Evolution of Animals 



433 



the natural and evolutionarily continuous origin for Reissner's 

 fiber that is present in vertebrates generally. 



In practically all cases the nemerteans show a relatively 

 large dorsal brain that consists of paired lobes. Each of these 

 lies directly above or somewhat behind the mouth^in forms 

 where the proboscis opens in front of and separately from the 

 mouth. But in some land nemerteans the main mass of the 

 brain lies in front of the common orifice of the oesophagus 

 and proboscis-sheath. The dorsal lobes also, as in the verte- 



FiG. 17. — a. Nervous system, to left, of Drepanophorus (after Hubrecht). C 

 dorsal brain; s, anterior sensory nerves; t, nerves to proboscis; ce nerves to digestive 

 tract; L, probable auditory organ and orifice o; T, lateral nerve trunks with their 

 branches. 6. Nervous system to right of Hoplonemertean (after Mclntosli) . 

 c, dorsal brain; s, probable oKactory and optic nerves; dc, dorsal commissure with 

 junction of dorsal nerve dn; vc, ventral commissure passing beneath proboscis 

 sheath ps; v, ventral ganglion; T, lateral nerve trunks. 



brates, may in higher types undergo subdivision into an anterior 

 lobe or thalamencephalon, and a posterior or mesencephalon as 

 in Ewpolia. The former are not only anterior to, they largely 

 overlap, the posterior lobes (Fig. 18 a). Anteriorly in some 

 genera of nemerteans, as in vertebrates, these lobes give off 

 branches that might appropriately be termed in the former 

 group, did we know their function exactly, the olfactory, the 



