CRAYFISH 



259 



Supraesoph- 

 -a^ecpil gang- 

 lion 



-Circumesoph- 

 ageal con- 

 nective 



~ Esophagus 



,Subesoph- 

 -ageaf gang- 

 lion 



organs. Other spaces around the alimentary canal which contain blood 

 and form what is known as a hemocoel are not truly coelomic. 



The alimentary canal consists of a buccal cavity, esophagus, stomach, 

 and intestine. The stomach is divided into two portions, one grinding 

 and the other digestive in function. Between 

 the two portions is a strainer composed of hairlike 

 setae which permits the food to pass only when 

 it has been ground into very fine particles. 



The circulatory system includes the heart, 

 seven arteries leading to various parts of the 

 body, and spaces in the tissues called sinuses 

 which communicate with a large space around the 

 heart known as the pericardial sinus. Valves are 

 present in the arteries and also guard the open- 

 ings from the pericardial sinus into the heart. 



The excretory organs of the crayfish are a 

 pair of bodies known as green glands situated in 

 the ventral part of the head in front of the 

 esophagus, the ducts from which open through 

 papillae on the basal segments of the antennae. 



The crayfish possesses a well-developed mus- 

 cular system, the muscles being attached to the 

 various portions of the exoskeleton. 



The nervous system (Fig. 165) is in many 

 respects similar to that of the earthworm, includ- 

 ing a supraesophageal ganglion; circumesophageal 

 connectives; a subesophageal ganglion, representing 

 a fusion of the ganglia of the metameres from III 

 to VII; and a ventral gangliated nerve cord with 

 ganglia in each segment posterior to the seventh. 

 This condensation of metameric ganglia in the 

 subesophageal ganglion promotes coordination of 

 all of the appendages used in connection with the 

 process of food taking. The largest sense organs 

 of the crayfish consist of a pair of eyes and a pair 

 of statocysts. Tactile organs also are well devel- Fig. 165.— The central 

 oped in different parts of the body, particularly ^Th" uTacT«)°' Vr7m 

 upon such parts as the chelae of the walking legs, Lang, "Text-book of Com- 

 the mouth parts, the ventral surface of the abdo- "v^^^'Za YungT'''' "^^"' 

 men, and the edge of the telson. There also seems 

 to be a general distribution of organs for the perception of chemical stimuli. 



Crayfishes are diecious. The vasa deferentia open on the median 

 side of the base of each last walking leg. The openings of the oviducts 

 are at the base of each third walking leg. 



-Sternal 

 artery 



- Verrtrat 

 nerve 

 cord 



