. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 636 RUSSELL-HUNTER, APLEY AND HUNTER ihc new dextral coiling which will be retained throughout spat, and juvenile and adult growth ( Figs. 5AB, 6ABC). The first shell secretion after metamor- phosis is .shown in Figures 5A and 5B. The reorientation of the spiral normal axis of growth can better he seen by comparing the .slightly older post- metamorphic spat of Figures 6A and (>B. with the orientation of the earlier post- larval shell (or protoconch) at the apex of the later juvenile shell shown in Figure 6C. All these lat
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 636 RUSSELL-HUNTER, APLEY AND HUNTER ihc new dextral coiling which will be retained throughout spat, and juvenile and adult growth ( Figs. 5AB, 6ABC). The first shell secretion after metamor- phosis is .shown in Figures 5A and 5B. The reorientation of the spiral normal axis of growth can better he seen by comparing the .slightly older post- metamorphic spat of Figures 6A and (>B. with the orientation of the earlier post- larval shell (or protoconch) at the apex of the later juvenile shell shown in Figure 6C. All these later illustrations (Figs. 4-6) are based on field specimens collected at appropriate times in Sippewisset marsh, and are previously undes- scribed. Appropriate comparisons of these figures show the shift from a sinistral to a dextral shell-coiling. As has often been pointed out, the growth and subse- quent configuration of the gastropod shell can be conceptualized as consisting of. FIGURE 6. Metamorphosis of mantle-shell in Mclainpus: (A) and (B) two views of the shell of a postmetamorphic spat (slightly older than that illustrated in Fig. 5) ; and (C) the postlarval shell or protoconch at the apex of a later juvenile shell. The sinistral growth of the postlarval shell is replaced by a dextral shell coiling which persists in juvenile and adult. For further discussion, see text. two somewhat distinct growth processes (Huxley, 1932; Russell-Hunter, 1953a, 1968; Fretter and Graham, 1962; Wilbur and Owen. 1964). One is growth along a spiral normal axis which can be visualized as running from the origin of the spiral (the apex of the shell in most cases) to the centroid of the plane of the aperture of the shell (sometimes more loosely defined as the midpoint of the foot). The other is the continued expansion of the mantle (and therefore of the shell which it secretes) by marginal increments to accommodate growth of the body. As a result of this accretionary method of growth, the form
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