568 ORDER ABDOMINALIA. 



its growth, moves a little downwards in its cavity, by means 

 of the new layers of the attached disc being formed, not 

 symmetrically with respect to the old layers, but beyond 

 them or at a greater depth in the shell ; hence when the 

 animal is removed quite perfect, by the Concholepas being 

 dissolved in acid, the upper and deserted margin of the 

 disc or surface of attachment generally projects as a free 

 edge, but in a tattered and worn condition. In full 

 grown specimens, which have ceased burrowing downwards, 

 nearly the whole disc, though occupying the same position 

 relatively to the animal, becomes in fact deserted, and is 

 lined by membrane continuous with, and like that, investing 

 the rest of the body, but furnished only with simple blunt 

 points, instead of with the sharp bifid and trifid triturating 

 points. 



The orifice is formed on each side by a toothed rim of 

 hard chitine (fig. 3, 4), which can be opened and shut, owing 

 to its being flexible at the rostral end, and folded inwards 

 at the carinal or posterior end. The teeth vary in arrange- 

 ment and sharpness : generally they form, taking a rim on 

 one side (fig. 3), two prominences or groups of points at the 

 rostral or anterior end, always separated by a broad notch, 

 from the bottom of which the lateral bar extends down- 

 wards, from the posterior, larger and less regularly toothed 

 half of the rim. Of these teeth the larger ones project 

 nearly straight up, and the smaller and lower teeth outwards, 

 graduating into the smaller teeth, just below the rim, which 

 again graduate into the minute points, studded over the 

 whole surface. These outer teeth probably serve to prevent 

 any creature crawling into the cavity, between the shell and 

 animal. Scattered bristles rise from all over the rim. The 

 lateral bar, above alluded to, consists of the general mem- 

 brane of the body, thickened, hardened, rendered elastic, 

 coloured yellow, and apparently formed into a fold : where 

 attached to the under side of the rim, at the above-men- 

 tioned notch, the bar is thinner and more flexible than else- 

 where : it runs half way down the animal, first straight and 

 then curved towards and closely approaching the disc or 

 surface of attachment. At the lower end, the bar, or more 

 strictly the thickened membranous margin of the bar {h\ 



