Freshwater Sponges, Hydroids & Polyzoa Part 32

GEOGRAPHICAL DISTRIBUTION.--_H. oligactis_ is widely distributed in Europe and N. America, but in India has only been found in and near the city of Lah.o.r.e in the Punjab.

BIOLOGY.--This species was found by Major J. Stephenson, I.M.S., in the basin of a fountain at Lah.o.r.e and in an ornamental ca.n.a.l in the Shalimar Gardens on the outskirts of the same city. Nothing is known as regards its habits in this country. In N. America, according to Downing, form B breeds in September and October and form A from October to December. The eggs of form B remain attached to the parent until the two cellular layers are formed and then drop off, whereas those of form A are fixed by the parent to some extraneous object, its column contracting until they are in a favourable position for attachment.

The colour of Indian examples of _H. oligactis_ apparently resembles that of the Calcutta winter brood of _H. vulgaris_ so far as visual effect is concerned, but I have noticed in specimens from Lah.o.r.e and the neighbourhood that very minute spherical bodies of a dark green colour are present in the endoderm cells.

PART III.

FRESHWATER POLYZOA

(CTENOSTOMATA & PHYLACTOLaeMATA).

INTRODUCTION TO PART III.

I.

STATUS AND STRUCTURE OF THE POLYZOA.

The Polyzoa const.i.tute a cla.s.s in the third great division of the animal kingdom, the so-called Triploblastea. In this division are included also the worms, molluscs, insects, crustacea, spiders, vertebrates, etc.; for heterogeneous as its elements appear, all these animals may be considered to have essential features in common, in particular a body consisting primarily of three cellular layers. Most of them also possess a body cavity distinct from the alimentary ca.n.a.l. Some authors regard the position of the polyzoa as near that of the higher worms, but the group is an isolated one.

In considering the anatomy of simple forms of animal life such as the sponges it is necessary to pay attention mainly to individual cells, but in discussing more complicated forms our notice is first attracted to tissues and organs, for the cells of which these tissues and organs are composed have each a definite position, a definite structure, and a definite function. The most characteristic feature of the polyzoa, considered from this point of view, is the fact that most of their organs fall into one of two categories and are connected either with what is called the "zooecium" or with what is known as the "polypide."

The zooecium is a cage in which the polypide is enclosed, but it is a living cage, differing from the sh.e.l.l of a snail or the tubes in which many worms encase themselves in being part of the animal itself. The polypide consists mainly of the organs connected directly and indirectly with nutrition and of part of the muscular system; its name is derived from the fact that it bears a superficial resemblance to a polyp such as _Hydra_.

The shape and structure of the zooecium differs greatly in different groups of polyzoa. In its simplest form it is merely a cylindrical tube of living matter which secretes an outer h.o.r.n.y or gelatinous covering.

It is open at the end furthest from its base, at which it is attached either to another zooecium or to some kind of supporting structure.

Certain parts of the polypide can always be extruded from the aperture, which is known technically as the "orifice," or withdrawn through it into the zooecium. When the polypide is retracted it draws in with it a portion of the zooecium. The dead outer layer or ectocyst lines part of the portion thus inv.a.g.i.n.ated and forms the walls of a cavity within the orifice. The base of this cavity consists in many forms of a transverse part.i.tion pierced in the middle by a circular hole and known as the "diaphragm." The diaphragm, however, does not const.i.tute the limit of the inv.a.g.i.n.ated portion of the zooecium, for the living inner wall or endocyst is dragged in still further and forms a sheath round the retracted tentacles. When the tentacles are protruded they emerge through the hole in the diaphragm, carrying with them their sheath of endocyst. The inv.a.g.i.n.ation above the diaphragm, consisting of both endocyst and ectocyst, is then everted.

The tentacles are a characteristic feature of the polypide. Together with the base to which they are attached they are known as the "lophoph.o.r.e"; they surround the mouth, usually in a circle. They differ widely from the tentacles of _Hydra_ in both structure and function, although they too serve as organs for the capture of prey; they are not highly contractile and are not provided with nettle-cells but are covered with cilia, which are in constant motion. When extruded they form a conspicuous calix-like crown to the zooecium, but in the retracted condition they are closely pressed together and lie parallel to one another. They are capable individually of motion in all directions but, although they usually move in concert, they cannot as a rule seize objects between them.

