204 L. Agassiz on the Natural Belations between 



Foraminifera, 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 animal kingdom, the so-called Hydroid 

 Polypi, as the lowest type of Acalephae, 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 classifica- 

 tion 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 cannot fail to be struck 

 \tith the circumstance, that all Acephala, with one single 

 exception, are aquatic, as are also Cephalopoda ; and that 

 we have only terrestrial representatives among Gaster- 

 opoda. Next, it must be obvious, that among Acephala we 

 have fewer fresh-water representatives, than among Gaster- 

 opoda, 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 fluvi- 

 atile and terrestrial types. 



The first thing which must strike us in this type, when 

 contrasting it with the Radiata, is the circumstance of a far 

 larger proportion of fresh-water forms, and of the introduc- 

 tion of a number of terrestrial ones. This simple fact, in 

 itself, would go to sustain the hint thrown out above, that a 

 higher organization in the animal kingdom is better adapted 

 to the fluviatile and terrestrial life than a lower structure ; 

 as among Kadiata we have not one single terrestrial type, and 

 only a single fluviatile one ; whilst the Mollusca, the struc- 

 ture of which is formed upon a plan decidedly 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 unexpected ex- 

 tent, if we consider which of the Mollusca are aquatic, and 

 marine, which are fluviatile, and which are terrestrial. Be- 

 ginning with the Acephala, we have, then, in the first place, 

 all the Polypi-like Bryozoa, and Tunicata, and the compound 

 Tunicata entirely marine, with the exception of a few genera 



