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and the annulus is composed of large hexagonal cells filled with air. If the whole be now crushed, under the microscope, multitudes of cells will escape, all filled with minute, strongly refracting corpuscles (fig. 41,41°,41'); but any further observation of the progressive development of the contents, up to the period of the opening of the statoblast and the final escape of a young polyzoon, is henceforth, in consequence of the opacity of the covering, impossible. The statoblast has now acquired the complete form characteristic of the species, and, breaking loose from the funiculus, it falls free into the perigastric space, still surrounded by the delicate external transparent membrane, which is soon torn and disappears. When exposed to conditions favorable for its further development, the two faces, after a longer or shorter period, separate from one another, as has been already said, and a young polyzoon gradually emerges and floats away freely through the surrounding water; this phenomenon I have observed in Cristatella, and several species of Plumatella (figs. 42, 43). The surface of the little polyzoon thus become free is destitute of cilia, except on the tentacula; and the motions of the young animal seem to be quite passive, except so far as they may be influenced by the ciliary action of the tentacula. At the period of its escape it possesses all the essential organization of the adult; the retractor muscles are well developed, and the polypide is capable of regular exsertion and retraction; but the endocyst is colourless and transparent, and free from the earthy particles which in the greater number of species are afterwards formed in it, and the little animal is still simple. It loses no time, however, in developing gemmae, which soon change it to the compound form of the adult. In many cases the two separated faces of the statoblast continue for some time to adhere to the posterior end of the young polyzoon, like the valves of a bivalve shell. In Cristatella the essential stages of the development of the statoblast are similar to those just described in Lophopus, but the external envelope acquires here over its whole surface minute cilia (Pl. I, fig. 3), and becomes separated from the rest of the statoblast by a considerable space, which is filled with semi-fluid granular contents. The statoblast acquires its full size still surrounded by the ciliated envelope, but as yet no trace of the spines is visible; these, however, shortly after show themselves growing out from the two faces of the statoblast (fig. 4); they penetrate the granular matter included within the external investment, and soon impinge on the last-mentioned membrane (fig. 5), which by this time has lost its cilia, and which now gives way, torn by the grapple-like extremities of the spines. The external and granular investments now rapidly disappear, and the statoblast presents itself as the elegant little spiny lenticular body (fig. 6) so characteristic of the genus Cristatella. I have sought in vain, in all the fresh-water Polyzoa, for some orifice through which the statoblasts or ova may escape from the cells; and yet, from the large size and incompressible nature of the former, such an orifice, were it present, could hardly escape detection. Meyen,” it is true, states that he has witnessed in Alcyonella fungosa the escape of an egg through an opening in the vicinity of the anus; but, notwithstanding a similar observation already noticed as made by Van Beneden on the marine Laguncula repens, this I feel certain has been an imperfect observation of Meyen, and that the escape of the egg was the result of some accidental laceration of the tissues in this spot. There is, then, no natural aperture through which either ova or statoblasts can escape, and their liberation, I am convinced, can only take
* “Naturgeschichte der Polypen, Isis, 1828.
place after the destruction of the soft parts of the polyzoon has afforded to them a mode of egress through the orifice of the cell. In two species of fresh-water Polyzoa, Plumatella emarginata and Alcyonella Benedeni, I have observed, besides the ordinary statoblasts, another kind which is characterised by some peculiarities. In both these Polyzoa the cells may be observed towards the end of summer loaded with statoblasts which lie loose within them. These are the ordinary ones, and, in the two species of Polyzoa now under consideration, are of an elongated oval figure, with a largely developed annulus which overlaps a considerable portion of the disc (Pl. IV, figs. 7–9; VII, figs. 7, 8). But, besides these bodies, others (Pl. IV, fig. 10; VII, fig. 9) may be observed which never lie loose in the cell, but are invariably attached to the internal surface of the walls, to which they adhere by means of a peculiar cement in which no trace of structure can be detected. These differ also from the unattached statoblasts in shape, being much broader in proportion to their length, while the annulus is exceedingly narrow and presents but slight traces of that highly developed cellular structure so remarkable in the others. After the decay of the coenoecium many of these attached statoblasts may be seen adherent to the stone or other body on which the specimen had developed itself, and to which they are now connected in lines (Pl. VII, figs. 5, 10) through the medium of a portion of the old cell in which they had been produced. I am unable to state whether the origin and destination of the last-described bodies is similar to that of the others, and I have not succeeded in witnessing the escape from them of the young. In Alcyonella fungosa and Lophopus crystallinus I have also occasionally seen bodies, which differ from the ordinary statoblasts of these Polyzoa by the possession of a regular elliptical aperture in the centre of their more convex face. They were always empty, and of their nature I have not been able to form any conclusion; they are most probably abnormal.
