184 ANIMAL BIOLOGY 



the anterior end. Hence, the animal in its natural position "stands 

 on its head," or at least where its head ought to be. From the pos- 

 terior end projects the tubular structure to which reference has been 

 made. 



Let us again drop some powdered carmine closer to the animal, 

 and watch the particles when they reach a point just above the tube 

 where we saw the particles enter. We shall now see the carmine 

 carried away from the animal instead of into it. A closer examina- 

 tion reveals the fact that the tubular structure has a second opening 

 above the first. Both of these tubes are called siphons, the lower 

 being the incurrent siphon, and the upper the excurrent siphon. 

 The stream of water forced out of the excurrent siphon carries with 

 it the carbon dioxid and other wastes of the body. 



138. Suggestions for study of the mussel. It is desirable to 

 have students see the mussel in its natural home. They should 

 tell where they found the animals and the positions in which they 

 were seen. It would then be well for the pupil to study in the 

 laboratory the shell, making out the points of structure described 

 above. A drawing of a side view of the mussel should be 

 made and labeled as follows: valve, umbo, hinge, lines of 

 growth, anterior region, posterior region, dorsal edge, ventral 

 edge. It is also desirable that a drawing of the animal in the 

 sand or mud be made and the incurrent and excurrent siphon 

 openings be labeled. 



The pupil might well follow the account as given above, verifying 

 the statements and experiments, and making drawings of the mussel 

 with the shell open and all the animal lying in one valve. Label: 

 mantle, muscles that close shell, incurrent siphon, excurrent siphon. 

 Also a drawing should be made of the mussel with the mantle re- 

 moved. Label: foot, abdomen, palps, mouth, gills. Write an 

 account of how the mussel moves or burrows, how it feeds and 

 breathes. 



139. Relatives of the mussel. Some of the relatives of the 

 mussel are the clams, oysters, salt water mussels, snails (Fig. 133), 

 and slugs. While the fresh water mussels are not much used for 



