'Tusk-shells, Mail-shells, and Limpets 179 



•situated the mouth. Tlie buccal mass may be found 

 at the base of the long, pointed, and bilobed foot, 

 which projects from the large end of the shell. 

 Witliin the buccal mass is the tooth-ribbon or raduhi, 

 similar to those described in pages 44-46 ; but in this 

 class it differs somewhat from all the others. It is 

 broad and oval, with only 5 teeth in a row ; the 

 central tootli a broad simple plate, the lateral strong, 

 arched, with two or three points, and the marginal 

 is a four-sided simple plate. The sexes are separate 

 in these creatures, the liver is paired and consists of 

 a number of radiating tubes, but there is neither 

 special respiratory organ, heart, nor circulatory system. 

 The nervous 

 system is similar ^ 



to that of the bi- 

 valves. Around 

 the mouth are 

 bunches of fila- 

 ments {capta- 

 cida), which are thrust out for the purpose of catch- 

 ing the Foraminifera and minute mollusks upon 

 which the Dentaliwin feeds. The edges of the 

 mantle are united, so that it forms a tube open in 

 front and behind, and the entire surface of this 

 appears to absorb oxygen for purifying the blood. 

 It will thus be seen that these mollusks form an 

 actual link connecting the bivalves with the uni- 

 valves. In most points they agree with the bivalves ; 

 in only two respects — the shell and the radula — do 

 they agree with the univalves, and even in the 

 matter of the shell they begin life as bivalves, for in 

 the embryo this consists of two plates which later 



Elephant's Tusk-shell 



