THE M0RPI10L0GICAL COMPOSITION OF ANIMALS. 117 



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

 radical distinction between them is this: — 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 which are simi- 

 larly composed; in the Mollusc the segmentation is limited 

 to the shell carried on its upper surface, and leaves its 

 body as completely undivided as is that of a common slug.* 

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

 section 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 

 homologous in the same sense as are the appendages of a 

 phsenogamie 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 

 of certain doctrines that have long been dominant, and have 

 still a wide currency. 



Among the Vertebrata, as among the Mollusca, homogene- 

 sis is universal. The two sub-kingdoms are like one another 

 and unlike the remaining sub-kingdoms in this, that in all 

 the types they severally include, a single fertilized ovum pro- 

 duces only a single individual. It is true that as the eggs of 

 certain gasteropods occasionally exhibit spontaneous fission 



* Prof. MacBride corrects this statement by saying that " The ctenidia or 

 gills (which in Mollusca generally are represented only by a single pair) are 

 here represented by a large number of pairs ; they do not, however, correspond 

 in either number or position to the shell plates." It may, I think, be con- 

 tended that if these had any morphological significance, they would not differ 

 in arrangement from the shell plates, and would not be limited to this special 

 type of Mollusc. 



