BIVALVE MOLLUSCS 



329 



Fig. 191. Shells of Fresh-water Mussel (Unto], seen from within 



ab, Impressions of anterior and posterior adductors; df, hinge- teeth; 



e, ligament; h, pallial line. 



their main features, though there are certain differences, especially 

 as regards the shell. 



External Characters. The animal is completely invested by 

 the two elongated oval pieces which make up the shell, and are 

 right and left respec- 

 tively (fig. 191). They 

 are united together 

 above, along what is 

 called the hinge -line, 

 and can either be 

 brought close together 

 elsewhere so as entirely 

 to cover the soft parts, 

 or may be separated so 

 as to "gape" more or 

 less, which is always the 

 case in dead specimens. 

 A series of closely-set 

 lines following the curve of the shell can be seen on the out- 

 side, and it will be found that these "lines of growth", which mark 

 successive additions of material, have as their centre a pointed 

 projection near the straight upper edge of the valve, known as 

 the beak or umbo. 

 This clearly marks 

 the oldest part, 

 and, as in bivalves, 

 usually projects 

 forwards, and is 

 nearer the front 

 than the back end. 

 When the shell is 

 removed from the 

 soft parts by cut- 

 ting through cer- 

 tain muscles, it will be found that each valve covers a soft flap 

 which lines it during life, and is really half the mantle (fig. 192), 

 here clearly divided into right and left halves, which may be 

 compared to the flaps of a man's coat, if the further assumption 

 be made that the coat is a part of the body and its flaps are big 

 enough to entirely cover the occupant. 



fO.ST.ERTO.R- AORT.A 



AITRIEC0 



ANTERIOR AORTA 



Kl 



ANTEfcJOTJ RETRACTOR 



Fig. 192. Structure of Fresh-water Mussel (Anodonta} 



Gi-Gs, ganglia: Go, opening of oviduct; Ki and K.2, internal and external 

 kidney-openings 



