CRISTATELLA. 97 



But now, by a wise provision of Nature, the ovum is brought to such 

 a position in its buoyancy, that the head of the originating animal dips 

 downwards in protrusion, as if to secure constant immersion amidst 

 the liquid element. The two shells remain close, or nearly so, for a con- 

 siderable time, though separating as the nascent hydra escapes, when they 

 become much more transparent than previously ; and both are thrown off 

 at last. 



The hydra, on quitting the ovum, is yellow, next converted to pale 

 green, which deepens with age. 



In common with the progeny of other zoophytes, the young of the 

 Cristatella consists of a single hydra, but the base disproportionately large, 

 and as if distended by internal contents, or tumid from approximated em- 

 bryonic elements — as it were the vitellus of the ovum of the oviparous 

 animals. — Plate XXVII. fig. 6. The form of the nascent creature is not 

 circumscribed by any regular outline, to be explained in words ; but, during 

 a favourable season, and when subjects are vigorous, it soon approaches 

 the shape of a spherical triangle, wherein the hydra may be said to spring 

 at the base. — Plate XXVII. fig. 7. In a few days the vertex becomes 

 less acute. After being divested of the valves, or shells of the ovum, the 

 young Cristatelhe frequently appear of irregular form, and very turgid, 

 fig. 8, a, b, c ; enlarged, figs. 9, 10, 11. Frequently, also, the base is de- 

 finitely circular, while three or four hydrse have developed from it ; and 

 then the nascent product is a beautiful object. But I feel induced to 

 conclude that the genuine original formation of the base or sole is tri- 

 angular. 



Some, in earlier stages, are of exactly the same specific gravity as 

 the water, remaining suspended amidst it at any given point from the 

 surface. Many reach the sides of the vessel containing them, where they 

 are prone to affix themselves, or they adhere to other solid foundations. 

 One had contrived to seat itself on the surface of an ovum, left empty by 

 its own nascent inmate, — the first hard substance, perhaps, with which it 

 came in contact, for the shells often close when freed of their contents, 

 and remain long united. — Fig. 12. The Cristatella seems to float only in 

 the earliest stages. 



VOL. II. N 



