48 THE AMERICAN MUSEUM JOURNAL 



Above the mouth opening, which by this time is clearly defined, 

 a ridge marks the beginning of the velum, or swimming organ, 

 of the larva ; below the mouth there is a large protuberance which 

 will form the foot ; at the sides of this two smaller knob -like 

 outgrowths form the larval kidneys. iVt a point directly op- 



model 19 

 The larva begins to assume its definitive form. The mouth opening is 

 formed ; above it the curved edge of the velum is defined ; below it the foot 

 begins to protrude; on either side of this the first appearance of the larva 

 kidney (EX) is indicated. At the lower pole of the model the shell gland 

 is shown. 



posite the apical, or head, end the shell gland develops (Model 

 1 8). Model 19 shows the shell beginning to be secreted by the 

 shell gland. 



The entoblast cells of the cavity of the gastrula by a process 



of unequal growth rapidly go to form the various parts of the 



digestive tube : stomach, liver, intestine etc. The oesophagus is 



formed by an invagination of the ectoderm from the exterior. 



The middle layer, the mesoblastic layer, forms the muscles, 



[24] 



