50 CEYLON PEARL OYSTER REPORT. 



fibre is a compound structure formed by the union of a number of these threads of 

 secretion. The figure shows a few of the narrow folds from one side only, the glands, 

 the convoluted threads of secretion and the close relation with groups of muscle 

 fibres on the periphery of the organ. 



THE MUSCULATURE. 



The pearl oyster is monomyary, possessing a single adductor (the posterior), the 

 largest and most important muscle in the body. The other muscles present are : 

 one pair of retractors of the foot, two pairs of weak pedal levator muscles (superior 

 retractors), the orbicular retractor muscle of the mantle (pallial muscles), the intrinsic 

 muscles of the foot and visceral mass, the branchial bands, and the heart or cardiac 

 muscle (which will be discussed under the vascular system below). 



The Adductor Muscle of the shell (Add., Plate VI., fig. 1) stretches transversely 

 across the body from valve to valve. It is a massive bundle, wedge-shaped in 

 section, and slightly curved (Plate II., figs. 3, 4). The narrow end points upwards 

 and lies immediately behind the ventricle of the heart. The terminal part of the 

 rectum runs in the middle line along the posterior surface (Plate VI., fig. 1, Int. 3). 



As the concavity of the muscle faces upwards and forwards, the wider ventral end, 

 which is rounded, is turned anteriorly. Its anterior margin marks, approximately, 

 the centre of each valve, while the dorsal and posterior apex lies a short distance in 

 front of the posterior sinus in the margin of the shell. Thus, as the lower end of 'the 

 adductor stretches from side to side in the widest region of the body, the fibres 

 decrease in length as they approach the dorsal and posterior extremity, where the 

 extent of the muscle is less by half than at the anterior and ventral end. 



The muscle is not homogeneous ; two distinct regions are obvious. The one, a 

 narrow tendinous strip made up of white glistening fibres, forms the posterior border 

 (Add. 1 , Plate VI., fig. 1, and sections on Plate V.) ; the other, broad and massive, of 

 colourless, semi-translucent fibres, occupies the remainder of the mass. Under the 

 microscope, the fibres of the latter are finer when teased up, and have an appearance 

 which has been described as striation, but is by no means distinct ;* those of the former 

 are about two to three times as thick, more fleshy and quite smooth. 



The substance of this muscle is permeated with lacunar spaces, penetrating into 

 and among the loosely compacted bundles of fibres. The blood supply is derived 

 from the posterior pallial arteries which arise from the terminal branches of the 

 posterior aorta and pass outwards to the mantle sunk within the substance of the 

 tendinous portion of the adductor (Art.p.p., 'Plate VII., fig. 4). 



The power exerted by the adductor in bringing the two valves together by its 

 contraction is very considerable, and the action is very rapid. Sir William Twynam 



* Very much less obvious than the striation of the corresponding fibres in, for example, Peden 

 opercvla/ris. 



%j 



