is about three-eighths of an inch long, one-tenth of an inch broad,
thin, and lancet-shaped. Lives, like the preceding kind, in the liver of
the sheep. Development unknown.
=Fourth Sub-Kingdom: MOLLUSCA= (MOLLUSCS).
This sub-kingdom, to which oysters, snails, cuttle-fish, etc., belong,
includes unsegmented animals devoid of internal skeleton, and with
bilaterally symmetrical (p. 16) embryo, while the adult may be much
modified, especially in snails, which are enclosed in spiral shells, and
also have their bodies partially coiled. The skin of molluscs possesses
a peculiar and characteristic covering. From a definite part of the body
a larger or smaller fold of skin (“mantle”) grows out, which encloses a
space termed the “mantle-cavity,” and also, from its function, the
“respiratory cavity.” In most molluscs this mantle secretes an external
or internal calcareous mass. In this way a “shell” is developed (Fig.
143, A, _S_), which is usually external. This is the case, for example,
with the edible, or Roman snail, and the common garden snail. In other
cases (cuttlefishes), the shell is formed inside the mantle, or it may
be replaced by a collection of calcareous granules (various slugs). In
most molluscs the chief organ of locomotion is the so-called “foot”
(Fig. 143, _F_). This is a very muscular region of the body, covered by
skin, and serving either for crawling (snails), or else for digging in
the sand, and even springing (bivalve molluscs). Its structure varies
according to its function. I mention three classes, those of
_Cephalopoda_ (Cuttlefishes), _Gastropoda_ (Snails and Slugs), and
_Lamellibranchiata_ (Bivalve Molluscs).
[Illustration:
FIG. 143.—Diagrams to explain the form of the body of a Mollusc. A, a
Bivalve Mollusc; B, a Snail (cross sections). In both; _K_, body;
_F_, foot; _S_, shell; _m_, mantle; _k_, _k_, gills.
]