2l6 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.
