xiv THE VOYAGE OF H.M.S. CHALLENGER. 



generations are not referable to one another, and have arisen independently of one another. 

 Among the Craspedotse, alternation of generation is the usual form of development of the 

 Leptolinse (or Acordyliaj), and the Polyp-nurses of the Anthomedusse are Tubularian Polyps, 

 whilst the Polyp-nurses of the Leptomedusse are Campanularian Polyps. On the other 

 hand, metagenesis only occurs rarely among the Trachylinse (orCordyliotse) (Lovenella clausa 

 is perhaps the Campanularian nurse of a Trachomedusa (?) — System, p. 653). Among the 

 Acraspedse, ontogenesis of the Tesseronise is still unknown. Alternation of generations 

 seems to be the usual form of development in the Ephyronice. The peculiar form of 

 terminal gemmation, by which the Discomedusa is developed from the Scyphopolyp nurse, 

 is, however, essentially different from the lateral gemmation by which the Craspedotse are 

 developed from the Hydropolyp nurse. 



§ 17. Hypogenesis or direct development. Direct development without alternation of 

 generations, which in one word we call hypogenesis, must not be regarded as the original 

 form of generation in the Medusae, but as a secondary, shortened and simplified mode 

 of development, — as cenogenesis ; it has arisen by lapse of the alternation of generations. 

 Among the Craspedotse, nearly all Trachylinse (Trachoinedusse and Narcomedusse) develop 

 in this manner, but only a very few Leptolinse {e.g., the Cannotid Dipleurosoma = 

 Ametrangia, System, p. 637). Among the Acraspedse probably many Tesseronia (Stauro- 

 medusse, Peromedusse, and Cubomedusse) are similarly developed directly from the ovum ; 

 their ontogeny is, however, unknown as yet. Among the Ephyronia (Discomedusse) 

 hypogenesis is as yet only known to be constant in Pelagia (whilst the closely allied 

 Chrysaora undergoes metagenesis). Aurelia is usually developed with alternation of 

 generations, but in isolated cases without it, directly from the ovum. 



§ 18. Medusse and Acalephse. Since the alternation of generations with Polpys has 

 been known in the Medusse, great difficulties have arisen in the classification of the Aca- 

 lephse ; and the natural class of Medusse, which can be so easily distinguished from other 

 classes of Cnidarise, has therefore sometimes been even abandoned by many more recent 

 authors. From the standpoint of the doctrine of evolution, however, these difficulties can 

 be easily solved, and the Medusse, at the same time reinstated as a class. According 

 to the present extent of our knowledge, it appears most logical, and at the same time, 

 natural to distinguish definitely the following five classes among the Acalephse : — Class I. 

 Polyps (Polypi) includes the common ancestral group of all Cnidarise ; it is divided first 

 of all into two sections, Hydropolyps and Scyphopolyps. To the Hydropolyps (without 

 tseniola) belong (a) the hypothetic ancestral form itself Archydra (also Gastrcea, closely 

 related to Hydra); (b) Hydropolyps without Medusa formation, and with simple genitaha 

 (Hydra, Clava) ; (c) Hydropolyps with spore-sacs or medusoid genitalia (Tubularise, Cam- 

 panularise) ; these spore-sacs or medusiform reproductive buds ("sporosacci")are Medusse, 

 which have undergone retrograde formation, without oral opening and without tentacles 



