124 HUGH WATSON. 



THE STRUCTUEE OF THE SKIN. 



Epidermis. — The outermost layer of the skin on the iDack 

 and sides of the animal consists of a well-marked cubical 

 epithelium, which in Apera dimidia attains a thickness of 

 rather less than '01 mm. (PI. XII, fig. 37). The cells composing- 

 this epithelium are approximately square in section ; they 

 stain rather deeply, and each contains a large rounded 

 nucleus. The outer walls of these cells are thickened to form 

 a cuticle, while the remaining walls are much thinner. In 

 this respect, thei'efore, the epidermis of Apera seems to 

 resemble that of Testacella rather than that of D au de- 

 bar dia, in which all the walls of the epidermal cells are 

 thickened,^ or of Paryphanta, in which a cuticle is said to 

 be absent. - 



The epidermis does not change its character on the dorsal 

 side of the foot-fringe, as in such forms as Daudebardia* 

 and Cystopelta,^ but on the sole of the foot it becomes 

 quite different. Here the epithelium is columnar rather than 

 cubical, and the inner ends of the cells taper and interdigitate 

 with the underlying connective tissue, so that the limits of 

 the epithelium are veiy ill-defined (PI. XII, fig. 38^). 

 Excepting towards the edges of the foot-sole, the outer end of 

 each cell is produced into a number of delicate cilia, which 

 in A. dimidia measure about •0035 mm. in length. 



Inner Layers of the Skin. — In the outer part of the con- 

 nective tissue, which everywhere underlies the epidermis, are 

 embedded the dermal mucous glands and the pigment- 

 granules. The mucous glands (PI. XII, fig. 37) are oval or 

 pear-shaped, and reach a considerable size, although each is 

 formed from a single cell. The protoplasm of the cell is 



' Plate, L. H., ' Zool. Jalirb.,' 1891. vol. iv, p. 529. 

 ••' Beutler, B., ' Zool. Jahrl).,' 1901, vol. xiv, p. 374. 

 ' Plate, op. cit., p. 527. 



^ Davies, Miss O. B., 'Proc. Roy. Soc. Victoria,' 1912, vol. xxiv, p. 332. 

 •'' I am indebted to Mr. W. Tams for kindly taking this photomicro- 

 graph. 



