14 THE ESSEX NATURALIST
A fairly early stage in this type of development can be seen
in the "keyhole limpet" (Diodora apertura) (Fig. 2)
from our rocky coasts. The body consists of the following
parts: a head with tentacles, eyes and mouth, a region called
the visceral hump where the digestive and reproductive organs
are located, and a powerful foot for movement and for firm
Fig. 2. Diodora, the key-hole limpet. Diagrams showing (a) dorsal and (b)
lateral views, a, anus: f, foot: g, gill: h, head: ka, kidney opening: kh, key-
hole : m, mantle: sh, shell: v, visceral hump. Arrows show the direction of
water currents.
attachment against wave action and the attacks of enemies. A
fold of the upper body wall forms a mantle within the cavity of
which are the gills and in the wall of which lie the kidneys and
heart. Excretory ducts and rectum also open here. Covering
the animal on this surface is a shell secreted by the mantle.
Both mantle and shell have apical apertures from which the
animal derives its common name.
The only essential differences between this animal and its
postulated ancestor (the so-called Archi-mollusk) appear to be
two. The ancestor had the mantle cavity behind and there was
no hole in the shell. We are fairly certain of this for four
reasons :—(a) three classes do not show the twisting which we
see in the gastropod, or show it only in part (viz. the Placo-
phora, the Lamellibranchiata and the Cephalopoda) ; (b) the
gastropods, which do show it, have undergone various adapta-
tions of structure in order to deal with the awkward situation
which arises when a head—with its sensory structures—occupies