The body of the Noctiluca (Fig. 333) is a nearly globular-shaped cyst, enclosed in a tough membranous wall, from a grooved opening in which a striated muscular flagellum or proboscis is projected forth, and it is by means of this the animal swims away even in rough seas. A fine whip-like flagellum is also located in the same groove. At the apex of the funnel there is a mass of protoplasm which extends itself as a widely-meshed, highly-vacuolated network to the inner wall of the cyst, whence it is believed the phosphorescent light emanates. It multiplies by self-division, first becoming encysted after withdrawing its flagellum, and then breaking up into numerous ciliated helmet-shaped swarm spores. Frequently two organisms fuse into one and then divide into spores.

Noctiluca mainly confines itself to the shallower seas, but there are related forms met with in the warmer open seas; these belong to the genus Pyrocystis (Fig. 334). In one variety the body is perfectly spherical and without the big flagellum or proboscis. Professor Butschli, however, regards this species as an encysted or resting phase of the commoner and better-known form.

The late Mr. Philip Gosse, F.R.S., was the first microscopist to describe the Noctiluca. After careful observation, he wrote in his “Naturalist’s Rambles” as follows:—“I had an opportunity of becoming acquainted with the minute animals to which a great portion of the luminousness of the sea is attributed. One of my large glass vases of sea-water I had observed to become suddenly at night, when tapped with the finger, studded with minute but brilliant sparks at various points on the surface of the water. I set the jar in the window, and was not long in discovering, without the aid of a lens, a goodly number of the tiny jelly-like globules of Noctiluca miliaris swimming about in various directions. They swam with an even gliding motion, much resembling that of the Volvox globator of our fresh-water pools. They congregated in little groups, and a shake of the vessel sent them darting down from the surface. It was not easy to keep them in view when seen, owing rather to their extreme delicacy and colourless transparency than to their minuteness. They were, in fact, distinctly appreciable by the naked eye, measuring from ¹⁄₅₀th to ¹⁄₃₀th of an inch in diameter.”

Among the numerous fresh-water members of the flagellate infusoria, there is one which especially calls for notice, Codosiga, discovered by the late Professor H. J. Clark. This minute body bears a delicate funnel-shaped protoplasmic expansion or collar, common to the several members of this organic series. The flagellum is placed at the base of the oral opening, and within the circumscribed area of the collar, which is of such extreme tenuity that its true form and nature can only be determined by a very careful adjustment of the achromatic condenser and accessory apparatus employed, together with a wide-angled objective. It is seen to greater advantage by supplying the animal with very fine particles of colouring matter. In this way it is found that the infundibuliform cup consists of protoplasm, through which the flagellum is protruded and withdrawn into the general substance of the Monad’s body (Fig. 335). As many as twenty or more zooids are attached to the extremity of a slender footstalk. The length of the body, exclusive of the collar, is ¹⁄₂₅₀₀th to the ¹⁄₁₂₀₀th of an inch. The habitat of these bodies is fresh water. Mr. Saville Kent in 1869 discovered some of these interesting infusoria in the London Docks.

“The more exact significance of the special organ, the collar, is manifest by the circulatory currents or cyclosis induced, and there can be no room for doubt that this structure finds its precise homologue in the pseudopodia of the foraminiferous group of the Rhizopoda, in which a similar circulation or cyclosis of the constituent sarcode is exhibited. The whole of this highly-interesting flagellate order, a comparatively small one as yet, are remarkable for their pale glaucous green or florescent hue, such colour assisting materially in their recognition, even when the magnifying power employed is insufficient for the detection of the very characteristic collar with its enclosed flagellum.”66

Ciliata.—Types of Ciliata obtained from hay infusions are very numerous. Ehrenberg’s animalcules were mainly of a large size, and of those belonging to the higher order of the Ciliata, pertaining to such genera as Paramecium, Colpoda, Cyclidium, Oxytricha, and Vorticella. These, however, represent but an insignificant minority of the hosts of flagellate forms which abound in our humid climate, and in hay infusions in particular. In such infusions, watched from day to day and produced from hay obtained from different localities, the number of types developed in regular sequence is found to be perfectly marvellous, commencing with the Monas proper, Amphimonas and Heteromita; while Bacteria, in their motile and quiescent forms, are invariably present and furnish an abundant supply of material for the microscope.67

