92 OKHAMANDAL MARINE ZOOLOGY REPORT 



is unfavourable for obtaining food without an undue admixture of unsuitable 

 particles. 



Backwaters, more or less brackish, are notable for a profusion of diatomacese ; 

 these organisms form the bulk of the food of Placuna, and to this fact we may 

 correlate the extraordinary development of the crystalline style in this mollusc. Much 

 enveloping gelatinous matter is needed to surround the sharp ends of the silicious 

 frustrules and so prevent injury to the delicate walls of the intestine, and such supply is 

 afforded by the continual wearing down of the end of the style where it projects into 

 the stomach. 



In its evolution Placuna, while retaining many archaic characters, as in the 

 primitive disposition of the heart and the simple structure of the branchial filaments, 

 has shown great plasticity in other directions, with the result that it is now one of the 

 most interesting of Lamellibranchs in its peculiarities of general- asymmetry and the 

 extreme specialisation of certain organs ; its ensemble is now such as adapts it most 

 perfectly to the habitat it has chosen — an adaptation to environment second only to 

 that most wonderful of mud-dwellers, Lingula, where the same end has been attained 

 by an absolutely different path. In the one case life in an environment of the softest 

 mud has been rendered possible by the adoption of the same principle as is embodied 

 in the use of snow-shoes ; here the animal rests upon the mud. In the other instance 

 a muscular stalk of great length has been developed to enable the animal to project the 

 edge of its valves a.bove the mud, and yet have an anchorage to some shell or stone 

 far down in the deeper and stiffer layers of the mud. At Balapur Bay, in Beyt Island, 

 I have seen these animals living together in great abundance. Such mud-flats ma}', 

 therefore, be characterised as distinguished by a Lingula-Placuna formation. 



