ALLIES OF THE EARTHWORMS 



253 



the tentacles, catch up grains of sand. These are added to 

 the little hummock that conceals the mouth of the tube. 



The Leech. Most species of leeches are known to be 

 temporary parasites on other animals. Placobdel'la rugo'sa 

 (Fig. 133) feeds very 

 commonly upon the 

 blood of turtles. It is 

 about two inches long 

 and broader near the 

 posterior end than 

 elsewhere. The an- 

 terior and posterior 

 sucking disks on the 

 ventral surface are 

 used for holding on to 

 a support and for lo- 

 comotion. Placobdella 

 "loops" itself along, 

 but it cannot swim, 

 as some other species 

 of leech can, although 

 no leeches have ap- 

 pendages. 



The body of a leech 

 is divided into thirty- 

 four somites, while 



these are superficially divided into two and sometimes more 

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

 the third somite. The mouth is on the ventral surface in 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. 



Fig. 132. Photograph of a tube worm. 

 (Slightly reduced) 



1, thread-like gills; 2, tentacles 



