524 Causes and Course of Organic Evolution 



jaws, which from their shape, position, and function probably 

 represent the incus completely separated into its halves. Alike 

 in rotifers and molluscs these plates are protrusible from the 

 mouth, and act as triturating organs. 



inc 



unc 



man 



Fig. 24 — Comparative view of masticatory plates from (a) a rotifer, (b) a 

 mollusc Dentalium and (c) a gasteropod mollusc Vermetus. 



But further in some Rotifera, e. g., Philodina, "there is a 

 multitude of minute ridges parallel to the teeth." These 

 we would suggest are the first beginnings of the elaborate 

 gasteropodous radula or tooth-ribbon. Now in the radula of 

 many molluscs it can truly be said that each radular row of 

 teeth is a vegetative repetition, in aspect and position, of the 

 masticatory teeth. For comparing Fig. 24 a from a rotifer, 

 with a row of teeth in Fig. 24 b from the mollusc Dentalium^ 

 and that of Fig. 24 c from Vermetus it will be observed that 

 the central radular teeth correspond to the halves of the incus, 

 the lateral teeth to the unci, and the marginal teeth to the 

 manubria. But again, where the teeth are small, uniform, 

 and harrow-like, these may represent a umiform development 

 of tubercles from the "multitude of minute ridges" spoken of 

 above. We incline therefore to regard the mandibulo-radular 

 apparatus of gasteropods as a direct evolutionary advance 

 on the rotiferan mas tax. 



We cannot here trace all of the changes in size and growth 

 of parts of the head, body, and foot, in the subdivisions of 

 gasteropods, as the larva changes into the more sluggish adult. 

 But it is of interest to note that the bilobed cement or pedal 



