MEDICINAL LEECH. 215 



slender core of hypodermic cells as they appear to be from a comparison with the 

 segmental papillae and from their histo-chemical properties (Whitman). The nerve 

 enters at the side but near the base, and at its point of entrance there appear to be 

 ganglion cells. The goblet-bodies and segmental papillae have a bulb-like 

 thickening of the hypodermis cells, no pigmented coat, and relatively few clear 

 cells. The former are exceedingly numerous (about sixty in number), especially 

 on the anterior margin of the prostomium. They are situated on branches 

 of the same nerves as the eyes. The Leech is sensitive to light, but the 

 prostomial region appears to be equally if not more sensitive to other impressions 

 (touch, taste ?). The eyes of Clepsine differ from the eyes of Hirudo, and consist, 

 according to Carriere, of a semi-globular cup of large pigmented retinal cells, con- 

 taining a number of transparent and smaller cells. They are situated in the 

 muscular coat. The eyes of Nephelis appear to be similar ; those of Pisticola, a 

 Leech in which they occur on the posterior as well as the anterior sucker, appear 

 to be more simple, judging from Leuckart's figures. The visual nature of these 

 eyes seems certain. 



The sub-hypodermic tissues consist of a connective jelly-like matrix imbedding 

 nucleated corpuscles with very fine branches, and more or less pigmented, as well 

 as ' vaso-fibrous ' tissue: The latter consists of nucleated branched cells, containing 

 a plentiful supply of pigment granules. Their branches sometimes become tubular, 

 and the nuclei project into the lumen, and even drop into it. These cells occur in 

 all parts of the body, and their processes pass out between the hypodermic cells. 

 According to Professor Lankester, the tubular cells may become continuous with 

 the thin-walled capillaries, in the cavities of which free nuclei may sometimes be 

 detected. The botryoidal or so-called ' hepatic ' tissue, which surrounds, but not in 

 immediate contact, the walls of the alimentary tract appears to be merely a modifi- 

 cation of the same tissue. It consists of largish vessels with cellular walls. The 

 individual cells are swollen up, nucleated, and pigmented. The vessels thus formed 

 partly end caecally, partly form a plexus continuous with the ordinary thin-walled 

 capillaries. The corresponding pigmented cells of Rhynchobdellidae never become 

 tubular, and usually remain more or less rounded. In the same group, vacuolated 

 cells and fat cells are found in the connective tissue matrix. They do not occur in 

 Gnathobdellidae. 



The muscles of the body are imbedded in the connective tissue matrix, leaving 

 a sub-hypodermic layer free. They are arranged in an outer circular and a deep 

 longitudinal series. Between the two are diagonal fibres* In Hirudo connective 

 tissue with longitudinal fibres intervenes between the circular and diagonal layers. 

 A set of dorso-ventral and radial fibres takes the place of the septa of Chaetopoda. 

 The fibres of this series near the centre of the body pass between the caeca of the 

 first portion of the alimentary canal. The outer ends of the radial fibres are 

 branched, and end close under the hypodermis in the sub-hypodermic connective 

 tissue. The muscle fibres themselves possess an outer fibrillate layer and an inner 

 granular medulla with a nucleus. Their ends are often much branched, especially 

 on the walls of the alimentary canal. 



The amount of connective tissue matrix present, as compared with muscular 

 fibres, is very variable in Hirudinea. Where there is little of it, as in Clepsine and 

 Nephelis, the worm is perfectly firm and rigid to the touch when living. In Aulo- 



