126 HUGH WATSON. 



zone of connective tissue is extremely thick, especially 

 towards the hind end of the animal; indeed, the most remark- 

 able thing about the skin of A per a is its great thickness and 

 strength. Only in the region of the head, where flexibility is 

 essential, is the skin moderately thin. Its thickness further 

 back in A. gibbonsi is well shown in PI. X, fig. 32. On 

 the other hand, A. dimidia has a somewhat thinner skin 

 than most of the other species, though even in this form its 

 thickness is not inconsiderable, as will be seen from PI. XI, 

 figs. 34 and 35. This unusual thickness of the skin can easily 

 be explained. In Apera the shell is so degenerate that it can 

 no longer fulfil its proper functions. Yet the slug still needs 

 protection from other animals seeking to prey upon it, and 

 from the pressure of the soil if it burrows in the ground like 

 Testacella, and also from excessive evaporation. And it still 

 needs a firm attachment for its powerful retractor muscles. 

 Therefore the skin has taken over the functions of the shell, 

 and has become adapted to fulfil these functions adequately. 

 The Rev. Dr. Cooke ^ has recently called attention to a case 

 among the Poly pi acoph ora in which the protection once 

 afforded by the shell has been transferred to the integument. 

 But in that case the mantle has taken the place of the shell, 

 while in the present instance it is the skin itself which has 

 been strengthened. 



The blood-vessels contained in the skin are irregularly 

 disposed, though most of them run in a longitudinal direction. 

 They communica,te with the body-cavity or hgemocoele by 

 minute pores, which can be seen puncturing the inner surface 

 of the skin (PI. X, fig. 32). In all parts of the skin these 

 vessels are provided at rather frequent intervals with power- 

 ful sphincters composed of circular muscles (PI. XIII, fig. 40). 

 Similar sphincters have been described by Dr. Simroth and 

 P. and F. Sarasin" in Veronicella (a genus in no way allied 

 to Apera) ; and it has been suggested that they serve to 

 prevent the reflux of the blood when the animal moves. 



1 ' Proc. Mai. Soc..' 1913. vol. x, pp. 321, 322. 



■^ ' Die Land-Molluskeu von Celebes.' 1899. p. 90. pi. xiii. fi^js. 122, 123. 



