164 M. Agassiz on the Relations between Animals 



higher degree of development of Mollusca than that of Acephala, 

 and the first terrestrial type in the animal kingdom in the gra- 

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

 teropoda. 



The Cephalopoda are highest among the Mollusca as a class. 

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

 of their structure, even some of the Vertebrata ; and strange to 

 say, we have among them only marine types, not a single fiuviatile 

 representative, nor a single terrestrial one. This fact would at 

 first seem to be in direct contradiction with the statements made 

 before, if it were not for the circumstance that this class in itself, 

 as represented in our days, does not seem altogether reduced in 

 comparison with the other two, if we could not be satisfied that 

 its perfect period of development were the former geological ages 

 when its numbers were far greater than at present, a circumstance 

 which places the whole class in peculiar relations to its type, 

 which must be rather appreciated under the point of view of the 

 conditions which prevailed in former ages, when the ocean co- 

 vered more extensively the whole surface of the globe than at 

 present ; so that the type with its high organization must be 

 considered more with reference to its development in former ages, 

 than to what it is now, as at present the class is proportionally 

 reduced ; and it is well known, and it will be further mentioned 

 with reference to other types, that in earlier periods, however high 

 animals might have ranked by their structure, they were all ma- 

 rine, as we know fishes to have been the only representatives of 

 Vertebrata in earlier periods. 



At this stage of the investigation, a comparison between Mol- 

 lusca and Radiata shows, that, though the former advance further 

 in their fluviatile development, and even reach with some few of 

 their types a terrestrial mode of existence, there is not yet a sin- 

 gle family among them which is entirely terrestrial, nor a single 

 class which is either entirely fiuviatile or terrestrial, this connec- 

 tion with the higher conditions of existence being only introduced 

 among some few of their representatives, which we are allowed 

 from other data to consider as the highest in their respective 

 groups. 



If we now pass to the great group of Articulata and begin as 

 before by revising their zoological arrangements as based upon 

 anatomical and embryonic data, we shall have at the outset to 

 settle the limits of their classes and their relative positions. 



The first point which we have here to investigate, is the ques- 

 tion whether the Articulata in the widest extension of this group 

 constitute one single natural type, or whether they should be sub- 

 divided into two equivalent groups, as has been proposed by those 

 who would restore the division of worms, in its widest sense, as a 

 great division equal in zoological importance to the type of Mol- 



