330 



ZOOLOGY 



SECT. 



a share in the formation of the mesoderm. Two lateral pouch- 

 like diverticula of the endoderm sac give rise to the mesoderm ; 

 in these a cavity appears the rudiment of the ccelome. A 

 second pair of sacs of the same character becomes developed 

 further back. A large prae-oral lobe is formed, and the anus 

 makes its appearance at a little distance behind the mouth as an 



FIG. 204. Phoronis, Development. A, young larva ; B, larva after the formation of the post- 

 oral circlet of tentacles ; C, larva with commencing pit-like involution ; D, larva with invagma- 

 tion partly everted ; E, invaginatiou completely everted. 1/1. mouth ; an. anus ; i-e. involution 

 to form the body. (From Balfour's Embryology.) 



imagination of the ectoderm ; it becomes surrounded by a circlet 

 of cilia (Fig. 264, A). The part of the body on which the anus 

 is situated becomes elevated into a conspicuous process. Behind 

 the mouth there is a circlet of cilia and from this region grow out 

 a circlet of processes the rudiments of the larval tentacles (B). 

 The larva has now reached the stage to which the term Actino- 

 trocha is applied. It has a large hoodlike lobe overhanging the 



