150 METAMORPHOSES OP MAN 



divisions of the sub -kingdom. Radiata, and the dis- 

 tinction between the two seemed a very justifiable 

 one. Indeed, it had been shown, that differences 

 exist between them of a more decided character than 

 those which separate reptiles from birds ; there is an 

 entire dissimilarity both as to internal organization and 

 external conformation. 



All acalephs are free and floating, and most of 

 them are solitary. The movements of polyps (which 

 are confined to a few) are, on the contrary, of a 

 creeping nature. Almost all these animals are per- 

 manently fixed, and the great majority of them live in 

 colonies. Trembley's Hydra is a type of the latter, 

 while the Medusa3 belong to the former, and may be 

 readily known by their peculiar iimbrella, shaped 

 like a bell or a mushroom, — in some instances colour- 

 less and transparent : in others, beautifully opaline, 

 and having an appearance not unlike that of tinted 

 enamel. This umbrella is at once the body and loco- 

 motive organ of the animal ; the digestive cavities and 

 circulating canals are buried in its substance, and 

 by its rhythmic contractions the animal is propelled 

 through the water. In the centre of the concave 

 surface — in fact, in the same position as the stalk of 

 the mushroom and the tongue of the bell — is placed 

 the mouth, which is almost always surrounded by 

 various appendages. The border of the umbrella is 

 frequently furnished with cirrhi, which are occasionally 

 both long and contractile, and serve as arms or fish- 

 ing-lines, by which the prey is seized, destroyed, and 

 carried to the animal's mouth. 



The observations on the reproduction of Medusae 

 had been few and isolated when Saars and Siebold 



