METAZOA CRUSTACEA. 347 



those of the thoracic region, while their dorsal and lateral 

 parts extend backward to form the protecting carapace. 

 Since fourteen pairs of app'endages have been found at- 

 tached to the cephalothorax, this part is supposed to be 

 made up of as many segments, which with the seven ab- 

 dominal segments and the six pairs of appendages make 

 in all twenty-one segments and twenty pairs of appendages. 



The circulatory system of the lobster is shown in part 

 in the preparation (No. 847) ; portions have been cut 

 away on the ventral side, exposing the sternal artery 

 which gives off branches that pass to the limbs. 



Nephrops norwegius Leach, or the Norway lobster (No. 

 848), is a peculiarly delicate and graceful crustacean. 

 The cephalothoracic region is more slender than in most 

 genera and the abdominal segments are finely sculptured. 

 With the exception of the sixth pair of swimmerets the 

 appendages are likewise long and slender ; especially is 

 this true of the second pair of antennae which extend 

 backward beyond the body and are thread-like in struc- 

 ture. The eyes are large and kidney-shaped, while the 

 stalks are small. 



One of the fresh-water crustaceans which closely 

 resembles the salt-water lobster in structure is the cray- 

 fish, Cambarus acutus God. (No. 849). It is in fact a 

 miniature lobster in general appearance, having the same 

 number of segments and appendages. The position of 

 the legs when the animal is walking is shown in the 

 specimens. The body of the male (specimen on the 

 left) is smaller than that of the female (specimen on the 

 right), but his arms, especially the claws, are much 

 longer than hers. The fifth pair of legs are without 

 branchiae. The development of this fresh-water crusta- 

 cean is accelerated and the nauplius stage is passed 

 through in the egg, evidence of which is found in the 

 membrane or delicate cuticle that is formed and after- 

 ward shed, while the embryo is still in its case. 1 When 



1 Huxley, The Crayfish, 1880, p. 215. 