Vorticellidæ constitute one of the most numerous families of the ciliate infusoria. All its members are at once recognised by their normal stationary condition, and by the structure of their oral system. In but few of the genera is there any marked divergence from this formula, and when any exists it is made manifest by an increase in development of some one of its elements at the expense of another. For instance, in the genus Spirochona, the external edge of the encircling border or peristome is suppressed, while the inner portion is abnormally developed into a transparent and highly elevated spiral membrane. The bell-animalcules usually possess stalks, and are either solitary or form branching colonies. Conichilus vorticella (Plate III., No. 80) is a well-known member of the colony stock, all the zooids of which are united on a slender branching pedicle, which consists of a central contractile cord enclosed within a tubular hyaline sheath. There are many other shrub-like colonies all variously modified in form and character. The Epistylis opercularia, or nodding-bell animalcule, is an interesting member of a numerous host of solitary short-stalked forms (Fig. 337). When the animal is disturbed, the heads drop down towards the stalk. This animalcule has been found to form a colony; and another, Carchesium, whose tiny branched tree-like colonies resemble little white globular masses of moulds, are seen at once to drop down towards the base of the colony with a jerky movement if the cell be touched. By a process of encysting, all the Vorticellæ and many of the more highly-organised ciliata have the means of what may be termed self-preservation. Should the water dry up in which they have been living, the little animal encases itself in mud at the bottom of the pool. Should this be baked by the sun not the least injury arises, for at this stage it crumbles into dust, and is carried by the wind to long distances, but the first shower of rain calls it back to active life, and soon after it is seen to issue forth as a free swimming bud.

Thuricola valvata (Plate III., No. 72) possesses a hinge-like process which closes up like a door when the animal contracts itself into its case. This very effectually protects it from assault. Both portions of the valve are capable of extension. Another group of ciliate infusoria also possess a limited number of cilia, but these, although restricted to the under surface of their bodies, have an unrestricted range of motion. The group are all free swimmers, belonging to the genus Oxytricha. They possess two separate alimentary orifices, neither of which are situated at the extremities or encased by a dense integument. Their locomotive organs consist either of setæ, vibratile cilia, or non-vibratile styles or uncini, variously situated, and all serving to make these infusorial animals very active (Plate III., Nos. 73 and 77). A typical species is the mussel-animalcule (Stylonychia, Fig. 338), common in all infusions and pools of water. Its body is oval and flattened, and about ¹⁄₁₀₀th of an inch in length. At one end a funnel-shaped depression or mouth, with a ciliated margin, leads to the inner part of the body, in which are two oval bodies, a nucleus and a contractile vacuole, which is seen to contract rhythmically. The creature can also stalk along by means of its cilia or setæ, and set up currents to the mouth. Plate III., Nos. 70, 71, 72, 73, and 74, are types of these interesting bodies.

Dr. Balbini believes a true sexual generation occurs among these organisms, but, with the exception of the Paramecium, this has not been seen to take place; even Gruber’s more recent investigations appear to be inconclusive on this point. Conjugation, however, it is said takes place among some attached forms, as in the Stentors. These have been seen to put forth a bud from the body base, and soon after become free swimming bodies. The trumpet-animalcule (Stentor), a conspicuous member of the ciliata, is comparatively large, being about the ¹⁄₂₅th of an inch in length when extended to the full size. It is usually found attached to the under sides of duckweed, and is continually changing its form from that of a small knob when contracted, to the trumpet shape seen in Fig. 339, No. 6, when fully extended, and from which it derives its name. The long cilia projected from the upper part form a spiral within the margin of the open mouth leading to the digestive sac. A contractile vacuole lies to the right of the oral opening. New individuals are produced by the process of budding, and in the form of ciliated embryos from the nucleus. Stentors are commonly met with in fresh water, and are usually of a brilliant green colour. These little bodies will bear cutting up: if only a fragment of the nucleus be included in the section, the injury is soon repaired.

