iby iS rariag nb nap tor ra riee dana) Cornell Mniversity Library THE GIFT OF A a San Cos har 2 QQ... eee eae Pie LL ck siocarnntoteistannetctnanerianniishomarscee le 4553 ‘Aca olin,anx Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031231800 EVENINGS AT THE MICROSCOPE. EVENINGS AT THE MICROSCOPE; oR, RESEARCHES AMONG THE MINUTER ORGANS AND FORMS OF ANIMAL LIFE. BY PHILIP HENRY GOSSE, F.RS. NEW YORK: D. APPLETON AND COMPANY, 346 & 348 BROADWAY. 1860. PREFACE. To open the path to the myriad wonders of creation, which, altogether unseen by the unassisted eye, are made cognisable to sight by the aid of the Microscope, is the aim and scope of this volume. Great and gorgeous as is the display of Divine power and wisdom in the things that are seen of all, it may safely be affirmed that a far more extensive prospect of these glories lay unheeded and unknown till the optician’s art revealed it. Like the work of some mighty genie of Oriental fable, the brazen tube is the key that un- locks a world of wonder and beauty before invisible, which one who has once gazed upon it can never for- get, and never cease to admire. This volume contains but a gleaning: the author has swept rapidly across the vast field of marvels, snatching up a gem here and there, and culling one and another of the brilliant blossoms of this flowery vi PREFACE. region, to weave a specimen chaplet, a sample coronal, which may tell of the good things behind. Yet the se- lection has been so made as to leave untouched no con- siderable area of the great field of Zoology which is under the control of the Microscope; so that the stu- dent who shall have verified for himself the observa- tions here detailed, will be no longer a tyro in micro- scopic science, and will be well prepared to extend his independent researches, without any other limit than that which the finite, though vast, sphere of study itself presents to him. The staple of the work now offered to the public consists of original observation. The author is far from thinking lightly of the labours of others in this ample field; but, still, it is true that, respecting very many of the subjects that came under his notice, he found, in endeavouring to reproduce and verify pub- lished statements, so much perplexity and difficulty, that he was thrown back upon himself and nature, compelled to observe de novo, and to set down simply what he himself could see. The ever accumulating stock of observed and recorded facts is the common property of science; and the author has not scrupled to reproduce, to amplify, or to abridge his own obser- vations which have already appeared in his published works and scientific memoirs, as freely as he would have cited those of any other observer, in which he had confidence, and which were germane to his pur PREFACE. vii pose Yet in almost all cases the observations so used have been subjected to renewed scrutiny, and have been verified afresh, or corrected where found defective. In order to relieve as much as possible the dryness of technical description, a colloquial and familiar style has been given to the work; which has been thrown into the form of a series of imaginary conversaziones, or microscopical so¢rées, in which the author is supposed to act as the provider of scientific entertainment and instruction to a circle of friends. It is proper to add, however, that the precision essential to science has never been consciously sacrificed. A master may be easy and familiar without being loose or vague. A considerable amount of information will be found incidentally scattered throughout the work, on micro- scopic manipulation—the selecting, securing, and pre- paring objects for examination ;—an important matter, and one which presents a good deal of practical diffi- culty to the beginner. Not a little help will be afforded to him, also, on the power to observe and to discriminate what he has under his eye. In almost every instance, the objects selected for illustration are common things, such as any one placed in tolerably favourable cireum- stances, with access to sea-shore and country-side, may reasonably expect to meet with in a twelvemonth’s round of research. Vill PREFACE. The pictorial illustrations are almost co-extensive with the descriptions; they are one hundred and thir- teen in number; all, with the exception of eighteen, productions of the author’s own pencil, the great ma- jority having been drawn on the wood direct from the Microscope, at the same time as the respective descrip- tions were written. He ventures to hope that they will be found accurate delineations of the objects repre- sented.* Torquay, February, 1859. * The subjects on pp. 48, 54, 112, 114 (the lower figures), and 175, have been copied, under the courteous permission of the publisher, from Dr. Carpenter’s valuable work, ‘‘ The Microscope, and its Revelations.” (Churchill, London.) LIST OF ILLUSTRATIONS. Hoe’s BRISTLE.....-...0084 6 Fisre or Sarer’s Woot. 8 Harr or Cat.... 9 HArrs OF MOLE.........cee ec eseeeeeeeees 9 HAIR OF SABLEQ.....ccececececsccnsecees - 10 HEATER OF MOUSE sisnsicicvdsvbawie reacaendaatarenmaleesteateremssates Leiveinataniiene 11 Trp OF SMALL HAIR OF MOUSE. ..cccseessecoessenssonecsscteasecesoscesenees 11 HAIR (OF Bat, ccccsctaseevocteascecsctes o gos aie veecen se ccustecar vgenicceysciee sees 11 Hate OF INDIAN BAT eiasisissccideanansssvageteensens snbansdetas ves sieeesnseass 12 Tre or Hair oF DERMESTES....... ines pnin en nabngun cia Buin tbe Scnansinaiek wesw 15. BARB OF CLOTHING FEATHER OF FOWL,.......ccseccseceescessescessees vee 16 Base SHON Gods OO Giccnsnsmexciarrsaicsaswaanes munis 17 SCALES OF PERCH .......cccecerecesccescncoencoeees roe er errs 19 SCALES: OF -GOLDFISH:..-sestateccssnssnnsencasnusteeuccierssa dois oadeansesee eee 21 SCALE OF FLOUNDER ..ecseeseseseeeeeenees echdoudeteastaveacetrelandeecnetaceaie 23 ScaLes oF PIKE....... adesigoaspeanemesenest bated uladnae seine saat veces wate dane 24 SPICULA OF GOLDFISH SCALE... ..cscesecseceeeceeeceueeeeeceencnssaececnstenses 25 BLOOD DISKS x caiddesaa stevenson san sedasanasin iden inane ddaeu oad se sidenseuhesdanniens 31 CIRCULATION IN FROG’S FOOT ......c:escssccatsecserseecasescesseeseseeerenses 35 PEROPHOR Mi ccnsoaecie Goidslarmeweinces Bes u yarns see su oPeeeeiniielesanend cuaeioaeeans 36 Curr iE: SHEis i .csseesasvanenres yiMeieiiwcimaans calls Oh neten akacoeevane sunee® 45 Secrion OF NACRE FROM PEARL OYSTER ....ceccccsesscsessenseersersooe 48 Toneur or "TROCHUS ..<-csasne seni nenars suiavisleaawneeraxKiweuneninenaniaen 54 SrrucTurRE OF EYE IN SNAILeeeeeeeeee Hittinmmimeus wacom. 62 Leary SEA-MAT....... Sadoecaas Sa deeenesneesets Tisecdeediesessmbeciseandatasense: “10. Dovsiuinc AND Hooxs 1n A BEES WING,......... sahaaoarecanwessetios we ~83 x LIST OF ILLUSTRATIONS. SCALES ON A GNAT’S WING 00.00. scccecneeceecseneeeeceenaeeneenereeueennenes 84 RIS TRRST AN jas cas cccaucnan sascenaciaieeatissis tiskaeant tein der ered ar OONIME 85 ScALE or BRISTLE-TAIL .. 86 BATTLEDOOR-SCALE OF POLYOMMATUS ALEXIS......:0cseereceeeeneeeeseeees 89 FRINGED SCALE OF PIERIS.....-.cececececesecacceneesaeaecsreneeessenanonees ees Scates or DrAmMonp-BEETLE... AIR=PIPE OF PLY scseciscensaven sors ancendcnriacsessesaduasnwsanessacaccinseesenacce Sprracte or Fry SPIRACLE OF LEATHER-COAT.......seseceeereeees sebvepiagidaplamcaakin veins 114 SPIRACLE OF COCKCHAFER-GRUB ...ssercereseeeeee ebiseaiaceaiiavevesbas eves 114 GRUB OF CHAMELEON FLY........ccscceseeceeeeneeeeseoes LeRoy © a Hane Ly POOP OF PLY sc.adsiasis tarcsineisewiaiupienvsncentasseniaey sigeidge aueattdaee easiae 132 Foor OF WATER-BEETLE .........0see00 sohwewsaceusudigurmsbaneasaareadensoniegs 134 STING: OF BEE. sciesiavedasis.caccxearep sia swornosinetacszicemmsasseredsaneessceeemees 144 GALL-FLY, AND MECHANISM OF OVIPOSITOR .......00s008 LnnRannerNeNene 146 OUTER (SAW OF SAWSELR cous cssinies sects ocd an ae Ed fj Mi ee ea => aw SS fe TIAIR OF MOUSE. = Mh vawecsawe 5 SY mY i 12 EVENINGS AT THE MICROSCOPE. that each is formed of two half-encircling scales ; for one scale occasionally springs from the level of its fel- low, so as to make the imbrication alternate. Even this, however, is far excelled by a species of Bat from India, of whose hair I have now specimens on the stage. The trumpet-like cups are here very thin and transparent, but very expansive ; the diameter of the lip being, in some parts of the hair, fully thrice as great as that of the stem itself. The margin of each cup appears to be undivided, but very irregularly notched and cut. In the middle portion of the hair, the cups are far more crowded than in the basal part, more brush- like, and less elegant; and this structure is continued to the very extremity, which is not drawn out to so at- tenuated a point as the hair of the Mouse, though it is of a neCDeEG sharpness. The trumpet-shaped scales Nl are, it seems, liable to be removed by accident ; for in these dozen hairs there are several, in which we see one or more cups rubbed off, and in one the stem is destitute of them for a consid- erable space. The stem so denuded closcly resembles the basal part of a Mouse’s hair in its normal condition. This character of being clothed with |{ overlapping scales, each growing out of "its predecessor, is common, then, to the hairs of the Mammalia, though it exists in different degrees of development. It may be readily detected by the unaided sense, even when the eye, though as- sisted by the microscope, fails to recognise it. Al- most every schoolboy is familiar with the mode MAIR OF INDIAN BAT, HAIRS, FEATHERS, AND SCALES. 18 by which the tip of any hair may be distinguished from its base; and even of the least fragment, the terminal end from the basal end. The initiated lad assembles a few younger ones, and says, “ Now you may make a mark with ink on one end of a white horse-hair, and Pll tell you, by feeling it, which end you have marked.” He does, infallibly. He rubs it to and fro between his thumb and finger, and the hair regularly travels through in the direction of its base: one or two rubs of course determine this, and the verdict is given oracularly. Now you sce the cause of this property lies in the imbricate structure ; the scales may be excessively thin and close, but still they project sufficiently in any specimen to present a barrier to mo- tion in the terminal direction when pressed between two surfaces, such as the fingers, while they very readily move in the opposite. But more than the success of a schoolboy’s magic depends on the imbricate surface of hairs. England’s time-honoured manufacture, that which affords the high- est seat in her most august assembly, depends on it. The hat on your head, the coat on your back, the flan- nel waistcoat that shields your chest, the double hose that comfort your ankles, the carpet under your feet, and hundreds of other necessaries of life, are what they are, because mammalian hairs are covered with sheath- ing scales. It is owing to this structure that those hairs which possess it in an appreciable degree, are endowed with the property of felting; that is, of being especially under the combined action of heat, moisture, motion, and pressure, so interlaced and entangled as to become inseparable, and of gradually forming a dense and cloth- 14 EVENINGS AT THE MICROSCOPE. like texture. The “body,” or substance of the best sort of men’s hats, is made of lamb’s wool and rabbit’s fur, not interwoven, but simply beaten, pressed, and worked together, between damp cloths. The same property en- ables woven woollen tissues to become close and thick : every one knows that worsted stockings shrink in their di- mensions, but become much thicker and firmer after they have been worn and washed a little; and the “stout broad-cloth,” which has been the characteristic covering of Englishmen for ages, would be but a poor open flimsy texture, but for the intimate union of the felted wool-fibres, which accrues from the various processes to which the fabric has been subjected. In a commercial view, the excellence of wool is tested by the closeness of its imbrications. When first the wool-fibre was submitted to microscopical examina- tion, the experiment was made on a specimen of Meri- no}; it presented 2,400 serratures in an inch. Thena fibre of Saxon wool, finer than the former, and known to possess a superior felting power, was tried: there were 2,720 serratures in an inch. Next a specimen of South-Down wool, acknowledged to be inferior to either of the former, was examined, and gave 2,080 ser- ratures. Finally, the Leicester wool, whose felting property is feebler still, yielded only 1850 serratures per inch. And this connection of good felting quality with the number and sharpness of the sheathing scales is found to be invariable. The hairs of many Insects are curious and interest- ing. Here you may see the head of the hive-bee, which is moderately clothed with hair; each hair is slender and pointed, and is beset with a multitude of subordi- nate short hairs, which project from the main stem, and HAIRS, FEATHERS, AND SCALES. 15 stand out at an angle: these are set on in a spiral order. Here again, is one of the hinder legs of the same bee: the yellow hair, which you can see with the naked eye, consists of strong, horny, curved spines, each of which is scored obliquely, like a butcher’s steel. These legs are used, as you are well aware, to brush off the pollen from the anthers of flowers, wherewith the substance called bee-bread, the food of the grubs, is made; and in this specimen, you may see hundreds of the beautiful oval pollen-grains entangled among these formidable looking spines. These rusty hairs are from a large caterpillar (that of the Oak Egger Moth, I believe); they appear, when highly magnified, like stout horny rods drawn out to an acute point, and sending forth alternate short pointed spines, which scarcely project from the line of the axis. But there is scarcely any hair more curi- ous than that of a troublesome grub in mu- seums and cabinets, the larva of Derimestes lardarius, which lives upon fur-skins, and any dried animal substances. It has a cyl- indrical shaft, which is covered with whorls of large close-set spines, four or five in each whorl, closely succeeding each other ; the upper part of the shaft is surrounded by a whorl of larger and more knotted spines, and the extremity is furnished with six or seven large filaments, which appear to have ar or main oF knob-like hinge in the middle, by which “ they are bent up on themselves. The feathers of Birds are essentially hairs. That shrivelled membrane which we pull out of the interior 16 EVENINGS AT THE MICROSCOPE. of a quill when we make a pen, is the medullary portion, dried. There is a beautiful contrivance in the barbs of most feathers, which I will illustrate by this feather from the body-plumage of the domestic fowl. Every one must have observed the regular arrangement of the vane of a feather, and the exquisite manner in which the beards of which it is com- posed are connected together. This is specially observable in the wing-feathers,—a goose-quill, for example; where the vane, though very light and thin, forms an exceedingly firm resisting medium, the individual beards maintaining their union with great tenacity, and resuming it immediately, when they have been violently separated. Now this property is of high importance in the economy of the bird. It is essential that bane oF cLormé-rearuze With great lightness and buoy- ancy—for the bird is a jlying creature—there be power to strike the air with a broad resisting surface. The wide vanes of the quill-feathers afford these two requisites, strength and lightness; the latter depending on the material employed, which is very cellular, and the former on the mode in which the individual barbs, set edgewise to the direction of the stroke, take a firm hold on each other. Now, in the body-feather which is under the micro- scope, we see that the central stem carries on each side a row of barbs, which interlock with each other. The WAIRS, FEATIERS, AND SCALES. 