134 GENERAL ZOOLOGY 



gills (Fig. 69, 4) are attached to and near the basal joints of 

 the thoracic appendages, and extend dorsally into partially 

 closed chambers bounded by the body-wall on the inside and 

 the ventral extension of the carapace on the outside. Since 

 it is sometimes necessary for the crayfish to make journeys 

 from pond to pond in dry seasons, the gills are thus protected 

 from sudden drying. Water is drawn into and out of this 

 gill-cavity by means of the action of the gill-bailers on the 

 second maxillae already referred to (Fig. 68, V, 3-4 ; Fig. 69, 1). 

 The blood flowing 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. 67, 8) in the 

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

 carries blood to them, and nitrogenous waste matter is sepa- 

 rated by them from the blood' and finds its way to the opening 

 on the basal segment of the antennae. 



The Nervous and Muscular Systems. The "brain," the 

 supraoesopliageal ganglion, of the crayfish is a mass of nervous 

 tissue resulting from the aggregation of several pairs of ganglia 

 (Fig. 67, 33). Pairs of nerves may be traced to the eyes, the 

 antennas, and the antennules. Two slender connectives, simi- 

 lar to those described for the locust, extend from the brain, 

 encircle the oesophagus, and join the sulxxsopliageal ganglion 

 (Fig. 67, 34) posterior to the mouth. This ganglion, too, is 

 really a combination of several pairs of ganglia, which send 

 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 (first thoracic ganglion, Fig. 67, 35), and to 

 the six ganglia in the abdomen, by the double nerve-cord which 

 is a continuation of the connectives that encircle the oesophagus. 



The muscles of the abdomen are arranged in a very com- 

 plicated fashion and are capable of powerful action. In all 

 parts of the body muscles are attached to the inner surface 

 of the exoskeleton. 



