Prof. J. D. Dana on Cephalization. 193 



Insect, being much larger, and more numerous in segments and 

 members*. 



The Lobster (or any ordinary Macrural Decapod Crustacean) 

 has an elongate body, and an abdomen well developed and fur- 

 nished below with a full series of members. In the male Crab, 

 also a Decapod, the body is very short, and the abdomen is 

 without its members, besides being so small that it folds into a 

 groove in the under shell of the body : this diminution of size 

 and increased compactness are a consequence of the higher 

 cephalization of the species (Method 4). Passing from Crabs to 

 the still higher Articulates, Bisects, there is an example of this 

 cephalization carried to its maximum, — it appearing in the ex- 

 treme diminution of size of body and members, in the very 

 small distinct head (comprising, normally, a third of the seg- 

 ments of the body, though so small), and in the thorax freed 

 from the viscera and devoted mainly to locomotion. By this 

 method an animal is made of the highest instincts under the 

 Articulate type. 



From these examples it is evident that, where there is a com- 

 pacting of the body connected with rise in grade, it is not merely 

 a general compacting of the different parts alike, or a general 

 concentration and perfecting of the system, but a true cephaliza- 

 tion of the system, — the compacting and perfecting showing it- 

 self primarily in a greater concentration, predominance, and 

 domination of the cephalic extremity. 



Among Articulates having feet, an Insect and a Limulus stand 

 at the opposite poles of cephalization. The mouth-organs and 

 feet in both correspond to those of the head (or the mouth- 



* There appears to be no reason to doubt that in all types, not degrada- 

 tional, each pah* of members (Avings excluded) corresponds to a separate 

 normal segment of the body. Audouin and Edwards are sustained in their 

 views on this point by the fact that, in a Sqidlla, three anterior cephalic 

 segments (those of the eyes and two pairs of antenna) and four posterior 

 thoracic are actually distinct ; and in an Erichthiis, other segments, ante- 

 rior to these four, are faintly indicated. (See the author's Expl. Exped. 

 Report on Crustacea, pi. 41.) 



Assuming the number of normal segments anterior to the mouth in an 

 Articulate from that (three) in the head of a Crustacean, the complete 

 number in an Insect is eighteen, and in a Crustacean twenty-one, three 

 abdominal being present which are obsolete in an Insect. In the former, 

 half (or nine) pertain to the head and thorax (onl)'^ three to the thorax) ; 

 in the latter, two-thirds (or fourteen), the rest being abdominal. In an 

 Insect, the viscera are abdominal; in a Crustacean (excepting some degra- 

 dational forms), thoracic. The separation of the viscera from the thorax 

 in an Insect leaves this part to higher purposes. It is to be noted that the 

 tenth to the fourteenth segments, inclusive, are visceral segments in both 

 Insects and Crabs, — being the first part of the abdomen in an Insect, and 

 the last (and lai'ge-foot-bearing) part of the cephalothorax in Crabs. 



