234 The Study of Animal Life PART m 



8th Class, Hirudinea 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 legs, 

 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 Chaetopods 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 (Hamopis sanguisugd) is common in Britain 

 and elsewhere. The voracious Aulastoma is rather carnivorous 

 than parasitic. The land -leeches (e.g. Hamadipsa ceylonica), 

 though small and thin, are very troublesome, sucking the blood of 

 man and beast. Among the others are the eight-eyed Nephelis of 

 our ponds, the little Clepsine which sometimes is found with its young 

 attached to it, the warty marine Pontobdella which fastens on rays, 

 Piscicola on perch and carp, Branchellion with numerous lateral 

 leaflets of skin, and the largest leech the South American Macro- 

 bdella valdiviana which is said to attain a length of over two 

 feet. 



Possibly related to the Annelid series are two other 

 classes 



9th Class Chaetognatha, including two genera of small arrow- 

 like marine " worms," Sagitta and Spadella. 

 loth Class Rotifera, " wheel animalcules," abundant and 

 exquisitely beautiful animals inhabiting fresh and salt 

 water and damp moss. The head-region bears a ciliated 

 structure, whose activity produces the impression of a 

 swiftly rotating wheel. Many of them seem to be 

 entirely parthenogenetic. Some can survive being made 

 as dry as dust. 

 Fifth set of Worms a doubtful combination including 



nth Class Sipunculoidea, " spoon-worms " living in the sea, 



freely or in tubes, e.g. Sipunculus. 

 1 2th Class Phoronidea, including one genus, Pkoronis. 



