ARTHROPODA 



99 



A typical crustacean consists of twenty segments, of 

 which five belong to the head, eight to the thorax, and 

 seven to the abdomen. 18 In the lobster, however, as in 

 all the higher forms, the joints of the head and thorax 

 are welded together into a single piece, called the 

 c^pkato thorax. On the front of this shield is a pointed 

 process or rostrum; and attached to the last joint of 

 the abdomen (the so-called " tail ") is the sole repre- 

 sentative of a tail 

 the telson. The 

 skeleton is a mix- 

 ture of chitin and 

 calcareous matter. 19 

 On the under side 

 of the body we find 

 numerous appen- 

 dages, feelers, jaws, 

 claws, and legs be- 

 neath the cephalo- 

 thorax, and flat 

 swimmerets under 

 the abdomen. In 

 fact, every segment 

 except the last, car- 

 ries a pair of mov- 

 able appendages, 

 consisting typically 



Of a Stalk Or protO- FIG. 54. - Under side of the Crayfish, or Fresh-water 



. ' Lobster (Astacus ftnviatilis} : a, first pair of an- 



Podlte, bearing tWO tennse; b, second pair; c, eyes; d, opening of 



v i ,1 kidney; e, foot jaws; f, g, first and fifth pair of 



branches, the eXOpO- thoracic legs; A, swimmerets; i, anus; k, caudal 



dite and the endopo- fin - 



dite. The five segments of the head are compressed 

 into a very small space, yet have the following mem- 

 bers : 2 the short and the long antennae ; the mandibles, 

 or jaws, between which the mouth opens; and the 



k 