17 magnifying power shows us that these barbs are not simple filaments, but are themselves doubly bearded in the same fashion ; and further, that these barbules of the second series are furnished with a third series. It ‘is in this third series of filaments that the tenacity in question resides. If we isolate one of the primary beards, by stripping away a few on each side of it, and again put it on the stage, we see that the secondary barbules of one side are armed differently from those of the other side. Those of the lower side carry short and simple barbulets, whereas those of the side which looks towards the point of the feather bear much longer ones ; and; moreover, many of them are abruptly hooked backwards. Now, whenever the primary beards are brought into contact, some of these hooks catch on the barbule next above, and, slipping into the an- gles formed by the barbulets, hold there, and thus the two contiguous beards are firmly locked together. In the beard of the goose-quill, the structure is essentially the same, but the barbulets are far more numerous and more closely set; they are also proportionally much larger,—both those which are hooked and those which are simple. Indeed, the latter manifest a tendency to the hooked form, and by all these peculiarities the interlocking power is aug- mented. Itis interesting to observe the great dilatation of the beard in a direction towards the inferior surface of the feather,—towards the stroke, as I just now ob- BARB FROM GOOSE-QUILL. om, 18 EVENINGS AT THE MICROSCOPE. served. This is to increase the resisting power, as a thin board set edgewise will bear a great weight with- out bending or breaking, provided it can be kept from yielding laterally. The barbules are arranged only on the very edge—the upper edge—of the beard. We will now examine some specimens of scales of Fishes, all of which are very interesting and beautiful objects under low powers of the microscope; though higher powers are requisite to resolve their structure. We will use both. The scales of almost all the Fishes with which we are familiar, fall under two kinds, which have been named ctenord (or comb-like), and cycloid (or roundish). The Perch affords us good examples of the former kind. On this slide are three scales from the body of this fish : the one on the left side is taken from the back (fig. a) ; the middle one from the lateral line (2); and the one on the right from the belly (¢). In order to understand these objects we must remember that the scales of fishes are horny or bony plates, developed in the substance of the proper skin, with a layer of which they are al- ways covered. In most cases (as, for example, the Perch), the hinder end of each scale projects, carrying with it the thin layer of skin with which it is invested ; and thus the scales overlay one another, like the tiles of a house, or like the feathers of a bird, and that for a like purpose. For as the rain, falling on the house-top, has a tendency to flow downwards, from gravitation ; and as the slope of the roof is in that direction, the current passing over each tile is deposited from its bottom edge on the middle of the next one, whence it still flows down to the free edge of this one, and so in succession. So the motion of the bird through the air, and of the fish, HAIRS, FEATHERS, AND SCALES. 19 through the water, produces the very same effect as if these fluids were in motion and the animals were still ; and therefore the bodies of the latter are, as it were, tiled with feathers or scales, the free edges of which, looking in the opposite direction to the coming of the current (that is, the same direction, as its flow), deposit the successive particles of the moving fluid in the midst of the successive feathers or scales. Thus two results ensue, both essential to the comfort of the animal: first, the air or water does not run upward between the feathers or scales to the skin ; and secondly, the surface presents no impediment to free motion. This latter advantage will be appreciated, if you take hold of a dead bird by the legs, and push it rapidly through the air tail-foremost: the feathers will instantly rise and ruffle up, presenting a powerful resistance to movement in that direction. These scales of the Perch have their hinder, or free edge, set with fine crystalline points, arranged in suc- cessive rows, and overlapping. ‘Their front side is cut SCALES OF PERCII. with a scolloped pattern, the extremities of undulations of the surface that radiate from a common point behind the centre. These undulations are separated by narrow 20 EVENINGS AT THE MICROSCOPE. furrows, across which, contrary to the ordinary rule, the close-set concentric lines that follow the sinuosities of the outline are not visible. Under the microscope they look as if they had been split in these radiating lines, after the whole number of layers had been completed, and the fissures had then been filled with new trans- parent substance. The middle scale is, as I have said, from the lateral line. Along each side, in most fishes, may be observed a line, known as the lateral line, formed by scales of peculiar form. They are commonly more bony than the other scales, and are pierced by a tubular orifice for the escape (as is generally supposed, though this has been denied) of a mucous secretion, which is poured out from glands beneath, and thus flows over the body for the double purpose of protecting the skin from the mas- cerating influence of the surrounding water, and of di- minishing friction in swimming. Let us now look at some scales of the cycloid kind. The great majority of our fishes are clothed with such as are of this description. This dead Gold-fish shall give us examples. The three scales in the upper row are from the lateral line, the left-hand one (@) taken just behind the head, the second (8) near the middle of the body, and the right-hand one (c) near the tail. Of the lower row, the first (d) is from the back, the second (¢) from the middle of the belly, and the last (/) from the throat. Thus we see there is considerable variety in form presented by the scales even of the same individ- ual fish. They all, however, differ from those of the Perch, in this respect ;—that their free overlapping edges are entire, or destitute of the crystalline points which we saw in the former examples ; while they agree HAIRS, FEATHERS, AND SCALES. 21 in having the front edges, by which they are during life imbedded in the skin, cut into waves or sinuosities. The lower part, as we now look at them, is the free por- tion of each, which alone is visible in the living fish, the SCALES OF GOLDFISH, other parts being concealed by the three neighbouring scales that overlap it,—above, in front, and below. In those from the lateral line, the tube already re- ferred to is seen to pervade each, running through it longitudinally, so that it opens posteriorly on the outer surface, and anteriorly on the inner or under surface of the scale. In the scales near the front of the line, just behind the head, the tube is large and prominent (a), while in the scales at the opposite extremity it becomes slender ; diminishing, in the very last scale, viz. at the commencement of the tail-fin, to a mere groove. The whole surface of each scale, when viewed under a lens of low power, is seen to be covered with concen- 22 EVENINGS AT THE MICROSCOPE. tric lines, following the irregular sinuosities of the out line. These lines are the edges of the successive layers of which the scale is believed to be composed, each layer being added in the process of growth to the under surface, and each being a little larger every way than its predecessor; thus the scale is a very depressed cone, of which the centre is the apex. There is a marked dif- ference (indicated in the figures) between that part of the surface which is exposed, and that which is covered by the other scales; the concentric marks in the former are much coarser and less regular, often being inter- rupted, and seeming to run into each other, and fre- quently swelling into oval scars. This may, perhaps, be owing to the surface having been partially worn down by rubbing against the gravel of the bottom, or against other objects in the water. Besides the concen- tric lines, there are seen on many of the scales, espe- cially those of the lateral line, radiating lines varying in number from one to twenty, or more, diverging fron, the centre towards the circumference, and frequently connected by cross lines forming a sort of net-work around the centre (see ¢). Under the microscope, these lines appear to be elevated ridges, dividing the concen- tric lines; but of their use I am ignorant. What I have just stated is the ordinary explanation of these fine concentric lines ; but a careful examination of the structure with much higher powers than we have been using, induces me to doubt its correctness. Re- verting to the scales of the Perch, let us notice the clear diverging bands, which look as if the whole scale had been split in several places, and the openings thus made filled with uniform clear substance. The same struc- ture is seen in many other scales, as in this cyclocd one UWAIRS, FEATHERS, AND SCALES. 23 from the Flounder, which, being coarsely lined, shows the structure well; or in these from the Green Wrasse. I will now apply to one of these a power of 600 di- ameters, concentrating the light thrown through the scale from the mirror by the achromatic condenser, and examine the scale anew. You now see two distinct layers ; the upper one which bears the con- centric lines, and a lower clear one which not only sbi aroRervoua eR: fills the radiating bands, cencnsarsciic: but underlies the whole of the lined parts. The con- centric lines of the upper layer do not now appear to be edges of successive plates, but irregular canals running through the solid substance. This, however, is illusory: .for, by delicate focussing, we perceive that each portion marked by these lines is really in a different plane from the others, that the highest is at the centre of radiation of the scale, and that each is successively lower till we reach the margin. But now, if with very sharp scissors we cut one of these scales longitudinally through the centre, and examine the cut edge, we find that each of these lines forms a distinct ridge. On the other hand, the under layer of clear substance is quite smooth, and always a little exceeds the margin of the concentrically lined portion. The clear substance that fills the radiat- ing slits agrees both in texture and level with this lower layer, and is manifestly continuous with it. Hence, I think that, in these slit scales, the upper layer is formed, as commonly believed, by successive 24 EVENINGS AT THE MICROSCOPE. deposits from beneath ; but that, after a few have been deposited, they begin to slit, probably by contraction in becoming solid; that the lower layer is formed after each upper one is hardened, exceeding its length by a little, and filling up the slit; that this lower layer be- comes the upper layer of the next course, slitting, and turning up its terminal edge as it hardens; that then the lower layer is deposited on this, filling up the slit as before; and that this process goes on as long as the fish lives. It is curious that, in the scales of the Pike, the por- tions thus separated by slitting, instead of expanding and leaving spaces to be filled up, actually close over each other, the divided parts overlapping considerably, as you may see in these specimens. The left hand scale (a) is from the back ; the central one (4), which has only SCALES OF PIKE, a deep narrow incision instead of a tube, is from the lateral line; and the third (¢) is from the belly of the fish. Let us return now to the scales of our Gold-Fish, and examine a highly interesting structure connected with them. The brilliant golden or silvery reflection that constitutes the beauty of these lovely fishes, de- pends not on the scales themselves, but on a soft layer HAIRS, FEATHERS, AND SCALES. 25 of pigment spread over their inner surface, and seen through their translucent substance. On carefully de- taching a scale, we see on the under side, opposite to that portion only which was exposed (all the concealed parts being colourless), a layer of soft gleaming sub- stance, easily separable, either silvery or golden, ac- cording to the hue of the fish. If now we remove a small portion of this substance with a fine needle, and spread it on a plate of thin glass, we shall find, by the aid of the microscope, that it consists of two distinct substances; the one giving the colour, the other the metallic lustre, With a power of 300 diameters, the former is seen to be a layer of loose membranous cells of an orange colour, in what are properly called the Gold-fishes, and whitish or pellucid in the Silver-fishes, If we now add a minute drop of water to the mass, and gently agitate it with the point of a needle, and again submit it to the microscope, we shall have a beautiful and interesting spectacle, The water around the mass is seen to be full of an infinite number of flat spicula or crystals, varying much in size, but of ey very constant form, a flat oblong prism with angular ends (as represented in the accompanying engraving), By transmitted light they are so transpa- rent and filmy as to be only just dis- cernible; but by reflected light, and S especially under the sun’s rays, they flash like plates of polished steel. But Shroot on eee what appears most singular, is that each spiculum is perpetually vibrating and quivering with a motion apparently quite spontaneous, but prob- ably to be referred to slight vibrations of the water in 2 26 EVENINGS AT THE MICROSCOPE. which they float; and each independently of the rest, so as to convey the impression to the observer that each is animated with life, though the scale be taken from a fish some days dead. Owing to this irregular motion, and consequent change of position, each spiculum, as it assumes or leaves the reflecting angle, is momentarily brightening or waning, flashing out or retiring into darkness, producing a magic effect on the admiring ob- server. To this property, I suppose, is to be attributed the beautiful pearly play of light that marks these lovely fishes, as distinguished from the light reflected by an uniformly polished surface. I have found the pearly pigment of the scales to be provided with similar spic- ula in fishes widely differing in size, structure, and habits; as the Gudgeon and Minnow, the Pike and the Marine Bream. The spicula of these fishes agree in general form with those of the Gold-fish; and also in size, with the exception of trifling variations in the comparative length and breadth. The colouring mat- ter is lodged in lengthened cylindrical cells, arranged side by side, and running across the scale; that is, in a direction at right angles to the lateral line. BLOOD. QT CHAPTER II. BLOOD. Tae microscope is daily becoming a more and more important aid to legal investigation. An illustration of this occurred not long ago, in which a murder was brought home to the criminal by means of this instru- ment. Much circumstantial evidence had been adduced against him, among which was the fact, that a knife in his possession was smeared with blood, which had dried both on the blade and on the handle. The prisoner strove to turn aside the force of this circumstance by asserting that he had cut some raw beef with the knife, and had omitted to wipe it. The knife was submitted to an eminent professor of microscopy, who immediately discovered the following facts:—1. The stain was certainly blood. 2. It was not the blood of a piece of dead flesh, but that of a living body ; for it had coagulated where it was found. 3. It was not the blood of an ox, sheep, or hog. 4. It was human blood. Besides these facts, however, other important ones were revealed by the same mode of in- vestigation. 5. Among the blood were found some vegetable fibres. 6. These were proved to be cotton fibres,—agreeing with those of the murdered man’s shirt and neck-kerchief. 7 There were present also numerous tessellated epithelial cells. In order to under- stand the meaning and the bearing of this last fact, I 28 EVENINGS AT THE MICROSCOPE. must explain that the whole of the internal surface of the body is lined with a delicate membrane (a continu- ation of the external skin), which discharges mucus, and is hence termed mucous membrane. Now this is composed of loose cells, which very easily separate, called epithelial cells; they are in fact constantly in process of being detached (in which state they consti- tute the mucus), and of being replaced from the tissues beneath. Now microscopial anatomists have learned that these epithelial scales or cells, which are so minute as to be undiscernible by the unaided eye, differ in ap- pearance and arrangement in different parts of the body. Thus, those which line the gullet and the lower part of the throat are tesselated, or resemble the stones of a pavement ; those that cover the root of the tongue are arranged in cylinders or tall cones, and are known as columnar ; while those that line some of the viscera of the abdomen carry little waving hairs (ci/éa) at their tips, and are known as ciliated epithelium. The result of the investigation left no doubt remain- ing that with that knife the throat of a lwing human being, which throat had been protected by some cotton fabric, had been cut. The accumulation of evidence was fatal to the prisoner, who without the microscopic testimony might have escaped. But what was there in the dried brown stain that determined it to be blood? And, particularly, how was it proved not to be the blood of an ox, as the pris- oner averred? To these points we will now give a moment’s attention. With this fine needle I make a minute prick through the skin of my hand. A drop of blood oozes out, with which I smear this slip of glass. The slip is now on BLOOD. 