ALLIES OF THE EARTHWORM 225 



(Fig. 109) feeds on plants that grow among stones and sticks 

 at the bottom of brooks. It is about two inches long and 

 broader near the posterior end than elsewhere. The anterior 

 and posterior sucking-disks on the ventral surface are used 

 for holding on to a support, and for locomotion. Placobdella 

 "loops" itself along, but it cannot swim, as some other species 

 of leech can, although no leeches have appendages. 



The body of the leech is divided into thirty-three somites, 

 and these are superficially divided into two and sometimes 

 three rings. Two eyes lie close together on the dorsal surface 

 of the third somite. The mouth is on the ventral surface near 

 the anterior sucking-disk. The pharynx can be rolled out as 

 in Nereis. The color of Placobdella is described by Professor 

 Moore, of the University of Pennsylvania, as a " pepper-and- 

 salt mixture of various light and dark browns, yellows, and 

 greens." Around the margin are light-colored patches. 



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 pos- 

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



Definition of Annulata (Lat. ^anmdus, a ring). The earth^ 

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

 ters 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 Annula'ta, also called 

 Anneli'da, is derived. 



Two classes are represented by these four animals: the 

 Chcetop'oda (bristle-footed), by Lumbricus, Nereis, and Am- 

 phitrite ; and the Hirudin'ea (Lat. hirudo, leech), by Placob- 

 della. 



The Flatworm. On the lower surface of submerged stones, 

 near the margin of ponds, there are many little black, or white, 

 flatworms. The most of these belong to a genus called Pla- 

 na'ria. Quite often a larger specimen (10 mm. to 15 mm.) is 



