102 THE LIFE OF THE MOLLUSCA 



valve, made their appearance early in the history of 

 the race. 



The last named, the Chitons, first occur in the 

 Ordovician (Priscochiton). They are, however, a 

 conservative race, and have not materially changed 

 their form since those far-off 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 Polyplacophora (Acanthochites, Amimla, Crypto- 

 chiton, and Cryptoplax, Plate IX., Fig. 3) the com- 

 ponent plates become wider and wider apart, and 

 the whole animal more vermiform, while in the 

 worm-like Aplacophora (Plate IX., Figs. 4-6) 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 exo- 

 gastric, and allowing for the Gastropod peculiarity 

 of spiral torsion, 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 of 

 shell like that of Terebra (Pate XII., Fig. 5) would 

 seem a positive disadvantage to the animal. Never- 

 theless, certain broad characteristics are observable. 



