380 



OILMAN A. DREW. 



ventral portion of the foot, just anterior to the heel-like 

 projection, makes its appearance (fig*. 39, hg.). This develops 

 into the byssal gland. It grows rapidly until it becomes 

 proportionately very large (fig. 45, hg.), then ceases to grow, 

 and possibly shrinks somewhat. In the adult it is compara- 

 tively insignificant (fig. 48). No signs of byssal threads have 

 ever been observed, nor have the secretions ever been seen 

 to protrude from the duct of the gland. 



The foot grows rapidly, and the projection that looks like 

 a heel becomes more marked (fig. 40, /.) . Anterior to this 



Text -FIG. K. — Keconstruction of a ten-day embryo of Yoldia iimatula. 

 Represented as seen from the right side with the right shell-valve and 

 mantle lobe removed, aa. Anterior adductor muscle, r^. Cerebral 

 ganglion. /. Foot. ^. Gill. int. Intestine. //. Left lobe of the di- 

 gestive gland. oL Otocyst. pa. Posterior adductor muscle, p^. Pedal 

 ganglion, rl. Right lobe of the digestive gland, sto. Stomach, v^. 

 Visceral ganglion. 



projection the sides grow ventrally faster than the interme- 

 diate portion, and finally from the side flaps that are so 

 characteristic of the foot of the adult. Movements of the 

 foot now become energetic. 



The gill (fig. 40, g.) becomes more pronounced, and soon 



