xxiv NERVOUS SYSTEM 285 



substance in which neither cells nor fibres can be made 

 out. 



Ramifying through the entire muscular layer of the body- 

 wall is a network of delicate nerve-fibres (int. muse, plx.) 

 with nerve-cells (Nv. (?) at intervals, the inter-muscular 

 plexus. Some of the branches of this plexus are traceable 

 to nerve-cells in the central nervous system, others (af. nv.) 

 to epidermic cells, others (Ef. nv.} to the delicate proto- 

 plasmic layer covering the muscle-plates. The superficial 

 cells of both brain and cord are also, as has been said, in 

 direct connection with the overlying epidermis, and from the 

 anterior end of the brain a bundle of nerve-fibres (Fig 67, B, 

 Nv.) is given off on each side to the corresponding tentacle, 

 constituting the nerve of that organ, to the epidermic cells of 

 which its fibres are distributed. 



We see then that, apart from the direct connection of 

 nerve-cells with the epidermis, the central nervous system is 

 connected, through the intermediation of nerve-fibres (a) 

 with the sensitive cells of the deric epithelium and (b) with 

 the contractile muscle-plates. And we can thus distinguish 

 two sets of nerve-fibres, (a) sensory or afferent (af. nv.) 

 which connect the central nervous system with the epidermis, 

 and (b) motor or efferent (Ef. nv.) which connect it with the 

 muscles. 



Comparing the nervous system of Polygordius with that 

 of a medusa (p. 243) there are two chief points to be noticed. 

 Firstly, the concentration of the central nervous system in 

 the higher type, and the special concentration at the anterior 

 end of the body to form a brain. Secondly, the important 

 fact that the inter-muscular plexus is not, like the peripheral 

 nervous system of a medusa which it resembles, situated 

 immediately beneath the epidermis (ectoderm) but lies in the 

 muscular layer, or, in other words, has sunk into the 

 mesoderm. 



