690 



MOLLUSCA. 



interior of each valve along a line running at a little distance within, 

 and parallel with, the ventral margin of the shell. In this way is 

 formed a more or less well-marked impression in the interior of each 

 valve, which is termed the " pallial line" (fig. 556, m). The form 

 assumed by the " pallial line " differs in different Bivalves accord- 

 ing as respiratory " siphons " are present or absent. In all those 

 Bivalves, namely, in which the mantle-lobes are free, and in which 



Fig. 556. — a, Interior of the left valve of Anodonta cygnea ; b, Interior of the right valve ot 

 Artemis exoleta (after Woodward); c, Interior of the left valve of Pecten varius (after Wood- 

 ward), a, Impression of the anterior adductor; pa, Impression of the posterior adductor; m, 

 Pallial line ; j, Sinus in the pallial line caused by the insertion of the retractor muscles of the 

 siphons ; pp, Scar of the protractor muscle of the foot ; rp, Scar of the anterior retractor muscle 

 of the foot ; rp' , Scar of the posterior retractor muscle of the foot. In Pecten varius (c), though 

 the shell is monomyary, the scar left by the posterior adductor (pa) is double, and there is a large 

 scar (pe) formed by the muscular base of the foot. 



there are consequently no siphons, the "pallial line" runs in an un- 

 broken curve round the lower part of the valve (fig. 556, a, m). 

 The pallial line is similarly unbroken in those Bivalves which possess 

 short siphons, but which do not possess a specially developed " re- 

 tractor muscle " for the withdrawal of the siphons within the shell. 

 The name of Integropallialia is given to all such Bivalves as the 

 above, in which the pallial line is " entire," or unindented, and there 



