LEECHES. 



441 



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 (Hirudo 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 towards 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- 

 wards and forwards 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 

 of these being very long and stretching back- 

 wards side by side with the narrow intestine, 

 which terminates close to the large cup-shaped 

 sucker. The structure of the organs that have been just described explains the 

 utility of the leech as a blood-letter. The creature adheres to the spot upon 

 which it is placed by means of its front sucker, which has the mouth in the middle 

 of it. The jaws are then brought to bear upon the skin and start sawing their 

 way into it, while the blood that flows from the wound passes into the sacs of the 

 stomach until they are all filled ; and since the walls of the body as well as those 

 of the alimentary canal are highly elastic, it is easy to understand how the creature 

 is able to expand to two or three times its normal size. Some of the structural 

 points enumerated above are shown in the illustration on the next page, in which 

 1 is the alimentary canal, with the oesophagus (a) and the sacs of the stomach (b 

 and c); 2 is the head end showing the eye-spots ; and 3 is part of one of the jaws. 



A, Myzostoma gigas from below ; B, portion 



OF ARM OF A SEA-LILY (Antedon), SHOW- 

 ING THE SWELLINGS PRODUCED BY 



Myzostoma. 



