TYPES OF C(ELENTERA--AUKEL1A AURITA, 151 



gradual widening of the central cavity renders the gullet tube less 

 obvious, and results in an increasing resemblance to the medusa 

 type. 



In late autumn, however, a more fundamental change occurs in the 

 history of the Hydra-tuba, (a] Occasionally, as has been observed by 

 Haeckel, the Scyphistoma becomes detached and converted into a free- 

 swimming Ephyra, which in turn becomes a jelly-fish, (b] Sometimes, 

 in unfavourable conditions, a furrow appears round the upper region of 

 the Scyphistoma, the upper portion is converted into an Ephyra, and 

 floats away, while the lower portion reforms its oral region by regenera- 



FlG. 69. Diagram of life history of Aitrelia. After 



Haeckel. 



i. Free-swimming embryo; 2-6, various stages of Hydra-tuba; 

 7, 8, Strobila stage ; 9, liberation of Ephyrse ; 10. n, growth of 

 Ephyrae into Medusae. 



tion, and produces another Ephyra. (c] In ordinary conditions the 

 Scyphistoma elongates, and displays a succession of annular constric- 

 tions. This stage, often compared to a pile of discs or saucers, is 

 called a Strobila. Each disc is separated off in its turn as a free- 

 swimming Ephyra, which becomes a jelly-fish. The still undivided 

 basal portion may rest for a time, and then undergo further con- 

 striction. This is probably an abbreviation of the primitive mode of 

 development. 



In the conversion of the Scyphistoma into the Ephyne, the diverticula 

 coalesce into a general cavity, the entrances to the septal imaginations 