The general structure and development of the statoblasts being now understood, the important question at once suggests itself, what is the true import of these bodies : All that we have seen of them is manifestly in accordance with the nature of a bud. The invariable absence of germinal vesicle and germinal spot, and their never exhibiting the phenomena of yelk-cleavage, independently of the conclusive fact that true ova and ovary occur elsewhere in the same individual, are quite decisive against their being eggs. We must then look upon them as gemmae peculiarly encysted and distined to remain for a period in a quiescent or pupa-like state. It was for this reason, therefore, that I proposed for them the name of statoblasts.”
How far the statoblasts of the Polyzoa admit of comparison with the “winter ova” of the Rotiferae and the ephippia of Daphnia remains yet to be determined. Huxley has studied the production of “winter ova” in Lacinularia, and, though he has shown these bodies to be derived from a portion of the ovary, he is yet of opinion that they must be regarded as gemmae..t He has carefully traced the early stages of their development in Lacinularia, and has shown that their contents are at one period divided into two masses. The reader will recollect that a precisely similar condition is presented by the statoblasts of the Polyzoa at an early period of their development; in the Rotiferae, however, the two masses appear to continue distinct, while in the Polyzoa they are subsequently fused into a single mass. The recent researches
of Mr. Lubbock” are against the supposition that the ephippia of Daphnia are gemmae. He has traced the formation of the ephippial ova in the ovary as true ova; but admits at the same time that they are possibly agamic. The external resemblance of these ephippia with the statoblasts of the Polyzoa is singularly striking; but, with the very different origin of the two sets of bodies, we must take care not to attribute a greater share of significance to this resemblance than it really deserves.t
The reproductive phenomena of the fresh-water Polyzoa may be thus classed under three distinct heads:
Sexual reproduction - - By true ova.
In Cristatella and Lophopus I have frequently witnessed the multiplication of a colony by a process of self-division. In Cristatella this commences by a constriction which takes place generally towards the middle of the colony, and which gradually deepens till at last it divides the entire mass into two separate portions, which move off in opposite directions. In Lophopus the process is very similar; large specimens of this polyzoon have the endocyst constricted at intervals so as to give to the colony the appearance of a variously lobed body enveloped by the gelatinoid ectocyst. It is at the point of these constrictions that the self. division takes place, separating the entire colony into two or more smaller ones.
It may, perhaps, be thought that I ought to have enumerated this multiplication of colonies by a process of self-division as a fourth form of reproduction; a little consideration, however, will show that this is nothing more than a reproduction by buds, with the separation of the buds in masses. It is analogous to the gemmiparous reproduction of Hydra, and must not be confounded with the true fissiparous reproduction of the lower forms of simple animals. In the Polyzoa the colony thus extends itself by the production of gemmae, which after development remain permanently adherent; it establishes new colonies by ova and statoblasts, and by ordinary gemmae which ultimately become detached.
If we attempt to correlate the individual phenomena now described in connection with the reproduction of the Polyzoa, we cannot but be struck with some remarkable analogies which would seem to bring the whole process of generation and gemmation in these animals within the domain of the so-called “Law of alternation of generations.” We have, first, as the immediate result of the development of the ovum, a ciliated sac-like embryo, resembling in form and habit an infusorial animalcule: it is a non-sexual zooid. From this is produced subsequently, by a process of gemmation, another form of zooid, namely, the polypide, with a
* An account of the two methods of reproduction in Daphnia, and of the structure of the “ephippium.” By John Lubbock, Esq. Abstract in ‘Proc. Roy. Soc.,’ Jan. 29, 1857, vol. viii.
t In the mode of development of the statoblasts from the funiculus of the polypide, we are involuntarily reminded of the development of the chains of salpa-buds from the stolon of the solitary salpae.
much more highly differentiated structure, in which the organs of digestion especially hold a dominant position, and which we may regard as sexual or non-sexual, according to the view we take of the relation between it and the testis, as will presently be seen.
