TE MORPHOLOGY. Ob Tit MOLLUSCA, 3 
his identifications of homologous organs in the different 
Molluscan groups are determined, not by a direct comparison 
of the organisation of these types one with another, but 
by independent comparisons of the organisation of the 
different Molluscan types with that of sucker-bearing 
Polyclads. The group Mollusca is thus made to lose its 
compactness, and characteristic organs, such as mantle and 
ctenidium, which have been regarded as homologous 
throughout the Molluscan series, are interpreted in different 
ways in the different types, as the exigencies of Thiele’s 
theory demand. One of the first propositions assumed by 
this writer is that the foot of the Mollusca is simply a colossal 
enlargementof the ventral sucker of the Polyclad; thesuctorial 
function of the foot in C#ztonx and the lower Gastropoda is 
pointed to in support of this comparison. A series of 
more revolutionary propositions is then promulgated in 
consequence of the necessity under which the author is 
placed of discovering the primitive body-edge of the 
Mollusca comparable to the edge of the body of the Tur- 
bellaria. This primitive body-edge Thiele identifies by 
means of the lateral sense-organs which characterise the 
epipodium in the Rhipidoglossa and the margin of the 
mantle in Pelecypoda. The epipodium in Gastropoda and 
the mantle edge in Pelecypoda are thus taken by this writer 
to represent the sides or edge of the body in the Tur- 
bellarian ancestor. The epipodium in Gastropoda and the 
mantle edge in Pelecypoda consequently separate the 
dorsal from the ventral regions of the body in those groups. 
It follows from this that the ctenidia of Gastropoda, which 
are supra-epipodial in position, are not homologous with 
the ctenidia of Pelecypoda, which are infra-pallial. How we 
are to regard the anus, which is dorsal in the one group and 
ventral in the other, is not explained. But since in oper- 
culate Rhipidoglossa the operculum, like the shell, is 
situated above the epipodium, we are told that the oper- 
culum must also be regarded as dorsal in position, as well 
