EVOLUTION OF THE NERVOUS SYSTEM 331 



namely, six abdominal ganglia, six thoracic ganglia, and one supra- 

 cesophageal 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 integument. 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 longitudinal 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, just behind the eyes, with the transversely elon- 

 gated 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 tritocerebron. 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, or subcesophageal, ganglion 

 gives origin to ten pairs of nerves which are distributed to the man- 

 dibles, to the jaws or maxillae, to the maxillipedes, and to the branchial 

 appendages of the latter. 



When we investigate the structural basis of such a nervous system 

 we find that, as in medusa, the starting-point of the reflex arc is 

 in certain neuro-epithelial cells (Fig. 139) lying on the surface 

 of the body. These cells are spindle-shaped, and have one short 

 process passing to the surface, and a long process which runs in a 

 nerve fibre or collection of fibres towards the ganglion of the segment. 

 Arrived at the ganglion it divides into two branches, which pass 

 towards the two ends of the body and become lost in the granular 

 material forming the inner part of each ganglion. The ganglia 

 themselves consist internally of this punctated substance or granular 

 material, and externally of a capsule of ganglion-cells. Each of 

 the ganglion-cells sends one thick process towards the centre, which 

 rapidly divides, some of the branches passing into the granular 

 material, while one branch passes outwards in a nerve to end in a 

 network of fine fibrils within the muscles on the surface of the body. 

 The nervous impulse excited in the sensory cell on the periphery 

 travels therefore up a nerve fibre into the granular substance of the 

 ganglion. From this granular substance it is collected by the fine 



