90 CRUSTACEANS 



82. Respiration. — Crayfish obtain oxygen from the water 

 by means of gills which are well covered by the overhanging 

 skeleton of the head-thorax region, but are really outside 

 of the body. Most of the gills are plume-like in shape 

 and are attached to the appendages, but some of them are 

 attached to the thorax. Water is made to circulate through 

 the gill chamber by means of the gill scoop or bailer. The 

 finely branched gill affords a large amount of surface for 

 the absorption of oxygen. 



83. Circulatory System. — The crayfish has a well-devel- 

 oped heart from which extend several arteries- that carry 

 blood to the various parts of the body. The blood returns 

 to the heart through veins and through several irregular 

 ducts called sinuses (si'-nus-es). As the blood flows 

 through the body it loses oxygen and receives carbon 

 dioxide. Fresh oxygen is absorbed by means of the gills 

 which, at the same time, pass off carbon dioxide from the 

 blood into the water. 



84. The Nervous System. — In the crayfish this is made up 

 of a brain, ventral nerve chain, and many nerves. The 

 eyes are borne on a pair of short movable stalks. The 

 special senses are well developed, and the sense of taste is 

 keener than that of most lower animals. 



85. Excretion. — The organs for excretion of waste are 

 the green glands that are found at the base of the antennas. 

 Blood going to these glands loses some of the waste which 

 it has gained in its course through the body. The method 

 of purification of the blood in these glands is much the 

 same as in the kidneys of the higher animals. 



86. Other Crustaceans. — Shrimps, lobsters, and crabs are 

 crustaceans of much economic importance, because of their 

 food value. The trade in these animals amounts to millions 

 of dollars each year. In order that these important food 

 animals may not become exterminated by careless and 



