37 



In the two species of bivalves of the family Pholadidae — Zirphaea crispate 

 (Warner, 1939) and Martesia striata (Boyle and Turner, 1976) — a ventral 

 tooth and alveolus form; these are situated on the ventral edge of the shell 

 facing the umbo (Figure 32). The ventral hinge is used only in the period of 

 metamorphosis and settling, after which it totally disappears. After termination 

 of metamorphosis, the clasping apparatus in mobile moUusks may become 

 reduced (Broom, 1985). 



In monomyarian mollusks the anterior adductor of the shell is destroyed 

 during metamorphosis and its place occupied by the posterior adductor. Upon 



Figure 3 1 : Posterior end of Bankia rochi Mali, witii siplions and palettes (from Turner, 1 966). 

 p — palettes; s — siphon. 



