52 CEYLON PEAEL OYSTER REPORT. 



more roomy than the right, and so the foot is more easily accommodated on that side 

 of the body (as shown in Plate II., fig. 6). 



The posterior pedal levators (Lcv.p., Plate III., fig. 26) are two short insignificant 

 bundles which originate high up in the fibres of the anterior levators, exactly on the 

 level of the mouth. Thence their course is backwards and upwards through the 

 visceral mass to an attachment to the valves behind the scar of the corresponding 

 anterior levator, but on a slightly lower plane. The whole course of each posterior 

 levator in an adult oyster is less than one-quarter of an inch from -fy" to \". The 

 contraction of the anterior levators causes the foot to be retracted and raised dorsally ; 

 the coincident shortening of the posterior levators introduces a drag towards the rear. 

 No protractor muscles are present, turgescence of the venous pedal spaces effecting, 

 in the main, the protrusion of the foot when muscular relaxation takes place. 



The Intrinsic Muscles of the foot and viscera are diffuse rather than in masses. 

 Those of the foot form a muscular enveloping sheath or interlacing net of 

 considerable thickness and complexity. It is formed of several ill-defined layers and 

 scattered bundles, shown in transverse section on Plate VIII. , figs. 3 and 4. A 

 number of the internal bundles run in the main longitudinally along the foot, some 

 fibres run circularly, there are groups diverging radially, and in some parts the fibres 

 interlace in various directions. 



In the visceral mass proper, small transverse muscle bundles pass from side to side, 

 binding its tissues together and providing a framework, slight though it be, wherein 

 ramify the tubules of the digestive gland and of the gonads. These transverse 

 intrinsic bundles are somewhat spindle-shaped, each end narrowing to a tendinous 

 insertion attached to the fibrous connective-tissue ensheathing the visceral mass. 



Of other intrinsic muscles the most important are the branchial, one of which, in 

 the form of a flat band of muscle fibres, runs within each ctenidial axis from end to 

 end, close to the dorsal edge, along with a large nerve. These bands have the effect 

 of retractor muscles, shortening the gills and withdrawing their posterior extremities, 

 an action assisted by other muscle fibres which radiate, fan-like, from a point just in 

 front of the anterior margin of the adductor. There are also muscle bundles running 

 longitudinally down each side of the principal filaments (Plate VIII., fig. 13, m.b.). 



The Pallial Muscles (Ret.pall., Plate VI., fig. 14) are all retractors, and together 

 constitute the orbicular muscle of the mantle. They are a series of fan-shaped muscles, 

 radiating outwards to the mantle edge from a number (15 to 18) of insertion centres 

 of various sizes, arranged circularly, and which together form the well-marked pallial 

 line of scars that runs parallel with the margin of the shell, upon the inner surface 

 of each valve (Plate IV., figs. 1 to 6). The fibres lie entirely within the layer of loose 

 connective-tissue, that is between the inner and outer epithelial surfaces of the free 

 portion of the mantle. Their ultimate ramifications form an anastomosing network, 

 the branches diverging and reuniting in the complex manner seen in Plate VII., 

 fig. 13. The bundles in some cases surround the branches of the pallial nerves. 



