752 PROCEEDINGS OF THE ACADEMY OF [DeC. , 



order as follows: " Development, hypogenesis (not metagenesis), 

 but usually with metamorphosis." Subsequent research into the 

 life-history of this group has shown that each clause of this state- 

 ment is open to emendation. In the first place, the " usually " is 

 superfluous. The exceptions which Haeckel supposed to exist and 

 Avhich caused him to say " usually with metamorphosis " have been 

 shown to be no exceptions, but cases of somewhat easily misunder- 

 stood metamorphosis. Such, for example, was the case of Liriope, 

 which has been studied by Metschnikoff^ and Brooks.^ The 

 larva is a true hydra, although its free swimming mode of life and 

 its superficial aspect caused it to be mistaken, formerly, for a 

 gonosome. My study of a jelly-fish which Haeckel includes in 

 his order " Trachomedusse " leads to the conclusion that the first 

 part of Haeckel' s statement also requires revision, and that meta- 

 genesis does occur among medusae of this order. Although there 

 may be different interpretations of the terms " metagenesis" and 

 " hypogenesis," the following notion of the process of alternation 

 of generations may be safely accepted as that which is generally 

 held by students of this group. The production by a larva of 

 offspring unlike itself, and its own ultimate death without 

 undergoing metamorphosis, are frequent accompaniments of the 

 intermediate as of the primary process of multiplication; but they 

 ai-e by no means essential to the process of metagenesis or alterna- 

 tion of generations. Creatures which multiply sexually at one 

 point of their life-history, and at another point non-sexually by 

 budding or fission, are said to have a metagenetic development. In 

 Gonionema a large number of adult individuals are produced from 

 a single egg through an intermediate process of multiplication (text- 

 figs. 2-10) ; buds are developed upon the body of the hydra-like 

 larva, become detached and, beginning as pianulse, follow exactly 

 the same course of development as the sexually produced parent. 

 Both parent and offspring later change into fully developed meduspe. 

 Gonionema has, then, a metagenetic form of development. It is, 

 of course, a mistake to regard the mere presence of a hydrula stage 

 enough to constitute alternation of generations (Murbach, 1895, 

 p. 496). 



These emendations of Haeckel's description of the order add 



2 Metschnikoff, Embryologische Studien an Medusen, 1886. 

 ^ Brooks, Life-History of the Hydromedum, 1886. 



