COlfTRTBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 429 



which may either preserve their position in the series or may 

 sink into the deeper layers (cf. infra — unicellular glands). Two 

 varieties of connective tissue may intrude upon the series of 

 epidermic cells and actually force their way up to the cuticle — 

 pigmented connective -tissue cells (figs. 9 — 14, pig.) and 

 capillaries (I.e. cap.) of the vascular system. 



No pigment is ever developed in the epidermic cells them- 

 selves, as was, on the contrary, shown to be the case in the 

 epidermic cells of Peripatus by the late Professor Balfour. 



The epidermic cells are always columnar, but vary much in 

 size and shape. 



In the Rhyncobdellidse they are short and wide, and possess 

 a large nucleus, while in the Gnathobdellidse they are more 

 specialised and longer cells with small nuclei, and are more 

 closely packed together. 



Piscicola (fig. 11) presents the most primitive arrangement 

 of the epidermic cells. They are small, and with the exception 

 of those which become glandular, to be described below, are 

 very regular in size. Their inner extremities are rounded off. 

 Instead of being closely packed, a small space exists between 

 neighbouring cells. The nuclei are relatively very large. No 

 connective tissue, either in the shape of pigmented cells or of 

 blood capillaries has penetrated between the cells. 



InClepsine (fig. 10) the cells remain fairly regular in size, 

 but are more closely packed than in Piscicola. The nuclei 

 are relatively smaller. 



The amount of pigment which intrudes upon the series varies 

 in the different species, some of which possess much more 

 colour than others; in Clepsine bi-oculata (fig. 10) no 

 intrusive pigmented tissue occurs. There are no intrusive 

 capillaries. 



In Pontobdella (fig. 9) the cells vary greatly in size, but 

 all retain large nuclei : a considerable amount of intrusive 

 pigmented tissue occurs, and capillaries may be seen forcing 

 their way in between the bases of the cells. 



In Branchellion (fig. 12) the cells vary immensely in size 

 and shape ; in the dorsal region they are similar to those of 



