ON THE DI-MYARIAN STAGE OF THE 

 " NATIVE • OYSTER. 



By WILFRED MARK WEBB, F.L.S.. 

 Technical Lalwratorics, County of Essex. 



It is, of course, a matter of common knowledge among mala- 

 cologists that bivalves have been divided into sub-classes, 

 according as the adult forms possess, two more or less equal 

 muscles for the closing of their shells, one muscle — the anterior — 

 nuich smaller than the other, or one — the posterior — only. 



From this state of things it might be deduced, without great 

 probability of error, that the last condition has been arrived at, 

 from the first, through the second. This assumption, however, 

 may be taken as proved, if one can find that during the early 

 development of the one-muscled, or iiioiio-iiiyan'aii species, they 

 pass through a two-muscled, or di-myarian, stage. Now, in the 

 year 1883, Professor Huxley' showed that the oyster must pass 

 through an undiscovered di-myarian stage, but none of our 

 countrymen seem to have made any further observations on this 

 fascinating case of '' Recapitulation." 



The evidence brought forward by Professor Huxley is 

 briefly this : — That there is but one adductor muscle closing the 

 shell of the oyster, in the embryo, as in the adult mollusc ; but 

 while the larval muscle is dorsal- to the alimentary canal, and is 

 consequently the anterior adductor (Figs. A and B, a. add.), that 

 in the adult, being on the ventral side of the intestine, is clearly 

 the posterior adductor. Therefore, the muscles being difterent ones 

 at different ages, and the oyster not being able to do without any 

 muscle at all, it stands to reason that there must be a time wlien 

 both are present, one " rising,"' so to speak, while the other is 

 " on the wane."" 



1 " Oysters and the Oyster Question " [a lecture delivered at the Royal Institution, May ii, 



1SS3, with additions], by T. H. Huxley. '■ The English lUustratnl Jlm/a-hir." 1S83. p. 112. 



2 In Dr. Jackson's paper, p. 299. the terms are accidentally reversed. 



