Bryozoa, Moss-animals.

PLATE XVII.ZOOPHYTES, ASTEROIDS, NUDIBRANCHIS, ACALEPS, ECHINOIDS, CTENOPHORA, TUNICATA, AND CRUSTACEANS.

PLATE XVII.

ZOOPHYTES, ASTEROIDS, NUDIBRANCHIS, ACALEPS, ECHINOIDS, CTENOPHORA, TUNICATA, AND CRUSTACEANS.

The Stinging Series,Cnidaria, comprise sea-anemones, corals, jelly-fish among marine animals, and Hydra among the fresh-water CÅ“lenterata; and derive their name from a remarkably curious feature, the so-called stinging capsules. These are not only offensive, but also defensive weapons with all the animals belonging to this group; the possession of which has converted the bell-like jelly-fish into a simple Cnidarian. The principal change is in the gelatinous layer between the outer wall and the inner digesting layer of the ectoderm. But without entering further into their structure and relations, the stinging-cells and batteries claim especial attention. These cells vary considerably in size without their characteristics being essentially changed. The protoplasm of the cell is modified into a tolerably firm substance, enclosing an oval or cylindrical vesicle. Closely associated with this is a pointed process, standing up far above the level of the outer covering, known as thecnidocil. Within the vesicle is found, either spirally rolled up or in an irregular tangle, a long filament or hollow tube, a prolongation of the vesicle, but turned outside in. This tube is more than twenty times as long as the cell, is pointed at the tip, and beset with two rows of fine spirally-arranged barbed hooks. When the cnidocil is touched or irritated, this filament is violently shot out, being then turned inside-out, like the fingers of a glove. So long as the thread remains rolled up within the vesicle the barbed hooks remain in their tube, but when shot out, they change to the outside. The rolled-up filament appears to be filled with some poisonous material, which is ejected when the tube is shot out, and where the point strikes a wound is inflicted, so that unless the prey is stronger than the attacker it cannot escape. The greater the struggle, the larger the number of capsules discharged in order to kill.

Fig. 347.â€”The Stinging Capsules of Cnidaria.1 and 2. Retracted filaments; 3. Partly protruded; 4. Fully protruded. Magnified Ã— 600. (Warne.)

Fig. 347.â€”The Stinging Capsules of Cnidaria.

1 and 2. Retracted filaments; 3. Partly protruded; 4. Fully protruded. Magnified Ã— 600. (Warne.)

PolypomedusÃ¦.â€”Among the higher development of the stinginggroup is the jelly-fish. The Siphonophora, as represented by the Portuguese man-of-war, are in their turn the highest development of swimming-bells, and exhibit many modifications and combinations of individuals. The tentacles of the Physalia, the best known, are stiff with batteries of stinging-capsules, the sting of which is more like the shock of the electric current. TheChallengersoundings brought to light some remarkably interesting forms, and these have furnished much work for the microscope, as all their larval forms are extremely curious. Among the HydromedusÃ¦ there are many different life histories. Take the jelly-fish, the eggs of which have given up forming stocks, and are hatched out at once as MedusÃ¦. There are others, the eggs of which form stocks; others, again, in which the sexual individuals do not swim away as jelly-fish. The last were at one time described under a new name, because of one or two curious forms being taken creeping on the ground. This creeping Medusa (Clavatella prolifera) has six arms, the tips of which are provided with true suckers, and on these it walks as on stilts, while from each arm a short stalk arises, the swollen end of which is beset with stinging capsules. It has an extensile mouth-tube, and feeds upon small crustaceans found on seaweeds.

Fig. 348.â€”Plumularia primata.Doris tuberculataseen clinging to a fucus.

Among the forms that swim away as jelly-fish a very curious example is presented inCorymorpha mutans. These swim about for a time, and then firmly attach themselves by numerous thread-like appendages, forced into the sand, and where the young prepare for their next metamorphosis. As an example of the stocks of those representatives which do not swim away as jelly-fish, take the beautifully-feathered, plant-like creatures found erect along the seashore, the Sertularia (Fig. 358, No. 12) and Plumularia.Plumularia primata,Fig. 348. Other members of these groups will be found inPlate IV., Nos. 95-99.

Fig. 349.â€”Group of female stock ofHydractinia echinata.a, a.Nutritive individuals;b, b.Female individuals and groups of eggs. Highly magnified.â€”(Warne.)

Fig. 349.â€”Group of female stock ofHydractinia echinata.

a, a.Nutritive individuals;b, b.Female individuals and groups of eggs. Highly magnified.â€”(Warne.)

In addition to the nutritive individuals, there are the egg-bearing; these do not become free-swimming individuals. One small family is neither branched nor featheredâ€”theHydractinia echinata, found in the North Sea and on the Norwegian coasts, where it attaches itself to the shells of gastropods, selecting those inhabited by hermit crabs. The part of the stock common to all the individuals is the skin-like portion which adheres to the surface of the shell. In some spiny processes are produced, and the nutritive canals running down the stems of the polyps are continued into the membrane belonging to the stock, as seen inFig. 349.

The nutritive individuals are distinguished by long tentacles, mouths, and digestive canals. The females have no mouths, and are supplied with food through the system of canals running to them from the nutritive males. These reproductive members are furnished with stinging threads instead of tentacles for the protection of their ova. The ciliated larvÃ¦, in a very short time, swim off to found new colonies.

Fig. 350.â€”MedusÃ¦, Jelly-fish.

