LIFE HISTORY OF AURELIA. 147 



this process there are formed first two and then four diverticula of the 

 general cavity, which are arranged round the gullet above, and open 

 freely into the digestive cavity below. In the gullet region these are 

 separated by broad septa, which are continued into the lower region of 

 the body as four interradial ridges or taeniolae. Although the develop- 

 ment is different, these may perhaps be compared to the mesenteries of 

 Anthozoa. The tentacles bud out from the region of the mouth, the 

 first four corresponding in position to the four pouches. Interradially 

 above the four septa, four narrow funnel shaped invaginations arise, these 

 are produced by the ingrowth of ectoderm, which then forms the muscle 

 fibres which run down the tceniolae (contrast the endodermic muscles of 

 Anthozoa). 



The larva now forms a " Hydra-tuba " or " Scyphistoma," it is about 

 an eighth of an inch in height. By lateral budding, or by the formation 

 of creeping stolons, it may give rise to larvae like itself. The 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. We 

 will begin with the simplest case, (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] In Aurelia, 

 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 regeneration, 

 and produces another Ephyra. (c) In ordinary conditions the Scyphis- 

 toma elongates, and displays a succession of annular constrictions. This 

 stage, often compared to a pile of saucers, is technically 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 constriction. This is probably an ab- 

 breviation of the primitive mode of development. 



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

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

 probably persist as the sub-genital pits, the gastric filaments sprout out 

 from the remains of the septa, and so mark the place where the ectoder- 

 mal gullet passed into the endodermal cavity. 



The first Ephyra differs from those which come after it in bearing the 

 original tentacles of the Hydra-tuba. From its margin eight bifid lobes 

 grow out, each embracing the base of a perradial or interradial tentacle. 

 The bases of these eight tentacles become the sense organs or rhopalia. 

 The other eight adradial tentacles atrophy. On the Ephyrae which fol- 

 low there are at first no tentacles, only the eight bifid marginal lobes 

 which bear the sense organs in their niches. 



This development illustrates alternation of generations. From the 

 fertilised ovum a fixed asexual Scyphistoma results. This grows into a 

 Strobila, from which transverse buds or Ephyrae are liberated. Each of 

 these grow into a sexual jelly fish, producing ova or spermatozoa. The 

 first two cases mentioned (a and b] show how readily this alternation 

 might pass into a " direct " development. 



Relatives of Aurelia. The Medusae, or true jelly fish, include forms 

 which agree with the Anthozoa, in relative complexity of structure as 



