36 



cells. The muscular system which served the velum is also destroyed (Bayne, 

 1971). In Placopectan magellanicus, large parts of the velum are discarded 

 during metamorphosis; only the apical part is retained, which participates in 

 the formation of the upper preoral lobe (CuUiney, 1974). As a result of these 

 processes, the larval mechanisms of swimming and catching food disappear. 

 The locomotor functions are usually passed to the foot and the function of 

 particle capture to the gills and preoral lobes; the lower lobe makes contact 

 with the first gill filaments. The change in mechanisms of food capture may 

 require several days, during which time the larva does not feed, and uses its 

 reserve nutrients. The foot in burrowing moUusks develops further, shifting 

 anteriorly in the mantle cavity. The larval muscles of the foot are replaced by 

 definitive ones. In attached (sessile) species, however, the foot is usually 

 totally or partly reduced, and its cells are phagocytized (Hickman and Gruffydd, 

 1971). 



The central nervous system generally retains its structure. Ganglia occupy 

 definitive positions. The visceral ganglion is fiirther developed, the relative 

 size of the cerebral ganglion decreases, and the pedal ganglia are reduced if 

 the foot is reduced. The cerebrovisceral connective develops. In many, espe- 

 cially immobile species, the eyes and statocysts disappear. Ultimately, in all 

 species the apical tuft of cilia disappears. In mobile forms defmitive sense 

 organs, lacking in larvae, are formed. 



Changes in the structure of the shell represent the concluding stage of 

 metamorphosis. The microstructure and mineralogy of the shell drastically 

 change; layers of the new defmitive shell — the dissoconch — are formed 

 (Wilbur, 1964). The larval hinge is also replaced by a definitive one. Changes 

 in the hinge system may precede metamorphosis, or follow it, depending on 

 the family. The shell changes considerably in specialized forms such as wood- 

 and rock-boring species (Kiseleva, 1970; Turner and Johnson, 1971; Boyle and 

 Turner, 1976). 



The defmitive shell is in a partially open state due to the ligament that first 

 appears in the prodissoconch or dissoconch (Lutz and Hidu, 1979). This liga- 

 ment is an elastic cord connecting the valves of the shell. It begins to form in 

 the ligament fossa of the shell. The position of this fossa and hence of the 

 ligament proper, differs in different families. In the vast majority of bivalves 

 it is situated ventral to the hinge line. It should be pointed out that the ligament 

 may not always be present in larvae; if present, it does not always develop in 

 the adult. 



In the family Teredinidae, new protective structures appear during meta- 

 morphosis: a calcified cone, made of detritus, over the inlet to the passage 

 made by the moUusk; a calcified bed in the passage; and plates or palettes near 

 the siphon (Figure 31) (Turner, 1966). 