The free-swimming jelly-fish (Fig. 350, and Plate XVII.,candd) belong to the order ScyphomedusÃ¦. These are characterised by their delicate colouring, and from the arrangement of their nervous system, which can only be made out by staining. Some new and curious forms were dredged from a depth of more than 6,000 feet off the coast of New Zealand, varying in size from an inch to twenty inches; many having from four to eight or ten eyes arranged along the margin.

Anthozoa.â€”From the free-swimming we turn to a group of permanently fixed polyp forms, the sea-anemones and corals. The development of Monoxenia commences with the egg, repeatedly dividing into many parts (Fig. 351,C,D, andE), by a process common to the animal kingdom, termed egg-segmentation, in this particular instance proceeding from an apparently hollow sphere,A, enclosing a single layer of cells,G. Each cell sends out a long cilia, or whip-like process,F, by means of which the larva turns about and swims in the body fluid of the parent polyp. One half of the spherenow becomes enfolded into the other half,H, and forms what is termed agastrula,I,K. The gastrula stage of Monoxenia is of the simplest kind, the larva forming a sac, with walls consisting of two layers, an outer, or ectoderm, and an inner, or endoderm. Thetransition from the flat dish shape,H, to the sac with a narrow mouth is at once clear, and the knowledge that all the CÅ“lenterates proceed from similar larvÃ¦, and that all the complications of their various systems are developed from a simple gastrula, throws much light on their anatomy. During these transitions the endoderm, whose cells multiply, continues as an uninterrupted lining to the stomach and its appendages, while the ectoderm yields the cuticular elements.

Fig. 351.â€”Stages in development ofMonoxenia Darwinii, Ã— 600.â€”(Warne.)

A third intermediate gelatinous layer, themesoglÃ¦a, arises between the two layers in which muscles and connective interstitial tissue appear. In the mesoglÃ¦a of one species of coral calcification takes place; this internal calcification has but a small share in the work of the great rock-making corals, their most important calcification being external. In Monoxenia, although the transition from the gastrula larva to the adult animal has not been seen, there can be no doubt as to how this is carried out, the transformations having been watched throughout in other species. The larva attaches itself with the end opposite the mouth, the cilia disappear, and after the mouth-tube has been formed by the folding in of the anterior end along the longitudinal axis of the body, and has thus become marked off from the stomach, eight hollow tentacles rise round the mouth as outgrowths of the body cavity, or as direct continuations of the stomach.

Like all other corals, Monoxenia periodically multiply by means of eggs, which are formed either in the walls of the radiating partitions or septa, or along the free edges. These are ejected through the oral opening. As a rule, the polyps are either male or female; but in stock-forming species individuals of the two sexes are often mixed. Monoxenia may be taken as the simplest type of the regularly radiate polyps; in all the different organs being repeated in regular rings round a central axis; the mouth also is circular. From this interesting account, drawn by Haeckel, of a simple polyp, it will be at once seen what kind of radiate animal it is that builds up the coral reefs. â€œNo garden on earth can match the gardens of the sea that circle the northern part of Australia. As the tide ebbs in azure sunset, coral-reefs peer out symmetrically arranged in beds and intersected by emerald pathways coursing through corals of all hues and tints fathoms deep in the channels.â€

In a growing polyp-stock the individuals usually remain in organic connection; that is to say, each first provides for itself and then shares its superfluity with others, sometimes by means of a continuous reticulated system of canals perforating the calcareous substance which often separates the members of one stock from another. The whole colony may thus be physiologically one creature with many mouths. There are others that remain single, as the inverted pyramidal-looking bodies, FungidÃ¦, commonly called â€œSea-mushrooms,â€ found in great variety. The colour of the polypidom is white, of a flattened round shape, made up of thin plates or scales, imbedded in a translucent jelly-like substance, and within is concealed a polyp; the footstalk, by means of which the animal is attached to the rock, is of a calcareous nature (Fig. 352, No. 1).

Fig. 352.â€”Sea-Anemones.1.Actinia rubra, tentacles displayed and retracted; 2.Heticictis bellis; 3.H. bellis, seen from above.

Fig. 352.â€”Sea-Anemones.

1.Actinia rubra, tentacles displayed and retracted; 2.Heticictis bellis; 3.H. bellis, seen from above.

Hexactinia(six-rayed polyps) are not limited to six rays, as the name given them may seem to imply; they are, in fact, very numerous in some of the largest and most gorgeous of the sea-anemones. All are distinguished by their solitary manner of life, their size, and their vivid and variedly beautiful colouring. The endoderm is firm, and when the animal withdraws its tentacles and shuts its body substance in, there is some difficulty in penetrating to the interior. It does not, however, secrete a calcareous skeletoninside or out, as do the true coral polyps. Among the Hexactinia the sea-anemone (Fig. 352) takes the first place.

These beautifully coloured creatures are, for the most part, found attached to the spot selected by the larvÃ¦; a few species bore into the sand with the posterior part of the body, or build a sheath, which they inhabit. They are voracious feeders, and devour large pieces of flesh, and even mussel and oysters, sucking them in by means of their long grasping tentacles. Well-fed anemones change their skin frequently, during which process they remain closely retracted; the shed skin forms a loose girdle around the base.Actinia bellisnot infrequently attach themselves to the shells of crabs and whelks, and are thus carried to pastures new.

Fig. 353.â€”LarvÃ¦ of Sea-Anemones,Actinia effÅ“ta, highly magnified.

