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Show 532 MR. B. B. WOODWARD ON THE GROWTH AND [June 14, not merely to the outer lip and columella alone as in normal growth, but all the way round in the plane of the outer lip, as first evinced by the deposition of periostracal layer along and over the outer margin of the enlarged callus (fig. 20), the lines of deposition, or growth, being continuous with those of the outer lip. In other words, were it not for the overlap of the callus the peristome would be complete, as in Neritina crepidularia and Tomostoma neritoides. Deposition also takes place over the whole surface of the callus. The direction of growth is in this manner completely changed. Instead of developing spirally, round an axis of which the protoconch forms the apex, the shell enlarges radially, the new axis being the pillarlike margin of the septum and its apex the point, on the exterior of the body-whorl, situated immediately over the junction of these two. In this new condition of affairs the callus, which is at right angles to the new axis, lies of course completely athwart the direction of growth and decidedly in the way of further extension, so that the animal must have found itself much in the same predicament that a limpet would do were it to be suddenly half-decked when its period of growth was still far from complete. A grave problem in its domestic architecture was thus raised, and the solution forms the most interesting feature in the life-history of this species; for layer by layer, as deposition of fresh shelly matter took place without, a corresponding amount of material was removed on the inner side of the callus, and the additional room required thus obtained. Put in homely phraseology, this mode of enlarging a tenement reminds one of nothing so much as of the Irishman, who raised his roof by digging out the floor of his cabin. The ultimate outcome of this novel mode of increase is that, in the adult Velates, that portion of the shell included between the margin of the outer lip and a line (A B, fig. 21) joining its extremities and passing round and a little below the apex on the further side is normal, whereas the remainder is formed out of callus past and present. This comes out very clearly in the various sections of the shell presently to be described. Of course the walls around the apex which require to be thickened as the shell increases in size, to make them as durable as the rest, are strengthened in the usual way by the deposition of fresh shelly matter within, so that, in an old shell, what was once the cavity inhabited by the young animal has become solid shell. The changes which take place in the external form of the test of Velates, as was to be expected, find their reflection in the intimate structure of the shell itself. An axial section whose plane passes close to and almost parallel with the edge of the columellar lip, but just misses the apex itself, has been made in each of three young shells of different ages, and the sections stained with picro-carmine to bring out the structure more clearly. In the first, a specimen of about 3 whorls (Plate XXXII. fig. 22 a), the shell-wall near the apex shows three readily distinguishable layers:-the outermost, or periostracal layer, the crystalline, and the innermost layer, which in this case consists of the material laid down not merely as a lining to |
