i8 THE HYDROMEDUSAE 



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

 the Anthomedusae 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). 



Fio. 2ti. 



Cteiuiria Ctenophvra (Haeckel), one of the Anthomedusae, presenting a. curious resem- 

 blance to the Ctenophora. A, lateral view of the entire medusa; B, two horizontal views, 

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

 passing nearly equatorially. a, the eight adradial rows of thread cells, corresponding 

 in position to the eight ctenophoral zones of Pleurohrachia ; b, jelly of the umbrella ; 

 c, circular muscle of the subumbrella ; d, longitudinal muscles of the subumbrella ; c, 

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

 generative glands in the stomach wall (manubrium) ; h, the four perradial primary radiating 

 canals ; i, the eight adradial bifurcations of the preceding ; fc, ring canal in the margin of the 

 umbrella ; I, velum ; m, the two lateral tentacle pouches ; n, the two lateral unilaterally 

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

 with its four primary and 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 Ctenaria homologous with the junction of the so-called funnel of Pleurobrachia with its 

 so-called digestive cavity. This last may bo the homologue of the subumbrellar cavity of 

 Ctenaria. The apical opening or openings of the funnel of Ctenophora suggest the stalk canal 

 of medusae, whilst the agreement between the tentacles and their pouches in Ctenaria and 

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



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

 in detail in different 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-shaped mass of cells ; this sinks below the level of the super- 

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

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



