58 THE WHENCE AND THE WHITHER OF MAN 



a snail or a clam with an insect and a fish, we find 

 clearly, I think, that the fundamental anatomical dif- 

 ference lies in the skeleton ; and that this resulted 

 from, and almost irrevocably fixed, certain habits of 

 life. 



We may picture to ourselves the primitive ancestor 

 of mollusks as a worm having the short and broad 

 form of the turbellaria, but much thicker or deeper 

 vertically. A fuUer description can be found in the " En- 

 cyclopaedia Britannica," Art., MoUusca. It was hemi- 

 ovoid in form. It had apparently the perivisceral 

 cavity and nephridia of the schematic worm, and a 

 circulatory system. In this latter respect it stood 

 higher than any form which we have yet studied. Its 

 nervous system also was rather more advanced. It 

 had apparently already taken to a creeping mode of 

 life and the muscles of its ventral surface were strongly 

 developed, while its exposed and far less muscular 

 dorsal surface was protected by a cap-like shell cover- 

 ing the most important internal organs. But the in- 

 tegument of the whole dorsal surface was, as is not 

 uncommon in invertebrates, hardening by the deposi- 

 tion of carbonate of lime in the integument. And this 

 in time increased to such an extent as to replace the 

 primitive, probably homy, shell. 



Into the anatomy of this animal or of its descend- 

 ants we have no time to enter, for here we must be 

 very brief. We have already noticed that the most 

 important viscera were lodged safely under the shell. 

 And as these increased in size or were crowded up- 

 ward by the muscles of the creeping disk, their portion 

 of the body grew upward in the form of a " visceral 

 hump." Apparently the animal could not increase 



