LESSONS IN ZOOLOGY. 287 



yellowish or whitish mass of flesh lying between the mantle lobes, 

 and forming the ventral portion of the animal is the foot. On 

 either side the foot find a pair of branchia or gills. Just in front 

 of the gills on each side are a pair of triangular processes called 

 the labial palpi, and between them just back of the anterior ad- 

 ductor muscle is the mouth. Toward the posterior of the ani- 

 mal notice that the mantle lobes coalesce so as to form two slit- 

 like openings called the ventral or inhalent siphon, and the dor- 

 sal or exhalent siphon. By the action of cilia in the ventral 

 siphon and gills, water is driven through the gill chamber, and 

 food materials pass with the water over the palpi to the mouth. 

 Commencing at the mouth with probe, blowpipe and scissors trace 

 out the alimentary canal. This can be done more easily with an 

 alcohol specimen. 



Pry open the shell of a live mussel and insert a little block of 

 wood to keep it from closing; then cut the adductor muscles, 

 separate the mantle lobes from the shell, and on the dorsal 

 border of the animal, near the region of the umbo, notice a thin 

 space in the mantle covering a cavity filled with fluid. In this 

 cavity, called the pericardium, a yellowish transparent sac, called 

 the heart, may be seen pulsating in the pericardial fluid. This 

 dissection can be made more satisfactorily with a live clam, but 

 most of the work can be as well or better done with the alcohol 

 specimen, or one that has been killed with boiling water.' 



Most mussel shells have hinge teeth with corresponding sockets 

 on the dorsal margin of the valves, near the ligament, by which 

 they are more firmly held together when the shell is shut. The 

 anterior is conical, sometimes sharp, the posterior narrow and 

 long. Notice a thin dark-colored membrane bordering the edge 

 of the shell, seemingly an extension of the outer covering or epi- 

 dermis of the shell. Burn some mussel shells in the open fire, 

 and note what properties they lost and what they gained from 

 burning. From the burned shells try to get some idea of how 

 the shell was built up. Test portions of the burnt shell with 

 acid and note results ; also test a fresh shell with acid and note 

 results. In observing the mussel note especially his methods of 



