162 M. Agassiz on the Relations between Animals 



we cannot fail to be struck with the circumstance, that all Ace- 

 phala, with one single exception, are aquatic, as are also Cepha- 

 lopoda, and that we have only terrestrial representatives among 

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

 have fewer freshwater representatives than among Gasteropoda, 

 as the freshwater types of Acephala belong truly to two groups, 

 one of which has very few freshwater 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 freshwater forms and of the introduction of a num- 

 ber 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 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 de- 

 cidedly higher than that of the Radiata, present already a large 

 increase of fluviatile types, with the addition of very many ter- 

 restrial ones. But this view will at once be sustained to a most 

 unexpected extent if we consider which of the Mollusca are 

 aquatic and marine, which are fluviatile, and which are terres- 

 trial. Beginning with the Acephala, we have then, in the first 

 place, all the Polyp-like Bryozoa and Tunicata, and the compound 

 Tunicata entirely marine, with the exception of a few genera of 

 freshwater Bryozoa. And it is very interesting to notice that 

 freshwater animals among Mollusca are of the lowest type of 

 their class, as also was the first and only freshwater 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 considera- 

 tion 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 Dimy- 

 arians. The reason for assigning to Naiades this higher rank, rests 

 upon the homology traced between the foot of Gasteropoda and 



