434 Causes and Course of Organic Evolution 



optic, the trigeminal, and the auditory nerves. Below the 

 dorsal brain masses and often closely ai)])osed to them are the 

 two ventral ganglia, that are like the dorsals united by a ven- 

 tral commissure. Through the circle thus formed the pro- 

 boscis-tube and proboscis pass. The mouth and alimentary 

 tract however lie ventral to them. The nervous system then 

 in all nemerteans resembles the other systems in showing 

 progressive advance from a comparatively simple to a com- 

 l)lex condition. It might truly be said further that, from the 

 simplest to the most complex nemertean brain, the advance is 

 almost as great as is the simplest vertebrate brain — that of 

 jMyxine — an advance on the nemertean. But all show an 

 evident homological continuity. We shall here concern our- 

 selves only with the most advanced nemertean types. 



In Drepanophorus (Fig. 17 a) the brain consists of two rela- 

 tively huge dorsal ganglionic lobes that are united by a dorsal 

 commissure ^dth each other, and also with two smaller ventral 

 or infraproboscidial lobes. From the dorsal lobes three main 

 sets of sensory nerves arise. The olfactory and optic are 

 given off anteriorly; the pair to the cerebral organs arises 

 posteriorly. The two last enlarge into special sacs, and for 

 reasons already given we would regard them as "auditory" 

 nerves with double audito-geotactic function. 



Such a dorsal brain system might therefore be regarded 

 as the primitive and evolving type of vertebrate swelling, 

 from which the fore and mid brain are to be evolved. 



In making such a comparison we readily grant that an ol- 

 factory function has not been demonstrated for the ciliated 

 pitSy nor an auditory function for the cerebral lobes, but, if 

 structure, position, and relation are to count for anything, 

 then such a view is warranted. 



The ventral lobes can now be examined. In most nemer- 

 teans they are distinct from the dorsals, and are united with 

 them and each other by a commissural thread below the proboscis. 

 In some genera, however, all are closely joined above the gul- 

 let and round the aperture of the proboscis sheath. From 

 their nerve distribution to the proboscis, oesophagus, and 

 surrounding as well as posterior parts, the suggestion is war- 

 ranted that these ventral ganglia may gradually join with the 

 dorsals to form a medullary element, from which nerves corre- 

 sponding in value to those of the glossopharyngeal and vagus 

 are given off. In such a case the ventral ganglia may then 

 represent the hind brain or myelencephalon of vertebrates, while 

 the dorsal divisions may evolve into the thalamencephalon 



