272 



SWAN MUSSELS. PHYLUM MOLLUSCA 



hinge the two valves are joined by an elastic ligament, which 

 pulls them together and thus causes them to gape below w^hen 



the adductor muscles are relaxed. To 

 open the shell of a living mussel the 

 handle of a metal scalpel is passed 

 between the valves and they are prised 

 apart ; they are then held open, for 

 example by turning the handle at 

 right angles, and the muscles can be 

 cut close to the shell on one side. The 

 body of the animal is then found to 

 be soft, without a cuticle, and pro- 

 vided with a flap of tissue which 

 hangs down on each side and covers 

 the other organs. This is the mantle 

 (Figs. 200, 201). It has a thick edge 

 which secretes the two outer layers of 

 the shell, while the pearly layer is laid 

 down by the whole outer surface of the mantle and skin of the 

 back. Pearls are formed in the same way in pockets of the mantle 



Fig. 198.— Part of the shell of 

 a swan mussel, seen from 

 above. 



l.g., Lines of growth ; lig., ligament; 

 ub., umbo. 



^r. mV. 



Fig. 199.— a swan mussel removed from its shell and lying on its right side with 



the greater part of the left lobe of the mantle cut away. 

 a.ad Anterior adductor muscle a.r., anterior retractor ; d.s., dorsal siphon ;/., foot ; l.i.g., left inner giU ; 



i'SuX"mu'c^e i r^^no 'J'^^' 'y'"f ^^'^ ' ^■'■^- ^'^' °"^^^ ^"^ ^ /./>. labial ps.lps;'p.ad., posterior' 

 pH««f fv, ' ^ •• posterior retractor; pro., protractor ; r.w/., right mantle lobe; r.m/'., thickened 



edge of the same ; v.s., ventral siphon with papilla. ' - & . '•"»» •, imcKeneu 



surface around foreign bodies which have intruded between 

 mantle and shell. The Hne of attachment of the mantle from the 

 side of the body is not straight but higher in the middle than near 



