164 



LECTURE XIV. 



tacea manifest their peculiarlj^ distinctive forms by different combi- 

 nations 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- 89, c), seven more obviously enter 

 into the formation of the thorax (^j^)j and the 

 remaining seven constitute the abdomen or 

 tail (a h). 



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 con- 

 stituent segments than seven : these Crustacea 

 form the class Entomostraca, which is subdivided 

 into the Xiphosura, in which the last segment of 

 Cymothoa. the body forms a long three-edged sharp-pointed 



weapon, and into the Entomostraca proper. 



The Xiphosura, typified by the Limulus or Molucca crab, have the 

 head and thorax more completely blended together than in the true 

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

 are peculiarly distinguished from all other Crustacea by having the 

 office of jaws performed by the first joint of the thoracic legs, which 

 surround the mouth. The large cephalo-thoracic segment is protected 

 above and laterally by an expanded semilunar shield, obscurely di- 

 vided by two longitudinal impressions into three lobes, supporting 

 the organs of vision on their highest part. The tergal parts of the 

 segments of the second division of the body are also blended 

 into one trilobate clypeiform piece, their original separation being 

 indicated by the branchial fissures, and the number of the segments 

 by that of the lamelliform appendages attached to their inferior sur- 

 face. The termination of the intestine beneath the last segment of the 

 second division of the body of the Limulus proves that division to 