Now, if the formation of the ovary be attended to, it will be seen that this body is developed at a late period from the walls of the original sac-like embryo, which have undergone slight changes, and have become the endocyst of the more mature Polyzoon; and it will be at once perceived that this development of the ovary takes place in a way which may obviously be compared with the formation of a bud; that at least in Alcyonella it occupies exactly the position in certain cells that the buds destined to become polypides do in others, and that at an early stage of polypide and ovary it is scarcely possible to distinguish one from the other; so that the idea is immediately suggested, that the body here called ovary is itself a distinct zooid, in which the whole organization becomes so completely subordinate to the reproductive function as to be entirely masked, and apparently replaced by the generative organs. This would then constitute a third zooid, which would therefore be a sexual zooid; it is, however, unisexual (female).
In the next place we find that upon the funiculus (in Alcyonella), which probably belongs rather to the polypide than to the endocyst, there is developed the mass here described as testis. Now, if we view this mass as a mere organ of the polypide, we must then regard the latter as the second sexual or male zooid; but the testis may perhaps be more correctly considered, like the ovary, as a distinct sexual bud, having the generative system so enormously predominant as to overrule and replace all the rest of the organization;" this bud, like the ovary-bud, being also unisexual, but with a male function. In confirmation of this view, it is to be remembered that the funiculus has the power of giving origin to a very remarkable form of undoubted bud, the statoblast, which, until ulterior development is excited in it, has no nearer resemblance to an ordinary polypide-bud than the testicular mass has ; and to this statoblast—so far at least as position is concerned—the male bud or testis in Alcyonella would therefore be related just as the female bud or ovary is related to an ordinary polypide-bud. In Paludicella the testis, though in immediate connection with the funiculus, is developed apparently from the endocyst.
If the above be the correct view, the complete comprehension of the Polyzoon will involve the conception of a ciliated sac-like embryo as a starting point, and a series of buds, of which the last term will consist of a pair of sexual buds, the others being non-sexual; from the sexual buds a true embryo like the first is again produced, which affords the point of departure for another similar cycle.
* Analogous instances of the dominant development of the generative system, so as to suppress more or less completely the development of all the other organs, occur in other members of the animal kingdom; as examples, may be mentioned the reproductive capsules (true buds) in certain Polypi, and the male of some of the Rotiferae.
HOMOLOGIEs of THE Polyzo A.
Before commencing the zoographical portion of the present memoir, there still remains to be considered a subject of great interest to the philosophical zoologist, namely, the exact position and affinities of the Polyzoa in the animal kingdom, a question which admits of much valuable elucidation from the study of the freshwater genera. We have seen in the historical sketch already given of the successive steps which resulted in the separation of the Polyzoa from the Polypes, that the molluscan relations of the Polyzoa began at last to be recognised in an obvious resemblance between their organization and that of the Ascideae. Guided by this relation, an important step was finally taken by M. MilneEdwards in distinguishing two primary series in the mollusca, characterised mainly by the relative grade of perfection of the nervous and circulatory systems. One of these, the Molluscoida, included the Polyzoa and Tunicata; the other, the Mollusca proper, embraced all the other members of the molluscan sub-kingdom. The affinities of the Tunicata and Polyzoa being thus fully recognised, it is of importance to know in what these affinities really consist; in other words, to determine the homologous organs in the two groups. I had elsewhere” attempted to show what I believed to be the correct view of the relation between the Tunicata and the Polyzoa. Additional opportunities of investigation have suggested a few modifications, but after a careful comparison of all that has been urged on this subject, I am still of the opinion that in its leading points view then taken was the true One. The great respiratory sac of the Tunicata is the most striking feature in the structure of this group, and I shall therefore first endeavour to point out where the respiratory sac of the Tunicata is represented in the Polyzoa. Now, the opinions entertained on this subject may be classed under two distinct heads. Under the one head it is maintained that the respiratory sac of the Ascidian has its representative in the pharynx of the Polyzoon, and that the rudimentary tentacula at the orifice of this sac are homologous with the tentacula of the Polyzoon. Under the other head it is asserted that the respiratory sac of the Ascidian is homologous with the tentacular crown of the Polyzoa. I have carefully examined all that has been urged in favour of each of these views, and I believe that the evidence preponderates on the side of considering the respiratory sac of the Ascidian as truly represented by the tentacular crown of the polyzoon, though the exact nature of the homology has not been correctly stated. The reader will call to mind that the respiratory chamber of an Ascidian, consists essentially of a membranous sac, having the inner surface of its walls covered by two sets of tubular bars, one running longitudinally, or from behind forwards, the other transversely, or from the neural to the haemal side, and thus crossing each other at right angles, while the membranous wall of the sac, in every one of the
* On the Homology of the Organs of the Tunicata and the Polyzoa. “Transactions of the Royal Irish Academy, vol. xxii, 1852.