248 ANIMAL BIOLOGY. [Part II. 



After the crayfish has been opened up as shown in Fig. 75, 

 and the relations of the alimentary system, the heart, and the 

 organs of generation (p. 252) have been studied, the abdominal 

 muscles should be carefully removed, the ganglia of this region 

 being thus displayed. On removal of the viscera of the cephalo- 

 thorax it is seen that the nerve-chain passes into the canal 

 formed by the apodemata of the endophragmal system. The 

 roof of this canal should be carefully cut away so as to expose 

 the whole chain, the anterior ganglion of which lies between the 

 eyes at the base of the rostrum. The chain is so easily exposed 

 and so readily understood that a figure is here unnecessary; 

 though the student should of course, in this as in all cases, draw 

 one from his own dissection. 



There are thirteen pairs of ganglia. In the abdomen there 

 are six, slightly bi-lobed, connected by commissures which, on 

 closer inspection, are seen to be double. In the thorax there 

 are six. The most anterior is obviously multiple, the result of 

 the fusion of two or more pairs. Lying behind and beneath 

 the gullet it is called sub-cesophageal. This, the second, and 

 the third thoracic ganglia, are connected by somewhat elongated 

 double commissures. Between the third and fourth the com- 

 missures are short. Between the fourth and fifth they separate 

 so as to allow of the passage of the sternal artery. The fifth 

 and sixth lie close together, almost in contact. From the sub- 

 cesophageal there pass forward two long commissures, which 

 separate to pass one on each side of the gullet, uniting anteriorly 

 in one large supra-cesophageal or cerebral ganglion. 



From the ganglia and inter-ganglionic commissures nerves are 

 given off. From the cerebral ganglion two antennary, two 

 antennulary, and two optic nerves are readily traced. From the 

 sub-cesophageal ganglion nerves may be traced to the muscles of 

 the mandibles, maxillae, and maxillipeds. Thus these two main 

 ganglia innervate the first nine segments. The other eleven 

 ganglia are segmentally arranged, and each innervates its own 

 somite and appendages. 



Just anterior to the gullet the commissures give origin to nerves, 

 which meet in the mid-line and pass backwards as an unpaired 



