I3 ELEMENTARY ZOOLOGY 



TECHNICAL NOTE. Some common water-leeches, alive or pre- 

 served in alcohol, should be examined by the class. The animals 

 are not unfamiliar to boys who "go in swimming" in the small 

 streams of the country. The body of a leech should be examined 

 carefully, and drawings of it showing the external structural charac- 

 ters should be made. 



The body of a leech is flattened dorso-ventrally, instead 

 of being cylindrical as in the earthworm, and tapers at 

 both ends. In the live animal the body can be greatly 

 elongated and narrowed or much shortened and broad- 

 ened. It is composed of many segments (not as many 

 as there are cross-lines however; each segment is trans- 

 versely annulated), and bears at each end on the ventral 

 surface a sucker, the one at the posterior end being the 

 larger. These suckers enable the leech to cling firmly 

 to other animals. The mouth is at the front end of the 

 body on the ventral surface and is provided with sharp 

 jaws. Leeches live mostly on the blood of other animals 

 which they suck from the body. The common leech 

 "fastens itself upon its victim by means of its suckers, 

 then cuts the skin, fastens its oral sucker over the wound 

 and pumps away until it has completely gorged itself with 

 blood, distending enormously its elastic body, when it 

 loosens its hold and drops off. ' ' Its biting and sucking 

 cause very little pain, and in olden days physicians used 

 the leeches when they wanted to " bleed " a person. A 

 common European species of leech much used for this 

 purpose is known as the "medicinal leech." All 

 leeches are hermaphroditic, that is, the sexes are not dis- 

 tinct, but each individual produces both sperm-cells and 

 egg-cells. Most of the leeches lay their eggs in small 

 packets or cocoons. This cocoon is dropped in soil on the 

 banks of a pond or stream so that the young may have a 

 moist but not too wet environment. The young issue 

 from the eggs in four or five weeks, but they grow very 