99 the stage of the instrument, with a power of 600 diam- eters. You see an infinite number of small roundish bodies, of a clear yellowish colour, floating in a colour- less fluid, but so numerous, that it is only here and there, as near the edges of the smear, that you can detect any interval in their continuity. These bodies are what we frequently call the blood- globules, or, more correctly, blood-disks ; since their form is not globular, but thin and flat, like a piece of money. ‘The slightness of their colour is dependent on their extreme tenuity: when a larger number lie over each other the aggregated colour is very manifest, as it then becomes either a full dark red, or bright rich scar- let ; for to these disks blood is entirely indebted for its well-known hue. All vertebrate blood is composed principally of these bodies, which, when once seen, are easily recognised again: the microscope then readily determines whether any given red fluid or dried stain is composed of blood. The disks in the blood of Mammalia are circular, or nearly so, and slightly concave on both of the surfaces. On the other hand, in Birds, Fishes, and Reptiles their form is elliptical, and the surfaces are flat, or slightly convex. This distinction, then, will at once enable us to determine Mammalian blood.* But to determine the various tribes of this great class among themselves, we must have recourse to another criterion,—that of dimensions. The blood-disks of Man nearly agree in size with those of the Monkey tribe, of the Seals and Whales, of * The Camels among Mammalia, and the Lampreys among Fishes, are exceptions to the above rule; the former having elliptical and convex blood-disks, and the latter circular, and slightly concave. 30 EVENINGS At THE MICROSCOPE. the Elephant, and of the Kangaroo. Most other quad- rupeds have them smaller than in Man; the smallest of all being found in the ruminating animals. The little Musk-deer of Java has disks not more than one- fourth as large as the human, but these are remarkably minute ; no other known animal approaches it in this respect: those of the Ox are about three-fourths, and those of the Sheep little more than half the human average. Tables have been made out showing the compara- tive size of these corpuscles in various animals, and such tables are very useful; but we must bear in mind that the average dimensions only are to be looked for ; since in any given quantity of blood, under examina- tion, we shall not fail to see that some disks exceed, while others come short of, the dimensions of the ma- jority. Generally speaking, the blood-disks in Birds and in Fishes are about equal in size: their form is, however, that of a more elongated ellipse in Birds than in Fishes. They may be set down as averaging in breadth the diameter of the human disks, while their length is about half as much again, or a little more, in most Birds. It is in Reptiles that we meet with the largest disks, and especially in those naked-skinned species, the Frogs and Newts. A large species inhabiting the American lakes—Siren lacertina—has disks of the extraordinary size of 1-400th of an inch long by 1-800th broad, or about eight times as large as those of Man. Our com- mon Newts afford us the largest examples among Brit- ish animals, but they do not reach above half the size just mentioned. BLOOD. 31 Taking this drop of blood from my finger as a standard of comparison, we find, on applying the mi- crometer, that the disks run from 1-2500th to 1-5000th of an inch; but that the great majority are about 1-3300th in diameter. On these slides are samples of other kinds. This is the blood of a Fish,—the common Blenny or Shanny (Blennius pholis). Here we see at once the oval form of the disks; their average is 1-2800th by 1-3300th of aninch. Here is the blood of a Frog (ana temporaria); these are more than twice the size of the fish’s; for they average 1-1250th by 1-1800th of aninch. And, finally, I can show you a drop of blood from this Smooth-newt (Lissotriton pune- tatus). The large size of the disks is now conspicuous, and so indeed is the elegance of their form: in this case, as in the last, we see in each disk a distinct round- ish nucleus. These run from 1-700th to 1-950th in length, by 1-1100th to 1-1600th in breadth; but the average are about 1-800th by 1-1300th of an inch. BLOOD-DISKS. @ Man. 6 Blenny. e Frog. ad Newt. Tt may interest you to sce these blood-disks in their proper situation, and to observe the motion which v2 EVENINGS AT ‘TILE MICROSCOPE. they possess during the life of their owners. It is, indeed, one of the most instructive modes of using this wonder-working instrument to look through it at liv- ing structures, and watch the different processes of life as they are earricd on under our cycs. Nor is this at all difficult to accomplish 5 for a large number of animals are so small that we can easily put them upon the stage of the microscope, and withal so transparent that their integuments and various tissues offer little or no impediment to our discerning the forms and movements of the contained viscera. And in cases where the entire animal is too large te be viewed mi- croscopically as a whole, it sometimes happens that, by a little contrivance, we can so secure the creature as to look, without interruption, on certain parts of the body which afford the requisite minuteness and trans- parency. Thave here a living Frog. You perceive that the web which councets the toes is exceedingly thin and translucent, yet arterics and veins meander through its delicate tissues, Which are then clothed on both surfaces with the common skin. But you ask how we ean in- duce the Frog to be so polite as to hold his paw up and keep it steady for our scientific investigation. We will manage that without difficulty. Most microscopes are furnished (among their acces- sory apparatus) with what is called a frog-plate, pro- vided for this very demonstration. Ilere is mine. It is a thin plate of brass, two inches and a half broad and seven long, with a number of small holes pierced through it along the margins, and a large orifice near one cnd, which is covered with a plate of glass. This is to be Froggy’s bed during the operation, for we BLOOD. 33 must mako him as comfortable as cireumstances will admit. Well, then, we take this strip of linen, damp it, and proceed to wrap up our unconscious subject. When we haye passed two or three folds round him, we pass a tape round the whole, with just sufficient. tight- ness to keep him from struggling. One hind-leg must project: trom the linen, and we now pass a needle of thread twice or thrice through the drapery and round the small of this free leg, so as to prevent him from re- tracing it. Tere then he lies, swathed like » mummy, with one little cold foot protruded. Lay him carefully on the brass plate, so that the webbed toes shall stretch across the glass. Now, then, we pass another tape through the marginal holes, and over the body, to bind it to the brass; of course taking care not to cut the animal, but only using just as much foreo as is needful to prevent his wrigelings. Now a bit of thread round each toe, with which we tie it to as many of the holes, so as to expand the web across the glass. A drop of cold water now upon the swathes to keep him cool, and a touch of the same with a feather upon the toes to prevent them from drying (which must be re- peated at intervals during the examination),—and he is ready. What a striking spectacle is now presented to us, as with a power of 3000 diameters we gaze on the web of the foot! There is an area of clear colourless tissue filling the field, marked all over with delicate angular lines, something like scales; this is the tessellated epi- thelium of the surface. Our attention is caught by a number of black spots, often taking fantastic forms, but ox 384 EVENINGS AT THE MICROSCOPE. generally somewhat star-like : these are pigment cells, on which the color of the animal’s skin is dependent. But the most prominent feature is the blood. Wide rivers, with tortuous course, roll across the area, with many smaller streams meandering among them; some pursu- ing an independent course below the larger, and others branching out of them, or joining them at different angles. The larger rivers are of a deep orange-red hue, the smaller faintly tinged with reddish-yellow. In some of these channels the stream rolls with a majestic evenness ; in others it shoots along with headlong impetuosity ; and in some it is almost, or even quite, stagnant. By look- ing with a steady gaze, we see that in all cases the stream is made up of a multitude of thin reddish disks, of exactly the same dimensions and appearance as those we saw just now in the Frog’s blood; only that here, being in motion, we see very distinctly, as they are rolled over each other, that they are disks, and not spherules; for they forcibly remind us of counters, such as are used for play, supposing they were made out of pale red glass. It is charming to watch one of these streams, select- ing one of medium size, where the density is not too great to sce the individual disks, and fixing our eye on the point where a branch issues from one side of the channel, mark the disks shoot by one after another, some pursuing their main course, and others turning aside into the branch, perhaps so small as to allow of only a single disk to pass at once. The streams do not pursue the same uniform direc- tion. The larger ones do indeed; and their course is from the extremity of the toes towards the body: these are the veins; but the smaller streamlets flow in any BLOOD. 35 direction, and frequently send out side-branches, which presently return into the stream from which they is- sued, or unite with others in a very irregular network. These are the capillaries, which feed the veins, and which are themselves fed by the arteries, whose course is in the opposite direction, viz., from the body. These, “@ however, are with difi- ayy culty seen: they are more deeply seated in am the tissues, and are less & spread over the webs, being generally placed & along the borders of the £ toes; they are, more- over, fewer and smaller than the veins; but the blood in them usually flows with more impetuous rapidity. The variations in the impetus of the current which we observe in the same vessel are probably owing to the mental emotions of the animal; alarm at its un- usual position, and at the confinement which it feels when it endeavours to move, may suspend the action of the heart,’and thus cause an interruption in the flow ; or analogous emotions may quicken the pulse. We will, however, now release our little prisoner, who, though glad to be at liberty, is, as you see, none the worse for his temporary imprisonment. Let us now look at the circulation of the blood in one of the Invertebrate Animals. In this thin glass cell “a IM y OKO MY Ay aps f: Yeas CIRCULATION LN FROG'S FOOT. 386 EVENINGS AT’ THE MICROSCOPE. of sea-water is a small fragment of sea-weed, and at- tached to one of its slender filaments you may see three or four tiny knobs of jelly, clustered together like a bunch of grapes. These are animals; each endowed with a distinct life, but associated together by a com- mon stalk, which maintains the mutual vital connexion of the whole. It is one of the Social Tunicata, and is named Perophora Listers. Though each globose knob is no larger than a small pin’s head, it is full of organs which carry on the various functions of life; and, because the whole tis- sues are as transparent as crystal, they allow us to watch the processes with perfect ease. Take a peep at it. It is a gelatinous sac, of a form intermediate be- tween globular and cubical, flattened on two opposite sides, with a sort of wart at the summit and another at the side, each of which is pierced with a purs- ed orifice. The up- per of these orifices admits water for re- spiration and food ; the latter passes " through a digestive system, and is dis- charged through the side orifice. The digestive organs lie on that flattened side, which is farthest from your eye, and are therefore dimly seen. PEROPHORA. BLOOD. 37 The globose body is inclosed in a coating of loose shapeless jelly, that passes off from one of the lower corners, and forms a short foot-stalk, which unites with similar foot-stalks from the sister-globules, and all to- gether are attached to the sea-weed. Each foot-stalk has an organic pore, into which a vessel passes upon the body. Your attention is first arrested by the breathing sac, with its rows of oblong cells all in wheel-like motion. It is indeed a wonderful object; but for the present neglect this, as we will return to it presently, and direct your consideration to the course of the blood. It is true the fluid which I so name is not red, like that of the Frog which you have just been gazing at, nor does it carry disks of the same elegantly regular form. But you have the advantage here of tracing, at one view, the whole Gourse of the circulation, from its first rush out of the heart to its return into that organ again. At the bottom of the interior, below the breathing sac, there is an oblong cavity, through whose centre there runs a long transparent vessel, formed of a deli- cate membrane, the appearance of which resembles that of a long bag, pointed (but not closed) at either end, and then twisted in some unintelligible manner so as to make three turns. This is the heart; and within it are seen many minute colourless globules, floating freely in asubtile fluid: this is the nutrient juice of the body, which we may, without much violence, des- ignate the blood. Now see the circulation of this fluid. The membranous bag gives a spasmodic con- traction at one end, and drives forward the globules contained there ; the contraction in an instant passes on- 38 EVENINGS AT THE MICROSCOPE. ward along the three twists of the heart (the part be- hind expanding immediately as the action passes on), and the globules are forcibly expelled through the nar- row but open extremities. Meanwhile, globules from around the other end have rushed in as soon as that part resumed its usual width, which in turn are driven forward by a periodic repetition of the systole and diastole. The globules, thus periodically driven forth from the heart, now let us watch and see what becomes of them. They do not appear to pass into any defined system of vessels that we may call arteries, but to find their way through the interstices of the various organs in the gen- eral cavity of the body. The greater number of globules pass immediately from the heart through a vessel into the short foot- stalk, where they accumulate in 4 large reservoir ; but the rest pass up along the side of the body, which (in the aspect in which we are looking at it) is the right. As they proceed (by jerks, of course, impelled by the contractions of the heart), some find their way into the space between the breathing surfaces, through narrow slits along the edges of the sac, and wind along between the oval ciliary wheels, which we will presently consider. Besides these, however, other globules wind along between the outer sur- faces of the sac and the inner surface of the body- walls. But to return to the current which passes up the right side: arriving at the upper angle of the body, the stream turns off to the left abruptly, principally passing along a fold or groove in the exterior of the breathing- sac until it reaches the left side, down which it passes, BLOOD. 