On account of the ease with which anemones are kept in captivity, their mode of reproduction can be closely observed. With but few exceptions they develop from eggs, and in the course of a few weeks are hatched into ciliated infusorial larvÃ¦, presenting most curious and exquisite representations of jugs and jars, with cover lids (as seen inFig. 353,Actinia effÅ“ta). These evince the handiwork of a master hand in the ceramic art. They are, however, of so translucent a nature as to permit of the internal structurebeing seen to consist of nerves and vessels, and which are rendered more apparent by staining. These settle down in a week or ten days, and then shed their cilia, the first tentacle appearing during the process of attachment.

In some species the young ActiniÃ¦ are seen to pass through their whole development within the body cavity of the parent. Most anemones are provided with several circles of more or less cylindrical tentacles, and there are a few specially beautiful species which, besides tentacles of the usual form, have, either within or without the ordinary circle of tentacles, lobed or leaf-like tactile and seizing organs. These belong to the family of the beautiful Crambactis of the Red Sea. Below these grasping tentacles comes a circle of thicker arms unlike the former, being spindle shaped. All the tentacles of the sea-anemones are hollow with a fine aperture at the tip, through which, on closing rapidly, it is seen to expel a jet of water.

True Corals.â€”It will have been noticed in the foregoing remarks that in the soft body-division of the Hexactinia there are both single individuals and colonies joined together to form stocks. The same diversity in this respect will be found among corals proper, with this difference, that the skeleton-forming polyps, by combining, build up substantial structures in the most secure and advantageous positions. Now it so happens that all the corals found about our coasts are generally small and solitary dwellers, one of the best known of which is the scarlet crisp coral, Flabellum, and is characterised by the slit-like form of the mouth. Viewed sideways it resembles a small fan fastened along the edges, and just inside a row of fully developed tentacles is seen protruding. An interesting form of budding occurs in these corals: the buds fall off, and in this budding condition the coral might pass, and indeed has been described as a different species of Flabellum. The colour of the coral is a beautifully transparent red. Remarkable as the solitary corals are, they are surpassed both in number and in form by those which form compound stocks, that is to say, in which the buds do not fall off, but go on building up coral islands and barrier reefs in the warmer seas. Some very few typical forms only are given in the group accompanying, shown inFig. 358.

A different kind of stock is developed in a number of forms,some producing many buds, as in the Madrepores, in which selected polyps spring up above the rest, their sides also becoming covered with small buds, each one of which is a living, feeding, coral animal surrounded by a crown of tentacles. These Madrepores play a very important part in the building up of coral reefs.

Fig. 354. Developmental stages of LarvÃ¦,Astroides calycularis, Ã— 40.

Another massive coral, theAstroides calycularis, has a different mode of growth, the tubes not being fused together. When seen standing out these yellowish-red polyps have been mistaken for small anemones. The larvÃ¦ of this coral leave the egg while still in the large chambered body cavity of the parent, where they swim about for a time, till they escape through the mouth. They are worm-like in form, and swim by means of cilia, which are thicker at the foremost end. The mouth first appears after leaving the parent, but as they soon become exhausted by the effort they assume a contracted form, and attach themselves, as do anemones, by pressing the thicker end of the body against a rock, the whole contracting into a thick round disc, while longitudinal furrows become visible at the upper part where the mouth sinks in. At the end of these furrows twelve tentacles appear. The accompanying illustration shows the various stages through which the larvÃ¦ pass in rapid succession (Fig. 354); at the same time it has already commenced to secrete its calcareous skeleton.This is not formed as a connected whole but from a number of separate centres of secretion formed between the polyp and the substance to which it has attached itself, and which become gradually fused into a perfect skeleton. A section of the polyp at this stage forms an interesting microscopical object.

The so-called eight-rayed corals consist of the one genus Tubipora, the members of which are few in number and not varied in form (Fig. 358, No. 10). In the structure, however, of skeletons they are unique among extant corals. Each individual secretes a smooth-walled tube without calcification of the vertical septa. These tubes, like the pipes of an organ, stand almost parallel, and are united to form a stock by means of transverse platforms. The formation of buds does not appear to take place in this family.

Another of the eight-rayed corals is GorgoniidÃ¦. These are permanently fixed to the spot on which they are found, and form a bush-like growth, giving no idea of the living coral, as it rises in graceful branching colonies, in deep water, and represents a portion ofGorgonia nobiliswith polyps expanded (Figs. 344 and 358, No. 9).

Other corals present numerous other departures from the types we have been considering, but so far modified in form as that of the Sea-pen, Veretillum (Fig. 355), the stock part of which is surrounded by polyps continued down a portion of the cylindrical stalk. The best known of the species isPennatula phosphoreaof the Mediterranean.

Fig. 355.1.Pennatula phosphorea; 2.Synapta chirodata; 3. Anchor-shaped spiculum and plate from the ectoderm of same.

Fig. 355.

1.Pennatula phosphorea; 2.Synapta chirodata; 3. Anchor-shaped spiculum and plate from the ectoderm of same.