Rotifera, or Wheel-animalcules (Fig. 339).—In this group we have a higher type of animal, with a more complex organisation than those previously noticed. The great majority inhabit fresh water, and are readily developed in hay infusions, in bog-moss, in house-top gutters, everywhere if looked for after a shower of rain. The rotating organs from which these fascinating animalcula derive their name consist of two disc-like bodies whose margins are fringed with rows of cilia, which create currents toward the oral aperture, and which have given rise to the optical delusion of rotating wheels. The disposition of the cilia is so arranged as to bring food to the rotifer and conduct it to the mastax or digesting apparatus—a muscular bulb moved by a series of muscles—the gastric glands and stomach. The great transparency of the whole structure permits of the animal economy being easily studied. The body is covered with a horny envelope of two layers, and is divided into segmental divisions, which slide into each other telescopic fashion. Consequently, as the water dries up, the animal is for a long time rendered indestructible and capable of resisting varying temperatures and the action of caustic reagents.

Rotifers are oviparous, and their eggs are conspicuous and of three kinds. The common soft-shelled eggs produce females, the smaller and more spherical produce males. The ephippial, or summer eggs, are often beset with spines or bosses; these have only a membranous covering, and are hatched soon after they are laid, or before leaving the ova sac. The male rotifer is but a third of the length of the female, often without cilia, and appears to have no alimentary tract; indeed, the only internal organ is a large sperm sac. Rotifers have been divided by Dr. Hudson and the late Mr. Gosse in their charming work on these very interesting “Wheel-animalcules” into four orders, according to their powers of locomotion, as follows:—(1) Rhizota, the rooted; (2) Bdelloida, the leech-like, that swim and creep like a leech; (3) Ploïma, the sea-worthy, that only swim with their ciliary wreath; (4) Scirtopoda, the skippers, that swim with their cilia and skip with arthropodous limbs. These, again, are subdivided into families. With such hardy creatures as Philodina, Adineta, Brachionus, &c., creatures to whom extremes of cold, heat, and drought are the ordinary conditions of life, nothing can be easier to keep going throughout the year. Mr. C. F. Rousselet, who has so thoroughly succeeded in mounting Rotifers with their cilia fully extended, recently exhibited at one of the evening meetings of the Royal Microscopical Society, London, no less than four hundred specimens in a natural and perfect condition, the nervous system being seen more clearly from its successful staining throughout the body than in the living rotifer.

There is also a family of Rotatoria with a single rotatory organ, disposed around the margin of the case. This comprises at present a very small group. The Œcistes is a member of the family (Plate III., No. 69). A single ciliary wreath leads to the alimentary canal, and a pharyngeal bulb or mastax comprises the apparatus of nutrition. The visual organs are red, as in other rotifers, and the ovarium contains several ova, shown in No. 69. The envelope is a gelatinous transparent sheath, into which the animalcule can withdraw itself, its attachment to the bottom being by the end of the foot-like tail. The most interesting among this genus are the Floscularians. These creatures may undoubtedly be described as among the most beautiful and interesting of infusorial animals.

The Stephanoceros, “crowned animalcule,” as it is termed, is about ¹⁄₃₆th of an inch in length, and enclosed in a transparent cylindrical flexible case, beyond which it protrudes five long arms in a graceful manner. These, touching at their points, give a form from which it derives its name. These arms are furnished with several rows of short cilia, which seize the food brought within their grasp until it can be swallowed. In addition to the rotatory organs, they have short flexible processes, or cornu, attached to the outside of one or more of their lobes. The water vascular system consists of two canals arising from a small pyriform contractile vesicle, situated below the stomach. The ova, after leaving the ova sac, remain quiescent until their cilia are developed. Floscularians, like Melicertans, have a certain affinity in form with Vorticellians and Stentors, and also with Campanulariæ, among polypes. Their cilia are less regular when in action than in other Rotatoria. When they retreat into their transparent cells they appear to fold themselves up. Their internal structure can be seen through the external case, and ova are observed enclosed in an ova sac; when thrown off they remain quiescent until the formation of their cilia. The whole family furnish interesting objects for microscopic investigation.

Melicerta ringens (“beaded Melicerta”).—Of all the Melicerta, or “horny floscularia,” this is the most beautiful. Its crystalline body is enclosed in a pellucid covering, wider at the top than the bottom, of a dark yellow or reddish-brown colour, which gradually becomes encrusted by zones of a variety of shapes, cemented together with a peculiar secretion that hardens in water. It derives its name from these pellets, which have the appearance of rows of beads. Mr. Gosse furnished an excellent account of the architectural instincts of Melicerta ringens: “An animalcule so minute as to be with difficulty appreciable by the naked eye, inhabiting a tube composed of pellets, which it forms and lays one by one. It is a mason who not only builds up his mansion brick by brick, but makes his bricks as he goes on, from substances which he collects around him, shaping them in a mould which he carries on his body.

