I\ 



PHYLUM CCELENTERATA 



159 



degraded reproductive zooids lodged in special chambers () of 

 the coral. 



The Hydrocorallina occur only in the tropical portions of the 

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



Fio. 119i Stylaster sanguineus. A, portion of skeleton, natural size; B, small portion, 

 magnified ; a. ampulla? ; <Lp. dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.) 



reefs " partly or entirely surrounding 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. 



Halistemma (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 thefloat or pncumatophore (pn). Next come a number 

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

 acters of unsymmetrical medusae without manubria, each being a 

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

 these swimming-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 organism 



