294 



PHYSIOLOGY 



the direct reaction excited in each segment by stimulation of its surface, and, 

 for this influence to be propagated, long tracks must be laid down, joining 

 up ganglion to ganglion and propagating impulses from the head ganglia 

 to the most distant part of the chain. As a type of such a system wo may 

 refer to the crayfish. 



In this animal the central nervous system (Fig. 137) consists of a chain of thirti -n 

 ganglia, namely, six abdominal ganglia, six thoracic ganglia, and one supracesophageal 

 or cerebral ganglion. In the abdomen and thorax the ganglia form a longitudinal 

 series situated in the middle line of the ventral aspect of the body close to the integu- 



OAN&UON - CHAIN 



FIG. 138. Diagram of nervous system of a segmented invertebrate (earthworm 



or crayfish). (From SCHAFER, after RETZIUS.) 



Ofl >r\ cells ; s, afferent nerve fibres; m, motor neuron ; i, central 

 or intermediate cell. 



ment. All rive origin to a variable number of nerves, which are distributed partly 

 ID tin IMIIX -li -s. partly to tin- skin and sense-organs. They are connected by longitu- 

 dinal bands of nerve fibres or eommissures, which arc double, each ganglion being bilobed. 

 The moM anterior of tin- thoracic ganglia, which is the largest, is marked at the side 

 by notches, as if it \\ere made up of several pairs of ganglia fused together. From this 

 <_ r a Million two commissures pass forward round the gullet to unite in front of this tube, 

 behind tin- eyes, \\ith the transversely elongated mass of ganglion cells and fibres 

 .ailed the MI j HM . . ->oj .ha u'ejil ganglion. This ganglion consists of three fused pairs of 

 LMiiL'lia. \\hii-h have been termed the /i/'nfnct , , hroti, the dcuterocerebron, and the trito- 

 cerebrou. The most anterior gives origin to the optic nerves, which run by the optic 

 -t ilk- t.i the yes. From the middle ganglion on each side a temimentary nerve passes 

 to ramify in the internment and from the inferior surface the antennulary nerves 

 pass to the internal antenna-. Krom these small Itranches are distributed to the organ 

 of hearing. The posterior protuberance of the brain gives origin to the antennary 

 \\hieh pa-s to the large external antenna' of the animal. The first thoracic, 