PennatulidÃ¦.â€”This family derives its name frompenna, a quill.Their spicula also resemble a penholder in appearance, shown inFig. 358, No. 3. The polyps are without colour, provided with eight rather long retractile tentacula, beautifully ciliated on the inner aspect with two series of short processes, and strengthened by these crystalline spicula, a row being carried up the stalk, together with a series of ciliated processes. The mouth, occupying the centre of the tentacula, is somewhat angular. The ova lie between the membranous part of the pinnÃ¦; these are globular, of a yellowish colour, and by pressure can be made to pass through the mouth. Dr. Grant wrote:â€”â€œA more singular and beautiful spectacle could scarcely be conceived than that of a deep purplePennatula phosphorea, with all its delicate transparent polyps expanded and emitting their usual brilliant phosphorescent light, sailing through the still and dark abyss, by the regular and synchronous pulsations of the minute fringed arms of the polyps.â€

The spicula are seen to be a continuous series of cones fitting into each other.

The exact position in which the Bryozoa, or moss-animals, should be placed in the animal kingdom has not been finally determined. They were at one time associated with corals; then with sponges; but, on further acquaintance, it became evident that they did not belong to either. Naturalists also claimed them as Rotifers and Ciliata, but this claim met with no better reception. Since they appear to have no settled classification, there can be no objection to linking them once more to corals, as they apparently resemble these animals by always living in colonies, the individual members of which are joined in a number of different ways to form stocks, the individuals themselves, however, being very much smaller than those of corals proper. The advantage is that the structure of the Bryozoans can be more readily studied, as many of them live in transparent chambers or cells, the walls of which, although somewhat firmly agglutinated together, are flexible enough to fold up, as the animals instantly withdraw their bodies and close up the top on the slightest alarm (Fig. 356).

Fig. 356.â€”Paludicella, tentacles expanded and cell closed.Fig. 357.â€”Sea-moss, Flustra, the body having been withdrawn from its cell.

Fig. 356.â€”Paludicella, tentacles expanded and cell closed.

Fig. 357.â€”Sea-moss, Flustra, the body having been withdrawn from its cell.

The general structure of the Bryozoan individual, figured attachedby its footstalk to a stem of wood, consists of a mouth at the anterior part of the body opening into a muscular pharynx in the alimentary canal, together occupying a considerable amount of space. The terminal portion turns upon itself towards the oral opening, its chief attachment being a short strand of tissue termed the funiculus (shown inFig. 358, No. 11). In all adults two masses of cells are found attached to the wall of the chamber; the upper yields the eggs, within the lower the male elements are developed. Moss-animals are hermaphrodite, fertilisation being effected by the two elements mingling together in the body fluid. These are the essential points in the structure of the whole seventeen hundred species. Among the larger colonies a number of fresh-water genera are found attached to the roots and branches of aquatic plants, most of which, however, are inconspicuous. The beauty of these minute bodies can only be seen under the microscope. Many consist of delicate branching growths, the Sea-mats (Flustra), for instance; others again appear as attractive lace corals, between the open meshes of which multitudes of minute apertures crowned with tentacles are displayed. The several individuals of the genus Lepralia are arranged in rows, and further distinguished by the animals being developed only on one side of the stock. The marvellous variety of forms presented by these small animals is in a measuredetermined by the particular manner of their buddings. The greater number of fresh-water moss-animals belong to the order PhylactolÃ¦mata, so called because the mouth is provided with a tongue-shaped lid. The crown of tentacles is furnished with rows of cilia, and is horseshoe-shaped, the whole being surrounded at its base by anintegument forming a kind of cup, which is either soft or horny. Those belonging to the wandering types (Cristatella,Plate IV., Nos. 95-98) form flattened elliptical colonies, some of which creep or move about on a kind of foot. A nervous system pervades the mass of polyps, while in each separate polyp a nerve ganglion is seen to be situated between the Å“sophagus and the posterior part of the alimentary canal. The colony nerve system regulates the movements of the stock.

Fig. 358.â€”Typical forms of Corals.1.Fungia agariciformis; 2. Alcyonium,Cydonium Mulleri; 3. Cydonium, polyps protruding and tentacles expanded, others closed; 4. A stock viewed from above; 5.Madrepore abrotanoide; 6. Madrepore, slightly magnified, showing oral opening; 7. CorallidÃ¦; 8. Coral, polyps protruding from cells; 9.Gorgonia nobilis, with polyps expanded; 10.Tubipora musica; 11. Tubes of same, with polyps expanded, one cut longitudinally to show internal structure; 12. Sertularia, polyps protruded, and withdrawn into their polypidoms.

Fig. 358.â€”Typical forms of Corals.

1.Fungia agariciformis; 2. Alcyonium,Cydonium Mulleri; 3. Cydonium, polyps protruding and tentacles expanded, others closed; 4. A stock viewed from above; 5.Madrepore abrotanoide; 6. Madrepore, slightly magnified, showing oral opening; 7. CorallidÃ¦; 8. Coral, polyps protruding from cells; 9.Gorgonia nobilis, with polyps expanded; 10.Tubipora musica; 11. Tubes of same, with polyps expanded, one cut longitudinally to show internal structure; 12. Sertularia, polyps protruded, and withdrawn into their polypidoms.

Fig. 359.1.Coryne stauridia; 2. A tentacle detached and magnified 200 diameters.

Fig. 359.

1.Coryne stauridia; 2. A tentacle detached and magnified 200 diameters.

There are many beautifully formed freshwater polyps deserving of more than a passing notice, as the slender Coryne (Coryne stauridia), found adhering to the footstalk of aRhodymenia(Fig. 359), about which it creeps in the form of a white thread. On placing both under the microscope, the thread-like body of the little animal appears cylindrical and tubular, perfectly transparent, and permeated by a central core, apparently cellular in texture, hollow, and within which a rather slow circulation of globules is perceived. The parent Coryne sends off numerous branches, the terminal head of which is oblong, cylindrical, and at the extreme end there are arranged four tentacles, long and slender, each being furnished with a nodular head. A magnified view of one detached is shown erect (Fig. 359, No. 2). This polyp is much infested by parasites, vorticella growing on it in immense numbers, forming aggregated clusters here and there, individuals of the parasitic colony adhering to each other, and projecting outwards in every direction.