39 and along the bottom, until it arrives at the entrance of the heart, and rushes in to fill the vacuum produced by the expansion of its walls after the periodic con- traction. This is the perfect circle; but the minor streams, that had forked off sideways in the course, as those within the sac for example, find their way to the entrance of the heart by shorter and more irregular courses. One or two things connected with this circulatory system are worthy of special notice. The first is, that its direction is not constant but reversible. After we have watched this course followed with regularity for perhaps a hundred pulsations or so, all of a sudden the heart ceases to beat, and all the globules rest in their circling course, that we had supposed incessant. Strange to behold, after a pause of two or three seconds, the pulsation begins again, but at the opposite end of the heart, and proceeds with perfect regularity, just as be- fore, but in the opposite direction. The globules, of course, obey the new impulse, enter at their former exit, and pass out at their former entrance, and per- form their circulation in every respect the same as be- fore, but in the reverse direction. Those globules that pass through the vessel into the foot-stalk appear to accumulate there as in a reservoir, until the course is changed, when they crowd into the heart again and perform their grand tour. Yet there is a measure of circulation here; for even in the con- necting vessel one stream ascends from the reservoir into the body as the other (and principal one) descends into it from the heart ; and so, vice versd. I have spoken of these motions as being performed with regularity ; but, if you look closely, you will see 40 EVENINGS AT THE MICROSCOPE. that this must be understood with some qualification. The pulsations are not quite uniform, being sometimes more languid, sometimes more vigorous ; perhaps forty beats in a minute may be the average; but I have counted sixty, and presently after thirty; I have counted twenty beats in one-half minute, and only fit teen in the next. The period during which one course continues is equally uncertain ; but about two minutes may be the usual time. Sometimes the pulsation in- termits for a second or so, and then goes on.in the same direction ; and sometimes there is a curious variation in the heart’s action—a faint and then a strong beat, a faint and a strong one, and so alternately for some time. The phenomena of respiration are so closely con- nected with those of circulation that it is not at all mal- apropos to turn from the latter to the former; not to say that it would be high treason against scientific cu- riosity if I were to remove this object without explain- ing to you that marvellous play of wheels that occupies the largest part of the area that you behold. As you look on the globe, you observe, hanging down from the upper extremity, and reaching nearly to the bottom in one direction and almost from side to side in another, a transparent square veil, which is indeed a flat mem- branous bag, having its sides pretty close together, with small openings along its edges, and an orifice at the bottom leading into the stomach. The mouth of this sac is in close connection with the upper or principal orifice, and therefore receives the water, which is constantly flowing in, while that aper- ture is expanded. This fluid then bathes the whole in- terior of the sac, but a portion of it escapes by the BLOOD. 41 lateral openings into the cavity of the body, between the sac and the mantle, and is discharged through the secondary, or side orifice. The inner surface of this transparent sac is studded with rings of a long oval figure, set side by side in four rows. These rings appear to consist of a slight eleva- tion of the general membranous surface so as to make little shallow cells, the whole edges of which are fringed with cilia, whose movements make waves, that follow each other round the course in regular succession. In truth it is a beautiful sight to see forty or more of these oblong rings, all set round their interior with what look like the cogs on a watch-wheel, dark and distinct, run- ning round and round with an even, moderately rapid, ceaseless motion. These black running figures, so like cogs and so well defined as they are, are merely an op- tical delusion; they do not represent the cilia, but merely the waves which the cilia make ; the cilia them- selves are extremely slender close-set hairs, as may be seen at the ends of the ovals, where a slight alteration of position prevents the waves from taking the tooth- like appearance. Sometimes one here and there of the ovals cease to play, while the rest continue ; and, now and then, the whole are suddenly arrested simultane- ously as if by magic, and presently all start together again, which has a most charming effect. A still more singular circumstance is, that while in general the cili- ary wave runs in the same direction in the different ovals, there will be one here and there in which the course is reversed ; and I think that the animal has the power of choosing the direction of the waves, of setting them going and of stopping them, individually as well as collectively. 42 EVENINGS AT THE MICROSCOPE. The object of these ciliary wheels is to keep up a constant current in the water. This fluid, as I have said, enters from without, through the upper orifice of the body, and is hurled over the whole surface of the breathing-sac by means of the ciliary waves, parting with its oxygen, as it goes, to the blood, which streams, as we saw, everywhere between the rows of wheels. But the water has another function: it carries particles of organic matter with it, which are suitable for the nourishment of the creature; these atoms are carried by the currents with the effete water to the bottom of the sac, and are poured into the stomach, where they are digested; the innutritive remains, together with the waste water, being discharged through the lateral orifice. Thus we see how closely connected are the three cardinal processes of circulation, respiration, and di- gestion. MOLLUSCA. 43 CHAPTER MII. MOLLUSOA : THEIR SHELLS, TONGUES, EYES, AND EARS. One of the most interesting aspects of microscopic study is that in which it reveals the intimate structure of objects, which to the unassisted eye appear simple or nearly so, but which prove, by the aid of magnifying power, to be complex. Thus we are often introduced to very curious contrivances (if I may use such a word in reference to the works of God), by which difficulties are overcome, and substances, which would seem, at first, wholly unfit for certain duties, are in the most admira- ble manner adapted to fulfil them. The combination of strength and lightness is always a difficult problem in human art; its successful solution always excites our admiration. In the Divine mechan- ics, too, it is very often required, and the variety of modes in which it is accomplished are, in the highest degree, novel and suggestive. We lately saw one of these in the structure of a feather, in the contrivance by which extreme lightness of material was made, by a most remarkable arrangement, to offer a firm resis- tance. to opposing force. I have now another example to show you, in which a material, in itself heavy, is by its arrangement made very light, while it preserves its aggregate strength. You have seen many times, when walking along the 44. EVENINGS AT THE MICROSCOPE. yellow sands kissed by the rippling waves, the shell, or bone as it is sometimes called, of the Cuttle-fish. You know that it consists of a shallow boat-shaped shell, the hollow of which is filled with a white substance, which can be scraped away even with the finger-nail, and which is sometimes use as pounce, to rub on paper from which writing has been erased. It is this sub- stance of which I mean now to speak. The possessor of this structure is a member of the numerous class Moxtusca, which are generally charac- terised by being inclosed in shells. Now shell, as we all know, is a solid, stony substance, much heavier than water; take into your hand that large Cassis on the mantel-piece, and observe its great weight and com- pactness. It is, in fact, real limestone ; differing from that of the rocks only in this, that it has been depos- ited by the living organic cells of an animal, and ar- ranged in a definite form. We will presently examine other examples. The “ cuttle-bone ” is a shell, not in- deed inclosing the animal, but inclosed by it; being contained within a cavity in the substance of the fleshy mantle; cut open the mantle, and the shell instantly drops out. The Cuttle isa rapid swimmer through the open sea. A shell so large as this, if solid and compact like that of the Casszs, would condemn it to grovel on the bottom, and frustrate all the instincts of its nature. On the other hand, it needs the strength and support of a solid column. Wonderful to tell, the calcareous shell is made not only to be no hindrance to its swimming, but to contribute greatly to its buoyancy: it is what the string of corks is to the bather who cannot swim— itis a float. Throw this entire cuttle-shell into water ; MOLLUSCA. 45 it floats on the surface as buoyantly as if it were actu- ally carved out of cork. Icut with a keen knife a little cube out of the “pounce,” and, fixing it on the end of the revolving stage-needle, apply a low power, say 70 diameters, using reflected light. We are looking now at the perpendic- ular section; is it not Me 7 a beautiful object? you might fancy yourself looking at one of the § noble icebergs that ma- f jestically navigate the polar seas, when it is § rendered porous and laminated, by the rains § of spring. You Bie @ Perpendicular; ® Horizontal section, number of thin horizon- tal tiers or stages, perfectly parallel and equi-distant, about one-fortieth of an inch apart, rising above each other like the floors of an edifice. These are connected together by an infinite multitude of thin pillars of erys- tal, or rather leaves, some of which show their edges towards us, others their broader sides, and others are broken off at various distances, the fragments standing up from the floor, or depending from the roof, like stalactites and stalagmites in a cavern. This whole series of crystal floors and supporting plates is formed of calcareous matter,—limestone, in short; but though the latter are set in such close array that the eye cannot penetrate to any appreciable dis- tance between them, their extreme thinness renders the whole structure very light, the interstices being oc- eupied by air. 46 EVENINGS AT THE MICROSCOPE. But now if I give the stage-needle half a revolution, we shall have the horizontal section presented to the eye. In this aspect we acquire much more information as to the structure. The cut has been made very close to one of the horizontal floors, which we see marked all over with a great number of lines, each of which runs hither and thither, in a very sinuous pat- tern. The lines are made up of a brilliant spark- ling substance; they are in fact the basal portions of what we saw in the other section as thin perpendic- ular plates; Ihave cut off the plates close to the bottom, and what we see is their insertion into the floor. Thus we perceive that what we took for a multi- tude of plates, were but the various doublings and in- foldings of a single plate of great length, running quite across the floor; an arrangement by which the strength of the material is greatly augmented. You have often seen the mode in which light walls are made of cor- rugated iron, especially at railway stations, and are doubtless aware that the corrugation, or bending in and out, imparts a strength to it which the mere sheet- iron, if set up as a smooth, plane surface, would in no wise possess. The principle is exactly the same in the two cases; but the corrugation of the limestone plates in the cuttle-shell is far more perfect than that of the iron ; added to which there is the other advantage, that the aggregate mass of material is made highly buoyant by the large bulk of empty space that in- tervenes between the sinuous folds of the crystal plates. It may be interesting to compare with this the structure of the more solid shells of bivalves, which MOLLUSCA. 47 have been so elaborately studied by Dr. Carpenter. In general, these consist of two very distinct layers, well seen in the valve of the Pearl Oyster, and its allies. The Pinna, or Wing-shell, the largest of our native bivalves, affords us a good example, especially of the external layer, since here this layer projects beyond the inner one, in thin transparent edges, which give us an opportunity of examining their structure, without any artificial preparation. This fragment, taken from the edge of one of those leafy expansions, we will examine with a low magnifying power. Each of its surfaces has a sort of faceted, or honeycombed ap- pearance, and the broken edges, which even to the naked eye appear fibrous, are seen to resemble a num- ber of basaltic columns. “The shell is thus seen to be composed of a vast number of prisms, having a toler- ably uniform size, and usually presenting an ap- proach 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 extremi- ties.” * The inner layer of such shells is remarkable for pos- sessing in different degrees the property of reflecting rainbow-like colours, often with great delicacy and splendour; and this is termed naere, or familiarly “mother-of-pearl.” This iridescent lustre depends, as Sir David Brewster has shown,t upon a multitude of grooves, or fine lines, which run in a very waved pat- tern, but nearly parallel to cach other, across the sur- face of the nacre. “As these lines are not obliterated * Carpenter. The Microscope, p. 590. + Phil. Trans. 1814. 48 EVENINGS AT THE MICROSCOPE. by any amount of polishing, it is obvious that their presence depends upon something peculiar in the tex- ture of this substance, and not upon any mere super- ficial arrangement. When a piece of nacre is carefully examined, it becomes evident that the lines are pro- duced by the cropping out of laminse of shell, situated more or less obliquely to the plane of the surface. The greater the dip of these lamin, the closer will their SECTION OF NACRE FROM PEARL OYSTER. 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. Herschel to the surface of a smoothed deal board, in which the woody layers are cut perpendicularly to their surface in one part, and nearly in their plane in an- other. Sir D. Brewster appears to suppose that nacre MOLLUSCA ! THEIR SHELLS. 49 consists of a multitude of layers of carbonate of lime, alternating with animal membrane, and that-the pres- ence of the grooved lines on the most highly polished surface, is due to the wearing away of the edges of the animal lamine, whilst those of the hard calcareous lamine stand out..... There is one shell, however, the well-known Haliotis splendens, which affords us the opportunity of examining the plaits without any dis- turbance of the arrangement, and thus presents a clear demonstration of the real structure of nacre. This shell is for the most part made up of a series of plates of animal matter, resembling tortoise-shell in its aspect, alternating with thin layers of nacre; and if a piece of it be submitted to the action of dilute acid, the calcare- ous portion of the nacreous layers being dissolved away, the plates of animal matter fall apart, each one carry- ing with it the membranous residuum of the layer of nacre that was applied to its inner surface. It will be found that the nacre-membrane covering some of these horny plates, will remain in an undisturbed condition ; and their surfaces then exhibit their iridescent lustre, although all the calcareous matter has been removed from their structure. On looking at the surface with reflected light under a magnifying power of seventy- five diameters, it is seen to present a series of folds or plaits, more or less regular; and the iridescent hues which these exhibit, are often of the most gorgeous de- scription. If the membrane be extended, however, with a pair of needles, these plaits are unfolded, and it covers a much larger surface than before ; but its irides- cence is then completely destroyed. This experiment, then, demonstrates that the peculiar lineation of the surface of nacre (on which its iridescence undoubtedly 3 50 EVENINGS AT THE MICROSCOPE. depends, as originally shown by Sir D. Brewster) is due, not to the outcropping of alternate layers of mem- branous and calcareous matter, but to the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface.” * Those beautiful objects,—so much prized for per- sonal adornment,—pearls, are concretions accidentally formed within the shells of such mollusks, and are wholly composed of the inner layer. Drs. Kelaart and Mobius have recently published some highly inter- esting observations on the causes both of the irides- cence and of the pearly lustre; and these I will cite from the abstract translation of them made by Mr. Dallas. “ The surface of pearls is not perfectly smooth, but covered with very fine microscopic elevations and de- pressions. These are more or less irregular in their al- titude, but approach most nearly to equality in pearls of the finest water. In pearls which exhibit a certain iridescence, and which, when turned in different direc- tions towards the eye, present even very faint bluish, greenish, and reddish tints, the surface is found to present delicate irregular curved furrows, which either run tolerably parallel to each other, or form small ir- regular closed curves. This is due to the mode of growth of the pearl, in which thin layers of nacre, of small dimensions, have been laid over each other. There is no continuous layer over the pearl, but a num- ber of small portions which sometimes overlie the mar- gins of the subjacent layers, and sometimes leave them uncovered. This structure is seen most distinctly in the pearl shell, where the conditions are rendered more * Carpenter. The Microscope, p. 594. MOLLUSCA:! THEIR SHELLS. 