THE LEECHES 



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to zoologists. The species are all of small size, the largest, Myzostoma gigas, measur- 

 ing only a little more than a quarter of an inch in length. The body is short and 

 oval. Its upper side, which is variously colored, is covered with fine threads, called 

 cilia, and its edges are prolonged into ten pairs of long, slender, flexible appendages, 

 while below there are five pairs of horny tipped parapodia, and four pairs of cup- 

 like suckers. All the members of this anomalous family are parasitic upon stone 

 lilies or crinoids, but the degrees of parasitism are various, some kinds wandering 

 freely about their hosts, while others cause those curious swellings which appear 

 upon the arms of the infested animal. 



THE LEECHES Class Hirudinea 



Leeches are worm-like animals which differ from the bristle worms mainly in 

 the absence of parapodia, and also of bristles, as well as in the presence of one cup- 

 like sucker at the hinder end of the body, and 

 usually of another at the anterior end. Examina- 

 tion of a leech's body shows that the skin is 

 divided into a number of close -set rings. 

 These, however, are not the true segments; for, 

 as the arrangement of the internal organs 

 shows, a true segment of the body such, for 

 instance, as have been described in the earth- 

 worm or the sea mouse is composed of four 

 or five of the dermal rings. The best-known 

 example is the common leech {ffirudo medi- 

 cinalis}, a species in common use fifty years ago 

 for blood-letting. The body is broadest in the 

 hinder third of its length, and from this point 

 it is gradually narrowed toward the head and 

 tail. The head end is furnished with ten eyes 

 arranged in pairs upon the first eight rings. 

 At the tail there is a large cup-shaped sucker 

 with a narrow neck; there is also a second 

 sucker placed upon the head round the mouth, 

 which is armed with three semicircular finely 

 toothed jaws capable of being worked back- 

 ward and forward like a saw. The ali- 

 mentary canal is of enormous extent and 

 occupies nearly the entire cavity of the body. 

 Its front part, or oesophagus, is a narrowish 

 tube, then follows the stomach which is ex- 

 panded into eleven pairs of sacs, the last pair 



,..,,. , , , . , , A. Myzostoma gigas PROM BEI,OW ; B. 



of these being very long and stretching back- PORTION OF ARM OF A SEA ULY (An . 



ward side by side with the narrow intestine, 

 which terminates close to the large cup-shaped 



tedon), SHOWING THE SWEUJNGS 

 PRODUCED BY Myzostoma. 



