124 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 
Cillery M’liicli may be found creeping on tlie rocks at low tide, has a 
jointed sliell, and looks like a wood-louse. Examination of 
the animals that live in these five types of shell shows that 
they are built on as many plans of structure, and to one or 
other of tliese plans all molluscs, except the Palaeozoic 
C(jnularida (p. 141), can easily he referred. Tlierefore the 
Mollusca are divided into the following five Classes 
I. AMPHINEURA, of which Chiton is an example, owe 
their name to the two nerve-cords that run down each side of 
the body, which is -elongated and symmetrical. The mantle 
always secretes little plates or spicules of shell-substance. 
1 hey are divided into two Orders : (a) Aplacophora, which 
have no shell other than the spicules, and therefore are not 
changes of sufficient importance to mention here. 
II. GASTROPODA, of which the snail Helix is an 
example, derive their name, meaning Belly-foot, from the 
position of the large foot beneath the stomach and viscera, 
which are contained in a hump on the animal’s hack; the 
surface and folded edges of this hum]) constitute the mantle 
that forms the shell. Thus the shell is a cone, sometimes 
short, as in the limpet (Jhtfclla), hut generally long, and 
coiled either ’in one plane as in the ram’s-horn snail 
(rianorhis), or spirally as in Helix (Fig. 68, 7). In the 
common snail it may easily he seen that the edges of the 
mantle fonn a cavity (the ])allial chamber) on the right side 
of the animal; and into this open the anus and genital duct, 
which have been brought towards the mouth end of the body 
by the curving upwards of the viscera into the hump. In 
many gastropods this twisting of the end of the gut forwards 
and to the right side has affected other organs and notably 
the nerve-cords. This affords a basis for dividing the 
Gastropoda into two Sub-Classes : — 
found fossil ; (h) Polypla- 
cophora, which liave a .sliell 
of eight larger ])ie.ces, sur- 
rounded by a flexible girdle 
formed of the mantle-edge, 
in which are usually smaller 
])lates or spicules (Fig. Gfi). 
Appearing finst in Ordovi- 
cian rocks, they have per- 
sisted till the present day, 
with increasing elaboration 
of the shell, but with no 
