200 CRUSTACEA. 



one in number, of which seven belong to the head, 

 seven to the thorax, and seven to the abdomen. 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 s,mall, and folded up beneath the enormous 

 thorax ; while in the remarkable king-crab or " horse- 

 foot " of warm climates (Limulus), the divisions 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, generally 

 of the limbs, takes their place. In the crabs, 

 lobsters, and shrimps, respiration is effected by a 

 number of branchiae, attached to the basis 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 en- 

 closed 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 opening, 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 



