122 



CCELENTERATA 



the upper end of which is expanded to form a float called the pneumato- 

 phore which contains air or a gas and serves to keep the colony right 

 side lip in the water: in the other type (Fig. 205) no such axial tube 

 is present, the various individuals budding off from the under side of 

 the enormously enlarged float. By far the greater 

 number of siphonophores are of the first type. The 

 individuals budding off from the axis immediately 

 back of the pneumatophore are swimming individ- 

 uals or nectophores; these are present in pairs, and 

 each has the form of a hydrozoan medusa without 

 the manubrium. Following the nectophores at in- 

 tervals on the axis are similar groups of individuals, 

 each group consisting usually of (1) the bract, a 

 flat, scale-like protective individual; (2) a club- 

 shaped palp; (3) a nutritive individual or gastro- 

 zooid, which is the mouth and stomach of the colony ; 

 (4) a long tentacle with nematocysts; (5) repro- 

 ductive individuals or gonozooids, which are usually 

 unisexual. A colony of this kind swims about slowly 

 in the sea and may be several feet in length and 

 contain thousands of individuals. A modification of 

 this type is seen in the deep-sea siphonophores of 

 the genera Stephalia and Rhodalia, in which the 

 pneumatophore is very large and the axis short and 

 thick. In Stephalia a mouth is present at the terminal end of the axis, 

 which forms the chief gastrozooid: the axis is thus in this case directly 

 comparable with the manubrium of a medusa, 

 of which the pneumatophore would be the bell. 

 In Rhodalia the axis has lost its character of a 

 gastrozooid, not having a mouth at the lower 

 end. These animals are probably primitive 

 siphonophores and seem to indicate the deriva- 

 tion of the group from a medusan instead of 

 from a mixed hydroid and medusan stock. They 

 also form a connecting link between the two 

 general types of Siphonophora, those with an 

 axial tube and those which have none. 



Siphonophores are essentially pelagic animals, although some forms 

 are found in deep water. They belong to the open ocean, especially of 

 the wanner parts of the world, and are among the most beautiful and 



Fig. 204 — Diagram 

 of a siphonophore (Mc- 

 Murrich). 1, pneuma- 

 tophore ; 2, necto- 

 phore ; 3, bract ; 4, 

 gonozooid ; 5, gastro- 

 zooid ; 6, club ; 7, ten- 

 tacle. 



Fig. 205 



Diagram of a Porpita 

 (Delage et H6rouard). 



conspicuous animals found there, 

 grouped in 4 suborders. 



The order contains about 250 species, 



