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2. Observations on the microscopic Anatomy of the Medicinal Leech 

 (Hirudo medicinalis). 



By Professor Ray Lankester, University College, London. 



Whilst we have from the pen and pencil of Ley dig a very de- 

 tailed account of the structure of the nervous system and sense organs 

 of the common medicinal Leech — which has been supplemented by 

 the researches of Hermann — the histology generally of the integu- 

 ment, of the connective tissue, blood-vessels, alimentary canal and 

 nephridia (excretory organs, Schleifencanäle) , has never been satis- 

 factorily investigated. In Leuckart's excellent »Menschlichen Para- 

 siten« we find a description of the anatomy of the medicinal leech in 

 which the earlier researches ofGratiolet, Brandt and L e y d i g are 

 cited and supplemented by the author's own observations made by 

 means of transverse transparent sections. 



During the past twelve months a certain amount of attention has 

 been given in my laboratory to the minute anatomy of the medicinal 

 Leech, by myself and by my pupils MM. J. E. Blomfield and 

 A. G. Bourne. Some of the results which we have obtained are new 

 and some in contradiction with the conclusions arrived at by 

 L eue kart. They may be briefly summarized as follows. 



1) Epidermis. The cells forming the epidermis are in a single 

 row as in the Earthworm but smaller and more completely cylindrical 

 than those constituting the epidermis of that animal. They do not pre- 

 sent a branched base as do the latter, nor are there goblet-cells scat- 

 tered at definite intervals amongst them. They are all, with the excep- 

 tion of the very large unicellular glands which sink far into the 

 body-wall, of the same cylindrical form and appearance and produce 

 on their free surface a thick cuticle which is readily separable. As 

 Leuckart has observed, in many places the branches of coloured 

 connective-tissue cells pass in amongst the single layer of epidermic 

 cells. This also occurs in the Lumbrici da e, — not ably in Z. oli- 

 clus. — An important fact which has not hitherto been recorded, is that 

 in all parts of the epidermis of the Leech, excepting the extreme ends 

 of the body, fine blood-vessels penetrate the epidermic 

 layer and take a horizontal course amongst the cylin- 

 drical epidermic cells. The respiratory function of the body- 

 surface is no doubt effected through this arrangement. 



The clitellus of the Earthworm exhibits a similar interpénétration 

 of blood-vessels and epidermic cells — but in this case the interpéné- 

 tration is in an exceptionally thickened and hypertrophied layer of the 



