27 



The nervous system, which in the ancestral Platyelminth 

 was probably diffused over the general surface, with per- 

 haps a tendency towards concentration around the mouth 

 opening, has become lost altogether in the very degraded 

 Cestodes, and is aggregated to form a pair of anteriorly 

 placed ganglionic masses from which nerves stretch back 

 along the sides of the body (see fig. 13, E), in the 

 Trematodes and the higher Turbellarians. In the Nemer- 

 teans the same general arrangement is found, but the 

 concentration is more complete and the anterior ganglionic 

 masses are more definite. The two longitudinal nerves are 

 long, and they are placed quite laterally. In some forms 

 there is in addition, a certain amount of diffused nervous 

 tissue in the body-wall, and in the walls of the proboscis. 

 The Palseonemertea are the most nearly related to the primi- 

 tive Nemerteans, while the Schizonemertea and the Hoplo- 

 nemertea form two divergent series at the top of the branch. 



The points of origin of the Mollusca and of the far-back 

 ancestors of the Vertebrata from the Vermes were probably 

 close together, and a little way above the origin of the 

 Platyelmia. We can obtain a certain amount of information 

 in regard to the probable ancestors of these higher groups 

 of the Metazoa by comparing their larval forms. MoUuscan 

 larvsB may be referred to the same type as the Trochosphere 

 (fig. 14), the typical larval form of the Chaetopoda and some 

 other groups of Vermes which will be discussed further on. 



This larva is a bilaterally symmetrical body with a more 

 or less rounded dorsal surface, which is prolonged anteriorly 

 in front of the mouth as a prae-oral lobe. The alimentary 

 canal is a slightly bent tube concave ventrally.* The mouth 

 is ventral and the anus posterior, and the tube may be 

 divided into three regions, the oesophagus, the stomach, and 



* Compare with the primitive worm represented by fig. 13, E. 



