4 SCIENCE PROGRESS. 
as serially homologous with the shell proper. This, in 
Thiele’s eyes, compares well with the condition of affairs in 
Chiton, whose shelly plates are without doubt serially 
homologous. Moreover, although the existence of an 
epipodium in C&zfon has not been hitherto recognised, 
Thiele argues that, since the pallial fold in this form re- 
presents the primitive body-edge, it must also, together with 
the series of ctenidia which are attached to its lower surface, 
be regarded as the homologue of the epipodium of the 
Rhipidoglossa. The ctenidia of Chzton are, in fact, re- 
garded as modified epipodial cirri. The consequence of 
this view is that while the mantle of C4z¢on and the mantle 
of Pelecypoda are regarded as homologous, the mantle of 
the Gastropoda is supposed to represent only a portion of 
the mantle in these other forms, and its projecting rim, 
similar as it appears to be in the two cases, is held to be a 
new and secondary formation unrepresented in the Am- 
phineura and Pelecypoda. 
Nowhere, however, do we find in Thiele’s voluminous 
writings any explanation of the anomaly which ought to 
have occurred to him, that while in CZzton the anus is 
“ventral,” and lies well beneath the ‘‘epipodium” and the 
last shell-plate, in operculate Gastropods the intestine opens 
not only above the epipodium, but between the operculum 
and the shell of the embryo—a relation which could only be 
represented in C/zton, if Thiele’s theories were correct, by 
the situation of the anus between two of the shell-plates 
upon the back of that animal ! 
The nervous system of the Mollusca is treated by Thiele 
with a ruthlessness no less than that which is meted out 
to the external organs of the body. Let us take the 
Amphineura first. In this group, if the relations of the 
nervous system in Cfzton be taken as typical, we have 
dorsal to the gut a great ganglionic nerve-ring whose lateral 
components are usually referred to as the lateral or pleuro- 
visceral cords. Connected anteriorly with the cerebral 
