96 MOLLUSC A. 



are correspondingly large. By pressing the gill the inter-fila- 

 mentar junctions can be pulled apart. 



Study prepared sections of the gill of Venus and notice: 



1. Lamellae. 



2. Inter-lamellar junctions. 



3. Water tubes. 



4. Filaments. 



5. Inter-filamentar junctions. 



6. Cilia. 



7. Inhalant ostia. 



8. Blood spaces. 



9. Chitinous rods. 

 Draw. 



Understand the direction taken by water in passing from the 

 branchial to the cloacal siphon. What makes the water mov^e? 



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 adductor 

 muscle. Open the pericardium, and notice the beating of the 

 heart. The heart consists of three parts: 



1. A central portion, the ventricle, that surrounds the intes- 

 tine and gives rise to a blood-vessel at each end. 



2. Two triangular portions, the auricles, that 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. 



