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MANUAL OF ZOOLOGY. 



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

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

 polypary (fig. 46, #, b) ; whilst the hydrosoma is in all cases 



Fig. 46. a Serlularia (Diphasia) pinnata, natural size ; a' Fragment of the same en- 

 larged, carrying a male capsule (p), and showing the hydrothecae (ft) ; b Fragment of 

 Campanularia neglecta (after Hincks), showing the polypites contained in their 

 hydrothecae (K), and also the point at which the coenosarc communicates with the 

 stomach of the polypite (c). 



composed of more than a single polypite. The mouth of the 

 hydrotheca is generally furnished with an operculum or valve 

 for its closure. Owing to the presence of " hydrothecae," the 

 name of " Calyptoblastic Hydroids" has been proposed by 

 Professor Allman for the Sertularians and Campanularians. 

 In all these forms, also, the generative buds are similarly en- 

 closed in chitinous receptacles the so-called " gonothecae " or 

 "gonangia." 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 sub-sessile, hydra-form, and in 

 all essential respects identical with those of the Corynida, 

 though usually smaller. Each polypite consists of a soft, con- 

 tractile 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 tentacles have an in- 



