1404 CRUSTACEA. 



Each secondary type embraces types of more subordinate character, 

 which it is unnecessary here to dwell upon. 



There is a tendency in the lowest species to a transfer of the two 

 posterior mouth appendages to the foot series, so as to leave but seven 

 cephalic annuli ; but it is only a modification of the primary type, as 

 the species have every mark of being degraded or imperfect forms, 

 and are not examples of a new type. 



In this primary type, the species vary in length from half an inch 

 to twenty inches. Two inches may be set down as the average length 

 and breadth for the Brachyura; while three inches is the average length 

 of the Macroura, the average breadth being half an inch or less. 



The second primary type among Crustacea is as well defined in its 

 limits, and as distinct in its characters as the first. Instead of having 

 nine annuli devoted to the senses and mouth, there are but seven, the 

 mouth, including a pair of mandibles, two pairs of maxillce, and one 

 of maxillipcds. The number is permanent and characteristic. There 

 are, consequently, seven pairs of legs in these species, instead of five, 

 the Decapod number; and the species have been appropriately styled 

 the Tetradecapoda. Instead of exhibiting any appearance of imper- 

 fection, or any obsolescent organs, like those lower Macroura that 

 show a transition to a fourteen-footed structure, the organs are all 

 complete, and the whole structure is perfect in symmetry and unique 

 in character. They have not a Macroural characteristic. The eyes 

 are not pedicellate; there is no carapax, but a body divided into as 

 many segments as there are legs (whence our name Choristopoda) ; 

 the antennae, legs, and whole internal structure are distinct in type. 

 The branchiae are simple sacs, either thoracic or abdominal. 



We have, therefore, in the Tetradecapods an expression of that 

 structure of body, and that size, which belongs to a system, in which 

 but seven annuli or segments are concentrated in the cephalic portion 

 of the structure. The structure is far inferior to the Decapodan. 

 The size rarely exceeds two inches, though in extreme cases three to 

 four inches ; and probably half an inch is the average length. The 

 contrast between the first and second of the primary types, is therefore 

 as distinct in the average size of their structures, as in their actual 

 grade or rank. 



Superior rank among the Tetradecapods may be distinguished by 

 some of the same points as in the Decapods. The short antennae, 



