40 



Figure 33: Lecithotrophic larva of Yoldia limatula Dall (from Drew, 1899). 

 A — external appearance; B — median section; ate — apical tuft of cilia; eg — cerebral 

 ganglion; fg — foregut; Im — larval mantle; m — mouth; mg — midgut; shg — shell gland. 



to lengthen the lecithotrophic phase is observed in the superfamily Ostreoidea. 

 In Crassostrea gigas and C. virginica, fertilization takes place in the external 

 medium (water) and the larva takes to active feeding in the straight hinge 

 stage. In Ostrea edulis, fertilization occurs in the mantle cavity and brooding 

 in the gills continues until the larval shell has reached a length of about 160 

 |j.m. In O. chinensis and O. lutaria, females bear larvae up to the pediveliger 

 stage which, soon after their emergence in the sea, settle and metamorphose 

 (Millar and HoUis, 1963); in the absence of a substrate for settling, metamor- 

 phosis may be delayed for several days (Chanley and Dinamani, 1980). In the 

 family Teredinidae, fertilization occurs in water in Bankia and the lecithotrophic 

 phase is cohfmed to the trochophore stage. In Lyrodus and Teredo, the duration 

 of the planktotrophic phase may be considerably shortened. This tendency 

 appears, obviously, independent of geographic distribution of the species and 



