378 Agassiz on the Relations between Animals — 
Polyp-like Bryozoa, and Tunicata, and the compound Tunicata, 
entirely marine, with the exception of a few genera of fresh water 
Bryozoa. And it is very interesting to notice that fresh water ani- 
mals among Mollusca are of the lowest type of their class, as also 
was the first and only fresh-water Radiate,—showing thus that the 
types to which they belong are not adapted to rise into any of their 
higher developments into the forms best fitted for other elements. 
Next we notice the Brachiopoda which are all, without excep- 
tion, marine. Next Lamellibranchiata, mostly marine, though 
some of their types are fluviatile. So the entire class of Acephala 
is aquatic and chiefly marine, and its fluviatile types belong to its 
lowest group, and to its highest. This circumstance has raised 
the question with me, what is the proper position to assign to the 
Naiades among the Lamellibranchiata, and upon due consideration 
of their peculiar characters, and especially of the circumstance 
that their mantle is entirely open, that they have no prolonged 
syphons whilst there are such even among Ascidia, I am inclined 
to suppose that they rank highest among Lamellibranchiata and 
that Monomyarians should rank between Brachiopoda and Dimya- 
rians. The reason for assigning to Naiades this higher rank rests 
upon the homology traced between the foot of Gasteropoda and 
that of Acephala, and between the reduction of the mantle upon 
the sides of the foot which it no longer encloses in Gasteropoda, 
and also the higher position of the gills under the margins of the 
mantle, all peculiarities in which Naiades bear closer resemblance 
to common Gasteropoda, than any other of the Acephala. Thus 
this class of Acephala, though chiefly marine, with a few repre- 
sentatives of its lowest types in fresh water, would reach its 
highest degree of development in one family, which is entirely 
fluviatile. 
Among Gasteropoda we have again Foraminifera as the lowest 
type entirely and without exception marine; Pteropoda, which 
rank next, entirely and without exception marine; Heteropoda . 
which follow, equally marine; and among true Gasteropoda, whic 
in their class are decidedly the highest, we find first, fluviatile and 
then terrestrial families. And now the question is, among these, 
what is the respective position of the marine families, of the flu- 
viatile families, and of the terrestrial families. "There are among 
them such structural peculiarities as will decidedly settle the ques- 
tion. If we set aside fora moment the few branchiate fresh 
water Gasteropoda, we have a large number left which are pul- 
monate, and which live in fresh water and upon land, and which 
as a whole we may contrast with the branchiate true Gasteropoda, 
which are almost all marine, with the few exceptions of Valvata 
and Paludina and Ampullaria. Now which of these two types 
rank highest will not be a matter of doubt as soon as it is remem- 
bered that Phlebenterata are among branchiate Gasteropoda, and 
