CRUSTACEANS 219 



Digestion. — Food which is not ground up into pieces smah enough for 

 the purpose of digestion is still further masticated by means of three teeth, 

 strong projections, one placed on the midline and two on the side walls of 

 the stomach. The exoskeleton of the crayfish extends down into the 

 stomach, thus forming the gastric mill just described. 



The stomach is divided into anterior and posterior parts separated fioni 

 each other by a constriction. The posterior part is lined with tiny pro 

 jections from the wall which make it act as a strainer for the food passing 

 through. Thus the unbroken particles of food are kept in the anterior end 

 of the stomach. Opening into the posterior end of the stomach are two 

 large digestive glands which further prepare the food for absorption through 

 the walls of the intestine. Once in the blood, the fluid food is circulated 

 through the body to the tissues which need it. 



Nervous System. — The internal nervous system of a crayfish consists of 

 a series of collections of nerve cells {ganglia) connected by means of a 

 double line of nerves. Posterior to the gullet this chain of ganglia is found 

 on the ventral side of the body, near the body wall. It then encircles the 

 gullet and forms a brain in the head region, the latter formed from several 

 ganglia which have grown together. From each of the ganglia, nerves pass 

 off to the sense organs and into the muscles of the body. These nerve 

 fibers are of two sorts, those bearing messages from the outside of the body 

 to the central nervous system (these messages result in sensations), and 

 those which take outgoing messages from the central nervous system (motor 

 impulses), w^hich result in muscular movements. 



Development. — The sexes in the crayfish are distinct. The developing 

 eggs, which are provided with a considerable supply of food material, are 

 glued fast to the swimmerets of the mother, and there develop in safety. 

 The young, when they first hatch, remain clinging to the swimmerets for 

 several weeks. 



Excretion of. Wastes. — On the basal joint of the antennae are found 

 two projections, in the center of which are found tiny holes. These are the 

 openings of the green glands, organs which have the function of the elimina- 

 tion of nitrogenous waste from the blood, the function of the human kidneys. 



Characters of Crayfish and its Allies. — Our study of crayfish shows 

 us that animals belonging to the same group as itself have several 

 well-marked characteristics. The most important are the presence 

 of a segmented limy exoskeleton, gills, jointed appendages, a pair 

 to each segment of the body (except the last) ; and, as we shall see 

 later, they pass through a metamorphosis or change of form before 

 they reach the adult state. We find that the Crustacea fall 

 naturally into two classes, those in which the number of pairs of 

 appendages is indefinite, and those in which the number is fixed 



