THE ANNELID THEORY 281 



haemal muscle-plates grow out dorsally round the alimentary 

 canal and the neural muscle-plates ventrally round the 

 nerve-cord (see Fig. 14). 



This strand of cells, he concludes, must clearly be the 

 notochord, and the type of development is obviously the 

 double-symmetrical met with in Vertebrates. 



The nervous system Semper found to develop in the 

 buds of Nais and CJitztogaster by an ectodermal thickening, 

 just as in some Vertebrates. The cerebral ganglion was 

 formed by the ends of the nerve-cord growing up round the 

 oesophagus and fusing with the paired "sense-plates" which 

 develop from the ectoderm of the head. The cerebral 

 ganglion is accordingly only secondarily haemal in position, 

 and there is no need therefore to seek in Vertebrates for the 

 homologue of the oesophageal commissures of Annelids, as, 

 for instance, Schneider did. 



Since the mouth opens on the neural surface in Annelids 

 and on the haemal surface in Vertebrates, Semper considers 

 that they cannot be equivalent structures, and he finds the 

 homologue of the Vertebrate mouth in a little pit on the 

 haemal surface of the head in the leech Clepsine (also in the 

 true mouth of Turbellaria and the proboscis-opening in 

 Nemertines). The primitive Annelid mouth, however, does 

 not appear in the embryogeny of Vertebrates, for the great 

 development of the brain crowds it out of existence. 



The homologues of the gill-slits Semper finds in two 

 little canals in the head of Chcetogaster, which open from the 

 pharynx to the exterior. In Sabellids he describes an 

 elaborate system of gill-canals, with a supporting 

 cartilaginous framework which forms a real Kiemenkorb 

 or gill-basket, comparable with that of Amphioxus. 



Gill-slits, notochord, relation of nervous system, meso- 

 nephric tubules, are thus common to Annelids and Vertebrates 

 what further proof could one desire of the close relation- 

 ship of these groups? Yet Semper enters into refinements 

 . of comparison, seeing, for instance, in the lateral portions of 

 the ventral ganglia (Fig. 14, sp. g.} the homologues of the 

 spinal ganglia of Vertebrates, and comparing the lateral line 

 of sense organs in Annelids with the lateral line in Anamnia. 

 He will not admit that Amphioxus and the Ascidians 



