MOLLUSCA : LAMELLIBRANCHIATA. 



flat space or area. When teeth are present, they differ 

 much in their form and arrangement. In some forms (fig. 

 129) the teeth are divisible 

 into three sets one group of 

 one or more teeth, placed im- 

 mediately beneath the umbo, 

 and known as the " cardinal 

 teeth;" and two groups on 

 either side of the preceding, 

 termed the " lateral teeth." 

 Sometimes there maybe lateral 

 teeth only ; sometimes the 

 cardinal teeth alone are pre- 

 sent ; and in some cases 

 (Arcad<z) 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 anterior 

 and posterior as are the man tie- 

 lobes of the Brachiopoda. To- 

 wards its circumference the 

 mantle is more or less com- 

 pletely united to the shell, 

 leaving in its interior, when 

 the soft parts are removed, a 

 more or less distinctly im- 

 pressed line, which is called 

 the "pallial line," or "impres- 

 sion" (fig. 131). 



There is no distinctly differ- 

 entiated head in any of the La- 

 mellibranchiata, and the mouth 

 is simply placed at the anterior 

 extremity of the body. It is 

 furnished with membranous 

 processes or "palpi" (usually 

 four in number), but there is no 

 dental apparatus. The mouth opens into a gullet, which con- 

 ducts to a distinct stomach. On the right side of the stomach, 



Fig. 130 Anatomy of a bivalve Mollusc 

 {My a arenaria) . The left valve and 

 mantle-lobe and half the siphons are re- 

 moved, .y j Respiratory siphons, the 

 arrows indicating the direction of the 

 currents ; a a' Adductor muscles ; b Gills ; 

 <h Heart ; o Mouth, surrounded by (p] 



. labial palpi ;f Foot ; v Anus ; m Cut edge 

 of the mantle. (After Woodward.) 



