84 THE FAUNA OF THE DEEP SEA 



fish that it is not easy to account for, and it is 

 necessary only to mention their occurrence without 

 attempting to offer any explanation of them. 



One of the most common of these is the very 

 dark pigment occurring in certain parts of the 

 epithelium of the mouth and respiratory passages 

 and the endothelium of the peritoneum. For ex- 

 ample, in Bathysaurus mollis, living at a depth of 

 2,000 fathoms, the mouth and buccal cavities are black. 

 The same thing occurs in Ij)nops MmTciyi, and indeed 

 in all the strictly deep-sea forms. 



Another important character of very frequent 

 occurrence is the reduction in size, length, and number 

 of the gill laminae. 



Among invertebrates we may mention as a fact 

 of some interest, dependent perhaps on the soft 

 character of the bottom, the preponderance of stalked 

 forms over those of more sessile habits. 



Thus among the Alcyonaria the characteristic 

 forms of the deep water are the Pennatulids, and 

 more particularly the genus Umhellula with its long 

 graceful stem and terminal tuft of polyps. Among 

 the Echinoderma w^e find many forms of stalked 

 Crinoids. Among the Tunicates several curious 

 genera characterised by their long peduncles. 



