88 SECTIONAL ADDRESSES. 



been achieved by the actual torsion. The greatest novelty of Prof. Naef's 

 theory now comes in. He supposes that the earliest Gastropods and their 

 immediate predecessors, owing to the flexibility of their ' necks,' were 

 able to twist their shells round, from back to front, or vice versa, at will 

 — as freely, he adds, as a bird can turn its head. On this theory the 

 difficulty as to intermediate stages disappears. A snail that has under- 

 gone torsion has not acquired something entirely new : it has merely 

 reversed what may be called the resting attitude of its shell. The power 

 of twisting its neck was not entirely lost until the process of reversal was 

 completed. By that time, however, the snail had given up swimming 

 altogether, no longer needed the Nautilus poise, and had settled down to 

 the monotony of a creeping life. As an intermediate stage it is suggested 

 that the symmetrical shells of Cambrian snails (e.g. Bellerophon) may have 

 rested sideways, i.e. with the spire over the left side of the body and the 

 open end over the right — a position from which it would require, as it 

 were, only half a pull on the left side to bring the shell back to the Nautilus 

 position for swimming, or half a pull on the right side to bring the open 

 end forwards into the position most suited to creeping. 



For the rest I ought perhaps to add that Prof. Naef explicitly rejects 

 the theory of the Veliger which I have adopted here, viz. that it is a 

 Trochosphere transformed by the incorporation of Molluscan characters, 

 and regards the Veliger as a ' phylogenetic reminiscence ' of the pelagic 

 ancestor of Gastropoda, which was adapted for swimming like a Pteropod 

 by means of an expanded and bilobed foot. 



You thus have before you two theories in explanation of the same 

 facts, and the only tests by which you can judge between them are the 

 degree to which they conform to well-established facts, and their con- 

 sistency with the order of events revealed by a wider survey. As I have 

 put before you a rival explanation, I may perhaps point out in what 

 respects Prof. Naef's theory seems to me to be lacking in cogency : (1) We 

 know that the morphological relations, both anatomical and embryo- 

 logical, of Gastropods to Scaphopods and Bivalves are much closer than 

 to Cephalopods, and we are on sure ground when we conclude that the 

 pree-torsional ancestor of Gastropods resembled primitive Scaphopods 

 and Bivalves more closely than it resembled any Cephalopod. This is, 

 in fact, what Prof. Naef's own classification means. By this test two of 

 Prof. Naef's principal assumptions fall to the ground : the adult prse- 

 torsional Gastropod did not possess a narrow flexible ' neck,' or the special 

 muscles required by his theory, or a highly coiled Nautiloid shell. At the 

 point when Gastropods diverged from Scaphopods, and Bivalves, the 

 shell can have been little more than a flat plate. The difference between 

 the two came in with the assumption of a lateral position of the gills in 

 the Scaphopod-Bivalve line and a posterior position in the Gastropod 

 line, thus leading to a preponderating lateral growth of mantle and shell 

 in the former, and a dominating posterior growth in the latter. This 

 entailed an anteriorly directed apex of the shell in prse-torsional Gastropods, 

 as in Nautilus, though the resemblance must have been one of simple con- 

 vergence, since the separation of the whole stock of Prorhipidoglosso- 

 morpha from that of Cephalopoda had taken place at an earlier stage, 

 when the shell was presumably still flat. It is apparent from his 



