i66 



ACRASPEDA. 



At the point of attachment there is developed a peculiar 

 pedal disc, and around the mouth there appears a fold of epiblast 

 which gives rise to an oral disc (fig. 78 D). Two tentacles first 

 make their appearance, but one of these is primarily much the 

 largest, though eventually the second overtakes it in its growth. 



FlG. 78. FOUR STAGES IN THE DEVELOPMENT OF CHRYSAORA. (After Claus.) 



A. Gastrula stage. 



B. Stage after closure of blast opore. 



C. Fixed larva with commencing stomodseum, 



D. Fixed larva with mouth, short tentacles, etc. 



ep. epiblast ; hy. hypoblast ; st. stomodaeum ; m. mouth ; bl. blastopore. 



A second pair of tentacles next becomes formed, giving to the 

 larva a 4-radial symmetry. Between these four new tentacles 

 subsequently sprout out, and in the intermediate planes four 

 ridge-like thickenings of the hypoblast, projecting into the cavity 

 of the stomach, make their appearance. They imperfectly divide 

 the stomach into four chambers, to each of which one of the 

 primary tentacles corresponds ; they may be regarded as homo- 

 logous with the mesenteries of the Actinozoa. The number of 

 tentacles goes on increasing somewhat irregularly up to sixteen. 

 All the tentacles contain a solid hypoblastic axis. Muscular 

 elements are developed from the epiblast. 



With the above changes the so-called Hydra tuba or Scyphi- 

 stoma form is reached (vide fig. 85). The peculiar strobilization 

 of this form is dealt with in the section devoted to the meta- 

 morphosis. 