Alcyonella, another fresh-water polyp, is found in the autumn of the year in all the London Docks adhering to pieces of floating timber.A. stagnorumpartakes of the character of a sponge rather than that of a polyp. It is usually found in gelatinous colonies, and when stood aside for a short time these put fortha number of ciliated tentacles (shown inFig. 360, magnified 100 diameters).

The ova contained within the sac, and viewed by transmitted light, appear as opaque spheres surrounded by a thin transparent margin; these increase in thickness as the ova is developed, and such of the ova as lie in contact seem to unite and form a statoblast. A rapid current in the water around each animal, drawing with it loose particles and floating animalcules, is seen moving with some velocity as in other ciliated bodies; and a zone of very minute vibrating cilia surrounds the transparent margin of each tentacle.

Fig. 360.â€”Alcyonella fluviatella.

Dr. Percival Wright discovered on the western coast of Ireland a new genus of AlcyonidÃ¦, which he named after the well-known naturalist Harte,Hartea elegans(Plate IV., No. 86). This polyp is solitary, the body cylindrical, and fixed by its base to the rock; it has eight ciliated tentacles, which are knobbed at their base and most freely displayed. It is a very beautiful polyzoon of a clear white colour, and when fully expanded stands three-quarters of an inch high.

Lophopus crystallinus(Plate IV., No. 98) displays beautiful plumes of tentacles arranged in a double horseshoe-shaped series. When first observed these polyps resemble in many respects masses of the water snail ova, for which they are often mistaken. On placing these jelly-like masses into a glass trough with some of the clear water taken from the stream in which they are found, delicate tubes are seen to cautiously protrude, and the beautiful fringes of cilia are quickly brought into play. The organisation ofL. crystallinusis simple, although it is provided with organs of digestion, circulation, respiration, and generation. The nervous70and muscular systems are well developed. This polyp increasesboth by budding and by ova, both of which conditions are shown inPlate IV., No. 98. The ova are enclosed in the transparent case of the parent. In Lophopus and some other fresh-water genera, Cristatella, Plumatella, and Alcyonella, the neural margin of the Lophopore is extended into two triangular arms, giving it the appearance of a deep crescent.

Another family presents a contrast: there is no lid to the mouth, and the tentacles are arranged in a circle on a disc. An important rise in organisation is found in the GymnolÃ¦mata, especially in the lip-mouthed forms; the individuals belonging to this order vary in structure and fulfil different physiological functions. There are structures known as zoÃ¦cia, stolons, avicularia, vibracula, and ovicells, some of which are merely modified individuals. The zoÃ¦cia are the normal individuals of the colony, fully developed for most of the functions of life; the stolons have a much humbler function, but are indispensableâ€”they are the root-like outgrowths of the stock, and serve for attaching the colony to foreign objects. The most remarkable are those known asavicularia, so called because they resemble the head of a bird. This process acts as a pair of forceps, the large upper blade of which is very like the skull and upper jaw of a bird, and the smaller lower blade (like the lower jaw) constantly opens and shuts by means of a complicated arrangement of muscles (shown inFig. 361). These avicularia are movably attached by short muscles to the neck, and are found near the entrance to a zoÃ¦cium. They turn from side to side, snapping in all directions, catching at every particle of food that may come near; at length the morsel is drawn into the mouth by the cilia on the tentacles. From this very peculiar structure the Chilostomata were originally named birdâ€™s-head corallines, then specifically shepherdâ€™s-purse corallines,Notamia bursaria. Equally interesting, again, are thevibracula, long thread-like structures, attached by short footstalks. These keep up a constant whip-like motion, the object of which is not quite clear. The ovicells, or egg receptacles, are found at the lower ends of the zoÃ¦cia in theform of shields, helmets, or vesicles. InPlate IV., Nos. 95 and 96, a front and edge view of the statoblast is shown highly magnified.

Fig. 361.1.Notamia bursaria, shepherdâ€™s-purse Bryozoa; 2. Polyp magnified and withdrawn into its cell; 3. Portion of a colony of Hydroid polyps.

Fig. 361.

1.Notamia bursaria, shepherdâ€™s-purse Bryozoa; 2. Polyp magnified and withdrawn into its cell; 3. Portion of a colony of Hydroid polyps.

Another sub-order consists of the Cyclostomata, or round-mouthed Bryozoans, of which the Tubulipora is the typical form. The stocks are cup-shaped incrustations, the individuals radiating outwards, as inPlate IV., No. 92.Tubularia dumortieriiis a very interesting form, the germinal bodies, statoblasts, being formed as cell masses on the strand, or funiculus, which also maintains the stomach in its place. They are round or oval in shape, and brown or yellow in colour, and consist of two valves fitted one upon the other like watch glasses, as shown in No. 96. A number of other statoblasts are shown, Nos. 97, 98, and 99. The edge running round No. 95 is seen to have barbed tips; the ring itself contains small air chambers, and is termed the swimming belt. It is, in fact, a perfect hydrostatic apparatus, giving support to the winter buds or statoblasts on the surface of the water. The barbed hooks apparently actas anchors, and by their means they catch on at points suitable for their development during the coming spring. As soon as the time comes, the two halves split apart and the germinal mass emerges forth. Out of these winter buds and statoblasts asexually produced individuals arise, which reproduce themselves sexually, their descendants again yielding winter germs. In short, an alternation of generations is a continually recurring process.

