XXIV NERVOUS SYSTEM 283 



In discussing the hydroid polypes we found that one of 

 the most important points of difference between the loco- 

 motive medusa and the fixed hydranth was the presence in 

 the former of a well-developed nervous system (p. 243) con- 

 sisting of an arrangement of peculiarly modified cells, to 

 which the function of automatism was assigned. It is 

 natural to expect in such an active and otherwise highly- 

 organised animal as Polygordius a nervous system of a 

 considerably higher degree of complexity than that of a 

 medusa. 



The central nervous system consists of two parts, the 

 brain and the ventral nerve-cord. The brain (Fig. 67, a and 

 B, Br}^ is a rounded mass occupying the whole interior of 

 the prostomium and divided by a transverse groove into two 

 lobes, the anterior of which is again marked by a longitu- 

 dinal groove. The ventral nerve-cord ( V. Nv. Cd.) is a 

 longitudinal band extending along the whole middle ventral 

 line of the body from the peristomium to the anal segment. 

 The posterior lobe of the brain is connected with the anterior 

 end of the ventral nerve-cord by a pair of nervous bands, 

 the oesophageal connectives {CEs. Con.) which pass respectively 

 right and left of the gullet. 



It is to be noted that one division of the central nervous 

 system — the brain — lies altogether above and in front of the 

 enteric canal, the other division — the ventral nerve-cord — 

 altogether beneath it, and that, in virtue of the union of the 

 two divisions by the oesophageal connectives, the enteric 

 canal perforates the nervous system. 



It is also important to notice that the nervous system is 

 throughout in direct contact with the epidermis or ectoderm, 

 the ventral cord appearing in sections (Fig. 67, c, and Fig. 

 70, a) as a mere thickening of the latter. 



Both brain and cord are composed of delicate nerve-fibres 



