182 
The British Leeches 
orifices are usually separated by three rings; they are never separated 
by more than three and a half rings or by less than two and a half rings. 
Length 50—70 mm.; width 4—6 mm. 
Distribution, etc. This species, widely distributed in Europe, is 
found in the same situations as H. octoculata and subsists on the same 
food. It is probably of frequent occurrence in the British Islands. In 
the neighbourhood of Cambridge it is nearly as common as H. octoculata. 
The egg-capsules closely resemble those of the latter leech but according 
to L. Johansson (1909, p. 81, fig. 144) the openings at the ends of the 
capsule are closed by spherical plugs which project into the interior. 
H. atomaria, it will be seen, differs from H. octoculata in the 
position of the genital openings, in its somewhat larger size, and 
generally in colouration and in small modifications in the external 
metamerism. Somite Y, which is triannulate in the latter species, is 
usually biannulate in H. atomaria ; colouration, which is variable in 
both species, is not always an infallible distinction between them and in 
doubtful cases we have to rely in the last resort upon the number of 
rings separating the genital apertures. 
Series 2. Epaetodesminae. 
Complete somite with an intercalated ring. 
Genus: Trocheta, Dutrochet , 1817. 
Synonymy: 
Trocketia, de Blainville (in Lamarck), 1818 . Hirudo (: Trochetia ), de Blainville, 1827 . 
Geobdella, de Blainville, 1828 . 
Amphibious Herpobdellidae with four pairs of eyes. Complete somite 
formed rarely of six rings all equal in size except the fourth (- intercalated) 
ring, which is half the width of the others; and generally of a varying 
number of rings not exceeding eleven, due to the subdivision frequently 
of rings b and 6 and occasionally also of rings 1, 2 and 3; the normal 
form consists of three equal large rings followed by five equal half 
rings. 