Fig. 362.â€”Lingula pyramidata.

Brachiopoda.â€”Here again we have to do with an enigmatical class of arm-footed animals, of which the Lamp-shells may be regarded as typical. These have remained unaltered from the earliest geological epochs. Brachiopods are divided into two orders: those having shells without hinges, and those with shells hinged together. On the whole they possess less interest for the microscopists than many other animals, except in their earliest developmental stages of existence.

One of the most interesting of the hinge-class group, living chiefly near the shores of the warmer seas, is the LingulidÃ¦. The valves are almost exactly similar, but are not hinged together, and have no processes for the support of the thick fleshy spiral arms of the animals. InL. pyramidata, found around the Philippine Islands (Fig. 362), the stalk is nine times longer than the body. The animal does not attach itself by this, but moves about like a worm, making tubes out of sand, into which it can withdraw itself and disappear. The cilia at the mantle edge form a fine sieve, thus preventing foreign particles from entering the gills. Its internal structure possesses points of interest, and the parasitic growths covering the cartilaginous structure, miscalled a shell, are curious, and excite the attention of the naturalist.

Another bivalve so unlike a crustacean, among which it has beenplaced, I may venture to describe among Lamp-shells. I refer to the barnacle (Lepas) generally met with covering the bottoms of ships. These, as in the former genus, are more interesting to the microscopist in the early stage of existence, and also for the curious parasites known to infest them. The barnacle protrudes through its two valves six pairs of slender, bristly, two-branched filamentous limbs, which keep up a constant sweeping motion, and whereby it secures its supply of food (Fig. 363). When first hatched the young are in the Nauplius stage, being furnished with a median eye and three pairs of flagellated appendages. After enjoying a free life the larva moults and passes into a second stage, in which with its two eyes and compressed carapace (shown inFig. 364) it so nearly resembles a Daphnia. Before these thoracic appendages entirely disappear they first change places, and then each is seen to be provided with a sucker; by this means the larva fixes itself to its permanent resting-place, while a cement gland pours out a secretion that glues it firmly to the point of attachment chosen. These Cirripedes are not true parasites, inasmuch as they do not extract nourishment from the body to which they are attached.

Fig. 363.1. Spat of oyster, some ciliated; 2. Barnacles attached by footstalks.

Fig. 363.

1. Spat of oyster, some ciliated; 2. Barnacles attached by footstalks.

One species, the Proteolepas, is in the adult stage a maggot-like, limbless, shell-less animal found living within the mantle chamber of other members of the same order, while the root-headed Cirripedes (Peltogaster curvatus, asFig. 364, No. 1) live parasitically upon higher crustaceans.

Echinodermata.â€”This sub-kingdom includes the star-fishes, stone-lilies, sea-urchins, feather-stars, and sea-cucumbers, some of which have been already alluded to, and are so well known that they need no lengthy description, while of the fossil sea-urchins of our chalk formations, the Pentremites and Crinoids, whose silicious remains are so abundant and so familiar to naturalists and geologists, but little remains to be said. They are chiefly interesting to the microscopist from their calcareous and silicious appendages, knownas spicula. In the sea-urchin, brittle-star, or feather-star, the outer body surface consists almost wholly of a deposit of calcium carbonate, combined in the form of little plates built up into a rigid â€œtest,â€ whereas in the star-fish it usually forms a kind of scaffolding, between the layers of which there stretches a firm leathery skin. Among the sea-cucumbers, the living specimens of which present extraordinary variations both in form and character, the deposit consists chiefly of small spicules which grate when the skin is cut with a knife. If a thin section of the skin is examined under the microscope, the spicules are seen to be profusely distributed in the middle layer. The same deposit takes place in the stalked column of a crinoid and in sea-urchins (Echinodermata), which has tended to preserve them in the fossilised state.Fig. 365is selected as exhibiting to perfection the Medusa-headed Pentacrinoid. This echinoderm differs in two characters: first, its microscopic structure is that of a meshwork deposited in the spaces of a network of soft tissue; secondly, that each element, whether a spicule or a plate, is, despite its trellised structure, deposited around regular lines of crystallisation (shown inPlate IV., Nos. 89 and 90). Owing to these characteristics the minutest portion of an echinoderm skeleton is readily recognised, even when fossilised, under the microscope. Even the species of the sea-cucumber can be determined by the shape of their spicules.

Fig. 364.â€”Parasitic Barnacles.1.Peltogaster curvatus; 2. Nauplius larva of Parthenopea; Ã— 200.

Fig. 364.â€”Parasitic Barnacles.

1.Peltogaster curvatus; 2. Nauplius larva of Parthenopea; Ã— 200.