51 simple by the layers being deposited on a flat, or but slightly curved, surface. The distance of the furrows from each other is not always the same; sometimes they may be recognised with the simple lens, whilst on other parts they approach within 5j5;th of an inch of each other. That the iridescence of nacre, or the na- creous colour, as distinguished from pearly lustre, is caused by the interference of the light reflected from these furrows and the intervening edges of the strata, is proved by the circumstance, ascertained by Brewster, that impressions of mother-of-pearl taken in red or black sealing-wax exhibit the same phenomena of col- our distinctly. In pearls, in consequence of their spher- ical form, the different masses of coloured light are so diffused that they unite to form white light; and this takes place with the greater perfection in proportion as the furrows are lost, and become converted into a surface of fine elevations and depressions. “ For their lustre, pearls are indebted to their being composed of fine layers, which allow light to pass through them, whilst the numerous layers lying one under the other, disperse and reflect the light in such a manner that it returns and mixes with that which is directly thrown back from the outer surface. It is the co-operation of light reflected from the surface, with light dispersed and reflected in the interior, that gives rise to lustre; for this reason the knots of window-glass exhibit pearly lustre, and the membranes of pearls de- prived of their lime are almost as lustrous as solid pearls, except that their whiteness is destroyed. ‘The two masses of light entering the eye act upon it from different distances. Now, as it adapts itself to the body seen through the transparent layer, it cannot dis- 52 EVENINGS AT THE MICROSCOPE. tinctly see the light reflected from the surface, and the consciousness of this infinitely perceptible reflection produces the phenomena of lustre.* The thinner and the more transparent the layers of which the pearl consists, the more beautiful is its lustre; and in this respect the sea-pearls excel those of our river-mol- lusks.” + We will pass now, by an easy transition, from the shells of the Mollusca to their tongues. Who that looks at the weather-worn cone of the Limpet, as he adheres sluggishly to the rock between tide-levels, would sus- pect that he carries coiled up in his throat a tongue twice as long as his shell? And that this tongue is armed with thousands of crystal teeth, all arranged with the most consummate art in a pattern of perfect regularity? It sounds almost like a fable to be told that the great Spotted Slug, which we sometimes find crawling in damp cellars, carries a tongue armed with 26,800 teeth! Yet there is no doubt of the fact. You see on this slip of glass a very slender band about two inches in length. This is the tongue of the common Periwinkle. While in the living animal, its fore-part occupied the floor of the mouth, whence it passed down below the throat, and turning towards the right side, formed a close spire of many whorls, exactly like a coil of rope, which rested on the gullet. Here we have it extracted, uncoiled, cleansed, and affixed to a slip of glass for microscopical examination. Only a small portion of the ribbon is visible ata time with such a power as is necessary to display the structure, but by means of the stage-movement we can bring the whole in succession under the eye, and dis- * Dove. Farbenlehre, 117. ¢ Ann. & Mag. N. H.; Feb. 1858. MOLLUSCA : THEIR TONGUES. 53 cover that, with some modifications of form, the same essential plan of structure, and even the same elements, exist throughout. Concentrating our attention on a single transverse series of the numerous curved lines that at first sight bewilder the mind, we perceive by delicate focussing, that the object before us consists of a number of hooks projecting from the surface of the translucent ribbon, and arching downward. In this case a single row consists of seven such hooked plates or teeth; one in the centre and three on each side. Each hooked plate has its arching tip cut into five toothlets, of which the central one is the largest; and its base is united with the cartilaginous substance of the ribbon. Only the middle plate is symmetrical; the lateral ones bend inwards towards the central one, and are symmetrical only when considered in pairs, each as- sociated with its opposite. The plates are perfectly transparent, but of a yellow horny colour; they are very hard, and as they are not dissolved by acids, it has been supposed that their substance is siliceous (having the nature of flint); but they are more probably chi- tinous, or formed of the substance of which the hard parts of insects are composed. The tongue before us has 600 rows such as these, each, as we see, closely fol- lowing, and indeed overlapping, its predecessor ; so that we can never look at a single row without at the same time seeing others which it overlaps, or by which it is overlapped. The specimen which I will now show you is broader, but shorter. It is the tongue of Trochus ziziphinus, a large and handsome shell of regularly conical form, not uncommon on our rocky shores. It is perhaps amore interesting study than that of the Periwinkle. 54 EVENINGS AT THE MICROSCOPE. There are here, you observe, three constituent elements in the pattern. First, a delicate glassy central tooth, tapering to a fine point, and cut into minute saw-teeth along cach edge. Then a series, of five on each side, of similar glassy pointed leaves, bending inward; and outside these, on either hand, are a great number of stout dark-coloured hooks, arching forward and inward,’ each notched with saw-teeth, and diminishing in thick- ness as they recede from the centre. The manner of using this elaborate organ is no less curious than is its structure. During life it is only the front portion—not more than one-third—of the TONGUE OF TROCHUS. ribbon that is in use; this is spread out on the floor of the mouth, with the teeth projecting and hooking backwards. The remainder has its edges rolled over towards each other, forming a tube closed at its extrem- ity, which, as I have already observed, is coiled away (in the long-tongued kinds) among the viscera. MOLLUSCA ! THEIR TONGUES. 55 The mode in which the tongue is used may be read- ily seen by watching the actions of a Periwinkle ina marine or a Pond-snail in a fresh-water aquarium. When the conferva has begun to form a thin green growth on the glass sides of the tank, the Mullusea are incessantly engaged in feeding on it, and rasping it away with this toothed ribbon. “The upper lip with its mandible is raised; the lower lip expands; the tongue is protruded, and applied to the surface for an instant, and then withdrawn; its teeth glitter like glass-paper, and in the Pond-snail it is so flexible that frequently it will catch against projecting points, and be drawn out of shape slightly as it vibrates over the surface.” * Perhaps every variety is accompanied by some vari- ation in food or manner of feeding. With the Zrochus, the proboscis, a tube with thick, fleshy walls, is rapidly turned inside out to a certain extent, until a surface is brought into contact with the glass, having a silky lus- tre: this is the tongue ; it is moved with a short sweep, and then tubular proboscis infolds its walls again ; the tongue disapearing, and every filament of conferva be- ing carried up into the interior from the little area which had been swept. The next instant, the foot meanwhile having made a small advance, the proboscis unfolds again, the tongue makes another sweep, and again the whole is withdrawn ; and this proceeds with great regularity. I can compare the action to nothing so well as to the manner in which the tongue of an ox licks up the grass of the field, or to the action of a mower cutting down swathe after swathe as he marches along. The latter comparison is more striking, for the * Woodward’s ‘ Mollusca,” 161. 56 EVENINGS AT TILE MICROSCOPE. marks of progress which each operator leaves behind him. Though the confervoid plants are swept off by the tongue of the Mollusk, it is not done so cleanly but that a mark is left where they grew; and the peculiar form and structure of the tongue, which I have above noticed, leave a series of successive curves all along the course which the Mollusk has followed, very like those which mark the individual swathes cut by the mower in his course through the field. The Periwinkle’s table-manners differ slightly from those of his relations. When he eats, he separates two little fleshy lips, and the glistening glass-like tongue is seen, or rather the rounded extremity of a bend of it, rapidly running round like an endless band in some piece of machinery ; only that the tooth-points, as they run by, remind one rather of a watch-wheel. For an instant this appears, then the lips close again, and pres- ently reopen, and the tongue again performs its rasp- ing. It is wonderful to see ;—perhaps not more won- derful than any other of God’s great works, never more great than when minutely great; but the action and the instrument, the perfect way in which it works, and the effectiveness with which the vegetation is cleared away before it, all strike the mind with more than usual force, as exhibitions of creative skill. As the Periwinkle moves along, mowing his sea- grass as he goes, he carries before him two soft and flexible horns, marked with zebra-like bands of black and white, which he constantly waves about. These are organs of some sense, probably of touch, and are therefore called tentacles (or tryers) ; but they bear on their outer sides, near the base, a pair of other organs, which are more closely analogous to what we ourselves MOLLUSCA : THEIR EYES. 57 possess. You sce on each tentacle a little wart, which when you look at it with a lens you perceive to have around black glossy extremity. This is the eye. By careful dissection under the microscope, we find it to contain a beautiful transparent crystalline lens, with a thick and glutinous vitreous humour adhering to it be- lind, bounded by a retina or curtain to receive the op- tic image, and an optic nerve. But much more attractive you will find the eyes in this little Scallop. It is a half-grown individual of what is provincially known as the Squin (Pecten oper- cularis), much prized for its delicate sapidity. Belong- ing to the bivalve class of the Mollusca, the animal is inclosed within two shallow shelly plates, concave in- ternally, and convex externally, which are united by a hinge, just as the works of a watch are protected by the case. When the little creature is at its ease, as when the water is pure and clear, it lies on one side, its valves being separated as we see them now, a quar- ter of an inch or so apart, allowing us to discern what is contained between them. Well, we see first a number of slender, white-pointed threads, peeping out from each valve and spreading on all sides, waving hither and thither, groping, now con- tracting, now expanding, with incessant but deliberate motion. These are tentacles. If we trace them to their origin, we find them attached to a fleshy sort of veil that lines each valve to near its edge, and then ab- ruptly falls at an angle towards the opposite valve, where it meets a corresponding veil. These two veils form the mantle. It is from each of these that the ten- tacles spring ; and we discover that there are four rows 3* 58 EVENINGS AT THE MICROSCOPE. of these organs, one row set along the angle, and one along the edge of each veil. But as we peer among these slender threads, our at- tention is riveted by some tiny points that are seated near their bases, which glitter like brilliant gems. They are secn only in those rows of tentacles which spring from the angles of the veils, and not in those which fringe their edges. Even the unassisted sight can detect the gleam and glitter of these little specks ; but it is only when we bring the lens to bear upon them that we see all their beauty. They then look like dia- monds or emeralds, each set in a broad ring of dark red substance, which greatly enhances their beauty. They are inserted into the mantle in the line of the ten- tacles, alternating with them, yet not with absolute regularity, for there are more tentacles than gem- points; they are about half as numerous again as the radiating ridges of the shell. Some are much larger and more prominent than others, but they have all the. same structure and appearance. These little organs are eyes. As its movements are far more extensive, and more fitful and rapid than is common in this class of animals, the little Pectan prob- ably needs these brilliant organs of vision to guide its wayward rovings, as well as to guard it from hostile assaults. The animal is very sensitive, withdrawing its tentacles and mantle, and bringing the valves of its shell together, on any shock being given to the vessel in which it resides. It manifests, however, a wisely measured degree of caution, for it does not actually close the valves, unless it be repeatedly disturbed, or unless the shock be violent, contenting itself with nar- rowing the opening to the smallest space appreciable ; MOLLUSCA ! THEIR EYES. 59 yet even then the two rows of gem-like eyes are dis- tinctly visible, peeping out from the almost closed shell, the two appearing like one undulating row from the closeness of their proximity. If you are familiar with the pin-cushions which children often make with a narrow ribbon round the edges of these very Scallop-shells, you can hardly fail to be struck with the resemblance borne by the living animal to its homely but useful substitute; and the beautiful eyes themselves might be readily mistaken for the two rows of diamond-headed pins, carefully and regularly stuck along the two edges of the pin-cushion ribbon,—the ribbon itself representing the satiny and painted mantle. A friend of mine, to whom I was once showing this object, compared it, not inaptly, to a lady’s ring set with diamonds. You will not fail to remark, how the position of these beauteous organs is suited for their most extensive usefulness consistent with their safety. In the ordinary condition of the animal’s expansion, and especially when it is about to make its sudden and vigorous leaps, the gemmeous points are so situated as just to project be- yond the margin of the shell. So that when we view the creature perpendicularly as it lies, our eyes looking down on the convexity of the upper valve, the minute eyes are seen, all round its circumference, just, and but just, peeping from under its edge. It is clear that this arrangement secures to them the widest range of vision with the least possible exposure. As Divine contriv- ance has been often most deservedly recognised in the projection of the bony ridge over the human eye, which we call the brow, we surely cannot fail to recognise, and admire it also in the position of these delicate organs, 60 EVENINGS AT THE MICROSCOPE. either beneath the margin of the solid shell, or, if pro- jected, projected only in the smallest degree, and en- dowed with the power of retreating beneath its barrier with the rapidity of thought on the least alarm. There can be no doubt that these points, numerous as they are, are true eyes, endowed with the faculty of vision in a well-developed degree. For when their structure is carefully examined by the skilful anatomist, each is found to be covered with the proper sclerotic tunic which becomes a perfectly transparent cornea in front, and to possess a coloured iris,—perforated with a well-defined pupil, and connected with a layer of pig- ment which lines the sclerotic tunic,—a crystalline lens, and a vitreous humour for the due refraction of the rays of light, and a retina in their focus, formed by an expansion of the optic nerve, and fitted to receive the picture ; the sensation of which is then conveyed by an optic nerve from each eye to the common nerve-trunk, which runs along the border of the mantle. Thus there exists in each of these lustrous points every element needful for the due performance of vision, though, prob- ably, the impressions thus conveyed may be neither so powerful, nor so distinct as those which are conveyed by the eyes of vertebrate animals. They are, how- ever, we may be sure, amply sufficient for the wants of the pretty Scallop, and are