92 SECTIONAL ADDRESSES. 



tracts, and there can be no question as to which of these methods is 

 primitive. Not only is the respiratory mechanism of Nautilus more 

 advanced than that of any Gastropod, but its use as a locomotive device is 

 also secondary, being but a further elaboration of the breathing movements. 

 The inferences we drew from the shells of Mollusca are thus confirmed by 

 a consideration of the gill-chambers. It is not Nautilus but Chiton that 

 shows us most nearly the form of Gastropod (and indeed Conchiferan) 

 ancestors. In Chiton the gill-chamber is hardly established as such : the 

 groove between mantle and foot is open at all points, being merely a 

 little deeper behind than in front. Water flows in at the sides, bathes 

 the gills hanging from the roof, and escapes behind where the anus lies 

 between the symmetrical pores of the kidneys, everywhere overhung by 

 the projecting mantle-frill. 



And now suddenly these excellent sanitary arrangements have been 

 turned round from back to front through the efforts of a pree-Veliger to 

 get its head into the hole before its foot (the Lamarckism may be excused !), 

 and by the persistence of the larval adaptation. Before the torsion the 

 gill-chamber opened freely behind ; after the torsion the free exit of water 

 and waste products was impeded by the snail's head and neck. There 

 were two possible solutions of this dilemma : either the snail must die, 

 and so wipe out the larval mutation altogether — in which event, had it 

 happened, there would have been no order of Gastropoda to perplex the 

 Zoological student — or a new exit must be provided. The latter alternative 

 was followed. Every member of the most primitive section of Gastropods 

 to-day, i.e. every snail which possesses paired gills and auricles (Zygobran- 

 chia), has in one form or another a slit or a hole piercing mantle and shell 

 where these overhang the gill-chamber in front. It is not present in the 

 larva ; it does not appear until the larva has settled down to its permanent 

 life on the bottom. It is an insignificant notch in the simpler cases, and 

 yet it is to the development of this ' breathing hole ' that the survival of 

 the whole order of Gastropoda must be ascribed. Bearing in mind the 

 direction of the ciliary currents in Chiton — in at the sides, and out at the 

 middle — we must picture the young snail with these arrangements, but 

 reversed now from back to front, and at the outset of its life on the bottom, 

 possibly with one of Dr. Bidder's Torridonian tides swirling over it. Also 

 we must recognise the effects of continuing growth and differentiation — 

 increase of size and gill-surface, multiplication of muscles between shell 

 above and foot-surface beneath, contractions of these muscles pulling the 

 shell down on to the animal's neck, increasing metabolism and output 

 from rectum and kidneys — all demanding increased ciliary activity, and 

 greater outpouring of waste water beneath the middle point of mantle 

 and shell. If I could start again as an Experimental Biologist I would 

 greatly like to try the effect on a growing epithelium of a continued stream 

 of deoxygenated water charged with a suitable quantity of metabolic 

 waste. Would it or would it not inhibit growth at the point affected ? 

 In any case, whatever the chain of cause and effect, that is what happens 

 now in the development of every young Zygobranch at the outset of its 

 adult life. Beginning with the intact edge of the larval mantle and shell, 

 the mantle grows less and less freely at the middle point where the waste 

 water is poured out and grows freely everywhere else, with the resultant 



