148 MOLLUSKS. 



rate the two pairs of gills on the median ventral line, and 

 find directly beneath the posterior adductor muscle a pair 

 of yellowish ganglia more or less united. These are the 

 visceral ganglia. Trace from them as a center a pair of 

 lateral nerves to the gills, a pair of posterior nerves 

 to the edges of the two mantle lobes, and a pair of very 

 long commissural nerves forward, to join a pair of cere- 

 bral ganglia which lie near the surface at the bases of 

 the labial palps, on either side of the mouth. From these 

 ganglia trace a pair of nerves downward to the pedal 

 ganglia, which lie deeply imbedded where the foot joins 

 the abdomen. The nerves appear as fine white threads ; 

 the ganglia are usually more easily seen because of their 

 yellowish color. These three pairs of ganglia, with their 

 connecting commissures, constitute the central nervous 

 system of the mussel, and of a large group of animals of 

 which it is a type. 



IV. Reproductive Organs lie in the posterior part of the 

 abdomen, one on each side, and each opens by a slit on the 

 upper surface just above the long commissural nerve traced 

 forward to the cerebral ganglia. 



The eggs of the river mussel are passed into the cavity 

 of the outer gill, where they hatch, and where the young 

 are retained for a time. If there be found a specimen in 

 which the outer gills appear swollen and dark colored, 

 open one of them, and mix a little of the contents with a 

 drop of water on a slide. Examine with low power, and 

 look for little mussels. Draw several of them. Notice 

 whether any of them move about. 



V. The Shell. Remove all muscles from the inside, 

 and dirt from the outside, of the shell, and observe again 

 the concentric lines of growth about the umbones. Hold 

 the shell up toward the light, and observe the fine lines 

 radiating from the umbones. 



Inside the shell, notice the large adductor muscle scars, 



