CLASSIFICATION OF CRUSTACEA. 14Q9 



a slight transformation of some Entomostraca an obliteration of the 

 legs and substitution of locomotive cilise would almost turn them 

 into Rotatoria. 



In the classification which has been developed, we have made out 

 five primary types of structure among Crustacea. A grand distinction 

 has been shown to consist in the different degrees of cephalization of 

 the normal Crustacean structure. The consecration of nine annuli, 

 out of the fourteen cephalo thoracic, to the senses and mouth, distin- 

 guishes the highest type ; of seven, the second type ; of six or jive, the 

 third and fourth; of five or four, the fifth. In connexion with other 

 distinctions in these types, we find that they correspond to structures 

 of different size, the size being directly related to the grade. These 

 particulars may be tabulated as follows : 



Typical number Mean normal length, in 



of cephalic an- twelfths of inches or 



null. lines. 



% 



Type I. PODOPHTHALMIA 1 Subtype I. Brachyura, ) g ( 24 (and breadth, 24). 



or DECAPODA, j II. Maeroura, j 1 36 (and breadth, 6). 



Type II. TETRADECAPODA, .... 7 6 



Type III. ENTOMOSTRACA, . . . 6-5 1 



Type IV. CIRRIPEDIA, .... 6-5 1 



Type V. ROTATORIA, . . . . 5-4 J 



The first type is alone in having true thoracic branchiae, and pedi- 

 cellate eyes. 



The second type has branchial sac-like appendages, either abdo- 

 minal or thoracic, and sessile eyes. 



The third type has generally no branchige, the surface of some part 

 or all of the body serving for aeration. A few species, however, are 

 furnished with special organs for this function. This is, however, no 

 mark of superiority in such species, for they occur even in the Limuli, 

 among the lowest of the Entomostraca. The necessity of them in 

 this case arises from the abnormal size of the species, both the mark 

 and occasion of its inferiority; for the system is thus too large for 

 the mode of surface aeration, found among ordinary Entomostraca; 

 moreover, the shell, which so large an animal possesses and requires 

 for the attachment of its muscles and its movements, is thick and 

 firm, and this is inconsistent with aeration by the exterior surface of 

 the body. The same remarks apply to the liver glands, which are 

 very small or wanting in the small species. 



353 



