116 HUGH WATSON. 



Respiratory Opening. — The aperture of the mantle-cavity 

 or " respiratory orifice " is situated towards the hind end of 

 the back, just to the right of the mid-dorsal line. Its posterior 

 position, about four times as far from the head as from the 

 hind end of the slug, is due to the lengthening of the anterior 

 part of the body and the retrogression of the pallial organs,, 

 as in Selenochlamys and Testacella. The dorsal posi- 

 tion of the opening is explained by the absence of any external 

 mantle or shield in Apera.^ 



The orifice is alwaj-s a little nearer the right side than the 

 left, a fact which previous writers seem to have overlooked, 

 although it is of considerable morphological importance, as 

 may be seen from PL X, fig. 33 (a transverse section through 

 the respiratory opening of Apera gibbonsi rubella). This 

 illustration also shows that the opening is surrounded b_v two 

 lips, one within the other. In A p e r a gibbonsi and A . p a r v a 

 the outer lip projects over the inner lip so as almost to con- 

 ceal it; but in the other four species the outer lip is much 

 narrower, and leaves a wide circular opening, within which 

 the light-coloured inner lip is exposed to view (PI. A^II, fig. 

 12; PI. VIII, figs. 24, 25, 26). The right anterior edge of 

 the inner lip is cleft by an oblique slit, running forwards and 

 outwards from the central aperture. 



Keels. — In most of the species of Apera the back is 

 rounded, but A. burnupi and A. sexangula both have four 

 longitudinal keels, two on each side of the body, which may 

 possibly serve to increase the rigidity of the skin. The upper 

 keels of A. sexangula are nearly parallel throughout the 

 greater part of their length, the distance between them measur- 

 ing about three-fifths of the breadth of the body. When followed 

 backwards, however, they diverge very slightly just in front 

 of the respiratory opening, and then gradually approach each 

 other behind it, until they unite to form a single median keel,. 

 3 or 4 mm. in length, at the hind end of the animal. In A. 

 burnupi the upper keels are further apart, and become very 

 prominent towards the hind end, divei'ging considerably and 

 ' See pp. 130. V.n. 