The mouth is a hole situated in the midst of the tentacles. It leads directly into a funnel-shaped oesophagus, the upper part of which is lined with cilia and is sometimes distinguished as the "pharynx," while the lower part, the oesophagus proper, is a thin-walled tube that connects the pharynx with the stomach, which it enters on the dorsal side. The stomach is a bulky organ that differs markedly in form and structure in different groups of polyzoa. It is lined internally with glandular cells and the inner wall is sometimes thrown into folds or "rugae." The part with which the oesophagus communicates is known as the "cardiac" portion, while the part whence the intestine originates is called the "pylorus" or "pyloric" portion. The intestine commences on the ventral side opposite the entrance of the oesophagus and nearly on a level with it, the bulk of the stomach depending between the two tubes.

This part of the stomach is often produced into a blind tube, the fundus or caec.u.m. The alimentary ca.n.a.l may therefore be described as distinctly Y-shaped. The proximal part of the intestine is in some forms lined with cilia, and the tube as a whole is usually divided into two parts--the intestine proper, which is nearest the stomach, and the r.e.c.t.u.m, which opens by the a.n.u.s not far from the mouth.

The nervous system consists of a central ganglion or brain, which is situated at the base of the tentacles on the side nearest the a.n.u.s and gives out radiating nerves in all directions. Close to the brain and providing a communication between the cavity of the zooecium and the cavity in which the tentacles are contained (or, in the case of an expanded polyp, the external world) is a ciliated tube known as the "intertentacular organ." Apparently it acts as a pa.s.sage through which the genital products are expelled; but contradictory statements have been made regarding it, and perhaps it is present only at certain seasons or in certain conditions of the polypide.

[Ill.u.s.tration: Fig. 30.--Vertical section through a polypide of _Alcyonidium_ with the polypide retracted (after Prouho).

A=orifice; B=contracted collar; C=diaphragm; D=parieto-v.a.g.i.n.al muscles; E=tentacles; F=pharynx; G=oesophagus; H=stomach; J=intestine; K=r.e.c.t.u.m; L=intertentacular organ; M=retractor muscle; N=testes; O=ovary; P=funiculus; Q=parietal muscles; R=ectocyst; S=endocyst.]

The muscular system is often of a complicated nature, but three sets of muscles may be distinguished as being of peculiar importance, viz., (i) the retractor muscles, which are fixed to the base of the lophoph.o.r.e at one end and to the base of the zooecium at the other, and by contracting pull the former back into the zooecium; (ii) the parieto-v.a.g.i.n.al muscles, which connect the upper part of the inv.a.g.i.n.ated portion of the zooecium with the main wall thereof; and (iii) the parietal muscles, which run round the inner wall of the zooecium and compress the zooecium as a whole. The parietal muscles are not developed in the Phylactolaemata, the most highly specialized group of freshwater polyzoa.

The cavity between the polypide and the zooecium contains a reticulate tissue of cells known as the "funicular" tissue, and this tissue is usually concentrated to form a hollow strand or strands ("funiculi") that connect the outer wall of the alimentary ca.n.a.l with the endocyst.

This rapid sketch of the general anatomy of a simple polyzoon will be the best understood by comparing it with fig. 30, which represents, in a somewhat diagrammatic fas.h.i.+on, a vertical section through a single zooecium and polypide of the order Ctenostomata, to which some of the freshwater species belong. The polypide is represented in a retracted condition in which the Y-shaped disposition of the alimentary ca.n.a.l is somewhat obscured.

In the great majority of cases the polyzoa form permanent colonies or polyparia, each of which consists of a number of individual zooecia and polypides connected together by threads of living tissue. These colonies are formed by budding, not by independent individuals becoming a.s.sociated together. In a few cases compound colonies are formed owing to the fact that separate simple colonies congregate and secrete a common investment; but in these cases there is no organic connection between the const.i.tuent colonies. It is only in the small subcla.s.s Entoprocta, the polypides and zooecia of which are not nearly so distinct from one another as they are in other polyzoa (the Ectoprocta), that mature solitary individuals occur.

