Branch Mollusca 8i 



motion, causing currents in the water to pass into the 

 mouth, so that the food in such currents may be taken 

 by the clam. The gill chamber is the large cavity in 

 which the gills are suspended : it is bounded mainly 

 by the mantle. The lower end of the siphon opens 

 into it. 



XVII. — It was stated above that currents of water enter- 

 ing the lower opening of the siphon escape through 

 the dorsal. To make this evident and to demonstrate 

 the relation between the cloacal and the gill chambers 

 hunt for small openings in the floor of the former. 

 Water passes through these from the gills into the 

 cloacal chamber and thence out. 



XVIII. — The Nervous System may be very difficult to 

 make out in a fresh specimen. Better results are likely 

 to be obtained by hardening the specimen in alcohol. 

 Remove the mussel entirely from the shell, and with 

 the needles search in the median ventral line between 

 the gills and directly under the posterior adductor 

 muscles for a pair of yellowish masses, or ganglia. Still 

 another pair of these nerve centres may generally be 

 seen, lying near the surface, one on each side of the 

 mouth, close to the bases of the labial palpi. The pedal 

 ganglion may be found in front of the intestine in the 

 foot. These nerve centres are connected by nerve 

 threads called connectives. Trace out as much of the 

 connecting system as possible. 



XIX. — The Reproductive Organs will not be easy to study. 

 They lie in the abdomen, below and behind the liver. 

 They open by ducts into the gill chamber. 



XX. — Make two drawings of the clam : a side view of 

 the shell, and a diagrammatic view of the internal 

 structure. 



