286 THE ANATOMY OF INVERTEBRATED ANIMALS. 



cord immediately behind the oesophagus (Fig. 75, c). The 

 size and form of the anterior thoracic ganglion would lead to 

 a suspicion of the complex nature which development shows 

 it to possess. It supplies the somites and their appendages 

 from the fourth to the ninth inclusively, and sends forward 

 delicate filaments to the oesophagus. 



Posteriorly it is connected with the ganglionic mass of 

 the tenth somite by two commissures, and the other thoracic 

 ganglia are similarly brought into communication, the com- 

 missures of the ultimate and penultimate only being remark- 

 able for their brevity. The abdominal, which are much 

 smaller than the thoracic ganglia, are, with the exception of 

 the last two, united by single cords, which represent coa- 

 lesced double commissures. Each of these ganglia supplies 

 the muscles and the appendages of the somite to which it 

 belongs, and the posterior abdominal ganglion sends branches 

 into the telson. 



The Crayfish possesses a remarkably well-developed sys- 

 tem of visceral or stomatogastric nerves, which has been the 

 subject of special study by Brandt, Milne-Edwards, Krohn, 

 and Schlemm, each of whom has described a larger or smaller 

 portion of the system with accuracy, but has omitted to men- 

 tion, or has denied, the existence of some other part. Each 

 of the great commissures (Fig. 75, 5), as it passes over the 

 sides of the oesophagus, becomes slightly swollen, and from 

 the enlargement four nerves arise ; one, external, passes 

 toward the mandibular muscles ; a second postero-lateral 

 branch (Fig. 75, g) runs upward and backward to the infero- 

 lateral regions of the stomach, and eventually enters into the 

 composition of the hepatic nerve (k) ; a third branch (f) 

 turns directly inward and upward, and unites upon the 

 oesophagus with its fellow and with an azygos nerve (d), 

 which passes up in the middle line of the anterior face of the 

 oesophagus and stomach, and enters a ganglion placed be- 

 tween the anterior gastric muscles (A), from whence a lateral 

 branch is given off on each side, while a posterior median 

 branch (d) continues the direction of the azygos nerve. 

 Having reached the cardiac ossicle, this nerve divides into 

 two branches (/), each of which passes downward and out- 

 ward, unites with the postero-lateral nerve of its side, and 

 thus forms the hepatic nerve (k). The fourth and last, or 

 antero-lateral branch (e), descends at first to near the mouth, 

 and then, curving forward, ascends to unite on the anterior 

 face of the oesophagus with the anterior continuation of the 



