12 
OYSTERS AND DISEASE. 
As to the probable function of this muscle in the existing oyster, its action 
clearly must be to draw the labial palps and the anterior end of the branchite forwards 
and outwards. When this action is imitated by means of forceps on the fresh oyster, 
the result is seen to be that the funnel leading to the mouth is widened and opened, 
and the " catchment area " by which the mouth is supplied becomes increased. We 
give a diagram, in Fig. 14, showing the relations of the protractor muscles of the two 
sides to the mouth and neighbouring parts, and we add in dotted lines the probable 
course of the muscles before the foot disappeared in ancestral oysters. It seems to us 
that when the muscle was no longer needed as a protractor pedis, it may readily 
have come to serve its present useful purpose by the posterior half of the muscle 
aborting, while the anterior half remained and acquired an attachment to the connective 
tissue of the palp and the gill through which it must have formerly passed, and where 
it is quite likely some of the fibres always ended. 
Our figure (PI. II., Fig. 14), which, although drawn as a diagram, is true in its 
proportions, and was sketched directly from the living animal, shows clearly the con- 
figuration of the labial palps when the two protractor muscles are slightly drawn upon. 
The inner or posterior palps curl together in the middle line behind the mouth, while 
the outer or anterior palps diverge widely. The anterior ends of both gills on each side 
are received between the outer and inner palps of that side ; so that the only avenue 
to the mouth, the passage on each side between the two palps, becomes much widened 
at its posterior end, more funnel-shaped, and evidently better adapted for collecting 
the food ]Darticles from the anterior ends of the gills, and passing them forward to 
the mouth. 
The action of the oyster in opening its shell {i.e., divaricating the valves), 
preparatory to feeding, separates the points of attachment of the two mu.scles, and so 
of itself, even without any muscular contraction, causes the opening up of the food 
avenues described above : the contraction of the muscles which doubtless takes place 
will emphasize the same action.* 
* The technical term " fishing " used by oyster-growers and others connected with tlie trade, to indicate the 
act of feeding in an oyster, is therefore so far a correct description, inasmiuh ns the action is comparable with the 
spreading of a net. 
