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 l. 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 



