16 W. K. BROOKS ON THE 



Theoretical Discussion of tee Observations. 



In a general view of the facts which have been detailed, the most prominent and 

 conspicuous feature seems to be the remarkable directness witli which the embryo 

 develops into the adult, and the total absence of anything like a metamorphosis. 

 Everything which does not contribute to the formation of the adult animal has been 

 dropped out of its life history. With the exception of the velum and the eye-stalks, there 

 are no larval or rudimentary organs, and the whole process of development is wonderfully 

 direct. 



When we bear in mind that the Cephalopoda are almost the most highly specialized 

 of Invertebrates, and that they must have had a long and complicated ^^hylogenetic 

 history, I think we must acknowledge that the embiyonic record has been simplified to a 

 degree which is without a parallel in the animal kingdom, and it is hardly too much to 

 say that the ontogenetic process furnishes us with no knowledge whatever of the 

 phylogeny of the group. 



The method of formation of the shell-area and of the shell ; the mode of origin of the 

 mantle and of the mantle-cavity, and the form and position of the gills of the Cephalopod 

 embryo are all of them features which show closer relationship to a typical Gasteropod 

 than could be inferred from the condition of these organs in the adult. They thus show 

 the affinity of these groups to each other, and help us to a clearer understanding of the 

 homology between the organs of the Cephalopod, and those of a typical Mollusc, but this 

 is about all. I do not see that they furnish any basis whatever for speculation upon the 

 origin of the Cephalopod, or give us the least information as to the manner in which its 

 peculiar specializations of structure have been brought about. 



We, undoubtedly, have an ancestral feature in the embrj^onic condition of the siphon, 

 at the time when the siphon folds are separated from each other upon the median line, 

 but in the present state of our knowledge it furnishes a very scanty basis for generali- 

 zation. It is true that there is a resemblance l>etween the adult condition of this organ in 

 the Tetrabranch and its embryonic condition in the Dibranch, and the evidence therefore 

 shows that in this respect, the former group is more embryonic, or less specialized, than 

 the latter, but it certainly is not sufficient to warrant the conclusion that the one group 

 has been derived from the other, rather than from a common ancestral form. 



I think that this is true of the development of the eye as well as of that of the siphon. 

 The history of this organ shows, as Grenacher has pointed out, that it is essentially 

 similar to the ordinary molluscan eye, and that it is less specialized in the Tetrabranch than 

 it is in the Dibranch, but the resemblance is not such as to indicate that the Tetrabranchs 

 are the ancestors of the Dibranchs. 



It is just such an embryonic resemblance as we should expect to find, if both are the 

 descendants of an unspecialized form, and while it is not inconsistent with the idea that this 

 common ancestry is very remote indeed, neither would it conflict with the view that the 

 divergence of the two groups was comparatively recent. 



The arms of the Dibranch embryo are hardly, if at all, more like those of a Tetra- 

 branch, than those of the adult, and although the shell is at first external, it is, in this 

 respect, no more like the Nautilus shell than any other molluscan shell is. 



