D.— ZOOLOGY. 



In the first Cambrian Gastropods it must have arisen as a marginal notch 

 almost immediately after the beginning of the adult life, just as it develops 



Fig. 7. — Shells of Adult Zygobrauchs. 

 A, BeUerophon (Cambrian) ; B, Pleurotomaria {Silurian onwards). 



now ; while there is every indication that torsion took place, as it takes 

 place now, during the embryonic or larval, and not the adult, stage. 



Prof. Naef has indeed attempted to trace a homology between the 

 apical hole of Fissurella and that of Dentalium, which, if it could be 

 sustained, would make the slit a pree-torsional instead of a post-torsional 

 modification. He even figures the slit as a feature of the shell before, as 

 well as after, torsion in his diagrams of this process. There is of course 

 a certain correspondence in the position of these two apertures or slits, 

 since both are morphologically median and posterior. But whereas the 

 hole in Dentalium is simply a remnant of the original gap between the 

 paired mantle-flaps of the larva, that of Fissurella is formed post-torsionally 

 at the extremity of a free median outgrowth of the mantle which has no 

 representative in Dentalium or the Bivalves. Moreover, in Gastropods the 

 slit is at right angles to the main mantle-edge : in those Scaphopods which 

 possess a slit as well as a hole, this slit, like the hole itself, is merely a 

 gap between the mantle-folds themselves. These differences are quite 

 sufficient to distinguish the two holes as examples of simple convergence. 



Having now put before you all the salient facts as to the history and 

 function of this Zygobranchiate slit, I need scarcely point out to you how 

 admirably these facts serve to disentangle the elements of truth and error 

 which Haeckel so confused in his ' Biogenetic Law.' Fissurella has an 

 apical hole which develops by fusion of the lips of a transitory marginal 

 slit. Emarginula retains a marginal slit throughout life. It is a statement 

 of simple fact to say in a general way, and with regard to this character, 

 that Fissurella goes through an Emarginula-sta.ge in its development. 

 But does it follow that the Emarginula-st&ge of Fissurella represents an 

 adult ancestral condition ? On this evidence clearly not, for the adult 

 ancestral condition, ex hypothesi, is that of Emarginula itself, and 

 Emarginula when definitely adult possesses a long ' slit-band ' which is 

 completely lacking in Fissurella. Let us condense the facts of the two 

 ontogenies symbolically and assume, as is not improbable, that one haa 

 been derived from the other. If this assumption is disputed the case for 



