D.— Z00L0C4Y. 91 



first appeared in a Veliger, that is merely to say that one larva went 

 wrong, whereas most larvae behave properly. Let the Gastropod go into 

 the same pen with Darwin's Niata cattle, and its veliger with the abnormal 

 embryo that produced the La Plata race — what then ? Stands not 

 ScotLmd where it did ? 



Yon will note, however, that Gastropods form no inconsiderable section 

 of Mollusca, and that MoUusca constitute one of the nine large phyla into 

 which the animal kingdom is di^'ided. A few years ago I brought a case 

 very similar to this, but without any touch of abnormality about it, to the 

 notice of the Linnean Society, and claimed that the carapace of Crustacea 

 was also in the first instance a larval adaptation in some primitive Trilobite. 

 If that case holds too, as I firmly believe, and as I hope before long to 

 establish in full detail, the whole phylum of Crustacea must be added to 

 Gastropoda and the Niata cattle. Last year, at Leeds, I put forward some 

 new grounds, now published with fuller details, for the conclusion that 

 Appendicularians are not primitive Tunicates, to be acknowledged by 

 Ascidian tadpoles as their ancestors, but Doliolids gone astray in their 

 development. The larval form in this case has ousted the adult from its 

 supremacy in the life-history and has created a free-swimming pelagic 

 creature out of originally sessile ancestors. 



In short the Gastropod and its Veliger loom large in this address, not 

 as ends in themselves, but as an additional example of a wide-ranging 

 phenomenon. The man in the street scoffs at the idleness of the question 

 ' Did the hen come first, or the egg 1 ' He thinks it one of Nature's 

 insoluble mysteries, but admits the priority of the egg when it hatches 

 into a monster with two heads or three legs. In a sense I have looked 

 around for a convenient monster, have found it in the snail, and now seek 

 to show that ' the exception proves the rule.' I stick to snails because 

 we are dealing with a problem which requires a certain amount of concen- 

 tration, and one point assists another. 



We left the ancestral Veliger creeping on its rock and growing up into 

 the first Gastropod, and we are to ask if the new position of its visceral 

 dome, twisted round through half a circle, was not attended by some 

 inconveniences. Before the torsion the gill-chamber lay behind, as in 

 Chiton and Nautilus. It was a more definite chamber than in Chiton, 

 but not so big as in Nautilus, and its ca^^ty opened downwards behind 

 the foot, not forwards as in Cephalopods, because its wails were not 

 specialised to propel the animal backwards through the water. This is 

 where bionomics comes in to help morphology. When Prof. Naef treats 

 the ancestor of Gastropods as a kind of Nautilus, he is putting the cart 

 before the horse, or, more exactly, the specialised condition before the 

 unspecialised, the higher before the lower. Before the mantle-cavity of 

 Nautilus was used as a locomotive organ, it must have been what it still 

 is in Gastropods, a simple shelter for the gills, and a passage for the 

 products of anus, kidneys, and gonads. This curious combination of 

 cloaca and respiratory chamber, easily explained by its evolution from 

 the condition seen in Chiton, implies arrangements for maintaining a 

 through circulation of water, as well as for preventing contamination of 

 the respiratory water by waste products. In Cephalopods the respiratory 

 current is maintained by muscular pulsations, in Gastropods by ciliated 



