ANNULOSA CRUSTACEA. 7 1 



regions head, thorax, and abdomen. In a typical speci- 

 men, it is composed of twenty-one somites. Seven of these 

 are assigned to the head, thorax, and abdomen, respectively. 

 The segments of the head and thorax are usually united 

 into a single mass, termed the cephalo thorax. The 

 skeleton is so varied, that no general description can be 

 given, which will apply to all the groups. The lobster 

 may be selected as a representative of the class. In this 

 animal the body is divided into two regions, which are 

 popularly termed the head and tail. The so-called 

 "head" is the cephalothorax, which is covered by a 

 large plate termed the carapace. The "tail" is the 

 abdomen, the joints of which are movable upon one 

 another. To the first segment of the head are attached 

 movable stalks which bear a pair of compound eyes. 

 The next two segments bear two pairs of antennae, called 

 respectively the antennules, and the great antennae. 

 The appendages of the mouth follow, and are of a very 

 complex character. There is a labrum or upper lip, a 

 labium or lower lip, a pair of biting jaws or mandibles, 

 and two pairs of maxilloB. One of these is furnished 

 with a spoon-like organ which is used in causing a 

 current of water to flow through the gill chamber. The 

 maxillse are followed by three pairs of maxillipedes, or 

 foot-jaws, one of which is attached to the last segment 

 of the head, the others to the first two segments of the 

 thorax. The maxillipedes are modified limbs, used as 

 masticating organs. The remaining five segments of the 

 thorax bear five pairs of walking legs. The first three 

 pairs are furnished at their extremities with nipping 

 claws. The front pair are much larger than the others, 

 and are termed the great claws. The two remaining pairs 

 of legs terminate in a single point, being without pincers. 

 To each of the segments of the abdomen, except the 

 last, paddle-like organs, called swimmerets, are attached. 

 The segment at the extremity of the body is termed the 

 telson (Gr. telson, a boundary), and is destitute of ap- 

 pendages. 



