130 HUGH WATSON. 



I have only been able to examine the minute structure of 

 the epithelium lining the mantle-cavity in one species, namely 

 A. dimidia. In this species (and probably also in the 

 others), that part of the posterior half of the cavity where 

 there is no respiratory tissue is lined by moderately large 

 cells, slightly higher than they are broad, and strongly ciliate 

 near the respiratory opening. These cells are continuous 

 through the aperture with those of the epidermis. The whole 

 of the remainder of the cavity, including both of the anterior 

 divisions, is lined by a pavement-epithelium of cells so thin 

 that the nucleus of each usually forms a knob-like projection 

 in the centre of its surface. 



The Mantle. — Apera has no external mantle — a remark- 

 able fact that was first pointed out by Heynemann.^ Yet it 

 cannot be entirely Avithout a mantle, for, as we have just seen, 

 it possesses both a mantle-cavity and a shell ; and all Gastro- 

 pods that have a shell also possess a mantle, as the 

 shell is always secreted by the mantle. I therefore regard 

 that part of the integument Avhich forms the roof of 

 the mantle-cavity beneath and around the shell as being 

 undoubtedly homologous with the mantle. Moreover I 

 believe that the mantle is not entirely concealed from view in 

 all the species of Apera, for it seems probable that it forms 

 the inner lip of the respiratory opening, and that the upper 

 surface of this lip may be regarded as part of the upper 

 surface of the mantle (PI. X, fig. 33). This theory is confirmed 

 by the occurrence of an oblique cleft in the inner lip of the 

 opening in a similar position to the cleft which runs from the 

 opening to the edge of the mantle in an ordinary slug. 



We may suppose that Apera has been evolved from a 

 snail, having on its back a mantle covered by a shell. It 

 might well be an advantage to such an animal to get rid of 

 this projection, especially if it burrowed in the ground in search 

 of worms. Accordingly both mantle and shell seem to have 

 sunk into the back and to have become covered by folds of the 

 surrounding skin which have grown over it on each side. 

 ' ' Jahrh. d. Deutsch. Mai. Ges.,' 1885, vol. xii, p. 19. 



