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J. D. Dana on the Classification of Crustacea. 



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 month, there are 

 but seven, the mouth, including a pair of mandibles, two pairs of 

 maxillae, and one of maxillipeds. 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. In- 

 stead of exhibiting any appearance of imperfection, or any ob- 

 solescent organs, like those lower Macroura that show a transi- 

 tion 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 di- 

 vided into as many segments as there are legs (whence our name 

 Choristopoda) ; the antennae, legs, and the whole internal struc- 

 ture 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 ce- 

 phalic 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, short compact bodies, and abbreviated abdomen of the 

 Isopods, are proofs of their superiority of grade. The abdomi- 

 nal appendages are simply branchial, and in the higher species 

 are naked or non-ciliated lamellae. The transitions to a lower 

 grade are seen in the elongation of these abdominal lamellae, 

 their becoming ciliated, and the abdomen being also more elon- 

 gated and flexible; then in the abdominal lamellae becoming 

 elongated natatory appendages, and the abdomen taking a length 

 usually not less than that of the thorax, as in the Amphipods, in 

 which the branchiae are appendages to the thoracic legs. And 

 while this elongation goes on posteriorly, there is also anteriorly 

 an enlargement of the antennae, which in the Amphipoda are 

 usually long organs. There are thus two secondary types of 

 structure among the Tetradecapods, as among the Decapods ; a 

 transition group between, analogous to the Anomoura, partakes 

 of some of the characters of both types, without being a distinct 

 type itself. These are our Anisopoda. The species graduate 

 from the Isopod degree of perfection to the Bopyri, the lowest 



