54 A LOBSTER OR A CRAYFISH. 



c) Two distinct regions of the body, cephalothorax and abdo- 



men, the former consisting of an immovably fused head and 

 thorax. 



d) The large shield-shaped carapace covering the first-named region 



of the body and showing no traces of segmentation, except, perhaps 

 in a depressed line, the cervical groove, which marks the bound- 

 ary between head and thorax. Anteriorly this shield is prolonged 

 into a more or less pointed structure, the rostrum. The lateral 

 portion of the carapace, gill cover, is not attached at its ventral 

 border, as can be demonstrated by inserting a flat object. 

 Between it and the body wall proper is the gill chamber enclosing 

 eight groups of gills. These will be studied later. 



e) The abdomen is composed of a number of movable segments, the 



last of which is termed the telson. How many are there alto- 

 gether ? Each segment consists of a dorsal piece, the tergum, two 

 lateral continuations of the same, the pleura, and a ventral piece, 

 the sternum. 



f) The posterior opening of the alimentary canal, anus, on the ventral 



surface of the telson. 



Execrise i. Pin out a specimen in a natural position and make a dorsal 

 sketch of the animal, enlarging or reducing the scale to a total length of 

 three or four inches. Label all the parts. 



g) The abdominal appendages. Not counting the stalked eyes, there 



are nineteen pairs of appendages. Six of these, the swimmerets or 

 pleopods, belonging to the abdomen. In the male the first two 

 pairs are modified to form secondary sexual organs. In the 

 female the first pair may be rudimentary or wanting, while some 

 of the others are modified for carrying the eggs and young. The 

 last pair in both sexes, uropods, is much broader than any of the 

 others, and with the telson forms the swimming fin. The telson 

 has no appendages. Study the fifth pleopod and note that it con- 

 sists of a basal piece, the protopod, a lateral branch, the exopod, 

 and a median branch, the endopod. This biramous type can be 

 traced in most of the other appendages. 



