THE CRAYFISH 153 



closed chambers bounded by the body wall on the inside and 

 the carapace on the outside. Since the crayfish frequently 

 journeys from pond to pond in dry seasons, the gills are 

 thus protected from sudden drying. Water is drawn into 

 and is forced out of this gill cavity by the gill-bailers on the 

 second maxillae (Fig. 84, V, 3-4; Fig. 85). The blood flow- 

 ing into the gills from the ventral sinus gives up its carbon 

 dioxide waste and receives oxygen from the water. 



The green glands are prominent organs (Fig. 83) in the 

 ventral region of the body cavity in the head. A large artery 

 carries blood to them, and nitrogenous waste matter is 

 separated by them from the blood and is discharged from 

 the opening on the basal segment of each antenna. 



The Nervous System. The central nervous system of the 

 crayfish is much like that described for the grasshopper. A 

 double cord extends the length of the body. On this cord 

 there are seven enlargements, or ganglia, in the cephalo- 

 thorax and six in the abdomen. The "brain," or supraeso- 

 phageal ganglion, is a large mass of nerve tissue (Fig. 83).. 

 Pairs of nerves may be traced from the brain to the eyes, 

 the antennae, and the antennules. Two slender cords, simi- 

 lar to those described for the grasshopper, extend from the 

 brain, encircle the esophagus, and join the subesophageal 

 ganglion posterior to the mouth. This ganglion sends off 

 the nerves to some of the head somites and to some of the 

 thoracic somites. There are five other ganglia in the thorax 

 joined to each other (Fig. 83) and to the six ganglia in the 

 abdomen by the double nerve cord which is a continuation 

 of the cords that encircle the esophagus. 



The Muscular System. The muscles of the abdomen are 

 arranged in a very complicated fashion and are capable of 

 powerful action. The compact muscle bundles of the ab- 

 domen, or "tail," of the crayfish and also of the shrimp are 

 the chief parts used by man as food. In all parts of the body 



