32 THE ANIMALS AND MAN 



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 car- 

 bon dioxide 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 carbon dioxide and the taking 

 up of oxygen. 



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

 These pincers are the chela, 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 recur- 

 rence of similar parts in successive segments. This serial 

 repetition of parts among animals is called metamerism. 



Internal Structure (fig. n). — Technical Note. — With a pair 

 of scissors cut through the dorsal wall of the cephalothor^x 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 pentagonal sac, the 

 heart, contained within a delicate membrane, the pericar- 

 dium. Remove the pericardium and note a pair of dorsal 

 openings into the heart, called ostia. (There are also two 



