CCELENTEBATA : HYDEOZOA. 



79 



group. The sub-class is divided into two orders, viz. the 

 Calycophoridce and the Physophoridce. 



ORDER I. CALYCOPHORID^J. This order includes those 

 Siphonophora whose hydrosoma is free and oceanic, and is pro- 

 pelled ~by ' nectocalyces ' attached to its proximal end. The 

 hydrosoma consists of several polypites, united by an unbranched 

 coenosarc, which is highly flexible and contractile, and never 

 develops a hard cuticular layer. The proximal end of the 

 hydrosoma is modified into a peculiar cavity called the " somato- 

 cyst." The reproductive organs are in the form of medusiform 

 gonophores produced by budding from the peduncles of the 

 polypites. 



In all the Calycophoridce the ecenosarc is filiform, cylin- 

 drical, unbranched, and highly contractile, this last property 

 being due to the presence of abundant muscular fibres. '-The 

 proximal end of the ecenosarc dilates a little, and becomes 

 ciliated internally, forming a small chamber' which communi- 

 cates with the nectocalycine canals. ' At its upper end this 



2. 



Fig. 19. Morphology of th Oceanic Hydrozoa. 1. Diagram of the proximal 

 extremity of a Physophond. a. Pneumatocyst. 2. Vogtia pc.ntacantliu, 

 one of the Calycophoridce. n. Nectocalyces ; p. Polypites ; /. Tentacle?. 

 3. Diagram of a Calycophorid. a a'. Proximal and distal nectocalyces; 

 b. Somatocyst ; c. Coenosarc ; d. Hydrophyllium or bract ; e. Medusiform 

 gonophore ; /. Polypite. The dark lines in figs. 1 and 3 indicate the 

 endoderm, the light line with the clear space indicates the ectoderm. 

 (After Huxley.) 



chamber is a little constricted, and so passes, by a more or less 

 narrowed channel, into a variously shaped sac, whose walls 

 are directly continuous with its own, and which will hence- 

 forward be termed the somatocyst (fig. 19, 3 b). The endo- 



