208 SYNOPTIC COLLECTION. 



This early fixation of the oyster has doubtless caused the 

 reduction of the foot, which exists as a mere vestige in an 

 early embryonic stage and is lost altogether before the 

 embryonic stages figured in PL 402. 



The straight hinge line of the embryonic shell has 

 given way to a curved line with high umbos (figs. 4, 5 ; 

 also fig. 6, which shows the shell attached by the left 

 valve in an almost vertical position). An important 

 change has taken place in the structure of the oyster, for 

 instead of having one muscle it now possesses two, a 

 posterior adductor (figs. 4, 5,/tf) having been developed. 



The gills (figs. 4, 5, g) are still in a primitive condition, 

 consisting of straight simple filaments (fig. 7). The sit- 

 uation of the mouth can be judged by the position of the 

 palpi (figs. 4, 5,//), but the anus has changed its place, 

 having moved dorsally before the development of the 

 posterior adductor muscle which took place on its ventral 

 side. 



The beginning of the nepionic stage is represented in 

 fig. 8. Here the left valve is below and the right above, 

 and on the edges of these valves the new growth of the 

 nepionic shell is being added to the prodissoconch. The 

 internal organs in the early nepionic stage are shown in 

 fig. 9. The single adductor muscle (fig. 9, ad) is in a 

 similar position to that of the adult. The gills have 

 passed througn the stage represented by fig. 10 and are 

 now in the condition illustrated by fig. n, in which the 

 filaments are connected by cross bars. 



The substages of the nepionic stage are so well shown 

 in the shell of the oyster, that contrary to our usual rule, 

 we illustrate them by figures. Fig. 12, /, represents the 

 prodissoconch succeeded by the first nepionic growth. 

 In figs. 13 and 14 two and a half of the third nepionic 

 substages are figured. At this time the right or upper 

 valve is convex (fig. 13) and the left or lower valve (fig. 14) 

 is flat. The tip of the left valve (fig. 15, x 87 diameters, 

 view of interior) shows the prodissoconch (fig. 15, /) 



