S WA N MUSSELS 



271 



which the}^ contain, which may be of considerable value. They 

 are not killed by the freezing of the water even if they themselves 

 be frozen solid, but can only survive a few hours of drought. 



SHELL AND MANTLE 



The shell has the same layers as in the snail. Lines of growth 



iib. 



.^ 



d.s 



v.s..-- 



,.. 1.9. 



f. 



B 



t.p.r. 



i.p.ad. 



Fig. 197. — The Swan Mussel. 



A , The shell with the animal, from the right side ; B, the left valve of the shell, from within. 



d.s. Dorsal siphon ; /., foot ; i., impressions of muscles ; i.a.ad., of anterior adductor ; i.a.r., of anterior 

 retractor ; i.p.ad., of posterior adductor ; i.p.r., of posterior retractor ; i.pro., of protractor ; l.g., lines 

 of growth ; pal., pallial line ; ub., umbo ; v.s., ventral siphon. 



parallel with its edge mark the outside of the shell (Figs. 197, 198), 

 centring upon a point about a quarter of its length from the 

 front end. This point is known as the umbo and shows the position 

 of the first shell of the young mussel. On the inside of the shell 

 may be seen the marks of attachment of the adductor, retractor, 

 and protractor muscles presently to be mentioned, and parallel 

 with its edge is a mark known as the pallial line, where the fold 

 of the body-wall known as the mantle is attached. Above the 



