220 



COMPARATIVE ANATOMY 



CHAP. 



FIG. 145. Terminal or (sucker) gan 

 glion of Hirudo, with preceding gan 

 glion, after Hermann. 



oesophageal ganglion, one in each segment. The 2 ganglia of each 

 double ganglion are connected together by short transverse commis- 

 issures, and with the corresponding ganglia of the preceding and 

 succeeding double ganglia by longitudinal commissures. 



Besides the cesophageal commis- 

 sures nerves proceed from the brain 

 to the integument and the sensory 

 organs of the head ; and from the 

 ganglia of the ventral chord nerves 

 supply the integument, sensory organs, 

 and musculature of the segments to 

 which they belong. 



The musculature of the fore -gut 

 (pharynx, proboscis, etc.) is provided 

 with nerves arising either direct from 

 the brain or from the oesophageal 

 commissures. These nerves are called 

 cesophageal nerves (often also nervi 

 vagi), and the plexus they form in 

 the fore -gut is called the cesophageal 

 nervous system. In the most various divisions there is also a further 

 plexus of ganglionic cells and nerve fibres in the walls of the mid-gut 

 (visceral nervous system, sympathetic nervous system) whose con- 

 nection with the central nervous system requires further investigation. 



Difference of opinion prevails as to the phylogenetic origin of the nervous 

 system of the Annulata. We prefer that view which not only derives the brain of 

 the Annulata from that of the Nemertina and Platodes, but also sees in the segmented 

 ventral chord with cesophageal commissures the further developed ladder nervous 

 system formed in these lower divisions by the longitudinal ventral trunks and their 

 transverse commissures. Ganglionic cells are found in the Platodes and Nemertina 

 everywhere in the larger longitudinal trunks, and are present in great numbers in the 

 Polyclada and Triclada at the points of divergence of the transverse commissures 

 and lateral branches. These latter are repeated in an extremely regular manner 

 and in a segmental arrangement as early as in certain Triclada (Gunda) and 

 Nemertina (Drepanophorus). If we imagine the ganglionic cells of the longitudinal 

 trunks crowded together at the points of divergence of the transverse commissures 

 and side branches, and their number perhaps increased still further, these points 

 of divergence must swell into ganglia and the longitudinal trunks become longi- 

 tudinal commissures between the successive ganglia. The longitudinal trunks with 

 their ganglia only need to move together into the ventral middle line, so as to 

 become the typical ventral chord of the Annulata. The mouth and oesophagus 

 prevent such a moving together in the anterior part of the body, and so the 

 first part of the longitudinal trunks becomes the cesophageal commissures of the 

 Annulata. 



An approximation of the longitudinal trunks, which, it is true, never leads to direct 

 contact, may be observed as early as the Nemertina, not to speak of the Platodes. 

 Whereas as a rule among these forms the paired longitudinal trunks lie laterally, 

 sometimes even over the enteric diverticula (Amphiporus Moseleyi), they are found 

 in Drepanophorus placed somewhat ventrally and nearer each other. On the other 



