430 ALFEED GIBBS BOtJEjTE. 



Pontobdella, but in the ventral region lose their columna 

 character, and are much flattened. 



The amount of intrusive pigmented tissue is so great that 

 even in very thin sections the epidermic cells are hardly visible. 

 There are no intrusive capillaries. 



In the Gnathobdellidse we find little diflference in the epi- 

 dermic cells among the genera inter se. The cells are long 

 and narrow, and the nuclei are very small. There is 

 naturally much variation in the amount of intrusive pigmented 

 tissue, this depending upon the amount of colouring in the 

 species, but diff'ering from point to point in accordance with 

 the pattern upon the surface. 



In all the genera capillaries force their way in between the 

 separate cells, the epidermis thus becoming a vascular mem- 

 brane.^ 



In Nephelis and Trocheta the epidermic capillaries (fig. 

 14, cap.) are exceedingly large and run for considerable 

 distances immediately underneath the cuticle. Very little 

 rough handling in the living state is sufficient to cause 

 extravasation. 



Two kinds of modification may take place in epidermic cells : 



1. They may become glandular. 



2. They may become sensory. 



Epidermic Glands. — These are of two kinds : (i) mucous 

 glands which remain dermal in position ; and (ii) glands which 

 secrete some special substance, and which take up a deep 

 position, either between the bundles of muscular fibres or even 

 entirely within the muscles of the body wall. Both varieties 

 always remain unicellular. 



i. Mucous Glands. — These occur all over the surface of 

 the body. They are easily recognisable as modified epidermic 

 cells. As is usual in all similar cells which store up their 

 secretion within themselves (e.g. fat-cells), the nucleus becomes 

 pushed up against the cell membrane, and is surrounded by a 



' E. Ray Lankester, 'Quart. Journ. Micr, Sci.,' 1880, p. 303; and 'Zool. 

 Anz.,' 1880, No. 49. 



