180 HUGH \VAT,S0N. 



Avhicli opened separately into the alimentary canal. But it is 

 more difficult to explain why half the liver should disappear 

 in this way. It is known, however, that the redu.ction of that 

 part of the body-cavity which usually occupies the spire of 

 the shell sometimes produces a corresponding reduction of 

 the left lobe of the liver which it contains.^ It seems pos- 

 sible^ therefore, that in A. dimidia the left division of the 

 liver, instead of merely pushing forward the right, as in the 

 other species of Apera, has disappeared altogether, just as 

 the right division has disappeared in some of the Pectini- 

 branchia. 



On the other hand, it is possible that the two divisions of 

 the liver in A. dimidia are actually homologous with the 

 two divisions in the other species. Blood-vessels are always 

 very subject to variation, and if by some mutation the so- 

 called posterior aorta had disappeared, the arteries supplying 

 the right division of the liver would probably extend to the 

 left also. Further, the posterior aorta passes to the left of 

 the intestine, and may to some extent hold it in position 

 (PI. IX, fig. 27). When, therefore, this vessel is not present, 

 there is nothing to prevent the intestine shifting to the left 

 and coming to occupy the position that it holds in A. 

 d i m i d i a . 



Possibly an examination of the liver of Apera pur colli 

 may show which of these views is correct. In this species the 

 intestine seems to occupy a similar position to that of A. 

 dimidia, but unfortunately the liver of the only specimen of 

 A. purcelli which I have been able to examine Avas in a 

 state of partial disintegration. 



Sections through the liver of Apera dimidia show that 

 the food is not confined to the alimentary canal itself, but 

 passes up the hepatic ducts into the lobes of the liver, and 

 probably digestion takes place chiefly inside this organ in 

 Apera as in At opus." Perhaps the disappearance of a 

 stomach in these genera may be pai^tly explained by this fact. 



' Pelseneer. P., ' Mem. Acad. Roy. Belg..' 1901. ex vol. liv, p. 55. 

 Sinu-otli. H., ' Naturwiss. Woohenschr..' 1901. vol. xvii. p. 121. 



