30 



ZOOLOGY 



SECT. 



to the oesophagus, stomach, and intestine. The mouth-opening is 

 formed shortly before the escape of the embryo from the egg ; an 



mes 



TIO to 



FIG. 689. Later stages in the development of Clavellina. A, approximately median optical 

 section of a larva in which the medullary canal (neuroccele) has become enclosed throughout, 

 communicating with the exterior only by the neuropore at the anterior end, and with the 

 archenterou by the neurenteric canal ; B, larva with a distinct rudiment of the tail and well- 

 formed mesoderm layer and notochord. Letters as in preceding figure ; in addition, mes. 

 mesoderm. (After Van Beneden and Julin.) 



ectodermal invagination is formed at the anterior end, and an 

 endodermal diverticulum from the archenteron grows out to meet 

 it ; the two coalesce, and the oral passage is thus formed. 



