W. A. Harding 
133 
Helobdella stagnalis, where the male and female genital apertures 
still emerge from their respective somites although opening by a 
common orifice between them. 
With regard to the sense organs, we are only concerned with the 
“ metameric sensillae ” already referred to, which are confined to the 
first or sensory ring of each somite and occur in strict series in definite 
longitudinal lines. In describing these lines the nomenclature adopted 
is that suggested by Livanow (1904) which again is but a modification 
of that given by Apfithy. These lines occur dorsally and ventrally in 
pairs with respect to a median line, and counting from the median line 
outwards we get (1) an inner and (2) an outer paramedian pair; (3) an 
intermediate pair; (4) an inner and (5) an outer paramarginal pair. 
Finally (6) a marginal pair coincide with the edges of the body. 
Ventrally the outer paramedian lines are slightly nearer together than 
dorsally. Sensillae are not present on all these lines in every species. 
By noting the position of any colour spot, papilla or other external 
feature with regard to its situation transversely on any particular ring 
and longitudinally on one of the above lines, we are able to locate 
precisety as it were its latitude and longitude on the surface of the 
body. 
The remaining diagnostic characters call for no special explanation 
and we may now proceed to a consideration of the species enumerated 
on pp. 186—187. 
Sub-order I. RHYNCHOBDELLAE. 
Marine and fresh-water Hirudinea with colourless blood, with an 
exsertile proboscis, without jaws. 
Family I. ICHTHYOBDELLIDAE. 
Body cylindrical or flattened, formed of two distinct regions, (i) a 
short narrow anterior portion or “ neck ” which includes the clitellum 
with the genital orifices and (ii) a long, large posterior region or 
“abdomen.” The anterior as well as the posterior sucker is a per¬ 
manent cupuliform or discoid organ distinct from the body. Eggs 
included in chitinous capsules which are attached to foreign bodies. 
Genus: Branchellion, Savigny, 1822. 
Synonymy: 
Branchiohdella, de Blainville, 1827 (not Branchiohdella , Odier, 1819). Branchellia, 
Gervais, 1845. 
Parasitology m 
9 
