10 J. D. Dana on the Classification of Crusiacea. 
. The second primary type among Crustacea is as well bens in 
its limits, and as distinet 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 peirs of 
maxille, and one of maxillipeds. 'lhe number is permanent 
and characteristic. 'lhere are, consequently, seven pairs of legs 
in these species, instead of five, the Decapod number; and the 
specles 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 à 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 antennz, legs, and the whole internal struc- 
ture are distinct in type. 'The branchis are simple saes, 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 1n the ce- 
phale portion of the strueture. '"lhe structure is far inferior to 
the Decapodan. "The size rarely exeeeds two inches, though in 
extreme cases three to four inches; and probably Aalf am ?nch 1s 
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 'T'etradecapods may be distinguished 
by some of the same points as in the Decapods. "The short 
antennz, short compact bodies, and abbreviated abdomen of the 
Isopods, are proofs of their superiority of grade. 'lhe abdomi- 
nal appendages are simply branchial, and in the higher species 
are naked or non-ciliated lamelle. "The transitions to a lower 
grade are seen in the elongation of these abdominal lamellse, 
their becoming ciliated, and the abdomen being also more elon- 
gated and flexible; then in the abdominal lamelle 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 branchiz are appendages to the thoracic legs. .And 
while this elongation goes on posteriorly, there is also anteriorly 
an enlargement of the antenns, which in the Amphipoda are 
usually long organs. "lhere 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 distinet 
iype itself. 'lhese are our Anisopoda. 'lhe species graduate 
from the Isopod degree of perfection to the Bopyri, the lowest 
« ruis did 
