curious parts of the organization of the Spiders, is the ' spinning-apparatus ' by means of which they fabricate their elab- orately constructed webs. This consists of the * spinnerets,' and of the glandular organs in which the fluid that hardens into the thread is elab- orated. The usual number of the spinnerets, which are situated at the posterior extremity of the body, is six; they are little teat-like promi- nences, beset with hairy appendages; and it is through a certain set of these appendages, which are tubular and terminate in fine-drawn points, that the glutinous secretion is forced-out in a multitude of streams of extreme minuteness. These streams harden into fibrils immediately on coming into contact with the air; and the fibrils proceeding from all the apertures of each spinneret coalesce into a single thread. It is doubtful, however, whether all the spinnerets are in action at once, or whether those of different pairs may not have dissimilar functions; for whilst the radiating threads of a spider's web are simple (Fig. 439, A) those which INSECTS AND ARACHNID A. 251 lie across these, forming its concentric circles, or rather polygons, are studded at intervals with viscid globules (B), which appear to give to these threads their peculiarly adhesive character; and it does not seem by any means unlikely that each kind of thread should be produced by its own pair of spinnerets. It was observed by Mr. R. Beck, that these viscid threads are of uniform thickness when first spun; but that undu- lations soon appear in them, and that the viscid matter then accumulates in globules at regular intervals . — The total number of spinning-tubes va- ries greatly, according to the species of the Spider, and the sex and age of FIG. 439. Ordinary thread (A), and viscid thread (B), of the common Spider. the individual; being more than 1000 in some cases, and less than 100 in others. The size and complexity of the secreting glandules vary in like manner: — Thus in the Spiders which are most remarkable for the large dimensions and regular construction of their webs, they occupy a large portion of the abdominal cavity, and are composed of slender branch- ing tubes whose length is increased by numerous convolutions; whilst in those which have only occasional use for their threads, the secreting organs are either short and simple follicles, or undivided tubes of moder- ate length. 252 THE MICROSCOPE AND ITS KEVELATIONS. CHAPTEE XX. VEKTEBRATED ANIMALS. 648. WE are now arrived at the highest division of the Animal King- dom, in which the bodily fabric attains its greatest development, not only as to completeness, but also as to size; and it is in most striking contrast with the Class we have been last considering. Since not only the entire bodies of Vertebrated animals, but, generally speaking, the smallest of their integral parts, are far too large to be viewed as Micro- scopic objects, we can study their structure only by a separate examina- tion of their component elements; and it seems, therefore, to be a most appropriate course to give under this head a sketch of the microscopic characters of those Primary Tissues of which their fabric is made-up, and which, although they may be traced with more or less distinctness in the lower tribes of Animals, attain their most complete development in this group.1 — For some time after Schwann first made public the remark- able results of his researches, it was very generally believed that all the Animal tissues are formed, like those of Plants, by a metamorphosis of cells; an exception being taken, however, by some Physiologists in regard to the 'simple fibrous' tissues (§ 668). There can be no longer any doubt, however, that this doctrine must be greatly modified;2 so that, whilst the Vegetable Physiologist may rightly treat the most highly organ- ized Plant as a mere aggregation of cells, analogous in all essential par- ticulars to those which singly constitute the f unicellular ' ProtopJiytes (§ 227), the Animal Physiologist does wrong in seeking a cellular origin for all the component parts of the Animal fabric; and may best interpret the phenomena of tissue-formation in the most complicated organisms, by the study of the behavior of that apparently-homogenous ' protoplasm ' of which the simplest Protozoa are made up, and by tracing the progres- sive ' differentiation ' which presents itself as we pass from this through the ascending series of Animal forms.3 1 This sketch is intended , not for the Professional student, but only for the amateur Microscopist, who wishes to gain some general idea of the elementary structure of his own body and of that of Vertebrate animals generally. Those who wish to go more deeply into the inquiry are referred to the following as the most recent and elaborate Treatises that have appeared in this country: — The translation of Striker's "Manual of Histology," published by the New Sydenham Society; the "Handbook for the Physiological Laboratory," by Drs. Burdpn- Sanderson, Michael Foster, Brunton, and Klein ; the translation of the 4th edition of Prof. Frey's " Histology and Histo-chemistry of Man;" the ' General Anatomy ' of the 8th edition of " Quain's Anatomy" (1874); and the "Atlas of Histology," by Prof. Klein and Mr. Noble Smith (1880-1). 2 The important 'Review of the Cell-Theory,' by Prof. Huxley, in the "Brit, and For. Med.-Chir. Review," Vol. xii. (Oct, 1853), p. 285, may be considered the starting-point of many later inquiries. 3 The study of Comparative Histology, prosecuted on this basis, promises to be VERTEBRATED ANIMALS. 253 649, Although tnere would at first sight appear but little in common between the simple bodies of those humble Monerozoa which constitute the lowest types of the Animal series (§ 392), and the complex fabric of Man or other Vertebrates, yet it appears from recent researches, that in the latter, as in the former, the process of ' formation' is essentially carried-on by the instrumentality of protoplasmic substance, universally diffused through it in such a manner as to bear a close resemblance to the pseudopodial network of the Ehizopod (Fig. 283); whilst the tissues pro- duced by its agency lie, as it were, on the outside of this, bearing 'the same kind of relation to it as the Foraminiferal shell (Fig. 314) does to the sarcodic substance which fills its cavities and extends itself over its surface. For, as was first pointed out by Dr. Beale,1 the smallest living ' elementary part ' of every organized fabric is composed of organic mat- ter in two states: the protoplasmic (which he termed germinal matter), possessing the power of selecting pabulum from the blood, and of trans- forming this either into the material of its own extension, or into some product which it elaborates; whilst the other, which may be termed formed material, may present every gradation of character from a mere inorganic deposit to a highly organized structure, but is in every case altogether incapable of self -increase. A very definite line of demarcation can be generally drawn between these two substances, by the careful use of the staining-process (§ 200); but there are many instances in which there is the same gradation between the one and the other, as we have have formerly noticed between the ' endosarc ' and the ' ectosarc ' of the Amoeba (§403). — Thus it is on the protoplasmic component that the existence of every form of Animal organization essentially depends; since it serves as the instrument by which the nutrient material furnished by the blood is converted into the several forms of tissue. Like the sarco- dic substance of the Rhizopods, it seems capable of indefinite extension; and it may divide and subdivide into independent portions, each of which may act as the instrument of formation of an 'elementary part.' Two principal forms of such elementary parts present themselves in the fabric of the higher Animals — namely, cells and fibres ; and it will be desirable to give a brief notice of these, before proceeding to describe those more complex tissues which are the products of a higher elaboration. 650. The cells of which many Animal tissues are essentially composed, consist, when fully and completely formed, of the same parts as the typi- exceedingly fertile in results of this most intereating character. Thus Dr. N. Kleinenberg, in his admirable " Anatomische entwickelungsgeschichtliche Un- tersuchung" (1872), on Hydra, gives strong reason for regarding a particular set of cells in the body of that animal as combining the functions of Nerve and Muscle. And the Author has been led by his study of Comatula to recognize the most elementary type of Nerve-trunk in a simple protoplasmic cord, not yet separated into distinct fibres with insulating sheaths. 1 Prof. Beale's views are most systematically expounded in his lectures « On the Structure of the simple Tissues of the Human Body," 1861; in his " How to Work with the Microscope," 5th edition, 1880; and in the Introductory portion of his new edition of " Todd and Bowman's Physiological Anatomy," 1867. The principal results of the inquiries of German Histologists on this point are well stated in a Paper by Dr. Duffin on ' Protoplasm, and the part it plays in the actions of Living Beings,' in " Quart. Journ. of Microsc. Science," Vol. iii., N.S., of any illogical to designate contractile muscular fibre (for example) as 'dead,' merely because it has not the power of self -reparation. 254: THE MICROSCOPE AND ITS REVELATIONS. cal cell of the Plant (§ 223); — viz., a definite 'cell-wall,' inclosing cell- contents (of which the nature may be very diverse), and also including a ' nucleus/ which is the seat of its formative activity. It is of such cells, retaining more or less of their characteristic spheroidal shape, that every mass of fat, whether large or small, is chiefly made up (Fig. 468). And the internal cavities of the body are lined by a layer of epithelium- cells (Fig. 466), which, although of flattened form, present the like combina- tion of components. But there is a large number of cases in which the cell shows itself in a form of much less complete development; the ' ele- mentary part ' being a corpuscle of protoplasm, of which the exterior has undergone a slight consolidation, like that which constitutes the ' pri- mordial utricle ' of the Vegetable cell (§ 223) or the 'ectosarc' of the Amoeba (§ 403), but in which there is no proper distinction between ' cell- wall ' and ' cell-contents.' This condition, which is characteristi- cally exhibited by the nearly globular colorless corpuscles of the Blood (§ 666), appears to be common to all cells in the incipient stage of their formation: and the progress of their development consists in the gradual differentiation of their parts, the ' cell-wall' becoming distinctly sepa- rated from the * cell-contents,' and these from the 'nucleus; ' and the ori- ginal protoplasm being very commonly replaced more or less completely by some special product (such as fat in the cells of adipose tissue, or haemoglobin in the red corpuscles of the blood), in which cases the nucleus often disappears altogether. — In the earlier stages of cell-develop- ment, multiplication takes place with great activity by a duplicative sub- division that corresponds in all essential particulars with that of the Plant- cell (§ 226); as is well seen in Cartilage, a section of which will often exhibit in one view the successive stages of the process1 (compare Fig. 470 with Fig. 139). Whether 'free' cell-multiplication ever takes place in the higher Animals, is at present uncertain. 651. A large part of the fabric of the higher Animals, however, is made up of fibrous tissues, which serve to bind together the other com- ponents, and which, when consolidated by calcareous deposit, constitute the substance of the skeleton. In these, the relation of the ' germinal matter ' and the ' formed material ' presents itself under an aspect which seems at first sight very different from that just described. A careful examination, however, of those 'connective-tissue-corpuscles' (Fig. 461) that have long been distinguished in the midst of the fibres of which these tissues are made up, shows that they are the equivalents of the corpuscles of 'germinal matter,' which in the previous instance came to constitute cell-nuclei; and that the fibres hold the same relation to them, that the 'walls' and ' contents' of cells do to their germinal corpuscles. The transition from the one type to the other is well seen in Fibro-cartilage, in which the so-called ' intercellular substance ' is often as fibrous as ten- don. The difference between the two types, in fact, seems essentially to consist in this — that, whilst the segments of ' germinal matter ' which form the cell-nuclei in cartilage (Fig. 470) and in other cellular tissues, are completely isolated from each other, each being completely sur- 1 Great attention has lately been given by many able observers, to the changes which take place in the nucleus before and during its cleavage. A full account of these is contained in the recently-published third Edition of Prof. Strassbur- ger's " Zellbildung und Zelltheilung " (1880). See also Dr. Klein's ' Observations on the Structure of Cells and Nuclei,' in "Quart. Journ. Microsc. Science," N.S., Vol. xviii. (1878), p. 315, and Vol. xix. (1879), pp. 125, 404; and Chap, xliv. of his " Atlas of Histology." VERTEBRATED ANIMALS. 255 rounded by the product of its own elaborating action, those which form the 'connective-tissue-corpuscles' are connected together by radiating prolongations (Fig. 461) that pass between the fibres, so as to form a continuous network closely resembling that formed by the pseudopodia of the Rhizopod (Fig. 283). — Of this we have a most beautiful example in Bone; for whilst its solid substance may be considered as connective tissue solidified by calcareous deposit, the ' lacunae' and 'canaliculi' which are excavated in it (Fig. 441) give lodgment to a set of radiating corpuscles closely resembling those just described; and these are centres of 'germinal matter/ which appear to have an active share in the for- mation and subsequent nutrition of the osseous texture. In Dentine (or tooth-substance) we seem to have another form of the same thing; the walls of its 'tubuli' and the ' intertubular substance' (§ 655) being the 'formed material' that is produced from thread-like prolongations of ' germinal matter ' issuing from its pulp, and continuing during the life of the tooth to occupy its tubes; just as in the Foraminifera we have seen a minutely-tubular structure to be formed around the individual threads of sarcode which preceded from the body of the contained ani- mal (Figs. 314, 335). It may now be stated, indeed, with considerabk confidence, that the bodies of even the highest Animals are everywhere penetrated by that sarcodic substance of which those of the lowest and simplest are entirely composed; and that this substance, which forms a continuous network through almost every portion of the fabric, is the main instrument of the Formation, Nutrition, and Reparation of the more specialized or differentiated Tissues. — As it is the purpose of this work not to instruct the professional student in Histology (or the Sci- ence of the Tissues), but to supply scientific information of general inter- est to the ordinary Microscopist, no attempt will here be made to do more than describe the most important of those distinctive characters which the principal tissues present when subjected to Microscopic examination; and as it is of no essential consequence what order is adopted, we may conveniently begin with the structure of the skeleton, l which gives sup- port and protection to the softer parts of the fabric. 652. Bone. — The Microscopic characters of osseous tissue may some- times may be seen in a very thin natural plate of bone, such as in that forming the scapula (shoulder-blade) of a Mouse; but they are displayed more perfectly by artificial sections, the details of the arrangement being dependent upon the nature of the specimen selected, and the direction in which the section is made. Thus when the shaft of a 'long' bone of a Bird or Mammal is cut-across in the middle of its length, we find it to consist of a hollow cylinder of dense bone, surrounding a cavity which is occupied by an oily marrow; but if the section be made nearer its ex- tremity, we find the outside wall gradually becoming thinner, whilst the interior, instead of forming one large cavity, is divided into a vast num- ber of small chambers, partially divided by a sort of ' lattice-work ' of osseous fibres, but communicating with each other and with the cavity of the shaft, and filled, like it, with marrow. In the bones of Reptiles and Fishes, on the other hand, this ' cancellated ' structure usually ex- tends throughout the shaft, which is not so completely differentiated into solid bone and medullary cavity as it is in the higher Vertebrata. 1 This term is used in its most general sense, as including not only the proper vertebral or internal skeleton, but also the hard parts protecting the exterior of the body, which form the dermal skeleton. 256 THE MICROSCOPE AND ITS REVELATIONS. In the most developed kind of ' flat ' bones, again, such as those of the head, we find the two surfaces to be composed of dense plates of bone, with a 'cancellated' structure between them; whilst in the less perfect type presented to us in the lower Vertebrata, the whole thickness is usu- ally more or less ' cancellated/ that is, divided-up into minute medullary cavities. — When we examine, under a low magnifying power, a longitu- dinal section of a long bone, or a section of a flat bone parallel to its surface, we find it traversed by numerous canals, termed Haver sian after their discoverer Havers, which are in connection with the central cavity, and are filled, like it, with marrow: in the shafts of 'long' bones these canals usually run in the direction of their length, but are connected here and there by cross-branches; whilst in the flat-bones they form an irregular network. — On applying a higher magnifying power to a thin transverse section of a long bone, we observe that each of the canals whose orifices present themselves in the field of view (Fig. 440), is the centre of a rod of bony tissue (1), usually more or less circular in its FIG. 440. FIG. 441. Minute structure of Bone, as seen in transverse section:—!, a rod surrounding an Haversian canal, 3, showing the concentric arrangement of the lam- ellae; 2, the same, with the lacunae and canaliculi; 4, portions of the lamellse parallel with the external surface. ft Lacunce of Osseous substance: —a, cen- tral cavity; 6, its ramifications form, which is arranged around it in concentric rings, resembling those of an Exogenus stem (Fig. 254). These rings are marked out and di- vided by circles of little dark spots, which, when closely examined (2), are seen to be minute flattened cavities excavated in the solid substance of the bone, from the two flat sides of which pass forth a number of ex- tremely minute tubules, one set extending inwards, or in the direction of the centre of the system of rings, and the other outwards, or in the direc- tion of its circumference; and by the inosculation of the tubules (or canali- culi) of the different rings with each other, a continuous communication is established between the central Haversian canal and the outermost part of the bony rod that surrounds it, which doubtless ministers to the nu- trition of the texture. Blood-vessels are traceable into the Haversian canals, but the 'canaliculi' are far too minute to carry blood-corpuscles; they are occupied, however, in the living bone, by threads of sarcodic substance, which bring the segments of 'germinal matter' contained in the lacunas into communication with the walls of the blood-vessels. VERTEBRATED ANIMALS. 257 653, The minute cavities or lacunce (sometimes, but erroneously termed ' bone-corpuscles/ as if they were solid bodies), from which the canaliculi proceed (Fig. 441), are highly characteristic of the true osse- ous structure; being never deficient in the minutest parts of the bones of the higher Vertebrata, although those of Fishes are occasionally des- titute of them. The dark appearance which they present in sections of a dried bone is not due to opacity, but is simply an optical effect, depen- dent (like the blackness of air-bubbles in liquids) upon the dispersion of the rays by the highly refracting substance that surrounds them (§ 153). The size and form of the lacunae differ considerably in the sev- eral Classes of Vertebrata, and even in some instances in the Orders; so as to allow of the determination of the tribe to which a bone belonged, by the Microscopic examination of even a minute fragment of it (§ 705). The following are the average dimensions of the lacunae, in characteris- tic examples drawn from the four principal Classes expressed in frac- tions of an inch : — Long Diameter. Man 1-1440 to 1-2400 Ostrich 1-1 333 to 1-2250 Turtle 1-375 to 1-1150 Conger-eel 1-550 to 1-1135 FIG. 442. Short Diameter. 1-4000 to 1-8000 1-5425 to 1-9650 1-4500 to 1-5840 1-4500 to 1-8000 Section of the Bony Scale of Lepidosteus :— hydrochloric acid, then macerating it for some time in pure water, and then tearing thin shreds from the residual substance, which will be found to consist of an imperfectly-fibrillated material, allied in its essential constitution to the ' white fibrous ' tissue (§ 668). 