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, Amicula, 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 Tevebva (Pate XII., Fig. 5) would 
seem a positive disadvantage to the animal. Never- 
theless, certain broad characteristics are observable. 