“The pellets composing the case are very regularly placed in position; in a fine specimen, about the ¹⁄₃₀th of an inch in length, when fully expanded, as many as fifteen longitudinal rows of pellets were counted, which gave about thirty-two rows in all. As it exposes itself more and more, suddenly two large rounded discs are expanded, around which, at the same instant, a wreath of cilia is seen performing surprising motions.

“On mixing carmine with the water, the course of the ciliary current is readily traced, and forms a fine spectacle. The particles are hurled round the margin of the disc, until they pass off in front through the great sinus, between the larger petals. If the pigment be abundant, the cloudy torrent for the most part rushes off, and prevents our seeing what takes place; but if the atoms be few, we see them swiftly glide along the facial surface, following the irregularities of outline with beautiful precision, dash round the projecting chin like a fleet of boats doubling a bold headland, and lodge themselves, one after another, in the little cup-like receptacle beneath. Mr. Gosse, believing that the pellets of the case might be prepared in the cup-like receptacle, watched the animal, and presently had the satisfaction of seeing it bend its head forward, as anticipated, and after a second or two raise it again; the little cup having in the meantime lost its contents. It immediately began to fill again; and when it was full, and the contents were consolidated by rotation, aided probably by the admixture of a salivary secretion, it was again bent down to the margin of the case, and emptied of its pellet. This process he saw repeated many times in succession, until a goodly array of dark-red pellets were laid upon the yellowish-brown ones, but very irregularly. After a certain number were deposited in one part, the animal would suddenly turn itself round in its case, and deposit some in another part. It took from two-and-a-half to three-and-a-half minutes to make and deposit a pellet.”

Melicerta may be found in clear pools, mill-ponds, and other places through which a current of water gently flows. If a portion of water-weed be brought home and placed in a small glass zoophyte-trough, and carefully examined with a magnifying power of about fifty diameters, a few delicate-looking projections of a reddish-brown colour will probably be seen adhering to the plant; these are the tubular cases of Melicerta, which, after a short period of rest, will be seen to be animals of ¹⁄₁₂th of an inch or more in length.

Porifera. Spongiadæ.

Sponges.—The term Porifera, or “canal-bearing zoophytes,” was applied by the late Dr. Grant to designate the remarkable class of organisms known as sponges, met with in every sea, and numbering about two thousand species, varying in size from a pin’s head to masses several feet in height; and weighing from a few grains to over a hundred pounds. Sponges assume an endless variety of shapes, as cups, vases, spheres, tubes, baskets, branched-like trees, but often as shapeless masses. When living they are all colours and all consistences, soft and gelatinous, fleshy, leathery or stony. A fuller knowledge of sponges was gained in 1825, when Dr. Robert Grant examined a fragment of living sponge under the microscope. On bringing it to the side of the glass cell in which he had preserved it, he beheld this living fountain pouring forth a torrent of liquid matter in rapid succession, and he was at once convinced that a current flowed out of the larger orifices. He introduced a small portion of fine chalk, and saw particles driven into the interior, and pass out again by different ways. To determine the cause of the currents, it was necessary to make a closer examination of the anatomy of the sponge. For this purpose he cut or peeled off thin sections, and saw that the whole substance was divided into flagellated chambers, enclosing spherical and other bodies, and perforated by pores. Each chamber proved to be about ¹⁄₅₀₀th of an inch in diameter, groups of them opening by a wider orifice into a common space, or canaliculus, and joining others to form canals terminating in larger oscular canals. The walls throughout are lined with flat cells, but in the flagellated chambers the living cells are more or less cylindrical, and each is provided at the free end with a whip-like appendage, or flagellum. Furthermore the upper margin was seen to be expanded into a thin hyaline collar, so that the whip appeared to have its origin in the centre of a basin or funnel. The currents of water traversing the body of the sponge are kept up by the movements of the flagella of the collar-cells. These beat the water in the flagellated chambers into the rootlets of the canals leading to the oscules. To replace this, water flows into the flagellated chambers from the rootlets of the canals passing down from the groups of pores in the skin. The currents entering the sponge bring in oxygenated sea-water and minute food particles, such as diatoms and infusorial organisms; the currents from the oscules contain an excess of carbonic acid of waste products, resulting from vital activity and indigestible remains. The cells lining the canals effect the exchange of gases, and take up food particles.

