294 



PHYSIOLOGY 



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 we may refer to the 

 crayfish. 



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

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

 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- 



&ANGLION - CHAIN 



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



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



s', sensory cells; s, afferent nerve fibres ; ra, motor neuron ; i, central 

 or intermediate cell. 



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

 to the muscles, partly to the skin and sense-organs. They are connected by longitu- 

 dinal bands of nerve fibres or commissures, which are double, each ganglion being bilobed. 

 The most anterior of the thoracic ganglia, which is the largest, is marked at the side 

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

 ganglion two commissures pass forward round the gullet to unite in front of this tube, 

 behind the eyes, with the transversely elongated mass of ganglion cells and fibres 

 called the supracesophageal ganglion. This ganglion consists of three fused pairs of 

 ganglia, which have been termed the protocerebron, the deuterocerebron, and the trito- 

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

 stalks to the eyes. From the middle ganglion on each side a tegumentary nerve passes 

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

 pass to the internal antennae. From these small branches are distributed to the organ 

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

 nerves which pass to the large external antennae of the animal. The first thoracic, 



