THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 33 



7. Nervous System. 

 (Figs. 6, 7, 8, 11, 13, 15, 27, 32—36, 53, 60, 61.) 



The structure of this agrees in the main with that described 

 by Leuckart in P. tsenioides, and by Stiles in P. probos- 

 cideum. There is one large ganglionic mass of a clearly 

 double nature, lying to the posterior side of the oesophagus, 

 about one third of the way down the latter from the point at 

 which it enters the ventral side of the mid-gut. From this 

 central mass a comparatively small commissure (figs. 35, 36, 

 60, A.) passes round the oesophagus, forming a ring in which 

 no trace of ganglionic enlargement or ganglionic cells is to 

 be detected in front of the oesophagus. There is thus nothing 

 comparable to a cerebral ganglion. 



From the suboesophageal nerve-mass nine nerves are given 

 off on either side. These are represented somewhat diagram- 

 matically in figs. 35 and 36, which have been drawn after 

 study of dissections, and of very numerous sections cut longi- 

 tudinally, in both a vertical and horizontal direction, trans- 

 versely and obliquely. 



The following is a list of the branches given off : 



(1)^ Close beliind the oesophageal commissure {A) arises on 

 each side a nerve which runs forwards and downwards, and 

 joins its fellow on the anterior side of the oesophagus. The 

 single nerve thus formed runs for some distance close to the 

 front wall of the oesophagus, and then sends branches to the 

 oral papilla, and the muscles concerned in moving this, as well 

 as to the strong muscles attached to the anterior wall of the 

 pharyngeal tube. 



In P. protelis, Hoyle describes the oesophageal commis- 

 sure as double. This appearance of a double commissure may 

 possibly be due to the union in that form, as in P. probos- 

 cideum (as described by Stiles) and P. teretiusculum, of 

 the two nerves in question. 



(2) From the latero-ventral surface on each side a nerve 

 ^ These numbers correspond to those used in figs. 35 and 36. 



VOL. XXXIV, PART 1. — NEW SER. C 



