xii BACKBONELESS ANIMALS 241 



Some of the Chaetopods multiply not only sexually, but 

 asexually by dividing into two or by giving off buds from 

 various parts of their body. Strange branching growths, 

 which eventually separate into individuals, are well 

 illustrated by the freshwater Nais, and still better by a 

 marine worm, Syllis ramosa, which almost forms a net- 

 work (fig. 68). 



Many sea-worms have much beauty, which some of 

 their names, such as Nereis, Aphrodite, Alciope, suggest, 

 and which is said to have induced a specialist to call his 

 seven daughters after them. 



Along with the Chsetopods, we include some other forms 

 too unfamiliar to find more than mention here, the Myzo- 

 stomata which form gall-like growths on the feather- 

 stars which they infest, the strange Bonellia where the 

 microscopic male lives as a parasite on or within the 

 female, and some very simple forms, e.g. Polygordius, 

 which are sometimes called Archi- Annelids. 



Class : Hirudiiiea or Discophora or Leeches. These are 

 blood-sucking animals, which often cling for a long time 

 to their victims. They live in salt and in fresh water, and 

 sometimes on land. The body is elastic and ringed, but 

 the external markings do not correspond to the internal 

 segments. There are no appendages, but the mouth is 

 suctorial, and there is another adhesive sucker posteriorly. 

 The body-cavity is almost obliterated by a growth of 

 spongy tissue, whereas that of Chsetopods is roomy. 

 Leeches are hermaphrodite, and lay their eggs in cocoons, 

 within which the young develop without metamorphosis. 



The medicinal leeches (Hirudo medicinalis) live in slow 

 streams and marshes, creeping about with their suckers 

 or sometimes swimming lithely, preying upon fishes and 

 amphibians, and both larger and smaller animals. They 

 fix themselves firmly, bite with their three semicircular 

 saw-like tooth-plates, and gorge themselves with blood. 

 When they get an opportunity they make the most of it, 

 filling the many pockets of their food-canal. The blood 

 is kept from coagulating by means of a secretion, and on 

 its store the leech may live for many months. 



The horse-leech (Hccmopis sanguisuga) is common in 



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