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 (Xephelis, 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. Malacobdellidae diiecious, soft, flattened, ciliated, not 



ringed ; pharynx protrusible ; anterior sucker none; the 



colourless blood flows in two lateral and one dorsal vessel : 



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



of whi .ited by long commissures; parasitic on -Mul- 



: in some respects allied to Amphiptyches. 2. Acan- 



thobdellidoj hermaphrodite, spindle-shaped, ilat ; anterior 



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



3. Histriobdellidae direcious ; hinder part 



split into two movable processes; head with tentacle-like 



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



.4. nranchiobdellidaj 



ind, unequally segmented, papillose; head- 



iteil from the oral sucker; two jaws, one dorsal 



.1, in the pharynx; anus over the jm.stcrior 



Ming medio-ventral. Branchiobdella is 



i the gills of crabs. Trmnocephalus has 



and tw<> Vpsinidiu moiuccious, 



d in front, with three rii: 



bsent, with no 



j.rotrusi 1 the hinder sucker; 



. .Mollus (S , I ; ivh. and 1 



native opcniiiL^ between the 

 :h and the zjth-Hth rings ; it carries its young 