655. Teeth. — The intimate structure of the Teeth in the several Classes and Orders of Vertebrata, presents differences which are no less remarkable than those of their external form, arrangement, and succes- sion. It will obviously be impossible here to do more than sketch some of the most important of these varieties. — The principal part of the substance of all teeth is made-up of a solid tissue that has been appro- priately termed dentine. In the Shark tribe, as in many other Fishes, the general structure of this dentine is extremely analogous to that of bone; the tooth being traversed by numerous canals, which are con- tinuous with the Haversian canals of the subjacent bone, and receive blood-vessels from them (Fig. 443); and each of these canals being surrounded by a system of tubuli (Fig. 444), which radiate into the sur- rounding solid substance. These tubuli, however, do not enter lacunae, nor is there any concentric annular arrangement around the medullary canals; but each system of tubuli is continued onwards through its own 1 See Prof. J. Quekett's Memoir on this subject, in the Transac. of the Microsc. Soc.," Ser. 1, Vol. ii.; and his more ample illustration of it in the " Illustrated Catalogue of the Histological Collection in the Museum of the Royal College of Surgeons," Vol. ii. 2 Some useful hints on the mode of making these preparations will be found in the "Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 258. VERTEBRATED ANIMALS. 259 division of the tooth, the individual tubes sometimes giving-off lateral branches, whilst in other instances their trunks bifurcate. This arrange- ment is peculiarly well displayed, when sections of teeth constructed upon this type are viewed as opaque objects (Fig. 445). — In the teeth of the higher Vertebrata, however, we usually find the centre excavated into a single cavity (Fig. 446), and the remainder destitute of vascular canals; but there are intermediate cases (as in the teeth of the great fossil Sloths) in which the inner portion of the dentine is traversed by pro- longations of this cavity, conveying blood-vessels, which do not pass into the exterior layers. The tubuli of the t non-vascular ' dentine, which exists by itself in the teeth of nearly all Mammalia, and which in the Elephant is known as 'ivory/ all radiate from the central cavity,. and pass towards the surface of the tooth in a nearly parallel course. Their diameter at their largest part averages 1-10, 000th of an inch; their smallest branches are immeasurably fine. The tubuli in their course present greater and lesser undulations; the former are few in number; Fto. 443. FIG. 444. Fig. 443. Perpendicular section of Tooth of Lamna, moderately enlarged, showing network of medullary canals. Fig. 443. Transverse section of portion of Tooth of Pristis, more highly magnified, show- ing orifices of medullary canals, with systems of radiating and inosculating tubuli. but the latter are numerous, and as they occur at the same part of the course of several contiguous tubes they give rise to the appearance of lines concentric with the centre of radiation. These * secondary curva- tures ' probably indicate, in dentine, as in the Crab's shell (§ 613), suc- cessive stages of calcification. — The tubuli are occupied, during the life of the tooth, by delicate threads of protoplasmic substance, extending into them from the central pulp. 656, In the Teeth of Man and most other Mammals, and in those of many Reptiles and some Fishes, we find two other substances, one of them harder, and the other softer, than dentine; the former is termed enamel; and the latter cementum or crusta petrota. — The enamel is com- posed of long prisms, closely resembling those of the ' prismatic ' Shell- substance formerly described (§ 563), but on a far more minute scale; the diameter of the prisms not being more in Man than l-5600th of an inch. The length of the prisms corresponds with the thickness of the layer of enamel; and the two surfaces of this layer present the ends of 260 THIS MICROSCOPE AND ITS REVELATIONS. the prisms, the form of which usually approaches the hexagonal. The course of the enamel-prisms is more or less wavy; and they are marked by numerous transverse striae, resembling those of the prismatic shell- substance, and probably originating in the same cause, — the coalescence of a series of shorter prisms to form the lengthened prism. In Man and in Carnivorous animals the enamel covers the crown of the tooth only, with a simple cap or superficial layer of tolerably uniform thickness (Fig. 446, a), which follows the surface of the dentine in all its inequali- ties; and its component prisms are directed at right angles to that surface, their inner extremities resting in slight but regular depressions on the ex- terior of the dentine. In the teeth of many Herbivorous animals, however, the enamel forms (with the cementum) a series of vertical plates, which dip down into the substance of the dentine, and present their edges alter- nately with it, at the grinding surface of the tooth; and there is in such teeth no continuous layer of enamel over the crown. This arrangement provides, by the unequal wear of these three substances (of which the enamel is the hardest, and the cementum the softest), for the constant FIG. 445. FIG. 446. A Transverse section of Tooth of Myliobates Vertical section of Human Molar Tooth: (Eagle Ray) viewed as an opaque object. a, enamel; b, cementum or crusta petrosa; c, dentine or ivory ; d, osseous excrescence, arising from hypertrophy of cementum ; e, pulp-cavity;/, osseous lacunae at outer part of dentine. maintenance of a rough surface, adapted to triturate the tough vegetable substances on which these animals feed. The enamel is the least constant of the dental tissues. It is more frequently absent than present in the teeth of Fishes; it is entirely wanting in the teeth of Serpents; audit forms no part of those of the Edentata1 (sloths, etc.) and Cetacea (whales) among Mammals. — The cementum, or crnsta petrosa, has the characters of true bone; possessing its distinctive stellate, lacunae and radiating canaliculi. Where it exists in small amount, we do not find it traversed by medullary canals; but, like dentine, it is occasionally furnished with them/ and thus resembles bone in every particular. These medullary canals enter its substance from the exterior of the tooth, 1 It has been shown by Mr. Charles Tomes, however, that the 'enamel organ' is originally present within the tooth-capsule of the Armadillo, though it under- goes an early degeneration; — a fact of no little interest in connection with the general doctrine of ' Descent with modification.' VERTEBRATED ANIMALS. 261 and consequently pass towards those which radiate from the central cav- ity in the direction of the surface of the dentine, where this possesses a similar vascularity, — as was remarkably the case in the teeth of the great extinct Megatherium. In the Human tooth, however, the cementum has no such vascularity; but forms a thin layer (Fig. 446, b), which envelops the root of the tooth, commencing near the termination of the cap of enamel. In the teeth of many herbivorous Mammals, it dips down with the enamel to form the vertical plates of the interior of the tooth; and in the teeth of the Edentata, as well as of many Reptiles and Fishes, it forms a thick continuous envelope over the whole surface, until worn- away at the crown. 657. Dermal skeleton. — The skin of Fishes, of most Reptiles, and of a few Mammals, is strengthened by plates of a horny, cartilaginous, bony, or even enamel-like texture; which are sometimes fitted-together at their edges, so as to form a continuous box-like envelope; whilst more com- monly they are so arranged as partially to overlie one another, like the tiles on a roof; and it is in this latter case that they are usually known as scales. Although we are accustomed to associate in our minds the ' scales' of Fishes with those of Reptiles, yet they are essentially-different structures; the former being developed in the substance of the true skin (with a layer of which, in addition to the epidermis, they are always cov- ered), and bearing a resemblance to cartilage and bone in their texture and composition; whilst the latter are formed upon the surface of the true skin, and are to be considered as analogous to nails, hoofs, etc., and other ' epidermic appendages.' In nearly all the existing Fishes the scales are flexible, being but little consolidated by calcareous deposit; and in some species they are so thin and transparent, that, as they do not pro- ject obliquely from the surface of the skin, they can only be detected by raising the superficial layer of the skin, and searching beneath it, or by tearing oft' the entire thickness of the skin, and looking for them near its under surface. This is the case, for example, with the common Eel, and with the viviparous Blenny; of either of which fish the skin is a very interesting object when dried and mounted in Canada balsam, the scales being seen imbedded in its substance, whilst its outer surface is studded with pigment-cells. Generally speaking, however, the posterior extremity of each scale projects obliquely from the general surface, carry- ing Before it the thin membrane that incloses it, which is studded with pigment-cells; and a portion of the skin of almost any Fish, but especially of such as have scales of the ctenoid kind (that is, furnished at their pos- terior extremities with comb-like' teeth, Fig. 448), when dried with its scales in situ, is a very beautiful opaque object for the low powers of the Microscope (Fig. 447), especially with the Binocular arrangement. Care must be taken, however, that the light is made to glance upon it in the most advantageous manner; since the brilliance with which it is reflected from the comb-like projections entirely depends upon the angle at which it falls upon them. The only appearance of structure exhibited by the thin flat scale of the Eel, when examined microscopically, is the presence of a layer of isolated spheroidal transparent bodies, imbedded in a plate of like transparence; these, from the researches of Prof. W. C. William- son1 upon other scales, appear not to be cells (as they might readily be 1 See his elaborate Memoirs ' On the Microscopic Structure of the Scales and Dermal Teeth of some Ganoid andPlacoid Fish,' in "Philos. Transact.," 1849; and ' Investigations into the Structure and Development of the Scales and Bones of Fishes,' in " Philos. Transact.," 1851. 262 THE MICROSCOPE AND ITS REVELATIONS. supposed to be), but concretions of Carbonate of Lime. "When the scale of the Eel is examined by Polarized light, its surface exhibits a beautiful St. Andrew's cross; and if a plate of Selenite be placed behind it, and the analyzing prism be made to revolve, a remarkable play of colors is presented. 658. In studying the structure of the more highly developed scales, we may take as an illustration that of the Carp; in which two very dis- tinct layers can be made-out by a vertical section, with a third but incom- plete layer interposed between them. The outer layer is composed of several concentric lamina of a structureless transparent substance, like that of cartilage; the outermost of these laminae is the smallest, and the size of the plates increases progressively from without inwards, so that their margins appear on the surface as a series of concentric lines; and their surfaces are thrown into ridges and furrows, which commonly have a radiating direction. The inner layer is composed of numerous laminge FIG. 447. FIG. 448. Portion of Skin of Sole, viewed as an opaque object. Scale of Sole, viewed as a trans- parent object. of a fibrous structure, the fibres of each lamina being inclined at various- angles to those of the lamina above and below it. Between these two layers is interposed a stratum of calcareous concretions, resembling those of the scale of the Eel: these are sometimes globular or spheroidal, but more commonly ' lenticular,' that is, having the form of a double-convex: lens. The scales which resemble those of the Carp in having a form more or less circular, and in being destitute of comb-like prolongations, are called cycloid; and such are the characters of those of the Salmon, Herring, Roach, etc. The structure of the ctenoid scales (Fig. 448), which we find in the Sole, Perch, Pike, etc., does not differ essentially from that of the cycloid, save as to the projection of the comb-like teeth from the posterior margin; and it does not appear that the strongly- marked divisions which Prof. Agassiz has attempted to establish between the 'cycloid' and the ' ctenoid' Orders of Fishes, on the basis of this difference, is in harmony with their general organization. Scales of every kind may become consolidated to a considerable extent by the calcifica- tion of their soft substance; but still they never present any approach to VERTEBRATED ANIMALS. 263 the true Bony structure, such as is shown in the two Orders to be next adverted-to. 659. In the ganoid Scales, on the other hand, the whole substance of the scale is composed of a substance which is essentially bony in its nature : its intimate structure being always comparable to that of one or other of the varieties which present themselves in the bones of the Vertebrate skeleton; and being very frequently identical with that of the bones of the same fish, as is the case with the Lepidosteus (Fig. 442), one of the few existing representatives of this Order, which, in former ages of the Earth's history, comprehended a large number of important families. Their name (from yavos, splendor) is bestowed on account of the smooth- ness, hardness, and high polish of the outer surface of the scales; which is due to the presence of a peculiar layer that has been likened (though erroneously) to the enamel of teeth, and is now distinguished as ganoin. The scales of this order are for the most part angular in their form; and are arranged in regular rows, the posterior edges of each slightly over- lapping the anterior ones of the next, so as to form a very complete defensive armor to the body. — The scales of the placoid type, which characterizes the existing Sharks and Rays, with their fossil allies, are irregular in their shape, and very commonly do not come into mutual contact, but are separately imbedded in the skin, projecting from its sur- face under various forms. In the Eays each scale usually consists of a flattened plate of a rounded shape, with a hard spine projecting from its centre; in the Sharks (to which tribe belongs the ' dog-fish ' of our own coast) the scales have more of the shape of teeth. This resemblance is not confined to external form; for their intimate structure strongly resem- bles that of dentine, their dense substance being traversed by tubuli, which extend from their centre to their circumference in minute ramifi- cations, without any trace of osseous lacunae. These tooth-like scales are often so small as to be invisible to the naked eye; but they are well seen by drying a piece of the skin to which they are attached, and mounting it in Canada balsam; and they are most brilliantly shown by the assistance of polarized light. — A like structure is found to exist in the 'spiny rays ' of the dorsal fin, which, also, are parts of the dermal skeleton; and these rays usually have a central cavity filled with medulla, from which the tubuli radiate towards the circumference. This structure is very well seen in thin sections of the fossil 'spiny rays,' which, with the teeth and scales, are often the sole relics of the vast multitudes of Sharks that must have swarmed in the ancient seas, their cartilaginous internal skeletons having entirely decayed away. — In making sections of bony Scales, Spiny rays, etc., the method must be followed which has been already de- tailed under the head of Bone (§ 654). 660. The scales of Reptiles, the feathers of Birds, and the hairs, hoofs, nails, claivs, and horns (when not bony) of Mammals, are all epidermic appendages; that is, they are produced upon the surface, not within the substance, of the true Skin, and are allied in structure to the Epidermis (§ 671); being essentially composed of aggregations of cells filled with horny matter, and frequently much altered in form. This structure may generally be made-out in horns, nails, etc.. with little difficulty; by treating thin sections of them with a dilute solution of soda; which after a short time causes the cells that had been flattened into scales, to resume their globu- lar form. The most interesting modifications of this structure are pre- sented to us in Hairs and in Feathers; which forms of clothing are very similar to each other in their essential nature, and are developed in the 264: THE MICROSCOPE AND ITS REVELATIONS. same manner — namely, by an increased production of epidermic cells at the bottom of a flask-shaped follicle, which is formed in the substance of the true skin, and which is supplied with abundance of blood by a spe- cial distribution of vessels to its walls. When a hair is pulled-out 'by its root/ its base exhibits a bulbous enlargement, of which the exterior is tolerably firm, whilst its interior is occupied by a softer substance, which is known as the 'pulp;' and it is to the continual augmentation of this pulp in the deeper part of the follicle, and to its conversion into the pe- culiar substance of the hair when it has been pushed upwards to its nar- row neck, that the growth of the hair is due. — The same is true of feath- ers, the stems of which are but hairs on a larger scale; for the 'quill ' is the part contained within the follicle answering to the ' bulb ' of the hair; and whilst the outer part of this is converted into the peculiarly-solid horny substance forming the ' barrel' of the quill, its anterior is occupied, during the whole period of the growth of the feather, with the soft pulp, FIG. 449. FIG. 450. FIG. 451. Fig. 449. Hair of Sa- ble, showing large rounded cells in its interior, covered by imbricated scales or flattened cells. 450. Hair of Musk-deer, consisting almost entirely of po- lygonal cells. A, Small Hair of Squirrel :— B, Large Hair of Squirrel: — c, Hair of Indian Bat. only the shrivelled remains of which, however, are found within it after the quill has ceased to grow. 661. Although the hairs of different Mammals differ greatly in the appearances they present, we may generally distinguish in them two ele- mentary parts — namely, a cortical or investing substance, of a dense horny texture, and a medullary or pith-like substance, usually of a much softer texture, occupying the interior. The former can sometimes be distinctly made- out to consist of flattened scales arranged in an imbricated manner, as in some of the hairs of the SaUe (Fig. 449) ; whilst, in the same hairs, the medullary substance is composed of large spheroidal cells. In the Musk-deer, on the other hand, the cortical substance is nearly undistin- guishable; and almost the entire hair seems made up of thin- walled poly- gonal cells (Fig. 450). The hair of the Reindeer, though much larger, has a very similar structure; and its cells, except near the root, are occu- pied with hair alone, so as to seem black by transmitted light, except when penetrated by the fluid in which they are mounted. In the hair of the Mouse, Squirrel, and other small Eodents (Fig. 451, A, B), the corti- VEKTEBRATED ANIMALS. 265 FlG- 452- cal substance forms a tube, which we see crossed at intervals by partitions that are sometimes complete,, sometimes only partial; these are the walls of the single or double line of cells, of which the medullary substance is made-up. The hairs of the Bat tribe are commonly distinguished by the projections on their surface, which are formed by extensions of the com- ponent scales of the cortical substance: these are particularly well seen in the hairs of one of the Indian species, which has a set of whorls of long narrow leaflets (so to speak) arranged at regular intervals on its stem (c). In the hair of thePec«n'(Fig. 452), the cortical envelope sends inwards a set of radial prolonga- tions, the interspaces of which are occupied by the polygonal cells of the medullary substance; and this, on a larger scale, is the structure of the 'quills' of the Porcupine; the radiating partitions of which, when seen through the more transparent parts of the cortical sheath, ffive to the surface of the latter a fluted appear- ance. The hair of the OrnitkorkynchuSiu a very curious object; for whilst the lower part of it resembles the fine hair of the Mouse or Squirrel, this thins away and then dilales again into a very thick fibre, having a central portion com- posed of polygonal cells, inclosed in a flattened sheath of a brown fibrous substance. 