224 PHYLUM ANNELIDA. 



ternally by rings which are much more numerous than the 

 true segments. The body cavity is much reduced and broken 

 up (except in Acanthobdelld], and may communicate indirectly 

 with the well-developed vascular system. The nephridia are 

 numerous and segmentally arranged. There are usually two 

 suckers, one at each end of the body, the anterior being formed 

 by the mouth. Almost all are hermaphrodite, the male 

 organs are numerous and segmentally arranged, and special 

 genital ducts are present. The genital openings are median. 

 The development is direct. Most live in fresh water or on 

 land, but a few are marine. 



Type, the Medicinal Leech (Hirudo medicinalis). 



Habits. This is the commonest and most familiar of 

 leeches, once so constantly used in the practice of medicine 

 that leech became synonymous with physician. It lives in 

 ponds and sluggish streams, and though not common in 

 Britain, is abundant on the Continent, where leech farms, 

 formerly of importance, are still to be seen. Leeches feed 

 on the blood of fishes, frogs, and the like, and are still 

 caught in the old fashion on the bare legs of the callous 

 collector. As animals are naturally averse to blood-letting 

 and hard to catch, leeches make the most of their 

 opportunities. They gorge themselves with blood, and 

 digest it slowly for many months, it may be indeed for 

 a year. Watched in a glass jar, the leech is seen to move 

 by alternately fixing and loosening its oral and posterior 

 suckers, and, on some slight provocation, it will swim 

 about actively and gracefully. At times it casts off from 

 its skin thin transparent shreds of cuticle, a process 

 which, in natural conditions, usually occurs after a heavy 

 meal, when the animal, as if in indigestion, spasmodically 

 contracts its body, or rubs itself on the stems of water- 

 plants. Numerous eggs are laid together in cocoons in 

 the damp earth near the edge of the pool. Thence, after 

 a direct development, the young leeches emerge and make 

 for the water. 



External features. The leech is usually from 2 to 6 inches 

 in length, and appears cylindrical or strap-like, according to its state 

 of contraction. The slimy body shows over one hundred skin-rings ; 



