100 INVERTEBRATE ZOOLOGY 
of the shell. Note the parallel lines of growth which extend 
from the umbo to the ventral and posterior sides of the shell. 
When the anterior, the right, and the left sides of the shell are 
known, the ventral and posterior sides can be easily determined. 
Exercise 1. Make an outline drawing of the right valve, indicat- 
ing the anterior, posterior, dorsal, and ventral aspects and 
showing the lines of growth. 
Remove the right valve in the following way: Break off the 
edge of the shell with a hammer, insert the blade of a scalpel 
and cut the large adductor muscle, which is not far from the 
edge but nearer the dorsal than the ventral margin. It is 
important to keep the blade close to the right valve so as not 
to mutilate the internal organs. Force off the right valve and 
examine its inner surface. 
Exercise 2. Draw the inner surface of the shell, showing the 
muscle scar with its lines of growth and the hinge liga- 
ment, and label the dorsal, ventral, anterior, and posterior 
sides of it. 
Study the animal as it lies in the left valve. Note the soft, 
shiny mantle, which covers the inner surface of the shell and 
has secreted it. The mantle is a double fold of the integument 
which extends ventrally from the dorsal side and covers the two 
lateral sides of the body. Its lower edge is bordered by a fringe 
of short, pigmented tentacles which are the principal sense organs 
of the animal; it is also provided with muscle fibers which 
enable it to be slightly extended beyond the edge of the shell. 
The most conspicuous organ in the body will be seen to be 
the large adductor muscle. Lying between it and the hinge liga- 
ment is the visceral mass, containing most of the viscera. Along 
the ventral side are the four gills. 
Put the oyster into a pan of water and with fine scissors and 
forceps remove the right mantle. Just in front of the adductor 
