MOLLUSCA : LAMELLIBRANCHIATA. 



399 



in some anomalous forms they are not near one another at all. 

 In the Arcades the two beaks are separated from one another 

 by an oval or lozenge-shaped 

 flat space or area. When 

 teeth are present, they differ 

 much in their form and ar- 

 rangement. In some forms 

 (fig. 207) the teeth are divi- 

 sible into three sets one 

 group of one or more teeth, 

 placed immediately beneath 

 the umbo, and known as the 

 "cardinal teeth;" and two 

 groups on either side of the 

 preceding, termed the 'flat- 

 era! teeth." Sometimes there 

 may be lateral teeth only; 

 sometimes the cardinal teeth 

 alone are present; and in 

 some cases (Arcadce) there is 

 a row of .similar and equal 

 teeth. 



The. body in the Lamelli- 

 branchiata is always enclosed 

 in an expansion of the dorsal 

 integument, which constitutes 

 the " mantle," or " pallium," 

 whereby the shell is secreted. 

 The lobes of the mantle are 

 right and left, and not ante- 

 rior and posterior as are the 

 mantle-lobes of the Brachio- 

 poda. Towards its circumfer- 

 ence the mantle is more or 

 less completely united to the 

 shell, leaving in its interior, 

 when the soft parts are re- 

 moved, a more or less dis- 

 tinctly impressed line, which 

 is called the " pallial line," or 

 ."impression" (fig. 210). 



There is no distinctly differ- 

 entiated head in anyof theZtf- 

 mellibranchiata, and the mouth is simply placed at the anterior 

 extremity of the body. It is furnished with ciliated and tactile 



"f 



Fig. 208. Anatomy of a bivalve Mollusc 

 (My a cirenaria). The left valve and 

 inantle-lobe and Half the siphons, are re- 

 moved, s s Respiratory siphons, the ar- 

 rows indicating the direction of the cur- 

 rents ; a of Adductor muscles ; b Gills ; h 

 Heart ; o Mouth, surrounded by (/) labial 

 palpi ; f Foot ; v Anus ; m Cut edge of the 

 mantle. 'After Woodward.) 



