476 



J. H. ORTON. 



Troschel describes the mandibles and gives a drawing of them in 

 the work cited. They are very small, being about "7 mm. long and 

 •16 mm. wide in the widest part. 



Fig. 20. — A^'entral view of the roof of the mouth of Crepidula. 

 A. Middle part of anterior edge of the roof of the mouth. 

 M. Mandible. 

 P. Prominence on which the mandible is situated. 



(x 25.) 



SUMMAKY. 



The gut-contents of Crepidula and the English oyster are similar with 

 regard to skeletal remains, and the commonest diatoms found in both 

 animals are the same, 



Crepidula is a marine Pectinibranch which settles down at an early 

 age to a sedentary life. 



Crepidula feeds in the same way, in principle, as the oyster, i.e. a food- 

 current of water is set up in the mantle cavity, while between the 

 entrance and the exit of the current the pectinate gill acts as a food- 

 sieve. The food-particles arrested in the inhalent chamber reach the 

 mouth in one of two ways : the fine particles by way of a food-groove 

 on the right side of the body, the coarse particles by way of a food-pouch 

 placed in front of the mouth. 



The radula is used for grasping the food-masses and conveying them 

 into the mouth ; its function has therefore changed from a rasping to a 

 grasping organ, hence adaptational developments of the radula may 

 be expected to occur in the allies of Crepidula. 



The mode of feeding may be easily observed in detail by inducing 

 animals to fix on glass and feeding them with carmine granules sus- 

 pended in methylene blue solution in sea water. 



The main food-current is caused chiefly by rows of cilia, the lateral 

 cilia, on the anterior and posterior faces of the gill-filaments : the food- 

 streams are caused by rows of cilia on the dorsal and ventral faces of 

 the gills, by cilia on the dorsal surface of the animal, and by cilia on 

 the inside of the mantle. 



