DISTRIBUTION 125 



the abyss. The sea floor between the shelf and the abyss contains 

 the bathyal or archibenthic fauna, which is to some extent an 

 intermediate type, but often contains species characteristic of deeper 

 waters. 



The Crustacea of the deep sea floor sometimes belong to primitive 

 groups. The decapod family Eryonidae, such as Stereomastis and 

 Polxcheles, are otherwise known only as fossils, dating back to the 

 Jurassic rocks, which were laid down over a hundred million years 

 ago. In fact the fossils were known before deep-sea exploration 

 revealed the presence of living members of the family. The modern 

 eryonids are all blind, but some of the fossils had well-developed 

 eyes, and lived in shallower water. The persistence of primitive 



seff $u*FOCE 



Fig. 55. Diagram of the main zones in the Sea. The various 



depths are not drawn to scale. The question mark in front of 



hadopelagic is necessary because pelagic animals are not yet 



known from the hadal regions. 



groups in deep water is probably due to the unchanging character 

 of the ocean floor over vast periods of time. 



Reduction of the eyes is frequent among the truly deep-water 

 species, but in the upper archibenthal zone, where a little light 

 may still penetrate, or where phosphorescence may provide sufficient 

 illumination for limited vision, the eyes are sometimes enlarged. It 

 has even been suggested that one large-eyed hermit crab, Para- 

 pagurus pilosimanus, finds it way about by the light emitted by the 

 phosphorescent sea anemones which it carries as a cloak over its 

 abdomen. 



An anatomical peculiarity which is often found in various bottom 

 dwellers of the deep sea is an enormous elongation of the legs and 

 antennae. Two examples are given in figs. 56 and 57. Long legs 

 might be useful for walking across the ooze which covers so much 



