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 Chelurse and Linnorise, 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 cheiura terebrans 

 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 

 words of notice ; but the highest order of the class, the 

 Thoracostraca, thus named from the carapace which 

 covers their thorax, so that only the abdomen presents 

 an annular structure, may justly claim a more ample 

 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 

 crystalline lens, receives its separate impression of froetsof 

 light, and is thus in itself a perfect eye. Approaches s °y UaTOS - 

 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 ||pBmj,- 

 the facetted appearance of the surface of the com- jjgglygg 

 pound eye, the facets being either square (Scyllari, 

 &c.) or more commonly hexagonal (Paguri, Squillse, 

 <&c). The auditory apparatus is likewise highly developed; 

 the sense of smell is known to be very acute; and the antennae 



are delicate organs of touch. 



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