38 



THE AMERICAN NATURALIST [Vol. XLIII 



Lying against the inner sides of the shell-valves are 

 right and left folds of the mantle (Figs. 7, 18, etc.), the 

 free edges of which secrete the shell material and may, 

 like velum and foot, at times protrude heyond the margin. 



Along each side underneath the mantle, past the base 

 of attachment of the foot to the body, lie the gills (Figs. 

 7, 8, 9, 19, 20, 21), extending backwards and downwards 

 to near the posterior edge of the shell ; in the oldest free- 

 swimming larva there are about eight filaments in each 

 series, diminishing in size from before backwards, the 

 last ones being mere knobs; their lower ends are free, 

 but their upper ends spring from one continuous fold 

 that, behind the foot, joins its mate of the opposite side, 

 near the margins of the mantle. They correspond to the 

 right and left inner gills of the adult oyster. 



The alimentary canal (Fig. 7) is much longer than the 

 body and in consequence has become folded, the greater 

 part lying to the left (Figs. 17, 19, 20) of the median 

 sagittal plane, but mouth, oesophagus and anus are 

 median. The mouth (Figs. 7, 13) is a funnel-shaped 

 opening lying immediately below and behind the velum, 

 to which its walls are attached and with which it is pro- 

 truded and withdrawn, so that it can only be functional 

 while the velum is to some extent expanded, when the ac- 

 tivity of its cilia may also contribute to the process of 

 feeding. The oesophagus (Figs. 7, 13, 14, 19) lies be- 

 tween velum and foot in the median sagittal plane as 

 well as in or very near the median transverse plane of 

 the body. Here it passes dorsalwards, between the first 

 gill-filamets, and opens into the stomach with its large 

 brown lateral liver-sacs. The intestine passes back- 

 wards towards the right and then forwards towards the 

 left, when it again turns backwards and upwards in the 

 left umbo and finally downwards in the median plane over 

 the posterior adductor muscle. 



In front and above the velum is an anterior adductor- 

 muscle (Figs. 7, 13, 18, etc.), running transversely be- 

 tween the valves, while below the posterior parts of the 



