/. D. Dana on the Classification of Crustacea. 17 



The most perfect state of it is seen in the Maia group, (the tri- 

 angular crabs,) in which the bases of the antennas and eyes are 

 crowded into the narrowest possible compass, and the mouth, 

 organs are well compacted within the buccal area, and the legs 

 and whole system have the highest completeness. 



The form of the body of a Maia is a somewhat flattened ovoid, 

 narrowest in front ; and the middle point between the mouth 

 and the second antennas, which we call the potential centre of 

 the animal, is situated near the front, say about half an inch 

 from the front outline (excluding the beak), supposing the ceph- 

 alothorax three inches long. We may call the part anterior to 

 this centre, A ; the part posterior, B ; and the length of the for- 

 mer, measured on the axis, a; of the latter, b. These parts 

 may be viewed, as regards development, as potentially equal ; 

 and yet the anterior, A, is six times shorter and as much nar- 

 rower and lower than the following. It would not, therefore, 

 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 antennas. 



These are all evidences of a slight relaxing of the concentrat- 

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

 to the Parthenopidas, and thence to the Cancridas. 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 sus- 

 taining 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 Grrapsoids. 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 



SECOND SERIES, VOL. XXII, NO. 64. — JULY, 1856. 3 



