CCELENTERATA I HYDROZOA. 99 



The Sertnlarida resemble the Corynida in becoming perma- 

 nently fixed after their embryonic condition by a hydrorhiza, 

 which is developed from the proximal end of the ccenosarc '; 



Fig. 23. Group of zooids of Hydractinia echiiiata. Enlarged (after Hincks). a a 

 Nutritive zooids ; b b Generative zooids, carrying sacs filled with ova. 



but they differ in the fact that the polypites are invariably pro- 

 tected by " hydrothecae," or little cup-like expansions of the 

 polypary (fig. 24, a, b} ; whilst the hydrosoma is in all cases 

 composed of more than a single polypite. The mouth of 

 the hydrotheca is generally furnished with an operculum or 

 valve for its closure. The ccenosarc generally consists of a 

 main stem or " hydrocaulus " with many branches ; and it 

 is so plant-like in appearance that the common Sertularians 

 are almost always mistaken for sea-weeds by visitors at the 

 seaside. It is invested by a strong corneous or chitinous 

 covering, often termed the "periderm." 

 The polypites are sessile or subsessile, hydra-form, and in all 



