110 MUSSELS, CLAMS, (lYSTERS. SNAILS, SQUIDS 



of ganglia with their i-adiating nerves. Two gangha, just 

 abo^'e the mouth, one on each side of the gullet, form the 

 brain. The}' are connected by a nerve cord running above 

 the gullet. P'rom the brain a ner\'e cor(_l runs posteriorly 

 along each side of the body and joins a pair of visceral 

 ganglia just under the posterior adductoi- muscle. Finally, 

 a pair of pn\al ganglia is buried in the tissues of the 

 foot near its base. Each of these is joined to the braiit 

 ganglion on the corresponding side by a nerve cord (Fig. 

 59). These different pairs of ganglia send nerves to all the 

 adjacent parts. 



This system of nerves gives sensation to all parts of the 

 body. The senses are not acute, but touch is well developed, 

 especially in the point of the foot and margin of the mantle 

 and around the edges of the siphons. There is no positive 

 evidence that the mussel can taste or smell and it cannot 

 hear, but it can feel vibrations through the soil or water. 



Reproduction and life history. — The sexes are separate 

 and may sometimes be distinguished by the greater con- 

 vexity of the shell of the female. The ovaries and sper- 

 maries are similar in ajjpearance and are situated around 

 and among the coils of the intestine. The ducts from 

 both oi)en on the side of the bod}' near the openings of the 

 ui-inary tubes. The sperms enter the shell of the female in 

 (he incoming currents of water and fert lize the eggs. The 

 eggs then di'oij into the troughs of the outer gills, which 

 serve as brood ])ouches and retain the eggs until they hatch. 

 The embryo mussels are called glochidia and ha\-(^ soft 

 bodies inclosed by two triangular valves. Each glochidium 

 finally passes out through the exhalent siphon and rests, 

 for a time, on the bottom, when it attaches itself to some 

 fi.sh by (he lower hooklike projections of the \'aK'i'.s and 



