92 MANUAL OF ZOOLOGY. 



often developed in chitinous receptacles known as "gono- 

 thecEE" (fig. 17, 0). The young Sertularian on escaping from 

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

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

 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 em- 

 bedded in it. 



Order IV. Campanularida. — The members of this order 

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

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

 chief difference consists in the fact that the hydrothecse of th? 

 Campanularida with their contained polypites are supported 

 upon conspicuous stalks, thus being terminal in position (fig. 

 17, b); whilst in the Sertularida they are sessile or subsessile, 

 and are placed laterally upon the branchlets. The gonophores 

 also in the Campanularida are usually detached as free-swim- 

 ming medusoids, whereas they remain permanently attached 

 in the Sertularians.. Each medusoid consists of a little trans- 

 parent glassy bell, from the under surface of which there is 

 suspended a modified polypi te, in the form of a "manubrium" 

 (fig. 15). The whole organism swims gaily through the water, 

 propelled by the contractions of the bell or disc (gonocalyx) ; 

 and no one would now suspect that it was in any way related 

 to the fixed plant-like zoophyte from which it was originally 

 budded off. The central polypite is furnished with a mouth at 

 its distal end, and the mouth opens into a digestive sac. From 

 the proximal end of this stomach proceed four radiating canals 

 which extend to the circumference of the disc, where they all 

 open into a single circular vessel surrounding the mouth of 

 the bell. From the margins of the disc hang also a number 

 of delicate extensile filaments or tentacles ; and the circum- 

 ference is still further adorned with a series of brightly-coloured 

 spots, which are probably organs of sense. The mouth of the 

 bell is partially closed by a dehcate transparent membrane or 

 shelf, the so-called " veil." Thus constituted, these beautiful 

 litde beings lead an independent and locomotive existence for 

 a longer or shorter period. Ultimately, the essential elements 

 of reproduction are developed in special organs, situated in 

 the course of the radiating canals of the disc. The resulting 