Professor Grant’s careful and instructive researches were begun on the smaller kind of British sponges hanging down from rocks (Spongia coalita), and on which he gazed for “twenty-five minutes, until obliged to withdraw his eyes from fatigue.” This sponge fixes itself by a root; and the currents enter through the stem and body, and leave principally by oscules placed on the branches.

At present too little is known as to the physiology of digestion in sponges to permit of a definite statement on the subject. In specimens fed upon carmine the collar-cells have been found loaded with granules; in others, again, the flat cells lining the subdermal cavities have been found gorged with colour granules. From Bowerbank’s monograph on the British Spongiadæ (1864 and 1874) nothing of importance can be gained on the subject; in fact, it relates almost entirely to the structure and organisation of sponges in their dried or preserved condition, and therefore is only of value for purposes of specific identification. One of the simplest of living sponges, the microscopic structure of which it is possible to trace, Ascetta primordialis, is found on seaweeds in the Mediterranean. In its simple unbranched condition it forms a minute white sac about one twenty-fifth of an inch in height, opening above by a wide round oscule and narrowing below to a stalk (Fig. 342). The walls are very thin and perforated by pores, through which the water passes into the interior. The walls of the sac are composed of two layers, an inner lining of collar-cells, and an outer layer consisting of a gelatinous matrix containing amœboid bodies and transparent three-rayed spicules. These serve to support the walls and as a frame-work for the pores, as in all the sponges. By eliminating the spicular skeleton, and by supposing the tube to be more globular, the “olynthus form” will be obtained, which has been regarded as the hypothetical ancestor of all sponges. A canal system arises when the walls grow thick or form folds, or give off pouches or tubes. From these channels arise incipient in-current canals, between the inside or lumen of the folds and that forming the out-current canal system.

There is a common ciliated Sycon found on seaweed round the British coast; it has the appearance of a white sac about an inch in height, with a crown of glassy spicules around the orifice. The vertical cavity of the sac is surrounded by a wall of closely-packed horizontal tubes, opening at their inner ends into the central cavity, but externally ending blindly. The central cavity of the sac is surrounded or lined with flat-cells, and the radial tubes with collar-cells, and the walls of the tubes are perforated. Here the spaces between and outside the densely-packed tubes are the in-current canals. In an equally common British sponge, Grantia, which forms small flat white bags, a rudimentary cortex covers the outer ends of the tubes. In Grantiopois, the cortex becomes quite thick; as the radial tubes in this species become more branched and the mesoderm thicker, so the passages or in-current canals become more complicated. Common silicious, sponges develop in a different manner from the calcareous ones, namely, from a hollow conical sac open at the top and with a flat base; the spherical flagellated chambers at a very early stage forming a mammillated layer in the walls. Plakina, one of the simplest silicious sponges, encrusts stones with a fleshy crust, consisting of a sac with a flat base attached to the stone in sucker-like fashion, and with the rest of the walls forming simple folds. The spaces between and outside the folds form the in-current, and those in the lumen of the folds the out-current, channels. Each of the flagellated chambers in the walls of the folds communicates with the in-current spaces through several pores, and opens into the out-current spaces by one large pore, the currents of water passing out by the central oscule. Here we have a general idea of the formation of all the commoner forms of sponges. In the more delicate species, as that of Venus’ flowerbasket, the cells are formed by a trellis work of large spicules of silica. Groups of cells congregate in the ground substance and secrete a network of cylindrical fibres and spicules, which, although they remain to a certain extent separate, are always beautifully adapted for purposes of support. In addition to the support these afford, the skeleton spicules afford a means of defence against the attacks of small animals.68

A fairly good idea will be gained of the internal structure of sponges from the section made of a Geodia Barretti, Fig. 343.

