114 MOLLUSCA. 



sitiveness. Upon being disturbed they quickly disappear be- 

 neath the surface of the mud. 



1. Notice the shape of the shell, the way it gapes at both 

 ends, and the way the lobes of the mantle are fused. 



2. With a pencil-point or seeker stroke the tentacles around 

 the siphon openings, while the animal is being held anterior end 

 downward. This will cause it to perform the burrowing move- 

 ments. Study the movements carefully and see what the effects 

 would be were they performed in mud. Thrust the anterior end 

 of the shell in mud and watch the result of the movements. 



3. Water is ejected by the sides of the foot to aid in burrow- 

 ing or to enable the animal to swim, but observations on its 

 method of ejecting it are not easily made, and are sure to take 

 much time. Notice the way the anterior margins of the lobes 

 of the mantle scrape mud from the foot when the foot is being 

 withdrawn. 



4. With a scalpel separate the united margins of the mantle 

 throughout their length. Slowly pry the valves apart, lift up 

 the free end of the foot and pull it posteriorly. 



The cerebral ganglia are plainly visible without further cutting. 

 They lie just posterior to the anterior adductor muscle and in front 

 of the mouth, and are widely separated. They are connected by 

 a narrow commissure, and each gives rise to a cerebro-visceral 

 and a cerebro-pedal connective and to a number of nerves. The 

 nerves that supply the anterior part of the mantle and the ante- 

 rior adductor muscle are especially easily seen. 



5. If the specimen is one that is nearly or quite dead, it is, 

 by cutting, easy to follow the cerebro-pedal connectives to the 

 pedal ganglia, which are not far from the base of the foot and 

 not deeply embedded. 



6. Allow the foot to return to its normal position and cut 

 along the line of union of the inner gills. Without further cut- 

 ting the visceral ganglia may be studied. Their connectives, 

 which may be followed easily as far forward as the palps, and 

 the posterior pallial and the branchial nerves may be seen. 



A drawing of the nervous system should be made. 