662. The structure of the human Hair is in certain respects peculiar. ***"<***« FIG. 453. Structure of Human Hair:— A, external surface of the shaft, showing the transverse striae and jagged boundary caused by the imbrications of the cuticular layer; B, longitudinal section of the shaft, showing the fibrous character of the cortical substance, and the arrangement of the pig- mentary matter; c, transverse section, showing the distinction between the cuticular envelope, the cylinder of cortical substance, and the medullary centre; D, another transverse section, showing deficiency of the central cellular substance. When its outer surface i •; examined, it is seen to be traversed by irregular lines (Fig. 453, A), which are most strongly marked in fcetal hairs; and these are the indications of the imbricated arrangement of the flattened cells or scales which form the cuticular layer. This layer, as is shown by transverse sections (c, D), is a very thin and transparent cylinder; and it incloses the peculiar fibrous substance that constitutes the principal part of the shaft of the hair. The constituent fibres of this substance, which are marked-out by the delicate striae that may be traced in longitudinal sections of the hair (B), may be separated from each other by crushing 266 THE MICROSCOPE AND ITS REVELATIONS. the hair, especially after it has been macerated for some Vime in sulphuric acid; and each of them, when completely isolated frora its fellows, is found to be a long spindle-shaped cell. In the axis of tins fibrous cylinder there is very commonly a band which is formed oi spheroidal cells; but this i medullary' substance is usually deficient in the fine hair scattered over the general surface of the body, and is not always present in those of the head. The hue of the Hair is due partly to the presence of pigmentary granules, either collected into patches, or diffused through its substance; but partly also to the existence of a multitude of minute air-spaces, which cause it to appear dark by transmitted and white by reflected light. The cells of the medullary axis in particular, are very commonly found to con- tain air, giving it the black appearance shown at c. The difference be- tween the blackness of pigment and that of air-spaces may be readily de- termined by attending to the characters of the latter as already laid-down (§§ 153, 154); and by watching the effects of the penetration of Oil of Turpentine or other liquids, which do not alter the appearance of pig- ment-spots, but obliterate all the markings produced by air-spaces, these returning again as the hair dries. — In mounting Hairs as Microscopic preparations, they should in the first instance be cleansed of all their fatty matter by maceration in ether; and they may then be put up either in weak Spirit or in Canada balsam, as may be thought preferable, the former menstruum being well adapted to display the characters of the finer and more transparent hairs, while the latter allows the light to pen- etrate more readily through the coarser and more opaque. Transverse sections of Hairs are best made by gluing or gumming several together, and then putting them into the Microtome; those of Human hair may be easily obtained, however, by shaving a second time, very closely, a part of the surface over which the razor has already passed more lightly, and by picking-out from the lather, and carefully washing the sections thus taken-off. 663. The stems of feathers exhibit the same kind of structure as Hairs; their cortical portion being the horny sheath that envelops the shaft, and their medullary portion being the pith-like substance which that sheath includes. In small feathers, this may usually be made very plain by mounting them in Canada balsam; in large feathers, however, the texture is sometimes so altered by the drying up of the pith (the cells of which are always found to be occupied by air alone), that the cellular structure cannot be demonstrated save by boiling thin slices in a dilute solution of potass, and not always even then. In small feathers, especially such as have a downy character, the cellular structure is very distinctly seen in the lateral barbs, which are sometimes found to be composed of single files of pear-shaped cells, laid end-to-end; but in larger feathers it is usually necessary to increase the transparence of the barbs, especially when these are thick and but little pervious to light, either by soaking them in turpentine, mounting them in Canada balsam, or boiling them in a weak solution of potass. In feathers which are destined to strike the air with great force in the act of flight, we find each barb fringed on either side with slender flattened filaments or 'barbules;' the barbules of one side of each barb are furnished with curved hooks, whilst those of the other side have thick turned-up edges; and as the two sets of barbules that spring from two adjacent barbs cross one another at an angle, and as each hooked barbule of one locks into the thickened edge of several barbules of the other, the barbs are connected very firmly, in a mode very similar to that in which the anterior and posterior wings of certain Hy- VERTEBRATED ANIMALS. 267 menopterous Insects are locked together (§ 638). — Feathers or portions of feathers of Birds distinguished by the splendor of their plumage are very good objects for low magnifying powers, when illuminated on an opaque ground; but care must be taken "that the light falls upon them at the angle necessary to produce their most brilliant reflection into the axis of the Microscope; since feathers which exhibit the most splendid metallic lustre to an observer at one point, may seem very dull to the eye of another in a different position. The small feathers of Humming-birds, portions of the feathers of the Peacock, and others of a like kind, are well worthy of examination; and the scientific Microscopist, who is but little attracted by mere gorgeousness, may well apply himself to the discovery of the peculiar structure which imparts to theseVbjects their most remark- able character. 664. Sections of horns, hoofs, daws, and other like modifications of Epidermic structure, — which can be easily made by the Microtome (§ 184), the substance to be cut having been softened, if necessary, by soaking in warm water, — do not in general afford any very interesting features when viewed in the ordinary mode; but there are no objects on which Polarized light produces more remarkable effects, or which display FIG. 454. a more beautiful variety of colors when a plate of Selenite is placed be- hind them and the analyzing prism is made to rotate. A curious modi- fication of the ordinary structure of Horn is presented in the appendage borne by the Rhinoceros upon its snout, which in many points resem- bles a bundle of hairs, its substante being arranged in minute cylinders around a number of separate centres, which have probably been formed by independent papillae (Fig. 454). When transverse sections of these cylinders are viewed by polarized light, each of them is Seen to be Transverse section of Horn of Rhinoceros, marked by a cross, somewhat resem- viewed by Polarized Light, bling that of Starch-grains; and the light and shadow of this cross are replaced by contrasted colors when the Selenite plate is interposed. — The substance commonly but erroneously termed whalebone, which is formed from the surface of the membrane that lines the mouth of the Whale, and has no relation to its true bony skeleton, is almost identical in structure with Ehinoceros horn, and is similarly affected by polarized light. The central portion of each of its component threads, like the medullary substance of Hairs, contains cells that have been so little altered as to be easily recognized; and the outer or cortical portion also may be shown to have a like structure, by macer- ating it in a solution of potass, and then in water. — Sections of any of the Horny tissues are best mounted in Canada balsam. 665. Blood. — Carrying our Microscopic survey, now, to the elemen- tary parts of which those softer tissues are made up, that are subservient to the active life of the body rather than to its merely-mechanical re- quirements, we shall in the first place notice the isolated floating cells contained in the Blood, which are known as Blood-corpuscles. These 268 THE MICROSCOPE AND ITS REVELATIONS. are of two kinds; the 'red,' and the ' white' or ' colorless.7 — The red present, in every instance, the form of a flattened disk, which is circular in Man and most Mammalia (Fig. 456), but is oval in Birds, Reptiles (Fig. 455), and Fishes, as also in a few Mammals (all belonging to the Camel tribe). In the one form, as in the other, these corpuscles seem to be flattened cells, the walls of which, however, are not distinctly differ- entiated from the ground-substance they contain; as appears from the changes of form which they spontaneously undergo when kept by means of a 'warm stage' 1 at a temperature of about 100°, and from the effects of pressure in breaking them up. The red corpuscles in the blood of Oviparous Vertebrata are distinguished by the presence of a central spot or nucleus; this is most distinctly brought into view by treating the blood-disks with acetic acid, which causes the nucleus to shrink and be- come more opaque, whilst rendering the remaining portion extremely transparent (Fig. 455, d). By examining unaltered red corpuscles of the Frog or Newt under a sufficiently high magnifying power, the nucleus is seen to be traversed by a network of filaments,, which extends from it FIG. 455. FIQ. 456. Red Corpuscles of Frog's Blood:— a a, their flattened face; 6, particle turned nearly edgeways; c, colorless corpuscle ; d, red corpuscles altered by diluted acetic acid. Red Corpuscles of Human Blood; represented at a, as they are seen when rather within the focus of the Microscope, and at b as they appear when precisely in the focus. throughout the ground-substance of the corpuscle, constituting an intra- collular reticulation.— The red corpuscles of the blood of Mammals, how- ever, possess no distinguishable nucleus; the dark spot which is seen in their centre (Fig. 456, b) being merely an effect of refraction, consequent upon the double-concave form of the disk. When these corpuscles are treated with water, so that their form becomes first flat, and then double- convex, the dark spot disappears; whilst, on the other hand, it is made 1 A very simple mode of applying continued warmth to an object under obser- vation is to lay the slide on a thin plate of brass or tin, about 3 inches longer than the breadth of the stage, and about 2 inches broad; which must be perfor- ated with a hole about 14th inch in diameter, at the distance of half the breadth of the stage from one end of it. When this plate is laid on the stage, and its hole is brought into the optic axis, so as to allow the light reflected upwards from the mirror to pass to the slide laid upon it, the plate will project about 3 inches on one side of the stage,— preferably the right. By placing a small lamp beneath this projection and keeping the finger of the left hand on the part of the plate close to the object (so as to feel the degree of warmth imparted to it), the heat given by the lamp may be regulated by varying its position. -For more exact and continuous regulation of the temperature, recourse may be had to the ' warm stage' devised by Prof. Schafer and made by Mr. Casella, which is traversed by a t °on7arm water> See "Quart- Journ. of Microsc, Sci.," N.S., Vol. xiv. p. o94. VERTEBRATED ANIMALS. 269 more evident when the concavity is increased by the partial shrinkage of the corpuscles, which may be brought about by treating them with fluids of greater density than their own substance. When floating in a suffi- ciently thick stratum of blood drawn from the body, and placed under a cover-glass, the red corpuscles show a marked tendency to approach one another, adhering by their discoidal surfaces so as to present the aspect of a pile of coins; or, if the stratum be too thin to admit of this, partially overlapping, or simply adhering by their edges, which then become polygonal instead of circular. The size of the red corpuscles is not alto- gether uniform in the same blood; thus it varies in that of Man from about the l-4000th to the l-2800th of an inch. But we generally find that there is an average size, which is pretty constantly maintained among the different individuals of the same species; that of Man may be stated at about l-3200th of an inch. The following Table » exhibits the average dimensions of some of the most interesting examples of the red corpuscles in the four classes of Vertebrated Animals, expressed in frac- tions of an inch. Where two measurements are given, they are the long and the short diameters of the same corpuscles. (See also Fig, 457). MAMMALS. Man 1-3200 Dog 1-3542 Whale 1-3099 Elephant 1-2745 Mouse 1-3814 Camel 1-3254, 1-5921 Llama 1-3361, 1-6294 Java Musk-Deer 1-12325 Caucasian Goat 1-7045 Two-toed Sloth. . . 1-2865 BIRDS. Golden Eagle 1-1812, 1-3832 Owl 1-1830, 1-3400 Crow 1-1961, 1 4000 Blue-Tit 1-2313, 1-4128 Parrot . 1-1898, 1-4000 Ostrich -1649, 1-3000 Cassowary -1455, 1-2800 Heron -1913, 1-3491 Fowl -2102, 1-3466 Gull -2097, 1-4000 REPTILES AND BATRACHIA. Turtle ..1-1231, 1-1882 Crocodile 1-1231, 1-2286 Green Lizard 1-1555, 1-2743 Slow-worm 1-1 178, 1-2666 Viper 1-1274, 1-1800 Frog 1-1108, 1-1821 Water-Newt 1-8014, 1-1246 Siren 1-420, 1-760 Proteus 1 400, 1-727 Amphiuma 1-345, 1-561 FISHES. Perch 1-2099, 1-2824 Carp 1-2142, 1-3429 Gold-Fish 1-1777, 1-2824 Pike 1-2000, 1-3555 Eel 1-1745, 1-2842 Gymnotus 1-1745, 1-2599 Thus it appears that the smallest red corpuscles known are those of the Musk-deer; whilst the largest are those of that curious group of Ba- trachia (Frog-tribe) which retain the gills through the whole of life; and one of the oval blood-disks of the Proteus, being more than 30 times as long and 17 times as broad as those of the Musk-deer, would cover no fewer than 510 of them. — Those of the Amphiuma are still larger.2 — Ac- cording to the estimate of Vierordt, a cubic inch of Human blood con- 1 These measurements are chiefly selected from those given by Mr. Gulliver, in his edition of Hewson's Works, p. 236 et seq. 8 A. very interesting account of the * Structure of the Red Corpuscles of the Amphiuma tridactylum ' has been given by Dr. H. D. Schmidt, of New Orleans, in the " Journ. of the Royal Microsc. Society," Vol. i. (1879*. pp. 57, 97. 270 THE MICROSCOPE AND ITS REVELATIONS. FIG. 457. tains upwards of eighty millions of red corpuscles, and nearly a quarter of a million of the colorless. 666. The white or ' colorless ' corpuscles are more readily distin- guished in the blood of Reptiles than in that of Man; being in the for- mer case of much smaller size, as well as having a circular outline (Fig. 455, c); whilst in the latter their size and contour are so nearly the same, that, as the red corpuscles themselves, when seen in a single layer, have but a very pale hue, the deficiency of color does not sensibly mark their difference of nature. The proportion of white to red corpuscles being scarcely ever greater (in a healthy Man) than 1 to 250, and often as low as from one-half to one-quarter of that ratio, there are seldom many of them to be seen in the field at once; and these may be recog- nized rather by their isolation than their color, especially if the glass cover be moved a little on the slide, so as to cause the red corpuscles to become aggregated into rows and irregular masses. — It is remarkable that, notwithstanding the great variations in the sizes of the red cor- puscles indifferent species of Verte- brated animals, the size of the white is extremely constant throughout, their diameter being seldom much greater or less than l-3000th of an inch in the warm-blooded classes, and 1 -2500th in Reptiles. Their ordin- ary form is globular; but their aspect is subject to considerable variations, which seem to depend in great part upon their phase of development. Thus, in their early state, in which they seem to be identical with the corpuscles found floating in chyle and lymph, they seem to be nearly homogeneous particles of protoplas- mic substance; but in their more advanced condition, according to Dr. Klein, their substance consists of a comparative sizes of Red Biood-corpuscies: reticulation of very fine contractile — 1. Man ^2. Elephant; 3. Musk-Deer; 4. Drome- protoplasmic fibres, termed the *in- tra-cellular network;' in the meshes of which a hyaline interstitial mate- rial is included; and which is contin- uous with a similar network that can be discerned in the substance of the single or double nucleus, when this comes into view after the with- drawal of these corpuscles from the body. In their living state, how- ever, whilst circulating in the vessels, the white corpuscles, although clearly distinguishable in the slow- moving stratum in contact with their walls (the red corpuscles rushing rapidly through the centre of the tube), do not usually show a distinct nucleus. This may be readily brought into view by treating the corpuscles with water, which causes them to swell up, become granular, and at last disintegrate, with the emission of VEKTEBRATED ANIMALS. 271 granules which may have been previously seen in active molecular move- ment within the corpuscle. — When the white corpuscles in a drop of freshly drawn blood are carefully watched for a short time, they may be observed to undergo changes of form, and even to move from place to place, after the manner of Amoeba (§ 403). When thus moving, they engulf particles which lie in their course — such as granules of vermilion that have been injected into the blood-vessels of the living animal, — and afterwards eject these, in the like fashion. Such movements will continue for some time in the colorless corpuscles of cold-blooded animals, but still longer if they are kept in a temperature of about 75°. The move- ment will speedily come to an end, however, in the white corpuscles of Man or other warm-blooded animals, unless the slide is kept on a warm stage at the temperature of about 100° F. A remarkable example of an extreme change of form in a white corpuscle of Human blood, is repre- sented in Fig. 458. Similar changes have been observed also in the cor- puscles floating in the circulating fluid of the higher Invertebrata, as the Crab, which resemble the ' white' corpuscles of Vertebrated blood, rather than its 'red' corpuscles, — these last, in fact, being altogether FIO. 458. peculiar to the circulating fluid of Vertebrated animals. 667. In examining the Blood mi- croscopically, it is, of course, impor- tant to obtain as thin a stratum of it as possible, so that the corpuscles may not overlie one another. This is best accomplished by selecting a piece of thin glass of perfect flatness, and then, having received a small drop of Blood upon a glass slide, to lay the thin-glass cover not upon this, but with its edge just touching the edge of the drop; for the blood Altered ^.^ c les of Blood< an Will then be drawn-Ill by Capillary hour after having been drawn from the finger. attraction, so as to spread in a uni- formly-thin layer between the two glasses. Such thin films may be pre- served in the liquid state by applying a cover-glass and cementing it with gold size before evaporation has taken place; but it is preferable first to expose the drop to the vapor of Osmic acid, and then to apply a drop of a weak solution of Acetate of Potass; after which a cover-glass may be put on, and secured with gold-size in the usual way. It is far simpler, however, to allow such films to dry without any cover, and then merely to cover them for protection; and in this condition the general charac- ters of the corpuscles can be very well made-out, notwithstanding that they have in some degree shrivelled by the desiccation they have under- gone. And this method is particularly serviceable, as affording a fail- means of comparison, when the assistance of the Microscopist is sought in determining, for Medico-legal purposes, the source of suspicious blood-stains; the average dimensions of the dried blood-corpuscles of the several domestic animals being sufficiently different from each other, and from those of Man, to allow the nature of any specimen to be pro- nounced-upon with a high degree of probability. 668. Simple Fibrous Tissues. — A very beautiful example of a tissue of this kind is furnished by the membrane of the common Fowl's egg; 272 THE MICROSCOPE AND ITS REVELATIONS. which (as may be seen by examining an egg whose shell remains soft for want of consolidation by calcareous particles) consists of two principal layers, one serving as the basis of the shell itself, and the other forming that lining to it which is know as the membrana putaminis. The latter may be separated by careful tearing with needles and forceps, after pro- longed maceration in water, into several matted lamellae resembling that represented in Fig. 459; and similar lamellae may be readily obtained from the shell itself, by dissolving away its lime by dilute acid.1 — The simply-fibrous structures of the body generally, however, belong to one of two very definite kinds of tissue, the 'white7 and the ' yellow, 'whose ap- pearance, composition, and properties are very different. The white fibrous tissue, though sometimes apparently composed of distinct fibres, more commonly presents the aspect of bands, usually of a flattened form, and attaining'the breadth of 1 -500th of an inch, which are marked by nume- rous longitudinal streaks, but can seldom be torn-up into minute fibres of determinate size. The fibres and bands are occasionally somewhat wavy in their direction; and they have a peculiar tendency to fall into FIG. 459. FIG. 460. Fibrous membrane from Egg-shell. White Fibrous Tissue from Ligament. undulations, when it is attempted to tear them apart from each other (Pig. 460). This tissue is easily distinguished from the other by the effect of Acetic acid, which swells it up and renders it transparent, at the same time bringing into view certain oval nuclear particles of 'ger- minal matter,' which are known as ' connective-tissue corpuscles ' (§ 651). These are relatively much larger, and their connections more distinct, in the earlier stages of the formation of this tissue (Fig. 461). It is per- fectly inelastic; and we find it in such parts as tendons, ordinary ligaments, fibrous capsules, etc., whose function it is to resist tension without yield- ing to it. It constitutes, also, the organic basis or matrix of bone; for although the substance which is left when a bone has been macerated sufficiently long in dilute acid for all its Mineral components to be re- moved, is commonly designated as cartilage, this is shown by careful Microscopic analysis not to be a correct description of it; since it does not show any of the characteristic structure of cartilage, but is capable of being torn into lamellae, in which, if sufficiently thin, the ordinary structure of a fibrous membrane can be distinguished. — The yellow 1 For an account of the curious form in which the Carbonate of Lime is dis- posed in the Egg shell, see§ 710. VERTEBRATED ANIMALS. 