ANATOMY OF THE PEARL OYSTER. 51 



relates (' Report on Ceylon Pearl Fisheries,' p. 6, 1900) how at the fishery ol 1891, 

 after the oysters had been landed and had lain in heaps in the Kottu for some time, 

 one gaping individual had still strength enough left to snap at and seize a hungry 

 sparrow which incautiously attempted to feed upon it. The oyster held on so tightly 

 that all the efforts of the trapped bird to escape were vain, and the strangely assorted 

 pair are now to be seen in Sir William Twynam's museum at Jaffna. Similarly, 

 Mr. Hornell reports that an oyster lying in the Kottu during the last fishery (1903) 

 captured a mouse (now in the possession of the Lieutenant-Governor the Honourable 

 Mr. E. im Thurn) ; and that he himself saw the foot of that agile animal, an inquisi- 

 tive mongoose, caught by an oyster which resisted all the efforts made to dislodge it 

 for nearly five minutes. 



Although the pearl oyster has not the power of moving rapidly through the water 

 or over the sea-bottom after the fashion of Lima and of some species of Pecten, by 

 the violent expulsion of water caused by a sudden closure of the valves, still it can 

 eject a jet of water with some force to a distance of 9 to 12 inches, as can be seen 

 when living oysters are watched in shallow vessels of water. This forcible ejection is 

 evidently useful in dislodging any small animals and other particles that may have 

 gained access to the branchial chamber. 



The Retractors of the foot are a pair of symmetrically disposed muscles lying in the 

 horizontal plane of the body. They originate (Ret., Plate III., figs. 24, 25) in the 

 walls of the byssal gland and, then diverging, pass backwards in V-like manner, to be 

 inserted, one into the right valve, the other into the left, within the concavity of the 

 adductor scar (compare Ret. on Plate IV., fig. 1, with other figures on same plate). 

 Neither retractor impresses a separate scar upon the nacre, the posterior edge of the 

 retractor impression blending indistinguishably with the anterior edge of that of the 

 adductor. 



In its anterior portion each retractor is sub-cylindrical, flattening to an oval, in 

 section, at the place of insertion. There is no decussation of the fibres of the two 

 bundles at their junction anteriorly. 



The Levators of the foot are four, two anterior and two posterior. Each of the 

 anterior pair (Lev. a., Plate III., fig. 26) has its insertion at the apex or inmost point 

 of the umbonal recess of its respective valve a point directly dorsal to the mouth 

 region. From this place the fibres pass vertically downwards, on either side of the 

 mouth, spreading laterally, fan-like, as they go. The external lateral fibres 

 eventually blend with the muscular sheath on the sides of the viscero-pedal mass, 

 while the inner or anterior fibres pass into the root of the foot. 



The left anterior levator is considerably stronger than its neighbour, a specially 

 strong cord of fibres passing on the inner side to the dorsal aspect of the root of the 

 foot. By the contraction of this cord the foot is drawn over to the left side, which is 

 its normal position when in a state of rest.- The explanation of this asymmetrical 

 arrangement is seen in the fact that the left valve is much deeper and consequently 



