v.] THE P^RESH-WATER MUSSEL. 339 



are so disposed that the ciliated groove (cf. supra) 

 leads directly to the mouth aperture. 



y. The foot; a fleshy mass, projecting forwardly, 

 immediately below the mouth. 



/. Dissect away the adductor muscle and labial palps 

 on one side, and note the relations of the retractor 

 aiid protractor pedis muscles. Their fibres are largely 

 derived from the muscular layer of the body- wall; 

 follow them to their attachments. (Cf. Sect. A.) 



Find the cerebral ganglion of the same side (see 

 Sect. I. c) and trace the course of the iiiter- 

 cerebral commissure ; it runs over the gullet, in 

 the substance of the labial-palps. Follow in like 

 manner the cerebro-pedal commissureSy until the 

 pedal ganglia are reached ; note that these — like 

 the parieto-splanchnic ones — are confluent in the 

 middle line. 



M. The reproductive elements and the larva. 



a. The animals are dioecious, but the reproductive 

 organs are similarly constructed in both sexes : 

 they vary much in size with the season, being 

 large in winter and spring, but small at other times. 



b. Tease up a small portion oT the ovary in eosin 

 solution, and examine under a high power. The 

 ripe ova are large, rounded, rarely ovoidal bodies, 

 and their special features are — 



a. The vitelline me??ibrane; usually separated from 

 the ovum by a cavity filled with a coagulate 

 albuminous fluid. 



(i. The micropyle ; a perforation, or frequently a 

 short neck-shaped prolongation of a. 



22 — 2 