273 fibrous tissue exists in the form of long, single, elastic, branching fila- ments with a dark decided border; which are disposed to curl when not put on the stretch (Fig. 462), and frequently anastomose, so as to form a network. They are for the most part between l-5000th and 1-10, 000th of an inch in diameter; but they are often met with both larger and smaller. This tissue does not undergo any change, when treated with Acetic acid. It exists alone (that is without any mixture of the white) in parts which require a peculiar elasticity, such as the middle coat of arteries, the * vocal cords/ 'ligamentumnuchae' of Quadrupeds, the elas- tic ligament which holds together the valves of a Bivalve shell, and that by which the claws of the Feline tribe are retracted when not in use; and it enters largely into the composition of areolar or connective tis- sue. 669. The tissue formerly known to Anatomists as 'cellular,' but now more properly designated connective or areolar tissue, consists of a net- FIG. 461. FIG. 462. Portion of young Tendon, show- ing the corpuscles of Germinal Mat- ter, with their stellate prolongations, interposed among its fibres. Yellow Fibrous Tissue from Ligamentum Nuchae of Calf. work of minute fibres and bands, which are interwoven in every direction, so as to leave innumerable areolce or little spaces that communicate freely with one another. Of these fibres, some are of the * yellow7 or elastic kind, but the majority are composed of the * white'' fibrous tissue; and, as in that form of elementary structure, they frequently present the con- dition of broad flattened bands or membranous shreds in which no dis- tinct fibrous arrangement is visible. The proportion of the two forms varies, according to the amount of elasticity, or of simple resisting power, which the endowments of the part may require. We find this tissue in a very large proportion of the bodies of higher Animals ; thus it binds to- gether the ultimate muscular fibres into minute fasciculi, unites these fasciculi into larger ones, these again into still larger ones which are obvious to the eye, and these into the entire muscle; whilst it also forms the membranous divisions between distinct muscles. In like manner it unites the elements of nerves, glands, etc., binds together the fat-cells into minute masses (Fig. 468), these into large ones, and so on; and in this 18 274 THE MICROSCOPE AND ITS REVELATIONS. FIG. 463. way penetrates and forms part of all the softer organs of the body. But whilst the fibrous structures of which the * formed tissue ' is composed have a purely mechanical function, there is good reason to regard the ' con- nective-tissue-corpuscles ' which are every where dispersed among them, as having a most important function in the first production and subsequent maintenance of the more definitely organized portions of the fabric (§ 650). In these corpuscles, distinct movements, analogous to those of the sarco- die extensions of Ehizopods, have been recognized in transparent parts, such as the cornea of the eye and the tail of the young Tadpole, by ob- servations made on these parts whilst living. — For the display of the characters of the fibrous tissues, small and thin shreds may be cut with the curved scissors (§ 183) from any part that affords them; and these must be torn asunder with needles under the simple Microscope, until the fibres are separated to a degree sufficient to enable them to be examined to advantage under a higher magnifying power. The difference between the ' white ' and the ' yellow ' com- ponents of connective tissue is at once made apparent by the effect of Acetic acid ; whilst the ' connective-tissue-corpuscles ? are best distinguished by the staining-pro- cess (§ 200), especially in the early stage of the formation of these tissues (Fig. 461). 670. Skin; Mucous and Serous Mem- branes. — The Skin which forms the external envelope of the body, is divisible into two principal layers ; the cutis vera or ' true skin,' which usually makes up by far the larger part of its thickness, and the ' cuticle/ ' scarf-skin,' or epidermis, which covers it. At the mouth, nostrils, and the other orifices nf J.I,P nrtp«t pavifipc jmrl p«nvil« nf flip hnriv C C tne. °Pen ca™ ?dy, the skin passes into the membrane that lines these, which is distinguished as the mucous membrane from the peculiar glairy secretion Vertical Section of Skin of Finger: —A, epidermis, the surface of which shows depressions a, a, between the , which are imbedded the perspiratory OI milCUS by which its SUriaCC IS protected. glands d, with their ducts e, and ag- T>nf 4-VincP o-rpaf rlr\ Fio. 499. Artificial Concretions of Carbonate of Lime. of the eggs of the Ostrich and Cassowary, this concretionary layer is of considerable thickness; and vertical as well as horizontal sections of it are very interesting objects, showing also beautiful effects of color under Polarized light. And from the researches of Prof. W. C. Williamson on the scales of Fishes (§ 657), there can be no doubt that much of the cal- careous deposit which they contain is formed upon the same plan. 713. This line of inquiry has been contemporaneously pursued by Prof. Harting, of Utrecht, who, working on a plan fundamentally the same as that of Mr. Eainey (viz., the show precipitation of insoluble salts of Lime in the presence of an Organic * colloid'), has not only confirmed but greatly extended his results; showing that with animal colloids (such as egg-albumen, blood-serum, or a solution of gelatine) a much greater variety of forms may be thus produced, many of them having a strong resemblance to Calcareous structures hitherto known only as occurring in the bodies of Animals of various classes. The mode of experimenting usually followed by Prof. Harting, was to cover the hollow of an ordi- nary porcelain plate with a layer of the organic liquid, to the depth of from 0.4 to 0.6 of an inch; and then to immerse in the border of the CRYSTALLIZATION. POLARIZATION. 325 liquid, but at diametrically opposite points, the solid salts intended to act on one another by double decomposition, such Muriate, Nitrate, or Acetate of Lime, and Carbonate of Potass or Soda; so that, being very gradually dissolved, the two substances may come slowly to act upon each other, and may throw down their precipitate in the midst of the ' colloid.' The whole is then covered with a plate of glass, and left for some days in a state of perfect tranquillity; when there begin to appear at various spots on the surface, minute points reflecting light, which gradually increase and coalesce, so as to form a crust that comes to adhere to the border of the plate; whilst another portion of the precipitate sub- sides, and covers the bottom of the plate. Round the two spots where the salts are placed in the first instance, the calcareous deposits have a different character; so that in the same experiment several very dis- tinct products are generally obtained, each in some particular spot. The length of time requisite is found to vary with the temperature, being gen- erally from two to eight weeks. By the introduction of such a coloring matter as madder, log-wood, or carmine, the concretions take the hue of the one employed. When these concretions are treated with dilute acid, so that their calcareous particles are wholly dissolved-out, there is found to remain a basis-substance which preserves the form of each; this, which consists of the * colloid' somewhat modified, is termed by Harting calco-globuline. — Besides the globular concretions with the pecu- liar concentric and radiating arrangement obtained by Mr. Rainey (Fig. 499), Prof. Harting obtained a great variety of forms bearing a more or less close resemblance to the following: — 1. The 'discoliths' and ' cya- tholiths' of Prof. Huxley (Fig. 293). 2. The tuberculated 'spicules' of Alcyonaria (Figs. 362, 363), and the very similar spicules in the man- tle of some species of Doris (§ 573). Lamellae of ' prismatic shell-sub- stance' (§ 363), which are very closely imitated by crusts formed of flattened polyhedra, found on the surface of the ' colloid.' 4. The spheroidal concretions which form a sort of rudimentary shell within the body of Limax (§ 573). 5. The sinuous lamellae which intervene between the parallel plates of the ' sepiostaire ' of the Cuttle-fish (§ 575); the imitation of this being singularly exact. 6. The calcareous concre- tions that give solidity to the ' shell ' of the bird's egg: the semblance of which Prof. Harting was able to produce in situ, by dissolving away the calcareous component of the egg-shell by dilute acid, then immersing the entire egg in a concentrated solution of chloride of calcium, and trans- ferring it thence to a concentrated solution of carbonate of potass, with which, in some cases, a little phosphate of soda was mixed.1 Other forms of remarkable regularity and definiteness, differing entirely from anything that ordinary crystallization would produce, but not known to have their parallels in living bodies, have been obtained by Prof. Harting. Look- ing to the relations between the calcareous deposits in the scales of Fishes (§| 657-659) and those by which Bones and Teeth are solidified, it can scarcely be doubted that the principle of * molecular coalescence ' is appli- cable to the latter, as well as to the former; and that an extension and variation of this method of experimenting would throw much light on the process of ossification and tooth-formation. — The inquiry has been 1 See Prof. Harting's "Recherches de Morphologic Synthetique sur la produc- tion artificielle de quelques Formations Calcaires Inorganiques, publiees par 1' Academic Royale Nederlandaise des Sciences," Amsterdam, 1872; and <; Quart. Journ. of Microsc. Sci.," Vol. xii., p. 118. 326 THE MICROSCOPE AND ITS REVELATIONS. further prosecuted by Dr. "W. M. Ord, with express reference to the for- mation of Urinary and other Calculi.2 714. Micro -Chemistry of Poisons. — By a judicious combination of Microscopical with Chemical research, the application of re-agents may be made effectual for the detection of Poisonous or other substances, in quantities far more minute than have been previously supposed to be recognizable. Thus it is stated by Dr. Wormley,2 that Micro- Chemical analysis enables us by a very few minutes' labor to recognize with unerr- ing certainty the reaction of the 100,000th part of a grain of either Hydrocyanic Acid, Mercury, or Arsenic; and that in many other in- stances we can easily detect by its means the presence of very minute quantities of substances, the true nature of which could only be other- wise determined in comparatively large quantity, and by considerable labor. This inquiry may be prosecuted, however, not only by the appli- cation of ordinary Chemical Tests under the Microscope, but also by the use of other means of recognition which the use of the Microscope affords. Thus it was originally shown by Dr. Guy3 that by the careful sublimation of Arsenic and Arsenious Acid, — the sublimates being deposited upon small disks of thin-glass, — these are distinctly recognizable by the forms they present under the Microscope (especially the Binocular) in extremely minute quantities; and that the same method of procedure may be applied to the volatile metals, Mercury, Cadmium, Selenium, Tellurium, and some of their Salts, and to some other volatile bodies, as Sal-Ammoniac, Cam- phor, and Sulphur. The method of sublimation was afterwards extended by Dr. Helwig4tothe Vegetable Alkaloids, such as Morphine, Strychnine, Veratrine, etc. And subsequently Dr. Guy, repeating and confirming Dr. Helwig's observations, has shown that the same method may be further extended to such Animal products as the constituents of the Blood and of Urine, and to volatile and decomposable Organic substances generally.5 By the careful prosecution of Micro-Chemical inquiry, especially with the aid of the Spectroscope (where admissible), the detection of Poisons and other substances in very minute quantity can be accomplished with such facility and certainty as were formerly scarcely conceivable. 1 See his Treatise " On the Influence of Colloids upon Crystalline Form and Cohesion," London, 1879. 2 " Micro-Chemistry of Poisons," New York, 1867. 3 * On the Microscopic Characters of the Crystals of Arsenious Acid,' in " Trans, of Microsc. Society," Vol. ix. (1861), p. 50. 3 " Das Mikroskop in der Toxikologie," 1865. 5 ' On Microscopic Sublimates; and especially on the Sublimates of the Alka loids,' in "Trans of Royal Microsc. Soc.," Vol. xvi. (1868),p. 1; also "Pharma. ceutical Journal," June to September, 1867. APPENDIX. 327 APPENDIX. 'NUMERICAL APERTURE' AND 'ANGULAR APERTURE.' THE introduction of the ' immersion system ' has rendered necessary a considerable modification in the mode of determining the real ' Aper- tures ' of Achromatic Objectives; which were formerly estimated entirely by their respective ' angles of aperture/ — such angles being (as formerly explained, § 10), those contained, in each case, between the most diverging of the rays issuing from the axial point of an object, that can enter the lens and take part in the formation of an image. A careful investigation of the whole subject of ( Aperture,' both theoretically and practically, has of late been carried out with the greatest ability by Prof. Abbe, of Jena; of whose important discovery of the dependence of ' re- solving power' upon dif fraction — not refraction — an account has been already given (§ 157). This investigation has enabled him to place the question on an exact basis; and not only to clear up a great deal that was formerly obscure, but to formulate a definite principle for the compari- son of ' immersion ' with ' dry ' or ' air ' objectives, which shows that the advantages obtainable from the use of the former are much greater than had been previously conceived. Prof. Abbe has also made an important contribution to the practical part of this inquiry, by the invention of an ' Apertometer ' for the pre- cise measurement of angular apertures,1 by which more exact and definite results can be obtained than by any of the methods previously in use: and he has further shown that a comparison of ' dry ' and of ' immer- sion ' lenses by their respective ' angles ' alone is so completely fallacious, as to necessitate the introduction of a new scale of ( numerical apertures/ to which, as to a common standard, both could be referred. — It is the object of this Addendum, in the first place, to explain to the readers of this treatise the precise meaning of Prof. Abbe's term; and then to put before them the new views in regard to the capacities of f immersion ' Objectives, to which his investigations have led him. As (for obvious reasons) conclusions only can be here stated, those who desire to master the train of reasoning by which those conclusions have been worked-out, are recommended to study the two most recent expositions of the doc- trine; one given by Prof. Abbe himself in his Paper ' On the Estimation of Aperture,' and the other by his disciple, Mr. Frank Crisp (one of the Secretaries of the Eoyal Microscopical Society) in his ' Notes on Aper- 1 " Journ. of Roy. Microsc. Soc.," Vol. i. (1878), p. 19. Another method devised by Prof . Hamilton Smith (Op. cit., Vol. ii., 1879, p. 775), gives nearly the same results as that of Prof. Abbe. And yet another has been proposed by Mr. Tolles (Op. cit., Vol. iii., 1880, p. 887), who does not, however, give any reason to ques- tion the accuracy of Prof. Abbe's instrument. 328 THE MICROSCOPE AND ITS REVELATIONS. tnre, Microscopical Vision, and the Value of Wide-angled Immersion Objectives;' contained in the "Journal of the Koyal Microscopical Society," for April and June, 1881. It can be easily demonstrated mathematically, that the ' aperture ' of a single lens used as a magnifying glass — that is, its capacity for receiv- ing, and bringing to a remote conjugate focus, the rays emanating from the axial point of an object brought very near to it—is determined by thenitio between its absolute diameter (or clear ' opening') and its focal length; while that of an ordinary Achromatic Objective, composed of several lenses, is determined by the ratio of the diameter of its back lens (so far as this is really utilized) to its focal length. This ratio is most simply expressed, when the medium is the same, by the sine of its semi- angle of aperture (sin u); and we hence see how different are the propor- tionate ' apertures ' of different lenses from their proportionate ' angles of aperture.' For as the sine of half 180°, — the largest possible theoreti- cal angle, whose two boundaries lie in the same straight line, — is equal to radius, and as the sine of half 60° is equal to J- radius, it follows that a lens having an angle of 60° has an aperture equal to half (instead of being only one-third) of the theoretical maximum. And as the sines of angles beyond 60° increase very slowly, an objective whose angle is 120° will have (instead of only two-thirds) as much as about 87-100ths of the aperture given by the theoretical maximum. When, however, the medium in which the Objective works is not air, but a liquid of higher refractive index — such as water or oil — an addi- tional circumstance has to be taken into consideration; for we may now have three angles of aperture expressed by the same number of degrees, which yet denote quite different 'apertures.' For instance, an 'angle' of 90° in oil will give a greater f aperture ' than one of 90° in water; and the latter a greater aperture than 90° in air. For since, when light is transmitted from any medium into another of greater refractive index (§ 1), its rays are bent towards the perpendicular, the rays forming a pencil of given angular extension in air, will, when they pass into water or oil, be closed-together or compressed; so that in comparing (for instance) an object mounted in balsam with one mounted dry, the balsam angle, though much reduced, may nevertheless contain all the rays that were spread-out over the whole hemisphere when the object was in the less dense medium. It follows, therefore, that a given ' angle ' in oil or water represents an increase in ' aperture ' over the same angle in air. The amount of this increase having been determined by Prof. Abbe to be proportional, in each case, to the index of refraction of the interposed medium, the comparative ' apertures ' of lenses working in different media are in the compound ratio of two factors, — the sines of their respective semi-angles of aperture, and the refractive indices of the interposed fluids. It is the product of these (n sin u) that gives what is termed by Prof. Abbe the Numerical Aperture; which serves, therefore, as the standard of comparison not only between ' immersion ' and ' dry ' objectives, but also between objects of like kind. For, when the medium is the same, the factor (w) which represents the refractive index may, of course, be neglected; the 'numerical apertures' of such objectives then being simply the sines of their respective semi-angles. Thus, taking as a standard of comparison a ' dry ' objective of the maximum theoretical angle of 180,° whose ' numerical aperture ' is the APPENDIX. 329 sine of 90°,=radius or 100, we find this standard to be equalled by a ' water ' immersion objective of only 96°, and by an ' oil ' or * homogene- ous' immersion lens of only 82°; the ' numerical apertures 'of these, obtained by multiplying the sines of their respective semi-angles by the refractive index of water in one case and of oil in the other, being 1.00 in both. Each, therefore, will have as great a power of receiving and utilizing divergent rays, as any ' dry ' lens can even theoretically possess, — an angle of nearly 70° being the limit of what is practically attainable. But as the actual angle of an ( immersion ' Objective can be opened-out to the same extent as that of an ' air' objective, it follows that the ( aper- ture ' of the former can be augmented far beyond even the theoretical maximum of the latter; the maxima of numerical aperture being 1.52 water-immersion, and the fourth an oil-immersion, their apertures have hitherto been designated, on the angular aperture notation, by (for instance) 47° and 74° air-angle; 85° water-angle; and 117° oil-angle; so that it is difficult witho.ut calculation to judge of their relative apertures. By the numerical notation, however, the apertures of the four are seen to be as .40, .60, .90 and 1.30; so that a comparison is readily made, and it is seen whether the two latter have larger or smaller apertures than the maximum of a dry objective. This important doctrine may be best made practically intelligible by a comparison (Fig. 500) of the relative diameters of the back lenses of ' dry' with those of ' water ' and ( oil ' immersion Objectives of the same 2)oiver, from an * air-angle ' of 60° to an ' oil-angle ' of 180°; these diam- eters expressing in each case, the opening between the extreme pencil- forming rays at their issue from the posterior surface of the combination, to meet in its conjugate focus for the formation of the image; the extent of which opening in relation to focal length (not that of the rays entering the Objective), is the real measure of the Aperture of the combination. The dotted circles in the interior of 1 and 2 are of the same diameter as 3; and therefore show the excess in the diameters of the back lenses of the ' oil ' and ' water ' immersion-objectives, over that of the ' dry ' at their respective theoretical limits. Now this difference is capable of being practically tested by a simple experiment originally suggested by Mr. Stephenson, and thus described by Prof. Abbe: — "Take any immersion-objective of balsam angle exceed- ing the critical angle, and focus it on a balsam-mounted object, which is illuminated by any kind of immersion-condenser, in such a way that the whole range of the aperture-angle is filled by the incident rays. Remove the eye-piece, and place the pupil of the eye at the place where the air- image is projected by the objective, and look down on the lens. You see a uniformly bright circle of well-defined diameter, which is the true cross section of the image-forming pencil emerging from the Microscope (for the eye receives now all rays which have been transmitted through a small central portion of the object — that portion which is conjugate to the pupil — and receives no other rays). After this, focus the same objec- 1 At p. 325 of Vol. i., Ser. 2 (1881) of the " Journ. of the Roy. Microsc. Soc.," will be found a Table calculated by Mr. Stephenson of the Equivalent Angles of Aperture of Dry, Water-immersion, and Oil (or homogeneous) immersion Objec- tives, with their respectiye Illuminating powers, and Theoretical Resolving powers, for every 0.02 of Numerical Aperture, from 0.40 to 1.52. 