106 THE FAUNA OF THE DEEP SEA 



The more recent discovery, however, of genera 

 allied to Brisinga has bridged over the gap 

 separating it from other star-fish, and it is now 

 considered simply as the type of a family of the 

 order. 



The numerous species of the genus that have been 

 found since Asbjornsen's original discovery are all 

 inhabitants of deep water, some of them going down 

 to the enormous depth of 2,000 fathoms ; indeed there 

 are very few genera in the animal kingdom, containing 

 so many species as the genus Brisinga, that have such 

 a uniform deep-sea habitat. 



The last group of Echinoderms that we have to 

 consider is the Holothurians. It contains one order — 

 the Elasipoda — that may be considered to be truly 

 bathybial, as there is only one species belonging to 

 it, Elpidia glacialis, that extends into water as shallow 

 as fifty fathoms. 



The Elasipoda are remarkable for their strongly- 

 developed bilateral symmetry. Adult Echinoderms 

 as a rule possess a well-marked radial symmetry, as 

 we see exemplified in the feather-star, star-fish, and 

 sea-urchin, but this radial symmetry is only adopted 

 when they undergo their metamorphosis from the free 

 swimming and bilaterally symmetrical larval stage. 



