LAMELLIBRANCHIATA. 



689 



which pass from the inner surface of one valve to that of the other, 

 one being placed anteriorly in front of the mouth, while the other 

 is situated posteriorly close to the termination of the intestine (fig. 

 555, a and a). Moreover, among 

 the Dimyary Bivalves the adductors 

 present two markedly different con- 

 ditions. In one group of forms — 

 hence termed Homoinyaria — the an- 

 terior and posterior adductors are 

 approximately equal in size. In a 

 second series — hence termed Hetero- 

 myaria — the anterior adductor is 

 very small, and the posterior adduc- 

 tor is very large. Examples of this 

 latter condition are found in the 

 Mussels, Pearl-mussels, &c. On the 

 other hand, in Ostrea, Pecten, and 

 certain other Bivalves — hence called 

 Monomyaria — the anterior adductor 

 is absent, and the posterior adductor 

 alone remains. The adductors leave 

 distinct " muscular impressions " in 

 the interior of the shell (fig. 556), 

 so that it is easy to determine, by a 

 simple inspection of the dead shell, 

 whether a given Bivalve has been 

 "dimyary" or "monomyary." 



Besides the scars left by the ad- 

 ductor muscles, or muscle, there are 

 other impressions in the interior of 

 the valves which are produced by 

 the attachment of muscles. Thus, 

 the " foot " is very commonly pro- 

 vided with " pedal muscles," which 

 leave small scars in the inside of the 

 shell. When they are well devel- 

 oped, the "pedal impressions" are 

 twofold, consisting of an impression 

 formed by the " protractor " muscle 

 which exserts the foot, and of another, 

 posteriorly-placed scar formed by the 

 "retractor" muscle which withdraws 



the foot (fig. 556, a, pp and rp), a second retractor being some- 

 times inserted behind the anterior adductor. 



Again, the muscular margin of the mantle is attached to the 

 vol. 1. 2 x 



Fig. 555.— Anatomy oiMya arenaria. 

 (After Woodward.) The left valve of 

 the shell and the left mantle-lobe are 

 removed, and half of the siphons has 

 been cut away, a, Anterior adductor ; 

 a , Posterior adductor ; b. Visceral mass ; 

 c, Chamber of the mantle-cavity into 

 which the anus {v) opens ; f, Foot ; g, 

 Branchiae ; h, Heart ; tn, Cut ventral 

 edge of the mantle ; o, Mouth ; s, In- 

 halant siphon ; s', Exhalant siphon ; t, 

 Labial palpi. The arrows indicate the 

 direction of the water-currents. 



