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Show 1878.] MR. T. J. PARKER ON PALINURUS VULGARIS. 443 struction ; and in this way Mobius attempts to account for the fact that no sound accompanies a downward movement. As to the first of these statements, one finds it rather hard to believe that hairs so fine1 that the surface they cover has a texture like that of satin, can produce visible scratches; their effect, one would rather think, would be to impart a fine polish. I should be inclined to suspect that the scratches must be due to fine particles of sand getting in between the flap and the ridge. With regard to the second statement, that it is the friction of the flap, and not that of the pad, which produces the sound, I can only say that I have removed the flap entirely without any sensible diminution of the noise. The mere observation of the parts while in action is enough to show the true state of things : when looked at from the front it is very evident that the flap exerts hardly any pressure upon the ridge, as, indeed, from the fact that it is a soft structure supported only along one edge, it could scarcely be expected to; while the pad, on the other hand, is completely flattened out against the smooth surface, and in the most perfect contact with it. It is not easy to see h o w such a mistake can have arisen; but Dr. Mobius's language is sufficiently explicit to leave no doubt of his meaning. He says:-" Das gefurclite Feld [the ridged pad] bringt keinen Ton hervor, wenn es fiber die Flache des Keiles [the antennulary sternum] gleitet. Es leistet aber Widerstand gegen die riickwarts gedrangten Haare, da es dicker und steifer als das behaarte Feld [the flap] ist. Ausserdem geben seine Riefen dem Gange der ganzen Platte grbssere Sicherheit, da sie diejenige Rich-tung haben, in welcher die Bewegungen ausgefiihrt werden." The fact that a downward movement of the antenna gives rise to no sound seems to m e to be explained by the mode of attachment of the pad. It is connected by its inferior edge (figs. 4 and 5, ii) with the lower boundary (fig. 5, b') of the slit or uncalcified portion of the basicerite, while at its superior edge it passes insensibly into the inner wall of the soft flap. The outer wall of the flap, again, is connected with the upper edge (fig. 5,b) of the slit in the basicerite. Thus, while the pad is immovably fixed at its inferior edge, it is capable of a considerable amount of displacement in the rest of its extent; and when pressed upon, by the downward movement of the antenna, in the direction from y to x (fig. 4), it is forced inwards upon the yielding tissue which underlies it, and never comes into sufficiently close contact with the opposing surface to produce a sound (fig. 5, A ) . But when it is pressed upon, in consequence of the upward movement of the antenna, in the direction from x to y, the only effect of the pressure is to cause it to bulge out over the line of attachment ii, and so come into closer contact with the opposing surface the stronger the force applied (fig. 5, B ). In the matter of histological structure, the pad does not differ from other chitinous membranes, being formed of fine superposed horizontal laminae, marked by a vertical striation (fig. 5). It is, bow-ever, of unusual thickness; and its horizontal laminae have, for some 1 They are not more than one seven-hundredth of an inch in length. 29* |
