FOSSIL CRABS. 295 
snocks 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 Alpheus, which live in holes 
in sponges, etc., the eyes are small. 
The eyes of the blind Willemeszia, Fig. 262.—Anthrapalemon gracilis. 
fete at great depths by the preeears Paneer att 
“‘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 pellucidus (Tell- 
kampf, Fig, 263) 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. 
Cuass II.—PopostoMaTa. 
Podostomata.—This class is proposed for the king- 
crab (Limulus), the only survivor of a large number of 
fossil Merostomata, which dominated the Silurian seas. 
