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



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 consecra- 

 tion of nine annuli, out of the fourteen cephalothoracic, to the 

 senses and mouth, distinguishes the highest type ; of seven, the 

 second type ; of six or five, 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 

 nuli. lines. 



Type I. Podophthalmia ) Subtype I. Brachyura, ? q (24 (and breadth, 24). 



or Decapoda, [ II. Macroura, ) (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 ^ 



The first type is alone in having true thoracic branchias, and 

 pedicellate eyes. 



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

 dominal or thoracic, and sessile eyes. 



The third type has generally no branchise, 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 Ento- 

 mostraca. 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 sur- 

 face 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. 



The third and fourth types show their inferiority to the second, 

 by the absence of a series of abdominal appendages ; and the 

 fifth a lower state still, in the absence of both thoracic and ab- 

 dominal legs. The more degraded Macroura (certain Mysidge) 

 show a transition in this obsolescence of abdominal organs to the 

 third type. 



Some of the conclusions from these facts are the following. 



I. Each type corresponds to a certain system of force more or 

 less centralized in the organism, and is an expression of that 

 force, — the higher degree being such as is fitted for the higher 



