118 



EMBRYOLOGY 



observed that the scyphopolj-ps of Aurelia aiirita, when 

 they are placed in unfavourable conditions (for example, in 

 aquaria), show little inclination to form polydiscous strobilce, 

 but frequently develop only monodiscous strobilae (Fig. 59 

 -4). In fact, Haeckel observed in certain cases that the 

 transverse division of the scyphopolyp metamorphosing 

 into an Ephyra is altogether sup- 

 pressed, so that the entire body of 

 the larva is converted into the adult 

 animal. This is Haeckel's so-called 

 Ephyra peduncii^ata (Fig. 59 B), which 

 was observed in the attached as well 

 as in the free-swimming condition. 

 Here therefore the alternation of 

 generations is omitted, and a simple 

 metamorphosis (hypogenesis) has 

 taken its place. 



The latter condition is the normal 

 and only one in Pelogia noctiluca, the 

 development of which has been made 

 known through Krohn (No. 109), 

 KowALEVSKY, and Metschnikoff (No. 

 12). In this case there is first formed 

 a blastula which has a large cleavage 

 cavity, and soon becomes covered on 

 the surface with Uagella. At the 

 same time an invagination from the 

 posterior pole is formed, which leads 

 to the development of a gastral cavity 

 which does not by any means completely fill the space of the 

 primitive cleavage cavity (Fig. 60 A). The blastopore does 

 not close, but becomes the mouth of the larva. A shallow 

 depression is very soon noticeable at the posterior end of the 

 free-swimming larva, in the middle of which the part sur- 

 rounding the mouth projects in the form of a cone (Fig. 

 60 B). This projection becomes the oral cone of the Ephyra 

 (Fig. CO C, m), and the circular depression surrounding it 

 the umbrellar cavity, while on the peripheral mai-gin a divi- 

 sion into eight marginal lobes is soon noticeable, into which 



Fig. 59.— a, Strobila mo- 

 nodisca of Cyanea capillnta 

 (after P. J. van Bknbden) ; 

 f, lobes of the Ephyra; t, 

 newly formed circle of 

 Scyphistoma tentacles on 

 the basal portion. 



B, Ephyra pedunculata of 

 Aurelia aurita (after Haec- 

 kel). 



