82 PHYLUM MOLLUSCA 



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 ligament, 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 ligament 

 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 

 muscle observe the pericardium. Carefully cut it away and see 

 the heart, which lies in the pericardial cavity ; it will be beating 

 if the animal is still alive. The ventricle is dorsal in position ; 

 the auricle is ventral, lying next to the gills, from which it re- 

 ceives the purified blood. The four gills lie close together, no 

 foot being present to separate the two right-hand from the two 

 left-hand gills. Just in front of the gills, at the front end of the 

 body, are the two pairs of large oral palps. The mouth is between 

 these palps, two being on each side of it. Find the mouth and 

 note that it lies between an under and a lower lip, each of which 

 is formed by the union of a pair of palps ; that is. a palp on the 

 right side joins one on the left and forms the upper lip, and the 

 other two palps join to form the lower lip. 



