. The causes and course of organic evolution; . Fig. 24—Comparative view of masticatory plates from (a) a rotifer, (b) amollusc Dentalium and (c) a gasteropod mollusc Vermetus. But further in some Rotifera, e. g., Philodina, there is amultitude of minute ridges parallel to the teeth. Thesewe would suggest are the first beginnings of the elaborategasteropodous radula or tooth-ribbon. Now in the radula ofmany molluscs it can truly be said that each radular row ofteeth is a vegetative repetition, in aspect and position, of themasticatory teeth. For comparing Fig. 24 a from a rotifer,with a row of
. The causes and course of organic evolution; . Fig. 24—Comparative view of masticatory plates from (a) a rotifer, (b) amollusc Dentalium and (c) a gasteropod mollusc Vermetus. But further in some Rotifera, e. g., Philodina, there is amultitude of minute ridges parallel to the teeth. Thesewe would suggest are the first beginnings of the elaborategasteropodous radula or tooth-ribbon. Now in the radula ofmany molluscs it can truly be said that each radular row ofteeth is a vegetative repetition, in aspect and position, of themasticatory 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 thatthe central radular teeth correspond to the halves of the incus,the lateral teeth to the unci, and the marginal teeth to themanubria. But again, where the teeth are small, uniform,and harrow-like, these may represent a umiform developmentof tubercles from the multitude of minute ridges spoken ofabove. We incline therefore to regard the
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