202 L. Agassiz on the Natural Relations between 



quite a number of terrestrial genera, and even a large num- 

 ber of fresh-water genera and families. 



Among Articulata we notice also large numbers of fresh- 

 water species, and a still larger number of terrestrial forms. 

 Finally, among Vertebrata we find the most promiscuous 

 occurrence of marine, fresh-water, and terrestrial forms. It 

 is now important to ascertain whether we may trace, beyond 

 Radiata, a direct relation between structure and the element 

 in which animals live, and whether the gradation of this 

 structure has any reference to the surrounding media as it 

 unquestionably has among Uadiata. 



2. General View ofMollusca. — Let us first consider Mollusca, 

 and perhaps revise their classes in a zoological point of view, 

 before undertaking the investigation of their relations to the 

 media in which they dwell, allowing in this revision a due in- 

 fluence to embryology, as far as it can influence this question 

 at present. 



The number of classes which should be admitted among 

 Mollusca, is the first point of importance we have to con- 

 sider. Since the Barnacles or Cirripedia, which Cuvier 

 still considered as a class among Mollusca, are now known 

 to belong to the type of Articulata, and to be most conve- 

 niently combined with Crustacea, we have five classes of 

 Mollusca left, if we follow Cuvier's arrangement of these 

 animals, as he distinguishes Cephalopoda, Pteropoda (Clio), 

 Gasteropoda (Univalve), Acephala (Bivalve), and Brachio- 

 poda (Terebratula), as so many distinct classes of the type 

 of Mollusca, in the order of gradation just mentioned. It 

 will hardly be necessary at present to insist upon the close 

 relation which exists between Brachiopoda and the other bi- 

 valve shells. Indeed, anatomical investigations of these 

 animals have shewn that they are not only constructed upon 

 the same plan, but that the difi'erence between Brachiopoda 

 and ordinary Acephala, is scarcely as great as the diff'er- 

 ences which exist between Ascidia and Lamellibranchiate 

 Acephala, (as Ostrea) which Cuvier, nevertheless, placed in 

 one and the same class. We shall therefore consider Tuni- 

 cata, Brachiopoda, and Diphyra, as one great natural class, 

 under the name of Acephala, to which we also refer, as men- 

 tioned above, the type of Bryozoa which has been so long 