Reproduction.—As regards the modes of reproduction, both male and female cells are found in the mesoderm. The male cells generally give rise by division of the nucleus to masses of spermatozoa, each of which possesses a conical head and a long vibratile filament. The ova appear as large round cells, and when conglomerated in masses, resemble those of Micro-gromia, which, after fertilisation, undergo segmentation or division, first into two cells, and again dividing and sub-dividing, until a cluster or mass of cells results (as seen in Fig. 343). The outer layer of the egg-shaped embryo becomes more cylindrical in shape, and is now provided with cilia, and soon appears as an independent minute oval body. If a bread-crumb sponge be cut open in the autumn, the embryos will be seen as bright yellow spots within the body-substance. By keeping specimens in a vessel of water, the embryos will be seen to escape from the oscules, and swim freely about with the broad end forwards. After twenty-four hours of independent existence, the embryo remains stationary, and fixes itself by its broad end, which becomes flattened out. By a remarkable transformation, the larger granular cells of the interior burst out and grow over the outer flagellate layer of cells, and the latter become the collar-cells of the adult sponge. A minute sponge with one oscule results from the development of the fertilised ovum. An extensive crust with numerous oscules may be regarded either as a colony in which each oscule represents an individual, or simply as one individual in which the growth of the body necessitates the formation of new channels for the conveyance of food materials. The embryos of some of the fresh-water sponges (Spongillidæ) living in ponds, canals, lakes and rivers all over the world, as soon as they become fertilised undergo segmentation, and form oval ciliated bodies, in appearance somewhat resembling the gastrula of Monoxenia, one of the simplest kinds of corals. Fresh-water sponges are green in colour, due to the granular bodies which crowd the cells near the surface of the sponge; that this colour is not due to the formation of chlorophyll is seen on keeping them in a shady place, when they become pale grey or yellowish-brown, and if kept quite in the dark they entirely lose all colour.

A few sponges possess no skeleton whatever, excepting the gelatinous ground substance; in some specimens the skeleton is mainly or entirely composed of foreign particles of sand or the remains of Foraminifera. Others are composed of calcium carbonate, and form the class Calcarea, the spicules of which are white, and opaque in mass; but on placing portions in hydrochloric acid, the skeleton is dissolved away with effervescence, and the spicules are left behind transparent and glassy. A great variety is seen in the different species, as will be gathered from the few typical forms shown in Plate XVI., and which even in their fossilised state remain unaltered, the silica which enters so largely into their composition being indestructible, the calcareous matter alone becoming separated in exposure to the action of air, or by boiling in hydrochloric acid. The only perceptible difference noticed is an increase in transparency, and this, on mounting them in Canada balsam, adds to their beauty when examined by polarised light.

Hyalonema, the “glass-rope” sponge of Japan, consists of a bundle of from 200 to 300 threads of transparent silica, glistening with a satiny lustre like the most brilliant spun glass; each thread is about eighteen inches long, in the middle the thickness of a knitting-needle, and gradually tapering towards either end to a fine point; the whole bundle coiled like a strand of rope into a lengthened spiral, the threads of the middle and lower portions remaining compactly coiled by a permanent twist of the individual threads; the upper portions of the coil frayed out, so that the glassy threads stand separate from each other. The spicules on the outside of the coil stretch its entire length, each taking about two and a half turns of the spiral. One of these long needles is about one-third of a line in diameter in the centre, gradually tapering towards either end. The spirally-twisted portion of the needle occupies rather more than the middle half of its entire length. In the lower portion of the coil, which is embedded in the sponge, the spicule becomes straight, and tapers down to an extreme tenuity, ultimately becoming so fine that it is scarcely possible to trace it to its termination.

Within the mesoderm, and in oscule, was noticed a deep brownish-orange coloured shrunken membrane; this was traced to a parasitic polyp. Since this was first observed on an early specimen of the Japanese glass-sponge, the same parasite has always been found growing on and in all these curious sponges. The surface of the stalk above the portion embedded in the mud is seen to be covered with a warty crust of parasitic polyps. All the specimens of Hyalonema in the European museums in 1860 had their stalks overgrown with Palythoa, while many had their bodies also covered with another parasite, and which, fortunately for the sponge, did not form a sandy crust. The polyps, having no skeleton, dry up entirely, and leave behind no trace except the stain first referred to. Unlike a parasite, however, the polyps do not feed upon the juices and soft parts of the sponge, nor indeed do they share its food, but simply settle upon the sponge and feed upon any food that may chance to come within their reach.

The dredgings of the Challenger brought to the surface many entirely new forms of glass-sponges and from great depths. One of the most beautiful, known as Carpenter’s glass-sponge (Pheronema), is composed of concentric laminæ of silica deposited around a fine central axial canal. These form a gauze-like network throughout, but with no regularity of structure.

