Animals and the Elements in which they Live. 201 



we cannot fail to perceive at once that all the remaining 

 animals which must be considered as truly radiate are not 

 only all aquatic, but, with a single exception of the genus 

 Hydra, all strictly marine ; from which we are allowed to 

 infer that, in the plan of the creation, the radiated structure 

 is incompatible with a terrestrial mode of life. We see that 

 the lowest degree of development of the whole animal king- 

 dom is entirely marine ; and that it has been so throughout 

 all ages in the history of our globe, is shewn by the large 

 numbers of Radiata found from the earliest period through 

 all geological epochs up to the most recent, and the entire 

 absence of radiated animals in any of the fresh-water de- 

 posits. The circumstance that no single genus among 

 Radiata contains fresh-water animals, further shews that 

 thia type in its main features is not better adapted for a 

 fluviatile existence ; or, we may say in other words, that the 

 plan involved in the structure of radiated animals is chiefly 

 adapted to the sea. We might, perhaps, even say, if, in this 

 stage of the investigation, it would not seem premature to go 

 so far, that the lower types of animals are not only entirely 

 aquatic, but exclusively marine. The fact of so large a 

 number of aquatic animals as Radiata being so exclusively 

 marine, undoubtedly shews that the connection of organic 

 structure with the ocean, involves peculiar circumstances, 

 which fresh waters by no means afford to a similar extent. 

 Whether this is especially connected with the greater den- 

 sity of the medium or not, I am not fully prepared to say, 

 though I am inclined to believe that it is so, from the 

 circumstance that Radiata are so constantly killed by the 

 contact of fresh water, as I have ascertained by direct ex- 

 periment upon Polypi, Medusae, and Echinoderms, some of 

 which are struck with almost instantaneous death, when 

 brought into fresh water, and decompose with astonishing 

 rapidity. I have seen on dropping an Ophiura into fresh 

 water, all the articulations dismembered and entirely separ- 

 ated within a few minutes. 



No one of the three other great types of the animal king- 

 dom is either so exclusively marine, or even so exclusively 

 aquatic as that of Radiata. For among Mollusca we have 



