INTRODUCTORY NOTE TO THE CRUSTACEA. 



Within 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 diflBcult 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 mqrphological 

 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 cephalotKoracic, 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 anterior 

 antennae longitudinal, and the base of the outer antennae soldered vMhxnit 

 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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