THE NERVOUS SYSTEM. 



193 



to form a commissure (the so-called supra-oesophageal commissure, 

 v. Kennel). This commissure is thus of secondary origin, and 

 seems also to involve parts of the brain lying further back. These 

 parts, however, do not belong to the cephalic segment, but are 

 formed by the (maxillary) segment that follows it. 



When the jaws become enclosed in the developing buccal cavity, 

 the ganglia of the maxillary segment also sink below the surface, 

 and pass toward the dorsal surface, so that they can soon be 

 recognised in viewing the embryo from the dorsal side (v. Kennel, 

 Fig. 97, g m ). It must be assumed that this upward displacement 

 takes place along the oesophageal commissures which are already 

 present. The maxillary ganglia are thus approximated to those 

 of the cephalic segment, with which they subsequently fuse. This 

 fusion is very intimate, and the maxillary ganglia can be recognised 

 as two moderate prominences behind the antennal ganglia (Fig. 

 97, g m ). The fibrous masses of the maxillary ganglia from the 

 two sides unite to form a com- 

 missure, the sub - oesophageal 

 commissure, this being facilitated 

 by the downward slope of the 

 posterior ends of these ganglia. 

 This method of formation of 

 the sub-oesophageal commissure 

 renders it very improbable that 

 it is a primitive structure. A 

 commissure lying further back 

 and consisting of cells (Fig. 97, 

 c) might rather be considered 

 as such. This latter commissure 

 connects two ganglionic swellings, 

 which may perhaps be attributed 

 to the segment of the oral 

 papillae. All the commissures 

 which follow this are, as already 

 mentioned, said to be derivatives 

 of the fibrous substance. 



The point that must be regarded as of most importance in the formation of 

 the brain in Peripatus is the fusion of the maxillary ganglia with the ganglia 

 of the cephalic segment, for this feature distinguishes Peripatus from the 

 Myriopoda and the Insecta so far as is at present known, and connects it 

 rather with the Crustacea, in which the ganglia of the segment of the second 

 antennae unite with the brain (Vol. ii., p. 165). It therefore seems likely that 



o 



Fig. 97. — Anterior part of the central nervous 

 system of an embryo of P. Edwardsii at a 

 somewhat earlier stage than that depicted 

 in Fig. 94 B, dorsal aspect (after v. 

 Kennel), at, antenna ; au, eye ; c, first 

 commissure after the sub-oesophageal com- 

 missure ; gj and g/j, cephalic portion of 

 the brain ; gm, portion belonging to the 

 maxillary segment ; g IV , the next follow- 

 ing ganglion ; op, oral papilla ; p /t first 

 foot ; s, passage for the oesophagus ; sd, 

 slime-gland. 



