CLASSIFICATION OF CRUSTACEA. 1399 



potentially equal; and yet the anterior, A, is six times shorter and as 

 much narrower and lower than the following. It would not, there- 

 fore, be far out of the way to say, in mathematical language, that the 

 functional importance of the two parts varies inversely as the cubic 

 contents of the parts. 



We pass now to the degradations from this, the highest type. 



These degradations are seen 



First, in a widening of the space between the antennae. 

 Second, in a slight enlargement of the outer maxillipeds, so that 

 they do not fit snugly over the buccal area. 

 Third, in an elongation of the antenna?. 



These are all evidences of a slight relaxing of the concentrating 

 element. The first, marks the transition of the Maia group to the 

 Parthenopidae, and thence to the Cancridae. The second, carries the 

 grade a step lower, to species of the old genus Cancer, also to the 

 swimming crabs and the Corystoids; and the third, marks off the 

 Corystoids as the lowest of the true Brachyura. 



While there are such marks of degradation exhibited through the 

 growth or. elongation of parts, there is also a mark, equally significant, 

 in the obsolescence of the posterior thoracic- legs, a peculiarity of many 

 Grapsoids. In the Maioids, the species are well balanced ; the type 

 is perfect in its development : the sustaining of the central functions 

 allows of the full and complete growth of all the other parts. But 

 the diminution of force may not only be attended with a loosening of 

 the cephalic hold on the remoter of the cephalic organs, but also, in a 

 failure in the production of the posterior organs of the body, or those 

 on the outer limits of the system : and this is what happens in many 

 Grapsoids. The swimming form of the legs in Lupa and allied species 

 is a similar mark of inferiority. 



Besides the above evidences of degradation, there are still others in 

 the Brachyural structure, which act conjointly with the preceding, 

 producing lower grades of species. They are all marks of a relaxa- 

 tion of the centralization. 



Fourth. An enlargement or widening of the sternum and abdomen. 



Fifth. The abdomen becoming somewhat relaxed from the venter 

 instead of remaining close-appressed to it. 



Sixth. The vulvae becoming more remote from one another, being 

 situated in the bases of the third pair of legs, instead of the sternum. 



