410 University of California Publications in Zoology. [ VoL - <> 



confused with that advocated by Cohn (1898) for merozoic ces- 

 todes. He believes that all of the longitudinal nerves, together 

 with their transverse connections in scolex and proglottides, con- 

 stitute the central nervous system. The branches from these to 

 the various organs and to the surface of the animal he regards 

 as the peripheral system. While all the evidence indicates that 

 such a division would be justifiable in Gyrocotyle, it is not in this 

 sense that the terms are here used, but purely with reference to 

 position, not at all with reference to structure or function. 



The peripheral nervous system consists of eight longitudinal 

 stems, lying in the intermediate muscle layers, just outside the 

 outer longitudinal set of fibres, and communicating with the cen- 

 tral system by means of the anterior nerve ring around the mar- 

 gin of the acetabulum. There are no ganglion cells in these 

 strands; they are very small and exceedingly difficult to trace. 

 With borax carmin and Lyon's blue they stain a very clear light 

 blue, and can be recognized with some ease in the neighborhood of 

 the acetabulum, especially near their junction with the anterior 

 ring. These strands have not been previously described. They 

 innervate the intermediate muscle layers (outer transverse and 

 outer longitudinal), wherever these occur in the body proper, 

 in the inner layers of the acetabulum, and in the inner layers of 

 the funnel. 



In dealing with the central system it is important to remem- 

 ber and recognize the existence of these extra-central nerves, for 

 it is only when the central system is clearly distinguished from 

 the others that the relations of its parts become intelligible. It is 

 perhaps because of their failure to take account of this division 

 that the results of investigators of the nervous system of Gyro- 

 cotyle have shown so little agreement in details. 



It is hardly necessary to say that the study of the nervous 

 system by means of serial sections, already made difficult by the 

 great contractility of the body, is rendered a much more serious 

 problem by the necessity of dealing not with two stems, their 

 branches and connectives, but with six or perhaps ten such stems. 

 To determine whether a complete ring is present in any part of 

 the body becomes a task of serious difficulty, and indeed one im- 

 possible without the assistance of the relations of the different 



