316 LEECHES 



adhering first by the large posterior sucker, then by the smaller 

 oral sucker and so forth. 



Nearly all leeches feed exclusively on blood. The digestive 

 tract (Fig. 60C, p. 197) is peculiar in that the oesophagus is sup- 

 plied with a series of " crops" or side pockets in which blood can 

 be. stored up as a reserve supply to be gradually drawn back into 

 the stomach and intestine and digested as needed. Since some 

 leeches can fill up with three times their own weight in blood, 

 and can live on this supply for a year or more, meals are few and 

 far between. The saliva of the leech has the power of prevent- 

 ing the coagulation of blood, and therefore blood continues to 

 flow for some time after the leech has " got his fill " and let go. 

 Like other annelids, leeches have a true blood system and a 

 series of nephridia, little coiled tubes, a pair in each segment, 

 which function as primitive kidneys. There are no special gills 

 or other respiratory organs; oxygen is absorbed directly through 

 the skin which is constantly kept moist. 



Leeches are hermaphroditic, i.e., both sexes are represented 

 in the same individual, but the egg of one leech is always ferti- 

 lized by a sperm from another. In most leeches the eggs are 

 deposited in a stiff mucous cocoon which is secreted by a por- 

 tion of the body. When the eggs are laid the cocoon is slipped 

 over the head like a jersey, the ends closing together to form 

 a capsule. After a little manipulation with the oral sucker the 

 mother leech imbeds the cocoon in moist soil, near the edge of 

 water in the case of aquatic species. 



Importance as Parasites. — The ordinary pond and river 

 leeches which adhere to bathers are of little or no economic im- 

 portance as human parasites. Of these the well-known medici- 

 nal leeches, Hirudo, used for sucking out infections or bad 

 blood, are the best known examples. They are furnished with 

 powerful suckers and sharp-pointed pincer-like jaws, and can 

 therefore easily penetrate the skin and suck blood from any part 

 of the surface of the body. They can usually be persuaded to 

 release their hold when removed from water. 



With the weak-jawed members of the genera Limnatis and 

 Hosmopis, commonly known as horse leeches, it is quite dif- 

 ferent. These animals seek to penetrate the natural openings 

 of the body and fasten themselves to the mucous membranes, 

 especially in the mouth and nasal cavities, where they may cause 



