Chap. II. LIMITS OF THE CLASS. 107 



bring forth buds from difFerent parts of their axis, in different families, and even 

 in difFerent genera ()f the same family. These buds start either from the stem 

 or from the upper part of the body, or even from the proboscis of the Hydra : 

 they gradually enlarge, and assume the appearance of Medusa?, even while still 

 connected with the H^'dra, and free themselves finally, and ))ecome independent 

 animals, undergoing but slight changes comparatively after their separation, except 

 that they grow larger, develo^J their sexual organs, and finally lay eggs, out of 

 which arise new Hydra?. The Hj-dra? themselves undergo no changes whatever 

 in consequence of this production of free Medusa? : they neither lose their tentacles 

 nor any part of their body, and continue to live for an indefinite period of time, 

 and may produce other ci'ops of free Medusa^, — although I have not traced directly 

 such a repetition of their reproduction. 



Here, as in the case of Aurelia and Cyanea, the connection of the free 

 Medusa) and the Hydra? is imquestionable, and hardly less direct in the one 

 than in the other ; for, though the Ephyra? are parts of the Ijody of Ilydrte, 

 the free Medusas of the common Hydroids are buds from Hydra?, some of which 

 differ but slightly from the Hydr£B of Avu^elia and Cyanea. If, therefore, the 

 Hydra? from which Ephyra? arise, belong to the class of Acalephs as young of 

 the highest type of Disco25hora^, surely the Hydra? born from the eggs of naked- 

 eyed Medus£B, though reproducing again the same kind of Meduste only through 

 buds, must equally belong to that class; and this the more since these Hydra? 

 themselves have already been shown to be strictly homologous to Acalephs, and 

 not to Polyps (p. 44). The doubts entertained by some naturalists respecting the 

 systematic position of the Hydroids have arisen from a belief that Hydroids were 

 Polyps, in connection with the fact, disclosed during the last twenty years, that 

 they produce free Medusa?, Avhen the following alternative seemed inevitable : either 

 must Polyps and Acalephs be united as a class, or, if considered distinct, it must 

 be acknowledged that Polyps produce Medusa?. But neither is true. Hydroids 

 are not genuine Polyps, and the true Polyps may be considered as a distinct 

 class, without forcing upon us the conclusion that they produce Medusa? ; since 

 the Polyp-like Radiates from which free Medusas arise are themselves a low type 

 of Acalephs, remarkable for the polymorphism of its representatives. And yet, 

 however great the diversity of the individuals of one and the same kind of these 

 Acalephs may he, it is easily reduced to two forms, one of which belongs to the 

 Hydra type, the other to the Medusa type. 



The genetic connection of certain Hydroids and certain free Medusa? once 

 established, it remains only to be settled what are the kinds of Acalephs which 

 should be considered as belonging to their type, among those Hydroids not known 

 to produce free Medusa? and among those Medusa? not known to originate from 



