18 MOLLUSCA. 



it are ciliated. Pigments are present principally upon the edges of 

 the mantle, which are frequently folded or beset by papillae and 

 tentacles. 



The outer surface of the mantle secretes a strong calcareous shell, 

 which is constituted of two valves corresponding to the two mantle 

 lobes. The two valves are united dorsally. They are rarely exactly 

 alike. Nevertheless, the term unequivalve is only applied to those 

 shells in which the asymmetry is very marked, and the valves can be 

 distinguished as upper and lower. The lower valve is the larger and 

 more arched, while the upper is smaller and flatter, closing up the 

 cavity of the lower after the manner of an operculum. The edges of 

 the two valves are generally closely applied to one another, still they 

 may gape more or less widely at various points for the exit of the foot, 



byssus and siphons. The 

 latter is especially the 

 case for those Molluscs 

 which bore in sand, 

 wood, or hard rock. In 

 extreme cases the shell 

 may, by a wide anterior 

 emargination and an 

 extended docking of its 

 posterior part, be re- 

 duced to an annular ru- 



FIG. 500. Avicula semitailtt", the valves are shifted over diment (Teredo\ while 

 one ano t her;M, muscle impression. ^ ^ ^^ end ^ 



applied a calcareous tube, which may intimately fuse with the shell 

 rudiments and receive the latter entirely into itself (Aspergittum). 



The two valves of the shell are always connected dorsally by an 

 external or internal ligament, which tends to keep the valves open. 

 The two shell valves are also firmly connected together dorsally by 

 interlocking teeth, which constitute the so-called hinge (cardo). 

 The hinge edge with the ligament is therefore to be distinguished 

 from the free edge of the shell, which is divided into an anterior, 

 inferior (ventral), and posterior or siphonal edge. The anterior 

 and posterior edge may generally be easily determined by the position 

 of the hinge-ligament with regard to the two umbones (nates), which 

 have the form of two prominencies projecting over the dorsal edge, 

 and indicate the point (apex) where the development of the valves 

 began. The area is behind the apex, and includes the dorsal 

 posterior side of the shell. The part of the dorsal edge m iroiiu of 



