27 



quickly obliterated (within a few hours). 



Accompanying the loss of the velum, the lonf^ byssus 

 thi'ead has been secreted. As soon as the lai'va ia secure, 

 it seeks a place to burrow, and in pr'-jparation for its fu- 

 ture life, its whole organization be{];ins to chanf^e. The 

 fusion of the mantle ventrally prorrtsses rapidly, and 

 within tv/o or tliree days (fig;. 3) only the opening for the 

 foot is left. The siphons elongate rapidly and very early, 

 the mantle grovrs out so as to project beyond the shell. 

 The latter changes rapidly after a ttaclirnent . V/ithin two 

 days the first row of teeth has been formed on either 

 valve; the greater grov^th on the ventral edges causes the 

 t'.vo valves to gape for the protrvision of the foot; tlie 

 knobs have been formed on dorsal and ventral portions; the 

 apophyses have been formed and the i-otractor muscles of the 

 foot have become attached to them. The foot, meanwhile, has 

 become pestle-shaped. 



The alimontai-y canal takes an important part in the 

 general change. The caecum of the stomach, present in the 

 larva as a rudiment, enlarges very rapidly, and even be- 

 fore woody material has been ingested in quantity, it pro- 

 jects I.- a large vesicle into the foot. In the early 

 sty"- - f-.' . (4 days attached, a half millimeter 



