68 Descriptive Zoology. 



higher still, arising from the side of the thorax, and called 

 the wall gills.) The lowest set, the foot gills, have leaflike 

 extensions along their borders, which perhaps serve to keep 

 the filaments of the gills from becoming entangled with 

 one another. The gills all have their free ends extending 

 upward. The direction in which the gills extend is in 

 keeping with the fact that the water which bathes the gills 

 enters about the bases of the legs, and, passing over the 

 gills, escapes near the anterior end of the cephalothorax, 

 back of the base of the antenna of each side. It has been 

 observed that the cephalic groove marks the distinction 

 between the head and the thorax. This groove also marks 

 the anterior limits of each gill chamber. In the extreme 

 anterior part of the gill chamber, extending obliquely up- 

 ward and backward, is the gill paddle or gill scoop. It is 

 a part of the second maxilla. It is attached by its middle 

 part, and each end is a somewhat spoon-shaped paddle. 

 By a constant back-and-forth motion this paddle continu- 

 ally bails the water out at the anterior end of the gill 

 chamber, and thus draws more water in at the lower border 

 of the gill cover, between the bases of the thoracic legs. 



Circulation in the Crayfish. The heart is situated in 

 the dorsal part of the cephalothorax. From its anterior 

 end arise five arteries : a single artery in the middle line 

 supplies the eyes ; back of this a pair run to the antennae ; 

 and, still further back, a pair lead to the digestive glands 

 of the two sides. At the posterior end of the heart arises 

 one artery, which almost immediately divides into two 

 branches; the first branch runs straight back, just above 

 the intestine, and is called the dorsal abdominal artery; 

 the other branch extends downward, passing through the 

 nerve cord. After passing through the nerve cord it again 

 divides into two branches, one running forward, the sternal 



