THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 129 



funnel-shaped prolongation of the body-cavity which extends 

 backwai'ds for a variable distance beneath the mantle-cavity, 

 and raises its floor along the middle. This feature is well shown 

 in PL XI, figs. 34, 35, and PI. XII, fig. 36, for the prolongation 

 is unusually large in Apera dimidia and A.purcelli. 



In horizontal section the mantle-cavity or lung is roughly 

 triangular, with the large pericardium occvipying the centre 

 of the broad anterior part, Avhich is thus divided into a right 

 and a left wing (PI. IX, figs. 27-31). The right division opens 

 widely behind into the posterior part of the mantle-cavity, but 

 the left division is cut off by the kidney, which extends 

 obliquely backAvards from the pericardium and fuses with 

 the left wall and the roof of the cavity. Thus the left 

 anterior division only communicates with the rest of the 

 nuintle-cavity by a narrow space round the top and front 

 of the pericardium and kidney. It is, therefore, not sur- 

 prising that the walls of this part of the cavity are without 

 respiratory tissue. But the right anterior division, which 

 communicates more freely with the air, and the greater part 

 of the posterior half of the cavity have their walls richly 

 supplied with pulmonary veins. These vessels branch 

 repeatedly, and in the larger species the fine branches form a 

 complicated network which divides the surface into numerous 

 alveoli. The thinness of the epithelium which separates the 

 blood in these vessels from the air in the mantle-cavity will be 

 seen from PI. XII, fig. 39. The only portion of the posterior 

 half of the cavity that is devoid of respiratory tissue is a 

 part of the roof ^ and the immediate neighbourhood of the 

 respiratory opening and the anus just below it. 



At the hind end the cavity tapers to a point. In Apera 

 gibbon si this point is below the extreme hind end of the 

 shell-sac; but in species such as A. sexangula and A. 

 dimidia it is within the papilla which is formed by the floor 

 of the shell-sac projecting into the hollow apex of the shell. 

 Hence in these species the hind end of the mantle-cavity is 

 inside the shell itself. 



' See p. 187. 



