280 



MANUAL OF ZOOLOGY. 



internal ligament, is lodged between the hinge-lines of the two 

 valves, generally in one or more " pits," or in special processes 



of the shell. It consists of 

 elastic fibres placed perpen- 

 dicularly between the surfaces 

 by which it is contained, so 

 that they are necessarily short- 

 ened and compressed when 

 the valves are shut. To open 

 the shell, therefore, it is simply 

 necessary for the animal to 

 relax the muscles which are 

 provided for the closure of the 

 valves, whereupon the elastic 

 force of the ligament and car- 

 tilage is sufficient of itself to 

 open the shell. 



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 mantle- 

 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 impres- 

 sed line, which is called the 

 "pallial line," or "impression" 

 (fig. TOO). 



There is no distinctly diffe- 

 rentiated head in any of the 

 Lamdlibranchiata, and the 

 mouth is simply placed at the 

 anterior extremity of the body. 

 It is furnished with membran- 

 ous processes or "palpi" (usu- 

 ally four in number), but there is no dental apparatus. The 

 mouth opens into a gullet, which conducts to a distinct 



Fig. 99. Anatomy of a bivalve Mollusc 

 (Mya arenaria). The left valve and 

 mantle-lobe and half the siphons are re- 

 moved. ^ J Respiratory siphons, the 

 arrows indicating the direction of the 

 currents ; a a' Adductor muscles ; b 

 Gills ; h Heart ; o Mouth, surrounded by 

 (/) labial palpi ; /Foot ; v Anus ; m Cut 

 edge of the mantle. (After Woodward.) 



