(Plate I, fig. 4.)

On more than one occasion I have found in my aquarium in Calcutta small sponges of a peculiar type which I am unable to refer with certainty to any of the species described above. Fig. 4, pl. I, represents one of these sponges. They are never more than about a quarter of an inch in diameter and never possess more than one osculum. They are cushion-shaped, colourless and soft. The skeleton-spicules are smooth, sharply pointed, moderately slender and relatively large. They are arranged in definite vertical groups, which project through the dermal membrane, and in irregular transverse formation. Small spherical gemmules are present but have only a thin chitinous covering without spicules or foramen.

These sponges probably represent an abnormal form of some well-known species, possibly of _Spongilla carteri_. I have seen nothing like them in natural conditions.

PART II.

FRESHWATER POLYPS

(HYDRIDA).

INTRODUCTION TO PART II.

I.

THE PHYLUM COELENTERATA AND THE CLa.s.s HYDROZOA.

The second of the great groups or phyla into which the metazoa are divided is the Coelenterata, in which are included most of the animals commonly known as zoophytes, and also the corals, sea-anemones and jelly-fish. These animals are distinguished from the sponges on the one hand and from the worms, molluscs, arthropods, vertebrates, etc., on the other by possessing a central cavity (the coelenteron or "hollow inside") the walls of which are the walls of the body and consist of _two_ layers of cells separated by a structureless, or apparently structureless, jelly. This cavity has as a main function that of a digestive cavity.

An ideally simple coelenterate would not differ much in general appearance from an olynthus (p. 27), but it would have no pores in the body-wall and its upper orifice would probably be surrounded by prolongations of the body-wall in the form of tentacles. There would be no collar-cells, and the cells of the body generally would have a much more fixed and definite position and more regular functions than those of any sponge. The most characteristic of them would be the so-called cnidoblasts. Each of these cells contains a capsule[AK] from which a long thread-like body can be suddenly uncoiled and shot out.

[Footnote AK: Similar capsules are found in the tissues of certain worms and molluscs, but there is the strongest evidence that these animals, which habitually devour coelenterates, are able to swallow the capsules uninjured and to use them as weapons of defence (see Martin, Q. J.

Micro. Sci. London, lii, p. 261, 1908, and Grosvenor, Proc.

Roy. Soc. London, lxxii, p. 462, 1903). The "trichocysts" of certain protozoa bear a certain resemblance to the nettle-cells of coelenterates and probably have similar functions.]

The simplest in structure of the coelenterates are those that const.i.tute the cla.s.s Hydrozoa. In this cla.s.s the primitive central cavity is not divided up by muscular part.i.tions and there is no folding in of the anterior part of the body to form an oesophagus or stomatodaeum such as is found in the sea-anemones and coral polyps. In many species and genera the life-history is complex, ill.u.s.trating what is called the alternation of generations. That is to say, only alternate generations attain s.e.xual maturity, those that do so being produced as buds from a s.e.xless generation, which itself arises from the fertilized eggs of a previous s.e.xual generation. The s.e.xual forms as a rule differ considerably in structure from the s.e.xless ones; many medusae are the s.e.xual individuals in a life-cycle in which those of the s.e.xless generation are sedentary.

An excellent general account of the coelenterates will be found in the Cambridge Natural History, vol. i (by Prof. Hickson).

STRUCTURE OF HYDRA.

_Hydra_, the freshwater polyp, is one of the simplest of the Hydrozoa both as regards structure and as regards life-history. Indeed, it differs little as regards structure from the ideally simple coelenterate sketched in a former paragraph, while its descent is direct from one polyp to another, every generation laying its own eggs[AL]. The animal may be described as consisting of the following parts:--(1) an upright (or potentially upright) column or body, (2) a circle of contractile tentacles at the upper extremity of the column, (3) an oral disk or peristome surrounding the mouth and surrounded by the tentacles, and (4) a basal or aboral disk at the opposite extremity. The whole animal is soft and naked. The column, when the animal is at rest, is almost cylindrical in some forms but in others has the basal part distinctly narrower than the upper part. It is highly contractile and when contracted sometimes a.s.sumes an annulate appearance; but as a rule the external surface is smooth.

