Dentalium, Or Tooth Shell, a gasteropod mol-lusk, usually placed near the limpets. The shell is tubular, symmetrical, curved like a long slender tooth, open at each end, attenuated posteriorly; the surface is either smooth or longitudinally striated; aperture circular. The animal is attached to the shell near the posterior anal surface; head rudimentary, without eyes, with teeth in mouth at the base of a long, conical foot; there are two symmetrical branchiae; sexes believed to be united. In the early stages they have wing-like expansions at the anterior part of the body, and many tentacles around the head. They are animal feeders, devouring minute bivalves and foraminifera; they are all marine, living in almost all seas, on sandy and muddy bottoms, in which they often bury themselves; they are found in from 10 to 100 fathoms. There are about 50 living and 125 fossil species, ranging from the Devonian forward. They were classed by the early zoologists with the worms, and even Cuvier placed them near serpula among the annelids, noticing, however, certain characters recalling the mollus-can structure.
This shell has recently been brought into notice by Prof. E. S. Morse, who has found in it many different characters interesting from a Darwinian point of view, and suggesting the development of the branch of mollusks from dentalium or some animal resembling it, which had been derived from some annelid form. Dentalium, according to him, points to the acephala in the absence of a head, to the gasteropods by the lingual teeth, to the pteropods by the wing-like expansions, and to the cephalopods by the many tentacles of the embryo. Whether these indicate derivation or not, the union of annelid peculiarities with those of all the classes of mollusks is certainly unusual and interesting. The common species of the Atlantic, the D. entalis (Linn.), is about 11 inch long and 1/4 inch in diameter at the anterior end, tapering to a dull point.
1. Shell of Dentalium entalis. 2. Shell magnified and fractured, showing animal contracted. 8. Animal at the moment of advancing from the shell. 4. Animal magnified, abdominal aspect. 5. Animal magnified and cut open, showing internal formation. 6. Animal magnified, dorsal aspect.