As representatives of both subcla.s.ses of polyzoa and of more than one order of Ectoprocta occur in fresh water, I have prefaced my description of the Indian species with a synopsis of the more conspicuous characters of the different groups (pp. 183-186).

CAPTURE AND DIGESTION OF FOOD: ELIMINATION OF WASTE PRODUCTS.

The food of all polyzoa consists of minute living organisms, but its exact nature has been little studied as regards individual species and genera. In _Victorella bengalensis_ it consists largely of diatoms, while the species of _Hislopia_ and _Arachnoidea_ possess an alimentary ca.n.a.l modified for the purpose of retaining flagellate organisms until they become encysted. Similar organisms form a large part of the food of the phylactolaemata.

Although the tentacles may be correctly described as organs used in capturing prey, they do not themselves seize it but waft it by means of the currents set up by their cilia to the mouth, into which it is swept by the currents produced by the cilia lining the pharynx. The tentacles are also able in some species to interlace themselves in order to prevent the escape of prey. Apparently they have the power of rejecting unsuitable food, for they may often be observed to bend backwards and forwards and thrust particles that have approached them away, and if the water contains anything of a noxious nature in solution the lophoph.o.r.e is immediately retracted, unless it has been completely paralysed. In the phylactolaemata the peculiar organ known as the epistome is capable of closing the mouth completely, and probably acts as an additional safeguard in preventing the ingestion of anything of an injurious nature.

In many genera and larger groups the food commonly pa.s.ses down the pharynx into the stomach without interruption, although it is probable that in all species the oesophagus can be closed off from the stomach by a valve at its base. In some forms, however, a "gizzard" is interposed between the oesophagus and the stomach. This gizzard has not the same function in all cases, for whereas in some forms (_e. g._, in _Bowerbankia_) it is lined with h.o.r.n.y projections and is a powerful crus.h.i.+ng organ, in others (_e. g._, in _Hislopia_ or _Victorella_) it acts as an antechamber in which food can be preserved without being crushed until it is required for digestion, or rough indigestible particles can be retained which would injure the delicate walls of the stomach.

Digestion takes place mainly in the stomach, the walls of which are of a glandular nature. The excreta are formed into oval ma.s.ses in the r.e.c.t.u.m and are extruded from the a.n.u.s in this condition.

Although the gross non-nutritious parts of the food are pa.s.sed _per anum_, the waste products of the vital processes are not eliminated so easily, and a remarkable process known as the formation of brown bodies frequently takes place. This process cannot be described more clearly and succinctly than by quoting Dr. Harmer's description of it from pp.

471 and 472 of vol. ii. of the Cambridge Natural History, a volume to which I have been much indebted in the preparation of this introduction.

The description is based very largely on Dr. Harmer's own observations[AW].

[Footnote AW: Q. J. Micr. Sci. x.x.xiii, p. 123 (1892).]

"The tentacles, alimentary ca.n.a.l, and nervous system break down, and the tentacles cease to be capable of being protruded. The degenerating organs become compacted into a rounded ma.s.s, known from its colour as the 'brown body.' This structure may readily be seen in a large proportion of the zooecia of transparent species. In active parts of the colony of the body-wall next develops an internal bud-like structure, which rapidly acquires the form of a new polypide. This takes the place originally occupied by the old polypide, while the latter may either remain in the zooecium in the permanent form of a 'brown body,' or pa.s.s to the exterior. In _Fl.u.s.tra_ the young polypide-bud becomes connected with the 'brown body' by a funiculus. The apex of the blind pouch or 'caec.u.m' of the young stomach is guided by this strand to the 'brown body,' which it partially surrounds. The 'brown body' then breaks up, and its fragments pa.s.s into the cavity of the stomach, from which they reach the exterior by means of the a.n.u.s."

