48 CEYLON PEARL OYSTER REPORT. 



means of the secretion of the byssal fihres ; lastly, on account of the general mobility 

 of the organ, and probably its sensory nature, the tip is of great use in cleaning the 

 gills and mantle from intrusive particles that cannot otherwise be got rid of. 



Locomotion. As has been explained in this Report, Part I., the pearl oyster is 

 capable of travelling short distances when it is freed from the byssal cable which 

 attache's it to some foreign body. When separated either by the byssus being 

 wrenched away, as may happen in diving operations or in dredging for transplantation, 

 or by its own action in sloughing the root of the byssus, it soon, under favourable 

 conditions, extends the foot as far beyond the shell as possible and begins to travel. 

 The tip of the foot circles round, bending first in one direction and then in another, 

 till it meets a body suitable for crawling upon. Then the groove immediately behind 

 the tip opens (see Plate III., figs. 18 and 19) and is flattened against the surface 

 selected, whereupon progression takes place by alternate extension and contraction, as 

 seen in the crawling action of the typical gastropod foot. Only that part of the 

 pedal groove anterior to the disc-pit takes part in the locomotion, an action therefore 

 strictly limited to the distal third of the organ. 



Cleansing. As to the use of the foot in freeing the gills, palps, and mantle from 

 intrusive bodies, it can be seen through the partly open valves of a living oyster that 

 the point is pushed between the gill plates, and over the inner surface of the mantle, 

 gently stroking the surface and insinuating itself into the crevices, thus freeing the 

 parts from any foreign bodies accumulations of debris, &c. that might cause 

 inconvenience. It is frequently on the move in this manner ; when at rest, it lies 

 shortened up, with the tip turned to the left, and tucked downwards between the 

 left gill and the mantle. In one pearl oyster, in which Mr. Hoknell had broken a 

 hole in the umbonal region of the valve, the mantle beneath being also pierced, the 

 foot was seen feeling gently round the edges of the wound and working off particles 

 of dirt that had gathered. The tongue-like tip was passed occasionally tit rough the 

 wound in the mantle and projected somewhat, at one time well beyond the hole in the 

 valve. The tip also freed the wound from dirt lying between the mantle and the 

 valve. 



Attachment by the Byssus. When a pearl oyster finds a place to re-attach to, 

 subsequent to sloughing a former byssus, it uses the locomotor region of the tip as a 

 suctorial organ to hold its body in position, while at the same time the disc-cup expands 

 and its edges press against the attachment surface, and the edges of the byssal groove 

 are tightly closed against one another. While in this position, the byssal gland pours 

 forth through the byssal groove a quantity of a fluid secretion which has the property 

 of coagulating and hardening upon contact with sea-water. This secretion sets in a 

 remarkably short time, and usually after the foot has remained pressed to the contact 

 surface for between 3 and 5 minutes, it is withdrawn and then reveals the presence 

 of a pale yellow, elastic strand stretching from the mouth of the byssal gland to the 

 point where it is attached by an oval disc, which is a model of the cavity of the 



