34 



In many bivalves metamorphosis with total or partial resorption of many 

 larval organs is characteristic. The digestive system, except for the trapping 

 apparatus, is subject to comparatively less change (Bayne, 1971). The oral 

 opening is shifted and occupies an anterodorsal position near the hinge line. 

 The anal opening likewise shifts and occupies a posteroventral position. The 

 digestive gland acquires a definitive structure. Disappearance of the larval liver 

 has been reported in protobranchs (Drew, 1899). 



The respiratoty function changes over from nonspecialized external tis- 

 sues to a gill apparatus that, in most bivalves except protobranchs, performs 

 simultaneously the function of catching food. After the foot has shifted forward 

 during metamorphosis, the gill filaments from each side of the mantle cavity 

 fijse and divide the mantle cavity into an incurrent and excurrent chamber. 

 This ensures efficient filtration of the incurrent water by the gill filaments. The 

 number of gill filaments increases after metamorphosis (Bayne, 1976). In most 

 bivalves in the superorder Autobranchia, the preoral lobes assist in directing 

 the food particles caught by the gill filaments to the mouth. In many mollusks 

 the mantle edges fiise along the median line and muscular tubules form ante- 

 riorly and posteriorly — incurrent and excurrent siphons — which regulate the 

 flow of water through the mantle cavity (Figure 29). Soon after settling, the 

 transport of substances within the organism is accomplished as in the larva. 

 The (blood) circulatory system develops later. 



The larval excretory system disintegrates and is replaced by definitive 

 kidneys originating from the pericardial cavity, which open through ducts into 

 the mantle cavity. 



The velum, or locomotor apparatus, of the veliger and the pediveliger 

 disintegrates (Figure 30). It is invaded by phagocytes, which absorb the velar 



Figure 29: Renopericardial rudiment in Dreissena polymorpha Pail. 



(from Meisenlieimer, 1901). 



hg — hind gut; m — mantle; rhp — rudiment of heart and pericardium; rr — renal rudiment; 



sh — shell. 



