THE MORPHOLOGY OF THE MOLLUSC A. 45 



surface of the Turbellarian. The foot is undoubtedly part 

 of the ventral surface of the Mollusc, and as such may be 

 compared, in a general way, with the creeping surface of a 

 Planarian ; but as a specialised organ, developed from the 

 fused lateral margins of a slit-like blastopore, it has no 

 homoloeue in the organisation of the Turbellaria. 



Let us now see what light has been thrown on the 

 problems of Molluscan morphology by the researches of 

 other investigators. 



The visceral commissure. — One of the greatest dif- 

 ficulties in comparing the Amphineura with the Gastropoda 

 or other Molluscan types has long been the fact that the 

 lateral or pleuro-visceral cords of Chiton, which innervate 

 the gills, viscera, and mantle, are united to one another 

 posteriorly by a "commissure" lying above the rectum; 

 whereas the visceral commissure of Gastropoda and Pelecy- 

 poda, etc., lies below the intestine. 



A little care in the use of words would have prevented 

 much of the confusion and controversy which has arisen on 

 this subject of the position of the visceral commissure. 

 Words, as Bacon phrases it, put constraint upon the in- 

 tellect, and there is no doubt that the disagreement and 

 perplexity of naturalists concerning this point have been 

 caused by one of the idola fori which they have themselves 

 set up, rather than by any intrinsic incompatibility in the 

 facts themselves. If the language must still be maintained, 

 I must at least point out that there are commissures and 

 commissures, and that one may be a commissure in fact, 

 and another only in name. The suprarectal ''commissure" 

 in Amphineura is ganglionic, and, like the rest of the 

 pleuro-visceral nerve-ring, is formed in situ by delamination 

 from the ectoderm (15). It is not a commissure in the 

 strict sense of the word, but an integral portion of an 

 annular central nervous system. But the visceral loop of 

 other Molluscs consists merely of nerve-fibres connecting 

 usually a couple of visceral ganglia with one another, and 

 with the pleural ganglia. Now nerve-fibres are outgrowths 

 from nerve-cells, and if two groups of nerve-cells should 

 happen to take a somewhat deep-seated position in the body 



