THE EARTHWORM AND OTHER SEGMENTED WORMS 175 



body. An animal with thick body walls needs organs for 

 getting rid of excretory substances and for carrying the germ 

 cells (eggs and spermatozoa) out of the body; the nephridia 

 and egg and sperm ducts develop in con- 

 nection with the ccelom. In insects and 

 other arthropods the body cavity is filled 

 with blood and is not considered a true 

 ccelom. 



Leeches. — The best-known segmented 

 worms besides the earthworms are the 

 leeches or " bloodsuckers," a name applied 

 to them because they suck blood from 

 fishes and other aquatic animals and from 

 human beings who wade about or swim 

 in the water. They do not poison people 

 nor inflict any injury except a very slight 

 wound. Leeches are characterized by 

 a flattened, segmented body, and two 

 suckers, one at the posterior end for 

 clinging to its prey and the other at the 

 anterior end, in which the mouth is 

 situated. Many leeches are provided 

 with jaws for biting through the skin; 

 then the blood is sucked into the ali- 

 mentary canal by means of a muscular 

 pharynx. 



The medicinal leech, Hirudo medic inalis 

 (Fig. 99), is only four inches long, but it 

 is capable of great contractions and elon- 

 gations. It moves along by means of its 

 suckers in loops like a measuring worm or swims through the 

 water by undulating movements. One meal of blood is suffi- 

 cient to last a leech for as long as a year. Formerly leeches 

 were used by physicians to " bleed " human beings, but this 

 practice has been discontinued. 



Fig. 99. 



— The medicinal 

 leech. 



1, mouth; 2, posterior 

 sucker; 3, sensory papilla?. 

 (From Shipley and Mac- 

 Bride.) 