fresh proofs of the Divine wisdom and benevolence. We have been accustomed, from childhood, to recog- nise as eyes the shining black extremities of the upper pair of “horns” in the Garden Snail. And though some naturalists have doubted, and even denied that the tentacle was anything more than a very delicate organ of touch, yet it has been abundantly proved by dissec- MOLLUSCA ! THEIR EYES. 61 tion, and is now incontrovertibly established, that its tip carries an eye even more completely developed than those of the Pecten, which we have just been looking at. The eye is situated, not indeed on the very sum- mit of the tentacle, but on one side of a movable bulb there placed. It is very minute, almost spherical, but slightly flattened in front. It is protected by a very thin transparent layer of the common skin and is sur- rounded at the side and behind, by a perfectly black membrane called the choriid, or pigment membrane. This black globule contains a transparent and semi-fluid substance, with which it is completely filled; towards the bottom it is of thinner consistence, and appears to contain many brilliant particles when the eye is dis- sected under the microscope; this may be considered as the vitreous humour. In the front part of the eye there is a crystalline lens, a small, circular, flattish, or rather lenticular body, perfectly clear and translucent, but a little more solid than the vitreous humour. Now protection for these so delicate organs is pro- vided in a way quite different from, yet equally effec- tive with, that which we just now admired in the case of the Pecten. You know that if you touch, though ever so tenderly, the eye of the Snail, it is in- stantly drawn into the horn by a most curious process of inversion. The action is performed by means of a long muscular ribbon, which originates from the great muscle that retracts the head within the shell, and which is inserted into the extremity of the hollow tentacle. When this ribbon contracts at the will of the animal, and still more forcibly, when it is aided by the contraction of the great head-muscle, the tip of the tentacle with its eye is drawn within the surrounding 62 EVENINGS AT TUE MICROSCOPE. parts, just like the finger of a glove. When the ani- mal would again protrude its eye, the fibres which sur- round the tentacle, like s0 many rings throughout its whole length, successively contract, and thus gradually squeeze out, as it were, the inverted part, until it is turned back to its original position. BTRUCTURE OF LY IN SNAIL, But the cars of this homely “creeping thing” are, perhaps, even more curious than its eyes; though far less elaborate in their structure. You will imagine now, that I refer to the other pair of tentacles, as you are accustomed to associate the idea of cars with pro- jecting organs situated on the head. No, you must not look there for them. Jere, in this young Garden Slug, which is s0 small as to be conveniently examined on the stage of the microscope, and so devoid of colour that we can readily look through its tissues,—we shall casily find its ears, though they are not quite so prominent as those of an ass. I subject the animal to a gentle pressure by means of the compressorium, just sufficient to flatten its soft body a little, without injuring it. And now, with this low power, you may see that Siebold, a learned zoolo- gist and comparative anatomist, familiar with the curi- ous phenomena of life, truly calls “a wonderful spec- tacle.” In the neck of the little animal you discern, MOLLUSCA : THEIR EARS. 63 deep-seated in the soft flesh, a pair of perfectly trans- parent globules, or bladders, without any opening, but filled with a clear fluid, in which there are some minute bodies performing the most extraordinary evolutions. They constantly keep up a series of swinging or balanc- ing movements, sometimes rotating, sometimes forcibly driven in a certain direction, then in the opposite, yet no single one ever by any accident touching the walls of the capsule in which they are contained. If the cap- sule be ruptured, the motions instantly cease. These little bodies are of a calcareous nature; and they are called otolithes, that is, ear-stones. The most that we know of these curious capsules, which are indubitably ascertained to be organs of hearing, we owe to the ob- servations of the eminent zoologist just named, and you may perhaps like to know a little more about them. Siebold says that a concentric depression is evident in these otolithes, and that there may be seen in the centre of the greater number of them a shaded spot, or rather a minute aperture, which penetrates through the concretion from the one flattened surface to the other. Subjected to a strong pressure, the otolithes crack in radiating lines, separating often into four pyra- midal pieces. This separation also ensues, after a longer time, when the otolithes are immersed in diluted nitric acid; and, if we touch them with the concentrated acid, they suddenly dissolve with the disengagement of a gas, whence Siebold concludes them to be composed of carbonate of lime. The size of the otolithes is not equal, and in the same capsule there are always some which are smaller than others. Within the capsule they have, during life, a very remarkable, and in some respects peculiar, lively, oscillatory movement, being 64 EVENINGS AT THE MICROSCOPE. driven about as particles of any light insoluble powder might be in boiling water. The otolithes in the centre have the appearance of being pressed together so as to form a sort of solid nucleus, and towards this centre the otolithes towards the cireumference seem even to be violently urged, their centripetal rush being invari- ably repulsed, and as often driven again into a centri- fugal direction. Removed from the capsule, the mo- tions of the otolithes instantly cease. The cause of these curious oscillations remain undiscovered. Siebold could detect no vibratile cidia on the surfaces of the capsule, and the cessation of the motion when the oto- lithes are removed, proves them to be unciliated them- selves, and, at the same time, distinguishes the motion from that of inorganic molecules. It has been more recently ascertained that the move- ments of the otolithes are due to very minute cilia with which the interior surface of the capsule is covered. This had been long suspected, and some eminent physi- ologists, as Wagner and Kolliker, have distinctly scen the cilia themselves. If you ask what can be the use of ears to a class of animals which are invariably dumb, I answer that though this is true with respect to the great majority, yet it may be only that our senses are too dull to per- ceive the delicate sounds which they utter, and which may be sufficiently audible to their more sensitive or- gans; and besides, some Mollusca can certainly emit sounds audible to us. Two very elegant species of Sea- slug, viz. Holis punctata, and Tritonia arborescens,* certainly produce audible sounds. Professor Grant, * Now called Dendronotus arborescens. MOLLUSCA : THEIR EARS. 65 who first observed the interesting fact in some speci- mens of the latter which he was keeping in an aqua- rium, says of the sounds, that “they resemble very much the clink of a steel wire on the side of the jar, one stroke only being given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water the sound is much obscured, and is like that of a watch, one stroke being re- peated, as before, at intervals. The sound is longest and oftenest repeated when the Tritonize are lively and moving about, and is not heard when they are cold and without any motion; in the dark I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke; the sound, when in a glass vessel, is mellow and distinct.” The Professor has kept these Tritonise alive in his room for a month, and, during the whole period of their confinement, they have continued to produce the sounds, with very little diminution of their original intensity. In a small apartment they are audi- ble at the distance of twelve feet. “The sounds obvi- ously proceed from the mouth of the animal; and, at the instant of the stroke, we observe the lips suddenly separate, as if to allow the water to rush into a small vacuum formed within. As these animals are her- maphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them, or, if they be of an electric nature, they may be the means of defending from foreign enemies one of the most delicate, defenceless, and beautiful Gastero- pods that inhabit the deep.” * * Edinb. Phil. Journ. xiv. 186. 66 EVENINGS AT THE MICROSCOPE, CHAPTER IV. SEA-MATS AND SHELLY CORALLINES. Wuen we were at the sea-side last summer we bought, you may remember, of a poor widow whom we met on the beach, a little basket of dried sea-weeds. Fetch it: it is on the chimney-piece upstairs. Now all of these objects are not sea-weeds. I mean they are not all plants; some of them are animals, and these I want to bring under your notice this evening for our microscopical entertainment. Here are exquisite- ly delicate crimson leaves, as thin or thinner than the thinnest tissue-paper, with solid ribs and sinuous edges. Here is a tall and elegant dark red feather, quite regu- larly pinnated. Here is a tuft of purple filaments as “fine as silkworm’s thread.” And here is a broad ir- regular expanse of the richest emerald-green, crumpled and folded, yet as glossy as if varnished. Well, all of these are plants, certainly: they are veritable Alga, or sea-weeds. But here are other plant- like objects of a pale brown, drab, or snowy-white hue. Let us take this flattened brown leaf, divided into irreg- ular broad lobes ; it looks almost like a thickish paper, and is about as flexible. But pass your finger over it, and you feel that its surface is evenly roughened ; and on close and careful scrutiny you discern, even by the naked eye, that its surface is covered with a delicate network of minute shallow cells. setA-MATS AND SHELLY CORALLINES. 67 “ Broad Hornwrack,” and ‘“ Leafy Sea-mat,” are the names which the old collectors gave to this object; and modern naturalists have given it the scientific appella- tion of Flustra foliacea, and arrange it in the class Polyzoa, a group of animate beings, which have much of the form of Polypes, and much of the structure of Mollusks. We cut off a little piece from the end of one of the lobes, and put this upon the stage of the microscope. We now see that the cells are disposed in nearly par- allel rows; but so that those of one row alternate with those of the next, guiéncunw fashion, the middle of one cell being opposite the end of its right and left neigh- bours ;—or like the meshes of a net. The cells extend over the whole leaf, and are spread over both its sur- faces in this case; the united depth of two cells consti- tuting the thickness of the leaf-like structure. There are other species, more delicate, which have but a sin- gle series of cells, all opening on the same side of the leaf. Each individual cell is shaped like a child’s cradle ; and if you will imagine 20,000 wicker cradles stuck together side by side in one plane, after the quincunx pattern I have just mentioned; and then the whole broad array turned over, and 20,000 more glued on to these, bottom to bottom, you will have an idea of the framework of this pale-brown leaf ;—dimensions, of course, being out of the consideration. The number may appear somewhat immense, yet it is no larger than the ordinary average, as I will soon show you. I meas- ure off a square half-inch of this leaf, which I carefully cut out with scissors; now with the micrometer count the cells in the square piece.—You find 60 longitudinal 68 EVENINGS AT THE MICROSCOPE. rows, each containing 28 cells, or thereabouts. Very well; a simple arithmetical process shows that there are 1,680 cells in this square half-inch; or 6,720 ina square inch. Now this very specimen, before I mutil- ated it, contained an area of about three square inches ; which would give 20,160 cells. This is the number on one surface; the other contains an equal number; and thus you see that I have not exaggerated the population of this tiny marine city. This, however, is by no means a specimen of unusual size. These cells, which I compare to cradles, are of shal- low depth, but the head-part rises to a much greater height than the foot. All round this elevated portion the margin is armed with short blunt spines, two on each side, which stand obliquely erect, projecting out- wards over the middle of the next cell, which thus, in concert with the spines of the cell on the opposite side, they protect. If you search carefully over the aggregation of cells with this pocket-lens, you will perceive that on some of them are seated minute white globules, which look like tiny pearls. These are not placed in any regular order, two being sometimes found on contiguous cells, but generally they are scattered at more or less remote in- tervals. If we now apply the microscope to these ap- pendages, each globule is seen to be flat on that per- pendicular side which faces the foot of the cradle; and this flat side is a movable door, with a hinge along its lower edge. The door is of a yellow hue; the globule itself being, as I said, of a pearly white hue. This is all that we can see in this dried specimen; but if we had been fortunate enough to have examined it when first it was torn from its attachment to an old SEA-MATS AND SHELLY CORALLINES. 69 shell at the bottom of the sea, you would have seen much more. And what would then have appeared, I will describe to you. Suppose, then, that a coverlid of transparent skin were stretched over each cradle, from a little within the margin all round, leaving a transverse opening just in the right place, viz. over the pillow, and you would have exactly what exists here. There is a crescent- form slit in the membrane of the upper part of the cell, from which the semicircular edge, or lip, can recede, if pushed from within. Suppose, yet again, that in every cradle there lies a baby, with its little knees bent up to its chin, in that zig-zag fashion that children, little and big, often like to lie in. But stay, here is a child moving! Softly! He slowly puslies open the semicircular slit in the cov- erlid, and we see him gradually protruding his head and shoulders in an erect position, straightening his knees at the same time. He is raised half out of bed, when lo! his head falls open, and becomes a bell of tentacles! The baby is the tenant-polype! “This is a very amusing romance,” you say. Nay, it is no romance at all. If you will excuse the homeli- ness of the comparisons, I will venture to affirm that a personal examination of the creature itself would justify their correctness, and you would acknowledge that they could scarcely be more apt. Moreover, the globular chambers show signs of life; their front doors suddenly open, gape widely, and then shut with a snap; and presently this opening and shut- ting is repeated. The meaning of this action you will better understand when we see analogous organs in an- other form of the same Glass of animals. Meanwhile, I 70 EVENINGS AT THE MICROSCOPE. will just point out a beautiful though minute proof of design in a point of the structure of the cells connected with these pearly chambers. If you look closely, you will see that the spines of the margin are not found on those cells that. carry the pearls; and moreover, that they are also wanting on the approximate edges of the two cells that lie behind every such pearl-bearing cell. Now the reason of this omission is obvious. The spines LEAFY SEA-MAT. (A portion magnified.) projecting obliquely would interfere with the gaping of the door; and hence they are invariably absent there. I happen to have in my aquarium a living individ- ual of another species belonging to the same class, and agreeing with this in all essential particulars of struc- ture, though widely different in form. The difference, however, is mainly dependent on a rather unimportant point of arrangement; for the cells, instead of being set side by side and end to end in quincunx fashion, to an indefinite extent, on two surfaces of a plane, are dis- posed on one single surface, and in longitudinal rows SEA-MATS AND SIIELLY CORALLINES. val of two or three cells abreast; thus narrow ribbon-like branches are formed, which now and then divide into two, then these into two more, and so on. These branches thus become fan-shaped, which, by being slightly curved, become segments of funnels; and the peculiar elegance of this coralline consists in the mode in which these branches are set on the stem, viz., in an ascending spiral curve, so that the effect is that of sev- eral imperfect funnels set one within another, but which yet you perceive, by turning the whole gradually round, to compose a