HIEUDINEA OR LEECHES. 83 



Hirudinea (Leeches). 



The last group of worms to examine are the Leeches. 

 Leeches are provided with a ventral sucker for attachment 

 to their host. They are all hermaphrodites, and parasitic during 

 some time of their life, but are chiefly ecto-parasites. The 

 body of the leech is ringed, but the outside annulations do not 

 correspond to the internal divisions. There may be two suckers, 

 one round the mouth, the other ventral. The oral aperture is 

 composed of three slit-like openings, in each of which is a 

 serrated jaw. 



Life-history of LeecJies. — The ova are fertilised in the worm. 

 Prior to depositing their ova the worms anchor themselves upon 

 a stone or plant, or in the case of the medical leech leave the 

 water and burrow into damp earth and mud. At this time the 

 genital rings get much enlarged by the swelling of the genital 

 glands, and the cells in the skin, of a glandular nature. "When 

 the time to oviposit comes the leech tightens its hold of the 

 plant or object on which it is fixed ; it then moves about with 

 much violence and covers the front of its body, especially the 

 genital rings, with a viscid mass, which hardens and produces a 

 fine membrane. When this membrane is complete the leech 

 passes out a number of ova and an albuminoid mass, and at 

 once withdraws its body from the barrel-shaped capsule, which 

 is left behind as a kind of cocoon full of eggs, each loose end 

 closing up and forming a safe shelter for the ova, of which there 

 are only a few in each cocoon. From these ova young leeches 

 appear, of the same form and appearance as the adult, but not 

 sexually mature. Most leeches live in water or in damp earth. 

 They move by a series of loops with the help of their suckers, 

 and also by swimming. Many are parasites on the giUs and 

 skin of aquatic animals, such as fish and crayfish ; others are 

 only occasionally parasitic on the skin of warm-blooded animals. 

 When young they live upon snails, insects, frogs, &c., but when 



