COMPOUND EYE OF CRABS. 247 
tion is executed by certain portions of the extremities, modified 
for this purpose in their structure. To this order belong among 
others the saltatorial sandhoppers (Talitrus), which so frequently 
jump up before our feet when walking on the wet sea-sand ; 
the ill-famed Chelure and Linnoris, whose devastations in sub- 
merged timber almost rival those of the ship-worm, and the 
parasitical Cyami, which gnaw deep holes into the skin of the 
whale. The sandhoppers are extremely frequent on the shores 
of the arctic seas, where they emulate the 
tropical ants in their speedy removal of 
decaying animal substances. Thus Captain 
Holboll relates that, having enclosed a piece 
of shark’s flesh in a basket, and let it down 
to a depth of seventy-five fathoms, in the Greenland sea, he by 
this means caught within two hours six quarts of these little 
creatures, while a vast number still followed the basket 
as it was hauled up. 
As the lower crustaceans offer but few points of in- 
terest to the general reader, they required but a few 4 
words of notice; but the highest order of the class, the 
Thoracostraca, thus named from the carapace which i 
covers their thorax, so that only the abdomen presents Limnoria 
an annular structure, may justly claim a more ample Bee 
description. The preceding orders had either sessile eyes or 
none at all; here the movable eyes are fixed on stalks and of a 
compound structure like those of the insects; each 
ocular globe consisting of a numberof-distinct parallel 
columns, every one of which is provided with its own B= = 
erystalline lens, receives its separate impression of es 
light, and is thus in itself a perfect eye. Approaches Ss: 
to this structure are seen in some of the lower crustaceans; but 
here the “ocelli,” as these minute individual eyes have been 
designated, are very numerous. They are at once 
recognised, under even a low magnifying power, by 
the facetted appearance of the surface of the com- 
pound eye, the facets being either square (Scyllari, 
&e.) or more commonly hexagonal (Paguri, Squille, Squilla. 
&e.). The auditory apparatus is likewise highly developed ; 
the sense of smell is known to be very acute; and the antenne 
are delicate organs of touch. 
s 2 
