468 



ZOOLOGY. 



at first escapes the sight, because it is folded under the thorax, 

 and is but small. Nevertheless, it is easy to demonstrate 

 that in the crab, as in the oniscus, there exist behind the 

 head seven easily recognisable thoracic rings, and that the 

 carapace is not a new organ created for the former, but 

 merely the dorsal portion of one of the rings of the head 

 which has acquired an extreme development, and encroached 

 on the neighbouring rings, In other animals of the same 

 class, the general form of the body is removed still further 

 from those of which we have just spoken. Thus the lim- 

 nadise are enclosed between two oval bucklers, united to- 

 gether after the manner of the valves of the oyster; and it is 

 after having removed this moveable cuirass, that the true 

 annulated structure of their bodies is observed (Fig. 519) ; 



Fig. 506. The Crab (Maia) Sea Spider. 



the cypris, abounding in stagnant waters, presents an analo- 

 gous disposition, only the rings of which their bodies are 

 composed are still more difficult to be recognised. Finally, 

 we may cite the lernsea, which at the adult age presents the 

 strangest forms (Fig. 155, 156), but which when young has 



