Animals and the Elements in which they Live. 205 



of fresh- water Bryozoa. And it is very interesting to notice 

 that fresh-water animals among Mollusca are of the lowest 

 type of their class, as also was the first and only fresh -water 

 Radiata, — shewing thus that the types to which they belong 

 are not adapted to rise into any of their higher develop- 

 ments into the forms best fitted for other elements. 



Next we notice the Brachiopoda, which are all, without 

 exception, 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 cir- 

 cumstance has raised the question with me, what is the pro- 

 per position to assign to the Naiades among the Lamelli- 

 branchiata, and upon due consideration of their peculiar 

 characters, and especially of the circumstance that their 

 mantle is entirely open, that they have no prolonged sy- 

 phons, whilst there are such even among Ascidia, I am 

 inclined to suppose that they rank highest among Lamelli- 

 branchiata, and that Monomyarians should rank between 

 Brachiopoda and Dimyarians. The reason for assigning to 

 Naiades this higher rank rests upon the homology traced 

 between the foot of Gasteropoda and that of Acephala, and 

 between the reduction of the mantle upon the sides of the 

 foot which it no longer encloses 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 

 representatives of its lowest types in fresh water, would 

 reach its highest degree of development in one family, which 

 is entirely fluviatile. 



Among Gasteropoda we have again Foraminifera as 

 the lowest type, entirely and without exception marine ; 

 Pteropoda, which rank next, entirely and without exception 

 marine ; Heteropoda, which follow, equally marine ; and 

 among true Gasteropoda, which in their class are decidedly 

 the highest, we find first fluviatile and then terrestrial fami- 

 lies ; and now the question is, among these, what is the re- 

 spective position of the marine families, of the fluviatile 



