250 ELEMENTS OF COMPARATIVE ZOOLOGY. 



rent siphon. Notice that it does not enter the branchial 

 chamber. Cut through the thin membrane between the 

 gills of the right and left sides, posterior to the abdomen. 

 This lays open the cloacal chamber into which the probe 

 extends. In the dorsal wall of this chamber, just below 

 the posterior adductor, see a pinkish or orange body, the 

 parieto-splanchnic ganglia. From this trace backward 

 nerves which curve forwards along the base of the gills. 

 Also trace two nerves forward, one on either side of the 

 body, until they meet in a pair of cerebral ganglia just 

 above the mouth. Are the two cerebral ganglia connected 

 directly with each other ? From the cerebral ganglia trace 

 a pair of nerves downward to the pedal ganglia lying 

 between the abdomen and the foot. Sketch the nervous 

 system. 



Beginning with the intestine where it leaves the heart, 

 trace it posteriorly. On which side of the posterior 

 adductor does it pass ? Where does it empty ? Trace it 

 forward from the heart, carefully picking away the sur- 

 rounding tissue with the needles, into and through the 

 abdominal mass, and plot the coils which it makes. It 

 will be found to pass into a rather large saccular stomach, 

 on either side of which is the dark-green liver.* Trace 

 the oesophagus from the stomach to the mouth. 



Take a clam which has been hardened for a couple of 

 weeks in strong alcohol or formol. Cut it transversely in 

 slices a quarter of an inch thick, using a sharp scalpel for 

 the purpose. Draw the sections and name all the parts 

 found. This can be done easily if the previous dissection 

 has been intelligently done. 



* In a pocket of the stomach in the long clam will be found 'a 

 structure of unknown function, the crystalline style, transparent, an 

 inch or more in length. 



