22 



Figure 1 8: Scheme of the hinge system of bivalves, 

 crest; f — flange; 1 — ligament; It — lateral tooth; p — provinculum; st 



special tooth. 



margin of one valve and corresponding alveoli in the hinge margin of the 

 other. The shape, number, and arrangement of teeth differ in different species. 

 Lateral to the provinculum lies the lateral hinge system. It consists of thick 

 dorsolateral margins on one valve — flanges — and thin internal crests run- 

 ning parallel to these margins. The lateral teeth — flange projections edging 

 the provinculum — comprise the lateral hinge system. In some species there 

 are so-called special teeth that belong neither to the provinculum nor to the 

 lateral teeth (Rees, 1950; Chanley and Andrews, 1971; Le Pennec, 1980). 



The anterior and posterior sides of the teeth usually bear ridges and 

 furrows that correspond to the ridges and furrows in the alveolar walls of the 

 other valve (Figure 19). The center of curvature of these ridges and furrows 

 lies roughly on the axis of rotation of the valves. It has been demonstrated in 

 adult bivalves that such a system of ridges and furrows decreases the gap 

 between partially open valves. Larvae of Rangia cuneata (Chanley, 1965), 

 Tiostrea (Ostrea) lutaria, T. chilensis (Chanley and Dinamani, 1980), Gemma 

 gemma, Lyonsia hyalina (Chanley and Andrews, 1971), and some other species 

 with lecithotrophic development lack teeth on the prodissoconch. 



Closure of the shell valves is achieved, first of all, by the working of the 

 anterior adductor. Initially, it is situated behind the dorsal edge of the velum 

 and is the most prominent of all the veliger muscles. The anterior adductor is 

 formed and functions also in those larvae whose adults have only a posterior 

 adductor developing later. In Planktemya henseni, the two adductors are simi- 

 lar in size (Allen and Scheltema, 1972). Besides the adductor, the velum 

 retractors and three lateral muscles are attached to the valves; they extend from 

 the valves in the region of attachment of the anterior adductor and muscles and 

 terminate in the body wall (see Figure 8) (Cragg, 1985). 



Rudiments of definitive organs : In addition to functional organs, the body 

 of the veliger contains rudiments of definitive organs that develop later and 

 begin to function after the larva has settled to the bottom; sometimes the 

 rudiments of certain larval organs are also present but not developed in the 



