IX 



MOLLUSCA 



343 



region surrounding the end of the intestine and extend into the 

 hinder region of the foot. 



The first rudiments of the gills appear as a row of short, knob-like, 

 ciliated protrusions from the roof of the mantle-groove, on each side 

 and parallel with the pos- 

 terior surface of the foot 

 (hr, Fig. 271). The coelomic 

 rudiment becomes divided 

 into a rounded mass of cells 

 on each side of the intestine, 

 which are the rudiments of 

 one kidney, and into an 

 arch of cells above the gut 

 connecting these two rudi- 

 ments. In the very last 

 stage before metamorphosis 

 this arch becomes a ring 

 of cells surrounding the 

 intestine (Fig. 272). 



The rudiment of the 

 cerebral ganglia becomes 

 detached from the cerebral 

 pit, and the cells forming 

 the apical plate degenerate, 

 cast off their cilia, and dis- 

 appear (Fig. 273). The loop 

 of the intestine becomes 

 very long, so as to extend 

 upwards parallel to the left 

 side of the stomach. 



The metamorphosis of 

 the Veliger into the mussel 

 takes place with startling 

 rapidity ; it is as sudden as 

 the change which converts 

 the late free - swimming 

 larva of Polygordius into 

 the adult worm. As in 

 that case, so here, the velar 

 cells die and are cast off, 

 the larval muscles break up 

 and disappear, the whole 

 anterior region in which 

 the mouth was situated shrinks, and the cells forming the mouth 

 cone degenerate and disappear. The result of this change is to bring 

 the mouth and the anterior adductor closer to one another, and thus 

 to swing the foot round so that its apex points forwards instead of 

 downwards. The rows of gill papillae are swung round from a vertical 



pit of 

 (After 



Fig. 273. — Sections through the cerebral 

 Veliger larvae of Dreissensia polymorpha. 

 Meisenheimer. ) 



A, longitudinal section through tlie pit of a larva, in 

 which the apical plate is fully developed. B, longitudinal 

 section through the pit in a larva in which apical plate is 

 degenerating. C, transverse section through the anterior 

 part of the pit in a Veliger to show its bilobed character. 

 a.p, apical plate ; eg, rudiment of cerebral ganglion ; c.j}, 

 cerebral pit. 