330 THE MICROSCOPE AND ITS REVELATIONS. FIG. 500. 180° Oil-angle. (1.52 Num. Ap.) tive on an ordinary dry-mounted preparation (or on one which is con- nected with the slide, the cover-glass being put on dry), and repeat the observation; you will now see a well-defined circle, a cross section of the emergent pencil, but of less diameter than in the former case, surrounded by a dark annulus, visible by faint diffused light only." 1 The explana- tion of this experiment is, that in focussing an immersion-objective on an object with air above it (i. e., between itself and the cover-glass), the under-surface of the . cover-glass acts as the plane front surface of the system, converting it into a true ' dry ' lens of 180° angular aperture, which gathers-in almost the whole hemisphere of light from the radiant in air- and yet the emergent pencil of rays is much narrower than when the same objective is used as an im- mersion, andfocussed on an object in balsam, the extreme divergence of whose rays is not more than 138°. A wide-angled ' immersion' Ob- jective can therefore utilize rays from an object mounted in a dense medium, such as balsam, which are entirely lost for the image (since they do not exist, physically) when the same object is in air, or is observed through a film of air. And this loss can- not be compensated-for by an in- crease of illumination; because the rays which are lost are different rays, physically, from those obtained by any illumination, however intense, in a medium like air. It is by increasing the number of ' diffraction-spectra/ that the rays, admitted from the object contribute to the 6 resolving power ' of the Ob- jective for lined and dotted objects; the truth of the image formed by the recombination of these spectra, being, as formerly shown (§ 157), essen- tially dependent upon the augmenta- tion of the number which the objec- tive can be made to receive. Upon the ' aperture ' of an objec- tive are dependent (1) its illuminating power, (2) its resolving power, and (3) its penetrating power; — the first varying as the square of the numerical aperture, the second being in direct, and the third in inverse proportion to the numerical aperture. Whilst Prof. Abbe's investigation has made it clear that the * aper- ture' of an immersion objective may exceed the maximum of that of a dry objective, it is hardly necessary to point out that the act of the excess is a distinct question from that of the 'value of the excess for particular- cases. As the penetrating power of the objective is diminished in pro- portion as the aperture is increased, it is seen that large apertures can 1 The diameter of the emergent pencil may be accurately measured by the use of an eye-piece Micrometer with the "auxiliary Microscope" of Prof. Abbe's. 180° Water-angle. U.33 Num. Ap.) 180° Air-angle. 96° Water-angle. 82° Oil-angle. 1.00 Num. Ap.) 97° Air-angle. (0.75 Num. Ap.) 0° Air-angle. (0.50 Num. Ap.) eye-piece Apertometric apparatus, already referred to APPENDIX. 331 only be obtained at the expense of a great reduction of penetration or focal depth, and consequently also of working distance, — qualities which, are essential in some of the most important kinds of Biological investiga- FIG. 501. tion; and the Author, ^nerefore, still holds to the opinion, that for objectives intended to be used for such purposes, ' moderate angles ' are? 332 THE MICROSCOPE AND ITS REVELATIONS. preferable; objectives of wide angle being kept for * critical ' investiga- tions upon objects specially demanding their use. In this view he is •entirely supported by Mr. Dallinger, whose unrivalled experience in Biological work of the highest kind, entitles his opinion on such a point to the highest respect. See Preface, pp. v., vi. MICROSCOPES, ETC. Messrs. Watson's New Models. — A new form of Large Compound Microscope (Fig. 501) has lately been brought out by Messrs. Watson (of Holborn), the peculiarity of which essentially consists in this, — that the horizontal axis on which it is suspended passes through the axial point of the plane in which the object lies; so that by inclination of the body and .stage — the source of light remaining fixed, — illuminating rays may be made to fall on the object at any degree of obliquity. The mechanical stage is so constructed (by placing the entire movement above the object platform) as to give it a thinness not otherwise attainable with the power of making a complete revolution. The mirror with its frame may be slipped off the swinging arm that ordinarily carries it, and slid into a fit- ting on the foot, on which it can be readily centred so as to reflect light upon the centre of the stage, whatever may be the inclination of the latter. And a further variety of illumination may be obtained by rotat- ing the whole instrument on its foot, the mirror retaining its fixed posi- tion in the centre. — The principle of these ingenious arrangements is to give to the stage, and all that is above it, every variety of position in rela- tion to a fixed source of light, instead of varying the position of the light in relation to the object. — Experience alone can test its advantages over the old models. The above-named Makers have also adapted a 'swinging sub-stage,' not merely to this large instrument, but to a smaller one on the scale of the ' Student's Microscope' of Messrs. Ross (Fig. 43); which is furnished, in addition, with a graduated disk for the precise measurement of the obliquity given to the illuminating apparatus. Having carefully exam- ined this instrument, with the Objectives supplied by the makers, the Author is able to speak favorably of its workmanship; and would desire to add the name of Messrs. Watson to those of whose Students' micro- scopes he has spoken with approval at p. 67. Messrs. Swiff s New Students' Microscope. — These excellent Makers, having adopted the general plan of the 'Wale' model (Fig. 44), of which the Author has spoken in terms of high commendation, have applied to it a new fine adjustment of their own, which gives to the ring that carries the objective a very delicate and steady movement; * replacing the iris- diaphragm of the Wale model with their own 'calotte ' diaphragm. M. Nachefs Objective-carrier. — Every working Microscopist has de- sired a ready means of varying his 'powers,' without the trouble of un- screwing one Objective and screwing on another. This difficulty has been partly met by the use of the ' nose-piece;' but this cannot be conve- niently made (at least, in the case of the heavily-mounted English objectives) to carry more than two powers. By Messrs. Parkes, of Birmingham, as already mentioned § 53, sliding tubes are substituted for screws; but the use of them requires the withdrawal of the nose of the microscope to a considerable distance above the stage. — The attention 1 See " Journ. of Roy. Microsc. Soc.," Vol. i., N.S. (1881), p. 297. APPENDIX. 335 of M. Nachet having been long directed to this point, he has recently brought out a form of < porte-objectif ' (an improvement on a suggestion originally made by Prof. Thury) which allows the change of objectives to- be readily made without as much raising of the body from the stage as is required in screwing and unscrewing. It consists (Fig. 502) of a fixed inner cylinder, whose top screws into the bottom of the body; this being embraced by a movable outer cylinder (A), that is kept closely pressed up to its lower end by a strong spiral spring between the two. The bottom of this outer cylinder is formed by a shoulder that is cut away for about one-fourth of its circumference, so as to allow a collar (B) at the top of the objective to be slipped into the opening as shown at c. When this is FIG. 502. Nachet's Objective carrier. done, the objective is held firmly in place by the pressure of the spring; and all that is needed to remove it is a slight pulling down of the outer cylinder, which enables the collar of the objective to be slipped out again. The inner cylinder is supplied by M. Nachet (when desired) with the Society's screw, and the 'collar' can be adapted to receive either M. Nachet's or any other Objectives. — Having been enabled, by the kindness of M. Nachet, to make a trial of this little apparatus, the Author is glad to be able to speak most favorably both of its simplicity and its effective- ness. INDEX. ABBE, Prof., on Homogeneous immer- sion, i. 17; on Diffraction spectra, i. 156-160; on Penetration of objec- tives, i. 163 note; on Numerical aper- ture of objectives, ii. 327; on Aper- tometer, ii. 327. Aberration, Chromatic, i. 8, 9. Spherical, i. 6, 7. means of reducing and cor- recting, i. 7-14. Absorption bands, i. 89-93. -Acalephs, see Medusce. Acanthometrina, ii. 113. Acarida, ii. 248, 249. Achlya prolifera, i. 251, 253. Achnanthes, i. 296, 297. Achromatic Condenser, i. 102, 103; use of, i. 144, 145. Achromatic Correction, i. 10, 11. Achromatic Objectives, see Objectives. Acinetina, ii. 39, 40. Acrocladia, spines of, ii. 143. Actinia, ii. 135; thread-cells of , ii. 135, 136. Actinocydus, i. 291. Actinomma, ii. 113. Actinophrys, ii. 10-12. Actinoptychus, i. 292. Actinosphcerium, ii. 13. Actinotrocha, ii. 198. ACTINOZOA, ii. 134-137. Adjustment of Focus, i. 82, i. 137-139. Adjustment of Object-glass, i. 11-15; i. 139-141. ^Ecidium tussilaginis, i. 324. Agamic eggs, of Rotifera, ii. 58-60; of Entomostraca, ii. 210, 211; of Insects, ii. 246, 247. Agrion. circulation in larva of, ii. 238. Air-bubbles, microscopic appearances of , i. 152; in microscopic preparations, i. 198, i. 216-217. Albuminous substances, tests for, i. 208, 209. Alburnum, i. 364, 370. Alcohol, as hardening agent, i. 202; as test, i. 209. Alcyonian Zoophytes, ii. 136, 137. Alcyonidium, ii. 161. ALGJE, higher, microscopic structure of, i. 331-335 (see Protophyta). Allman, Prof., on Sarcode organisms, ii. 22 note; on Noctiluca, ii. 34; on Peridinium, ii. 37; on Myriothela. ii. 122; on Tubularida, ii. 129 note; on fresh- water Polyzoa, ii. 162 note; on Appendicularia, ii. 169, 170. Alternating Circulation of Ascidiaris, ii. 166, 168. Alternation of Generations, ii. 134. Alveolina, ii. 73. Amaranthus, seeds of, i. 386. Ambulacral disks of Echinida, ii. 141. Amici, Prof., his early construction of Achromatic lenses, *i. 11, 15; his in- vention of the immersion system, i. 16; his drawing Camera, i. 97; his Prism for oblique illumination, i. 106. Amoeba, ii. 15-17. Amceboids of Volvox, i. 244; of proto- plasm of Chara, i. 262 note; of proto- plasm of roots of Mosses, i. 339; of Myxomycetes, i. 326; of Sponges, ii. 117-119; of Polypes, etc., ii. 122, 123; of colorless Blood-corpuscles, ii. 271. Amoroucium, ii. 165. Amphipleura pellucida, resolution of, i. 171. Amphistegina, ii. 91. Amphitetras, i. 295. Amplifiers, i. 86. Anacharis alsinastrum, formation of cells in, i. 356; cyclosis in, i. 356, 357. Anagallis, petal of, i. 383. Androspores of (Edogonium, i. 258. Angle of Aperture, i. 8; limitation of, for Binocular, i. 35-38; its relation to Angular Aperture, i. 160 note; to Numerical Aperture, ii. 327. Anguillulce, ii. 193. Angular Apperture of Object Glasses, i. 160 note; its relation to resolving power, i. 157, 163; its real meaning, ii. 327; limits to its value, Preface, v. Anguliferece, i. 295. Aniline dyes, as staining agents, i. 206. Animal Tissues, formation of, ii. 252- 255. Animalcule-cage, i. 123. ANIMALCULES, ii. 24 (see Infusoria, Monerozoa, Rhizopoda, and Rotifera). Animals, distinction of, from Plants, 336 INDEX. i. 222, 223; links connecting with Plants, i. 325-328. ANNELIDA, ii. 192-204; marine, circu- lation in, ii. 196, 197; metamorphoses of, ii. 197, 198; remarkable forms of, ii. 199-202; luminosity of, ii. 202; fresh-water, ii. 202, 203. Annual layers of Wood, i. 369-371. Annular Ducts, i. 365, 366. ANNULOSA, ii. 192 — see Entozoa, Turbel- laria, and Annelida. Anodon, shell of, ii. 174; parasitic em- bryo of, ii. 183; ciliary action on gills of, ii. 189. Anomia, fungi in shell of, i. 321. Ant, red, integument of, ii. 220. Antedon, development of, ii. 152-154. Antennae of Insects, ii. 232, 233. Antheridia, of Chara, i. 262; of Mar- chantia, i. 337; of Mosses, i. 340; of Ferns, i. 347; — see Antherozoids. Antherozoids, i. 229; of Volvox, i. 242; of Vaucheria, i. 251; of Sphaeroplea, i. 255, 256; of GEdogonium, i. 258; of Characese, i. 261, 266; of Fuci, i. 333; of Marchantia, i. 337; of Mosses, i. 339; of Ferns, i. 347. Anthers, structure of, i. 383, 384. Anthony, Dr., on scale of Gnat, i. 155; onbattledoor scales, ii. 223; on tongue of Fly, ii. 235 note. Antirrhinum, seeds of, i. 386. Apertometers, ii. 327. Aperture, Angular, see Angular Aper- ture; Numerical, ii. 327. Aphides, agamic reproduction of, ii. 246. Aphthae, fungus of, i. 320. Aplanatic Searcher, i. 8 note. Apothecia of Lichens, i. 330. Appendicularia, ii. 169, 170. Apple, cuticle of, i. 376. Apus, ii. 209. Aquatic Box, i. 123. ARACHNIDA, microscopic forms of, ii. 248, 249; eyes of, ii. 250; respiratory organs of, ii. 250; feet of, ii. 250; spinning apparatus of, ii. 250, 251. Arachnoidiscus, i. 293. Arachnosphcera, ii. 114. Aralia, cellular parenchyma of, i, 354. Arcella, ii. 18, 19. Archegonia, of Marchantia, i. 336; of Mosses, i. 340; of Ferns, i. 347. Archer, Mr., on zoospores of Desmi- diaceae, i. 267 note; on Chlamido- myxis, i. 327-328; on Clathrulina, ii. 14. Arenaceous Foraminifera, ii. 77-85. Arenicola, ii. 196. Areolar tissue, ii. 273. Argulus, ii. 212. AristolocJiia, stem of, i. 375. Artemia, ii. 207, 210. Ascaris, ii. 193; fungous vegetation on, i. 319. I Asci, of Lichens, i. 330; of Fungi, i. 322, 323. ! Ascidia parallelogramma, ii. 164. ' Ascidians, solitary, ii. 164; compound ii. 165-167; social, 166-167; develop- ment of, ii. 168-170. Ascogonia, of Fungi, i. 322; of Lichens ,. i. 330. Ascomycetes, i. 322. Asphalte- varnish, i. 178. Aspidisca-form of Trichoda, ii. 48. Aspidium, fructification of, i. 345. Asplanchna, ii. 57, 58. Astasia, ii. 33. Asteriada, skeleton of, ii. 145; meta- morphoses of, ii. 150, 151. Asterolampra, i. 291. Aster omphalus, i. 291, Astromma, ii. 112. Astrophyton, ii. 145. Astrorhiza, ii. 78. Auditory vesicles of Mollusks, ii. 190; development of, ii. 186. Aulacodiscus, i. 293. Auxospores of Diatomaceae, i. 283. Avicula, nacre of, ii. 174. Avicularia of Polyzoa, ii. 162. Axile bodies of sensory papillae, ii. 28o. Axis-cylinder of Nerve-fibres, ii. 284- 286. Azure-blue butterfly, scales of, ii. 222, 223. Bacillariaparadoxa, i. 287; movements of, i. 283. Bacillus, i. 308-311. Bacteria, i. 308-315. Bacteriastrum, i. 296. Badcock, Mr., on metamorphosis of Acinetina, ii. 40. Bailey, Prof., his Diatomaceous tests,. i. 171; on siliceous cuticle, i. 349; on internal casts of Foraminifera, ii. 93 note. Baker, Mr., his Students' Microscope, i. 59; his Students' Binocular, i. 60; his Travelling Microscope, i. 81, 82; his Pond-stick, i. 219. Balanus, metamorphosis of, ii. 213. 214. Balbiani, M., on generation of Infu- soria, ii. 49-52. Balsam, Canada, see Canada Balsam. Banksia, stomata of, i. 380. Barbadoes, Polycystina of, ii. 109, 114. Bark, structure of, i. 373. Barnacle, metamorphosis of, ii. 213, 214. Basidia of Fungi, i. 322. Bat, hair of, ii. 264; cartilage of ear of, ii. 279. Batrachospermece, i. 258, 259. Battledoor scale of Polyommatus, ii- 222 223. Bathybius, ii. 20. Beading of Diatom-valves, i. 277-279; of Insect-scales, Dr. Royston Pigott on, ii. 224, 226. INDEX. 337 Beale, Prof., his Pocket Microscope, i. 80; his Demonstrating Microscope, i. 81; his views of viscid media, i. 210-212; his Views of Tissue-forma- tion, ii. 253-255. Beck, Messrs., their Economic Micro- scopes, i. 67; their Popular Micro- scope, i. 68; their Large Compound Microscope, i. 77; their improved ditto, i. 80; their Achromatic Con- densers, i. 102, 103; their arrange- ment of Polarizing apparatus, i. 113; their Compressors, i. 126, 127; their Binocular Magnifier,-!. 187 note; their Microtome, i. 192. Mr. Joseph, on scales of Thysa- nurse, ii. 223-226. Mr. Richd., his Dissecting Micro- scope, i. 48; his Disk-holder, i. 121: his Side-Reflector, i. 116; his Vertical Illuminator, i. 118, 119; on scales of of Thysanurae, ii. 224; on Spider's threads, ii. 251. Bee, eyes of, ii. 229, 230; hairs of, ii. 227; proboscis of, ii. 235; wings of, ii. 242; sting of, ii. 245; reproduction of, ii. 247. Berg-mehl, i. 302. Bermuda-earth, i. 292, 302. Beroe, ii. 137, 138. Biddulphia, i. 293; growth of, i. 275 note; surface-marking of , i. 276; self- division of, i. 280, 281. Bignonia, seed of, i. 386. Biliary Follicles, ii. 280. Biloculina, ii. 71. Binary subdivision of Vegetable Cells, i. 227, 228; of Animal Cells, ii. 253; see Cells, Animal and Vegetable. Binocular Eye-piece, i. 33. Magnifier, Nachet's, i. 48, 49; Beck's, i. 187 note. - Microscopes, Stereoscopic, principles of construction of, i. 25-28; advantages of, i. 35-38; Objectives suitable for, i. 35-37; different forms of, Nachefs, i. 28-29; Wenham's, i. 29-31; Stephenson's, i. 30-33. Non-Stereoscopic, Powell and Lealand's, i. 84; Wenham's, i. 85. Stereo-Pseudoscopic, Nachet's, i. 33-35. Vision, i. 25-28. Bipinnaria-larva of Star-fish, ii. 151. Bird, Dr. Golding, on preparation of Zoophytes, ii. 131. BIRDS, bone of, ii. 257; feathers of, ii. 266; blood of, ii. 269; lungs of, ii. 30". Bird's-head processes of Polyzoa, ii. 162. Bisulphide of Carbon, mounting Dia- toms in, i. 291. Bivalve Mollusks, shells of, ii. 171-178. Black ground illuminators, i. 106-110, i. 147. Blackham, Dr., on Focal Depth, i. 163 note. 22 Blankley, Mr., his Selenite Stage, i. 113. Blenny, viviparous, scales of, ii. 261. Blights of Corn, i. 323. Blood, Absorption-bands of, i. 92, 93. Blood-disks of Vertebrata, ii. 267-271; mode of preserving, ii. 271; circula- tion of, see Circulation. Blood-vessels, injection of, ii. 292-297;. disposition of, in different parts, iu 297-301. Bocket Lamp, i. 131. Bone, structure of, ii. 255-258; mode of making sections of, i. 196-199, ii. 258. Bones, fossil, examination of, ii. 311. Botryllians, ii. 160. Botrytis, of SilkVorms, i. 317-326. Botterill, Mr. , his Growing-slide, i. 122, 123; his Zoophyte-trough, i. 125. Bowerbankia, ii. 161. Brachionus, ii. 54, 62. BRACHTOPODA, Shell-structure of, ii. 177, 178. Brady, Mr. H. B., on Saccammina, ii. 78; on Loftusia, ii. 84; on Globigerina, ii. 87. Braithwaite, Dr., on Sphagnacese, i. 344. Branchiopoda, ii. 207-209. Branchipus, ii. 209. Braun, Prof., on development of Pedi- astrese, i. 270-272. Brittan, Dr., on Fungus-germs, i. 321. Brownian Movement, i. 153, 154. Browning, Mr., his Platyscopic Lens, i. 21; his smaller Stephenson Binocu- lar, i. 72-73; his Rotating Microscope, i. 65; his Micro-Spectroscope, i. 89- 91. Bryozoa, see POLYZOA. Buccinum, palate of, ii. 182; egg cap- sules of, ii. 184; development of, ii. 187. Buckthorn, stem of, i. 369. Bugs, ii. 219; wings of, ii. 242. Bugula avicularia, ii. 162, 163. Built-up Ceils, i. 182. Bulbels, of Chara, i. 261; of Marchantia, i. 336, 337. Bulimina, ii. 89. Bull's-eye Condenser, i. 114, 115; use of, 148-150. Burdock, stem of, i. 375. Busk, Mr. G., on Volvox, i. 238-243; on Polyzoa, ii. 162, 163. Butterflies, see Lepidoptera. Cabinet for Microscopic Apparatus, i. 130; for Objects, i. 218. Cacao-butter, for imbedding, i. 195. Cactus, raphides of, i. 363. Calcaire Grossier, ii. 305. 307. Calcareous Deposits, organic origin of, ii. 308, 309. Calcareous Sponges, ii. 120. Calcarina, ii. 90. Calycanthus, stem of, i. 374. Calyptra of Mosses, i. 339. 338 INDEX. Cambium- layer i. 373, 374. Camera Lucida, i. 96-98; use of, in Micrometry, i. 99. CampanularidcR, ii. 129. Campy lodiscus, i. 288. Canada Balsam, use of, as Cement, i. 176, i. 197-199; mounting of objects in, i. 209, 314, 215. Canaliculi of Bone, ii. 256, 257. Canal-system of Foraminifera, ii. 70, 90-99. Capillaries, circulation in, ii. 286-292; injection of, ii. 293-298; distribution of, ii. 298-300. Capsule of Mosses, i. 340. Carbolic Acid, as preservative, i. 210; use of for dehydration, i. 215. Carmine, as staining agent, i. 205; in- jection with, ii. 295, 297. Carp, scales of, ii. 262. Carpenteria, ii. 88. Carrot, seeds of, i. 387. Cartilage, structure of, ii. 278, 279. Caryophyllia, ii. 135. Caryophyllum, seeds of, i. 386. Caterpillars, feet of, ii. 245. Cedar, stem of, i. 371. Cells for mounting objects, i. 179-183; mounting objects in, i. 215-217. Cells, Animal, formation, of, ii. 253; binary subdivision of, ii. 253, 279; in Protozoa, ii. 1, 9, 12, 16, 26-30, 37, 38, 45, 47. Vegetable, i. 224-227; origin and multiplication of, i. 227, 228; binary subdivision of, in protophyta, i. 230, 233, 241, 246, 254, 264-266, 279; in Phanerogamia, i. 352-356; cyclosisin, i. 260, 263, 356-359; thickening de- posits in, i. 359-361; spiral deposits in, i. 361; starch-grains in, i. 361, 362; raphides in, i. 363. Cellular Tissue, Animal, ii. 273; Vege- table, ordinary forms of, i. 353-356; stellate, i. 354, 355; formation of, i, 356. Cellulose, i. 225; tests for, i. 208, 209. Cements, Microscopic, i. 178, 179. Cement-Cells, i. 180. Cementum of Taeth, ii. 259. CEPHALOPODS, shell of, ii. 180; chroma- tophores of, ii. 191. Ceramiacece, i. 334. Ceratium, ii. 38, 39. Cercomonas, development of, ii. 29, 30. Cestoid Entozoa, ii. 192, 193. Chcetocerece, i. 295. Chcetophoracece, i. 257, 258. Chalk, formation of, ii. 305-308. 