GENERAL ANATOMY OF A LAMELLIBRANCH. 281 



gently pressing it under a cover, examine it with a power 

 of about eighty diameters. Having found the pedal gan- 

 glion, search carefully for the auditory organs. These are 

 a pair of spherical microscopic pouches, each of which 

 contains a round, highly refractive calcareous ossicle. 

 After the auditory organ has been found in a small speci- 

 men, carefully dissect out the pedal ganglion of a full- 

 grown specimen under the microscope, and try to find the 

 auditory organs and the small nerves which join them to 

 the cerebro-pedal commissures. If a young Unio or 

 Anodonta cannot be found for microscopic examination, 

 any other very small marine or fresh-water lamellibranch 

 will answer. 



m. The reproductive and renal openings. On each side 

 of the abdomen, above the cerebro- visceral commissure, 

 notice a small slit (Fig. 143, ), through which the repro- 

 ductive organs open into the ckmcal chamber, and just 

 above and close to these a second pair of openings (Fig. 

 143, bo), the external apertures of the renal organs, or 

 Oryans of Bojanus. 



n. Open a fresh specimen, and remove the body from 

 the shell, exercising great care to avoid injuring the soft 

 parts. Place it in water with the dorsal surface above, 

 and notice on the middle line the transparent pericardium 

 (Fig. 144, o). Carefully open this, and notice that the 

 dark-colored intestine runs through it longitudinally. 

 The greater part of the cavity of the pericardium is occu- 

 pied by the transparent heart (Fig. 144, n), which con- 

 sists of a median ventricle wrapped around the intestine, 

 and two lateral auricles. 



1 . The ventricle is a large oval transparent pouch which 

 gives rise to an anterior aorta dorsal to the intestine, and 

 a posterior aorta ventral to. the intestine. 



