THE HYDROMEDUSAE 



If, as seems probable, the product of the fertilised ovum of 

 the Authomedusae is always a hydroid, there is an invariable 

 alternation of an asexual generation (the hydroid) with a sexual 

 generation (the medusoid) ; this alternation of generations, or 

 metagenesis (Brooks, 14), is not disturbed by the fact that the sexual 

 generation may in a few cases reproduce asexually (Sarsia). 



J-'ai. -lu. 



Ctriiaria Ctenophora (Haeckel), one of tlie Aiitlionipdusae, iireseiiting <a curioiis respin- 

 blaiice to tlie Ctenophora. A, lateral view of the entire nieclusa ; 7>', two horizontal views, 

 thai to the left representing the surface of the aboral hemisphere, that to the right a section 

 passini; nearly equatfirially. a, the ei>;ht adradlal rows of thread cells, corresjiondinj; 

 in iiositioii to the ei};ht ctenophoral zones of rieurohrachia ; li, jelly of the umbrella ; 

 <■, circular muscle of the snbunibrella ; d, lonjiitudinal muscles of the subumbi-ella ; i\ 

 stomachal dilatation of the enteric cavity ; /, the sixteen oral tentacles; (/, the four perradial 

 generative glands in tlie stomach wall (manubrium); h, the four perra<iial primary radiating 

 canals ; i, tlie eight adradial bifurcations of the preceding ; ).; ring canal in the margin of the 

 umbrella; /, velum; »i, the two lateral tentacle jKiuches ; ii, the two lateral unilaterally 

 fringed tentacles; n, the apical gastric cavity above the stomach. The canal system, 

 with its four ])rimary ami eight secondary rami, resembles that of Pleurobrachia. The mouth 

 of the latter may be homologous with the margin of the umbrella of the former, and the mouth 

 of Ctcnaria homnlogous witli the junction of the so-calle<l funnel of IMeurolirachia with its 

 so-called digestive cavity. 'I'liis last may be the homologue of the subinnhrellar cavity of 

 Ctenaria. The apical opening or oi)enings of the funnel of Ctenophora s\iggest the stalk canal 

 of meilusae, whilst the agreement between the tentacles and their pouclics in Ctenaria and 

 Pleurobrachia is comidete. Cf. p. 14, infra. (After Haeckel.) 



The method of formation of a medusoid (Weismann, 10) varies 

 in detail in ditlerent genera, but the following account of Bougain- 

 villea may be taken as typical. A rapid proliferation of cells at 

 the apex of a simple bud (Fig. 27, I) results in the production of 

 a lens-sh,aped mass of cells ; this sinks below the level of the super- 

 ficial ectoderm, pressing the cndodermal wall in front of it into 

 the shape of a cup. This mass of ectoderm is termed the ento- 