'Challenger' Expedition, use of tow- net in, i. 221 note; collection of Glo- bigerinee in, ii. 86, 87; observations in, on Bathybius, ii. 20 ~ on deep-sea sediments, ii. 312, 313. Characece, i. 259-262; cyclosis of fluid in, i. 260, 261; multiplication of, by gonidia, i. 262; sexual apparatus of, i. 260-262. Cheilostomata, ii. 162. Chemical Microscope, i. 82-83. Re-agents, i. 208, 209. Chemistry, microscopic, ii. 326. Cherry-stone, cells of, i. 360. Chilodon, teeth of, ii. 43; self -division of, ii. 46. Chirodota, calcareous skeleton of, ii. 149. Chitine of Insects, ii. 220. Chlamidomyxis, i. 327, 328. Choroid, pigment of, ii. 275. Chromatic Aberration, i. 8, 9; means of reducing and correcting, i. 10-16; re- sidual, in high-angled Objectives, i. 173. Chromatophores of Cephalopods, ii. 191. Chromic acid, as solvent, i. 201; use of, for hardening, i. 203. Chyle, corpuscles of, ii. 270. Cienkowski, on Myxomycetes, i. 327; on Noctiluca, ii. 37. Cidaris, spines of, ii. 135. Ciliary action, nature of, ii. 41; in Pro- tophytes, i. 250; in Infusoria, ii. 41; on gills of Mollusks, ii. 189; on epithe- lium of Vertebrata, ii. 277. Ciliate Infusoria, ii. 41. Cilio-flagellata, ii. 37-39. Circulation of Blood, in Vertebrata, ii. 286-292; in Insects, ii. 237, 238; alter- nating, in Tunicata, ii. 164, 167 Circulation, Vegetable, see Cyclosis. Cirrhipeds, metamorphosis of, ii. 213, 214. Cladocera, ii. 209. Clark, Prof. H. James, on flagellate In- fusoria, ii. 32; on Sponges, ii. 118 note. Clathrulina elegans, ii. 13, 15. Clavellinidce, ii. 166-168. Cleanliness, importance of, to Micro- scope, i. 134; in mounting objects, i. 217. Clematis, stem of, i. 368,- 376. Closterium, cyclosis in, i. 264; binary subdivision of , i. 264, 265; conjugation of, i. 267-269. Clypeaster, spines of, ii. 144. Coal, nature of, ii. 303, 304. Coalescence, molecular, ii. 323-325. Cobweb-Micrometer, i. 92, 93. Coccoliths and Coccospheres, ii. 19, 20, 325. Cocconeidce, i. 296. Cockchafer, cellular integument of, ii. 220; eyes of, ii. 229; antenna of, ii. 232, 233; spiracle of larva of, ii. 240. Cockle of Wheat, ii. 193. Coddington lens, i. 20. Codosiga, life history of, ii. 32, 33. Ccelenteratd, ii. 2. Coenurus, ii. 192, 193. INDEX. 339 Cohn, Dr., his researches on Protococ- cus, i. 232-236; onVolvox, i. 242,243; on Stephanosphaera, i. 242 note; on Sphae- roplea, i. 255; on Schizomycetes, i. 307, 308; on reproduction of Rotifera, ii. 59; his cultivation-solution, i. 307 note. Coleoptera, integument of. ii. 220; an- tennae of, ii. 232, 233; mouth of, ii. 233, 234. Collection of Objects, general direc- tions for, i. 219-221. Collins, Mr., his Harley Binocular, i. 70-71; his Eye-piece caps, i. 70; his Students' Microscope, i. 59; his Grad- uating Diaphragm, i. 102, 104. Collomia, spiral nbres of, i. 361. Collozoa, ii. 115. Colonial nervous system of Polyzoa, ii. 160. Colorless corpuscles of Blood, ii. 270, 271. Columella of Mosses, i. 342. Comatula, metamorphosis of, ii. 152- 156; nervous system of, ii. 285. Compound Microscope, optical princi- ples of, i. 22-25; mechanical construc- tion of, i. 40-42, 51, 52; Educational, i. 52-55; Students', i. 55-67; Second Class, i. 67-73; First class, i. 73-80; for special purposes, i. 80-85. Compressor, i. 126, 127; use of, i. 142. Concave lenses, refraction by, i. 5, 6; use of, in Achromatic combinations, i. 10-15. Conceptacles of Marchantia, i. 337. Concretions, calcareous, ii. 323-325. Condensers, Achromatic, i. 102-104: Webster, i. 103; Swift's new combi- nation, i. 113. for Opaque objects, ordinarv, i. 114; Bull's-eye, i. 115; mode of using, i. 147-150. Confervacece, i. 253; self -division of, i. 254; zoo-spores of, i. 255; sexual re- production of, i. 254, 255. Conidia of Fungi, i. 322. Coniferce, peculiar woody fibre of, i. 364; absence of ducts in, i. 369; struc- ture of stem in, i. 371, 372; pollen- grains of, i. 385 note; fossil, ii. 302. Conjugatece, i. 236, 237. Conjugation, of Paknoglsea, i. 232; of Desmidiaceae, i. 267, 268; of Diato- maceae, i. 281, 282; of Conjugates, i. 236, 237; of Monadina, ii. 28-31; of Noctiluca, ii. 37; of Vorticellina, ii. 52; — see Zygosis. Connective Tissue, ii. 273; corpuscles of, ii. 254, 273, 274. Contractile vesicle, of Volvox, i. 238; of Actinophrys, ii. 11; of Amoeba, ii. 16; of Infusoria, ii. 25, 45. Conversion of Relief, i. 27, 28, 33, 34. Convex lenses, refraction by, i. 3-5; formation of images by, i. 6. Copepoda, ii. 208. Coquilla-nut, cells of, i. 360. Coral, cutting sections of, with animal i. 200. Corallines, true, i. 335; Zoophvtic, ii. 129. Cork, i. 373. Corn, blights of, i. 323; ii. 193. Corn-grains, husk of, i. 388. Corns, structure of, ii. 276. Cornuspira, ii. 70. Corpuscles of Blood, ii. 267 -271. Correction of Object-glasses, for Sphe- rical Aberration, i. 8, 9, 173; for Chro- matic Aberration, i. 10, 11, 173; for thickness of covering glass, i. 11, 14, 135-141. Coscinodiscece, i. 289-291. Cosmarium, binary subdivision of, i. 265; conjugation of, i. 267; develop- ment of, i. 268. Cover-correction of Objectives, i. 11, 14, 135-141. Covering-glass, i. 176, 177. Crab, shell-structure of, ii. 214, 215; metamorphosis of, ii. 216. Crabro, integument of, ii. 220. Crag-Formation, ii. 309. Cricket, gastric teeth of, ii. 237; sounds produced by, ii. 242. Crinoidea, skeleton of, ii. 146; meta- morphosis of, ii. 153-156. Cristatella, ii. 162. Cristellaria, ii. 85. Crouch, Mr., his Educational Micro- scope, i. 53; his Students' Binocular, i. 65, 66; his stage-centering adjust- ment, i. 80; his adapter for Beck's side-reflector, i. 116. Crusta Petrosa of Teeth, ii. 259. CRUSTACEA, ii. 205-217; lower forms of, ii. 205-206; Entomostracous, ii. 206- 211; Suctorial, ii. 212; Cirrhiped, ii. 213, 214; Decapod, shell of, ii. 214, 215; metamorphosis of, ii. 215, 216. CRYPTOGAMIA, general plan of structure of, i. 331, 351, 352;— see Protophyta, Algae, Lichens, Fungi, Hepaticas, Mosses, Ferns, etc. Crystallization, Microscopic, ii. 318, 323. Ctenoid scales of Fish, ii. 261, 262. Ctenophora, ii. 137-139. Culture of Protophytic Fungi, i. 123 note, i. 307; of Flagellate Infusoria, ii. 31. Curculionidce, scales of, ii. 220, 228; elytra of, ii. 128; foot of, ii. 244. Cuticle of Animals, ii. 275-277. of Equisetaceae, i. 349; of leaves, i. 379. Cutis Vera, ii. 274. Cuttle-fish, shell of, ii. 180; chromato- phore of, ii. 191. - Cyanthus, seeds of, i. 387. Cyclammina, ii. 82. Cycloclypeus, ii. 70. 340 INDEX. Cycloid scales of Fish, i. 261, 262. Cyclops, ii. 208, 209; fertility of, ii. 210. Cyclosis, in Vegetable cells, i. 226; in Closterium, i. 263; in Diatomaceee, i. 273; in Chara, i. 259, 260; in cells of Phanerogamia, i. 356-359; in Rhizo- pods, ii. 6. Cydippe, ii. 137, 138. Cymbellece, i. 297. Cynipidce, ovipositor of, ii. 245. Cypris, ii. 207. Cyprcea, structure of shell of, ii. 179. Cystic Entozoa, ii. 192, 193. Cysticercus, ii. 192, 193. Cytherina, ii. 207. Dactylocalyx, ii. 121. Dallinger, Mr., on flagellum of Bacte- rium termo, i. 309: his Microscope Lamp, i. 133; on qualities of Objec- tives, i. 162, 165; Preface, v., vi. Dallinger and Drysdale, their researches on Monadina, ii. 26-32. Dallingeria Drysdali, ii. 26-28. Dalyell, Sir J. G., on development of Medusae, ii. 132-134. Dammar- Varnish, i. 178, 213. Daphnia, ii. 209, 210. Darker's Selenites, i. 113. Davies, Mr., on Microscopic Crystalliza- tion, ii. 319-321 note. Dawson, Dr., on Eozoon, ii. 102. Deane's Gelatine, i. 211. De Bary, Dr., on Myxomycetes i. 327 note. Decalcification, i. 201. Decapod Crustacea, shell of, ii. 214, 215; metamorphosis of, ii. 215, 216. Defining power of Objectives, i. 161- 163, 173. Dehydration, by Alcohol, i. 195; by Carbolic Acid, i. 215. Delsaux, Rev. J., on Brownian move- ments, i. 154 note. Demodex folliculorum, ii. 249. Demonstrating Microscope, Beale's, i. 81. Dendritina, ii. 72. Dendrodus, teeth of, ii. 311. Dentine of Teeth, ii. 258-260. Depressions, distinction of, from eleva- tions, i. 151. Dermestes, hair of, ii. 228. Desiccation, tolerance of, by Proto- phytes, i. 235; by Infusoria, ii. 31, 49; by Rotifera, ii. 59, 60; by Entomos- traca, ii 210. Desmidiacece, general structure of, i. 262-264; cyclosis in, i. 263; binary subdivision of, i. 264-266; formation of gonidia in, i. 267; conjugation in, i. 267, 268; classification of, i. 268, 269; collection of, i. 269, 270. Deutzia, stellate hairs of, i. 379. Development, of Annelids, ii. 197-201; of Anodon, ii. 183, of Ascidiaris, ii. 168; of Cirrhipeds, ii. 213, 214; of Crab, ii. 215; of Desmidiaceae, i. 267; of Diatomaceae, i. 279-281; of Echin- odermata, ii. 150-156; of Embryo(Ani- mal), ii. 1, 122, of Embryo (Vegeta- ble), i. 351, 352; of Entomostraca, ii. 211-213; of Ferns, i. 348; of Gastero- pods, ii. 184-189; of Insects, ii. 247, 248; of Leaves, i. 356; of Medusas, ii. 126, 127; of Mosses, i. 342; of Nudi- branchiata, ii. 185; of Palmoglsea, i. 230; of Pollen-grains, i. 384; of Pro- tococcus, i. 232; of Sponges, ii. 118; of Stem, i. 374; of Vegetable-cell, i. 227, 228; of Volvox, i. 238-241. Diagonal Scales, i. 94, 99. Diamond-beetle, scales of, ii. 220; elytra of, ii. 228; foot of, ii. 243. Diaphragm Eye-piece, Slack's, i. 95. Diaphragm-Plate, i, 101-103. Diatoma, i. 286. Diatomacece, general structure of, i. 273, 274; silicified valves of, i. 274- 276; surface-markings of, i. 276-279; binary subdivision of , i. 279, 280; con- jugation of, i. 281, 282; gonidia of, i. 282; auxospores of, i. 282; move- ments of, i. 283; classification of, i. 284; general habits of, i. 301, 302; fos- silized deposits of, i. 302, 303, ii. 304; collection of, i. 302-304; mounting of, i. 305, 306. Diatoms, as Tests, i. 169-172, i. 276-279. Dichroism, ii. 322. Dicotyledonous Stems, structure of, i. 369-375. Dictyoloma, seeds of, i. 387. Didemnians, ii. 166. Didymoprium, i. 269; self-division of, i. 265; conjugation of, i. 269. Difflugia, ii. 18. Diffraction, errors arising from, i. 154- 157; production of microscopic images by, i. 157-160. Diphtheria, fungus of, i. 320. Dipping-tubes, i. 127. Diptera, mouth of, ii. 235; halteres of, ii. 243; ovipositors of, ii. 246. Discorbina, ii. 89. Disk-holder, Beck's, i. 121; Morris's, i. 122. Disk-illuminator, Wenham's, i. 105. Dispersion, chromatic, i. 8, 9. Dissecting Instruments, i. 187; Trough, i. 187; Microscopes, i. 44-50. Dissection, Microscopic, i. 186-188. Distoma, ii. 194. Dog, epidermis of foot of, ii. 276. Doris, palate of, ii. 182; spicules of, ii. 179; development of, ii. 185. Dorsal Vessel of Insects, ii. 237. Double-staining, i. 207. Doublet, Wollaston's, i. 20; Steinheil's, i. 21. Dragon- fly, eyes of, ii. 230; larva of, ii. 238-240. INDEX. 341 Drawing Apparatus, i. 96-99. Draw-Tube, i. 87. Dropping Bottle, i. 211. Drosera, hairs of, i. 379. Dry-mounting of Objects, i. 179, 183. Drysdale, Dr. , see Dallinger. Ducts, of Plants, i. 365, 366. Dujardin, M., on Sarcode of Foramini- fera, etc., i. 222 note; on Rotifera, ii. 60-62. Dunning's Turn-Table, i. 184. Duncan, Dr., on Fungi in coral, i. 321. Duramen, i. 370. Dytiscus, foot of, ii. 244; trachea and spiracle of, ii. 339. Eagle-Ray, teeth of, ii. 260. Earwig wings of, ii. 242. Eccremocarpus seeds of, i. 386. Echinida, shell of, ii. 140, 141 ; ambula- cral disks of, ii. 141, 142; spines of, ii. 142, 143; pedicellariae of, ii. 144; teeth of, ii. 144, 145; metamorphosis of, ii. 151, 152. ECHINODERMATA, skeleton of, ii. 140- 145; metamorphoses of, ii. 150-153. Echinus-spines, cutting sections of, i. 196-200, ii. 146-148. Ectocarpacece, i. 332. Ectoderm, ii. 1. Ectosarc of Rhizopods, ii. 7, 15. Ectoplasm of Vegetable cell, i. 225. Edmunds, Dr., his immersion-parabo- loid, i. 109; his parabolized gas-slide, i. 125. Educational Microscopes, i. 53-55. Eel, scale of, ii. 262; gills of, ii. 299. Eels, of paste and vinegar, ii. 193. Eggs of Insects, ii. 246; — see Winter- Egg-shell, fibrous structure of, ii. 272; calcareous deposit in, ii. 324. Ehrenberg, Prof., his researches on In- fusoria, ii. 24, 25; on Rotifera, ii. 24; on Polycystina, ii. 109, 116; on com- position of Greensands, ii. 93 note, ii. 309 Elastic Ligaments, ii. 272. Elaters of Marchantia, i. 338. Elementary Parts of Animal body, ii. 253-255 ;— see Tissues. Elevations, distinction of, from depres- sions, i. 151. Elytra of Beetles, ii, 242. Embryo, see Development. Embryo-sac of Phanerogamia, i. 352. Empusa musci, i. 318. Enamel of Teeth, ii. 259. Encrinites, see Crinoidea. Encysting process, of Protophytes, i. 230-236; of Infusoria, ii. 47-49". End-bulbs of sensory Nerves, ii. 285. Endochrome, of Vegetable cell, i. 225; of Diatomaceae, i. 273. Endoderm, ii. 1. Endogenous Stems, structure of, i. 367, 375. Endoplasm of Vegetable cell, i. 225. Endosarc of Rhizopods ii. 7, 15. Endosperm of Phanerogams, i. 353. Enterobryus, i. 319, 820. Entomostraca (Crustacea), ii. 205-211; classification of, ii. 207-210; repro- duction of, ii. 210-212. Entophytic Fungi, i. 323, 324. Entozoa, ii. 192-194; Cestoid, ii. 192; Cystic, ii. 192; Nematoid, ii. 193, 194; Trematode, ii. 194. Eosin, as staining agent, i. 206. Eozoic Limestone, ii. 310. Eozoon Canadense, ii. 101-106. Ephemera, larva of, ii. 219, 237, 240. Ephippium of Daphnia, ii. 211. Epidermis, Animal, ii. 275, 276. Vegetable, i. 377-380. Epithelium, ii. 276; ciliated, ii. 277. Epithemia, i. 285; conjugation of, i. 281. Equisetacece, cuticle of, i. 349; spores of, i. 349. Erecting Binocular, see Stephenson. Erecting Prism, Nachet's, i. 88. Erector, Lister's, i. 87. Errors of Interpretation, i. 150-156. Euglena, ii. 33. Eunotiece, i. 285. Euplectella, ii. 121. Euryale, skeleton of, ii. 145. Ewart, Prof., on Bacillus, i. 311-313. Exogenous Stems, structure of, i. 369- 375. Eyes, care of, i. 134; Preface, vi., vii. Eyes of Mollusks, ii. 190, 191; of Insects, ii. 229-231; of Trilobite, ii. 310. Eye-piece, i. 22; Huyghenian, i. 23, 24; Keilner's, i. 24, 25; solid, i. 25; Rams- den's, i. 25; Binocular i. 33; Erecting, i. 88; Spectroscopic, i. 89; Microme- tric, i. 92-95; Diaphragm i. 95. Collins's shades for, i. 70. Eye-piecing, deep, disadvantage of, i. 136, 137, Preface, vi. Falconer, Dr., on bones of fossil Tor- toise, ii. 312. Fallacies of Microscopy, i. 150-156. Farrant's Medium, i. 211, 213. Farre, Dr. Arthur, his researches on Bowerbankia, ii. 161. Fat-cells, ii. 278; capillaries of, ii. 298. Feathers, structure of, ii. 263, 266. Feet of Insects, ii. 243-245; of Spiders, ii. 250. Fermentation, influence of vegetation on, ii. 315. FERNS, i. 344-349; scalariform ducts of, i. 344; fructification of, i. 344-346; spores of, i. 346; prothallium of, i. 347; antheridia and archegonia of, i. 347; generation and development of, i. 347, 348. 34:2 INDEX. Fertilization of ovule, in Flowering plants, i. 352, 385. Fibre-cells of anthers, i. 384; of seeds, i. 360, 361. Fibres, Muscular, ii. 281-283. Nervous, ii. 284-286. — Spiral, of Plants, i. 360, 361, i. 364-366. Fibrillse of Muscle, structure of ii. 281. Fibro-Cartilage, ii. 279. Fibro- Vascular Tissue of Plants, i. 363. Fibrous Tissues of Animals, ii. 271-273; formation of, ii. 254. Field's Dissecting and Mounting Micro- scope, i. 49, 50; his Educational Mi- croscope, i. 53. Filiferous Capsules of Zoophytes, ii. 135. Finders, i. 99; Maltwood's, i. 100. Fine Adjustment, i. 40; uses of, i. 137- 139. FISHES, bone of, ii. 257, 258; teeth of, ii. 258, 259; scales of. ii. 261-263; blood of, ii. 268, 269; circulation in, ii. 288; gills of, ii. 299. Fishing tubes, i. 127. Flagella, of Protococcus, i. 233; of Vor- vox, i. 237; of Bacteria, i. 308. Flagellata (Infusoria), ii. 26-37; their relation to Sponges, ii. 117. Flatness of field of Objectives, i. 164. Flints, organic structure in, ii. 308; ex- amination of, ii. 308. Flint Glass, dispersive power of, i. 10; use of, in Objectives, i. 15, 16. Floridece, i. 334. Floscularians, ii. 60, 61. Flowers, small, as Microscopic objects, i 382; structure of parts of, i. 382-385. Fluid, mounting objects in, i. 215-217. Fluke, ii. 194. Flustra, ii. 157-161. Fly, fungous disease of, i. 318; number of objects furnished by, ii. 218; eye of ii. 230; circulation in, ii. 238; tongue of, ii. 234; spiracle of, ii. 239; wing of, ii. 241; foot of, ii. 243; development of, ii. 248. Focal Adjustment, i. 137-139; errors arising from imperfection of, i. 151- 153. Focal Depth of Objectives, i. 163; in- crease of, with Binocular, i. 38. Follicles of Glands, ii. 280. Foot of Fly, ii. 243: of Dytiscus, ii. 244; of Spider, ii. 250. FORAMINIFERA, ii. 64-106; their relation to Rhizopods, ii. 7, 66; their general structure, ii. 66-70; porcellanous, ii. 70-77; arenaceous, ii. 77-85; vitreous, ii. 85-106; collection and mounting of, ii. 107-109; fossil deposits of, see Fos- sil Foraminifera; mode of making sections of, i. 198 note. Forceps, i. 128; Stage, i. 120; Slider, i. 185. Forficulidce, wings of, ii. 242. Formed Material, Dr. Beale on, ii. 253- 255. Fossil Bone, ii. 311. Diatoms, i. 302, 303, ii. 304, 305. Foraminifera, ii. 72, 73, 78, 82-84, 90, 96-106, 304-310. Radiolaria, ii. 109. Sponges, ii. 307, 308. Teeth, ii. 310, 311. Wood, i. 371, 373, ii. 302-304. Fowl, lung of, ii. 300. Fragillariece, i. 286. Free Cell formation in Plants, i. 227, 228. Freezing Microtome, i. 191, 192. Frog, blood of, ii. 268-271; pigment cells of, ii. 275, 276; circulation in web of, ii. 286-288; in tongue of, ii. 288; in lung of, ii. 288; structure of lung of, ii. 299, 300. Fructification, of Chara, i. 260-262; of Fuci, i. 332-334; of Florideae, i, 334- 335; of Lichens, i. 329; of Fungi, i. 321. 324; of Marchantia, i. 837, 338; of Mosses, i. 339-342; of Ferns, i. 344- 348; of Equisetacese, i. 349; of Lyco- podiacesB, i. 350. Fucacece, i. 331-334; sexual apparatus of, i. 332-334; development of, i. 334. FUNGI, relation of, to Algae, i. 229, 307; to Animals, i. 307, 325-329; to Li- chens, i. 329; simplest forms of, i. 307- 316; in bodies of living Animals, i. 316-320; in substance, or on surface, of Plants, i. 323, 324; amoeboid states of, i. 325, 326; universal diffusion of sporules of, i. 313-321; culture of, i. > 23 note, i. 307. Furcularians, ii. 62. Fusulina, ii. 90, 91. Gad-flies, ovipositor of, ii. 246. Gall-flies, ovipositor of, ii. 245. Galls of Plants, ii. 245. Ganglion-Cells, ii. 284. Ganoid scales of Fish, ii. 263. GASTEROPODA, structure of shells of, ii. 178; palates of, ii. 180-183; develop- ment of, ii. 183-189; organs of sense of, ii. 190, 191. Gastrula, ii. 1. Geikie, Prof., on Geographical evolu- tion, ii. 313 note. Gelatine, see Glycerine jelly. Gelatinous Nerve fibres, ii. 284. Generation, distinguished from Growth, i. 227-229; in Cryptogams, i. 350; in Phanerogams, i. 352. Geology, applications of Microscope to, ii. 302-314. Geranium-petal, peculiar cells of, i. 383. Germ-cell of Cryptogams, i. 350; of Phanerogams, i. 352. Germinal Matter, Dr. Beale on, ii. 233. INDEX. 343 Gills, of Mollusks, ciliary motion on, ii. 189, 190: of Fishes, distribution of vessels in, ii. 299; of Water-newt, cir- culation in, ii. 288. Gizzard of Insects, ii. 237. Glands, structure of, ii. 279, 280. Glandular woody fibre of Conifers, i. 364. Glass Slides, i. 175, 176. Stage-plate, i. 122. Thin, i. 176, 177. Glaucium, cyclosis in hairs of, i. 359. Globigerina, ii. 86, 87. Globigerina-mud, ii. 86; its relation to Chalk-formation, ii. 305-309. Globigerinida, ii. 86-91. Glochidium, ii. 183. Glue, Liquid, i. 179. , Marine, uses of, i. 179, 181. Glycerine, for mounting objects, i. 210- 213. Glycerine Jelly, i. 211; mounting in, i. 212. Glycerine and Gum medium, i. 211, 213. Gnat, scale of, i. 155: transparent larva of, ii. 237. Gold-Size, use of, i. 178. Gomplionemece, i. 298. Goniometer, i. 95. Gonidia, i. 229 note. i. 230; multiplica- tion by, in Desmidiaceae, i. 267; in Pediastreee, i. 270; in Diatomacese, i. 280; in Hydrodictyon, i. 253; inChara, i. 261; in Lichens, i. 329; in Fungi, i. 322, 324; in Volvox, i. 241. Gordius, ii. 193. Gorgonia, spicules of, ii. 136, 137. Gosse, Mr., on mastax of Rotifers, ii. 56, 57; on sexes of Rotifers, ii. 58; on Melicerta, ii. 61; on thread-cells of Zoophytes, ii. 136. Grainmatopliora, i. 288; its use as test, i. 171. Grantia, structure of, ii. 120, 121. Grasses, silicified cuticle of, i. 379. Gray, Dr., on palates of Gasteropods, ii. 182; on development of Buccinum, ii, 187. Green Sands, Foraminiferal origin of, ii. 309, 310; Prof. Ehrenberg on com- position of, 11. 