216 



COMPARATIVE ANATOMY 



CHAP. 



two longitudinal trunks, proceeding from the brain and running some- 

 what ventrally in the lateral parts of the body ; these end near the 



anus, or else coalesce immediately in 

 front of the anus above the hind-gut. 

 The brain consists of two large 

 lateral ganglia connected by a trans- 

 verse commissure and further united 

 by a commissure which runs over the 

 proboscis, so that the proboscis at its 

 anterior end is embraced by a nerve 

 ring. Each cerebral ganglion carries 

 a lobe, usually sharply demarcated 

 and placed behind, seldom to the 

 side or in front; the relations of 

 this with the lateral grooves (ciliated 

 organs, olfactory organs) will be de- 

 scribed later. These are known as 

 the olfactory lobes, and usually lie 

 above the first portion of the lateral 

 nerves. From the brain various 

 nerves proceed forwards to the apex 

 of the head, to the eyes, and so on. 

 Special nerves innervate the oeso- 

 phagus and the proboscis. In Paleo- 

 nemsrtina and Schizonemertina, between 

 the longitudinal and circular muscle 

 layers, there is a continuous nerve 

 sheath (nerve plexus) entering the 

 longitudinal trunks. A nerve aris- 

 ing anteriorly in the dorsal cerebral 

 commissures often runs in the dorsal 

 middle line ; l>eneath this nerve we 

 can occasionally observe a second 

 dorso-median nerve, the nerve of the 

 proboscis sheath. In the general 

 nerve sheath, which innervates the dermo -muscular tube, we can, 

 in the Schizonemertina, observe thicker strands, which form annu- 

 lar commissures between the three principal nerves running in 

 the longitudinal direction. In the Hoplonemertina the nerve sheath is 

 wanting ; the commissures run separately, and sometimes show a 

 markedly metameric arrangement. 



In the whole structure of this nervous system a considerable agreement with that 

 of the Platodes (Folydada, Triclada, Trematoda) cannot be ignored. The brain and 

 the longitudinal trunks of the Nemertina answer to the brain and the ventral longi- 

 tudinal trunks of the Platodes. Whether the impaired dorso-median longitudinal 

 nerve of the Nemertina answers to the two dorsal longitudinal nerves of the Platodes 

 cannot yet be decided. The longitudinal trunks (and also the brain) lie either 



Fig. 140. — Nervous system of tlie an- 

 erior Tpart of tlie toody of a Nemertian 

 (Drepanophonis Lankesteri), after Hub- 

 recllt. g, Cerebral ganglia ; os%>, [aperture 

 of the lateral organs (ciliated grooves) ; Iso, 

 cerebral lobes of the ciliated organs ; nt, 

 proximal portion of the proboscis nerves ; 

 kn, nerves of the head ; s%, oesophageal 

 nerves ; sii (further down), lateral longi- 

 tudinal trunks ; 70, transverse commissures ; 

 sft, lateral branches of the longitudinal 

 trunks. 



