OF THE IIYDROMEDUSJ3. 421 



in the nutritive hydra. It is true that the hydranths have, as the theory requires, no re- 

 productive function, but this is no more than we should expect, if the hydra is a medusa 

 larva. 



Balfour says that rr it would obviously be advantageous for the species that the de- 

 tached buds with generative organs should be locomotive, so as to distribute the species 

 as widely as possible, and such buds in connection with their free existence would natu- 

 rally acquire a higher organization than the attached trophosomcs." It seems at first 

 sight as if this must be true, but more careful examination will give us many reasons for 

 questioning whether the high organization of the medusa has been acquired for the pur- 

 pose of distributing the species, rather than for the benefit of the individual. We know 

 that, in many species, of all the great groups of hydroids, the medusa; have become de- 

 graded into sessile gonophores which have lost their locomotor power, and in many 

 cases all their complicated organization as well. This degradation must be for the ad- 

 vantage of the species, and in view of its prevalence I think we must hesitate to believe 

 that the production of free reproductive zooids would be for the good of the species, and 

 that after such free zooids were produced, they might be expected to acquire a compli- 

 cated organization and highly specialized locomotor and sensory organs. "We know 

 that changes in the opposite direction have been to the advantage of the species, since 

 they have been preserved, and if sessile gonophores are so useful that free medusa' have 

 been degraded into sessile gonophores, there is no a priori reason for believing that it 

 would be to the advantage of the species for reproductive zooids to become locomotor. 

 The distribution of the species is well provided for in the swimming planula and the hab- 

 its of the medusa often carry it very far from any proper habitat for the hydra, and as a 

 matter of fact, genera and species without free medusa' are as widely distributed as 

 those in which the medusa is a perfect swimming organism. Eudendrium and Cordy- 

 lophora have no means of dispersal except the cilia of the planula, yet Cordylophora is 

 found on both sides of the Atlantic and from Boston to Baltimore, and Eudendrium is 

 found all over the world. Turritopsis is an extremely active medusa, living in the open 

 sea. and it is often swept by the gulf stream as far north as Cape Cod, yet its hydra has 

 been found nowhere except upon the North Carolina coast. 



I think that we may safely conclude that while the view that the complex structure of 

 the medusa has been acquired as a means for distributing- the species seems at first sight 

 to be very plausible, more careful examination renders it probable that this is not the 

 case, but that the purpose of the organization of the medusa is to enable it to live out its 

 own life; that it has been acquired and preserved on account of its direct benefit, rather 

 than from any indirect advantage to the species as a whole. 



In 1871 Koch advanced an hypothesis which escapes this difficulty, since he believes 

 that the medusa stage has been acquired to prevent self-fertilization rather than to secure 

 the distribution of the species, and his hypothesis is therefore more satisfactory than 

 those which have been noticed. 



He says (38) that the ancestral form was a hydra, with solid scattered tentacles, re- 

 producing both sexually and asexually, and that in some species the new buds were set 

 free as in Hydra, while in others they remained attached and formed communities. 



