182 PHOLADIDA. 
Lamarck’s Dimyal arrangement is strictly untenable, as 
the Pholades, having only a medial adductor, ought to be 
removed therefrom, and many of his Monomye, 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 poimt where it 
becomes the origin of the siphonal sheath, and adheres by its 
large subcireular 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; 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 \ can only find two inconsiderable ganglia ; 
the anterior one is the largest, consisting of a white pulpy 
mass, situated on the centre of the cesophagus just above the 
buccal aperture; from it two distinctly visible threads curve 
