and the Elements in which they live. 159 



and will consider their separation from Polypi and Medusae, as 

 proposed by Dr. Leuckardt, rather as a retrograde step, than as an 

 improvement upon the general classification of animals. To me 

 the type of Radiata, embracing the three classes of Echinoderms, 

 Medusse and Polypi, constitutes, in its circumscription illustrated 

 above, a most natural group of the animal kingdom, all the mem- 

 bers of which are intimately connected by a close uniformity in 

 the plan of their structure, but present a remarkable gradation 

 of their types in the manner in which this structure is developed 

 in each of their classes. And the circumstance that even in the 

 higher ones, which contain chiefly free moveable animals, we have 

 some few representatives attached permanently to the soil upon a 

 Polyp-like stalk bearing the radiated animal crown, shows further 

 the intimate connection which exists between them all. Radiata 

 consist therefore of three classes only, which in theii' natural gra- 

 dation rank as follows : Polypi lowest, next Medusae, and highest 

 Echinoderms. 



As soon as we have removed in this way all the classes or fami- 

 lies which do not sti'ictly belong to the type of Radiata, we can- 

 not 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 marme ; 

 from which we are allowed to infer, that, in the plan of the crea- 

 tion, the radiated structure is incompatible with a terrestrial mode 

 of life. We see that the lowest degree of development of the 

 whole animal kingdom is entirely marine ; and that it has been so 

 throughout all ages in the history of our globe, is shown by the 

 large numbers of Radiata found from the earliest periods through 

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

 sence of radiated animals in any of the freshwater deposits. The 

 circumstance that no single genus among Radiata contains fresh- 

 water animals, further shows that this 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 pre- 

 mature 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 shows that the connection of organic structure with 

 the ocean involves peculiar circumstances, which fresh waters by 

 no means afibrd to a similar extent. Whether this is especially 

 connected with the greater density 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 di- 



