Scaphopods get their name from the keel-shaped curvature of their shell. The shell is wider at the anterior end in most species, which explains the group's common English name of tusk or tooth shell. In some scaphopods, the widest part of the shell is about one-third of the way back from the anterior opening. The shells of most scaphopods, which range from about an eighth of an inch to upwards of 7.8 in (20 cm) in length, are smooth or have faint circumferential growth lines. Some species have longitudinal or annular (ringlike) ribbing. Generally, the shell is white, but a few scaphopod species have strongly colored shells that are green or banded in red and yellow. In species with thin translucent shells, the pink or yellow color of the gonads (eggs) may show through.
A muscular foot and numerous thin feeding tentacles, or captacula, extend from the front opening of a typical scapho-pod. The narrower posterior end of the animal is also open, providing access to the mantle cavity for respiratory currents and the outflow of gametes and waste products. A small siphon-like extension of the mantle edge extends to and sometimes slightly beyond the posterior opening. The mantle cavity extends the length of the shell, partially encircling the body but otherwise quite narrow in its dorsoventral dimension. The restricted space has led to the loss of gills and the osphradium (chemosensory organ), which are structures found in the mantle cavity of most other mollusks. In turn, the loss of gills in scaphopods is reflected in the absence of auricles in the heart; in addition, the ventricle is much reduced.
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