CRUSTACEANS 



321 



at the thorax. The eyes are attached to " stalks" on the 

 head. All these appendages consist of joints, so that 

 they may be bent. The segments of the abdomen have 

 smaller appendages for use in swimming. The jointed 

 abdomen can itself be 

 bent very easily. Thus 

 the crayfish is able to 

 make a very swift 

 backward motion (to 

 "crawfish") by giving 

 its "tail" a downward 

 and forward stroke. 



The crayfish has powerful 

 jaws for seizing and tearing 

 food, and a large stomach 

 containing a tooth-like struc- 

 ture for crushing food. 



The blood is circulated 

 by a heart, which beats vig- 

 orously. The heart contains 

 only one chamber, and is set 

 into a receiving space. This 

 space contains blood brought 

 from the gills. When the 

 heart muscles contract, they 

 make the heart cavity smaller. The compressed blood in the cavity 

 then opens the valve leading to the arteries, and at the same time 

 closes the valve from the receiving space in which the heart is sus- 

 pended. When the heart muscles are relaxed, the pressure of the 

 blood in the arteries closes the valves opening out of the heart, and 

 the pressure in the receiving vessel opens the valves leading into the 

 heart. Hence the heart is refilled with blood that has passed through 

 the gills and been purified. This blood it pumps to all the organs 

 of the body. The gills are placed below the head-thorax covering, 



FIG. 262. The Lobster. 



