PHVUTM CCKLENTERATA 



159 



degraded reproductive zooids lodged in special chambers (a) of 

 the coral. 



The Hydrocorallina occur onl}' in the tropical portions of the 

 Pacific and Indian Oceans, where they are found on the " coral- 



Fin. us< — Stylaster sanguineus. A, portion of skeleton, natural size; B, small portion, 

 magnified ; a. ampulla ; (?.p. dactylopores ; g.p. gastrupores. (After Nicholson and Lydekker.) 



reefs" partly or entirely surrourding many of the islands in 

 those seas. Fossil forms arc found as far back as "the Triassic 

 epoch. 



Order 4. — Siphoxophora, 



The diversity of form exhibited by the members of this order is 

 so great that anything like a general account of it would only be 

 confusing to the beginner, and the most satisfactory method of 

 presentation will be by the study of a few typical genera. 



Ilalisiemma (Fig. 119 A) occurs in the Mediterranean and other 

 seas, and consists of a long, slender, floating stem, to which a 

 number of structures, differing greatly in form, are attached. At one 

 ■ — the uppermost — end of the stem is an ovoid, bubble-like body con- 

 taining air — ihejioat or pncumatojplwre {pn). Next come a number 

 of closely set, transparent structures {net), having the general char- 

 acters of unsyrametrical medusse without manubria, each being a 

 deep, bell-like body, with a velum and radiating canals. During life 

 these stvimming -bells or nectocalyccs contract rhythmically — i.e. at 

 regular intervals — drawing water into their cavities, and immedi- ' 

 ately pumping it out, thus serving to propel the entire organisHT 



