64 OKHAMANDAL MARINE ZOOLOGY REPORT 



The intrinsic muscles of the foot form an extremely complex system, comparable 

 in general arrangement with the. musculature of any mobile liguliform muscular organ. 

 Towards the base the fibres in large measure radiate outwards to form a somewhat 

 laterally compressed discous base. In the cylindrical trunk region of the foot the 

 muscle fibres run in every possible direction ; many run longitudinally, some run 

 circularly, others radiate outwards, and the. remainder interlace in apparently hopeless 

 confusion. Circular and radiating muscle fibres preponderate in the sucker-like free 

 extremity ; the rim is capable of closure as by a sphincter muscle. 



Few other intrinsic muscles are to be found in Placuna ; the principal are the 

 small paired cylindrical muscle bundles which traverse the axes of the branchise 

 longitudinally in the floor of the efferent branchial vessels (fig. 24). 



Slender muscle fibres also pass down the interior of the individual gill filaments, 

 enabling them to be retracted to some slight extent. 



The ORBICULAR PALLIAL MUSCLE is composed of a small and variable series of 

 loosely compacted fan-shaped muscles radiating outwards to the mantle edge from 

 some four to six insertion centres of varying size and shape grouped in a roughly 

 semi-circular manner around the ventral half of the adductor muscle. The dispo- 

 sition of these centres (and of the corresponding scars which their insertion imprints 

 upon the inner surface of the valves) lacks alike the regularity of disposition seen in 

 the Meleagrinidse, or the continuity characteristic of Cardium ; the insertion scars are 

 extremely faint on the valve surface in Placuna, while in a dissection the insertion 

 centres are difficult to trace owing to their irregularity and to the layer of repro- 

 ductive tissue which accompanies and masks them. One of the most frequent 

 dispositions is where there are two large insertion centres anterior (M.ins.a.) and 

 two small ones posterior (M.ins.p.} to the base of the adductor, with a long and 

 extremely narrow band insertion skirting and apparently fused with the ventral 

 edge of the adductor at its insertion in the valve. Fig. 19 graphically depicts 

 the relative positions and forms of these centres, as well as the course and branching 

 of the ill-defined fans of fibres which radiate towards the pallial margin dividing 

 into finer and finer branches as they go. 



From the radial course pursued by these bundles, it follows that they are attached 

 to the shell at a very acute angle. Their whole course lies in the filling of connective 

 tissue forming the thick middle layer of the distal region of the mantle. IVlany of the 

 branches anastomose, and as they approach the pallial edge they divide into two sets 

 of fibres, one passing to the inner aspect of the margin to provide for the movements 

 of the velum, the other to the outer margin to serve the two folds of the true pallial 

 edge. Many of the principal of these radial muscles impress a record of their course 

 and branching in corresponding shallow but distinct groovings upon the inner surface 

 of the valves. 



