and the Elements in which they live, 163 



that of Acephala, and between the reduction of the mantle upon 

 the sides of the foot which it no longer incloses in Gasteropoda, 

 and also the higher position of the gills under the margins of the 

 mantle, all peculiarities in which Naiades bear closer resemblance 

 to common Gasteropoda than any other of the Acephala. Thus 

 this class of Acephala, though chiefly marine, with a few repre- 

 sentatives of its lowest types in fresh water, would reach its 

 highest degree of development in one family, which is entirely 

 fluviatile. 



Among the Gasteropoda we have again the Foraminifera as the 

 lowest type, entirely and without exception marine ; Pteropoda, 

 which rank next, entirely and without exception marine ; Hete- 

 ropoda, which follow, equally marine ; and among the true Gas- 

 teropoda, which in their class are decidedly the highest, we find 

 first, fluviatile and then terrestrial families. And now the 

 question is, among these, what is the respective position of the 

 marine families, of the fluviatile 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 freshwater Gasteropoda, we have a large num- 

 ber left which are pulmonate, and which live in fresh water and 

 upon land, and which as a whole we may contrast wdth the bran- 

 chiate 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 structure rank be- 

 low 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 fa- 

 miliar 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, and the development of their 

 organs of sense, to be satisfied that Helices and Limax rank above 

 Planorbis and Limncea ; 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 exclu- 

 sively marine ; the highest, equally fluviatile 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 a 



