38 



Figure 32; Valves of the pediveliger I of Martesia striata L. with tooth and alveolus on 

 ventral side (from Boyle and Turner, 1976). 



completion of metamorphosis, the attachment system in mobile moUusks may 

 be reduced. 



The development of the genital system lags behind the development of all 

 other systems and occurs only after the termination of metamorphosis. The 

 primary germ cells in the freshwater mollusks Sphaerium are already visible 

 in the early stages of development of the embryo, in the mesodermal bands; 

 in other species, these cells become visible only after metamorphosis in the 

 region of the ventral part of the pericardium (Okada, 1936; Coe and Tumer, 

 1938; Lucas, 1975). 



On the whole, metamorphosis is a distinct sequence of processes trans- 

 forming the larva into a juvenile. A disturbance of this sequence or blockage 

 of any stage leads to deformity or mortality (Tumer, 1976). 



Lecithotrophic Larvae and Direct Development 



In evolution, primary development with planktotrophic larvae is typical of 

 bivalves (Jagersten, 1972), yet in many families a lecithotrophic development 

 phase is observed, which is usually linked with larviparity of ovoviviparity 

 (Thorson, 1936; Ockelmann, 1962). This phenomenon has been detailed for 

 bivalves by Sellmer (1967) and Blacknell and Ansell (1974). Duration of the 

 lecithotrophic phase can be determined from the size of the eggs and 

 prodissoconch I. In most bivalves the duration of this phase terminates in the 

 early differentiation of the digestive tract and formation of the straight-hinged 

 veliger with emergence from its envelopes. However, in more than 25 families. 



