SUB-CLASS SIPHONOPHORA. 



73 



the bell is also furnished with a delicate ledge, which runs round its 

 circumference, and is known as the " veil." The structure, therefore, 

 of the nectocalyces, is very similar to that of an ordinary medusif orm 

 gonophore, the chief difference being the absence in the former of 

 the central polypite or manubrium. The nectocalyces are highly 

 muscular, and have the power of alternately contracting and dilat- 

 ing, thus driving the whole organism through the water. In Praya 

 (fig. 39), which may be taken as a good example of the group, the 



Fig, 39. — Calycnphoridce. A, 'Upper portion of the colony of Fraya maxima, of the 

 natui-al size : n The proximal nectocalyces ; Ti'tt' Mouths of the same ; c c Cceno- 

 sarc, carrying polypites (p p) at intervals, along with their swimmicg-bells (s s) 

 the openings of these being indicated by the letters m m; t Tentacles. B, A single 

 polypite of the same (p), separated from the coinosarc, and enlarged, with its swim- 

 ming-bell (s), the opening of the bell (m), and the tentacles (t). (After Gegenbaur.) 



nectocalyces (n) are succeeded by a series of polypites. Each poly- 

 pite (p) is furnished with a mouth, opening into a digestive sac, 

 which in turn communicates with the cavity of the coenosarc. In 

 this form each polypite has a little swimming-bell of its own ; but 

 in many forms, each is protected by an over-arching plate, termed 

 a "bract." Each polypite has in general a single tentacle, rising 

 from near its base, usually of great length, and provided with lateral 

 branches armed with numerous thread-cells. The reproductive organs 