Another noticeable feature in the radiate structure is that in many cases it gives to the animal a star-shape, to which the names of star-fish and brittle-star are given (seePlate IV., No. 91, andPlate XVII.,fandn). The ordinary five-rayed star-fish is found everywhere around the English coasts. This constant arrangement of organs holds good in the majority of the echinoderms; it can be detected in theHolothurians, where, beside the feathery tentacles of the head, rows of shorter sucker-like processes will be found, which in some instances extend the whole length of the body, the fixed number of rows being also five in their internal organs. Hence these animals were formerly grouped under Radiata. But if a sea-cucumber or sea-urchin be dissected, a marked distinction will be found between them, in one portion of the organism in particular: the intestine is shut off from the rest of the body-cavity, often coiling round inside. Examine a star-fish or sea-urchin on the under-surface of the rays, and, passing in five bands from top to bottom, a number of small cylindrical processes are seen gently waving about; these lie in two rows with a clear space between them, and aretermed in consequenceambulacrum. They end in sucker-like discs, which enable the animal to attach itself, or pull itself against strong currents.

Fig. 365.â€”Medusa-headed Pentacrinoid.a.Crown and part of stem;b.Upper surface of body, the arms broken away, showing the food grooves passing to the central mouth.â€”(Warne.)

Fig. 365.â€”Medusa-headed Pentacrinoid.

a.Crown and part of stem;b.Upper surface of body, the arms broken away, showing the food grooves passing to the central mouth.â€”(Warne.)

Just one other special feature should be noticed: radial canals pass along under the ambulacra, and join a ring-canal around the mouth, well supplied by nerve cells.

Fig. 366.1. Transverse section of a branch of Myriapore; 2, and the others Section of the stem ofVirgularia mirabilis; 3, Spiculum from the outer surface of Sea-pen; 4, Spicula fromIsis hippuris; 5, fromGorgonia elongata; 6, from Alcyonium; 7, and fromGorgonia umbraculum; 8, Calcareous remains of a Crinoid.

Fig. 366.

1. Transverse section of a branch of Myriapore; 2, and the others Section of the stem ofVirgularia mirabilis; 3, Spiculum from the outer surface of Sea-pen; 4, Spicula fromIsis hippuris; 5, fromGorgonia elongata; 6, from Alcyonium; 7, and fromGorgonia umbraculum; 8, Calcareous remains of a Crinoid.

Crinoids(stone-lilies), on the other hand, are formed of a series of flat rings, pierced through by a narrow canal. The ossicles, as they are termed, are joined by ligaments passing through their solid substance and endowed with muscular power; the central part serves for the passage of blood-vessels, and is surrounded by a sheath of nervous tissue that controls the movements of the stem, the latter being encrusted by a number of fine rootlets. The stems possess a limited power of bending. In the words of Professor Agassiz, â€œThestem itself passes slowly from a rigid vertical attitude to a curved or even a drooping position; the cirri move more rapidly than the arms, and the animal uses them as hooks to catch hold of objects, and on account of their sharp extremities they are well adapted to retain their hold of prey.â€ The rosy-feather star-fish is often found clinging to a tube of the Sabella worm; the food of crinoids consists of foraminifera, diatoms, and the larvÃ¦ of crustaceans. There are so many curious features in connection with the Echinodermata that my readers may with advantage consult â€œTheChallengerReportsâ€ and Warneâ€™s â€œNatural Historyâ€ on other points of interest.

Holothuroidea(sea-cucumbers) are elongated slug-like creatures, the skin being in structure similar to that of the slug, with a comparatively small amount of calcareous matter. Usually this occurs in small spicules, which assume very definite shapes, as the anchors of Synapta (Plate IV., No. 87, and inFig. 355). There are also rings of calcareous plates around the gullet, five of which have the same relation to the radial water-vessels as the auricles round the jaws of a sea-urchin, and which likewise serve for the attachment of muscles. These plates are seen inPlate VIII., Nos. 171 and 172, as they appear coloured by selenite films under polarised light. Around the mouth in Cucumaria is a fringe of branched tentacles connected with the water-vascular ring; these appear to be used as a net to intercept floating organisms.

Correlated with the star-fishes is a small family based on the character of their pincer-like organs, called pedicellariÃ¦, on the surface of the test (shown inPlate IV., Nos. 93 and 94, magnified Ã— 25). Movable spines cover the surface of these echinoderms, varying in size from minute bristle-like structures to long rods. The pedicellariÃ¦ are, it is believed, derived from the smaller spines, and two of them are united at the base by muscles, slightly curved, and made to approach each other at their extremities. There is a gradual modification of this type through the whole series. Many uses have been assigned to them, as the holding of food, as they have been seen to hold to the fronds of seaweed and keep them steady until the spines and tube feet can be brought into action. The inner surface of the pedicellariÃ¦ are known to be the most sensitive, and the blades close on the minutest object touching the inner surface. Beside thesepeculiar bodies the surface of the skin has small tubular processes, and tubular feet with suckers at the end. At the extremity of each arm is a single tube-foot with an impaired tentacle, and above this again is a small eye coloured by red pigment.

Passing by many other points of interest in the EchinoidÃ¦, the spines are seen to be attached to the test or shell by a ball and socket joint and well-arranged muscles, whereby the spines can be moved in any direction. The tubercles, however, do not cover the whole test, but are disposed chiefly in five broad zones extending from one pole to another. When a transverse section of a spine is examined by a medium power it is seen to be made up of a series of concentric and radiating layers (shown inPlate XVIII., Nos. 1 and 2), the centre being occupied by reticulated structure and structureless spots arranged at equal distances; these may be termed ribs or pillars. Passing towards the margin are other rows conveying the impression of a beautiful indented reticulated tissue. Many of the spines present no structure, while others exhibit a series of concentric rings of successive growth, which strongly remind one of the medullary rays of plants. When a vertical section of a spine is submitted to examination, it is seen to be composed of cones placed one above the other, the outer margin of each cone being formed by the series of pillars. In certain species of Echinus the number of cones is very considerable, while in others there are seldom more than one or two to be found; from these, transverse sections may when made show no concentric rings, only the external row of pillars.

