THE CAKNIVOKOUS SLUGS OF SOUTH AFRICA. 127 



Perhaps iu Apera they also assist the circulation ot the blood 

 in a similar manner. But it seems possible tliat their chief 

 function may be to enable the animal to increase the turgidity 

 of some or all of the dermal vessels when it contracts, and thus 

 to give greater rigidity to the skin either as a whole or in part. 

 For if the slug by closing the sphincters could thus increase 

 the pressure in the dermal blood-vessels, the skin would 

 become much more rigid in consequence, just as the strength 

 of the stem of an herbaceous plant is largely due to the 

 turgidity of its cells. 



THE SHELL, THE MANTLE-CAVITY, AND THE 

 MANTLE. 



The Shell. — The small shell of Apera lies deep in a little 

 pocket in the skin just behind and to the left of the respiratory 

 opening. As a rule its length is only about one-fourteenth 

 of that of the animal (iu alcohol). When viewed from above 

 the somewhat oval contour of the shell is seen to be inter- 

 rupted by a broad sinus which extends backwards from the 

 front end more than half way along the right side. The 

 posterior part of the shell is thus broader than the anterior 

 half, which projects forward on the left side of the respiratory 

 opening. The sinus is deepest in Apera gibbonsi, and 

 shallowest in A. purcelli and A. dimidia. It will be 

 seen from PI. XIII, figs. 42, 45, 48, and 51, that the shell of 

 Apera is often much less flattened than is usually the case 

 with the iuternal shells of slugs. In A. burnupi and A. 

 sexangula it is very convex, and culminates in a conical 

 apex, which overhangs the posterior margin of the shell. In 

 A. purcelli the apex projects still further, but it is rounded 

 instead of conical, as is also the case in A. dimidia, in 

 which the apex is less prominent. Lastly, in A. gibbonsi 

 and A. parva the shell is more depressed, and the apex is in 

 front of the posterior margin. 



The shell is extremely thin and translucent iu Apera 

 purcelli and A. dimidia. In the other species it is thicker 