93 note, ii. 309. Gregarinida, ii. 21, 22. Gromia, ii. 8-10. Growing-Slide, i. 122. Growth, distinguished from Generation, i. 227-229. Guano, Diatomaceae of, i. 303. Gulliver, Mr., on Raphides, i. 363; on sizes of Blood-disks, ii. 269-271. Gum Arabic, i. 179. Guy, Dr., on sublimation of Alkaloids, ii. 326. Gymnosperms, i. 352, 353. Haeckel, Prof., on Gastrsea theory, ii. 22 note,' on Monerozoa, ii. 2; on Ba- thybius, ii. 19; on Radiolaria, ii. 110; on Infusoria, ii. 26 note; on Calcare- ous Sponges, ii. 120 note. Hcematococcus, i. 245; its relations to Protococcus, i. 233. Haematoxylin, as staining agent, i. 206. Hairs, of Insects, ii. 227; of Mammals, ii. 263-266. of Vegetable cuticles, i. 378; rota- tion of fluid in, i. 358, 359. Halichondria, spicules of, ii. 119. Halifax, Dr., on making Sections of Insects, ii. 219. Haliomma, ii. 113. Haliotis, palate of, ii. 182. Haliphysema, ii. 80. Halodactylus, ii. 161. Halophragmium, ii. 81. Halteres of Diptera, ii. 243. Hand-Magnifiers, i. 19-22, 43, 44. Hard Substances, cutting Sections of, i. 196-199. Hardening of Animal Substances, i. 202. Harley Binocular, i. 70. Harting, Prof., on Calcareous Concre- tions, ii. 324. Hartnack, M., his diagonal Micrometer, i. 94; onSurirella, i. 171. Harvest-bug, ii. 249. Haversian Canals of Bone, ii. 256. Haustellate Mouth, ii. 236. Hay craft, Mr., on Muscular fibre, ii. 286. Hazel, stem of, i. 370. Hearing, organs of (?), in Insects, ii. 233. Heart-wood, i. 370. Heat, tolerance of, by Bacteria, etc., i. 313; by Infusoria, ii. 31, 32. Heliopelta, i. 292. Heliozoa, ii. 8, 11-14. Helix, palate of, ii. 181. Hemiptera, wings of, ii. 242. Hepaticce, i. 335-338; see Marchantia. Hep worth, Mr., on feet of Insects, ii. 244. Hertwig, Dr., on Rhizopods, ii. 8 note, ii. 9; on Foraminifera, ii. 64 note. Heteromita, ii. 30. Heterostegina, ii. 99. Hexiradiate Sponges, ii. 121. Hicks, Dr., on Volvox, i. 244; on Amoe- boid production in root-fibres of Mosses, i. 339; on eyes of Insects, ii. 230; on peculiar organs of sense in Insects, ii. 233 note, ii. 243. Hincks, Rev. T., on Hydroid Zoophytes, ii. 126; on Polyzoa, ii. 162 note. Hippocrepian Polyzoa, ii. 161, 162. Hogg, Mr., on development of Lym- naeus, ii. 186. Hollyhock, pollen-grains of, i. 37, 167, i. 385. Holothurida, skeletons of, ii. 148-150. Holtenia, ii. 121. Homogeneous Immersion, i. A 7 18. 344 INDEX. Hoofs, structure of, ii. 267. Hooker, Sir J. D., on Antartic Dia- toms, i. 301. Hoop, of Diatoms, i. 275, 279, 280. Hormosina, ii. 81. Hornet, wings of, ii. 242. Horns, structure of, ii. 267. Houghton, Rev. W., on Glochidium, ii. 183. Hudson, Dr., on in Pedalio, ii. 58, 63. Huxley, Prof., on Protoplasm, i. 222; on cell formation in Sphagnaceae, i. 342; on Bathybius, ii. 19; on Coccoliths, ii. 19, 20; on Rotifera, ii. 59, 63, on Thalassicolla, ii. 116 note; on Nocti- luca, ii. 34 note; on Shell of Mollusca, ii. 173; on Appendicularia, ii. 169; on Blood of Annelida, ii. 197; on Shell of Crustacea, ii. 214 note; on Repro- duction of Aphides, ii. 246. Huyghenian eye piece, i. 23, 24. Hyalodiscus, i. 171, i. 289. Hydatina, ii. 62; reproduction of, ii. 58. Hydra, life-history of, ii. 122-126. Hydra tuba, developments of Acalephs from, ii. 132-134. Hydrodictyon, i. 252, 253. Hydrozoa, simple, ii. 123; composite, ii. 126-131; their relation to Medusee, ii. 126, 131-134. Hyla, preparation of nerves of, ii. 286. Ice-Plant, cuticle of, i. 378. Ichneumonidce, ovipositor of, ii. 245. Illumination of Opaque objects, i. 147- 150; of Transparent objects, i. 144- 147; diverse effects of, on lined ob- jects, i. 146-147. Illuminators, Black-ground, i. 106-110, i. 147. Oblique, i. 104, 105, i. 145- 147. Parabolic, i. 107, 108. Reflex, i. 109, 110. Side, i. 114-117. Vertical, i. 117-119. "Wenham's Disk, i. 105; his Reflex, i. 109, 110. White Cloud, i. 111. Imbedding processes, i. 194-196. Immersion-Lenses, i. 16-18. Images, formation of, by convex lenses, Index of Refraction, i. 1-3. Indigo-carmine, as staining agent, i.207. Indian Corn, cuticle of, i. 377, 380. Indicator, Quekett's, i. 96. Indusium of Ferns, i. 346. INFUSORIA, ii. 25-53; Flagellate, ii. 26; Suctorial, ii. 39; Ciliate, ii. 41; move- ments of, ii. 43, 44; internal struc- ture of, ii. 44, 45; binary subdivision of, ii. 46; encysting process of, ii. 47- 49; sexual generation (?) of, ii. 49-52. Infusorial Earths, i. 302. Injections of Blood-vessels, mode of making, ii. 292-298. INSECTS, great numbers of objects furnished by, ii. 218; microscopic forms of, ii. 219; antennae of, ii. 232, 233; circulation of blood in, ii. 237, 238; eggs of, ii. 246, 247; eyes of, ii. 229-231; feet of, ii. 243-245; gastric teeth of, ii. 237; hairs of, ii. 227; in- tegument of, ii. 219, 229; mouth of, ii. 233-236; organs of hearing in, ii. 233; of smell in, ii. 243; of taste in, ii. 236; ovipositors of, ii. 245, 246; scales of, ii. 220-228; spiracles of, ii. 239, 240; stings of, ii. 245; tracheae of, ii. 238-240; wings of, ii. 241-243. Interference-spectra, i. 156-160. Intermediate Skeleton of Foraminifera, ii. 70, 86, 91, 94, 103. Internal Casts of Foraminifera, ii. 89, 92, 93, 99, 100, 105, 309. Interpretation, errors of, i. 150-155. Inverted Microscope, Dr. L. Smith's, i. 82. Iodine, as test, i. 208. Iris, structure of leaf of, i. 380, 381. Iris-diaphragm, i. 102. Isthmia, i. 293; markings on, i. 276; self -division of, i. 280. Itch-Acarus, ii. 249. lulus, fungous vegetation in, i. 319. Jackson, Mr. G., his model for Com- pound Microscope, i. 52; his Eye-piece Micrometer, i. 93, 94. Jevons, Prof., on Brownian movement, i. 154. Jukes, Prof., on Foraminiferal reef, ii. 305. Kellner's Eye-piece, i. 24, 25. Kerona silurus, ii. 42. Kent, Mr. S., on Flagellate Infusoria, ii. 32, 33; on Sponges, ii. 118 note. Kidney, structure of, ii. 280. Klein, Dr., on Cells arid Nuclei, ii. 252 note. Kleinenberg, Prof., on Hydra, ii. 126 note, ii. 253 note; his preparing fluid, i. 203; his staining fluid, i. 206. Koch, on Sections of hard and soft sub- stances, i. 200. Kolliker, Prof., on Fungi in Shells, etc., i. 320 note. Kovalevsky, on development of Asci- dians, ii. 169 note. Kuhiie, on contraction of Vorticella- stalk, ii. 44. Labelling of Objects, i. 218, 219. Laboratory Dissecting Microscope, i. 47. Labyrinthodon, tooth of, ii. 311. Lachmann, see Claparede and Lach- mann. Lacinularia, Prof. Huxley on, ii. 58 note. INDEX. 345 Lacunas of Bone, ii. 256, 257. Lagena, ii. 66, 85. Laguncula, ii. 157-160. Lamellicornes, antennae of, ii. 232. Lamps for Microscope, i. 131, 132. Lankester, Prof. E. Ray, on amoeboids in fresh- water Medusa, ii. 122; on de- velopment of Limnaeus, ii. 186. Larvae of Echinoderms, ii. 150-156. Laurentian Formation of Canada, ii. 101, ii. 310; of Europe, ii. 101 note. Leaves, structure of, i. 380-382. Leech, teeth of, ii. 203. Leeson, Dr. , his double-refracting Go- niometer, i. 95; his Selenite-plate, i. 112. Legg, Mr., on collection of Foramini- fera, ii. 107, 108. Leidy, Dr., on Enterobryus, i. 319; on Rhizopods, ii. 19 note. Lenses, refraction by, i. 3, 4. Lepidocyrtus, scales of, see Podura. Lepidoptera, scales of, ii. 220-229; pro- boscis of, 236, 237; wings of, ii. 220, 242; eggs of, ii. 247. Lepidosteus, bony scales of, ii. 257, 263. Lepidostrobi, i. 350. Lepisma, scales of, ii. 223, 224; diffrac- tion-spectrum of, i. 159. Lepralia, ii. 158, 162. Lerncea, ii. 213. Levant-Mud, microscopic organisms of, ii. 305. Lever of Contact, i. 177. Lewis, Mr. B., his freezing Microtome, i. 192. Libellula, eyes of, ii. 230; respiration of larva of, ii. 240. Liber, i. 374. Lichens, composite nature of, ii. 329, 330. Lichmophorece i. 286. Lieberkiihn (speculum), i. 117, 118; mode of using, i. 150. Lieberkuhnia, ii. 6, 7. Ligaments, structure of, ii. 272. Light, for Microscope, i. 131-134; ar- rangement of, for Transparent ob- jects, i. 143-147; for Opaque objects, i. 147-150. Light-modifiers, i. 110. Ligneous Tissue, i. 363, 364. Umax, shell of, ii. 179; palate of, ii. 181. Limestones, organic origin of, ii. 308, 309; Fusuline, ii. 90, 91, 309; Nummu- litic, ii. 94, 96, 309; Milioline, ii. 309; Orbitoidal, ii. 100; Eozoic, ii. 101, 102, 310. Limiting Angle, i. 3. Limpet, palate of, ii. 182. Liquid Glue, i. 179. Lined Objects, diverse effects of Illumi- nation on, i. 146, 147. Tests, resolution of, i. 169-172. Lister, Mr. J. J., his improvements in Achromatic lenses, i. 14; his Erector, i. 87; his observations on Zoophytes, ii. 127; on Social Ascidians, ii. 167, 168. Lister, Prof., on Bacteria, etc., i. 314. Lituolida, ii. 81-85. Live-box, i. 123. Liver, structure of, ii. 280, 281. Liverwort, i. 335-338. Lobb, Mr., on binary subdivision in Micrasterias, i. 266. Lobosa, ii. 8, 14-19. Loftusia, ii. 84. Logan, Sir W., on Laurentian Forma- tion, ii. 101 note, ii. 310. Lophophore of Polyzoa, ii. 158. Lophyropoda, ii. 207, 208. Lowne, Mr., on feet of Insects, ii. 244 note; on eyes of Insects, ii. 231 note; on development of Insects, ii. 248. Lubbock, Sir J., on Daphnia, ii. 211; on Thysanura, ii. 223. Liiders, Mad., on fermentation, i. 316. Luminosity of Noctiluca, ii. 33, 36; of Anelida, ii. 202. Lungs of Reptiles, ii. 299; of Birds, ii. 300; of Mammals, ii. 300. Lyccenidce, scales of, ii. 221, 223. Lyeopodiacece, i. 350. Lymnceus, development of, ii. 184, 186. Lymph, corpuscles of, ii. 270. Machilis, scale of, ii. 224. Macro-gonidia, i. 229 note; of Volvox, i. 241; of Pediastreae, i. 271; of Hydro- dictyon, i. 252. Maddox, Dr., his Growing-Slide, i. 123; on cultivation of Microscopic Fungi, i. 123 note. Magnifying power, augmentation of, i. 136, 137; determination of, i. 173, 174. Magenta, as staining agent, i. 206. Mahogany, section of, i. 372. Malpighian bodies of Kidney, ii. 280. layer of Skin, ii. 275. Maltwood's Finder, i. 100. Malvacece, pollen-grains of, i. 385; their use as tests, i. 37, 167. MAMMALS, bone of, ii. 255-258; teeth of, ii. 259-261; hairs, hoofs, etc., of, ii. 263-267; blood of, ii. 267-271; lungs of, ii. 300. Man, teeth of, 259-261; hair of, ii. 265, 266; blood of, ii. 267-271. Mandibulate mouth of Insects, ii. 233. Marchantia, general structure of, i. 335; stomata of, i. 336; conceptacles of, i. 337; sexual apparatus of, i. 338. Margaritacece, shells of, 173, 174. Marine Glue, uses of, i. 179, 181. Marsh, Dr. S., his section-lifter, i. 205; on Section-cutting, etc., i. 194, 213. Mastax of Rotifera, ii. 56. Mastogloia, i. 299, 300. 346 INDEX. Matthews, Dr., his Micro-megascope, i. 88; his saw for Section-cutting, i. 197. Media, Preservative, i. 209-211. Medullary Rays, i. 354, 371-273. Sheath, i. 364, 369. Medusce, their relation to Polypes, ii. 126, 131-134; fresh-water, amceboids in, 122. Megalopa-larvsi of Crab, ii. 216. Megatherium, teeth of, 261. Melanospermece, i. 332. Melicertians, ii. 60, 61. Melolontha, see Cockchafer. Melosira, i. 289; auxospores of, i. 282. Menelaus, scale of, ii. 221, 222. Meniscus Lenses, refraction by, i. 6. Meridian circular e, i. 285. Mesembryanthemum, cuticle of, i. 378. Mesocarpus, i. 236. Metamorphosis, of Annelids, ii. 198- 201; of Ascidians, ii. 168-170; of Cir- rhipeds, ii. 213, 214; of higher Crus- tacea, ii. 215, 216; of Entomostraca, ,ii. 211; of Echinoderms, ii. 150-153; of Infusoria, ii. 47-49; of Insects, ii. 248; of Mollusks, ii. 183-189. Metazoa, ii. 2. Mica-Selenite Stage, i. 113. Micr aster ias, binary sub-division of, i. 266; stato-spores of, i. 267. Micro-Chemistry, ii. 326. Micrococcus, i. 308. Micro-gonidia, i. 229 note; of Protoc- cus, i. 234; of Desmidiaceae, i. 267; of Hydrodictyon, i, 253. Micro-megascope, i. 88. Micrometers, Ramsden's, i. 92, 93; Jack- son's, i. 93, 94; Hartnack's, i. 95. Micrometry, by Micrometer, i. 93-95; by Camera Lucida, i. 99. Micropyle of Vegetable Ovule, i. 353, 386. MICROSCOPE, support required for, i. 130, 131; care of, i. 134, 135; focal ad- justment of, i. 137-141 ; arrangement of, for Transparent objects, i. 141- 147; for Opaque objects, i. 147-150. — Binocular, see Binocular Microscope. Compound, see Compound Microscope. Simple, see Simple Micro- . scope. — Chemical, i. 82, 83. Demonstrating, i. 81. Dissecting, i. 44-50. — Educational, i. 53-55. • Inverted, i. 82-84. — Mineralogical, ii. 315-317. — Pocket, i. 81 . — Popular, i. 68. — Portable- Binocular, i. 83. — Student's, i. 55-67, ii. 332. — Travelling, i. 81, 82. Microscopic Dissection, i. 186-188. Micro-Spectroscope, i. 89-92. Microtome, Simple, i. 189; Hailes's, i. 190, 191; Strassburg, i. 191; freezing, i. 192; Rivet-Leiser, i. 192, 193. Microzymes, i. 314. Mildew of Corn, i. 323. Miliolida, ii. 70. Millon's test for Albuminous substances, i. 208. Milne-Edwards, M., on Compound As- cidians, ii. 166, note. Mineral Objects, ii. 313-326. Minnow, circulation in, ii. 288. Misinterpretation of microscopic ap- pearances, causes of, i. 150-156. Mites, ii. 249. Mivart, Prof., on Radiolaria, ii. 110. Moderator, Rainey's, i. 110. Molecular Coalescence, ii. 323-325. Movement, i. 153, 154. MOLLUSCA, shells of, ii. 171-180; palates of, ii. 180-183; development of, ii. 183-189; ciliary motion on gills of, ii. 189, 190; organs of sense of, ii. 190, 191. Molybdate of Ammonia, i. 207. Monadina, ii. 26-32. Monerozoa, ii. 2-7. Monocotyledonous Stems, structure of, i. 367, 368. Monothalamous Foraminifera, ii. 66. Morula, ii. 1. Morehouse, Mr., on Lepisma-scale, ii. 224, ii. 227 note. Morris, Mr., his Object-holder, i. 121, 122; on mounting Zoophytes, ii. 131. MOSSES, structure of, i. 338. 339; sexual apparatus of, i. 340-342; development of spores of, i. 341. Mother-of -Pearl, structure of, ii. 174. Moths, see Lepidoptera. Moulds, fungous, i. 321, 322. Mounting of objects, i. 211; in Canada Balsam, i. 214, 215; in cement cells, i. 215; in deep cells, i. 216. Mounting-Instrument, Smith's, i. 186. Microscope, Field's, i. 49, 50. Mounting- Plate, i. 185. Mouse, hair of, ii. 264; cartilage of ear of, ii. 278; vessels of toe of, ii. 297 Mouth of Insects, ii. 233-236. Mucor, i. 323. Mucous Membranes, structure of, ii. 275; capillaries of, ii. 298. Miiller, Dr. Fritz, on Polyzoa, ii. 160. Miiller, Prof. J., on Radiolaria, i. 110; on Echinoderm-larvse, ii. 150-153. Miiller s fluid, for hardening, i. 203. Muscardine, of Silk-worms, i. 317, 318. Muscular Fibre, structure of, ii. 281- 284; mode of examining and prepar- ing, ii. 282; capillaries of, ii. 298. Musk-deer, hair of, ii. 264; minute blood- corpuscles of, ii. 269. Mussel, ciliary action on gills of, ii. 189; development of, ii. 183. INDEX. 347 Mya, structure of hinge-tooth of, ii. 175. Mycelium of Fungi, i. 320-825. Myliobates, teeth of, ii. 258, 259. Myriapods, hairs of, ii. 228. Myriothela, amoeboids in, ii. 122. Myxomycetes, i. 325-327. Nachet, M., his Stereoscopic Binocular, i. 28, 29; Stereo-pseudoscopic Binocu- lar, i. 83-35; Binocular Magnifier, i. 48, 49; Student's Microscope, i. 63, 64; Chemical Microscope, i. 82, 83; Min- eralogical Microscope, ii. 315-317; Erecting Prism, i. 88; Camera, i. 98. Porte-Objectif, ii. 333. Nacre, structure of, ii. 173, 174. Nais, ii. 202, 203. Nassula, teeth of, ii. 43. Navicellse of Gregarinida, ii. 22. Naviculce, i. 298; movements of, i. 283. Needles for Dissection, i. 188. Nematoid Entozoa, ii. 193, 194. Nemertes, larva of, ii. 198, 199. Nepa, trachea! system of, ii. 239. Nepenthes, spiral vessels of, i. 364. Nervous Tissue, structure of, ii. 284, 285; mode of examining, ii. 286. Net, Collector's, i. 219-221. Nettle, sting of, i. 379. Neuroptera, circulation in, ii. 237, 240; wings of, ii. 241. Neutral-tint Reflector, i. 98. Newt, circulation in larva of, ii. 288. Nicol-Prism, i. 111. Nitella, i. 259. Nitzschiece, i. 287. Nobert's Test, i. 169, 170. Noctiluca, ii. 33-37. Nodosaria, ii. 85. Nonionina, ii. 94. Nose piece, i. 99. Nostochacece, i. 249. Nucleus, of Vegetable cells, i. 225-228; of Animal cells, ii. 254. Nudibranchs, development of, ii. 185, 186. Numerical Aperture of Objectives, ii. 327. Nummulinida, ii. 69, 91-101. Nummulite, structure of, ii. 94-98. Nummulitic Limestone, ii. 94, ii. 309. Nuphar lutea, parenchyma of, i. 854, 355. Oak, galls of, ii. 245. Object-Glasses, Achromatic, principle of, i. 7-10; Angular aperture of, i. 8, i. 161-163; ii. 327; Numerical aperture of, ii. 327; construction of, i. 11-16; immersion, i. 16-18; adjustment of, for covering glass, i. 14, i. 139-141; adaptation of, to Binocular, i. 35-38; working distance of, i. 161; defining power 'of, 161-163; focal depth of , i. 163; increase of, with Binocular, i. 38; resolving power of, i. 164; flatness of field of, i. 164; comparative value of i. 161-165; Preface, v., vi.; different powers of, tests for, i. 165-173; de- termination of magnifying power of, Object-Holder, i. 121, 122. Objects, mode of mounting, dry, i. 179, 183; in Canada balsam, i. 213, 214; in preservative media, i. 209-213; in cells, i. 215-217; see Opaque and Transparent Objects. labelling and preserving of, i. 218, 219. collection of, i. 219-221. Oblique Illuminators, i. 104, 105. Ocelli of Insects, ii. 229-231. Octospores of Fuci, i. 333. (Edogoniece, i. 257, 258. Oidium, i. 323. Oil-globules, microscopic appearances of, i. 152, 153. Oil-immersion Objectives, i. 17, 18; ii. 329. Oleander, cuticle of, i. 378; stomata of, i. 380. Oncidium, spiral cells of, i. 361. Onion, raphides of, i. 363. Oogonia of Fucaceae, i. 333. Oolite, structure of, ii. 282. Oospores, i. 228, 229; of Volvox, i. 243; of Achlya, i. 251 ; of Sphaeroplea, i. 255; of (Edogonium, i. 258; of Batracho- spermeae, i. 259; of Chara, i. 262; of Fucaceae, i. 333. Opaque Objects, arrangement of Micro- scope for, i. 147-150; modes of mount- ing, i. 179, 183. Operculina, ii. 94-96. Ophiocoma, teeth and spines of, 'ii. 145. Ophioglpssece, prothallium of, i. 848. Ophiurida, skeleton of, ii. 145; develop- ment of, ii. 151. Ophrydince, ii. 46. Orbiculina, ii. 72, 73. Orbitoides, structure of, ii. 100, 101. Orbitolina, ii. 90. Orbitolites, structure and development of, ii. 70, 73-77; fossil, ii. 305. Orbulina, ii. 86. Orchideous Plants, i. 360. Ord, Dr. W. M., on Calculi, ii. 326. Ornithorynchus, hair of, ii. 265. Orthoptera, wings of, ii. 242. Osmic acid, uses of, i. 203. Osmunda, prothallium of, i. 348 note. Oscillatoriacece, i. 247, 249. Ostracea, shells of, ii. 175, 176. Ostracoda, ii. 207. Otoliths of Gasteropods, ii. 190; of Fishes, ii. 324. Ovipositors of Insects, ii. 245, 246. Ovules of Phanerogamia, i. 352; fertili- zation of, i. 385; mode of studying, i. 385, 386. Owen, Prof., on fossil Teeth, ii. 311; on fossil Bone, ii. 312. 348 INDEX. Oxytricha form of Trichoda, ii. 47-49. ter, shell of, ii. 176, 177. Pachymatisma, spicules of, ii. 120. Pacinian corpuscles, ii. 286. Palates of Gasteropods, ii. 180-183. Palm, stem of, i. 367, 368. Palmellacece, L 245, 246. Palmodictyon, i. 246. Palmoglcea macrococca, life-history of, i. 230, 231. Papillae of Skin, structure of, ii. 274, 285; capillaries of, ii. 298; of Tongue, ii. 285. Parabolic Speculum, i. 116. Parabolized Gas-Slide, i. 125. Paraboloid, i. 107, 108; immersion, i. 108, 109. Paraffin, imbedding in, i. 194-196. Paramecium, ii. 42; contractile vesicles of, ii. 45; binary subdivision of, ii. 46; sexual generation (?) of, ii. 49. Parasitic Fungi in Animal bodies, i. 317-321; in Plants, i. 323, 324. Parker, Mr. Jeffery, on Hydra, ii. 122. Parkeria, ii. 83-85. Passulus, fungous vegetation in, i. 321. Paste, Eels of, ii. 93. Pasteur, M., his researches on ferments, i. 313; on pebrine, i. 315. Patella, palatal tube of, ii. 182. Pearls, structure of, ii. 174. Pebrine, i. 315. Pecari, hair of, ii. 265. Pecten, eyes of, ii. 190; tentacles of, ii. 191. Pedalion, ii. 58, 63. Pedesis, Prof. Jevons on, i. 154. Pediastrece, structure of, i. 270-273; multiplication and development of, i. 270, 271; varieties of, i. 272. Pedicellariae of Echinoderms, ii. 144. Pedicellina, ii. 162. Pelargonium, cells of petal of, i. 383. Pelomyxa palustris, ii. 17. Peneroplis, ii. 66, 71. Penetrating power of Object-glasses, i. 163, increase of, with Binocular, i. 38. Penicillium, i. 323. Pentacrinoid larva of Comatula, ii. 152- 156. Pentacrinus, skeleton of, ii. 146. Perennibranchiata, bone of, ii. 257; blood-corpuscles of, ii. 269, 270. Peridinium, ii. 37, 38. Peristome of Mosses, i. 389-342. Peronospora, i. 323. Perophora, ii. 167, 168. Petals of Flowers, structure of, i. 383. Petrology, Microscopic, ii. 312-317. Pettenkofer's test, i. 208. PHANEROGAMIA, distinctive peculiari- ties of, i. 352, 353; elementary tissues of, i. 353-367 (see Tissues of Plants); Stems and Roots of, i. 367-377; Cuti- cles and Leaves of, i. 377-382; Flowers of, i. 382-386; Seeds of, i. 386-388. Phyllopoda, ii. 209. Picric acid, for hardening, i. 203. Picro-aniline, as staining agent, i. 207. Picro-carmine, as staining agent, i. 206. Pieridce, scales of, ii. 221, 222. .Pigott, Dr. Royston, his Aplanatic Searcher, i. 8 note; his Micrometers, i. 92 note; on angle of aperture, i. 162; on scales of Insects, ii. 224, 226. Pigment-cells, ii. 275, 276; of Cuttle- fish, ii. 191; of Crustacea, ii. 215. Pigmentum nigrum, ii. 275. Pilidium-larva, of Nemertes, ii. 199. Pillischer, Mr. , his International Micro- scope, i. 59, 60. Pilulina, ii. 79. Pinna, structure of shell of, ii. 171-173; fossil, in Chalk, ii. 307. Pinnularia, i. 298. Pistillidia, see Archegonia. Pith, structure of, i. 354, 368. Placoid scales of Fish, ii. 263. Planaria, ii. 194, 195. Planorbulina, ii. 89. Plantago. cyclosis in hairs of, i. 359. Plants, distinction of, from Animals, i. 222-224. Plasmodium, of Myxomocetes, i. 326; of Protomyxa, ii. 3. Plate-glass Cells, i. 182. Pleurosigma, i. 298; nature of markings on, i. 274-279; value of, as Test, i. 170- 172; diverse aspects of, i. 146-151; diffraction-spectrum of, i. 160. Pluteus-larvsi of Echinus, ii. 152. Plumules of Butterflies, i. 221. Pocket Microscope, Beale's, i. 80. Podophrya quadripartita, ii. 39-41. Podura, scale of, ii. 223-227; use of, as Test object, i. 172, 173. Poisons, detection of, ii. 326. Polarization, Objects suitable for, i. 318-323. Polarizing Apparatus, i. 111-114. Polistes, fungous vegetation in, i. 318. Pollen-grains, development of, i. 383; structure and markings of, i. 383- 385. Pollen-tubes, fertilizing action of, i. 386. Poly celis, ii. 195. Polyclinians, ii. 165. Polycystina, ii. 109, 113-116. Polygastrica, see Infusoria. Polymorphina, ii. 85. Polyommatus argus, scale of, ii. 222, 223. Polypes, see Hydra and Zoophytes. Polypide of Polyzoa, ii. 157. Polypodium, fructification of, i. 345. Polystomella, ii. 92-94. Polythalamous Foraminifera, ii. 66-68. Polytoma uvella, ii. 29. Polytrema, ii. 90. INDEX. 