260 THE SEA SHOBE 



joint ; and when the moulting period arrives, the new limb, still 

 very small, is exposed to view. It then rapidly enlarges, though 

 not to anything like its proper size, and its surrounding skin 

 becomes hardened by the deposit of the calcareous secretion 

 simultaneously with that of the rest of the body. Further enlarge- 

 ments of the new appendage take place at subsequent moults, with 

 the final result that it is but slightly inferior to its fellow either in 

 size or in power. 



The eye of a crustacean is a very complicated structure, 

 commonly described as a compound eye. It consists of a large 

 number of conical, radiating, crystalline rods, collected together 

 into a mass that presents a convex outer surface. This surface is 



covered with a transparent layer 

 of chitin which naturally pre- 

 sents a more or less distinct 

 netted appearance, the bases of 

 the rods being in contact with 

 its inner surface, and visible 

 through it. Each rod is sur- 

 rounded by a layer of pigment 

 that prevents light from passing 

 from one to another, and the 

 FIG. 189. SECTION THROUGH THE optic nerve passing into the base 

 COMPOUND EYE OP AN ARTHRO- O f the compound structure sends 



a sensitive filament into each 

 one. 



In many crustaceans this compound eye is situated on the end 

 of a movable stalk that generally allows it to be protruded or drawn 

 under cover as occasion requires, but in others the organ does not 

 project beyond the general surface of the body. Thus we hear of 

 the animals of this class being divided into the stalk-eyed and 

 the sessile-eyed groups ; the former being represented by crabs, 

 lobsters, shrimps, &c. ; and the latter by sandhoppers and sand- 

 borers. 



Crustaceans undergo metamorphoses while very young, the 

 body being altered considerably in form at several successive 

 moults. Some, in their earliest stage, consist of a little oval body 

 that shows no signs of a division into segments. It swims about 

 by means of three pairs of appendages, and has only one eye. 

 Others start life with four pairs of limbs, attached to the front 

 portion of the body, a segmented abdomen, as yet perfectly limbless, 



