FEEDING OF CREPIDULA. 477 



The gill of Crepidula, like that of Lamellibranchs, is at the same time 

 a respiratory organ, a water-piimp and a food-sieve. 



The food-streams of Crepidula are comparable to those of Lamelli- 

 branchs. 



A partial selection of the coarser food-particles is effected in the 

 oyster — and Lamellibranchs generally — in the same way in principle as 

 in Crepidula. 



There are special morphological arrangements in Mytilus and Cardium 

 to assist in the automatic selection of the finer food-particles. 



The main food and respiratory stream in Lamellibranchs is caused by 

 the " lateral " cilia on the gill-filaments, while the collection and trans- 

 portation of food is effected mainly by the frontal cilia of the filaments. 

 The latero-frontal cilia in Nucula, Anomia, Mytilus, Tapes, and 

 Cardium, and therefore probably in all the Lamellibranchs in which 

 tliey occur, are true straining cilia. 



The gill-leaflets of Nucula and most Protobranchia possess similar 

 cilia having a similar function to those on the gills of higher Lamelli- 

 branchs, and those of Nucula nucleus have also ciliated discs. 



The gill of Nucula divides the mantle cavity into infra- and supra- 

 branchial chambers, and the ventral surface of the gill is used for 

 food-collection in the same way as in higher Lamellibranchs. Thus 

 the gill of Nucula is essentially similar to that of some Filibranchs — 

 for example, some species of Anomia and Dimya. 



Hence the Protobranchia cannot now be considered as a group 

 co-equal with the Filibranchia, and should be elevated to a sub-division 

 of the Filibranchia, 



Evolution in Lamellibranchs, which has occurred mainly on the 

 principle of folding and consolidating the gill-filaments, comprises a 

 series of adaptations tending towards a more perfect mode of 

 feeding. 



Calyptraea chiiunsis feeds in exactly the same way as Crepidula, and 

 Capulus hunfjaricus feeds in a similar way. 



Doubtless, therefore, all the Calyptraeidae, all the Capulidae, and 

 there is reason to suspect that all sedentary Pectinibranchs, feed in 

 the same or in a similar manner. 



Most Gastropods have gill-filaments essentially the same in structure 

 and function as those of Lamellibranchs, i.e. lateral cilia occur gene- 

 rally and produce the main current in the mantle cavity ; frontal and 

 ab-frontal cilia are found, and these collect food-particles from the 

 ingoing current. In many Gastropods, and probably in most branchiate 

 forms, the mantle cavity is divided by the gill into inhalent and 

 exhalent chambers. 