single corkscrew band of successive fans. This whole structure stands upright in its natural state, like a little compact shrub growing from a root; and as a good many are commonly associated together, they form a sort of mimic grove, fringing the sides of dark rocky sea-pools. The species is called the Corkscrew Coralline, or sometimes the Bird’s-head Coralline, the latter name being assigned to it for a reason which you will pres- ently perceive. The appellation by which it is known to naturalists is Bugula avicularia. We drop our specimen into a very narrow cell, com- posed of parallel walls of thin glass, a very minute flat- tened tank, in fact, such as can be put on the stage of the microscope. Here, bathed in its native seawater, as clear as crystal, we shall see it opening and expand- ing its numerous polypides with the utmost activity and evident enjoyment. You gaze; but you know not what you see. The presence of many lines representing transparent vessels of strange and dissimilar shapes, overlying each other ; and the swaying to and fro of curious objects, which strike now and then forcibly across the field of view, 72 EVENINGS AT THE MICROSCOPE. are quite bewildering. I must act the showman, and tell you what to see. The cells are oblong, shaped much like a sack of corn, with a spine ascending from each of the upper corners. Each stands on the summit of its predecessor in the same row, and side by side with those of its fel- low-rows, in such an order that the top of one cell comes opposite the middle of the one beside it. The top of the sack is rounded, and appears closed, but we shall presently find an opening there. The broad side that faces inwardly has a large elliptical transparent space occupying nearly its whole surface; this is covered with a very thin and elastic membrane, and answers a peculiar end. Just below one of the spines that crowns the summit of the cell on one of the edges, is situated a little lump, to which is attached, by a very free joint, an object which you will perceive to bear a remarkable resemblance to the head of a bird of prey. It has a beak strongly hooked, with two well-formed mandibles, of which the lower is movable, shutting into the cavity of the upper; you observe it deliberately opening, like that of a bird, only stretching to an enormous width of gape, and then closing with a strong and sudden snap. Now and then the whole head sways backward and for- ward on its joints; and these movements, combined with the fitful and apparently spiteful snappings, per- formed by many birds’ heads scattered about the branch, are highly curious and amusing. The birds’ heads, however, are not the living inhab- itants of the cells; they are not integral parts of them. The cells have their proper inhabitants, each dwelling in its own; and each essentially formed on the same plan as the “baby with the tucked-up knees,” that makes the Sea-mat for his cradle-house. SEA-MATS AND SHELLY CORALLINES. 43 In order to get a good view of the tenant here, you must move the stage about till you find that the branch is presented to your eye sidewise. Directing your at- tention then to the lateral edge of asingle inhabited cell, its summit is seen to protrude diagonally towards the inner side (. e. towards the axis of the spire), a tu- bular mouth, which is membranous and contractile. When the animal wishes to emerge, this tubular orifice is pushed out by evolution of the integument, and the tentacles are exposed to view, closely pressed into a parallel bundle; the evolution of the integument, that is attached at their base, goes on till the whole is straight- ened, when the tentacles diverge and assume the form of a funnel, or rather that of a wide-mouthed bell, the tips being slightly everted. They are furnished with a double row of short cilia in the usual order, one set working upward, the other downward. Their base sur- rounds a muscular thick ring, the entrance to a funnel- shaped sac, the substance of which is granular, and evidently muscular, for its contractions and expansions are very vigorous, and yet delicate. Into this first stomach passes, with a sort of gulp, any animalcule whirled to the bottom of the funnel by the ciliary vor- tex, and from thence it is delivered through a contract- ed, but still rather wide gullet, into an oblong stomach, the lower portion of which is obtuse. An extremely attenuated duct connects this, which is probably the true stomach, with a globular, rather small, intestine, which is again connected by a lengthened thread with the base of the cell. By an arrangement common to the ascidian type of the digestive function, the food is returned from the intestine into the true stomach, whence the effete parts are discharged through a wide 4 44. EVENINGS AT TITE MICROSCOPE. and thick tube that issues from it close behind the point where the gullet enters. This rectal tube passes up- wards parallel to the gullet, and terminates by an orifice outside and behind the base of the tentacles. All these viscera are beautifully distinct and easily identified, owing to the perfect transparency of the walls of the cell, the simplicity of the parts, and their density and dark yellow colour. All of them are manifestly gran- ular in texture, except the slender corrugated tube which connects the stomach with the globose intestine: this is thin and membranous, and is doubtless, if I may judge from analogy, capable of wide expansion for the passage of the food-pellet. The sudden contraction of the polypide into its cell upon disturbance or alarm, and its slow and gradual emergence again, afford excellent opportunities for studying the forms, proportions, and relative positions of the internal organs. In contraction, the globular intestine remains nearly where it was, but the stomach slides down into the cell behind it, as far as the flexible duct will allow, and the thick gullet bows out in front, showing more clearly the separation between it and the rectum, and the insertion of both into the stomach. This retraction is, in part, effected by a pair of longitudinal muscular bands, which are inserted at the back of the bottom part of the cell, and into the skin of the neck below the tentacles. The contraction of these bands draws in the integument, like the drawing of a stocking within itself, and forces down the viscera into the cavity of the cell, which is probably filled with the vital juices. Besides the hind bands, there is one, or a pair of similar muscular bands attached on each side of the front part of the base of the cell, and inserted similarly SEA-MATS AND SHELLY CORALLINES. 45 into the neck. By watching the contraction of these, you will be enabled to determine the use of the mem- brane-covered aperture up the front of the cell. At the moment of the retraction of the viscera into the cell, a large angular membrane is forced outward from the front side, which is protruded more or less in proportion to the degree of withdrawal of the polypide; and asthe latter emerges again, the membrane falls back to its place. It is evident, then, that this is a provision for enlarging the cavity ; the walls are horny, and probably almost inelastic; but when the stomach forces the in- testine forward, and the thick gullet is bent outward by the withdrawal of the neck and tentacles, the needful room is provided by the bulging out of this elastic mem- brane, which recovers its place by the pressure of the surrounding water, when the pressure of the fluids within is removed. Now, after watching these movements of the poly- pides, and the various structures whose forms and limits those movements reveal, it will become manifest to you that there is no visible organic connexion between the animal distinctively so called and the curious bird’s head. This latter has a muscular system of its own, by means of which its energetic motions are performed ; but it appears quite isolated on the outside of the cal- eareous cell, and wholly cut off from the interior by the knob on which it works, and by the thickness of the cell-wall. Both knob and wall appear quite imperfo- rate; and yet we cannot but presume that some con- nexion exists, perhaps throngh the medium of an ex- cessively delicate and subtile, but living tissue, which may be presumed not only to dine, but also to cover the strong cell; just as the strong envelope and spines of 16 EVENINGS AT THE MICROSCOPE. a Sea-urchin are covered with a thin film of living flesh. The functions and use of these singular processes are as obscure as their connexions with the animal. Yet that they play some important part we may almost certainly infer, from the general prevalence of similar or analogous appendages among the various forms of this class. The globular pearls which you lately saw on the sea-mat, is but another form of bird’s head ; and the falling-door answers to the opening and shutting mandible. The forms, indeed, of these organs are very diverse, and sometimes they are greatly disguised. But what about their function? More than one observer has noticed the seizure of small roving animals by these pincer-like beaks; and hence the conclusion is pretty general, that they are in some way connected with the procuring of food. But it seems to have been forgotten, not only that these organs have no power of passing the prey thus seized to the mouth, but also that this latter is situated at the bottom of a funnel of ciliated tenta- cles, and is calculated to receive only such minute prey as is drawn within the ciliary vortex. I have ventured to suggest a new explanation. The seizure of a passing animal, and the holding of it in a tenacious grasp until it dies, may be a means of attracting the proper prey to the vicinity of the mouth. The presence of decom- posing animal substance in water invariably attracts crowds of infusory animaleules, which then breed with amazing rapidity, so as to form a cloud of living atoms around the decaying body, quite visible in the aggre- gate to the unassisted eye; and these remain in the vicinity, playing round and round until the organic matter is quite consumed. Now a tiny Annelid or SEA-MATS AND SHELLY CORALLINES. qT other animal caught by the bird’s head of a Polyzoan, and tightly held, would presently die; and though in its own substance it would not yield any nutriment to the capturer, yet by becoming the centre of a crowd of busy infusoria, multitudes of which would constantly be drawn into the tentacular vortex, and swallowed, it would be ancillary to its support, and the organ in question would play no unimportant part in the econ- omy of the animal. 78 EVENINGS AT THE MICROSCOPE. CHAPTER V. INSECTS: WINGS AND THEIR APPENDAGES. I provosE now to reveal to you some of the microscopic marvels of the insect world; a race vastly more popu- lous than all of the other animate tribes put together ; for the most part so minute as to be peculiarly suitable subjects for our present investigations, and so furnished with elaborate contrivances and exquisite pieces of mechanism, as to elevate our thoughts at every turn to the majesty of the Divine wisdom displayed in the most minute of His creatures. Let us begin with their wings. The most perfect fliers in existence are Insects. The swallow and the humming-bird are powerful on the wing, and rapid; but neither these nor any other “winged fowl” can be compared with many of the filmy-winged Insects. The common House-fly, for example, will remain for hours together floating in the air beneath the ceilings of our dwelling-rooms, hov- ering and dancing from side to side, without effort and without fatigue. It has been calculated that in its ordinary flight the House-fly makes about 600 strokes with its wings every second, and that it is carried through the air a distance of five feet during that brief period. But, if alarmed, the velocity can be increased six or seven-fold, as every one must have observed, so INSECTS: WINGS AND THEIR APPENDAGES. v63) as to carry the insect thirty or five and thirty feet in the second. In the same space of time, observes Mr. Kirby, a race-horse could clear only ninety feet, which is at the rate of more than a mile a minute. Our little fly, in her swiftest flight, will in the same space of time go more than the third of a mile. Now compare the in- finite difference of the size of the two animals (ten mil- lions of the fly would hardly counterpoise one racer), and how wonderful will the velocity of this minute creature appear! Did the fly equal the race-horse in size, and retain its present powers in the ratio of its magnitude, it would traverse the globe with the rapid- ity of lightning.* Bees, again, are accomplished masters of aérial mo- tion. The Humble-bees, notwithstanding their heavy bodies, are the most powerful fliers of this class. The same excellent entomologist tells us that they “ traverse the air in segments of a circle, the are of which is al- ternately to right and left. The rapidity of their flight is so great that, could it be calculated, it would be found, the size of the creature considered, far to exceed that of any bird, as has been proved by the observa- tions of a traveller in a railway carriage proceeding at the rate of twenty miles an hour, which was accom- panied, though the wind was against them, for a con- siderable distance by a Humble-bee (Bombus subinter- ruptus), not merely with the same rapidity, but even greater, as it not unfrequently flew to and fro about the carriage, or described zig-zag lines in its flight. The aérial movements of the Hive-bee are more distinct and leisurely.” + You have doubtless often admired the noble Dragon- * Intr. to Entom. Lett. xxii. t Ibid. 80 EVENINGS AT THE MICROSCOPE. fly, with its four ample and wide-spread wings of gauze, hawking in a green lane, or over a pool in the noon of summer. It sails, or rather shoots with arrowy fleetness hither and thither, now forwards, now back- wards, now to the right, now to the left, without turn- ing its body, but simply by the action of its powerful and elegant wings. Leeuwenhoek once saw an insect of this tribe chased by a swallow in a menagerie a hun- dred feet long. The Dragon-fly shot along with such astonishing power of wing, to the right, to the left, and in all directions, that this bird of rapid flight and ready evolution was unable to overtake and capture it, the insect eluding every attempt, and being in general fully six feet in advance of the bird. A Dragon-fly ° has been known to fly on board a ship at sea, the nearest land being the coast of Africa, five hundred miles distant, a fact highly illustrative of its power of wing. It is a point of interest to know the structure of the organs by which such results are accomplished, and therefore we will devote an hour to the microscopical examination of the wings of one or two Insects. Let us begin with the common Fly, one of which, a fine blue-bottle, is somewhat noisily buzzing in the win- dow— “The blue-fly sung i’ the pane,”— as if to invite our attention to him. Well, we will bor- row one of his wings for the lesson: and putting it into the stage-forceps, we shall be able to turn it in any di- rection for observation beneath the microscope. At first it seems a very thin transparent membrane, of a shape between triangular and oval, with a few fine INSECTS: WINGS AND THEIR APPENDAGES. 