THE CRAYFISH 235 



the fluid food is circulated through the body directly to the tissues 

 which need it. 



The gills. The plume-like gills are outside of the body, but are 

 kept moist by being well protected by the overhanging carapace. 

 The blood of the crayfish passes by a series of veins into the long 

 axis of the gill, where the blood vessels divide into very minute 

 tubes, the walls of which are extremely delicate. Oxygen, dis- 

 solved in the water, passes into the blood by osmosis, during which 

 process the blood loses some carbon dioxide. 



Circulation. The circulation of blood takes place in a system 

 of thin- walled open vessels which allow the blood to come in direct 

 contact with the tissues. The heart lies on the dorsal side of the 

 body, inclosed in a delicate bag (see diagram). 



Excretion of wastes. On the basal joint of the antennae are 

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

 are the openings of the green glands, organs which eliminate the 

 nitrogenous waste from the blood, corresponding to the human 

 kidneys. 



Practical Exercise 7. Study the diagram on page 234 and make a diagram 

 of a cross section through a crayfish in the region of the walking legs. Ex- 

 plain how a crayfish might become aware of the presence of food. How might 

 it catch living prey? 



Nervous system. The internal nervous system of a crayfish 

 consists of a series of collections of nerve cells called ganglia 

 (gan' gli a), connected by means of a nerve cord. Posterior to 

 the gullet, this chain of ganglia is found on the ventral side of 

 the body. At the anterior end it encircles the gullet and forms a 

 brain in the head region. 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, which result in muscular 

 movements. 



Life history. The sexes in the crayfish are distinct. The eggs 

 as they pass to the outside of the body of the female are fertilized 

 by the sperm cells. The eggs, which are provided with yolk 



