OF THE HYDROMEDUSiE. 405 



which any given problem presents depend, to a very great degree, upon the accidents 

 which direct attention to examples and illustrations which are simple and easy to under- 

 stand, or to those where the simple laws are obscured or hidden under secondary com- 

 plications. 



Just as Werner's geological speculations were colored by the peculiar nature of the 

 region where he lived, so the speculations of zoologists upon the origin of the medusae, 

 and their relation to hydroids, have been complicated by the accident which has directed 

 their attention to the wrong end of the problem and has caused the almost total neglect 

 of the groups which furnish its solution. 



The typical Hydromedusae, the Tnbnlarians or Anthomedusae, and the Campanularians 

 or Leptomedusae, are found in abundance on every coast and the shortest visit at the 

 seashore must bring them before the eyes of the naturalist; while the pelagic Tracho- 

 medusae and Narcomedusae, which are seldom found near the shore, are usually regarded 

 as minor or aberrant groups and they usually occupy a very subordinate and secondary 

 position in our general conception or mental picture of the Hydromedusac', although they 

 include nearly one-third of all the known species of Acraspecla and arc, so far as diver- 

 sity of structure is concerned, fully as important as the more familiar groups. 



Most of the writers who have discussed the origin of the medusas, and the significance 

 of the alternation between them and the hydroids, have entirely ignored the Narcome- 

 dusae and the Trachomedusse; or else they have made only an incidental reference to 

 these two groups, which actually furnish the clearest, simplest and most direct evidence 

 which is attainable. 



As soon as we perceive that there is no reason why we should believe that the me- 

 dusa 1 which are set free from fixed hydroid communities are the most primitive, simply 

 because they are the most familiar; and that, Liriope, vEgina and Ounina are not, as 

 Balfour (65) and Grobben (74) assert, medusae which have lost their ancestral hydra 

 stage, but simply solitary floating or swimming hydras which gradually grow into medusae 

 and which repeat, more or less exactly during their own ontogenetic development and 

 gradual metamorphosis from the egg to the adult, the phylogenetic history of the medusa: 

 the complicated problem disentangles itself and we feel at once that Ave have found the 

 right end of the thread. 



In ^Eginopsis, as Metschmkoff shows (30), the egg gives rise to a ciliated swimming 

 planula, which acquires a mouth and tentacles and thus becomes directly and gradually 

 converted into a floating hydra or actinula which is at first ciliated like the planula. The 

 tentacular zone of the floating hydra now grows out into a flange or umbrella which 

 carries the tentacles with it; sense organs and a veil are soon acquired and the hydra be- 

 comes a medusa. 



The whole process is perfectly simple and direct; there is nothing like an alternation 

 of generations and the single egg becomes a single medusa with an actinula stage, a 

 floating hydra-like larval stage and a swimming medusa stage. The life-history is as sim- 

 ple and uninterrupted as that of any other animal which undergoes a metamorphosis, 

 and it may be represented by the following simple diagram in which the sign of equality 

 (=) denotes that the change is direct growth or metamorphosis rather than multiplica- 

 tion. 



