INVERTEBRATA. 335 



more interesting and instructive if it were compared with 

 that of members of other Molluscan classes, such as the 

 snail, whelk, or cuttle-fish. Here we can only consider it 

 as illustrating a type of structure so utterly different from 

 that of any Vertebrate that only by contrasting both with 

 that of some form like Hydra can we find points of agree- 

 ment between the two. If Mollusca and Vertebra ta have 

 had a common ancestor, it must be in the immensely distant 

 past. And indeed we find that whereas the whole history 

 of Vertebrate development is recorded for us in the rocks, 

 in the earliest rocks in which fossils have been found there 

 are Lamellibranchs closely allied to some of the most primi- 

 tive Lamellibranchs of the present day. The divergence of 

 the Lamellibranchs from the other classes of Mollusca must 

 have taken place at a still earlier period, and that of the 

 primitive Mollusca from the primitive forms of other 

 groups much earlier still. 



There can be little doubt that the most primitive 

 Mollusca were much more active organisms than Anodonta, 

 which affords us an example of a degenerate group one 

 in which specialization, in adaptation to a simple mode of 

 feeding, has involved a general simplification of structure 

 instead of a general elaboration. It is probable that Hydra 

 is also a degenerate form. Degenerate forms can usually 

 be distinguished from primitive forms by the fact that 

 along with general simplicity of structure they exhibit an 

 undue elaboration of some organs ; whereas primitive forms 

 show equal simplicity in all their parts. 



