BODY OF CRUSTACEA. DIVISION INTO SEGMENTS. 



233 



FIG. 566. CANCER PAGURCS, Liim., with the tail of the 

 male, a , and of the female, b. 



ments (as}, whose size diminishes rapidly, but whose form is 



nearly the same as in 

 the thorax. In the 

 Crab (Fig. 566), on 

 the contrary, the head 

 is not separated from 

 the thorax ; and it 

 forms, with the whole 

 middle part of the 

 body, but a single 

 mass covered by a 

 large solid buckler 

 called the carapace. 

 The abdomen, at first, 

 escapes observation ; 

 for it is bent down 

 under the thorax, and 



is of small size. Yet it is easy to show, that in the Crab, as in 

 the Woodlouse, there are seven thoracic rings, and that the cara- 

 pace is not a new part introduced instead of the former, but 

 merely the dorsal portion of one of the rings of the head, so ex- 

 tremely developed as to encroach upon all the neighbouring rings. 

 845. In other animals of the same class, the general form of 

 the body differs still more widely from those of which we have 

 just spoken. Thus the Limnadice are inclosed between two oval 

 shields, joined like the valves of an Oyster, and it is only after 

 having raised this moveable cuirass, that we first perceive the 

 annular structure of the body (Fig. 598) ; the Cypris (Fig. 603), 

 which abounds in stagnant waters, presents a similar arrange- 

 ment : but the rings of which its body is composed are still 

 more difficult to recognise. The Lernece, which at their adult 

 age present the strangest forms, in the earlier part of their exist- 

 ence possess a regular annular structure (Figs. 609, 610). And 

 lastly, we may advert to the Cirrhopods, in which the characters 

 of annulose animals are so singularly masked, that for many years 

 they were placed amongst the Mollusca ; but which, on first 

 quitting the egg, are completely Crustacean in their character 