Clionæ.—Not the least wonderful circumstance connected with the history of sponges is the power possessed by certain species of boring into substances, the hardness of which might be considered as a sufficient protection against such apparently contemptible foes. Shells (both living and dead), coral, and even solid rocks are attacked by these humble destroyers, gradually broken up, and, no doubt, finally reduced to such a state as to render substances which would otherwise remain dead and useless in the economy of nature available for the supply of the necessities of other living creatures.

These boring sponges constitute the genus Cliona of Dr. Grant. They are branched in form, or consist of lobes united by delicate stems, and after having buried themselves in shells or other calcareous objects, preserve their communication with the water by means of perforations in the outer wall of the shell. The mechanism by which a creature of so low a type of organisation contrives to produce effects so remarkable is still doubtful, from the great difficulties which lie in the way of coming to any satisfactory conclusions upon the habits of an animal that works so completely in the dark as the Cliona celata. Mr. Hancock, in his valuable memoir upon the boring sponges, attributes their excavating power to the presence of the multitude of minute silicious crystalline particles adhering to the surface of the sponge; these he supposes are set in motion by ciliary action. In whatever way this action may be produced, however, there can be no doubt that these sponges are constantly and silently effecting the disintegration of submarine calcareous bodies—the shelly coverings, it may be, of animals far higher in organisation, and in many instances they prove themselves formidable enemies even to living molluscs, by boring completely through the shell. In this case the animal whose domicile it so unceremoniously invades has no alternative but to raise a wall of new shelly matter between himself and his unwelcome guest, and in this manner generally succeeds in barring him out.

From a close examination of the structural and developmental characters of the Spongideæ, it must be conceded that they belong rather to the flagellata Protozoa than to any other order. This was the view held by the late Professor Clark, and Mr. Saville Kent quite concurs in it.69 Summing up the entire evidence adduced, scarcely a shadow of doubt is admissible concerning the intimate relationship that subsists between the Choano-flagellata and other flagellate Protozoa and that of sponges. The primary and essential element of the apparently complex sponge stock is the assemblage of collared flagellate zooids that inhabit its interstitial cavities under various plans of distribution. Individually these collared zooids correspond structurally and functionally in every detail with the collared units of such genera as Codosiga, Salpingœca, and Proto-spongia. The collar in either case presents the same structure and functions, exhibits the same circulatory currents or cyclosis, and acts in the same way for the capture of food. The body contains an identical centrally located spheroidal nucleus or endoplast, and a corresponding series of rhythmically pulsating contractile vesicles. The developmental reproductive phenomena are also strictly parallel. Both originate as simple Amœba or simple flagellate Monads, exhibiting no trace in their earliest stage of the subsequently acquired characteristic collar. Both again after a time withdraw their collar and flagellum, and assume the amœboid state; then, coalescing, enter upon a quiescent or encysted condition, and break up into a number of sporular bodies, and thus provide for the further existence and distribution of the species. The whole process again is much akin to that which obtains in the protophytic type, Volvox globator, which liberates from its interior free swimming gemmules that take the form of spherical aggregation of biflagellate daughter-cells. In their isolated state, on the other hand, the swarm gemmules of the sponge stock are directly comparable with the free swimming subspheroidal colony stock of the flagellate infusoria Synura, Syncrypta, and Uroglena, or with the attached subspheroidal clusters of Codosiga and Anthophysa.

CHAPTER III.

Zoophytes, Cœlenterata, Medusæ, Corals, Hydrozoa.

A study of the earliest growth of the Cœlenterata has shown that their internal cavities are nothing more than regular radiate out-growths of the internal structures. The result of this development is a condition which does not occur again in the whole of the animal kingdom. There is a system of cavities all in open communication one with another, no closed blood vascular system, and no specialised respiratory apparatus. Again, all the animals that constitute this large group are radiate in structure, that is, when viewed from above they are typically star-shaped, and if cut across, every horizontal section shows a symmetrical arrangement of the several parts around a centre. There are other radiate animals, as the Echinoderms, but while in these five is the fundamental number of rays, in the Cœlenterata the rays are a multiple of four, six and upwards. The skeleton or framework of each differs, and when the Cœlenterata form calcareous structures, these are quite different from the tests of the sea-urchins; and in all cases the anterior portion of the body is crowned with one or more circles of tentacles, which remain perfectly flexible and flower-like. The most highly-developed of the free forms are the sea-anemones and the jelly-fish. These have no hard or calcareous skeleton whatever, but withal they are, in the opinion of naturalists and microscopists, the most beautiful objects among Zoophytes.

