46 CRUSTACEA. 



two, partaking neither of the typical characters of the former or latter, 

 a number of forms are arranged, that are as stepping-stones from one 

 to the other. A subdivision into a higher, and a lower, and a transi- 

 tion group, is therefore, true to nature and convenient to the mind. 



It is of great interest to trace out these relations; and, in order to 

 appreciate their true value, we must first comprehend in what way, 

 or by what characteristics, superiority of grade is exhibited. Concen- 

 tration in the nervous system, has been well shown to be the basis of 

 it, and simplicity (under certain limitations) in number of external 

 members or parts, its exhibition. Prof. Agassiz has shown that the 

 larger number of articulations in the body, and of limbs appended to 

 them, form a mark of inferiority of grade. This characteristic is a 

 mark of the vegetative quality prevailing over the animal. The 

 caterpillar, with its long, many-jointed body and numerous legs, is an 

 inferior grade or condition of the butterfly; in the former, the abdo- 

 minal and digestive part predominates ; in the latter, the cephalo- 

 thoracic, or those parts by which the higher functions of the animal 

 are performed, and the head is a single centre, in which the senses 

 and organs of the mouth are closely grouped. 



The Eubranchiata, among Crustacea, afford another illustration of 

 this principle. It is well known, since the investigations of Thomp- 

 son and Rathke, that the imperfect Crab has the long tail of the 

 Macroura. The fact, that the Macroura are a lower grade, is there- 

 fore obvious from this analogy. Like the larvae among insects, the 

 abdominal portion of the body is largely developed and furnished with 

 appendages; and usually this portion far exceeds in extent the cepha- 

 lothoracic. But, in the Crab, the abdomen is reduced to its minimum, 

 and in males is memberless ; the whole force of the system is concen- 

 trated in the cephalothorax ; and, even the nervous ganglia of the 

 members, as Edwards and Audouin have well exhibited, are gathered 

 into a single mass, while they are like distant knots in a long cord in 

 the Macroura. 



But there are other marks of superiority beside these well-known 

 and often-mentioned facts; and the additional points to which we now 

 allude must be understood, before we can explain the gradations 

 among the Anomoura. 



It is one distinction between Crustacea and true insects, that the 

 former have no proper head. Among those Crustacea that rank highest, 

 we ought to find the nearest approximation to the concentrated or 



