446 J. H. ORTON. 



schia, Navicula, and Grammatophora, and occasional specimens of a 

 few other species. As the majority of these forms were found living 

 in the washings from the shells of Crepidula and the oyster, there is 

 no doubt that the animals were feeding on at least most of the forms 

 mentioned. A species of Prorocentrum, probably P. micans, was how- 

 ever the organism found in the greatest numbers in the gut of these 

 animals at the time they were examined, i.e. about the month of 

 October, 1911. 



It therefore became evident that Crepidula takes the same kind of 

 food as oysters, and as the oyster has no radula, I appeared to have 

 gained my point about the radula of Crepidula, namely, that it is a 

 useless organ. However, while examining Crepidula one evening I 

 detected a current in the mantle cavity, and subsequently observed 

 the mode of feeding, which established beyond doubt both the nature 

 of the food-material and the use of the radula, as will be shown in 

 the following account. 



II. THE MODE OF FEEDING IN CEEPIDULA. 



Crepidula feeds in the same way in principle as the oyster — that is, 

 an ingoing and an outgoing current of water is established in the 

 mantle cavity along a definite pathway, while between the two 

 currents the gill acts as a strainer, retaining even very fine particles 

 of suspended matter which may eventually reach the mouth. The 

 gill consists of a row of free filaments — more than four hundred fila- 

 ments were counted in the gill of an adult specimen — placed parallel 

 to one another, midway between the dorsal and ventral surfaces. 

 The filaments stand out in a line along the left side of the mantle 

 cavity, extending almost in a horizontal line across this cavity ; their 

 tips rest along the edge of the right epipodium anteriorly, but 

 posteriorly on the dorsal surface of the visceral mass. The gill thus 

 forms a sheet across the mantle cavity, which it divides into a left 

 ventro-lateral inhalent chamber, and a right dorso-lateral exhalent 

 chamber. Fig. 1 gives a ventral view of the animal in the act of 

 feeding ; the arrows indicate the direction of the food-current. In 

 feeding, the front end of the shell is raised slightly and a current is 

 set up in the mantle cavity by the cilia on the gill-filaments. Water 

 is drawn in along the anterior half of the edge of the shell on the 

 left, passed through spaces between the gill-filaments, and is expelled 

 along the front half of the edge of the shell on the right (see 

 Fig. 1). 