[Footnote AL: The statement is not strictly accurate as regards the Calcutta phase of _H. vulgaris_, for the summer brood apparently does not lay eggs but reproduces its species by means of buds only. This state of affairs, however, is probably an abnormality directly due to environment.]

The tentacles vary in number, but are never very numerous. They are disposed in a single circle round the oral disk and are hollow, each containing a prolongation of the central cavity of the column. Like the column but to an even greater degree they are contractile, and in some forms they are capable of great elongation. They cannot seize any object between them, but are able to move in all directions.

The disk that surrounds the mouth, which is a circular aperture, is narrow and can to some extent a.s.sume the form of a conical proboscis, although this feature is never so marked as it is in some hydroids. The basal disk is even narrower and is not splayed out round the edges.

[Ill.u.s.tration: Fig. 27.--Nettle-cells of _Hydra_.

A=capsules from nettle-cells of a single specimen of the summer phase of _H. vulgaris_ from Calcutta, 480: figures marked with a dash represent capsules with barbed threads. B=a capsule with the thread discharged, from the same specimen, 480. C=capsule with barbed thread, from a specimen of _H. oligactis_ from Lah.o.r.e. D=undischarged nettle-cell of _H. vulgaris_ from Europe (after Nussbaum, highly magnified).

E=discharged capsule of the same (after the same author).

_a_=cnidoblast; _b_=capsule; _c_=thread; _d_=cnidocil. Only the base of the thread is shown in E.]

A section through the body-wall shows it to consist of the three typical layers of the coelenterates, viz., (i) an outer cellular layer of comparatively small cells, the ectoderm; (ii) an intermediate, structureless or apparently structureless layer, the mesogloea or "central jelly"; and (iii) an internal layer or endoderm consisting of relatively large cells. The cells of the ectoderm are not h.o.m.ogeneous.

Some of them possess at their base narrow and highly contractile prolongations that exercise the functions of muscles. Others are gland-cells and secrete mucus; others have round their margins delicate ramifying prolongations and act as nerve-cells. Sense-cells, each of which bears on its external surface a minute projecting bristle, are found in connection with the nerve-cells, and also nettle-cells of more than one type.

The mesogloea is very thin.

The endoderm consists mainly of comparatively large cells with polygonal bases which can be seen from the external surface of the column in colourless individuals. Their inner surface is amoeboid and in certain conditions bears one or more vibratile cilia or protoplasmic lashes.

Nettle-cells are occasionally found in the endoderm, but apparently do not originate in this layer.

The walls of the tentacles do not differ in general structure from those of the column, but the cells of the endoderm are smaller and the nematocysts of the ectoderm more numerous, and there are other minor differences.

A more detailed account of the anatomy of _Hydra_ will be found in any biological text-book, for instance in Parker's Elementary Biology; but it is necessary here to say something more as regards the nettle-cells, which are of great biological and systematic importance.

A nettle-cell of the most perfect type and the structures necessary to it consist of the following parts:--

(1) A true cell (the cnidoblast), which contains-- (2) a delicate capsule full of liquid; (3) a long thread coiled up in the capsule; and (4) a cnidocil or sensory bristle, which projects from the external surface of the cnidoblast.

A nerve-cell is a.s.sociated with each cnidoblast.

In _Hydra_ the nettle-cells are of two distinct types, in one of which the thread is barbed at the base, whereas in the other it is simple.

Both types have often two or more varieties and intermediate forms occur, but generally speaking the capsules with simple threads are much smaller than those with barbed ones. The arrangement of the nettle-cells is not the same in all species of _Hydra_, but as a rule they are much more numerous in the tentacles than elsewhere on the body, each large cell being surrounded by several small ones. The latter are always much more numerous than the former.

CAPTURE AND INGESTION OF PREY: DIGESTION.

The usual food of _Hydra_ consists of small insect larvae, worms, and crustacea, but the eggs of fish are also devoured. The method in which prey is captured and ingested has been much disputed, but the following facts appear to be well established.

If a small animal comes in contact with the tentacles of the polyp, it instantly becomes paralysed. If it adheres to the tentacle, it perishes; but if, as is often the case, it does not do so, it soon recovers the power of movement. Animals which do not adhere are generally those (such as ostracod crustacea) which have a hard integument without weak spots.

