102 ZOOLOGY. 



pieces, followed hy two smaller pieces with a fifth dorsal piece. Sometimes 

 there are accessory pieces at the base, and the whole are in some cases so 

 niuch reduced in size as to be rudimentary like the shell in CMtonelhcs (p. 

 81). Sowerby's genus, Lithotrya^ occupies holes in rocks; but it is not 

 known whether it forms them, or takes possession of those previously made 

 by boring mollusca. 



Order 2. Acamptosomata. In this order the animal is short and conical, 

 without a peduncle, the shell solid and conical, sometimes sub-cylindrical, 

 with the base or attached portion open or closed, the aperture provided with 

 a two-valved or four-valved operculum. Those of the order known to the 

 ancients were named Balanus {pi. 76, fig. 54), on account of their 

 resemblance to an acorn, a name which is still retained. They were a 

 favorite article of food with the ancients. The natives of Chili eat a very 

 large species, Bolanus i^sittacus.^ which is five and a half inches high by 

 three and a half in diameter ; it has much the taste of a crab. The young 

 of this species are attached to the adults, and in turn support their desendants, 

 so that they occur in large masses of fifty or a hundred individuals. 



Some genera, as Pyrgoma., are buried in coral ; Acasta inhabits sponges ; 

 (Jhelonobia is attached to turtles ; and Tuhidnella and Coronula {pi. 7G. 

 f,g. 53) are imbedded in the skin of whales. 



Glass 3. Cittstacea. 



In this class the sexes are separate; the body and limbs are distinctly 

 articulated ; the breathing is by means of gills, or more rarely (in some of 

 the lower forms) by the external surface. The larger forms, as the lobsters 

 and crabs, and the great majority of the smaller ones, inhabit the sea, where 

 they take the place of the insects which are so abundant on land. Some 

 species inhabit the fresh waters, and a few the land. The larger species 

 are many times the bulk of the largest insects, from which they decrease to 

 forms of microscopic size. In some of them the characters of the class are 

 so obscure that the}' have been placed with the parasitic worms. Named 

 from the hard integument, this aifords a prominent characteristic, being a 

 calcareous exterior skeleton of consideral)le thickness and strength in the 

 larger species; becoming more delicate, and often transparent in the 

 smaller ones, to disappear, or to escape observation in some of the obscure 

 forms. This covering is periodically cast off and renewed, like the integu- 

 ments of certain reptiles, and the larvas of insects. 



In comparing various members of this class {pi. 78), the number of 

 segments, and the consequent ability to bend the body, will be found to be 

 very variable, so that whilst the body of some {figs. 1-9) is a solid box 

 incapable of flexure, that of others is composed of a number of loosely 

 connected segments moving freely, and chiefly downwards from the 

 horizontal position. This allows some of the members of the class to roll 

 themselves into a ball by approximating the head and tail. 



The normal number of the segments in the body of the Crustacea is 

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