124 THE FRESH-WATER MUSSEL 



carbon dioxide and other soluble wastes are discharged. After enter- 

 ing by way of the incurrent siphon, the water bathes the organs of the 

 mantle cavity. From here it passes through microscopic openings, the 

 ostia, which lead from the mantle cavity through the inner and outer 

 surfaces of all the gills into the water tubes. Passing upward in the 

 water tubes, water enters the suprabranchial chambers and, flowing 

 posteriorly in these, it reaches the outside through the excurrent 

 chamber and excurrent siphon. The water that passes out has been 

 strained of microorganisms and other bodies too large to pass through 

 the ostia of the gills. Upon coming in contact with the surfaces of the 

 gills, foot, or mantle, this food material is entangled in a mucous se- 

 cretion and carried by cilia along definite lines which converge toward 

 the palps. Passing between the inner and outer palp on each side, 

 the food is at length delivered to the slitlike mouth, an opening which 

 lies below the anterior adductor muscle and between the continuations 

 of the inner and outer palps, 



(1) Transverse sections, about 5 mm. thick, of whole specimens are 

 valuable in helping you to understand the relationship of structures 

 here and in your further dissection. If such sections are available, 

 identify on them, as far as possible, the structures found in your 

 dissection. 



II. THE INTERNAL STRUCTURE 

 Exercise 5. — The Nervous System. 



(a) Remove the right gills and palps by cutting near their attach- 

 ments. Look on the ventral surface of the posterior adductor muscle 

 for a pale yellowish body, the fused right and left visceral ganglia. 

 Using your needles, carefully trace any nerves found extending from 

 the ganglion on the right side. The paired cerebral ganglia will be 

 found by carefully picking away the tissue below the base of the 

 palps. From the cerebral ganglion trace a nerve, the cerebro-pedal 

 connective, posteriorly and ventrally into the soft tissue in the upper 

 part of the foot to find the pedal ganglia. The cerebral ganglia are 

 also connected to the visceral ganglia by the cerebro-visceral connec- 

 tives. These may be difficult to follow. Add these structures to your 

 previous figure. 



Exercise 6. — The Circulatory System. 



(b) Turn the specimen dorsal surface up and open the pericardial 

 cavity, or ccElom, carefully by cutting with the scissors in front of the 

 posterior adductor muscle. Remove the roof of the pericardial cavity 

 and expose the heart, which consists of a median ventricle, wrapped 



