14 J. 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. .À grand 
distinetion has been shown to consist in the different degrees of 
cephalization of the normal Crustacean structure. 'l'he conseera- 
tion of nine annuli, out of the fourteen cephalothoracie, 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. PorornrHBarMiA | Subtype I. Brachyura, 9 24 (and breadth, 24). 
or DECAPODA, IL Macroura, 36 (and breadth, 6). 
Type IL TzgrRADECAPODA,  - . - - 6 
Type IIL. ENroMosrRACA, : - - - 6-5 1 
(Type IV. CigRIPEDIA, - - : - : 6-5 1 
Type V. Roraronia, -05* - yd s 6-4 H 
The first type is alone in having true thoracic branchie, and 
pedicellate eyes. 
The second type has branchial sac-like appendages, either ab- 
dominal or thoracie, and sessile eyes. 
The third type has generally no branchiz, the surface of some 
pus or all of the body serving for aeration. .A few species, 
owever, 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 
inferjority ; for the system 1s 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 Mime and the 
fifth a lower state still in the absence of both thoracic and ab- 
dominal legs. 'lhe more degraded Macroura (certain Mysidze) 
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 
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