PLACTTR a'' FISHERY. 131 



ovaries packed full of microscopic eggs, each in its own follicle, to 

 the wall of which it was attached by a stalk. I looked in vain for 

 this condition in Placuna in June and October (1907), but found it 

 prevailing on all the beds in March {1908). Male and female gonads 

 are in separate individuals, as they are in the IVIannar pearl oyster 

 (Margaritifera) * and are either quite mature or submature in this 

 month of March. 



I am not aware that there exists any previous statement regarding 

 the periodicity of the maturation of the gonads in the window-pane 

 oyster, so that this definite observation may be regarded as a signi- 

 ficant contribution to our knowledge of the natural history of this 

 species ; and, moreover, it yields information which will relieve future 

 fisheries from arbitrary restrictions, affording a biological basis for 

 whatever regulations may be drafted. The fact that the largest 

 oysters examined in October last were immature (this does not mean 

 that they were spent) was somewhat surprising, and seems to indicate 

 that Placuna does not produce an annual brood, but that one 

 generation succeeds another at intervals longer than one year, the 

 exact period remaining for future discovery ; and that sexual 

 maturity is only attained after the completion of the superficial 

 growth of the shell. 



In the specimens examined in October I noted evidence of new 

 growth at the shell-margin, chiefly in the form of a delicate lace-like 

 fringe on the left valve. This was not conspicuous in March, and 

 although the cessation of peripheral growth of the shell cannot be 

 timed exactly, yet it would appear, from the evidence at hand, 

 to coincide with the period of sexual maturitJ^ In this respect, 

 therefore, P/aci«? a offers a striking contrast to the true pearl oyster, 

 which attains sexual maturity at a very early age.t 



The gonad of Placuna is a single organ chiefly connected with the 

 right half of the mantle, and, contrary to what is observed in the 

 common pelecypod (axe-footed) lamellibranchs, is mainly indepen- 

 dent of the foot, which, in Placuna, has an acetabular structure 

 at the end, is very protrusible, and remarkably hke a proboscis in 

 position and superficial appearance. Its action is probably like 

 that of a contractile nmscular proboscis when the animal imbeds 

 itself in the mud ; but I have not observed this directly. In the 

 fresh condition the colour of the gonad varies commonly from a 

 very pale, almost whitish yellow, to rich orange, the average colour 

 being a creamy yellow. The differences in colour are not related 



* W. A. Herdman. Observations and Experiments on the Life-history and 

 Habits of the Pearl Oyster ; in Herdman, " Ceylon Pearl Fisheries," Part I., 

 p. 125, 1903. 



t J. Homell. Biological Results of the Ceylon Pearl Fishery of 1904 ; 

 in Reports from the Ceylon Mar. Biol. Lab., No. 1, p. 8, 1905. Also printed in 

 Sessional Paper XTTT. of 1 904. 



