CLASSIFICATION 41 
way, and the shell is principally held in place and 
protected on the exterior by the anterior pair of 
arms, which are furnished with web-like expansions 
for the purpose. The ends of the remaining arms, 
which are carried folded back over the animal’s body, 
are tucked into the shell, and seemingly also assist 
in retaining it by the aid of the suckers. It is 
commonly stated that the shell is entirely the pro- 
duct of the pair of webbed arms, and that once the 
animal] quits its tenement it cannot re-occupy it; but 
both statements are incorrect. The latter has been 
shown by actual observation to be the reverse of 
fact, while the occurrence of fractured shells, showing 
evidences of repair from the inner side, upsets the 
former statement, which is otherwise wanting in 
probability. The more reasonable supposition is 
that the shell is mainly secreted by the mantle cover- 
ing the visceral dome, and that the webbed arms, 
which are also furnished with secretory cells, con- 
tribute to the exterior and assist in moulding it, 
during formation, to the animal’s body. 
The Cephalopoda are divided into— 
Order I.: TETRABRANCHIA (Plate XXII.), or those 
having four gills and an external shell. The funnel 
is in two parts, and the eyes are open, having no 
crystalline lens. 
Sub-Order 1: NAUTILOIDEA, or the Nautilus and 
its allies. 
Sub-Order 2: AMMONOIDEA, or the extinct group 
