214 NATURAL SCIENCE. Sept., 



which may well be supposed to occur "fortuitously"; and is one 

 which confers an advantage upon the species undergoing the trans- 

 formation — however numerous these species may be. 



It is not necessary to go more into detail in this question, as the 

 view here urged is, I believe, adopted almost unanimously : difference 

 of opinion, if there be any, will probably be confined to the question 

 of the possibility, or perhaps probability, of such a transformation 

 occurring independently in several species. 



There are, however, not a few who have only adopted this view 

 as to the origin of tentaculocysts at a moment when they were not 

 thinking of their homologues in a supposed Haliclystus -like ancestor. 

 These, no doubt, will require that any hypothesis or conclusion as to 

 the course of evolution of tentaculocysts shall regard the marginal 

 bodies of Haliclystiis (or something very like them) as having served 

 as a stage in the evolution. To such I commend the following 

 considerations. 



(i) The evolution of marginal bodies in a Lucernarian ah initio 

 offers difficulties already considered, and apparently insurmountable. 



(2) The evolution of these bodies directly from tentacles in ancestors 

 of Lucernarians offers difficulties no less formidable. Natural 

 Selection will not on this assumption account for the evolution of two 

 genera. Lucernaria and Haliclystus in the same seas, and in like positions 

 in those seas. If the marginal bodies are advantageous, then their 

 evolution from tentacles would be accounted for easily enough, 

 provided we had Haliclystus alone to deal with. The evolution of the 

 two side by side negatives any such hypothesis. 



We may therefore turn in another direction for the solution of our 

 problem — there is only one left ! The marginal bodies may be vestiges 

 of the tentaculocyst-rudiments of ancestral scyphistomata. 



This involves, of course, the supposition that Haliclystus is 

 descended, not merely from scyphistomata, but from free-swimming 

 medusae. 



The evolution of tentaculocysts in free-swimming medusae I have 

 explained. In that explanation is involved an explanation of the 

 evolution of their rudiments in the young. The evolution of stro- 

 bilating from non-strobilating medusan types appears to me to offer 

 no serious difficulty, and I leave this portion of my argument (which 

 seems almost obvious) to be filled in by my readers. If it should be 

 called for I will publish it. 



Given, then, that there has been evolved a type of medusa in 

 which the formation of a scyphistoma, strobilation, and the separation 

 of Ephyvce provided with rudimentary tentaculocysts have been 

 phases in the life-history, the origin of Lucernarians from such a type 

 offers no further difficulty of great magnitude. {Re the meaning of 

 " rudimentary," see footnote, p. 211.) 



On this hypothesis the community of origin or " homogeny " (as 

 Professor Lankester has well called it) of the marginal bodies and the 



