The Lobster as a type of Crustacea. 



13 



which, in the case of the body, are termed body-segments or 



" somites." 



A study of the various modifications of structure presented by Wall- 

 cases 

 Nos. 1-3. 



Crustacea and other Arthropoda has led to the conclusion that ^^^^^ 



they are to be regarded as built up of a series of somites or body- 

 segments, which may be distinct or soldered together, and each of 

 which bears typically a single pair of limbs or appendages. 



Thus, in the Lobster (Fig. 1), the hinder half of the body (or 

 abdomen) is plainly made up of six somites (besides a tail-piece or 



Cefihalothorax 



The Common Lobster (Ilomarn.s <j(i))imurns). Female, from the side. 

 [Wall-case No. l.J 



" telsoii "), each of which carries on the under side a pair of 

 " swimmerets." The front half of the body is not so divided, but 

 is covered by a large shield or " carapace " which projects between 

 the eyes as a toothed beak or " rostrum." Since, however, this 

 part of the body also bears a number of appendages constructed 

 on the same plan as the swimmerets of the abdomen, it is con- 

 cluded that here also we have to do with a series of somites, 

 although they are so completely fused together as to be indistin- 

 guishable except by their appendages. That this conclusion is 

 correct is proved by comparison with some of the lower Crustacea, 

 for instance, Anaspides (see Table-case No. 5), in which there is no 



