CCELENTERATA : HYDROZOA. 87 



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 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- 

 distinctly alternate arrangement. The mouth opens into a 

 chamber which occupies the whole length of the polypite, and 

 is to be regarded as the combined body-cavity and digestive 

 sac. At its lower end this chamber opens by a constricted 

 aperture into a tubular cavity which is everywhere excavated 

 in the substance of the ccenosarc (fig. 17, b}. The nutrient 

 particles obtained by each polypite thus serve for the support 

 of the whole colony, and are distributed throughout the entire 

 organism. The nutritive fluid prepared in the interior of each 

 polypite gains access through the above-mentioned aperture to 

 the cavity of the ccenosarc, which by the combined exertions 

 of the whole assemblage of polypites thus becomes filled with 

 a granular nutritive liquid. This ccenosarcal fluid is in con- 

 stant movement, circulating through all parts of the colony, 

 and thus maintaining its vitality, the cause of the movement 

 being probably due, in part, at any rate, to the existence of 

 vibrating cilia. 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 receptacles known as "gono- 

 thecae" (fig. 17, o). The young Sertularian on escaping from 

 the ovum appears as a free-swimming ciliated body, which 

 soon loses its cilia, fixes itself, and develops a young ccenosarc, 

 by gemmation from which the branching hydrosoma of the 

 perfect organism is produced. 



In Plumularia and some of its allies there occur certain 

 peculiar organs, probably offensive, to which the name of 

 " nematophores " has been applied. Each of these consists of 

 a process of the ccenosarc, which is invested by the horny 

 polypary, with the exception of the distal extremity, which 

 remains uncovered, and contains many large thread-cells im- 

 bedded in it. 



ORDER IV. CAMPANULARIDA. The members of this order 

 are closely allied to the Sertnlarida ; so closely, indeed, that 

 they are very often united together into a single group. The 



