216 MORPHOLOGY OF INVERTEBRATE TYPES 



Reproductive system. The sexes are separate and the 

 reproductive organs consist of a pair of simple gonads with a 

 right and left reproductive opening just in front of the corre- 

 sponding nephridiopore under the gill. The gonads when fully 

 developed fill all the space between the loops of the alimentary 

 canal, extending backward to the posterior end of the body. 

 The reproductive cells are discharged into the mantle cavity 

 and then pass with the water current through the cloacal si- 

 phon to the outside. Development is indirect, combined with 

 a larval stage. 



Instructions 



1. Place a specimen of Venus mercenaria on edge against the 

 side of a tray, hinge uppermost. Make a full size drawing 

 showing both valves, the umbones and the ligament holding 

 the valves together. Label right and left valve. 



2. Make a full size drawing showing the side view of the left 

 valve. Label umbo, lines of growth, anterior and posterior end, 

 dorsal and ventral surface. 



3. Cut the ligament longitudinally with a very strong scalpel. 

 Press the ventral edges of the valves somewhat apart, carefully 

 introduce a fine scalpel under the left valve, between it and the 

 mantle, cut the anterior adductor of the valves and retractor 

 of the foot, and the posterior adductor and retractor as close 

 to the valve as possible, press down the mantle with the handle 

 of the scalpel and remove the left valve. Place the right valve 

 with the animal in it into a deep crystallization dish with water 

 for future examination. Examine the inner surface of the left 

 valve and make a full size drawing showing the umbo, ligament, 

 cardinal and lateral teeth of the lock, pallial line showing at- 

 tachment of the mantle, attachment area of anterior adductor 

 of the valves and above it attachment area of the much smaller 

 anterior retractor of the foot, attachment area of the posterior 

 adductor and, if visible, above it the small area for the pos- 

 terior retractor of the foot, pallial sinus showing the position 



