ALLIES OF THE EARTHWORMS 255 



considered good medical practice, and living leeches of a 

 species known as the medicinal leech were regularly carried 

 in stock by the apothecary shops. 



Leeches are hermaphroditic. Placobdella rugosa carries 

 its eggs in a gelatinous mass on the ventral surface. When 

 the young hatch they live for several weeks attached by the 

 posterior sucker to the parent, as shown in the picture. 



Definition of Annelida (Lat. annulus, "ring "). The earth- 

 worm, sandworm, tube worm, and leech have certain char- 

 acters in common. The body is divided into somites of 

 nearly uniform shape, the characteristic form being a ring. 

 From that resemblance the name of the phylum Annel'ida 

 is derived. A conspicuous space, the body cavity, or ccelom, 

 lies between the digestive tract and the body wall in every 

 somite. 



