CCELENTERATA. 



161 



primitive gastrovascular cavity of the adult appears in the 

 general hypoblast between the epithelial and nutritive layers in 

 the immediate neighbourhood of its attachment. This cavity 

 becomes prolonged into the nectocalyx to form the four gastro- 

 vascular canals; while the hypoblast at the upper end of the 

 nectocalyx forms the somatocyst (fig. 74 C, so). The primitive 

 enteric cavity once formed rapidly extends, especially in an oral 

 direction (fig. 74 C), and forms a widish cavity in the oral part of 

 the embryo. At the pole of this part (fig. 74, pd] is eventually 

 formed the opening of the mouth, and the contained cavity 

 becomes in a special sense the gastric cavity. This region of the 

 embryo may be spoken of as the polypite. The nectocalyx 

 grows with great rapidity and soon forms by far the most 

 prominent part of the larva (fig. 75). The true gastric region or 

 polypite (fig. 75,/<?) continues also to grow, and a mouth becomes 

 formed at its extremity. The aboral end of the original body of 

 the embryo gradually atrophies. 



FlG. 76. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM. 



(After Metschnikoff.) 



A. Stage after the delamination. ;'/. epiblastic invagination to form pneuma- 

 tocyst. 



B. Later stage after the formation of the gastric cavity in the solid hypoblast, 

 po. polypite ; A tentacle ; pp. pneumatophore ; cp. epiblastic invagination to form 

 pneumatocyst ; hy. hypoblast surrounding pneumatocyst. 



At the junction of the nectocalyx and polypite the coenosarc 

 becomes formed, and rudiments of a second nectocalyx (nc) and 

 B. II. 1 1 