Brown bodies are rarely if ever found in the phylactolaemata, in which the life of the colony is always short; but they are not uncommon in _Hislopia_ and _Victorella_, although in the case of the former they may easily escape notice on account of the fact that they are much paler in colour than is usually the case. When they are found in a ctenostome the collar-like membrane characteristic of the suborder is extruded from the orifice (which then disappears) and remains as a conspicuous external addition to the zooecium, the ectocyst of which, at any rate in _Bowerbankia_ and _Victorella_, sometimes becomes thickened and dark in colour.

It is noteworthy that the colouring matter of the brown bodies is practically the only colouring matter found in the polypides of most polyzoa. Young polypides are practically colourless in almost all cases.

REPRODUCTION: BUDDING.

Polyzoa reproduce their species in three ways--(i) by means of eggs, (ii) by budding, and (iii) by means of bodies developed as.e.xually and capable of lying dormant in unfavourable conditions without losing their vitality.

Most, if not all species are hermaphrodite, eggs and spermatozoa being produced either simultaneously or in succession by each individual, or by certain individuals in each zoarium. The reproductive organs are borne on the inner surface of the endocyst, as a rule in a definite position, and often in connection with the funiculus or funiculi. It is doubtful to what extent eggs are habitually fertilized by spermatozoa of the individual that has borne them, but in some cases this is practically impossible and spermatozoa from other individuals must be introduced into the zooecium.

Budding as a rule does not result in the formation of independent organisms, but is rather comparable to the proliferation that has become the normal method of growth in sponges, except of course that individuality is much more marked in the component parts of a polyzoon colony than it is in a sponge. In the genera described in this volume budding takes place by the outgrowth of a part of the body-wall and the formation therein of a new polypide, but the order in which the buds appear and their arrangement in reference to the parent zooecium is different in the different groups. In the freshwater ctenostomes three buds are typically produced from each zooecium, one at the anterior end and one at either side, the two latter being exactly opposite one another. The parent zooecium in this formation arises from another zooecium situated immediately behind it, so that each zooecium, except at the extremities of the zoarium, is connected with four other zooecia, the five together forming a cross. The two lateral buds are, however, frequently suppressed, or only one of them is developed, and a linear series of zooecia with occasional lateral branches is formed instead of a series of crosses. In the phylactolaemata, on the other hand, the linear method of budding is the typical one, but granddaughter-buds are produced long before the daughter-buds are mature, so that the zooecia are frequently pressed together, and lateral buds are produced irregularly. In _Victorella_ additional advent.i.tious buds are produced freely near the tip of the zooecium.

Reproduction by spontaneous fission sometimes occurs, especially in the Lophopinae, but the process differs from that which takes place when a _Hydra_ divides into two, for there is no division of individual zooecia or polypides but merely one of the whole zoarium.

The production of reproductive bodies a.n.a.logous to the gemmules of sponges appears to be confined in the polyzoa to the species that inhabit fresh or brackish water, nor does it occur in all of these.

All the phylactolaemata produce, within their zooecia, the bodies known as statoblasts. These bodies consist essentially of ma.s.ses of cells containing abundant food-material and enclosed in a capsule with thick h.o.r.n.y walls. In many cases the capsule is surrounded by a "swim-ring"

composed of a ma.s.s of h.o.r.n.y-walled chambers filled with air, which renders the statoblast extremely light and enables it to float on the surface of the water; while in some genera the margin of the swim-ring bears peculiar hooked processes, the function of which is obscure. The whole structure first becomes visible as a ma.s.s of cells (the origin of all of which is not the same) formed in connection with the funiculus, and the statoblast may be regarded as an internal bud. Its origin and development in different genera has been studied by several authors, notably by Oka[AX] in _Pectinatella_, and by Braem[AY] in _Cristatella_.

[Footnote AX: Journ. Coll. Sci. Tokyo, iv, p. 124 (1891).]

[Footnote AY: Bibliotheca Zoologica, ii, pt. 6, p. 17 (1890).]

The external form of the statoblasts is very important in the cla.s.sification of the phylactolaemata, to which these structures are confined. In all the genera that occur in India they are flattened and have an oval, circular, or approximately oval outline.