FRESH-WATER MUSSEL 



209 



covered by a bivalve shell, the valves being lateral in position. When 

 buried in the mud only a small part of the posterior end is left free, and 

 at this point may be seen projecting from between the two valves the 

 open ends of the siphons, by means of which water is circulated through 

 the animal, the current entering through the ventral siphon and leaving 

 through the dorsal one. When the animal is moving, a muscular foot 

 is projected ventrally and anteriorly between the two valves. This foot 

 ends in a blunt point where its anterior and ventral margins meet, and this 

 point is capable of being extended to form a sort of hook by means of 

 which the animal pulls itself along. 



244. Shell. — The two valves of the shell are fastened together along 

 the dorsal side by a hinge ligament which is elastic and which tends to 



///h^e li'giamer?-/- 



Umbo 



Aifcichmenf area 

 of an far /or 

 adductor 

 n7i/sde 



Hinge tooth 



Aitachmet^t area 



o-f pos feri'or pro -f-racfor 



musc/e 



Affachmenf 

 area of 

 posfer/or 

 aa/ducfor 

 musc/e 



Periostracum 



Pa///a/ 

 //rre 



Fig. 117. — Anodonta grandis Say. Showing inner surface of a left-hand valve. X 3^. 



draw the valves together dorsally and to cause them to gape ventrally. 

 Close to this ligament, on either side and nearer the anterior than the 

 posterior end, is a point known as the umbo, which represents the oldest 

 portion of the shell. On the outer surface of the shell (Fig. 116) may be 

 seen a great number of concentric lines arranged around the umbo as a 

 center and gradually increasing in distance from the umbo until the 

 margin of the shell is reached. They represent lines of growth, several of 

 which may be formed during one year, though the annual lines are some- 

 what more prominent than the intermediate ones. In older animals the 

 shell at the umbo is very often eroded. Elsewhere it is covered by a skin- 

 like horny layer called the periostracum, which gives color to the shell. 

 The periostracum extends a little way beyond the margin of the shell 

 except at the hinge ligament. 



The inner surface of each valve (Fig. 117) often possesses elongated 

 sharp ridges and toothHke projections which hold the valves from slipping 

 out of position. These are known as hi72ge teeth. They vary greatly in 

 degree of development in different types and may be absent. There are 

 roughened areas serving for the attachment of the anterior and posterior 

 adductor muscles, which run from one valve to the other and hold the two 



