54 



American Seashells 



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 mmutes; 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. How- 

 ever, the primary method of feeding is by means of a number of long, ce- 

 phalic 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 



-CftPTPiCULUM 



Figure 19. a, Diagrammatic drawing of the internal anatomy of Deiitalmm; b, 

 radular teeth of Dentalimn; c, the central tooth found in the radula of the 



Sipho?iodentalndae. 



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 Siphonoden- 

 talium, 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 



