CRUSTACEAN METABOLISM 217 



energy store ; when it is exhausted, hpid reserves are drawn upon, while 

 tissue protein is utilized last of all. This picture is less well established for 

 invertebrates, but some insects appear to use protein quite as readily as 

 they use carbohydrate or lipid in fasting (Chauvin, 1949) . The presence of 

 glycogen in the digestive gland of crustaceans was first noted by Bernard 

 (1879); reserves of lipid in this organ were demonstrated by Cuenot 

 (1893). In 1922, Morgulis found reducing substances in the blood of 

 PanuUrus argus, and in 1923 he showed that added glucose disappears 

 from the blood. Hemmingsen (1925) performed similar experiments with 

 fresh-water crayfish and followed the time course of glucose disappear- 

 ance. He concluded that glucose is removed from the blood more rapidly 

 then it can be oxidized, and hence that regulatory mechanisms must exist. 

 In 1924 Verne showed that glycogen accumulates in the integumentary 

 tissues immediately before a molt, but is lacking from these tissues at other 

 times; as chitin is formed, glycogen disappears. He therefore concluded 

 that the glycogen serves as a precursor of chitin. Hoet and Kerridge ( 1926) 

 observed a decrease in muscle glycogen immediately before a molt, parallel- 

 ing an increase in the reducing value of the blood. Krishnan (1954) has 

 found fructose in the muscles of Carcinus maenas; the content is minimal 

 in the C stages, and rises as the molt approaches. 



Renaud (1949) made a very tho.rough study of the carbohydrate con- 

 tent of the tissues of Cancer pagurus in relation to the intermolt cycle. In 

 the digestive gland, the level is low following ecdysis, and continues to de- 

 cline immediately afterwards, reaching a steady low level after water ab- 

 sorption is completed (stage Bi). The carbohydrate content begins to in- 

 crease sharply after tissue growth is completed, reaching a maximum in 

 stage D2, when formation of the new integument begins, after which it 

 declines. The glycogen content of the hypodermis follows a similar course, 

 but reaches its maximum earlier, at the beginning of stage Di (resorption 

 of inner layers of old integument) . The reducing value of the blood (blood 

 sugar) rises steadily from a minimum at 24 mg. per 100 ml. immediately 

 after ecdysis to a maximum of 48 in stage Dj. This increase in reducing 

 value is the result of two factors — first, the decrease in water content of 

 the animal and hence of blood volume during the intermolt cycle, and, sec- 

 ond, the increase in nonfermentable reducing substances in the later stages 

 of the cycle. When the absolute amount of fermentable sugar in the blood 

 is calculated for an animal weighing 100 gm. immediately after molting, 

 it is found to decrease steadily from a value of 7.7 mg. in stage Ao to one 

 of 2.7 mg. in Do. The nonfermentable reducing substances increase from a 

 minimum of about 2 mg. glucose equivalents in stage C3 to a maximum of 

 1 1 mg. in stage D2. Both the fermentable and nonfermentable constituents 

 reach a maximum at the actual time of ecdysis. The glucosamine content. 



