THE CRAYFISH 21 



the blood is purified. The blood of the crayfish flows into 

 the large vessels on the outer sides of the gill and thence 

 into the fine vessels in the little leaf-like lamellae. At the 

 same time the air which is mixed with the water bathing 

 the gills passes freely through the thin membranous walls 

 of these lamellae and blood-vessels, and the blood gives 

 off its carbonic acid gas to the water and takes up oxygen 

 from the air in the water. Thus it will be seen that the 

 office of the gill is like that of the lung in the toad, 

 namely, to act as an organ for the elimination of carbonic 

 acid gas and the taking up of oxygen. 



Note the pincer-like appendages of the first pair of legs. 

 These pincers are the chclce, with which food is torn into 

 bits and placed in the mouth. In the basal segment of 

 each of the last pair of legs of the male note the genital 

 pore. In the female the genital pores are in the basal 

 segments of the next to last pair of legs. Is the crayfish 

 bilaterally symmetrical ? Note the repetition of parts in 

 the crayfish, that is, the recurrence of similar parts in 

 successive segments. This serial repetition of parts among 

 animals is called metemerism. 



Internal Structure (fig. 4). TECHNICAL NOTE. With a pair 

 of scissors cut through the dorsal wall of the cephalothorax into the 

 body-cavity. Cut the body-wall away from both sides and remove 

 the middle portion. 



At the anterior end of the cephalothorax note the large 

 membranous sac, the stomach. Attached to each end of 

 this are sets of muscles which control its movements. 

 To the right and left of the stomach notice attached to 

 the shell large muscles which connect by stout ligaments 

 at their lower ends with the mandibles. Note a yellow 

 fringe-like structure, the digestive gland, which fills most 

 of the region about the stomach. It connects by a pair 

 of small tubes, the bile-ducts, with the alimentary canal. 

 Within the posterior portion of the cephalothorax note a 



