428 ALFEED GIBBS BOUENE. 



iii. Skin — Epidermic Glands — Sensory Cells — Dermis. 



The previous observations upon the histology of the skin are 

 to be found scattered through the works of Leydig, and a 

 complete account of the epidermis in Hirudo in Professor 

 Lankester's memoir (16). 



The skin consists of: 



1. Cuticle. 



2. Epidermis. 



3. Sub-epidermis or dermis. 



Cuticle (see figs. 9— 14, cm.). — This presents similar cha- 

 racters throughout the group; it is a structureless, hyaline, 

 somewhat elastic envelope, marked here and there with stria- 

 tions. It is easily separable by maceration. Besides the 

 pores in it, which correspond to the mouth, anus, generative 

 apertures, and the apertures of the nephridia, it exhibits 

 all over its surface small rounded pores which correspond 

 to the apertures of the various unicellular glands described 

 below. 



In some genera — Clepsine, Piscicola, and Pontobdeila 

 — the skin can be voluntarily thrown into papillae at certain 

 definite spots. In such condition the cuticle becomes much 

 stretched at these spots, but these can be completely with- 

 drawn, the surface becoming perfectly smooth as in Hirudo, 

 Hsemopis, Aulostoma, Heemadipsa, Nephelis, and Tro- 

 cheta, thus bringing very markedly into play the elasticity of 

 the cuticle. 



The cuticle is continually undergoing regeneration, new 

 layers being secreted by the epidermic cells, the old cuticle 

 being peeled off. This process is most easily seen in Hirudo, 

 which, when kept in cavity, is continually shedding its skin. 

 This cuticular ecdysis takes place, no doubt, in all leeches, but 

 is more frequent in some genera than in others. 



Epidermis (see figs. 9 — 14, ep.). — This consists through- 

 out the group of a. single layer of nucleated cells; certain 

 of these may become converted into unicellular glands, 



