No. 6 (I921) COMMON MOLLUSCS OF SOUTH INDIA 1 53 



Usually the hinge is formed by small interlocking projections, the 

 hinge teeth. The valves are kept shut by the contraction of large 

 adductor muscles, usually two in number, one towards each end of 

 the shell. These pass from one valve to the other; when they 

 relax slightly, the margins of the shell open, the normal condition; 

 after death these muscles relax completely and the shell gapes 

 widely, owing to the elasticity of the ligament which acts precisely 

 as though a small pad of indiarubber were inserted in the hinge. 

 In burrowing forms, where the shells lie mouth downwards in the 

 sand, the two mantle folds join posteriorly to form two apertures — 

 the inhalent and the exhalent. In bivalves that bury themselves 

 deeply these apertures are extended into a pair of long tubes or 

 siphons. Through the inhalent or branchial siphon water laden 

 with microscopic food is drawn within the mantle cavity, where it 

 filters through the gills and then passes out through the exhalent 

 siphon. , 



The two chief orders of the LameUibranchs are the Filibranchia 

 and the Eulamellibranchia. In the former the thread-like gill 

 filaments lie parallel to one another and are bent upon themselves 

 (reflected upwards) at about half their length. In the latter the 

 filaments instead of being free are united at regular intervals by 

 cross bars containing blood vessels, and thus form a more efficient 

 breathing and food-collecting apparatus — a very perfect form of 

 filter. 



Order i.— Filibranchia. 



The Indian Auoiniidae include the FalSE-OYSTER {Anoinia achaens) 

 and the Window-pane oyster, Placuna placenta. Anomia is a small 

 shell usually about iVz inch diameter, frequently found adherent to 

 edible oysters in the backwaters on the East Coast. It has a 

 beautiful pearly shell often golden in colour, very like a young 

 edible oyster in shape. It rests with its right or flat valve down- 

 wards ; the left valve is convex. Through a hole in the lower 

 valve, a shelly plug passes ; by this it attaches itself to fixed 

 objects. This plug is a modification of the byssus which in most 

 other bivalves consists of numerous elastic horny filaments. 



The WiNDOW-PANE OYSTER grows to a much larger size, often 



5 and even 6 inches in diameter. It is orbicular in outline when 



adult, very much compressed, and is remarkable for a very 



strong A -shaped tooth to which the ligament is attached. 



8 



