1922] MADRAS PEARL FISHERIES 



12 



in bathing the tubular filaments containing blood, parts with some 

 of its oxygen and receives waste carbon dioxide, so effecting 

 respiration. The general inner surface of the mantle also partici- 

 pates in this function, for it too is highly vascular and separated 

 from the incurrent water by an extremely tenuous epithelium. 



The pearl oyster feeds upon the minuter organisms found in 

 the plankton of the surrounding water. There is at least partial 

 selection exercised, chiefly by means of the filaments along the 

 mantle edge, for these bar the way to copepods and small worms 

 and similar small animals that might be of more trouble than use. 

 But all algal matter is admitted freely. Whatever organisms pass 

 in are carried with the current into the natural filter formed by the 

 gill filaments and there are collected by certain sets of cilia and 

 directed into definite ciliated pathways leading towards the palp 

 and the mouth ; these progress much like a football trundled along 

 the ground. When the particles reach the palps they are rolled 

 together to form a tiny pellet and this is finally forwarded by a 

 ciliated pathway into the mouth. 



The excretory system is a pair of simple tubular chambers lined 

 with secretory cells. Each opens at one end into the pericardium, 

 at the other to the outside. 



The blood system. — In the pearl oyster the blood is colourless. It 

 passes at each pulsation of the ventricle into two main channels 

 or arteries, one leading forwards, the other posteriorly. These 

 arteries distribute blood to all parts of the body ; thence it passes 

 to the gills and the mantle to be purified and from there is returned 

 to the auricles. Only purified blood passes through the heart. 



As pearl oysters are sedentary animals, mere passive machines 

 for eating and reproducing, and are well protected against all but 

 particular enemies by the strength of their shell, they require no 

 elaborate nervous system. As a consequence there are no conspi- 

 cuous sensory organs and in particular no eyes. In this last 

 respect they are less specialised than their near relatives, the 

 Scallops, for these have very numerous well-developed eyes upon 

 the margins of the mantle. Three nerve centres exist, each 

 composed of a pair of ganglia (small rounded aggregations of nerve 

 cells). One is found on the sides of the oesophagus, one at the 

 base of the foot and the third on the anterior face of the adductor 

 muscle. These centres are joined by long nerve trunks, the 

 connectives, and fro.n them arise a network ofdelicate nerve fibres 



