DIVISIONS OF THE HYDROZOA. 



67 



amongst the Gorynida, the whole coenosarc is enveloped in a horny 

 or chitinous envelope or polypary (fig. 34, a), and this is the struc- 

 ture which is most familiarly known to sea-side observers. The 

 Sertularida, however, are distinguished from the Gorynida by two 

 points. Firstly, none of the Sertularida are simple, but all are com- 

 pound, consisting of more or less numerous polypites, united by a 

 branched coenosarc. Secondly, the polypary of the Sertularida 

 differs from that of the Gorynida in not simply reaching to the 

 bases of the polypites, but in being prolonged to form a number 

 of little cups or " hydrothecse " (fig. 34, a, h) within which the poly- 

 pites are lodged. Each polypite has a hydrotheca of its own, within 

 which it can entirely withdraw, and from which it can protrude its 

 distal extremity. 



The polypites of the Sertularida have essentially the same structure as in 

 the Gorynida, and each may te compared to a little Hydra. Each, namely, 

 consists of a soft contractile and extensile body, which is furnished at its distal 

 extremity with a mouth and a circlet of prehensile tentacles, richly furnished 

 with thread-cells. The mouth opens into a chamber which occupies the whole 

 length of the polypite, and which is to be regarded as the combined body-cavity 

 and digestive sac. At its lower end this chamber opens by a constricted aper- 

 ture into a tubular cavity, which is everywhere excavated in the substance of the 

 coenosarc (fig. 34, 6). The nutrient particles obtained by each polypite thus serve 

 for the support of the entire colony, and are distributed throughout the entire or- 

 ganism. The nutritive fluid prepared in the in- 

 terior of each polypite gains access through the 

 above-mentioned aperture to the cavity of the 

 cosnosarc, which, by the combined exertions of 

 the whole assemblage of polypites, thus become 

 filled with a granular nutritive liquid. This 

 ccenosarcal fluid i.s in constant movement, cir- 

 culating through all parts of the colony, and thus 

 maintaining its vitality — the cause of the move- 

 ment being probably due, in part, at any rate, to 

 the existence of vibrating cilia. 



The process of reproduction varies some- 

 what in different members of the order. In 

 all alike, however, the ordinary polypites 

 are incapable of producing the essential 

 elements of reproduction, and for this pur- 

 pose special generative buds have to be 

 developed. In the typical Sertularians the 

 reproductive buds are developed at certain 

 seasons in great numbers, and they con- ' 

 stitute vrhat used to be called the " ovarian vesicles " or " capsules " 

 (fig. 35). These reproductive buds are enclosed in horny cups or 

 receptacles, often of a very beautiful shape, and much larger in size 



Fig. 35. — Ovarian capsule of 

 Dlj)hasia {Sertularla) oper- 

 culata, Liim. (after Hincks). 

 Greatly enlarged. 