349 POLYZOA, general structure of, ii. 157- 163; classification of, ii. 162. Polyzoary, ii. 157. Pond-Stick, Baker's, i. 219. Poppy, seeds of, i. 386, 387. Popular Microscope, Beck's, i. 68. Porcellanous Foraminifera, ii. 68, 70- Porcellanous shells of Gasteropods, ii. 178. Porcupine, quill of, ii. 265. PORIFERA, see Sponges. Portable Binocular, i. 83. Potato-disease, i. 323. Powell and Lealand's Microscopes, i. 67, 68, 77, 79; their non-stereoscopic Binocular, i. 85; their Achromatic Condenser, i. 103; their Light-modi- fier, i. 110; their Oil-immersion objec- tives, i. 18; their Vertical Illuminator, i. 116. Prawn, shell of, ii. 215. Preservative Media, i. 209-211 Primordial Utricle, i. 225, 356. Pringsheim, Dr., his observations on Vaucheria, i. 251; on Hydrodictyon, i. 252; on CEdogonium, i. 258. Prismatic Shell-substance, ii. 171, 172. Prism, Amici's, i. 106; Nachet's Erect- ing, i. 88; Wenham's Binocular, i. 30, 85; Stephenson's Binocular, i. 31; Camera Lucida, i. 96-98; Spectro- scope, i. 90; Polarizing, 111, 112. Proboscis, of Bee, ii. 234, 235; of Butter- fly, ii. 236; of Fly, ii. 234. Proteus, blood- corpuscles of, ii. 269, 270. Prothallium of Ferns, i. 346-348. Protococcus, life-history of, i. 231-236. Protomyxa, ii. 2, 3. Protoplasm, i. 222; of Vegetable cell, i. 224-228; of Aninfals, ii. 253-255. PROTOPHYTA, general characters of, i. 222-228. Protophytic Algee, i. 229-306. Protophytic Fungi, i. 229, 307; relation of, to Protozoa, i. 307; cultivation of, i. 123, 307. PROTOZOA, ii. 1, 2; their relations to Protophyta, i. 224. Pseud-embryo of Echinoderms, ii. 150. Pseudo-navicellse of Gregarinida, ii. 21. Pseudopodia of Rhizopods, ii. 2-19, dif- ferent forms of, ii. 7. Pseudoscope, i. 27, 28. Pseudoscopic Microscope of MM. Nachet, i. 33-36. Pteris, fructification of, i. 345; prothal- lium of, i. 346. Pterodactyle, bone of, ii. 3 12. Puccinia, i. 323. Purpura, egg-capsules of, ii. 184; devel- opment of, ii. 187-189. Pycnogonidce, ii. 205-207. Quadrula symmetrica, ii. 19. 3, M. de, on luminosity of Annelids, ii. 202. Quekett, Prof. J. , his Dissecting Micro- scope, i. 45; his Indicator, i. 96; on Raphides, i. 363; on structure of Bone, ii. 258, 311. Quinqueloculina, ii. 71. Radiating Crystallization, ii. 320, 321. Radiolaria, ii. 109, 110; their relation to Heliozoa, ii. 109; their general structure, ii. 110, 111; their classifica- tion, ii. 112, 113; collection and mounting of, ii. 115, 116. Rainey, Mr., his Light modifier, i. 110; on Molecular coalescence, ii. 313-325. Ralfs, Mr., on Desmidiacese, i. 263 note; on Diatomaceae, i. 284 note. Ralph, Dr., his mode of mounting, i. 215. Ramsden's Micrometer, i. 92, 93. Raphides, i. 363. Re-agents, Chemical, use of, in Micro- scopic research, i. 208, 209, ii. 326. Red Corpuscles of blood, ii. 267-270. Red Snow, i. 245. Reflection by Prisms, i. 2, 3, Reflex Illuminator, Wenham's, i. 109. Refraction, laws of, i. 1-3; by convex lenses, i. 3-5; by concave and menis- cus lenses, i. 5-6. Reindeer, hair of, ii. 264. Reophax, ii. 82. REPTILES, bone of, ii. 257, 258, 311; teeth of, ii. 259, 311; scales of, ii. 263; blood of, ii. 268-271; lungs of, ii. 299, 300. Resolving power of Object-glasses, i. 158, 164. Reticularia, ii. 7-11. Reticulated Ducts, i. 366. Rhabdammina, ii. 80. Rhinoceros, horn of, ii. 267. Rhizocarpece, i. 350. RHIZOPODA, ii. 7-19; their subdivions, ii. 7, 8; their relation to higher Ani- mals, ii. 252, 253. Rhizosolenia, i. 296. Rhizostoma, ii. 134. Rhodospermece, i. 334. Rhubarb, raphides of, i. 363. Rhynchonellidce, structure of Shell of, ii. 178. Rice, starch-grains of, i. 362. Rice-paper, i. 354, 355. Ricinice, ii. 249. Ring-Cells, i. 181. Ring-Net, i. 219-221. Rivet-Leiser Microtome, i. 192, 193. Roasted Corn, detection of. in Chicory, i. 388. Robin, M., on Noctiluca, ii. 34 note, ii. 37 note. Rochea, epidermis of, i. 378. Rocks, structure of, ii. 304-310 313- 317. 350 INDEX. Roots, structure of, i. 375, 376; mode of making sections of, i. 376. Ross, Mr., on correction of Object- glass, i. 14, 15; his First-class Micro- scopes, i. 73-76; his Achromatic Con- denser, i. 103; his Students' Micro- scope, i. 60, 61; his Simple Microscope, i. 43-45; his Lever of contact, i. 177; his Compressor, i. 126. Ross-Model for Compound Microscope, i. 51, 52. Rotalia, ii. 67, 68, 89, 90. , Rotaline Foraminifera, ii. 67, 89, 90. Rotating Microscope, Browning's, i. 64, 65. Rotifer, anatomy of, ii. 55-58; repro- duction of, ii. 58, 59; tenacity of life of, ii. 59; occurrence of, in leaves of Sphagnum, i. 343, ii. 53. ROTIFERA, general structure of, ii. 53- 63; reproduction of, ii. 58, 59; desic- cation of, ii. 59; classification of, ii. 60-63. Royston-Pigott, Dr., see Pigott. Rush, stellate parenchyma of, i. 354, 355. Rust, of Corn, i. 323. Rutherford, Prof., his freezing Micro- tome, i. 192, 196. Sable, hair of ii 264. Saccammina, ii. 78, 79. Saccharomyces, i 315. Saccolobium, spiral cells of, i. 361. Safety-stage, Stephenson's, i. 120. Salpingceca, ii. 32. Salter, Mr. Jas., on teeth of Echinida, ii. 144. Salts, crystallization of, ii. 318-323. Salyia, spiral fibres of seed of, i. 361. Salicylic Acid, as preservative, i. 210. Sand-wasp, integument of. ii. 220. Sandy tests of Foraminifera, ii. 77-85. Sarcina ventriculi, i. 316. Sarcode, of Protozoa, i. 222 note, i. 222. Sarcoptes scabiei, ii. 249. Sarsia, ii. 127. Saw-flies, ovipositor of, ii. 245. 246. Scalariform ducts of Ferns, i. 344, .366. Scales, of cuticle of Plants, i. 378, 379. of Fish, ii. 261-263. of Insects, ii. 220-228; their use as Test-objects, i. 167-173. of Reptiles and Mammals, ii. 263. Schiek's Compressor, i. 126. Schizomycetes, i 307-313; their Zymotic action, i. 313-315. Schizonemece, i. 299. Schultz's test, i. 208. Schultze, Prof. Max, on Protoplasm, i. 222 note; on movement of fluid in Diatoms, i. 273; on surface-markings of Diatoms, i. 277 note. Schulze, Mr. A., on use of Illuminators, i. 110. Schwann, doctrines of, ii. 252. Schwendener, on Lichens, i. 329. Scissors, for microscopic dissection, i. 188; for cutting thin sections, ii. 188. Sclerogen, deposit of, on walls of Cells, i. 359, 360. Scolopendrum, sori of, i. 345. Sea Anemone, ii. 135, 136. Section-cutting Instruments, i. 189-193. Section-lifter, Marsh's, i. 205. Sections, thin, mode of making, of Soft substances, i. 188-196; modes of mounting, i. 212-214; of Hard Sub- stances, i. 196-200. of Foraminifera, i. 198 note; of Leaves, i. 382; of Wood, i. 376; of Echinus-spines, ii. 146, 147; of Insects, ii. 219; of Bones and Teeth, ii. 258; of Hairs, ii. 266. Seeds, testae of, i. 386-388; spiral cells in, i. 361. Segmentation of Yolk-mass, ii. 185, 187. Selaginellece, i. 350. Selenite Stages, i. 112-114. Sepiola, eggs of, ii. 191. Sepiostaire of Cuttle-fish, ii. 180, 325. Serialaria, colonial nervous system of, ii. 160. Serous Membranes, structure of, ii. 274. Serpentine-Limestone, ii. 101-107, 310. Sertularidce, ii. 129-131. Sexual Generation, lowest forms of, in Protophytes, i. 229, 230, 236, 237; in Infusoria, ii. 26-30. Shadbolt, Mr., on Arachnoidiscus, i. 293: his Annular condenser, i. 107 note; his Turn-table, i. 184. Shark, teeth of , ii. 258,259; scales, etc., of, ii. 263. Shell, of Crustacea, ii. 214, 215; of Echinida, ii. 140, 141; of Foramini- fera, ii. 68-70; of Mollusca, ii. 172- 180; Fungi in, i. 321. Shrimp, shell of, ii. 215. Side Illuminators, i. 114-116. Side- Reflector, Beck's, i. 116, 117. Siebert and Kraft's Dissecting Micro- scope, i. 46. Siebold, Prof., on reproduction of Bee, ii. 247. Silica crack-slide, i. 152, 162, ii. 321. Siliceous Epiderms, i. 349, 379. - Sponges, ii. 120, 121. Silk-worm diseases, i. 315-317. Silver, crystallized, ii. 319. Simple Microscope, optical principles of, i. 18-22; various forms of, i. 43-51. Siphonacece, i. 250, 251. Siricidce, ovipositors of, ii. 245, 246. Skin, structure of, ii. 274, 275; papillae of, ii. 284, 285, 298. Slack, Mr., on Pinnularia, i. 298; on artificial Diatoms, i. 277 note; his Diaphragm-Eyepiece, i. 95; his Light- modifier, i. Ill; his Stage- vice, i. 121; his Compressors, i. 126, 127; his Silica INDEX. 351 crack-slide, i. 152, 162; his crystalliza- tions from silicated solutions, ii. 321. Sladen, Mr. P., on preserving Echino- derm larvae, ii. 153. Slider-Forceps, i. 185. Slides, Glass, i. 175, 176. Wooden, i. 183. Slug, rudimentary shell of, ii. 179; pal- ate of, ii. 181, 182; eyes of , ii. 190. Smith, Mr. Jas., his Mounting Instru- ment, i. 186; his use of Bull's-eye Condenser, i. 118. Smith, Dr. Lawrence (U. S.), his In- verted Microscope, i. 82. Smith, Prof. H. L.^U. S.), on Binocular Eyepiece, i. 33; his vertical Illumin- ator, i. 118; his cells for dry-mounting, i. 180; on mounting Diatoms, i. 306. Smith, Prof. J. Edwards (U. S.), on de- velopment of (Edogonium, i. 257; on wide-angled Objectives, Preface, vi., vii. Smith, Prof. W., on Diatomaceae, i. 170, 273, 301 note. Smith and Beck, see Beck, Messrs. Smut, of Wheat, i. 323. Snail, palate of, ii. 181, 182; eyes of, ii. 190. Snake, lung of , ii. 299. Snow crystals, ii. 318. Social Ascidians, ii. 166-168. Soemmering's speculum, i. 97. Sole, skin and scales of, ii. 261, 262. Sollitt, Mr., on Diatom-tests, i. 170. Sorby , Mr. , on skeleton of Echinoderms, ii. 146 note; his Spectroscope Eye- piece, i. 90; his Microscopic examina- tion of Rocks, ii. 314, 315 note. Soredia of Lichens, i. 329. Sori of Ferns, i. 344-346. Spatangidium, i. 291. Spatangus, spines of, ii. 143. Spectacles, for Dissection, i. 187. Spectro-Micrometer, Browning's, i. 91. Spectroscope Eye-piece, i. 90. Spectroscopic Analysis, principles of, i. 89-92. Speculum, Parabolic, i. 116, 117. Spermogonia of Fungi, i. 322; of Li- chens, i. 330. Sphacelaria, i. 332. Sphceria, development of, within Ani- mals, i. 318. Sphceroplea, sexual reproduction of, i. 255. Sphcerosira volvox, i. 243. Sphcerozosma, i. 266. Sphcerozoum, ii. 115. Sphagnacece, peculiarities of, i. 342-344; occurrence of parasites in leaf-cells of, i. 327, ii. 53. Spherical Aberration, i. 6, 7; means of reducing and correcting, i. 7, 8. Spicules of Sponges, ii. 120-122; Alcy G-- man Zoophytes, ii. 136; of Doris, ii. 179. Spiders, eyes of, ii. 250; respiratory organs of, ii. 250; feet of, ii. 250; spin- ning apparatus of, ii. 251. Spines of Echinida. ii. 142, 143; mode of making sections of, ii. 146, 147; of Spatangus, ii. 143. Spinning apparatus of Spiders, ii. 251. Spiracles of Insects, ii. 239-241. Spiral Cells of Sphagnum, i. 343; of Orchideae, i. 360; of anthers, i. 384. Crystallization, ii. 321. Ducts, i. 366. Fibres, i. 361. Vessels, i. 364; in petals, i. 383. Spiriferidce, Shell-structure of, ii. 178. Spirillina, ii. 85. Spirillum, i. 312. Spirolina, ii. 72. Spiroloculina, ii. 71. SPONGES, general structure and relations of, ii. 117, 118; reproduction of, ii. 118; skeleton of, ii. 119-122; fossil, ii. 307, 308. Spongilla, ii. 118, 121. Spongiole of Root, i. 375. Spores, different kinds of, i. 228-230; of Fungi, general diffusion of, i. 321- 323; of Hepaticae, i. 338; of Mosses, i. 342; of Ferns, i. 345-348; of Equise- taceae, i. 349; — see Oospores and Zy- gospores. Spot-Lens, i. 107. Spring-Clip, i. 186. Press, i. 186. Scissors, i. 188. Squirrel, hair of, ii. 264. Stage-centering adjustment, i. 80. Stage, Glass, i. 63. Stage, Safety, i. 120. Stage-Forceps, i. 120. Stage-Plate, glass, i. 122. Stage- Vice, i. 121. Staining Processes, i. 204-208. Stanhope Lens, i. 21. Stanhoscope, i. 21. Star- Anise, seed-coat of, i. 360. Starch-granules, in Cells, i. 361, 362; appearance of, by Polarized light, i. 362. Star-fish, Bipinnarian larva of, ii. 150, 151. Stato-spores, i. 230 note; of Volvox, i. 244; of Hydrodictyon, i. 253. Staurastrum, prominences of, i. 263; self-division of, i. 265; varieties of, i. 272. Stauroneis, i. 299. Steenstrup, Prof., on Alternation of generations, ii. 134. Stein, Dr., his doctrine of Acineta forms, ii. 41 note, ii. 52 note; his re- searches on Infusoria, ii. 63 note. Steinheil Doublet, i. 21. Stellaria, petal of, i. 343. Stellate cells, of Rush, i. 354, 355; of Water lily, i. 354, 355. 352 INDEX. Stemmata of Insects, ii. 231. Stem, i. 367; Monocotolydonous, struc- ture of, i. 367, 367; Exogenous, struc- ture of, i. 368-374; development of, i. 374, 375; mode of making sections of, i. 376. Stentor, ii. 44; its conjugation, ii. 52. Stephanoceros Eichornii, ii. 60, 61. Stephanosphcera, i. 243 note, i. 244 note. Stereoscope, i. 25. Stereoscopic Spectacles, i. 187. Vision, principles of, i. 25- 28; application of, to Compound Mi- croscope, i. 27-39; to Simple Micro- scope, i. 48, 49. Stephenson, Mr., his suggestion of homogeneous immersion Objectives, i. 17; on diffraction-doctrine, i. 157- 160; his Binocular Microscope, i. 31- 33; his safety-stage, i. 120; on mount- ing in bisulphide of carbon, i. 279; on Coscinodiscus, i. 290. Stewart, Mr., on internal skeleton of Echinodermata, ii. 148. Stick-net, i. 220. Stigmata of Insects, ii. 239, 240. Stings of Plants, structure of, i. 379; of Insects, ii. 245, 246. Stokes, Prof., on Absorption bands of blood, i. 91, 92. Stomata/of Marchantia, i. 336; of Flow- ering Plants, i. 379, 380. Stones, for polishing Sections, i. 198. Stones, of Fruit, structure of, i. 360. Strassburger, Dr., on cell-division, ii. 254 note. Striatellece, i. 288. Student's Microscopes, principles of construction of, i. 55-57; Objectives suitable for, i. 57-58; various forms, of, i. 59-67. Suctorial Crustacea, ii. 212, 213. Suctorial Infusoria, ii. 39. Sulphate of Copper and Magnesia, radiating crystallization of, ii. 320. Sulphate of Copper, spiral crystalliza- tion of, ii. 321. Sulphuric Acid, as test, i. 208. Sundew, hairs of, i. 379. Sunk Cells, i. 182. Surirella, i. 287; conjugation of, i. 281; use of, as test, i. 171. Swift, Mr., his Challenge Microscope, i. 71; his Portable Binocular, i. 82, 83; his swinging Sub-stage, i. 71 note; his combination Sub-stage, i. 113, 114; his Aquatic box, i. 124; his Micro- scope lamp, i. 122; his Wale Students' Microscope, ii. 332. Synapta, calcareous skeleton of, ii. 149. Syncoryne, ii. 127. Syncrypta, i. 244. Synedrece, i. 287. Syringe, small glass, i. 128; uses of, i. 142, 204, 208, 212, 217, ii. 188 note, ii. 293. Syringe, injecting, ii. 293. Tabanus, ovipositor of, ii. 246. Table for Microscope, i. 130. Tadpole, pigment cells of, ii. 276; circu- lation in, ii. 288-292. Tcenia, ii. 192, 193. Tardigrada, ii. 62. Teeth, of Echinida, ii. 144, 145; of Ophiocoma, ii. 145; of Mollusks, ii. 181-183; of Leech, ii. 203; of Verte- brata, structure of, ii. 258-261; fossil, ii. 811, 312; mode of making sections of, ii. 258. Tendon, structure of, ii. 273. Tenthredinidce, ovipositor of, ii. 245. Terebella, circulation and respiration in, ii. 196, 197. Terebratula, shell-structure of, ii. 177, 178; muscular fibre of, ii. 283. Terpsinoe, i. 288. Tests, of Rhizopods, ii. 18, 19; of Foraminifera, ii. 77-85. Test-Liquids, i. 208, 209. Test-Objects, i. 165; for low powers, i. 166, 167; for medium powers, i. 167- 169; for high powers, i. 169-173. Tetramitus rostratus, ii. 30, 31. Tetraspores of Ceramiacese, i. 334. Textularia, ii. 68, 88. Thalassicolla, ii. 115. Thallus of lower Cryptogamia, i. 246, 329, 331. Thaumantias, ii. 131. Thecse, of Ferns, i. 345; of EquisetaceaB, i. 349. Thin Glass, i. 176, 177. Thomas, Mrs. H., on Cosmarium, i. 266. Thomas, Mr. R., on microscopic crys- tallization, ii. 321. Thompson, Mr. J. V., on development of Comatula, ii. 153; on metamor- phosis of Cirrhipeds, ii. 213; on meta- morphosis of Crustacea, ii. 216. Thomson, Sir Wyville, on Giobigerina, ii. 87; on Siliceous Sponges, ii. 121; on development of Pentacrinoid larva, ii. 156; on Chalk-formation, ii. 306. Thread-cells of Zoophytes, ii. 137, 138. Thrush, fungous vegetation of, i. 320. Thurammina, ii. 81. Thwaites, Mr., on conjugation of Dia- toms, i. 281, 282; on filamentous ex- tensions of Palmelleae, i. 246 note. Ticks, ii. 248. Tinea favosa, fungus of, i. 320. Tinoporus, ii. 89, 90. Tipula, larva of, ii. 240. Tissues, Elementary, of Animals, mi- croscopic study of, ii. 252; formation of, ii. 253-255; see Blood, Bone, Ca- pillaries, Cartilage, Epidermis, Epi- thelium, Fat, Feathers, Fibrous Tis- sues, Glands, Hair, Horn, Mucous Membranes, Muscle, Nervous Tissue, INDEX. 353 Pigment-cells, Scales, Serous Mem- branes, Teeth. Tissues, Elementary, of Plants, i. 353; Cellular, i. 353-363; Woody, i. 363, 364; Vascular, i. 365, 366; dissection of, i. 366; preparation of, i. 201. Tolles, Mr., his Binocular Eye-piece, i. 33; his Amplifier, i. 86; his vertical Illuminator, i. 119. Tomopteris, ii. 199-201. Tongues of Gasteropods, ii. 181-183; of Insects, ii. 234-236. Torula cerevisice, i. 315. Tous-les-mois, Starch-grains of, i. 362. Tow-net, i. 220. Tracheae of Insects, ii. 238-240; mode of preparing, ii. 240, 241. Tradescantia, cyclosis in hairs of, i. 358. Transparent Objects, arrangement of Microscope for, i. 141-145; various modes of Illuminating, i. 141-146. Travelling Microscopes, i. 81, 82. Trematode, Entozoa. ii. 194. Triceratium, i. 295; markings on, i. 277. Trichoda, bristles of, ii. 43; metamor- phosis of, ii. 47-49. Trichogyne, of Lichens, i. T.30; of Flori- dese, i. 335. Trilobite, eye of, ii. 310. Triloculina, ii. 71. Triple Staining, i. 207. Trochus, palate of, ii. 181. Trout, circulation in young of, ii. 289. Tube-cells, i. 180. Tubular Nerve-substances, ii. 284, 285. Tubularia, ii. 127. TUNICATA, general organization of, ii. 163, 164; see Ascidians. Turbellaria, ii. 194-196. Turn-tables, i. 184. Tyndall, Prof., on Bacteria, etc., i. 313, 314, 321. Ulvacece, i. 246, 247. Unicellular nature of Infusoria, ii. 25. Unicellular Plants, i. 229. Unionidce, shells of, ii. 174-176. Uredo, i. 323. Urns of Mosses, i. 340. Uvella, i. 235. Vacuoles, i. 225; microscopic appear- ances of, i. 153. Vallisneria, cyclosis in, i. 356, 357. Vampyrella, ii. 3-5. Van Beneden, Prof. Ed., on gigantic Gregarina, ii. 21. Vanessa, haustellium of, ii. 236. Variation, tendency to, in Desmideacese, j i. 271; in Diatomacea3, i. 282; in For- aminifera, ii. 72, 77, 96; in Polycys- tina, ii. 112, 113 note. Varnishes and Cements, i. 178, 179. Vaucheria, zoospores of, i. 250; sexual reproduction of, i. 251. Vegetable Ivory, i. 360. Vegetable Kingdom, differentiated from Animal, i. 222-233. Vegetable substances, preparation of, i. 201. Ventriculites, ii. 307. Vermilion injections, ii. 294, 295. VERTEBRATA, elementary structure of, ii. 252 (see Tissues) ; blood of, ii. 267- 271 circulation in, ii. 286-292. Vertical Illuminators, i. 118, 119; Vesicular Nerve Substance, ii. 284. Vessels of Plants, i. 365, 366. Vibracula of Polyzoa, ii. 163. Vibrio, i. 311, 312. Villi of intestine, injections of, ii. 295. Vine-disease, i. 323. Vinegar, Eels of, ii. 193. Vitreous Foraminifera, ii. 69, 85-107. Volvox, structure of, i. 237-240; devel- opment and multiplication of, i. 240, 241; generation of, i. 241-243; amoe- boid state of, i. 241-243. Vorticella, ii. 43, 46; encysting process in, ii. 47; conjugation of, ii. 52. Wale's New Working Microscope, i. 61- 63. Wallich, Dr., on making sections of Foraminifera, i. 198 note: on Diatoms, i. 275 note, i. 277 note; on Cocco- epheres, ii. 19; on nucleus in Gromia, ii. 9; on Globigerinaa, ii. 86; on Poly- cystina, ii. 113 note. Warts, structure of, ii. 276. Water-Bath, i. 185. Water-immersion Objectives, i. 16, 17; ii. 327, 330. Water-Lily •, stellate cells of, i. 354, 355; leaf of, i. 382. Water-newt, circulation in larva of, ii. 288. Water- Vascular system, of Rotifera, ii. 57; of Planaria/ii. 195. Watson, Messrs., their new form of Microscope, ii. 331, 832. Weber's Annular Cell, i. 124. Webster-Condenser, 1. 103, 104. Wenham, Mr., his new Achromatic combination, i. 15; his suggestion of homogeneous immersion, i. 17; his Binocular Microscope, i. 19*31; his Non-Stereoscopic Binocular, i. 84; his Disk-illuminator, i. 105; his Parabolic Illuminator, i. 107, 108; his Reflex Illuminator, i. 109; on adjustment of Object-glasses, i. 140: his observations on Pleurosigma, i. 277 note; on Cyclo- sis, i. 358, 359; on Podura scale, ii. 226. Whalebone, structure of, ii. 267. Wheat, blights of, i. 323, ii. 193. Wheatstone, Sir C., his invention of the Stereoscope, i. 25-27; of the Pseudo- scope, i. 27, 28. Wheel-animalcules, see Rotifera. 354: INDEX. White-cloud Illuminator, i. 111. White Corpuscles of blood, ii. 270, 271. White Fibrous tissue, ii. 272. Whitney, Mr., on circulation in Tad- pole, ii. 289-292. Williamson, Prof. W. C., on Volvox, i. 243 note; on shells of Crustacea, ii. 215 note; on scales of Fishes, ii. 261, 262; on Coal-plants, ii. 303; on Levant- mud, ii. 305. Wings of Insects, ii. 241-243. Winter-eggs, of Rotifera, ii. 59; of Hydra, ii. 125; of Entomostraca, ii. 210. Wollaston, Dr., his Camera Lucida, i. 96. Wood, of Exogenous stems, i. 369, 370. Woodward, Col. Dr., his Prism, i. 105; his resolution of Amphipleura pellu- cida, i. 171; of Surirella gemma, i. 172; on scale of Gnat, i. 155, on Podu- ra-scale, ii. 227. Woody Fibre, i. 363; glandular, of Con- ifers, i. 364. Working-distance of Objectives, i. 161. Wormley, Dr., on Micro-Chemistry, ii. 326. Wyth's Amplifier, i. 86. Xanthidia of Flints, i. 267 notet ii. 308. Yeast-plant, i. 315. Yellow Fibrous tissue, ii. 273. Yucca, epidermis of, i. 377; stomata of, i. 380. Zeiss's oil-immersion Objectives, i. 17, 18; his adjusting Low-power, i. 136, 166; his Sub-stage Condenser, i. 104 note. Zentmayer, Mr., on denning power, i. 162 note; his swinging tail-piece, i. 61, 75; his glass stage, i. 63. Zoea-larva of Crab, ii. 216. Zoantharia, ii, 135. Zooglcea, i. 308. Zoophyte-Trough, i. 125. ZOOPHYTES, ii. 122-137; see Actinozoa, Alcyonaria, and Hydrozoa. Zoospores, i. 230; note; of Protococcus, i. 2o3, 234; of UlvaceaB, i. 247; of Vau- cheria, i. 250; of Achlya, i. 251; of Confervaceae, i. 254; of Chaotophora, i. 258; of Pediastreaa, i. 271; of Fuca- cese, i. 334. Zygnemaceoz, i. 236, 237. Zygospores, i. 229; of Conjugatese, i. 232, 236; of Desrnidiacese, i. 267, 268; of Diatomacese, 281, 283. Zygosis of Actinophrys, ii. 12; of Amoe- ba; ii. 17; of Gregarina, ii. 22. Zymotic action of Bacillus-organisms, 313-315. ERRATUM. The first sentence in the Note to p. 163. vol. i., should run thus:— The Author is informed by Prof. Abbe, that the ' penetration ' of Objectives de- creases in a corresponding ratio with the increase of their respective Numerical Apertures; or, when Objectives of the same class are compared, with the increase in the sines of their respective semi-angles of aperture. 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