ADDITIONAL ANIMAL STUDIES 



181 



136. Relatives of the earthworm. Two forms of animals that 

 formerly were classed with the earthworm under the head of "worms * 

 are the tapeworm (Fig. 129) and trichina. The tape worm is some- 

 times present in beef and trichina 

 (Fig. 130) in pork. Meats, there- 

 fore, should be well cooked to kill 

 all such parasites. The trichina, if 

 it gets into the human system, 

 causes great suffering. When a 

 tapeworm becomes attached to the 

 human intestine by the suckers and 

 hooks on its anterior end, it is diffi- 

 cult to dislodge. 



D. Mollusca 



137. Fresh water mussel. The 

 fresh water mussels are mollusks 

 that are sometimes called clams. 

 They are often quite abundant on 

 the bottom of creeks, 

 rivers, ponds, or lakes. 

 Usually they are partly 

 covered with sand or 

 mud, sometimes even 

 more than is shown in 

 Figure 131. It will be 

 seen at once that the 



mussel is inclosed by a 



* FIG. 129. 



B, tapeworm, about 15 feet long, 

 omitted portions being indicated 



The tapeworm. 

 MacBride.) 



(Shipley and 



shell. This consists of 

 two parts called valves; 

 hence these animals, as well as salt water mussels, clams, and 

 oysters are called bivalves (Latin bis = two + valve). The two 

 valves are held together along one margin -by a tough material 

 that serves as a hinge. On each valve near the hinge, a promi- 

 nence, known as the beak or umbo, may be readily seen. Around 