In spite of their variety of forms, the Cœlenterata seem to be as incapable of higher development as do the Echinoderms, and they have failed to make headway in fresh water, but it is not improbable that some of the simplest forms of the whole group may have given rise to higher animal forms, while the sea anemones, corals, &c., being those descendants of the primitive simple form, have retained the original type of organization almost unchanged.

The type of the group is the Hydra, a fresh-water polyp, commonly found attached to the leaves and stems of many aquatic plants, or floating pieces of stick. Two species are well known to microscopists, the H. viridis, or green polyp, and the H. vulgaris, somewhat darker in colour, probably dependent upon the nature of its food. The third, less common species, the H. fasca, is distinguished from both by the length of its tentacles, which, when fully extended, greatly exceed those of either of the before-mentioned. The fresh-water group measures from one-eighth to the one-third of an inch in length, and form simple stocks of one, two or more branches. They almost exactly resemble in form the polyps of the Hydractinia, which are provided with a circle of tentacles. When placed in a vessel of water and left undisturbed they often attach themselves to the side, where they may be examined with a moderate power at leisure. They are then seen to spread out their tentacles like fine threads, and seize upon any small creature that may come in their way, and by the same means convey it to a mouth capable of great extension. All Hydra possess stinging-cells, by means of which they paralyse their prey. Many Hydra attain to a large size, and shoot out long poisonous filaments; they also possess smaller kinds of smooth cells, which appear to be employed for an entirely different purpose, but for what is not positively known. Hydra usually multiply by means of buds, an out-growth from the body, and these remain attached to the mother stalk for some time, often long enough to give rise to one or two smaller buds. Single eggs are also developed in the ectoderm beneath capsules, or wart-like prominences. The adult animal can be cut to pieces, and from each piece a new individual will be developed. This method of reproduction was first tried by the naturalist Trembley in 1739, whose experiments in this direction excited the greatest interest among the naturalists of the middle of the last century. Hydra fusca in various stages of development is given in outline in Fig. 346.

In the polyps belonging to this family the body-structure for the most part consists of a homogeneous aggregation of vesicular granules, held together by an intercellular sarcode, and capable of great extension and contraction, so that these animals can assume a variety of forms and extend their body and tentacles until the latter become almost invisible. It was the resemblance in this respect to the fabled Hydra that originated the name. Its organ of prehension is termed the hasta; this consists of a sac or opening at the terminal end of the tentacle, within which is seen a saucer-shaped vesicle, supporting a minute ovate body, which carries a sharp calcareous piece termed a sagitta or arrow. Although the fresh-water Hydra may be regarded as typical of this group of animals, marine fauna furnish a far more extensive group in the corals, jelly-fish, and sea-anemones.

A smaller group, the Ctenophora, although members of this sub-kingdom, have not yet found their true position; nevertheless they are interesting glassy, transparent creatures, either shaped like apples, melons, or Phrygian caps, or else forming bands of some considerable length; all are wonderfully transparent, with the single exception of the Beroë. These inhabit the open sea, and are only seen inshore when driven in by currents or strong winds. Their position in the water is usually more or less vertical, the mouth being turned downwards. The portion from which this group derives its name is the ribs, which are symmetrically arranged, and consist of rows of short transverse combs, each forming rows of cilia, which, as they wave to and fro, constitute a swimming or rowing plate, their activity in the water depending upon the will of the animal. They are also provided with an oral umbrella, and capturing filaments or tentacles with hair-like branches. These tentacles, attached to the sides of the animal, are capable of erection or withdrawal into pockets. Great variety is seen in these accessory organs of locomotion; for instance, the Cydippidæ (Plate XVII.) have only arms, but these are remarkable for their length, and serve for the purpose of capturing food as well as for steering. The most interesting, if not the most beautiful of the Ctenophora, are the Beroidæ; it is this family that bear a resemblance to the Phrygian cap (Plate XVII., e). The mouth is wide, but it appears to have no capturing tentacles, and yet their habits are carnivorous; they will even devour their own relations. Many of the genus are phosphorescent, and in place of stinging-cells have small spherical knobs beset with sticky globules, in which their food becomes entangled, and these are apparently in constant use.