OF MASSACHUSETTS. 



35 



The Gills, — The early veliger has no gills. They first begin to 

 develop coincidently with the formation of the foot as simple bars or 

 ciliated filaments, capable of extension and contraction from the dorsal 

 point of attachment. Starting from beneath the stomach they lie in 

 folds along the upper part of the foot. When first seen, at the begin- 

 ning of the degeneration of the velum, they scarcely consist of two 

 folds, but before the velum has disappeared they number from four to 

 five. The edges of the folds are lined with active cilia which keep up 

 an incessant motion. These primitive bars, as seen in the prodissoconoh 

 (Mg. 18), are the paired beginnings of the inner gills. The outer gills 

 develop at a later stage. 



The Mantle. — ■ At the time of the formation of the gills the mantle 

 becomes noticeable as a thin, transparent covering just under the shell, 

 although it has been functional before this period. By the time the 

 dissoconch stage is reached, the free edge has thickened into fine folds 

 and is lined with small cilia. 



The Digestive Tract. — The digestive apparatus of the early veliger 

 consists of a funnel-shaped mouth lined with active cilia, leading into a 

 broad sac, the stomach, also lined with minute cilia, from which arises 

 a two-lobed liver. The intestine is merely a straight tube opening pos- 

 teriorly. With the prodissoeonch veliger the digestive tract is obscured 

 by the growth of the liver, which has assumed a greenish yellow color 

 so that the coils of the intestine are difficult to distinguish. The mouth 

 has travelled forward in a dorsal direction, the edges apparently having 

 formed the palps, while the ciliated funnel has become the oesophagus. 

 The intestine now has one or more coils, and, in order to carry on the 

 more complicated process of digestion, opens dorsal to the adductor 

 muscle. 



Summary of Veliger Stage. 



