208 



COLLEGE ZOOLOGY 



Carapace 



Pericardial sinus 



Heart 



Branchiostegite 



Muscle 



Intestine 



Digestive 

 gland — 



Podobranch 



Arthrobranch 



Ventral 

 nerve cord 



Skeleton 



Coxopodite 



Basipodite 



Branchio- 



cardiac 



canal 



Efferent 



channel 



Afferent 



channel 



Sterna! sinus 



Artery to leg 



Figure 116. Cross section of a crayfish showing arrangement of the gills and some of the 

 internal organs. 



branchial filaments. Crayfishes do not drink 

 when in water; the water diffuses through 

 the gills into the body. 



Excretory system 



The excretor}- organs are a pair of rather 

 large bodies, the green glands (Fig. 114), 

 situated in the ventral part of the head an- 

 terior to the esophagus. Each green gland 

 consists of a glandular portion which is 

 green in color, a thin-walled dilatation, the 

 bladder, and a duct opening to the exterior 

 through an excretory pore on the basal seg- 

 ment of the antenna. 



Nervous system 



The general structure of the nervous sys- 

 tem (Fig. 114) of the crayfish is in many 

 respects similar to that of the earthworm but 

 further developed in the head and thorax. 



The central nervous system includes a dorsal 

 ganglionic mass, the brain ( supraesophageal 

 ganglia), in the head, and two circume- 

 sophageal connectives which pass to the 

 subesophageal ganglion. This is the most 

 anterior ganglion of the ventral nerve cord. 

 Following the subesophageal are 6 ganglia 

 in the thorax and 6 ganglia in the abdomen, 

 all joined to each other by the ventral nerve 

 cord. 



The brain is a compact mass, larger than 

 that of the earthworm, which supplies the 

 eyes, antcnnules, and antennae with nerves. 



The ganglia and connectives of the ventral 

 nerve cord are more intimately fused than 

 in the earthworm; it is difficult to make out 

 the double nature of the connectives except 

 between thoracic ganglia 4 and 5, where the 

 sternal artery passes through. 



The visceral nervous system consists of 



