HORNELL ANATOMY OF PLACUNA 45 



developed specialisations, the chief being, in Anomia, the calcified byssus, and in 

 Placuna the single genital aperture, and the development of the foot into a very long 

 and trumpet-shaped organ. 



In both genera the unequal development of the two nephridia and the obliteration 

 of the pericardium are most noteworthy ; in Anomia Felscneer held (loc. cit.) that the 

 pericardia! coelomic cavity is reduced to an insignificant connecting channel bctwrm 

 the kidneys, but Sassi contends (loc. cit.) that the coelomic cavity is represented by 

 certain groups of saccate glands opening into each nephridium ; in Placuna, as I 

 believe and suggest, it is to be correlated with a glandular body on each side, which 

 opens into the anterior horn of the kidney of its side a view which strengthen - 

 Sassi's contention regarding the organs he describes in Anomia. 



Before entering upon a detailed description of the organisation of Placuna 

 placenta, it may be useful to give here a brief resume of the distribution of the species 

 treated of, with notes upon the economic uses to which it is put. 



DISTRIBUTION AND HABITAT. 



Placuna placenta, Linn, has a range extending from the Arabian Sea on the 

 west through the Indian Ocean and Malayan Seas to the coast of China on the east. 



The special habitat favoured by this mollusc is the muddy bottom of coastal b;iys ; 

 in many cases these bays are more or less completely landlocked, with a characteristic 

 association of mangrove swamps along the shores ; frequently the water of such bays 

 is slightly brackish by reason of the rivers and streams that debouch thereinto. 

 Placuna placenta appears, indeed, to be able to withstand a considerable admixture of 

 fresh water, as we find it in such muddy estuaries as Karachi Harbour and other bays 

 and creeks on the outer edge of the delta of the Indus ; even in tidal rivers and 

 backwaters cut off from the sea by surf-beat sandbars Placuna thrives, as, for example, 

 in the Sambore River and the adjacent backwaters near Trincomalee in Ceylon. In 

 Tampalakam Bay, also in Ceylon, I have found the specific gravity of the water over 

 the Placuna beds to range during the dry season as low as TO 19 to TO 15 at 

 temperatures 87 to 90 F., the open sea being 1-023 ; in the rainy season it must be 

 notably lower. 



On the other hand, the past fishery experiences of the Tampalakam beds have 

 shown that an unusually large influx of fresh water during an exceptionally wet season 

 entails the destruction of the oysters. The same adverse influence of an excessive 

 admixture of fresh water has been noted at the window-pane oyster fishery near 

 Tetabuan, on the east coast of British North Borneo, where the flow of the adjacent 

 rivers in flood not infrequently causes widespread mortality. 



With all this adaptation to life in distinctly brackish shallow water areas, I have 

 found that Placuna placenta is capable of attaining large individual size, and of living 



