INTRODUCTORY NOTE TO THE CRUSTACEA. 
Wiruin a short time, the.class Crustacea has received a contribution of 
so valuable a character that I cannot omit to mention it specially in a note. 
I refer to the large and comprehensive work of Dana, published this year 
(1853). This work, aside from its high zoological value, includes anatomi- 
cal details and the discussion of principles in animal morphology, of great 
importance to the student of this interesting yet difficult class of animals, 
It will be found that constant reference has been made to the anatomi- 
cal details, in my notes ; but the doctrines advanced as to the morphological 
structure of these animals, more than equally important, could be here 
given only in a separate form. I have been the more induced to include 
‘them here, from the fact that the work in question will have a very 
‘limited circulation, comparatively, and can be accessible only to a few. 
With these views, I have solicited Professor Dana to put his particular 
principles into a condensed form for this work, and he has kindly favored 
me with the following account : 
The several types of structure among Crustacea are distinguished, pri- 
marily, by the different degrees of centralization or cephalization in the 
species, which degrees of cephalization are exhibited in the form of the 
body, and position, number, form or length of the appendages. The 
higher cephalization is seen in the larger number of organs that are 
pressed into the service of the senses and mouth; in the closely-crowded 
position and small size of these organs; in the little elongation of the 
antennae; and in the obsolescence of the abdomen and absence of abdominal 
appendages. Thus, in the Brachyura, nine segments and their pairs of 
appendages, out of the fourteen cephalothoracic, belong to the senses and 
mouth; they are all small, and gathered into a short space; the antennae 
are exceedingly small, excepting the basal joint which is the seat of 
sense; the abdomen in the males is small and without appendages. In the 
Maioids, the highest Brachyura, the head is very narrow, with the anterioz 
antennae longitudinal, and the base of the outer antennae soldered without 
suture to the shell. The concentration is here most complete. The wid- 
ening of the front in the Cancroids shows a relaxation of the concentration, 
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