THE MORPHOLOGY OF THE MOLLUSCA. 9 
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 zdola_forz 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 zz se¢« 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 
before their fibres have grown out (which is not a rare 
embryological phenomenon), there should be nothing in- 
comprehensible in their fibres taking the shortest route and 
meeting beneath the gut instead of over it. Clearly, there- 
fore, the ventral position of the visceral commissure in 
most Mollusca by no means precludes the possibility of the 
essential homology between the visceral loop of these 
forms and part of the pleuro-visceral ring of Amphineura. 
The other differences between the visceral loop of most 
Mollusca and the pleuro-visceral ring of Amphineura are 
principally differences in the degree of segregation and 
concentration of ganglion-cells and nerve-fibres. The 