Nematocysts of both kinds shoot out their threads against prey with considerable violence, the discharge being effected, apparently in response to a chemical stimulus, by the sudden uncoiling of the thread and its eversion from the capsule. Apparently the two kinds of threads have different functions to perform, for whereas there is no doubt that the barbed threads penetrate the more tender parts of the body against which they are hurled, there is evidence that the simple threads do not do so but wrap themselves round the more slender parts. Nussbaum (Arch.

mikr. Anat. xxix, pl. xx, fig. 108) figures the tail of a _Cyclops_ attacked by _Hydra vulgaris_ and shows several simple threads wrapped round the hairs and a single barbed thread that has penetrated the integument. Sometimes the cyst adheres to the thread and remains attached to its cnidoblast and to the polyp, but sometimes the thread breaks loose. Owing to the large ma.s.s of threads that sometimes congregate at the weaker spots in the external covering of an animal attacked (_e. g._, at the little sensory pits in the integument of the dorsal surface of certain water-mites) it is often difficult to trace out the whole length of any one thread, and as a thread still attached to its capsule is frequently buried in the body of the prey, right up to the barbs, while another thread that has broken loose from its capsule appears immediately behind the fixed one, it seems as though the barbs, which naturally point towards the capsule, had become reversed. This appearance, however, is deceptive. The barbs are probably connected with the discharge of the thread and do not function at all in the same way as those on a spear- or arrow-head, never penetrating the object against which the projectile is hurled. Indeed, their position as regards the thread resembles that of the feathers on the shaft of an arrow rather than that of the barb of the head.

Adhesion between the tentacles and the prey is effected partly by the gummy secretion of the glands of the ectoderm, which is perhaps poisonous as well as adhesive, and partly by the threads. Once the prey is fast and has ceased to struggle, it is brought to the mouth, which opens wide to receive it, by the contraction and the contortions of the tentacles, the column, and the peristome. At the same time a ma.s.s of transparent mucus from the gastral cavity envelops it and a.s.sists in dragging it in. There is some dispute as to the part played by the tentacles in conveying food into the mouth. My own observations lead me to think that, at any rate so far as _H. vulgaris_ is concerned, they do not push it in, but sometimes in their contortions they even enter the cavity accidentally.

When the food has once been engulfed some digestive fluid is apparently poured out upon it. In _H. vulgaris_ it is retained in the upper part of the cavity and the soluble parts are here dissolved out, the insoluble parts such as the chitin of insect larvae or crustacea being ejected from the mouth. Digestion is, however, to a considerable extent intracellular, for the cells of the endoderm have the power of thrusting out from their surface lobular ma.s.ses of their cell-substance in which minute nutritive particles are enveloped and dissolved. The movements of the cilia which can also be thrust out from and retracted into these cells, keep the food in the gastral cavity in motion and probably turn it round so as to expose all parts in turn to digestive action. Complete digestion, at any rate in the Calcutta form, takes several days to accomplish, and after the process is finished a flocculent ma.s.s of colourless excreta is emitted from the mouth.

COLOUR.

In _Hydra viridis_, a species that has not yet been found in India, the green colour is due to the presence in the cells of green corpuscles which closely resemble those of the cells of certain freshwater sponges.

They represent a stage in the life-cycle of _Chlorella vulgaris_, Beyerinck[AM], an alga which has been cultivated independently.

[Footnote AM: Bot. Zeitung, xlviii (1890): see p. 49, _antea_.]

In other species of the genus colour is largely dependent on food, although minute corpuscles of a _dark_ green shade are sometimes found in the cells of _H. oligactis_. In the Calcutta phase of _H. vulgaris_ colour is due entirely to amorphous particles situated mainly in the cells of the endoderm. If the polyp is starved or exposed to a high temperature, these particles disappear and it becomes practically colourless. They probably form, therefore, some kind of food-reserve, and it is noteworthy that a polyp kept in the unnatural conditions that prevail in a small aquarium invariably becomes pale, and that its excreta are not white and flocculent but contain dark granules apparently identical with those found in the cells of coloured individuals (p. 154).

Berninger[AN] has just published observations on the effect of long-continued starvation on _Hydra_ carried out in Germany. He finds that the tentacles, mouth, and central jelly disappear, and that a closed "bladder" consisting of two cellular layers remains; but, to judge from his figures, the colour does not disappear in these circ.u.mstances.