CCELENTERATA : SIPHQNOPHORA. 131 



one of the principal sources 'of the luminosity of the sea. It 

 does not seem, however, that they phosphoresce, unless irri- 

 tated or excited in some manner. 



CHAPTER IX. 



SIPHONOPHORA. 



SUB-CLASS II. SIPHON OPHORA. 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, contrac- 

 tile, unbranched or slightly-branched ccenosarc, the proximal end 

 of which is usually furnished with ' nectocalyces] and is dilated 

 into a 'somatocyst' 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 

 Calycophoridce. and the Physophoridce. 



ORDER I. CALYCOPHORIDCE. This order includes those 

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

 pelled by " necto calyces " attached to its proximal end. TJie hydro- 

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

 sarc, which is highly flexible and contractile, and nei'er 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 coenosarc is filiform, cylindrical, 

 unbranched, and highly contractile, this last property being due 

 to the presence of abundant muscular fibres. " The proximal 

 end of the ccenosarc 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. 54, 3 b). The endoderm of this sac is cili- 

 ated, and it is generally so immensely vacuolated as almost to 

 obliterate the internal cavity, and give the organ the appearance 



