206 L. Agassiz on the Natural Relations between 



families, and of the terrestrial families. There are among 

 them such structural peculiarities as will decidedly settle the 

 question. If we set aside for a moment the few branchiate 

 fresh-water Gasteropoda, we have a large number left, which 

 are pulmonate, 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 AmpuUaria. Now 

 which of these two types rank highest will not be a matter of 

 doubt, as soon as it is remembered that Phlebenterata are 

 among branchiate Gasteropoda, and by their general struc- 

 ture rank below the others. So that we shall have the marine 

 branchiate Gasteropoda follow immediately the Heteropoda 

 to which they are more or less closely allied through the 

 Phlebenterata, and, above all, the Pulmonata. But here 

 arises a new question. This family of Gasteropoda is partly 

 fluviatile and partly terrestrial ; and we may further ask, 

 which should rank higher \ No one familiar with the forms 

 of these animals will hesitate in answering this question. 

 We need only compare the development of their tentacles, 

 their forms and position, the development of their organs of 

 sense, to be satisfied that Helices and Limax rank above 

 Planorbis and Limnsea. So that the natural gradation esta- 

 blished by their structure among the upper groups in the 

 class of Gasteropoda, agrees with their natural connection 

 with the elements in which they live in the order which I 

 have assigned to these, the types of Gasteropoda, which are 

 lowest, being exclusively marine, — the highest, equally fluvia- 

 tile and terrestrial ; and among these the fluviatile, ranking 

 immediately above the marine, and the terrestrial ranking 

 highest, and the proportion of the fluviatile in the whole class 

 being still larger than in the class of Acephala, inasmuch as 

 the structure of Gasteropoda is also in a higher degree of 

 development of Mollusca than that of Acephala, and the first 

 terrestrial type in the animal kingdom in the gradation of 

 its structure, making its appearance in the class of Gas- 

 teropoda. 



The Cephalopoda are highest among Mollusca as a class. 

 They rank so high, as to rival in the complication and de- 



