46 CEYLON PEARL OYSTER REPORT. 



of energy to the animal. This provision for thus disposing of unsuitable particles is 

 especially useful when the water is disturbed and laden with sand or mud. Under 

 such circumstances the oyster feeds slowly, rejecting nearly everything that comes. 

 When, as happened sometimes in our tanks at the Galle Marine Laboratory, the 

 in-flowing water was laden with decaying vegetable debris in great quantity, or when 

 mud was present, the palps accumulated the particles till they had enough to form a 

 small bolus. This while forming was revolved constantly by the palpar surfaces. 

 When large enough, a twist seemed to be given to it whereby it passed from the palps 

 to the anterior end of the pallial ciliated path, along which it advanced rapidly, and 

 was shot out in a few seconds. In the course of a few hours it has been noticed, 

 when the tank water remained still, that a conical pile of ejecta more than \ inch 

 high accumulated on the ground just behind the oyster (Plate VI., fig. 14, Pel.). 



Pigmentation. The free pallial margin together with the velum is in most cases 

 deeply pigmented (Plate II., figs. 3, 6), usually in black, grey, and shades of yellows 

 and browns, mostly chestnut brown, though orange is also frequent. The pigmenta- 

 tion is usually in the form of large alternate blotches which give a certain appearance 

 of regularity to the colouring ; hut the exact pattern varies much. It is noticeable, 

 however, that there is considerable difference in the degree of pigmentation in oysters 

 from quite shallow water and in those from depths of 6 to 10 fathoms. The latter have 

 the colouring mainly confined to the velum and the pallial margin, whereas in the former 

 the pigment may extend widely over the general inner face of the mantle. We find 

 that orange and chestnut tints are much more frequent amongst the pearl oysters of 

 Trincomalee Harbour, which live in shallows averaging from 6 to 18 inches at low 

 tide, than amongst those from the deeper banks in the Gulf of Manaar. The gills 

 also are more frequently and more extensively pigmented at Trincomalee than amongst 

 individuals from deeper water. 



THE FOOT AND BYSSAL OROAN. 



The Foot is a highly mobile tongue-shaped organ capable of great elongation and 

 contraction. It arises from the anterior region of the visceral mass nearly midway 

 between the mouth and the intestinal lobe and has the anterior extremities of the 

 branchiae flanking it on either side (Plate VI., fig. 1). The greater part of its bulk is 

 composed of networks of muscle fibres running in various directions, thus ensuring a 

 wide range of movement, and is so extensively penetrated by blood spaces that the organ 

 is highly cavernous. When these spaces are rendered turgid through an influx of 

 blood, the foot becomes erected and is then quite three times as long as in the 

 completely contracted, non-turgid state. In the latter condition it has the form of a 

 slender elongated cone tapering gently from a wide base to a pointed apex (Plate III., 

 fig. 13). The dorsal and ventral surfaces are clearly distinguishable, the former 

 convex in section ; the latter, which is grooved longitudinally, is also convex in 



