182 PHOLADID.E. 



Lamarck's Dimyal arrangement is strictly untenable, as 

 the Pholades, having only a medial adductor, ought to be 

 removed therefrom, and many of his Monomyce, having two 

 muscles, must be deposited therein. The medial adductor of 

 the Pholades is a most influential organ ; it is fixed to and is 

 an integral component 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 man- 

 tellar 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 posterior part of the 

 shell ; it likewise supplies the posteal parts of the body with 

 the minor muscular threads j and finally it is the organ of a 

 limited relaxation to allow the valves to be opened in concert 

 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 

 respiratory 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, crossed at right angles by 

 minor ones, and at the posterior extremities they throw off 

 the special annulated retractors of the terminal cirrhi of the 

 branchial orifice, 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. 



Nervous Influences. 



The powerful and diffusively distributed muscles of this 

 species would lead us to expect that the medullary masses 

 would be of corresponding importance ; this is not the case, as 

 in Pholas dactylus I can only find two inconsiderable ganglia ; 

 the anterior one is the largest, consisting of a white pulpy 

 mass, situated on the centre of the oesophagus just above the 

 buccal aperture ; from it two distinctly visible threads curve 