81 black lines running through it, and along one edge. But on bringing a greater magnifying power to bear on it, we see that the clear surface is covered with minute short stiff hairs, each of which has an expanded base. And still further, by delicate focussing, we find that there are two sets of these hairs, which come into view alternately, those of one row projecting upward to- wards our eye, those of the other downward. They are placed on both the upper and under-surface, and are in fact appendages of two distinct membranes, applied to each other. There is some reason to believe that these hairs are delicate organs of touch communicating im- pressions through the skin to a sensitive layer beneath ; at least such seems their function on the body, and we may judge from analogy that it is not different here. The black lines are elastic, horny tubes, over which the membranes are spread and stretched, like the silk of an umbrella by its ribs. The upper membrane is firmly attached to the tubes (which are called ner- vures); the lower has but a slight adhesion, and is easily stripped from them. The nervures originate in the body, and diverge like a fan to various points of the tip, and to the upper and lower edges; some of them, however, terminate in the substance of the wing without reaching the edge, and some send off cross branches by which two are connected together. They generally maintain the same thickness throughout, but there are enlargements where the branches join the main trunks. These nervures are hollow, and are, dur- ing life, filled with a subtile fluid, which is supplied from the vessels of the body. They contain also rami- fications of the exquisite spiral air-vessels, which we 4* 82 EVENINGS AT THE MICROSCOPE. shall presently consider, so that both air and blood cir- culate in them. In this wing of the Bee all of these structures may be seen to greater advantage. The membrane appears perfectly homogeneous by transmitted light, even with so high a magnifying power as 600 diameters, at least on a cursory examination; though, by careful manipula- tion, we may discern faint traces of angular lines which divide the whole surface into irregular areas. But by using reflected light at an oblique angle, this areola- tion, which indicates the primary cells of the structure, is much plainer, and each area is perceived to carry a single hair in its centre. The hairs themselves here take the character of curved spines, not unlike those of arose tree. Along the front edge of the wing they are straight, stout, densely crowded, and overlapping in an inclined po- sition; but the most interesting modification of these organs is seen at the front edge of the posterior pair. Unlike the Fly, which has but a single pair of wings, the Bee has two pairs, of which the fore pair is the larger and more horny, the hinder pair seeming to be, as it were, cut out of the hinder and inner side of the fore ones. The two edges—the hinder edge of the fore pair and the front edge of the hind pair—then corres- pond, but it is necessary that, during flight, when the wings are expanded, the two wings on each side should maintain this relative position, neither overlapping the other, but together presenting one broad surface, where- with to beat the air. There must be, therefore, some contrivance for locking together the two edges in ques- tion, which yet shall be capable of being unlocked at the pleasure of the animal ; for the wings during repose slide over one another. This contrivance is furnished INSECTS: WINGS AND THEIR APPENDAGES. 83 by a series of hairs or spines running along the front edge of the hind-wing; they are bent up into strong semicircular hooks, arching outwards, looking, under a high power, like the hooks on a butcher’s stall. On the other hand, the margin of the fore-wing is strength- ened, and is turned over with a shallow doubling, so as to make a groove into which the hooks catch; and thus, while the fore-wings are expanded, the hooks of the other pair are firmly locked in their doubled edge, while, as soon as flight ceases, and ae wings are re- laxed, there is no hin- : drance to the sliding of the front over the hind pair. The wings of many in- sects are interesting on ° account of the organs with which they are clothed. 4 OF conrNactis, ities are obtuse, but in others, as in 7. erassicornis [an example of which I now show you] the posterior ex- SEA-ANEMONES : THEIR WEAPONS. 425 tremity runs off to a finely attenuated point, the whole of the spire visible even to the last, the whole bearing no small resemblance to a multispiral shell; as one of the Cerithiadw or Turritellade. The ecthorwum is dis- charged reluctantly from this form, and I have never seen an example in which the whole had been run off. So excessively subtile are the walls of the cnzda, that it was not until after many observations that I detected them ; in an example from 7. crassicornis, which had discharged about half of the wire, I have not seen the slightest sign of armature on the ecthorweum. So far as my investigations go, these Spiral Cnide are confined to the walls of the tentacles, in which, however, they are the dominant form.” Such, then, is the form and armature of these organs. But I have not yet done with them. The emission of the wire, strange to say, is a process of distinct eversion from beginning to end. The ecthoreum is not a solid, but a tubular, prolongation of the walls of the enida, turned-in, during its primal condition, like the finger of a glove drawn into the cavity. Of this fact you may convince yourself by a careful watching of the phenomena before you. Many of the ecthorwa from the tangled cnide now under your eye run out, not in a direct line, but in a spiral direction. Select one of these, and you will perceive that each bend of the spire is made, and stereotyped, so to speak, in suc- cession, while the tips go on lengthening ; the tip only progresses, the whole of the portion actually discharged remaining perfectly fixed ; which could not be on any other supposition than that of evolution. In the discharge of the chambered kind—to revert to those which we were just now examining—we saw 426 EVENINGS AT THE MICROSCOPE. the ventricose basal part first appear; the lower barbs flew out before the upper ones, and all were fully ex- panded before the attenuated portion began to lengthen. “This, again, is consistent only with the fact of the evolution of the whole. On several occasions of obser- vation on the chambered enide of Cyathina Smithii, I have actually seen the unevolved portion of the ectho- reum running out through the centre of the evolved ventricose portion. But perhaps the most instructive and convincing example of all was the following. One of the large tangled cnidw of worynactis virdis had shot about half of its wire with rapidity, when a kind of twist, or ‘kink,’ occurred against the nipple of the cnida, whereby the process was suddenly arrested. The projectile force, however, continuing, caused the impediment to yield, and minute portions of the thread flew out piecemeal, by fits and starts. By turning the stage-screw I brought the extremity of the discharged portion into view, and saw it slowly evolving, a little atatime. Turning back to the cnzda, J saw the kink gradually give way, and the whole of the tangled wire quickly flew out through the nipple. I once more moved the stage, following up the ecthoreum, and presently found the true extremity, and a large portion of the wire still inverted ; slowly evolving, indeed, but very distinct throughout its whole course, within the walls of the evolved portion. “ From all these observations there cannot remain a doubt of the successive eversion of the entire ecthor- cum.” You ask, What is the nature of the force by which the contained thread is expelled? “That it is a potent force is obvious to any one who makes the sudden SEA-ANEMONES: THEIR WEAPONS. 427 explosive violence with which the nipple-like end of the cnida gives way, and the contents burst forth; as also the extreme rapidity with which, ordinarily, the whole length is evolved. A curious example of this force once excited my admiration. The ecthorwum from a enida of Corynactis virdis was in course of rapid evolution, when the tip came full against the side of another cnida already emptied. The evolution was momentarily arrested, but the wall of the empty capsule presently was seen to bend inward, and sud- denly to give way, the ecthoreum forcing itself in, and shooting round and round the interior of the enida. ‘“‘ The most careful observations have failed to reveal a lining membrane to the cnida. I have repeatedly discerned a double outline to the walls themselves, the optical expression of their diameter; but have never detected any, even the least, appearance of any tissue starting from the walls, as the ecthorwum bursts out. My first supposition, reluctantly resigned, was, that some such lining membrane, of high contractile power, lessened, on irritation, the volume of the cavity, and forced out the wire. “The enida is filled, however, with a fluid. This is very distinctly seen occupying the cavity when, from any impediment, such as above described, the wire flies out fitfully; waves, and similar motions, passing from wall to wall. Sometimes, even before any portion of the wire has escaped, the whole mass of tangled coils is seen to move irregularly from side to side, within the capsule, from the operation of some intestine cause. The emission itself is a process of injection ; for I have many times seen floating atoms driven for- cibly along the interior of the ecthorwum, sometimes 428 EVENINGS AT THE MICROSCOPE. swiftly, and sometimes more deliberately. Nothing that I have seen would lead me to conclude that the wall of the cnida is ciliated. “T consider, then, that this fluid, holding organic corpuscles in suspension, is endowed with a high degree of expansibility ; that, in the state of repose, it is in a condition of compression, by the inversion of the ecthoreum , and that, on the excitement of a suit- able stimulus, it forcibly exerts its expansile power, distending and, consequently, projecting, the tubular ecthoreum—the only part of the wall that will yield without actual rupture.” It has been proved that the execution of these weapons is as effectual as their mechanism is elaborate. The wire shot with such force penetrates to its base the tissues of the living animals which the Anemone attacks, when its barbs preclude the withdrawal of the dart. Butthe entrance of bodies so excessively slen- der would of itself inflict little injury; there is evi- dently the infusion at the same time of a highly subtile poison into the wound; some venomous fluid escaping with the discharge of the ecthorewm, which has the power, at least when augmented by the simultaneous intromission of scores, or hundreds, of the weapons, of suddenly arresting animal vigour and speedily de- stroying life, even in creatures—fishes for example—far higher than the zoophyte in the scale of organization. I have seen a little fish in perfect health come in accidental contact with one of the acontia of an irritat- ed Sagartia, when all the evidences of distress and agony were instantly manifested; the little creature darted wildly to and fro, turned over, sank upon the bottom, struggled, flurried, and was dead. SEA-ANEMONES ! THEIR WEAPONS. 429 “ Admitting the existence of a venomous fluid, it is difficult to imagine where it is lodged, and how it is injected. The first thought that occurs to one’s mind is, that it is the organic fluid which we have seen to fill the interior of the enida, and to be forced through the everting tubular ecthoreum. But if so, it cannot be ejected through the extremity of the ecthoreum, because if this were an open tube, I do not see how the contraction of the fluid in the cnida could force it to evolve; the fluid would escape through the still inverted tube. It is just possible that the barbs may be tubes open at the tips, and that the poison-fluid may be ejected through these. But I rather incline to the hypothesis, that the cavity of the ecthorwum, in its primal inverted condition, while it yet remains coiled up in the cnida, is occupied with the potent fluid in question, and that itis poured out gradually within the tissues of the victim, as the evolving tip of the wire penetrates farther and farther into the wound.” I do not think that the whole range of organic ex- istence affords a more wonderful example than this of the minute workmanship and elaboration of the parts ; the extraordinary modes in which certain prescribed ends are attained, and the perfect adaptation of the contrivance to the work which it has todo. We must remember that all this complexity is found in an animal which it is customary to consider as of excessively simple structure. But the ways of God are past finding out. These are but parts of His ways. 430 EVENINGS AT THE MICROSCOPE. CHAPTER XX. PROTOZOA AND SPONGES. WE are s0 accustomed to see certain of the vital functions of animals performed by special organs or tissues, that we wonder when we find creatures which move without limbs, contract without muscles, respire without lungs or gills, and digest without a stomach or intestines. But thus we are taught that the function is independent of the organ, and, as it were, prior to it; though in nine hundred and ninety-nine cases out of a thousand it be associated with it. In truth, the simplest forms of animal life display very little of that division of labour, the minuteness of which increases as we as- cend the organic scale; the common tissue is not yet differentiated (to use the awkward term which is becoming fashionable among physiologists) into organs, but is endowed with the power of fulfilling various offices, and performing many functions. In all probability, the function is but imperfectly performed ; the specialization of certain tissues, and their union into organs, and the complexity of such combinations, no doubt, perform the given function in a far more complete degree; and it is the number and elaborate- ness of these that constitute one animal higher in the scale than another. The human lung is no doubt a more complete breathing apparatus than the entire PROTOZOA AND SPONGES. 431 ciliated surface of an Infusory, and the human eye sees more perfectly than the loose aggregation of pigment granules on the edge of a Medusa. But this diversity is essential to creation, as the great and wondrous plan which we see it to be; and, meanwhile, we may rest satisfied that the humble requirements of the lowest organism are met adequately by its humble en- dowments. This evening I propose to show you some of these humble conditions of animal life—the lowest of the lowly. I have here two or three phials of very rich water dipped from the fresh-water ponds in the neigh- bourhood. All collections of water are not equally productive ; and very far indeed is the popular notion from correctness, that every drop of water which we drink contains millions of animaleules. You may find many collections of clear water, springs, streams, and pools, from which you may examine drop after drop in succession, with the highest powers of the microscope, and scarcely discover a solitary animalcule. Again, it is not stagnant and fetid pools that are the richest in vitality; though no doubt you will always obtain some forms abundantly enough in such conditions. According to my own experience—an experience of many years—the paucity or profusion of animal life in any given collection of water can never be determined beforehand ; the season, the situation, the aspect, the character of the country, and many other unsuspected conditions, may influence the result; which yet one may often give a shrewd guess at. Generally speak- ing, small ponds, in which a good deal of sub-aquatic vegetation grows—and particularly if this be of a minutely-divided character, such as Myriophyllum, 432 EVENINGS AT THE MICROSOOPE. Chara, &c., and whose surface is well covered with duckweed (Zemna), yields well; and, in collecting, it is desirable so to dip as that some of the fine loose sedi- ment of the bottom may flow into your phial, and then to pluck up one or more of the filamentous water-plants, and introduce these into your vessel. Now, to examine such a collection, proceed as Iam about to show you. JI hastily glance with the pocket- lens over the foliage, and selecting such filaments as seem the most loaded with dirty floccose matter, I pluck off with pliers one or two, together with one or two of the cleaner ones that are higher up on the plant, nearer the growing point. Having laid these on the lower glass of the live-box, I take up with the tip of a fine capillary tube, or a pipette, a minute quantity of the water at the bottom, which flows in as you see, carrying a few granules of the sediment. This drop I discharge upon the glass of the live-box, put on the cover, and place the whole on the stage of the micro- scope. First let us use a low power—one hundred diame- ters or so—in order to take a general glance at what we have got. Here is an array of life, indeed! Motion arrests the eye everywhere. ‘The glittering swift and the flabby slow” are alike here ; clear crystal globules revolve giddily on their axes; tiny points leap hither and thither like nimble fleas; long forms are twisting to and fro; busy little creatures are regularly quarter- ing the hunting-ground, grubbing with an earnest de- votedness among the sediment, as they march up the stems; here are vases with translucent bodies protrud- ing from the mouths; here are beamteous bells, set at the end of tall threads, ever lengthening and short- PROTOZOA AND SPONGES. 433 ening ; here are maelstréms in miniature, and tempests in far less than a teapot; rival and interfering currents are whirling round and round, and making series of concentric circles among the granules. Surely here is material for our study. I see an object slowly creeping along the glass, which will be just the thing for our purpose. It is the Proteus (Ameba diffluens). Let me put on a higher power, and submit it to your observation. You see a flat area of clear jelly, of very irregular form, with sinuosities and jutting points, like the out- line of some island in amap.