CLAM 147 



ever, are conveyed entirely away from the body of the male with 

 the current of water passing out through the exhalent siphon. 

 Some of the sperm thus discharged into the surrounding water 

 are drawn into the mantle cavity of the female by the current of 

 water which is continually taken in through the inhalent siphon. 

 This water containing the sperm bathes the gills of the female, 

 and there the sperm come in contact with the attached eggs and 

 fertilization occurs. 



The fertilized eggs develop in the gills of the mother, and in some 

 species remain there during the following winter. In all cases the 

 eggs undergo a considerable period of development in the gills, 

 and are then discharged into the water, where they begin their 

 independent existence. The eggs of the Clam, when fertilized, 

 undergo complete segmentation and pass through the various 

 embryonic stages with which we are familiar. In the fresh- 

 water Mussel there finally develops a parasitic stage, known as 

 a glochidium, which is adapted for attaching itself to a fish by 

 means of teeth-like structures which are present on the edges of the 

 valves. When a fish comes in contact with the glochidium, the 

 toothed shells snap shut in the fish tissues, and the embryo is thus 

 attached to the body of the fish, where it stays for a time as a 

 parasite. The skin of the fish grows over the glochidium, and it 

 receives nourishment from the surrounding tissues. Finally, it 

 liberates itself, falls to the bottom of the lake or stream and 

 takes up its independent existence as a mature Clam. 



6. Nervous System 



The nervous system of the Clam departs somewhat widely in its 

 structure from that of the animals previously studied. It consists 

 of a number of paired ganglia situated in various regions of the 

 body and connected by nerve cords. In the region just posterior 

 to the anterior adductor muscle is a pair of ganglia, the cerebro- 

 pleural ganglia, one of which lies on either side of the esopha- 

 gus. These ganglia are connected with each other by a nerve 

 cord, the cerebral commissure, which passes around the esopha- 

 gus just before the latter opens into the stomach. Running ven- 

 trally and posteriorly from each of these ganglia is a nerve cord, 

 the cerebro-pedal connective, which connects in the basal 

 portion of the foot with a ganglion, known as the pedal ganglion. 

 This ganglion is really paired, but the two parts have become 



