■VII MOLLUSGA—THE NERVOUS SYSTEM 135 



5. There is'no nerve in Chiton homologous with the visceral ganglion or ganglia of 

 the Gastropoda ; this is the chief difficulty in the comjiarison of the two nervous 

 systems. In the Ampldncura, the pleuro visceral cords unite above the intestine ; 

 in all other Molluscs the point of junction (which is the visceral ganglion) lies below 

 the intestine. 



In Proneomenia the posterior commissures between the i)leurovisceral cords are 

 merely a more strongly developed part of a general commissural system. 



Origin of the Crossing- of the Pleuroviseeral Connective 

 (Chiastoneury) (Figs. 113-11 6). 



Several attempts have been made to explain the peculiar crossing 

 of these connectives in the Prosobmnchia. The one here given is in a 

 high degree probable if not altogether satisfactory. 



We must start with a supposed racial form which was perfectly 

 symmetrical, even in its nervous system, and possessed an organisation 

 somewhat like that of our hypothetical primitive Mollusc (p. 26). 

 Such an organisation agrees in most important points with that of the 

 extant Chitonida' ; only one gill, however, was present on each side. 



Further, the parietal ganglia innervated the gills and the osphradia, 

 and were thus closely connected with these organs. 



The racial form of the Gastrojmla may have been surrounded by 

 a mantle border which widened posteriorly, i.e. covered a somewhat 

 deeper mantle cavity which contained the pallial complex, viz. the 

 median anus, to the right and left of which were the ctenidia and 

 osphradia, and between the ctenidium and anus on each side the 

 nephridial aperture. 



If we suppose this pallial complex to have changed its position, 

 shifting gradually forward along the right mantle furrow, each cteni- 

 dium would drag along with it its parietal ganglion. The heart and 

 its auricles which are coimected with the ctenidium would also become 

 shifted. 



As long as the pallial complex had not moved far forward to the 

 right, the pleuroviseeral connectives would not cross, but would only 

 be shifted to the right (Fig. 114). We find the Tectibranchia among 

 the Opisthobranchia apparently at this stage, the only difference being 

 that they have already lost the original left ctenidium and also the 

 original left auricle (Fig. 43, p. 33). 



If the pallial organs are still further shifted forward along the 

 mantle furrow (Figs. 115, 116) till they come to lie quite ante- 

 riorly, and once more symmetrically, above and behind the neck, the 

 original left ctenidium comes to lie on the right, and the original right 

 ctenidium on the left in the anteriorly placed mantle cavity. The 

 original right ctenidium has, however, dragged its parietal ganglion 

 over the intestine to the left side, and the latter becomes the 

 supraintestinal g-angrlion. The original left ctenidium, on the 

 contrary, has dragged its ganglion below the intestine to the right 



