PATELLA. 223 



(b) A pouch — the Radular Sac, formed of ectodermic 

 epithelium, with a little subjacent connective tissue. 



(c) A horny tooth-studded ribbon, the Radula, of which 

 the part in use rests on the surface of the cushion, while 

 the parts in reserve and in course of formation are lodged 

 in the radular sac. 



The Cushion is the projection on which the radula rests, 

 and whose movements enable that organ to perform' its 

 scraping function. It consists essentially of muscles and 

 four pairs of cartilages, and is covered above by the fore 

 gut epithelium and that of its diverticulum, the radular 

 sac. The radula is placed in a median groove on the 

 dorsal surface between the paired cartilages (fig. 12). 



The cartilages are best named Anterior, Aiitero-lateral, 

 Yentro-lateral and Posterior. Their shapes and relations 

 are most suitably explained by diagrams (fig. 13). Each 

 anterior cartilage has a ridge towards its front end, acting 

 as a lateral pad for the radula. The cartilage is of the 

 type called spongy, as it consists of fairly large, clear, 

 nucleated cells, separated by a comparative^ small amount 

 of structureless intercellular substance. The antero- 

 lateral are placed at the sides of the anterior cartilages 

 near their front ends, and give the cushion its charac- 

 teristic broad front. The ventro-lateral cartilages, first 

 noticed by Amauclrut, are very small, and on the ventral 

 surface of the cushion. The posterior cartilages are well 

 developed, and not so ventral as in some Rhipidoglossa. 



The muscles of the Odontophore include (1) Extrinsic 

 Muscles, connecting the odontophore with the walls of the 

 buccal chamber, (2) Intrinsic Muscles, connecting the 

 cartilages with one another and with the sub-radular 

 membrane. 



The chief Extrinsic Muscles are : — 



(a) The large ventral protractor muscles attached 



