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The
mollusks (also spelled molluscs) are a fairly
successful group, with maybe well over 100,000 species alive
today. Since most species have shells and live in water, they
are extremely well represented in the fossil record, and more is
probably known about their evolution than is known about any other
group.
There are several subgroups of mollusks, but only
three major ones:
- the
Gastropods,
made up mostly of
snails and
slugs.
- the
Bivalves,
a group including
clams,
scallops,
mussels, and
oysters.
- the
Cephalopods,
a group including
octopus and
squid.
Mollusks are arranged somewhat differently from
other animals, although they are bilaterally
symmetrical (sort of) and often show cephalization.
Their basic body plan consists of three parts: a visceral
mass, where most of the internal organs are; the mantle,
which wraps around the visceral mass and produces the shell
if there is one; and the foot (sometimes
referred to as the head-foot), that protrudes out of
the mantle and is modified for different uses in the different
groups.
The visceral mass is where the digestive,
reproductive, and excretory (removing metabolic wastes from
the blood) systems are. These systems can be quite
complex. Everyone reading this should thank the quirks of
evolution that, of the various types of mollusks, only the snails,
slugs, and clams have been able to adapt to fresh water
systems; if the cephalopods had easily moved upstream, it's difficult
to imagine that our fishy ancestors would have been able to outcompete them, and the evolutionary course that led to people
might never have begun.
There is usually a space called the mantle
cavity between the visceral mass and the mantle. This
cavity is often used as a breathing space, equipped with pumps to
move water past the gills, and is sometimes modified to produce a
sort of jet propulsion, as happens in the squids and mildly in
things like scallops. For bivalves, which are filter
feeders, it also houses the filters
through which food is strained out of the water.
Shells are produced by the mantle
in various forms: as
a coiled "horn," as snails have (and also a cephalopod
called a
Nautilus), a hinged pair of discs, as bivalves have,
or an internal structural rod, found in squids. Slugs and
octopus have no shells, but mollusks with external shells make up
about 99% of known species.
The foot isn't really much like any
other animal's foot: it is more a mass of muscles and nerves
that often includes the main nervous system processors.
Mollusks range from the fairly dim clams to the very intelligent
squids and octopus. The foot takes different basic shapes in
the main subgroups: in gastropods, it's a muscular mass but
flat on one side and covered with cilia,
which help the muscles push the
animals along on a carpet of secreted mucus; in
bivalves, the foot often has less of a role, but it is commonly a
tongue-shaped structure used for pushing and/or digging; in
cephalopods, the foot is subdivided into tentacles,
commonly with suckers for holding onto surfaces or
prey. The
outer "skin" of some mollusks, especially cephalopods,
may contain complicated pigment cells called
chromatophores.
These
cells, often under very precise control, are used for
camouflage (for
both
colors and
patterns), for startle displays
to frighten off potential predators, and sometimes for
communication. When schooling squid flash
moving patterns of
various colors to each other, the messages look like they are pretty
complicated, but of course we don't know what they are
"saying." |