104 MORPHOLOGICAL DEVELOPMENT. 



It may be well to note, however, more especially because 

 it illustrates a danger of misinterpretation presently to be 

 guarded against, that there are certain Mollusks which si- 

 mulate the segmented structure. Externally a Chiton, Fig. 



188, appears to be made up 

 of divisions substantially like 

 those of the creature Fig. 

 189 ; and one who judged 

 only by externals, would say 

 that the creature Fig. 190 

 differs as much from the 

 creature Fig. 189, as this 

 does from the preceding one. But the truth is, that while 

 190 and 189 are closely- allied types, 189 differs from 188 

 much more widely than a man does from a fish. And the 

 radical distinction between them is this ; that whereas in the 

 Crustacean the segmentation is carried transversely through 

 the whole mass of the body, so as to render the body more 

 or less clearly divisible into a series of parts that are similarly 

 composed ; in the Mollusk the segmentation is limited to the 

 shell which it carries on its upper surface, and leaves its 

 body as completely imdivided as is that of a common slug. 

 Were the body cut through at each of the divisions, the sec- 

 tion of it attached to each portion of the shell would be unlike 

 all the other sections. Here the segmentation has a purely 

 functional derivation — is adaptive instead of genetic. The 

 similarly-formed and similarly-placed parts, are not homolo- 

 gous in the same sense as are the appendages of a phoonoga- 

 mic axis or the limbs of an insect. 



§ 210. In studying the remaining and highest sub-king- 

 dom of animals, it is important to recognize this radical dif- 

 ference in meaning between that likeness of parts which is 

 produced by likeness of modifying forces, and that likeness 

 of parts which is due to primordial identity of origin. On 

 our recognition of this difference depends the view we take 



