OUBUN VATDBAL UI8T0BY SOCIETr. 159 



seated by the tentacula of Hjdra, which consists of two distinct layers, besides 

 muscular fibres, and thus, so far as structure is concerned, correspond with that 

 of the gastru-vascular canals of the Medusie, while the relation of the tentacula 

 in the one, and the gastro-vascular canals in the other, to the digestire system, 

 is also precisely similar in the two groups. The peduncle of the Meduste is 

 exactly represented, both in internal structure and external form, by the ante- 

 rior portion of the body of the Hydra. We have thus, in order to convert a 

 Hydra into a Medusn, only to suppose the external layer or dermis of the ten^ 

 taculato be extended at each side into a kind of web, uniting the tentacula to 

 each other, and the posterior elongation of the body with its adhesive disk to 

 become suppressed, neither of which conditions involves any essential modifica- 

 tion of structure. The hydroid polypes, generally, are fixed animals, incapable 

 of locomotion — in Hydra an imperfect locomotion is permitted, and the creature 

 can detach itself from the bodies to which it adheres, and slowly creep about by 

 means of its tentacula and adhesive disk ; while in the Medusae the locomotive 

 powers are carried to their maximum, and the tentacula, which in the polypes 

 are free, are united by a membrane in the Medusa), and thus constitute the large 

 locomotive disk by which these beautiful animals swim with rapidity and ease 

 through the surrounding water. 



