HORNELL ANATOMY <>K I'LACUNA 17 



suberitid sponge known as l\<l<il i><il n> (sea-fruit) to the Tamil divers. In the Gull 

 of Kutch ;i similar or closely related suberitid is associated with /V^r//////, and in the 

 same locality we also find Lingula and a large yellow Dendronepkthya living on the 

 same ground. Algse are scarce, and those seen are all green and mostly fleshy 

 genera, such as Codium. The valves of Placuna seldom bear any crusting or parasitic 

 growths such as are so familiar on the shells of the true pearl oyster (Margaritifera 

 vulgaris). When such are present they consist chiefly of sheets of encrusting Bryozoa. 



I have never seen the valves attacked by the burrowing sponge Clione, but the 

 tubular burrows of the polychset Polydora are sometimes not uncommon. The sandy 

 tubes of another polychset, Eitnice indica, are also at times fairly abundant attached to 

 the valves. 



Placuna does not appear to have many enemies ; fishes are the principal, and 

 their damage is largely confined to biting pieces out of the margin. Shells so 

 mutilated are riot infrequent ; photographs of two shells so damaged are shown in 

 Plate I, fig. 2. Larval parasitic worms, both cestodes and trematodes (figs. 20 and 21), 

 are frequently seen encysted on the mantle edge, and as fragments of this are ingest ed 

 every time a fish bites a piece out of the margin of a Placuna shell, it is clear how the 

 parasitic infection is passed on to a fish host in which the parasites will become sexually 

 mature. 



A remarkable feature in this connection is the early age at which the infection of 

 Placuna may take place. I have sections through individuals of which the shell and 

 the visceral mass are but 1'5 cm. and 4 mm. in diameter respectively, showing well 

 developed larval cestodes (/ Tetrarhyiichus) encysted in the liver. 



DETAILS OF DISTRIBUTION. 



The following list comprises all the localities of which I have knowledge. It will 

 be seen that it gives a chain of habitat practically unbroken from the mouths of the 

 Indus to the south coast of China : 



India. Karachi Harbour and numerous inlets and creeks along the Sind coast ; 

 (Julf of Kutch (Balapur Harbour in Bcyt Island! and Rann Bay!); Bombay 

 Harbour and vicinity ; Malabar Coast ; muddy creeks south of Tuticorin ! Buckingham 

 Canal and Pulicat Lake (Madras Presidency) ! open sea off Negapatam in seven 

 fathoms! thrown up on sea, beach, Ennore, near Madras! Mergui Archipelago. 



Ceylon. Tampalakam Lake! Sambore River and adjacent backwaters (Willey); 



open sea off Nilaveli ! (these three localities near Trineomalee) ; off Delft Island, Palk 

 Hay, six to .seven fathoms! dead valves in nine fathoms, Periya Par, Mannar Pearl 

 Banks ! 



