| OCR Text |
Show 536 MR. B. B. WOODWARD ON THE GROWTH AND [June 14, a point situated a little way in from the posterior angle, their course as they approach the margin becoming uniformly deflected to the left, i. e. in the direction of the anterior angle of the aperture. The nature of this arrangement is, however, more easily gathered from the figure than realized from a mere description. Seen from the inner aspect the relationship of the internal septum to this structure becomes apparent (fig. 29). The point by the posterior angle of the aperture whence the plates radiate in three directions marks the junction of the septum with the outer wall; the centre of radiation a little further in falls just beside the septum in the middle of its curved inner side, so that the direction of the plates in the septum itself very nearly corresponds with the curvature of its walls, or, to put it in another way, the ' graining' of the septum is but slightly ' on the cross.' The whole system of construction of the callus, therefore, would appear to foreshadow the future requirements of the animal, and its component plates to be so arranged that when by erosion in the course of growth its unabsorbed portions form part and parcel of the walls and septum of the shell these plates shall be in the right position to impart the greatest strength and durability to the whole that is possible under the circumstances, for the lines of growth in the callus-formed portion are of necessity lines of weakness. Mr. H . A. Miers was so good as to investigate a portion of the crystalline layer from the callus, testing its specific gravity by means of density fluids. It floated in a liquid in which aragonite and tourmaline sank; but foundered in one which would support beryl and calcite. On account of its porous nature, the observed specific gravity of the material must in reality be too low and the crystalline calius is therefore more likely to be aragonite than calcite. Mr. Miers further reports :-" By taking some of the very finest powder and examining it with a twelfth oil immersion, I am able to find some specks which are undoubtedly calcite (by cleavage and opt. char.). In section it is impossible to determine the substance- because the use of the same objective with polarized light shows that it consists of very minute overlapping fibres even where it appears homogeneous under the quarter. Some of the powder looks to me more like aragonite-little ragged fibres with straight extinction, no cleavage, and the double refraction of aragonite. " The final result is then-an extremely fine fibrous structure; the presence of calcite proved; the presence of aragonite highly probable." The manner in which the myophore and the callus shift forward with the growth of the shell in Neritina can now be readily understood. Fresh shelly matter is added to the outer and a corresponding quantity removed from the inner side. No section can well be taken to prove this to demonstration in the majority of the species of the genus, but in JV. crepidularia it can be shown that the callus does thus change its position and move through a segment of a circle, keeping pace in this way with the growth of the rest of the shell (fig. 30); and the same is true of Septaria [=Navicella]. In |
