456 J. H. ORTON. 



V. FUNCTION OF THE EADULA IX CEEPIDULA. 



In the process of feeding it will now be seen that the radula plays 

 a very important part; it is used for seizing and conveying to the 

 pharynx all the food that the animal takes; while the mandibles, it 

 may be noted, assist in retaining the food temporarily in the pharynx. 



Thus the radula of Crepidula, far from its being, as I thought, an 

 obsolete organ, is one which is in constant use and of the first im- 

 portance in the life of the animal, but, instead of its being used for 

 rasping, as in its allies and presumably in its ancestors, it is now used 

 for grasping. The function of the radula in Crepidula has therefore 

 changed, and the failure to imagine the probability of such a change 

 led me to a wrong conclusion with regard to its present importance to 

 the animal. The change in function is, however, interesting, as it adds 

 one more instance to the economy practised by nature in making use 

 of the material that is to hand. Signs of degeneration iu the radula 

 are nevertheless appearing, as may be gathered from the following 

 independent observations by Haller (5): "Die Auffallende kurze Eadula 

 {of species of Crepiclulet) liegt in einem sehr dickwandigen Eadularsacke. 

 Der Munddarm und die Buccalmasse ist bei alien von mir un tersuchten 

 Calyptraeiden ungemein klein." Hence one might expect to find among 

 the allies of Crepidula some forms which are evolving out of their radular 

 apparatus a more efficient organ adapted to the present needs; and 

 such a change is the more to be expected as the radula is a specific 

 variant in the group to which the animal belongs. 



VI. EESEMBLAXCE OF THE FUNCTION OF THE GILL OF 

 CEEPIDULA TO THAT OF THE LAMELLIBEAXCH GILL. 



A change in function — or rather an additional function — has also been 

 taken on by the gill of Crepidula. The ancestral gill was probably 

 mainly an organ of respiration, but now the gill serves also as a food- 

 collector. The gill of Crepidula has, therefore, exactly the same function 

 as that of typical Lamellibranchs. The phenomenon is thus apparently 

 presented of two independent trends of evolution arriving in principle 

 at exactly the same result : both groups of animals having utilized the 

 respiratory organ in a similar way as a water-pump and as a food-sieve.* 



VII. THE MODE OF FEEDING IN THE OYSTEE AND OTHEE 



LAMELLIBEANCHS. 



The mode of feeding in Lamellibranchs has been described by several 

 writers. Stenta (6) described a number of forms fairly fully. Kellogg 



* If, however, the gill of the ancestors of Lamellibranchs and Gastropods were already 

 a food-collecting organ— as seems possible from the observations here made on the gill of 

 Nucula and many Gastropods (see pp. 467-73)— then the "convergence" is homogenic and 

 not homoplastic. 



