170 LEECHES 



transverse lacunae linked to a row of pulsatile vesicles on each side 

 (Fig. 27, p. 53); diverticula^ the elongated posterior pair of crop 

 diverticula which may be more or less fused together; eyes, eyes 

 on head; ganglia, number of ganglia in the ventral chain, not 

 including the suboesophageal and caudal masses; ocelli, simple 

 eyes on posterior sucker; pulsatile vesicles, externally visible 

 respiratory vesicles like those found in Piscicola (Fig. 17, p. 28); 

 trachelosome, anterior part of body including clitellum. 



Key to the Genera of the World 



1. Body with many conspicuous branchiae or tubercles . . 2 



— Body smooth or with a single fin or row of vesicles on 

 each side 6 



2. Body with lateral branchiae 3 



— Body with numerous tubercles 4 



3. Branchiae finger-shaped and branching 



Ozobranchus (Fig. 19, p. 31) 



— Branchiae leaf-shaped and unbranched 



Branchellion (Fig. 18, p. 30) 



4. One annulus of each somite bears large tubercles, most 

 of the other annuli bear small tubercles; 4-6 very dis- 

 tinct annuli per somite 5 



— All tubercles small and arranged longitudinally in 6 

 rows dorsally and in 6 rows or absent ventrally; 12-14 

 annuli per somite Oxytonostoma 



5. Body rounded with regions ill-defined 



Pontohdella (Fig. 19, p. 31) 



— Body sharply divided into slender trachelosome and 

 broad flattened abdomen Pontohdellina 



6. Body with a flattened, fin-like expansion extending 

 along each side 7 



— Body without lateral fins 8 



7. Fins confined to anterior 2/3 of body ; genital ducts with 



a common opening Pterohdella (Fig. 19, p. 31) 



— Fins extending whole length of body; separate genital 

 openings Pterobdellina 



