282 ^^^ ANIMAL KINGDOM 



attachment to the host. They creep by moving the posterior sucker up 

 close to the anterior one, and then stretching the anterior sucker for- 

 ward. They also swim well by vertical undulations of the flattened body. 

 Their powerful suction is known to anyone who has tried to pull a leech 

 off his skin. Most leeches live in fresh water, feeding on fish, amphibians 

 and other animals. In the absence of blooded prey most leeches can 

 subsist indefinitely on small worms and arthropods which they capture 

 and swallow whole. Once leeches find blood, however, they take enough 

 to last for weeks. 



The suckers are not used for sucking blood, but only for attach- 

 ment. In the mouth are three cutting teeth that make a Y-shaped 

 incision in the skin. Numerous small salivary glands around the mouth 

 secrete a substance that prevents the coagulation of blood. This 

 substance, hirudin, is commercially extracted from leeches and used 

 medicinally when anticoagulants are indicated. Once assured of a con- 

 tinuing flow of blood, the leech sucks with a powerful pharynx built like 

 that of the earthworm with radiating muscles to the body wall. The 

 esophagus, which in the earthworm forms a modest crop, in the leeches 

 is expanded into an enormous, branched crop that fills much of the body 

 and which can be greatly distended. Blood is stored here during feeding, 

 and over the following weeks trickles slowly into the small stomach and 

 on into the intestine that ends in a short rectum and anus. 



The other organ systems are similar to those already described for 

 Nereis and Lumbricus, except that the coelom is secondarily reduced by 

 the invasion of loose connective tissue to a series of sinuses that become 

 connected with the circulatory system. The circulatory system includes 

 longitudinal vessels and networks of capillaries, but the capillaries of the 

 skin, containing oxygenated blood, drain into the sinuses. These sinuses 

 parallel the digestive tract and the ventral nerve cord. 



The body is composed of a fixed number of segments (36 in the large 

 medicinal leech) each of which is superficially subdivided into several 

 rings, giving the external appearance of many more segments. 



The male reproductive system, comparable to that of the oligo- 

 chaetes terminates at a single median duct that opens on the 11th seg- 

 ment through a curved, muscular, eversible penis. Seminal receptacles 

 are absent from the female system. The oviducts terminate at a single 

 median duct that opens on the 12th segment as a vagina. Mutual cross 

 fertilization is followed by the secretion of a cocoon (by the 9th to 11th 

 segments) into which eggs, sperm and albuminous fluid are placed. The 

 cocoon is slipped off the head and attached to a rock. The fertilized 

 eggs develop into tiny leeches which eventually hatch from the cocoon. 

 Some of the larger leeches attach the cocoons to the ventral surface of 

 the body, and after the young emerge they remain attached to the parent 

 for some time. 



In moist tropical forests leeches are terrestrial. They climb up the 

 vegetation and stand with the posterior sucker attached, and the anterior 

 end held over a pathway, waiting for some mammal to go by. They 

 sometimes occur in stich numbers as to pose a serious threat to animals 

 because of the amount of blood they can remove in a short time. 



