Introduction to Animal Morphology. 203. 



develops, each ganglion being primitively double ; 

 later appear the sex-organs and a series of solid 

 oblique cell buildings, which, becoming hollowed, 

 form the segmental organs. In some (Piscicola) the 

 ova are cells detached from the wall of the ovary ; in 

 others (Nephelis, Clepsine, Hirudo) a thin, tortuous 

 protoplasmic rachis forms in the ovary, from which 

 the eggs bud as grape-like masses, and become de- 

 tached. Fossil leeches have been found in the Litho- 

 graphic slate. 



The forty genera may be grouped in a single order, con- 

 taining six families : — 



I. IMalacobdellidae — dioecious, soft, flattened, ciliated, not 

 ringed ; pharynx protrusible ; anterior sucker none ; the 

 colourless blood flows in two lateral and one dorsal vessel ; 

 two chains of lateral ganglia (Fig. 27, C), the terminal ones 

 of which are united by long commissures ; parasitic on Mol- 

 lusca; in some respects allied to Amphiptyches. 2. Acan- 

 thobdellidse — hermaphrodite, spindle-shaped, flat ; anterior 

 end wdth a bundle of bristles at each side ; anus in the pos- 

 terior sucker. 3. Histriobdellidae — dioecious ; hinder part 

 split into two movable processes ; head with tentacle-like 

 processes ; two horny jaws in the pharynx, and a simple in- 

 testine ; they live on lobsters' eggs. 4. Branchiobdellidae — • 

 monoecious, round, unequally segmented, papillose ; head- 

 lobe separated from the oral sucker ; two jaws, one dorsal 

 and one ventral, in the pharynx ; anus over the posterior 

 sucker ; genital opening medio-ventral. Branchiobdella is 

 eyeless, and lives on the gills of crabs. Temnocephalus has 

 a lobate head and two eyes. 5. Clepsinidae — monoecious, 

 short, flat, gradually enlarged in front, with three rings in 

 each metamere ; anterior sucker present or absent, with no 

 jaws and a protrusible pharynx ; anus over the hinder sucker; 

 eyes present ; parasitic on IMolluscs, Fish, and Frogs. 

 Clepsine bioculata has the generative openings between the 

 25th-6th and the 27th-8th rings ; it carries its young fastened 



