WOODWARD : INAUGURAL ADDRESS. 247 



exhibited in the Vertebrate kingdom ; the very plasticity of the 

 Mollusca has thwarted progressive development, as we understand the 

 phrase, and they readily retrograde or branch off into bye-paths. 



Hence the study of evolution in this group is an exceedingly 

 complex one, offering, like a very tangled skein, so many clues to 

 follow out that one is in doubt which thread to pursue first. 



On the present occasion it will suffice to take certain leading 

 features and organs, to summarize what is known concerning them, 

 and to endeavour to ascertain how far, if at all, any definite con- 

 clusions can be based upon them. 



It may fairly be conceded that the tidal zone was in all probability 

 the cradle of the race, and that thence the various members gradually 

 betook themselves, mostly to deeper and deeper water on the one 

 hand, but also, though perhaps more tentatively and gradually, to 

 fluviatile and terrestrial conditions on the other. 



Now the first requirement of a soft-bodied animal, and especially of 

 one considered by its fellows to be good eating, is protection. In 

 early days, however, enemies were far fewer than now, and it was 

 rather from the force of the elements that preservation was needed. 



This first requirement is supplied by the shell, and all three types, 

 univalve, bivalve, and multivalve, make their appearance early in the 

 history of the race. 



The last named, the Chitons, first occur in the Ordovician [Prisco- 

 chiton). They are, however, a conservative race, and have not 

 materially changed their forai since those far-ofi" days. Still, taking 

 the Amphineura as a whole, the class shows a desire to disburden 

 itself of its coat of-mail. Through the successive genera of one branch 

 of the PolypUicophora {Acajithochites, Amicula, Cryptochiton, and 

 Cryptoplax) the component plates become wider and wider apart, 

 and the whole animal more vermiform, while in the worm-like 

 Aplacophora the shell has disappeared, though numerous calcareous 

 spicules remain scattered over the mantle. 



As regards the Gastropoda, when it is borne in mind that the 

 embryonic shell is nautiloid and exogastric (and allowing for the 

 gastropod peculiarity of spiral torsion, which is a deep-seated 

 phenomenon, foreshadowed early in the cleavage of the egg-cell), the 

 number and variety of forms assumed in the adult state is remarkable. 

 Seeing that departures from this embryonic and therefore primitive 

 type are pronounced, even in the earliest known gastropods, it is not 

 possible to say how far environment or other forces may have played 

 part in their development. Certain elongate forms like Terehra would 

 seem a positive disadvantage to the animal, and still more so to the 

 Hermit-crab, who, with mistaken notions of levity, occupies an empty 

 example. 



Nevertheless, certain broad characteristics are observable. Primarily 

 among the inhabitants of a rough foreshore the massive strength of 

 the shell is noticeable, the object being, of course, to withstand the 

 battering action of the waves and hard substances like stones cast up 

 by them. 



