vii MOLLUSGA^THE ASYMMETRY OF THE GASTROPODA 149 



mantle. Numerous nerves radiate into the mantle from this ganglion, one of them, 

 which runs dorsally, looking like the direct continuation of the pallial nerve through 

 the ganglion. The pallial nerve often divides into two branches sooner or later after 

 it has left the parietal ganglion ; one of the branches running to and through the 

 stellate ganglion, to unite beyond it with the other branch which runs past the 

 ganglion. The two stellate ganglia are often connected by a transverse commissure. 



The visceral ganglia give off, near the middle line, two visceral nerves, which 

 innervate the rectum, the ink-bag, the gills, the heart, the genital apparatus, the 

 kidneys, and certain parts of the vascular system. The two genital branches of 

 these nerves are connected by a commissure. 



The sympathetic nervous system consists of a buccal ganglion lying beneath 

 (behind) the oesophagus in the buccal mass ; this ganglion is joined to the upper 

 buccal or pharyngeal ganglion by a buccal connective. Two nerves run up along 

 the oesophagus from the lower buccal ganglion to the gastric ganglion, which lies 

 on the stomach, and innervates the greater portion of the intestine and the digestive 

 gland (liver). 



XIV. An Attempt to explain the Asymmetry of the Gastropoda. 



1. 



Chiastoneury, i.e. the crossing of the two pleuro-visceral connectives in the 

 Prosohranchia, may be explained on the three following assumptions. 



1. The ancestors of the Prosohranchia were symmetrical animals ; the mantle 

 cavity lay behind the visceral dome and in it the pallial complex, that is, the ctenidia, 

 osphradia, nephridial apertures, genital apertures, and, in the centre, the median 

 anus. 



2. The visceral commissure or ganglion lay beneath the intestine. 



3. The pallial complex shifted gradually from behind forward, along the right 

 side of the body {cf. p. 136). 



The position of the pallial complex in the Tcctibranchia, among the Opisthohranchia 

 on the right side, can also be thus explained. The pallial complex in its forward 

 movement in these animals has either not yet reached the anterior position or, 

 having reached it, has shifted back again. ^ The visceral connectives are therefore 

 not crossed. 



The above assumptions do not, however, explain — 



1. The asymmetry which is brought about in some Gastropoda by the dis- 

 appearance of one ctenidium, one osphradium, and one renal aperture. 



2. The coiling of the visceral dome and shell, especially the dextral or sinistral 

 spiral twist. 



3. The relation existing between the manner in which the visceral dome and 

 shell are coiled, on the one hand, and the special asymmetry of the asymmetrical 

 organs (ctenidia, osphradia, nephridia, anus, genital organs) on the other. 



4. The cause of the shifting forward of the pallial complex. 



2. 



It is unnecessary to discuss the first of the above assumptions, viz. that the 

 ancestors of the Gastropoda were symmetrical animals, since all Molluscs except 

 the Gastropoda are symmetrical, i.e. the Amphineura, the Lamellibranchia, the 

 Scaphopoda, and the Cephalopoda. 



1 See note to § 13, p. 158. 



