92 HTDEOZOA. 



The coenosarc, in all cases, excretes a very firm 

 chitinous polypary, usually of a pale horny colour, 

 which may either remain throughout in close conti- 

 guity with the ectoderm, or become separated from 

 it at regular intervals, so as to impart an elegant 

 ringed appearance to portions of the tree-like 

 structure {fig. 1 9, h). This semi-transparent, horny, 

 sheath persists long after the destruction of the 

 soft parts of the organism, so that, among the 

 larger species of Sertularidce, the peculiar form 

 of the hydrosoma is sufficiently well seen in dried 

 specimens. Here, therefore, the polypary differs 

 from that of the Gorynidai in its firmer texture, 

 but the most important distinctive feature of the 

 present order is found in the occurrence of the 

 hydrothecse ; organs which do not exist in any 

 other group oi Hydrozoa {fig. 18, 6). The nature 

 of these appendages has already been explained. 

 Their numerous, often beautiful, diversities of 

 form and mode of arrangement afford aids to the 

 definition of the minor types of structure 

 which occur within the limits of this circumscribed 

 group. In Campanularia fastigiata the distal 

 end of the hydro theca forms, according to Mr. 

 Alder, a sort of " operculum, which, when closed, 

 slopes down on each side like the roof of a house, 

 the two opposite angles forming the gables. When 

 the operculum is fully open, the folds disappear, 

 and the edges unite into a continuous rim round 

 the top of the cell." 



The polypites of the Sertularidce, more minute 

 than those of the Corynidcc, differ little from one 

 another, either in form or the general arrangement 

 of their tentacles. In Sertulariadce proper the 

 polypites are sessile, while in the Campanulariadw 



