26 



]){' seen, apjjeais as an appendag-e (jnite distinct from the 

 rest of tke visceral mass, and contains no extensions of the 

 reproductive organs. 



The pericardium (tig. 1, I^er.) is situated posterior to 

 the digestive gland. The rectum (fig. A/, c. 5) passes 

 through tlie ventricle of the heart, which is enclosed by 

 the pericardiiim, and is continued over the adductor 

 muscle, to which it is attached, bending to one side of the 

 median line and eventually terminating in a lipjoed anus. 



The aperture of the mouth is placed not far above the 

 fo(jt on the anterior surface of the digestive srland between 

 the two ver}' conspicuous dendritic lips, pigmented with 

 an orange colour (fig. 1, L. 'p.). 



At each side where the gills terminate dorsally are 

 two flaps, also pigmented with a yellowish broAvn colour. 

 These are the Labial Paljjs (fig. 1, L. y. e.) \ they become 

 continuous dorsally with the lips. 



The renal organs (fig*. 1, R. o.) are situated on each 

 side of the reproductive portion of the viscera between it 

 and the gills, and the external opening at their ventral 

 end serves both as renal and reproductive aperture (fig. 1, 

 Uo. r-p.). 



The positions of these various organs in relation to 

 the shell are not the same as those in the Dimyaria. Thus 

 the pericardium is posterior, the digestive gland (" liver ") 

 is dorsal and the foot and visceral mass are situated 

 anteriorly, the hinge line being considered as dorsal. 



Owing to this, some authors have regarded the hinge 

 line as dorso-anterior, and the antero-posterior diameter 

 as represented by a line drawn from the front corner of the 

 hinge line to the point where the rectum ends. The 

 position of the organs is regarded as due to an increase in 

 size of the posterior adductor after the disappearance of 

 the anterior adductor, together with a movement of the 



