582 EVOLUTION OF ANIMALS Part V 



ally accumulates in the large sternal sinus which appears like a coelom but is an 

 unlined blood cavity (hemocoel) (Fig. 29.12). It then flows into the gills 

 through thin walled incurrent vessels and out through excurrent ones. From 

 the gills the now fully oxygenated blood flows back through vessels (branchio- 

 cardiac) to the pericardium and the heart. 



Respiratory System. The plumy gills of crayfishes and lobsters are pro- 

 tected by the sides of the carapace that covers them like a jacket (Fig. 29.12). 

 They are washed by water bailed back over them by the scoops on the 2nd 

 maxillae and are moved by the legs and mouth parts to which they are attached 

 (Table 29.1 ). When water in an aquarium becomes too warm, the scoop beats 

 more rapidly. In response to the sparsity of oxygen in the warm water — the 

 crayfish is "out of breath." The same response is noticeable in lobsters caged 

 in tanks at summer lobster pounds. 



Excretion. At least for animals beyond the embryonic stage, crayfishes 

 and lobsters are unconventional in having kidneys, "green glands," in their 

 heads, each one of the pair opening on a basal segment of an antenna just 

 below the eye (Figs. 29.10, 29.11). Like all kidneys these are closely asso- 

 ciated with blood. They carry on the characteristic functions of kidneys, re- 

 move metabolic waste, and take part in keeping the water content of the body 

 normal. Each consists mainly of a sac crowded with blood vessels, minute 

 blood sinuses, all closely associated with the coils of microscopic kidney tu- 



CAROIAC 

 PORTION 



•STOMACH 



"■"^p'oflTION DIGESTIVE GLAND 



ANTENNA ^ ' ' """^ 



ANT^NNULE I BRAIN 



DORSAL ARTERY 

 INTESTINE 



Fig. 29.11. Internal structure of the male crayfish (very similar to that of the 

 lobster). The green gland is the secretory or working part of the kidney; the 

 "bladder" of the diagram refers to the urinary bladder that opens externally below 

 the eye. Note the sperm duct opening externally in the basal segment of the fifth 

 walking leg. Sperm cells are placed in the seminal receptacle of the female by the 

 slender first and second abdominal appendages here shown. These are easy recogni- 

 tion marks of male crayfishes and lobsters. (Courtesy, MacDougall and Hegner: 

 Biology. New York, McGraw-Hill Book Co., 1943.) 



