58 



LAMELLIBRANCHIA UNIONIDAE. 



place, the foot now appears as a blunt cone and soon grows rapidly. 

 The wall-like outer margins of the two lateral pits also increase in 

 height. These prominences become the rudiments of the gills which 

 first appear in the form of two knobbed papillae (F. SCHIMDT). 



Fig. 26 shows the rudiments of the gills at a somewhat later 



stage. The foot is here 

 f 



K 



found well developed, 

 and both it and the 

 gills are ciliated. The 

 posterior ciliated area 

 of the embryo (;), 

 which was still visible 

 when the foot had 

 attained a considerable 

 size, now disappears. 

 Of the larval organs, 

 the shell- hooks and the 

 large adductor muscle 

 are still to be seen. 

 The first are for the 

 present retained, the 

 shell in other respects 

 also retaining its em- 

 bryonic form until the 

 young Laniellibranch 

 leaves the fish ; indeed 

 the embryonic shell can 

 still be made out in the 

 shell of the adult. The 

 longer of the two free 

 sides of the three-sided 

 embryonic shell must 

 be considered to corre- 

 spond to the anterior 

 end of the animal, and 

 in this position it can 

 actually be found as a small prominence on the umbo of the adult 

 shell (BRAUN). 



The powerful adductor muscle of the larva agrees in position with 

 the anterior adductor of the marine Trochophore larva. It is, accord- 

 ing to BRAUN and F. SCHIMDT, merely a larval organ, and degene- 



FIG. 26. A-C, larvae of Anodonta (after SCHIERHOLZ). 

 d, rudiment of the intestine ; /, larval filament ; 

 fu, foot ; g, lateral pits ; k, gills ; m, mouth ; sh, 

 shell-hooks ; sm, adductor muscle ; .so, sensory 

 organs ; w, ventral plate (ciliated area). 



