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EVOLUTION IN THE MOLLUSCAN RADULA. 



By The REV. A. H. COOKE, Sc.D., F.Z.S. 



(President'al Address delivered at the Annual Meeting, October i6th, 1920). 



Those familiar processes of development, by which modifications are 

 brought about in one or more parts of an organism, appear to have 

 acted upon the digestive implement, which in the mollusca is called 

 the radula. It may be interesting to endeavour to bring together 

 some of the facts which bear upon evolution as seen in this important 

 organ. 



The radula is one of those many devices by which an animal breaks 

 up its food before subjecting it to the processes of digestion. 'To 

 swallow the food whole was probably the rule in very early stages of 

 animal development, and still persists in certain cases. But in time, 

 the advantages of a digestive system whose functions were more 

 specialised must have established themselves, and various mechanical 

 means were evolved for masticating, triturating, carding or otherwise 

 mincing up the food, before it passed into the digestive tract. Hence 

 the origin of teeth in the widest sense, whether set in a jaw, in the 

 roof or sides of the mouth, on a tongue, or on a special organ dis- 

 tinct from any of these. To some such process of development 

 must be ascribed the origin of the molluscan radula. 



The oldest known form of radula is believed to be that of the 

 Rhipidoglossa, a section containing, amongst marine groups, Tut'bo, 

 Trochus, Halioiis, Fissurel/a, Nerita, amongst the land forms JVeri- 

 tiiia, Helicina, and Pfoseipina. Its characteristic is the multiplication, 

 to an indefinite number, of identical small ribbon-shaped marginal 

 teeth. To this section belongs also Pleiirotomaria, a genus long 

 regarded as extinct, four living species of which have been discovered 

 within the last sixty years. Pleurotoinaria first appears in the Cam- 

 brian epoch, and more than four hundred species are known from 

 Palaeozoic strata, since which age it has gradually diminished in 

 numbers. There can thus be no doubt that in the radula of Pleuro- 

 toinaria^ which is markedly rhipidoglossate, we have an extremely 

 early form, exhibiting the multiplication of many identical or nearly 

 identical teeth. It seems only natural to hold, on a priori 'grounds, 

 that when an organ like the radula was developed, it was furnished at 

 first with teeth of small or even minute size, the gradual coalescence or 

 fusion of which gave rise, in process of time, to teeth of larger size 

 and more effective cutting capacity. 



The evidence of the geological record supports this view. The 

 Rhipidoglossa appeared first, earliest of all radula-bearing Gasterb- 

 poda (in the Lower Cambrian), the Tsjnioglossa, with fewer and 



