OF THE HYDROMEDUSiE. 



417 



digestive cavity of the hydroid; in Hydractinia this outgrowth is a true proboscis or 

 manubrium, which projects into a sub-umbral, ectodermal chamber opening to the ex- 

 terior, and gives rise to a peripheral chamber which corresponds to the canal system of 

 medusae. In some species of Tu'bularia the peripheral chamber is not continuous, but is 

 divided into four radiating canals and a circular canal, and in other species the opening 

 of the umbrella is furnished with tentacles, so that we have all the characteristic struct- 

 ures of a locomotor medusa, although the medusa-buds of Tubularia are never set free, 

 and serve simply to mature the eggs and embryos. In a closely related form, Ecto- 

 pleura, the ova or spermatozoa are matured before the medusa is set free, as is the case 

 in Tubularia, but the medusae of Ectopleura are nevertheless set free, and live for some 

 time as swimming medusa', while in still other forms the reproductive elements are very 

 immature at the time the medusa is set free, and are gradually developed and ripened 

 during its swimming life. 



The series of forms is so complete that we cannot doubt that there is a genetic rela- 

 tion between them, and that they are actually steps in a process of modification; and it 

 at first seems natural to conclude that the simplest forms show us the first steps in this 

 process, and the more complex forms the later stages, and that they therefore prove that 

 the free locomotor medusa has been gradually evolved from the simple sexual organ; 

 but we must remember that the process of modification may possibly have gone in the 

 other direction, and that the simple reproductive buds of Hydractinia and Eudendrium 

 may possibly be degraded medusae which have gradually become sessile and have lost, by 

 successive slight modifications, their locomotor apparatus. 



So far as I am aware, Koch (38) was the first to point out, in 1873, that this is not 

 only possible, but that there are facts which compel us to believe that it is actually true, 

 such as the homology between a medusa and a hydroid and the fact that the medusae of 

 widely separated hydroids are fundamentally alike, while closely related species of hy- 

 droids may give rise to sexual buds which are very different from each other. For ex- 

 ample, the hydroid communities of Hydractinia and Podocoryne (Dysmorphora) are so 

 much alike that they can be distinguished only by the most careful examination, but Hy- 

 dractinia produces sessile medusa-buds without radiating canals or tentacles, while 

 Podocoryne sets free perfect locomotor medusa?. A very similar case is presented by 

 Tubularia with its sessile medusa-buds and Ectopleura with its free medusae; and the 

 medusa-buds of Tubularia are essentially like those of Hydractinia, while the free me- 

 dusa? of Ectopleura and Dysmorphora (Podocoryne) are again very much alike. If we 

 put these facts into tabular form, we shall have something like this: 



1 

 2 



Hydractinia 

 Podocoryne 



Sessile buds 

 Medusae 



Tubularia 

 Ectopleura. 



3 

 4 



1 and 2 are much more closely related to each other than to either 3 or 4, while 3 and 4 

 stand in a similar relation to each other; and, if we believe that medusa? have been pro- 

 duced by the gradual specialization of reproductive buds, we must believe that 2 has 



