zUb CCELENTERATA. 



than the preceding ones. After an interval four new folds appear, one 

 on each side of the two primary mesenteries (tig. 170). The twelve 

 gastrovascular chambers thus formed gradually become equal in 

 size, and can be separated into two unpaired chambers situated in 

 the median plane, and into five pairs placed symmetrically on 

 cither s-ide of it. 



FIG. 170. Frcm the history of the development of Actinia meeembryaniJieiiwnt (after Lacaza 

 Duthiers). a, Larva with eight mesenteries ;mcl two coiled bunds; O, mouth. >/, Slightly 

 more advanced larva with the commencement of eight tentacles, c, d, Young Ai-thiia 

 with twenty-four tentacles, two longitudinal sections, at right angles to one anothei. e, 

 Mouth and tentacles seen from the oral surface. 



The tentacles begin to develop before the appearance of the fifth 

 and sixth pairs of mesenteries. They appear at the oral end of the 

 gastrovascular chambers, and the tentacle of the anterior unpaired 



