HYDRACTINIA. 19 



Genus HYDRACTINIA, Van Beneden. 



Der. Hydra, a genus of Hydroicla, and Actinia, a Sea-anemone. 



EciiiNoc'iiORiUM, Hassall, Annals N. H. for July 1841. 

 SYNIIYDRA, De Quatrefages, Ann. Sc. Nat. xx. 232. 



GENERIC CHARACTER. Polypites claviform, sessile, with 

 a single verticil of filiform tentacula surrounding the base of 

 a conical proboscis, developed at intervals from the cosno- 

 sarc, which forms a naked expansion above, and below con- 

 sists of a mass of anastomosing stolonic tubes clothed with 

 a chitinous polypary, which are adnate to one another and 

 form a continuous crust : reproduction by means of fixed 

 sporosacs, which are borne on partially developed polypites*, 

 destitute of tentacles and furnished with many spherical 

 clusters of thread-cells round the oral extremity. 



POLYMORPHISM reaches its height amongst the Hydroicla 

 in the genera Hydractinia and Podocoryne. In each 

 colony of these zoophytes several distinct kinds of zooid 

 with separate functions are united together, presenting a 

 wonderful variety of form and structure. We have in 

 Hydractinia, (1) the alimentary polypites, whose sole 

 office it seems to be to procure and digest nutriment for 

 the commonwealth; (2) the fertile polypites, which are 

 small and attenuated and only furnished with rudimentary 

 tentacles, and which support the true reproductive zooids ; 

 (3) the fixed reproductive sacs, which differ in shape and 

 colour in the two sexes ; (4) the spiral appendages, snake- 

 like organs, endowed with great muscular power, and 

 localized in certain regions of the common basal crust; 



* This is not always the case. Gegenbaur mentions that he has found 

 gonophores on fully developed polypites (Grundziige der vergleichenden 

 Anatomic, p. 99, & p. 94. fig. 15). 



C 2 



