BDELLIDEA. 37 



rescent ; and the skin rarely and feebly reflects the light. Pro- 

 gression is effected by alternate fixations and loosenings of the 

 suckers, and by corresponding contractions and extensions of the 

 intermediate segments or rings. Many species can also swim in the 

 water by undulatory eel-like movements of the extended body. 



The British genera may be arranged as follows : — 



Tribe I. Hirudinacea, Grube. 

 Mouth without a protrusile proboscis. 



* Oral sucker entire, exannular, strictured at its origin, with the mouth at 

 the bottom of the cup on its ventral side. 



Family I. BRANCHELLID.ffi. Branchiseform lobes on each side 

 of the segments. 



1. Branchellion. The only genus. 



Family II. PISCICOLIDJE. Segments not marginated. 



2. Pontobdella. Segments distinct : eyes none. 



3. Piscicola. Segments indistinct : eyes on the sucker. 



** Oral sucker incomplete, continuous with the body, and formed by a 

 moulding of the anterior rings. 



Family III. NEPHELID^. Anus large: the gullet long. 



4. Nephelis. Mouth edentulous: gullet with three plaits: eyes 



eight. 



5 . Trocheta. Jaws rudimentary, not denticulated : gullet with 



three plaits : eyes eight. 



6. Aulastoma. Jaws three, minute : gullet with twelve plaits : 



eyes ten. 



Family IV. HIRUDINID^. Anus very small : the gullet short. 



7. Hsemopsis. Jaws three, not compressed, sparingly and bluntly 



denticidate : eyes ten. 



8. Hirudo. Jaws three, compressed, multidentate : eyes ten. 



Tribe II. Clepsinea, Grube. 

 Mouth with a protrusile proboscis. 



Family V. GLOSSOPORID^. The only family. 

 0. Glossophonia. The only genus. 



