GEMMATION. 39 



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 develop- 

 ment 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 phe- 

 nomenon I have observed in Cristatella, and several species of Plumafella (figs. 42, 43). The sui-- 

 face 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 after- 

 wards 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 Lojjhojjus, but the external envelope acquires here over its whole surface 

 minute cilia (PI. I, fig. 3), and becomes separated from the rest of the statoblast by a considei'able 

 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 Aki/onella fwngosa 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 Layunnda 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 



* ' Naturgescbichte der Polypen,' Isis, 1828. 



