130 MOLLUSCA 



Understand the direction taken by water in passing from 

 the branchial to the cloacal siphon. What makes the water 

 move? 



Labial Palps. — The positions of these organs have already 

 been noted. 



1. Examine a piece of the palp with a microscope, and 

 notice that the side turned toward the adjacent palp is 

 thrown into ridges and grooves, and is densely ciliated. 



2. The space between each outer and inner palp is con- 

 tinuous with the "corners" of the mouth. The free margins 

 come close to the borders of the gills and normally inclose 

 them. 



Understand how food is gathered and carried to the 

 mouth. 



Circulatory System. — The pericardium, in which the heart 

 lies, is a somewhat triangular space that appears clear, 

 through the mantle. It lies just anterior to the posterior ad- 

 ductor muscle. Open the pericardium, and notice the beating 

 of the heart. The heart consists of three parts: 



1. A central portion, the ventricle, which surrounds the 

 intestine and gives rise to a blood vessel at each end. 



2. Two triangular portions, the auricles, which receive 

 blood from the gills and open into the sides of the ventricle. 



Notice the sequence and power of the contractions. 



Just posterior to the pericardium is an enlarged portion 

 of the alimentary canal. This has no relation to the heart, 

 for which it is sometimes mistaken. 



Excretory and Genital Systems. — The excretory system 

 consists of a pair of dark colored glandular organs that lie 

 beneath the pericardium. Each communicates with the peri- 

 cardium by a small opening that is not easy to demonstrate 

 in dissections, and with the cloacal chamber by another small 

 opening. 



By turning the two gills (of Venus) dorsally a very small 

 papilla may be seen, just beneath the free border of the inner 

 gill, lying in the cloacal chamber. On the tip of this papilla 



