( lo ) 
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 (PI. II., Fig. 2, jt/.) on the 
mantle, near the anterior end of the visceral mass, and of a slight depression or mark 
on the shell over the spot (PI. II., p^ig. 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 
PI. 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 PI. 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,! figures what is clearly our 
spot as a pedal muscle ; but it is not mentioned by Kellogg in his work on the 
Morphology of Lamellil-)ranchiate Mollusks.I 
Huxley showed, in 1883, 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. Huxlej' 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 virginica, Gxx\.) proves the truth of Huxley's view. 
* Contributions to the Life-History of the Oyster, 1893, p. 719. 
t Memoirs Boston Soc. Nat. Hist., vol. IV., No. VHI., 1890. 
J Bulletin U.S. Fish Commission, vol. X. (1892). 
