150 CRUSTACEANS. 



spiring through a watery medium, and thus adapted to a residence 

 in the recesses of the deep. Many species, it is true, are met with 

 abundantly in the fresh waters around us ; but these form rather 

 exceptions to the general rule, and we may fairly regard the 

 Crustaceans as marine representatives of the insects and spiders, 

 with which they form a parallel series. These animals are divided 

 into segments, typically twenty-one in number, of which seven 

 belong to the head, seven to the thorax, and seven to the abdo- 

 men. The segments, however, are generally consolidated or sol- 

 dered together in various degrees, so that in the great majority 

 of cases only a few of these divisions are obvious. Thus, in the 

 lobster, the whole head and thorax are united into one great 

 shield, on the under side of which, however, the divisions can be 

 traced ; in the crab the consolidation of these parts is still further 

 conspicuous, and the segments of the abdomen are small, and 

 folded up beneath the enormous thorax ; while in the remarkable 

 king-crab or "horse-foot" of warm climates (Limulus), the divi- 

 sions of the abdomen are lost, the body being covered by two 

 large shields, terminating in a long, sharp spine. The higher forms 

 of Crustaceans breathe by means of gills, and when these organs 

 are 'wanting, the integument of certain parts of the body, gene- 

 rally of the limbs, takes their place. In the crabs, lobsters, and 

 shrimps, respiration is effected by a number of brancJdce attached 

 to the bases of the locomotive limbs. The structure of these gills 

 is very curious : they consist of a central stem, to which are 

 attached numerous appendages, disposed like the bristles of a 

 brush, or else piled on each other like the leaves of a book, thus 

 presenting in the aggregate a very extensive surface for exposure 

 to the surrounding element. These organs are enclosed in two 

 large chambers situated on the sides of the thorax, into which 

 the water is freely admitted by a wide aperture situated between 

 the bases of the legs and the margin of the shell, and expelled 

 through another orifice placed at the side of the mouth. In 

 order to insure the renewal of the respired water, a valve of 

 beautiful construction is placed in the vicinity of the latter open- 

 ing, and attached to the root of the second pair of foot-jaws. The 

 construction of this valve is precisely that of the Archimedean 

 screw, or of the propelling laminae of a screw steamer, so that by 

 its every movement it drives a powerful current through the open- 

 ing in which it is situated. In other Crustaceans, such as the 

 mantis crabs, the gills have the form of bunches of feathers, and 

 instead of being enclosed within the thorax, float freely from the 



