OF THE HYDROMEDUS^. 423 



quite possible that neither hypothesis may be correct, and that the primitive form may 

 have been neither a sessile hydra nor a highly specialized medusa, but something mid- 

 way between; for example, a ciliated locomotor organism with simple hydra-like struct- 

 ure. But overlooking this third alternative, and deciding that the homology between 

 the hydra and the medusa can only be explained on the hypothesis that one has been 

 derived from the other, they go on to show, quite correctl}', that the view that the me- 

 dusas have been produced by the gradual specialization of medusa-buds, or medusiform 

 gonophores, leads us to a series of untenable positions, and that we are compelled to be- 

 lieve that the medusa-buds are degraded medusas. They therefore conclude that the 

 medusa 1 , must have originated as modified hydroid persons, which have become adapted 

 to a swimming life, and have assumed the function of sexual reproduction, which has at 

 the same time been lost by the sessile unmodified hydras. 



In the absence of all direct evidence, this reasoning could be termed "proof" only by 

 showing that we are compelled to accept one of the two Ivypotheses, and a very great 

 step was made towards the solution of the question when Bohm showed in 1878 (9, p. 

 153) that the primitive form may have been intermediate between medusae and hy- 

 droids, and that both these forms may have been developed from this common form in 

 two divergent directions. 



Bohm points out that the long path from the slightly specialized sessile hydra to the 

 highly complex swimming medusa is greatly shortened by the assumption that they are 

 both derived from an intermediate form; and on p. 174 he says that an additional reason 

 for the belief in such a form is to be found in the fact that in certain families, as in the 

 Eucopidae, the very simply organized medusas are so much alike that it is difficult to find 

 any distinctive specific characters, although the hydroids are often very different from 

 each other, thus proving that they have diverged more than the medusas. 



As he justly remarks, it is much more difficult to understand the origin of locomotor 

 medusas by the modification of sessile polyps or the reverse, than it is to understand the 

 origin of both from an intermediate form which has served as a basis for two lines of 

 modification; and he therefore believes that both the hydra and the medusa are de- 

 scended from a free, solitary hydra-like organism with solid tentacles and with lasso- 

 cells, which were peculiarly abundant at the tips of the tentacles. The life-history of 

 Liriope, as I have described it, furnishes us with a stage of development which is ex- 

 actly like Bohm's hypothetical form in every particular, except that its endoderm and 

 ectoderm are separated from each other by a thick gelatinous layer. The ciliated, ten- 

 taculated larvae of ^Eginopsis and ^Egineta which Metschnikoff has figured (51), and 

 the Cunina larvae shown in Plate 43 of this paper, are also free hydra-like larvae which 

 become directly converted into medusas without the intervention of a sessile "nurse" 

 stage, and without metagenesis. Bohm himself does not refer to the Trachomedusae or 

 the Narcomedusae in this connection, although he calls attention on p. 1G2 to the fact 

 that the actinula of Tubularia is an example of the persistent retention of this locomotor 

 ancestral stage. 



He says very little about the actinula larva, however, and he selects Eleutheria as the 

 best modern representative of the hypothetical ancestral form, and treats of its structure 

 at considerable length. This selection seems an unfortunate one to me, for Eleutheria 



9 



