W. A. Harding 
131 
most readily placed at my disposal. Finally I desire to express my 
thanks to Mr Edwin Wilson, of Cambridge, who has spared no pains 
to make the coloured illustrations accompanying this memoir exact 
representations of the living examples placed before him. 
With regard to the number of the British leeches, it may be stated 
of the fresh-water forms with some degree of certainty that ten species 
now occur in these Islands. An eleventh species, Himdo medicinalis, 
is included in the following list, although there is very little doubt that 
it no longer occurs in the wild state. We cannot speak so positively 
regarding the number of marine species. Our knowledge of the marine 
Ichthyobdellidae leaves much to be desired and few European species 
are well established. Pontobdella alone is frequent upon our shores; 
Branchellion has been noted twice, and there remain a number of forms 
which have been recorded from time to time, chiefly from the coast 
of Scotland, the descriptions of which are generally not sufficiently 
adequate to enable us to do more than guess at the species upon which 
they have been based. Some of these have been referred provisionally 
to Trachelobdella lubrica (Grube, 1840) and it seems not improbable 
that at least one other species has been observed in British waters in 
addition to the three marine leeches already noted. 
The descriptions here given apply to features which may be seen 
either by the naked eye or with the assistance of a good dissecting 
lens. The form described, unless otherwise stated, is that assumed by 
the leech when in a moderate or average state of extension. The 
treatment of the various morphological features indicated in the series 
of figures supplementing the coloured illustrations is purely schematic. 
Whilst every endeavour has been made to make the synonymical 
tables as complete as possible, the accompanying bibliographical refer¬ 
ences make no pretence of being exhaustive. 
The Classification here adopted is that laid down by Professor 
Raphael Blanchard in a well-known monograph on the leeches of Italy 
(1894) and subsequently modified by him in other works. The same 
authority also has generally been followed with respect to the synonymy 
of the species referred to. 
Diagnostic Characters. 
In the leech, as is well known, the number of rings exceeds the 
number of somites or segments into which the body is divided, and 
throughout the greater part of its length these riugs resolve them- 
