14 



Guide to Crustacea. 



Wall- 

 cases 

 Nos. 1-3. 



carapace, and the fore part of the body has eight distinct somites 

 each bearing a pair of walking legs. In front of these eight 

 somites, which form what is called the " thorax," is the " head," 

 a part of the body which is never, in any Crustacean, distinctly 

 segmented, but which, since it bears five pairs of appendages, 

 must contain at least five somites. The part of the body covered 

 by the carapace of the Lobster includes the head and the thorax 

 and is known as the " cephalothorax." It is necessary to remark, 

 however, that the regions of the body named head, thorax, and 

 abdomen in the Crustacea are by no means exactly equivalent to 

 those so named in the other Arthropoda, for instance in Insects, 



and still less to the parts bearing 



the same names among Vertebrate 



animals. 

 This 



body, or 



not only 



covering, 



Terpum. 



Pleu Tvn. 



Ajifiendage 



One of the abdominal somites of 

 the lobster, with its appendages, 

 separated and viewed from in 

 front. [Wall-case No. 1.] 



"segmentation" of the 

 division into somites, is 

 shown by the external 

 but affects some of the 

 internal organs as well. Leaving 

 these aside for the present, how- 

 ever, and considering only the 

 exoskeleton, the structure of a 

 typical somite will be best un- 

 derstood by examining one of the 

 separated abdominal somites of the 

 Lobster (Fig. 2). This consists of 

 a ring of shelly substance, connected 

 with the rings in front and behind by areas of thin membrane 

 which permit movement in a vertical plane. For convenience 

 of description the upper or dorsal part of this ring is called 

 the " tergum " (or " tergite ") and the under or ventral part the 

 " sternum " (or " sternite "). To the sternum are attached the 

 appendages (or swimmerets), while the tergum overhangs the 

 base of the appendage on each side as a flap called the 

 "pleuron." The terminal segment of the body or " telson " never 

 bears typical limbs, and on this account and also because of 

 its mode of development in the embryo, it is not regarded as a 

 true somite. 



The carapace of the Lobster is not formed simply by the terga of 

 several adjacent somites becoming soldered together. This is 

 shown by a comparison with some of the lower shrimp-like 

 Crustacea (Mysidacea, sec Table-case No. 5), in which the carapace 



