132 MOLLUSCA 



portion of the foot. The pedal ganglia are connected with 

 each other by a broad commissure and with the cerebral 

 ganglia by connectives. 



By careful dissection it is possible to trace the connec- 

 tives and many of the nerves. The razor clam, Ensis, is es- 

 pecially favorable for dissections of the nervous system, as 

 the ganglia, connectives, and many important nerves lie very 

 near the surface and can be seen without cutting the tissues 

 above them. 



Make a drawing, indicating the positions of the ganglia. 



Digestive System. — This may be traced by following a 

 guarded bristle that has been inserted into the mouth of a 

 specimen that has been killed in hot water (not boiling), or 

 by very carefully picking off the tissue from one side. The 

 intestine where it penetrates the heart has already been seen, 

 and may easily be followed to the anus. 



The general arrangement of the alimentary canal is well 

 shown by a median sagittal section of a preserved specimen. 



The brownish digestive gland, commonly called the 

 "liver," will be seen surrounding a portion of the stomach. 



The enlargement on the intestine in the posterior portion 

 of the pericardium is of unknown function. In some forms 

 a special diverticulum from the stomach bears a transparent 

 cylindrical rod, the crystalline style. This can easily be 

 found in Mya. Probably all lamellibranchs have similar 

 structures more or less well developed, but many do not have 

 special pouches for their formation. 



Draw the alimentary canal. (This may be included with 

 your sketch of the nervous system.) 



Cut a preserved specimen into transverse sections about 

 a quarter of an inch thick, and place the sections in their 

 proper order and position. (They should be placed in a dis- 

 secting pan in very little water.) 



Study these sections for the arrangement of organs. The 

 relation of the gills to the branchial and the cloacal chambers 

 should be understood. 



