21 



size of the shell increases rapidly and already in the early veliger stage it 

 entirely covers the larval body. When this happens, the formation of the 

 prodissoconch I is completed. The outer surface of each valve in ostreids has 

 a punctate-stellate appearance; a small punctate zone occurs in the center and 

 an extensive stellate-radial zone along the periphery (Figure 17). Possibly, the 

 punctate zone is formed as a result of the activity of the cells of the shell gland 

 and the stellate zone because of the activity of the mantle cells after eversion 

 of the shell gland (Waller, 1981). The valves of the prodissoconch I are usually 

 similar in size; they are calcified and transparent. However, there is no calci- 

 fication on the dorsal side of their place of attachment; this area of the shell 

 is replaced by the thickened periostracum. The hinge margin is straight and 

 usually (except in mytilids) nondentate. The valve is D-shaped. 



The cells of the free margin of the external mantle fold and the thickened 



Figure 17: Early prodissoconch I of Ostrea ediilis soon after eversion of the shell gland and 



before bending along the hinge line (from Waller, 1981). 



Dots — two centers of calcification. 



margin of the mantle under the hinge continue to discharge a secretion from 

 which a new area of the larval shell is formed — ^the prodissoconch II. It differs 

 from the prodissoconch I in hinge development, appearance of lines on the 

 shell surface (repeating the outline of the mantle margin), and a change in shell 

 shape. The most significant change in shell shape is the appearance of a 

 prominence (umbo) on both valves. 



The umbones are arranged dorsal to the hinge margin. Like the 

 prodissoconch I, the prodissoconch II is also calcified; one exception is the 

 prodissoconch II of Planktomya henseni (Allen and Scheltema, 1972). 



In Penimytilus purpuratus, umbones do not develop and, at the time of 

 metamorphosis, the shell is mainly represented by the prodissoconch I and is 

 D-shaped (Ramorino and Campos, 1979). 



Structural features of the larval hinge (Figure 18) and size and shape of 

 the shell valves make it possible to identify the species of different larvae. 

 These will be discussed in detail later. The central straight part of the larval 

 hinge — the provinculum — consists of different cardinal teeth along the hinge 



