BODY OF CRUSTACEA. DIVISION INTO SEGMENTS. 



233 



ring provided with a pair of legs (/?, pp,) : and at the posterior 

 part of the body we see an abdomen, also composed of seven seg- 

 ments (as), whose size 

 diminishes rapidly, 

 but whose form is 

 nearly the same as in 

 the thorax. In the 

 Crab (Fig. 458), 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 



FIG. 458. CANCKR PAOURUS, Linn., with the tail of the Q^JJgjJ ^Q CCLTdDCHCQ 

 male, a ; and of the female, b. * 



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 very recognisable thoracic rings, 

 and that the carapace is not a new part introduced instead of 

 the former, but merely the dorsal portion of one of the rings of 

 the head, so extremely developed that it has encroached upon 

 all the neighbouring rings. 



770. 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 Limnadias 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. 490) ; the Cypris (Fig. 495), 

 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. Lastly, we may advert to the 

 Lernem, which at their adult age present the strangest forms ; 

 but which, in the earlier part of their existence, possess a regular 

 annular structure (Figs. 501, 502). This comparative study of 



