272 



PHYLUM ANNELIDA 



Type, the Medicinal Leech {Hiriido medicinalis) 



Habits. — This is the commonest and most famihar 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 oppor- 

 tunities. 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 alter- 

 nately 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 ; 

 its dorsal surface is beautifully marked with longitudinal pigmented 

 bands, while the ventral surface is mottled irregularly ; the suctorial 

 mouth is readily distinguished from the unperforated hind sucker, above 

 which, on the dorsal surface, the alimentary canal may be seen to end. 



According to Whitman's precise investigations, there are 102 skin- 

 rings and 26 somites or true segments. The hind sucker is supposed 

 to consist of 7 fused segments, making the total number 33. 

 These segments may be recognised externally by conspicuous 

 pigment spots (" segmental papillae "), which in the middle region of 

 the body occur on every fifth ring. In type, therefore, five rings 

 correspond to a segment, but at either end of the body the number of 

 rings is abbreviated. In the head region there is a pair of " eyes " 

 on the ist, 2nd, 3rd, 5th, and 8th rings : these are homologous with 

 " segmental papillae," and therefore in this region eight rings corre- 

 spond to five segments. 



