68 ORGANS OF MOVEMENT IN THE PELECYPODA. 



In the burrowing species, the foot is large and powerful, and 

 in some of those which excavate cells within hard substances, it 

 is studded with siliceous spicula, which make it an efficient 

 instrument for boring: although it is very doubtful whether this 

 is the only or even the principal means by which the animal 

 works. 



Adductor Muscles. These attach the animal to the valves of 

 the shell upon either side, and their contraction serves to close 

 them together. Their insertion is shown by scars upon the 

 valves lii, 31, 32; iii, 51), and these indicate an important 

 difference in position and in number, useful in the classification 

 of bivalve mollusca. In the Monomyaria (ii, 32), the adductor 

 is single and usually subcentral : Ostrsea, Tridacna, etcL, being 

 examples; whilst in the Dimyaria (ii, 31), including most of 

 the bivalves, there are two adductor muscles, situated anteriorly 

 and posteriorly near the margin. The action of the adductors 

 is antagonistic to that of the ligament which connects the valves 

 of the shell, and which passively opens them. Their contraction 

 is rapid and powerful, exerting a force which is enormous, when 

 compared with their size and weight ; according to the experi- 

 ments of M. Leon Yaillant made at Suez, a Tridacna, 21 centi- 

 metres long, and the muscle of which weighed but 39 grammes, 

 lifted a weight of 4914 grammes by its contraction. In Fecten 

 varius (ii, 32), two large independent impressions are formed by 

 the adductor, the muscle itself being composed of two elements ; 

 there is also in the left valve a third impression produced by the 

 foot. So in Anomia, whilst the right valve has the single 

 muscular impression onl}^, the left valve shows three additional 

 scars, the large central one being that of the muscle of the plug 

 (the equivalent of the byssus of Pinna and Modiola), a smaller 

 one within the umbo, and one in the disk being caused by the 

 retractors of the foot. These foot retractors cause scars -upon 

 the valves of many of the pelecypoda ; in the dim3^arians the 

 anterior pair are attached within the umbones as in Modiola or 

 Mytilus (iii, 51), or nearer the adductor, as in Astarte and Unio ; 

 the posterior pair are often close to the adductor so that their 

 scars are not separated from it. The Unionidae have two 

 additional retractors of the foot attached laterally behind the 

 anterior adductor (iii, 52) ; in Leda, Solenella, and a few others, 

 this lateral attachment forms a line extending from the anterior 

 adductor backwards into the umbonal region of the shell. 



At a certain distance within, and parallel with the margin of 

 the shell, is an impression caused by the muscular mantle-margin, 

 and termed the pallial line (iii, 31, 32) ; it connects the scars of 

 the adductors. In the monomyaries it is broken up into an 

 irregular chain of spots. The presence of a sinus (iii, 31) in 



