298 LECTURE XIV. 



degree, so as more or less to obliterate the traces of their primitive 

 distinctness. The segments are variously proportioned and combined, 

 so as commonly to permit the entire body to be divided into three 

 parts, viz., the head, the thorax, and the abdomen ; and most of the 

 Crustacea manifest their peculiarly distinctive forms by different 

 combinations and proportions of the same number of primary rings 

 or segments. Each ring, again, consists of certain elementary parts, 

 which, by varying their proportions, contribute to the peculiar form 

 of the region of the body into the formation of which they enter. 



There may be distinguished in the annular segment of a Crus- 

 tacean, a dorsal arch and a sternal arch, each consisting of a median 

 and two lateral elements : the lateral elements in the upper arch 

 are called " epimeral," and in the lower one " episternal," pieces ; 

 the middle element above, or " tergum," consists of two pieces united 

 in the middle line, and that below, or the " sternum " has the same 

 structure. In a great proportion of the class the body consists of 

 twenty-one of these rings, of which seven are more or less blended 

 together to form the head {fig^ 129, c), seven more obviously enter 

 into the formation of the thorax {g, g), and the 

 remaining seven constitute the abdomen or tail 

 (a b)* 



The Crustacea, with seven thoracic and seven 

 abdominal segments, form the sub-class Malacos- 

 traca^ ; but a few large species and a very great 

 proportion of the smallest members of the class 

 have the thorax and the abdomen composed re- 

 spectively of a greater or a less number of consti- 

 tuent segments than seven : these Crustacea form 

 the sub-class Entomostraca. The best-marked 

 group, that including the largest species of this sub- 

 Cymothoa. class, is named Xiphosura, because the last seg- 



ment of the body forms a long three-edged sharp-pointed weapon : it 

 is typified by the Limulus, or Molucca crab, in which the head and 

 thorax are more completely blended together than in tlie true crabs, 

 which they resemble in the general form of the body ; but they are 

 peculiarly distinguished from all other Crustacea by having the office 



* CCXXXII. 



t In the larger species of Crustacea, where the chitine is combined with such 

 a proportion of carbonate and phosphate of lime as to be firm and brittle, it is, 

 as Aristotle has observed, less hard and less brittle than the shell of the moUusks, 

 ■whence that philosopher called those Testacea Ostracoderma, but gave to the 

 Crustacea the name oi Malacostraca, which name is still retained for that division 

 of the class which alone was known to the Greek naturalist 



