REPORT ON THE DEEP-SEA MEDUSAE. X1U 



§ 14. Monophyletic origin of the Acraspedse. Whilst the section of the Craspedotse 

 is probably of polyphyletic origin, and their chief groups have arisen independently of one 

 another, the conditions of relationship among the Acraspedse or Scyphomedusse is, on the 

 contrary, so close that the monophyletic origin of these sections may be admitted with the 

 greatest probability. The existing Tessera has even retained a very old primordial form, 

 which has the same significance for the section of the Acraspedse as Hydra has for the 

 whole tribe of the Cnidaria. The order of Stauromedusse in which the sense clubs or 

 rhopalia are completely wanting, is developed first of all from Tessera. The two orders 

 of Peromedusse and Cubomedusse, spring as two diverging principal branches from the 

 ancestral group of the Stauromedusse ; in the former the four interradial tentacles are 

 transformed into sense clubs or rhopalia ; in the latter, on the contrary, it is the four per- 

 radial tentacles. The fourth order of Acraspedse, the extensive group of the Discomedusse, 

 is distinguished by the fact that all the eight principal tentacles are transformed 

 into rhopalia ; it has probably originated from the Stauromedusse or Cubomedusse, per- 

 haps rather from the Peromedusse. The Cannostomse form the ancestral group among the 

 Discomedusse ; the Semostomse have probably arisen later from these, and the Rhizostomse 

 still later from the Semostomse (System, p. 450). 



§ 15. Ontogenesis of the Medusse. The individual development or ontogenesis, 

 which, according to the biogenetic fundamental law, affords us the surest explanation of 

 the phylogenesis or the original development of the tribe, appears in the Medusse in 

 two distinct principal forms, as metagenesis and hypogenesis. Metagenesis (or indirect 

 development) includes the alternation of generations of the Medusse ; the Medusa is 

 formed here asexually by gemmation from a sessile polyp ; and the ova of the free-swim- 

 ming Medusse do not again develop into Medusa, but into the Polyp, which functions 

 as bud-bearing " nurse." On the other hand, hypogenesis (or direct development) of the 

 Medusa consists of the immediate development of the Medusse from the ova of the 

 Medusse ; the intermediate stage of the Polyp-nurse is wanting. As then the compara- 

 tive anatomy indubitably shows that the wandering, more highly developed Medusa 

 form is to be derived phylogenetically from the sessile Polyp form, we must consider 

 the metagenesis of the Medusse as their original or pabngenetic mode of development, 

 but their hypogenesis as the modified, shortened, and simplified cenogenetic form of 

 gemmation. It is the metagenesis, not the hypogenesis, which repeats in epitome the 

 method of historical development. 



§ 16. Metagenesis or indirect development. The alternation of generations or meta- 

 genesis of Medusse must be considered their original form of generation or primary mode 

 of development, — their palingenesis; it appears in different forms in the two sections of 

 the class. The Craspedotse originate from Hydropolyps by lateral gemmation, the Acras- 

 pedse from Scyphopolyps by terminal gemmation. These two forms of alternation of 



