62 CRUSTACEA. 



pace, between the Macroura and the higher Brachyura, 

 and this coincides also with its sluggish style of walk- 

 ing, long, weak appendages, and generally immature 

 characteristics, as compared with those of the more 

 perfect walking forms of the latter. 



That curious little parasite, the Oyster crab (Fig. 

 30), also belongs among the Brach- 

 yura, but can be more appropriately 

 described farther on. 



In the Brachyura we get the 

 highest expression of the concen- 

 tration of the regions, and not only 



r.n^tturrsoslrtu,,, {W ]m{ ft tffl] mQrc rc . mal kablc 

 (after Morse). Female. 



change, since the cephalothorax 

 tends to become broader from side to side. The gen- 

 eral concentration seems to be well accounted for by 

 the change of function from a swimming to a walking 

 type, but how shall we account for this lateral spread- 

 ing? The animal, having a narrow body, could not 

 move with as great rapidity as if it used the legs of one 

 side in pushing and those of the other in pulling the 

 body simultaneously. Any effort to do this would cer- 

 tainly cause a great increase in lateral development, and 

 materially alter the form, so that in its progress side- 

 wise it would offer as little resistance to the surround- 

 ing medium as the elongated bodies of animals which 

 habitually move forwards. In other words, the reason 

 why the crab crawls sidewise is, that it is the direction 

 in which its type can move with the greatest readiness 

 and acquire the greatest strength of limb and the maxi- 

 mum of speed. 



The history of the development of the Crustacea, in 



