CHAP. II] A NERVOUS SYSTEM. 49 



Thus far concerning the simplest elements of a nervous 

 mechanism. It should now be stated, however, that even 

 when the Nervous System consists of a mere multiplication 

 of the simplest combinations necessary for the excitement 

 and execution of ' reflex actions,' the groups of these 

 nervous arcs are almost always arranged in pairs, one on 

 each side of the middle line of the body. The body of an 

 animal is for the most part divisible by a median longi- 

 tudinal plane into two symmetrical halves, and the integral 

 parts of the nervous system are, in the main, similarly 

 double. In some lower organisms, such as certain Mollusks, 

 Worms, and Crustacea, these halves of the nervous system 

 are distinctly separated from one another (figs. 23, 32, 

 34), though in Vertebrate Animals they are always more or 

 less fused into one axial ' cerebro-spinal ' system (fig. 20). 



FIG. 17. Nervous System of one of the Eolidae (Fiona atlantica). (Gegenbauer after 

 R. Bergh). A, Supra-oesophageal sensory ganglia, composed of two pairs of ganglia 

 fused, the cerebral in front and the branchial behind ; each pair united by its own 

 commissure. B, Great motor ganglia, in connection with the sensory ganglia, and 

 with one another by the commissure e. 0, Buccal ganglia. D, Gastro-oesophageal 

 ganglia, a and 6, Nerves from the sensory tentacles, c, Nerves from the genital 

 organs, d, Principal motor nerves of the body, e', Commissure of the branchial 

 ganglia. 



These lateral halves of the nervous system are connected 

 with one another by means of shorter or longer transverse 

 fibres, which, gathered into thick or thin bundles, are 



E 



