CCELENTERATA : HYDROZOA. 89 



CHAPTER IX. 

 SIPHONOPHORA. 



SUB-CLASS II. SIPHONOPHORA. The members of this sub- 

 class constitute the so-called "Oceanic Hydrozoa;" and are 

 characterised by the possession of a "free and oceanic hydro- 

 soma, consisting of several polypites united by a flexible, contractile, 

 unbranched, or slightly branched ccenosarc, the proximal end of 

 which is usually furnished with ' nectocalyces] and is dilated 

 into a * somatocysf or into a ' pneumatophore! " (Greene.) 



All the Siphonophora are unattached, and permanently free, 

 and all are composite. They are singularly delicate organisms, 

 mostly found at the surface of tropical seas, the Portuguese 

 man-of-war (Physalia) being the most familiar member, of the 

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

 Caly cop hor idee, and the Physophoridce. 



ORDER I. CALYCOPHORID^E. This order includes those 

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

 pelled by " nectocalyces" attached to its proximal end. The hydro- 

 soma consists of several polypites, united by an unbranched cceno- 

 sarc, which is highly flexible and contractile, and ntver develops a 

 hard cuticular layer. The proximal end of the hydrosoma is 

 modified into a peculiar cavity called the " somatocyst" The re- 

 productive organs are in the form of medusiform gonophores pro- 

 duced by budding from the peduncles of the polypites. 



In all the Calycophoridce the ccenosarc is filiform, cylindrical, 

 unbranched, and highly contractile, this last property being due 

 to the presence of abundant muscular fibres. " The proximal 

 end of the coenosarc dilates a little, and becomes ciliated in- 

 ternally, forming a small chamber" which communicates with 

 the nectocalycine canals. " At its upper end this 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 henceforward be termed 

 the somatocyst (fig. 18, 3, b\ The endoderm of this sac is ciliated, 

 and it is generally so immensely vacuolated as almost to obli- 

 terate the internal cavity, and give the organ the appearance 

 of a cellular mass." (Huxley.) The polypites in the Calyco- 

 phoridce often show a well-marked division into three portions, 

 termed respectively the proximal, median, and distal divisions. 

 Of these, the " proximal" division is somewhat contracted, and 

 forms a species of peduncle, which often carries appendages. 

 The "median" portion is the widest, and may be termed the 



