Mr. W. Clark on the Pholadidae. 319 



apophysary and pedal apparatus are not the substitutes for an 

 anterior adductor muscle ; for if they did act as such, their mus- 

 cular powers must remain in quietude from the necessary con- 

 traction, and the excavating action would be destroyed : the two 

 actions are completely antagonistic. The posterior part of the 

 valves is only opened slightly to allow the issue of the basal 

 portion of the branchial sheath, to assist by attrition in the en- 

 largement of the posterior part of the chamber whilst the foot is 

 operating in front. 



The foot and pedicle, which in a living state appear almost 

 hyaline, when they have become exsiccated, will be found to consist 

 of a mass of longitudinal elastic fibres, the principal portion of 

 which centre on the umbonal excrescences, and the remainder 

 supply muscular threads to the anterior part of the body : the 

 basal area of the foot is by far the most coriaceous portion. 



Lamarck's Dimyal arrangement is almost untenable, as the 

 Pholades having only a medial adductor are removed therefrom, 

 and many of his Monomyse having two muscles renders the posi- 

 tion still more doubtful. The medial adductor of the Pholades 

 is a most influential organ ; it is fixed to and is an integral com- 

 ponent of the mantle at that point where it becomes the origin of 

 the siphonal sheath, and adheres by its large subcircular flaps to 

 each side of the valves, showing when removed two well-marked 

 cicatrices : this muscle extends its influence to each extremity of 

 the animal, as from it the mantellar marginal supports emanate ; 

 it also supplies the siphons with powerful retractors, and furnishes 

 the tube into which the rectum discharges with a sphincter ; it is 

 the main support and connection of the animal with the poste- 

 rior part of the shell ; it likewise supplies the posteal parts of the 

 body with the minor muscular threads ; and finally it is the organ 

 of a limited relaxation to allow the valves to be opened in con- 

 cert with the cartilage and ligament for the issue of the basal 

 portion of the branchial sheath, when it is required to assist in 

 excavation, and of their closure to expel the water from the re- 

 spiratory sac. 



The whole mass of the branchial and anal tubes is a tissue of 

 coriaceous muscles which are composed of layers of strong close- 

 set longitudinal fibrous cords, ci-ossed at right angles by minor 

 ones, and at the posterior extremities they throw ofi" the special 

 annulated retractors of the terminal cirrhi of the branchial ori- 

 fice, which appear each to have a minute sheath, and they also 

 provide for the retraction of the anal orifice. — We have next to 

 examine the nervous influences. 



The Nervous Influences. 

 The powerful and difi'usely distributed muscles of this species 



