8 SCIENCE PROGRESS. 
middle of the ventral surface, and opened into a gastral 
cavity devoid of an anal orifice (Polyclads) ; while along the 
line of descent which led to the Annelida and Mollusca the 
blastopore elongated along the ventral surface, as Sedg- 
wick has so ably contended, its lips coalesced except at the 
two extremities, and these open ends constituted the mouth 
and anus of the Ccelomate descendants. Thiele has 
altogether overlooked the significant behaviour of the blas- 
topore in Annelidan and Molluscan embryos; and since 
no similar modification of the blastopore is known in the 
case of Turbellarians and Trematodes, in which groups the 
absence of an anus is so marked a characteristic, we are 
amply warranted, I think, in drawing the conclusions which 
I have emphasised above. 
The admission of this distinction is however fatal to 
any theory of the Polyclad ancestry of the Mollusca. The 
foot of the Mollusca is a development of the fused lips of 
the elongated blastopore, and can in no case be homo- 
logised with the ventral sucker of Turbellarians which lies 
entirely behind the blastopore. The same remark applies 
to Lang’s comparison of the Molluscan foot with the ventral 
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 
homologue 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 C4z¢on, which innervate 
the gills, viscera, and mantle, are united to one another 
posteriorly by a “commissure” lying above the rectum ; 
