FOSSIL CRABS. 



295 



shocks and sounds. When alarmed, lobsters are said to 



cast off their large claws, but the latter are again re- 



newed. It is more probable, however, that the claws are 



torn off during their contests with each other. Hensen 



found that crabs and shrimps liv- 



ing in water do not notice sounds 



made in the air. The hairs about 



the mouth are the organs of tac- 



tile sense, and have been made by 



Hensen to vibrate to certain sounds. 



The eyes may be greatly devel- 



oped in shrimps living at great 



depths ; thus Thalascaris, a shrimp 



living near the bottom of the At- 



lantic Ocean, is remarkable for the 



large size of its eyes. In the spe- 



cies of Alplieus, which live in holes 



in sponges, etc., the eyes are small. 



The eyes of the blind WilUmossi 



J 



dredged at great depths by the 

 "Challenger" Expedition, are rudimentary, though in the 

 young the eyes are better developed. This is the case with 

 the young of the blind craw-fish Cambarus pettucidus (Tell- 

 kampf, Fig, 203) of Mammoth and other caves. The fact 

 that the eyes in the young are larger than in the adult indi- 

 cates that this species has descended from other forms living 

 in neighboring streams, and well endowed with the sense 

 of sight. The eye (Fig. 264) of the blind craw-fish differs 

 from that of the normal species in its smaller size, conical 

 form, the absence of a cornea (indicated by the dotted lines 

 in A), the pigment cells being white instead of black, and 

 by differences in the form of the brain, that of the blind 

 species being fuller on the sides. Crabs breathe by gills, 

 but the palm crab breathes by lungs. 



Natural size. Restored. 



CLASS II. PODOSTOMATA. 



Podostomata. This class is proposed for the king- 

 crab (Limulus), the only survivor of a large number of 

 fossil Merostnmuta, which dominated the Silurian seas. 



