THE CARN1V(3R0US SLUGS OF SOUTH AFRICA. 135 



varies from about 1 mm. in the smaller species to more than 

 5 mm. in a large specimen of A p e r a b u r n u p i, and its breadth 

 is usually about one-third of its length. The walls of the 

 vesicle are thin and semi-translucent like those of the duct, 

 with which they are continuous. A few gland-cells occur along 

 the ventral^ surface in A. burnupi, A. sexangula, and 

 A. purcelli, but none at all in the other species. The 

 structure of the walls of the vesicle is similar to that of the 

 walls of the duct, excepting that the cells of the epithelium 

 become somewhat higher on the dorsal side, and among the 

 ordinary epithelial cells there are scattered a few rather 

 small vacuolated cells. 



From the dorsal wall of the vesicle there projects into its 

 cavity a remarkable fold, resembling a typhlosole, which fills 

 up a large portion of the interior, so that the cavity is cres- 

 centic or horse-shoe-shaped in section (PI. XIII, fig. 62). If 

 the wall of the vesicle in Apera sexangula be cut open, it 

 will be seen that the fold has a spongy and somewhat lami- 

 nated appearance. Serial sections through the vesicle of 

 A. dimidia show that in this species the fold is rounded 

 excepting at the ends, where it becomes wrinkled and divided 

 by furrows (cf . PL XIII, figs. 61, 62, PI. XIV, fig. 66). Fig. 

 QQ shows the structure of the fold in A. dimidia. It will be 

 seen that it is entirely composed of vacuolated tissue, which 

 is not bounded by a distinct epithelium. The pedal artery, 

 which always becomes intimately connected with the dorsal 

 surface of the vesicle, gives off a branch into the middle of 

 the fold (PI. XIII, fig. 62). In A. gibbonsi the structure 

 of the fold is different, for in this species it is divided into a 

 complicated series of narrow lamellas. Fig. 58 shows the 

 appearance of these as dimly seen through the wall of the 

 vesicle, Avhile PI. XIV, fig. 65, shows them in section. Each 



' I use the terms '" dorsal " and " ventral " in a morphological sense, 

 the dorsal side being that to which the artery is attached. Generally, 

 however, the vesicle lies on its side, owing to the twisting of the duct, 

 and the attachment of the artery to the floor of the body-cavity beyond 

 the vesicle. 



