( io ) 



V.— THE PROTRACTOR PEDIS MUSCLE. 



(See Plate II) 



In opening large numbers of American oysters, usually a hundred at a time, we 

 noticed the constant presence of a more or less distinct spot (PL II., Fig. 2, sp.) on the 

 mantle, near the anterior end of the visceral mass, and of a slight depression or mark 

 on the shell over the spot (PL II., Fig. i, sp.). The spot is circular, and about 2 mm. 

 in diameter ; it is rather lighter and less opaque than the surrounding mantle, and its 

 edges may be coloured with a deposit of dark brown or black pigment granules (see 

 PL II., Fig. 3). At first we were inclined to think this the opening of a gland, or 

 an aggregation of minute glands, as a watery fluid continued for some time after 

 opening the oyster to ooze from the spot, and collect in a drop, which re-formed on 

 removal. Dissection and the examination of sections showed, however, that we had to 

 do with a band of muscular tissue, and that the " spot " was the insertion of the 

 muscle, and the mark on the shell the scar produced by that insertion. This insertion 

 mark has exactly the appearance and structure of the adductor impression. (Compare 

 Figs. 4 and 5 with 6 and 7, on PL II.). 



This muscle has been noticed before. J. A. Ryder,* in 1884 and 1893, writing 

 of the mark on the shell, says : " It gives attachment to a feeble muscular bundle which 

 springs out of the mantle on either side of the visceral mass, and when the animal is 

 torn loose a slight whitish scar on the soft part marks its position on the surface of 

 the mantle. I have been informed that Mr. W. H. Dall, who has investigated the 

 matter, has identified this muscle with the pedal muscle of some other acephalous 

 molluscs." R. T. Jackson, in his Phylogeny of Pelecypoda.t figures what is clearly our 

 spot as a pedal muscle ; but it is not mentioned by Kellogg in his work on the 

 Morphology of Lamellibranchiate Mollusks.t 



Huxley showed, in 1S83, that the first formed adductor muscle of the oyster, 

 seen in the free-swimming larval stages, cannot from its position be the adductor 

 present in the adult. The larval muscle, which is clearly an "anterior adductor" 

 muscle, lies antero-dorsally in the body, and would require to traverse the alimentary 

 canal to attain to the position of the adult muscle. Huxley predicted that a new 

 second adductor muscle would be found to develop in the young oyster, and he argued, 

 therefore, that oysters must be derived from dimyarian ancestors, and that their single 

 large muscle is the persistent posterior adductor muscle, the anterior one being lost in 

 early life. Jackson's discovery of a transitory dimyarian stage in the very young 

 American oyster (Ostrea virghrica, GmJ proves the truth of Huxley's view. 



•Contributions to the Life-History of the Oyster, 1S93, p. 719. 

 t Memoirs Boston Soc. Nat. Hist., vol. IV., No. VIII., 1S90. 

 % Bulletin U.S. Fish Commission, vol. X. (1S92). 



