TYPES OF CCELENTERA AURELIA AURITA. 143 



and ectodermal muscles, thus confirming his view of the intimate 

 relation between the Actinozoa and Scyphomedusse. 



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 forma- 

 tion of creeping stolons, it may give rise to larvee 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 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- 

 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 Ephyrce, 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 ecto- 

 dermal 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 Ephyrse which 

 follow 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 Ephyrse are liberated. Each of 

 these grows 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 compared with Hydrozoa, and in the possession of an ectodermal gullet 

 (see footnote on p. 142), but differ in possessing ectodermal septal muscles 

 and in some histological featuies. If Gcette's discovery of rudimentary 

 ectodermal muscles in the larv;e of certain sea-anemones be confirmed, 

 however, it would greatly increase the probability of a close relationship 

 between the two sets. Among the ScyphomedusDe closely allied to 

 Aurelia, some, e.g. Pelagia, have a direct development without the 

 intervention of Scyphistoma or Strobila stages, but this may occur 



