254 



METAZOAN PHYLA 



Supraesoph- 

 ' ageal going- 

 lion 



'Ci re u me soph- 

 age a I con- 

 nective 



- Esophagus 



organs. Other spaces around the ahmentary 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. 155) is in many 

 respects similar to that of the earthworm, 



,Subesoph- 

 -ageal gang- 

 lion 



-Sferncfl 

 artery 



- Ventral 

 nerve 

 cord 



Ommatjplium 

 Cornea 



Basement 

 membrane 



-Optic nerve 



Fig. 155. Fig. 156. 



Fig. 155. — The central nervous system of a crayfish (Astacus) . {From Lang, 

 'Text-book of Comparative Anatom,y," after Vogt and Yu7ig.) 



Fig. 156. — Diagrammatic representation of the eye of a crayfish. 



including 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 condensa- 

 tion of metameric ganglia in the subesophageal ganglion promotes 



