HORNELL— ANATOMY OF PLACUNA 65 



THE ALIMENTARY CANAL. 



The alimentary canal in Placuna consists of a slit-like asymmetric mouth, 

 a rather long and wide cesophagus, a capacious stomach surrounded by a well 

 developed digestive gland, an enormously developed pyloric caecum, a short intestine 

 making a single visceral loop, and a short and slightly curved rectum ending in 

 an anal opening in the centre of a broad and widely everted membranous collar 

 (fig. 14). 



The mouth is a long slit-like aperture situated vertically in a deep cleft 

 between two delicate membranous folds — the lips or labia — to the right side of 

 the median plane of the body at about midway between the base of the foot and 

 the dorsal margin of the body. The mouth is concealed by the labia. These, 

 which in typical Lamellibranchs are disposed transversely to the vertical plane of 

 the body, here follow the asymmetery of the mouth ; what is morphologically the 

 lower one in Placuna bounds the mouth along the left side ; the upper lies to 

 the right of the mouth. At each angle of the mouth the labia close in and pass 

 into the labial palps. The labia are smooth on both surfaces ; the labial palps are 

 smooth on the faces turned away from each other, while on those turned to one 

 another they are furrowed with many fine transverse ciliated furrows of ordinary 

 typical form such as seen in Margaritifera. The free edges of the labial palps 

 face forwards (anteriorly) while the posterior margins are attached to the front 

 of the visceral mass. Those of each pair are also closely approximated at their 

 insertion into the visceral mass, forming thus a very narrow deep-walled gutter 

 along which food pellets are propelled by ciliary action from the anterior extremity 

 of the gills to the corners of the lips and thence into the mouth. 



Unlike the labia which are thin and membranous, the palps are considerably 

 thickened with a filling of connective tissue. They are unusually long, reaching 

 well below the base of the foot. The ridges are of the same form as seen in 

 Margaritifera and densely ciliated. 



The mouth leads into a long and rather wide ciliated cesophagus, slightly 

 compressed laterally, which curves posteriorly and ventrally and opens abruptly 

 without change of calibre into the anterior dorsal region of the stomach. 



The stomach is relatively capacious and occupies a sub-central position in 

 the visceral mass. In form it is irregularly ovoid and much compressed laterally ; 

 its long axis lies somewhat obliquely dorso-ventrally in such fashion that the 

 ventral part is directed slightly anteriorly, while the dorsal exhibits a corresponding 

 backward inclination. 



Several well marked depressions occur in the walls of the stomach, so that 

 we may roughly divide it into four subsidiary chambers (vide fig. 29) ; the first 

 is shallow and lies dorsal and anterior ; into this the cesophagus opens. The second 



