and the Elements in which they live. 375 
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 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 absence 
of radiated animals in any of the fresh water deposits. The cir- 
cumstance that no single genus among Radiata contains fresh wa- 
ter 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 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, un- 
doubtedly shows 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 density of the medium or not, lam 
not fully prepared to say, though [ 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, Meduse 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 
ismembered and entirely separated within a few minutes. 
_ No one of the three other great types of the animal kingdom 
Is either so exclusively marine, or even so exclusively aquatic as 
that of Radiata. For among Mollusca we have quite a number of 
terrestrial genera, and even a large number of fresh water genera 
and families. 
Among Articulata we notice also large numbers of fresh water 
ation be 
live, and whether the gradation of this structure has any refer- 
ence to the surrounding media as it unquestionably has among 
iata. } 
_ Let us first consider Mollusca, and perhaps revise their classes 
in a zoological point of view before undertaking the investigation 
