METAZOA MOLLUSCA. 209 



and the nepionic growth with the cartilage pit or furrow 

 running through the middle. On either side of the true 

 nepionic shell there is a flange-like extension (fig. 15,^) 

 of the margin which passes over the object of support. 



The shell, as it appears at the end of the four nepionic 

 substages and the neanic stage, is represented in fig. 16, 

 right valve, and fig. 17, left valve. The sinus (figs. 16, 17, 

 s) is clearly indicated, and it marks the position of the 

 outgoing current of water. Changes take place in the 

 shape of the shell during the ephebic stage, the variations 

 depending largely upon the surroundings. As a rule the 

 lower or left valve becomes deep and cup-shaped, while 

 the right valve flattens. This is seen in fig. 18, which 

 represents two adults. The upper specimen is growing 

 with the left, deep valve below, and the right, flat valve 

 above. The right hand specimen, however, reverses this 

 condition, so that the left, deep, attached valve is above 

 and the flat valve below. In both cases the same relative 

 form of the valves is maintained. 



The adult oyster (No. 403, alcoholic specimen ; 404, 

 model; 405, shells, young and adult) has but one muscle, 

 the anterior adductor having disappeared. The posterior 

 adductor holds a more central position (Nos. 403, 404) 

 and the dark purple scar is seen in No. 405. The palpi 

 have moved dorsally and are nearer the hinge line. The 

 model shows the two leaves of the mantle, one of which 

 is thrown back exposing the gills. These organs have 

 become more complicated in structure, as shown by PI. 

 402, fig. 19. 



Unionidae. It is very rare to find parasites at any 

 period of life among molluscs. The fresh-water Lamp- 

 silis(=Unio) (No. 406, Z. nasutus Say; Nos. 407, 408, 

 Z. radiata Gmel.) and Anodonta (No. 409) however, 

 pass the young stage attached to fishes. Their develop- 

 ment is, therefore, more complicated than that of most 

 molluscs. The eggs are carried in little pouches in the 

 gill cavities of the parents. No. 407 is the female of 



