208 SEGMENTED WORMS OR ANNELIDA. 



Class Hirudinea or Discophora. Leeches. 



This class includes forms in which the body is oval and 

 flattened, usually devoid of seta, or gills, and marked externally 

 by rings which are much more numerous than the true 

 segments. The body cavity is much reduced, and may com- 

 municate 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 her- 

 maphrodite, — the male organs are numerous and segmentally 

 arranged, and special genital ducts are present. The develop- 

 ment 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 bloodletting 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 develop- 

 ment, the young leeches emerge and make for the water. 



External features. — The leech usually measures from 2 to 6 ins. 

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

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



