190 ANIMALS WITH JOINTED LEGS— THE ARTHROPODS 



head region. The green glands are excretory organs somewhat like ne- 

 phridia. Each gland consists of a ventral glandular portion which ex- 

 tracts the wastes of metabolism from the blood, a thin-walled bladder 

 which stores the waste, and a short tube which empties the wastes to the 

 outside through a small pore on the base of the antenna. 



Circulatory System. Circulation in the crayfish is called the open 

 type system, where blood flows through sinuses in the body, in contrast 

 to the closed type system, where the blood does not leave the blood ves- 

 sels in its circuit around the body. The earthworm and vertebrates have 

 the closed type of system, whereas the fresh-water clam has a combina- 

 tion of both. The heart lies just dorsal to the anterior portion of the 

 intestine. It connects to arteries, which lead from it in all directions; 

 there are no veins. Blood is taken into the heart from the surrounding 

 pericardial sinus through three pairs of openings in the heart, the ostia. 

 When the heart expands, blood flows in through the ostia ; when the 

 heart contracts, valves close the ostia so that the blood cannot pass out 

 through them, but flows out into the arteries instead. The main arteries 

 form smaller branches which empty the blood into the body sinuses. 

 The most important of these is the sternal sinus, from which the blood 

 flows out into the gills for respiration and then flows back into the peri- 

 cardial sinus to repeat the circuit of the body. The blood contains cor- 

 puscles, but is clear because there is no hemoglobin. There is an oxy- 

 gen-absorbing substance in the plasma, hemocyanin, which is colorless 

 in the body, but turns blue when removed and allowed to stand awhile. 



Muscular System. The body of the crayfish is liberally supplied with 

 muscles which move the body and its appendages. The crayfish can 

 crawl along rather slowly in any direction, using its walking legs, but if 

 it really gets in a hurry it darts backward for several feet so rapidly that 

 it can hardly be seen. It is able to do this because the greater portion 

 of the abdomen is filled with a strong flexor muscle. When the fan-like 

 portion at the rear of the body is suddenly pulled forward against the 

 water by the flexing of this muscle, the animal is jerked backward at 

 great speed. There is a much smaller extensor muscle lying on top of 

 the flexor which extends the abdomen. 



Nervous System. This system is very much like that found in the 

 annelids. The brain is in the dorsal part of the head region, and two 

 circumesophageal connectives circle the esophagus and join ventrally to 

 form the nerve cord. There are enlargements of the nerve cord, ganglia, 

 in the different segments, and the sensory and motor nerves are given 

 off from the ganglia. The ganglia in the second through the sixth seg- 

 ment are fused to form one large subesophageal ganglion, but the re- 



