CCELENTERATA : HYDROZOA. 



77 



like expansions of the polypary (figs. 17, 18) ; whilst the 

 hydrosoina is in all cases composed of more than a single poly- 

 pite. The coenosarc gene- 

 rally consists of a main 

 stem or ' hydrocaulus ' 

 - with many branches ; 

 and it is so plant-like in 

 appearance that the com- 

 mon Sertularians are al- 

 most 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, 

 andin all essential respects 

 identical with those of the 

 Qorynida, though usually 

 smaller. The tentacles are placed below the mouth, and have 

 an indistinctly alternate arrangement. The generative buds 

 (gonophores or ovarian vesicles) are usually supported upon 

 gonoblastidia, and seldom, if ever, become detached in the 

 true Sertularids. They are often developed in chitinous recep- 

 tacles known as 'gonothecae' (fig. IS). The young Sertu- 



Flg. 17. Sertularia abietina, the sea-fir. 



Fig. 18. a. Fragment of Sertu- 

 li'ria abietina magnified, b. 

 Portion of Sertularia rosacea, 

 enlarged, showing the hydro- 

 thecse and ovarian vesicles. 



larian on escaping from the ovum appears as a free-swimming 

 ciliated body, which soon loses its cilia, fixes itself, and deve- 

 lops a young coenosarc, by gemmation from which the 

 branching hydrosoma of the perfect organism is produced. 



