122 MADRAS FISHERIES BULLETIN [VOL. XVI, 



one end in a circular disc of attachment upon the solid object to 

 which the oyster is affixing and at the other in the deep byssal pit 

 sunk in the base of the foot. The process is repeated indefinitely 

 till a strong cable of byssal threads — the byssus — has been formed. 

 I have watched young pearl oysters repeatedly form within an hour 

 or two, a tuft of byssal fibres and then free themselves for another 

 journey by casting off the root from the byssal pit. Old oysters 

 do not discard their byssus and form new ones save under stress of 

 circumstances. 



The alimentary canal is comparatively simple in form and 

 structure. The mouth, a simple opening guarded on either side by 

 two long flaps which embrace at their lower ends the tips of the 

 gills, opens into a straight horizontal gullet which in turn leads 

 into the stomach, located in the centre of the visceral mass. The 

 digestive gland, dark green in the living condition, surrounds the 

 stomach and empties its secretion thereinto by several ducts. The 

 anterior section of the intestine forms a loop in the lower part of 

 the visceral mass and then merges into a wide postertor section 

 which passes through the ventricle and then turns downwards as 

 the rectum along the hinder surface of the adductor muscle, to 

 end in an anal orifice prolonged dorsally into an ear-shaped 

 directive projection. Parallel with the straight part of the an- 

 terior section of the intestine is a long diverticulum, lodging 

 a long crystalline style, whereof the outer end projects into the 

 stomach. 



The gills are two in number, each folded upon itself longitudi- 

 nally in such a manner that there appears to be a pair on each 

 side. Each is made up of a multitude of slender tubular filaments 

 placed side by side. No cross bars or branches connect them. 

 When dead, the filaments are readily separated, but in life opposing 

 pads of strongly ciliated cells on adjacent filaments form an 

 interlocking mechanism that prevents displacement, an arrange- 

 ment somewhat analogous to that seen in the vane of a feather. 

 Respiration is effected by a current of sea-water, admitted through 

 the open margins of the shell and mantle, and set up by the 

 rhythmic lashing of cilia covering the inner surface of the mantle 

 folds and also upon the gill filaments. By this means a continuous 

 current of fresh sea-water passes between the gill filaments 

 into the suprabranchial cavity and thence outwards, past the 

 anus back into the sea. In its passage through the gills the water 



