C(ELENTERATA. 



137 



pearance, but one of these is primarily much the largest, though 

 eventually the second overtakes it in its growth. A second pair of 



FIG. 78. FOUR STAGES IN THE DEVELOPMENT OF CHKYSAORA. (After Glaus. ) 



A. Gastrula stage. 



B. Stage after closure of blastopore. 



C. Fixed larva with commencing stomodneum. 



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



ep. epiblast; hy. hypoblast; st. stomodseiun; in. rmrath; bl. blastopore. 



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 regard- 

 ed as homologous 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 Scyphistoma 

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

 is dealt Avith in the section devoted to the metamorphosis. 



Aurelia is stated by Kowalevsky to develope in the same way as 

 Cassiopea ; and the one stage of Rhizostoma observed is that in which it 

 has a (probably iuvagiuate) gastrula form. 



In Pelagia the ovum directly gives rise to a form like the parent. The 

 segmentation and the invaginatioii take place nearly as in Cassiopea, but the 

 archenteric cavity is relatively much smaller, and the large space between it 

 and the epiblast becomes filled with the gelatinous tissue which forms the 

 umbrella. The blastopore does not appear to close but to become directly 

 converted into the mouth. As in Cassiopea the larva takes a somewhat 

 four-sided pyramidal form. The mouth is placed at the base. The pyramid 

 becomes subsequently flatter, and at the four corners four tentacles grow 

 out which increase to eight by division. The flattening continues till the 