The skeleton of echinoderms contains but a small amount of organic matter, as will be seen on dissolving out the calcareous portion in dilute nitric or hydrochloric acids. The residuum structure will appear to be meshes or areolÃ¦, bounded by a substance having a fibrous appearance, intermingled with granulous matter; in fact, it bears a close resemblance to the areolar tissue of higher animals, and the test may be considered as formed, not by the consolidation of the cells of the ectoderm, as in the mollusc, but by the calcification of the fibro-areolar tissue of the endoderm. This calcification of a simple fibrous tissue by the deposit of a mineral substance, not in the meshes of areolÃ¦ but in intimate union with the organic basis, is a condition of much interest to the physiologist; it presents anexample of a process which seems to have an important share in the formation and growth of bone, namely, in the progressive calcification of the fibrous tissue of the periosteum membrane covering of the bone.

The development of the sea-urchin from the fertilised egg first divides and then sub-divides, and in a short time the embryo issues forth with a small tuft of cilia, by means of which it swims off freely. The larvÃ¦, in its full development, measures about one millimetre in diameter, and is a curious and remarkable creature.

The sub-kingdom Molluscacomprises some fifty thousand species, and fresh forms are being constantly discovered, the number of the aquatic genera being more than double that of the terrestrial species, for it matters not to what depth of ocean the dredge is let down, some new form is certain to be gathered. TheChallengerexpedition has enriched our knowledge of the deep-sea fauna to an enormous extent; so much so, that fifty volumes have already been published descriptive of animals brought to the surface. Nevertheless, we are told that the great coast lines of South America, Africa, Asia, and parts of Australia have been but imperfectly explored for smaller kinds of Mollusca.

Molluscs are soft-bodied, cold-blooded animals, without any internal skeleton, but this is compensated for by the external hardened shell, which at once serves the purpose of bones, and is a means of defence. These bodies are not divided into segments like those of worms and insects, but are enveloped in a muscular covering or skin, termed the mantle, the special function of which in most species is the formation and secretion of the shell. The foot, which serves the double purpose of locomotion and burrowing in the sand or rock, is an organ particularly characteristic of most molluscs. There are many departures from this rule, as, for instance, in the group ChitonidÃ¦, where the shell takes the form of a series of eight adjacent plates; and in another, the PholadidÃ¦, there are one or more accessory pieces in addition to the two principal valves. Some are bivalved, others univalved, and concealed beneath the skin. All shells are mainly composed of carbonate of lime, with a small admixture of animal matter. Their microscopic examination reveals a great diversity of structure, as we shall presently see, and they are accordingly termed porcellaneous, nacreous, glassy, horny, and fibrous. Mostmolluscs have the power of repairing injuries to their shells; many exhibit an outer coat of animal matter, termed theperistracum, the special function of which is to preserve the shell from atmospheric and chemical action of the carbonic acid in the water in which they dwell.

The shells of gastropods are enlarged with the growth of the mollusc by the addition of fresh layers to the margin. In some species the periodic formation of spines occurs; a typical case will be found among MuricidÃ¦. The varied colours of shells are due to glands situated on the margin of the mantle, and beneath the peristracum; occasionally the inner layer of porcellaneous shells is of a different colour to the outer, as, for example, in the helmet-shells (Cassis), much used by carvers of shell cameos. Light and warmth, as in the vegetable kingdom, are the great factors in the production of brilliant colours. In cold climates land snails bury themselves in winter time in the ground or beneath decaying vegetable matter, and in hot seasons they close up the aperture of the shells with a temporary lid, called anepiphragm. These exhibit great tenacity of life, as, for instance, in the Egyptian desert-snail,Helix desertorum. The reproductive system is in all cases effected by means of eggs. The ova are usually enclosed in capsules, and deposited in masses, and the number of eggs contained in the squid and the whelk have been stated to be thirty or forty thousand. The ova of molluscs may be gradually developed into the adult, or there may be a free-swimming ciliated larval stage, or a special larval form, as in the fresh-water mussel. Most are provided with a more or less distinct head; both cephalopods and gastropods are furnished with eyes. In land snails these are found placed on projecting stalks. In most cases the utility of molluscs far outweighs the injury occasioned by a few species, as, for instance, the Teredo, and the burrowing habits of the Pholas and Saxicava, compact marble having been found bored through by them.

Mr. J. Robertson wrote me in 1866:â€”â€œHaving, while residing here (Brighton), opportunities of studying thePholas dactylus, I have endeavoured during the last six months to discover how this mollusc makes its hole or crypt in the chalkâ€”by a chemical solvent? by absorption? by ciliary currents? or by rotatory motions? My observations, dissections, and experiments set at rest controversyon this point. Between twenty and thirty of these creatures have been at work in lumps of chalk in sea water in a finger glass and a pan, at my window for the last three months. ThePholas dactylusmakes its hole by grating the chalk with its rasp-like valves, licking it up when pulverised with its foot, forcing it up through its principal or branchial siphon, and squirting it out in oblong nodules. The crypt protects the Pholas from ConferveÃ¦, often found growing parasitically not only outside the shell but even within the lips of the valves, thus preventing the action of the siphons. In the foot there is a spring, or style, which when removed is found to possess great elasticity, and this seems to be the mainspring of the motion of the Pholas.â€

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