and the Elements in which they live. 377 
The third class is that of Cephalopoda, which has always been 
circumscribed within natural limits, since Foraminifera have been 
removed from it. The position which I ascribe here to Foram- 
inifera will appear very natural to those who are equally conver- 
sant with the succession of fossil types in geological periods, and 
with embryology, and who know, as we have seen it to be the 
-case also among Radiata, that the higher classes reproduce in their 
lower forms, types analogous to the lower ones. F'or the great 
number of fossil chambered shells, existing in earlier geological 
riods, is very striking when we compare those old representa- 
‘tives of the class of Cephalopoda with their condition in the pres- 
ent period of the creation, and the natural gradation and analogy 
between Bryozoa as the lowest type of Acephala, with the For- 
aminiféra as the lowest type of Gasteropoda, and the chambered 
shells of old ages as lower types of Cephalopoda will remind us 
of similar relations between Polypi as the lowest type of the ani- 
mal kingdom, the so-called Hydroid Polypi as the lowest type of 
Acalephze, and Crinoids as the lowest type of Echinoderms, which 
are strictly parallel cases in two of the great types of the animal 
kingdom. 
If we now start from these modifications in the classification of 
Mollusca which rest entirely upon anatomical and embryological 
considerations, to appreciate the relations between the three classes 
of this type, and the media in which they naturally live, we can- 
not fail to be struck with the circumstance, that all Acephala, with 
one single exception, are aquatic, as are also Cephalopoda; and that 
we have only terrestrial representatives among Gasteropoda. Next 
it must be obvious, that among Acephala we have fewer fresh 
water representatives, than among Gasteropoda, as the fresh water 
types of Acephala belong truly to two groups one of which has 
very few fresh water families, whilst among Gasteropoda we have 
quite a variety of fluviatile and terrestrial types. 
The first thing which must strike us in this type, when con- 
trasting it with the Radiata, is the circumstance of a far larger 
proportion of fresh water forms and of the introduction of a num- 
ber of terrestrial ones. This simple fact in itself would go to sus- 
tain the hint thrown out above, that a higher organization in 
the animal kingdom is better adapted to the fluviatile and terres- 
trial life, than a lower structure; as among Radiata we have not 
one single terrestrial type, and only a single fluviatile one ; whilst 
the Mollusca, the structure of which is formed upon a plan deci- 
dedly higher than that of Radiata, present already a large increase 
of fluviatile types, with the addition of very many terrestrial 
ones. But this view will at once be sustained to a most unex- 
pected extent if we consider which of the Mollusca are aquatic, 
marine, which are fluviatile, and which are terrestrial. Be- 
ginning with the Acephala, we have then, in the first place, all the 
Srconp Series, Vol. IX, No. 27.—May, 1850. 48 
