272 Professor Owen on Metamorphosis and Metagenesis. 



proves to be an example of a condition of procreation to which 

 the greater part of organised Nature is subject. 



The Lecturer was inevitably limited in his choice of illus- 

 trations ; and proceeded to an instance of metagenesis from 

 the radiated sub-kingdom of animals. 



The stages of this metagenesis have been best and most 

 completely traced in the Medusa aurita, by Siebold, Daly ell, 

 Sars, and others. 



The first step was made by Siebold, who, in 1839, traced 

 the development of the Medusa aurita from the egg to a stage 

 resembling a ciliated monad, then to a lobed rotifer, and next 

 to a long-armed polype. 



This polype stage of the Medusa had been previously re- 

 cognised in 1788, but without a suspicion of its true nature, 

 by 0. F. Miiller, who called it Hydra gelatinosa. 



It was next observed, and its habits more fully described, 

 by Sir John Dalyell, in 1834, as Hydra tuba : and in 1836 

 he made known the singular metamorphoses into forms which 

 Sars had previously described as Scyphistoma and Strobila ; 

 and Dalyell saw the spontaneous division of the latter into a 

 pile or series of small Medusae. All the stages of the meta- 

 genesis were independently noted by Sars who described 

 them in 1841. 



The difficulty of accounting for the presence of Entozoa in 

 the interior parts of animal bodies is rapidly disappearing as 

 the knowledge of their course of development advances. 



The principal stages of this development were described 

 in a small worm (^Monostoma mutabile), parasitic in the air- 

 cells, intestines and peritoneal cavity of many water-fowl. 



The ovum is converted into a ciliated monadiform embryo ; 

 which escapes from the bird, and swims about freely in the 

 water. A clear mass may be discerned in the interior which 

 exhibits independent movements. This body is liberated, 

 grows rapidly, and generates in its interior a number of in- 

 dependent organisms provided with a cephalic spiculm and 

 a caudal appendage, referable by their form to the genus 

 Cercaria. They are very active and insinuating, could even 

 bore through the skin by the sharp needle-like armature of 



