posterior end of the tube-like shell. The water slowly builds up inside the mantle cavity of the animal over a period of about ten minutes; then, after a short period of rest, the water is suddenly expelled in the opposite direction. As in the manner of feeding among the bivalves, ciliated ridges within the mantle ensure passage of food particles to the region of the mouth. However, the primary method of feeding is by means of a number of long, cephalic filaments or captacula which are anchored to the two flattened lobes flanking the mouth. The club-shaped ends of these tiny filaments are tactile and prehensile and are capable of capturing Foraminifera and other similar minute organisms. These captacula project out in all directions from the
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Figure 19. a, Diagrammatic drawing of the internal anatomy of Dentalium; b, radular teeth of Dentalium; c, the central tooth found in the radula Siphonodentaliidae.
larger, anterior end of the shell. Frequently, they are broken or torn off in the searchings through the sand but are soon regenerated. This accounts for the difference in length of the captacula in many specimens.
The embryonic shell or prodissoconch of the scaphopods is cup-shaped and consists of two shelly valves, which subsequently unite to form a tube. They may still be seen at the initial end in some specimens of Siphonodentalium, but are always absent in adult Dentalium. The adult shell is open at both ends. It is added to at the larger, anterior end by the mantle edge, while at the posterior end there may be a gradual loss of shell through wear and absorption. The tiny posterior slits or notches that are characteristic of some species are formed by reabsorption of the previously solid shell wall. The shell wall is made up of three thin layers of calcareous material; this is in contrast to the similar-appearing worm-tubes that have only two layers. In cross-section, the shell may be round, slightly elliptical, octagonal or polygonal in shape, depending upon the species. The presence or absence of
