I
will not expand too much the description of this 4 classes because
the seashells world is huge and I want to focus in gastropods. But it
is good to have a general idea of all the classes of the Philum
Mollusca that are related with seashells.
Class
Polyplacophora
The
members of the class Poliylacophora are primitive mollusks known as
chitons. The chitons have elongated, flattened, bilaterally
symmetrical bodies covered by a shell of eight overlapping transverse
plates or valves that are surrounded by a muscular band called
girdle. Their foot is long and muscular and has a long mantle cavity
on both sides that contains from 6 to 88 pairs of gills. The head of
chitons is reduced lacking eyes and tentacles. Light-sensing cells,
that are exclusive to this class, pass trough tiny canals in the
shell plates. Today exists about 1000 species of chitons and their
size range from 3 mm to 40 cm. All
members of this class live in the ocean, normally on rocks and hard
bottoms in shallow waters of tropical and temperated regions where
they feed on algae and sponges.
Class
Monoplacophora
Also
known as gastroverms are relatively small, ovate, bilaterally
symmetrical mollusks that have a single conical, limpetlike shell
with eight pairs of serial repeated muscle scars. Until 1957 they
were thought to be extinct, but after this it was discovered thirty
living species, almost of all from deep sea habitats. All of the
gastroverms feed on organic matter and on small animals in the
sediment and they inhabit muddy, rocky, or gravelly bottoms.
Class
Scaphopoda
This
class is also known as scaphopods or tusk shells, because the shells
resemble elephant tusks. This class comprises about 600 living
species and all of them inhabit the oceans at shall to abyssal
depths. They have tall, bilaterally symmetrical bodies contained in a
long, curved, tapering tubular shell that is open at both ends. Some
smaller spicemens are broader in the middle than at the ends and the
size of this animals range from 3 mm to 15 cm. Tusk shells can be
polished and smooth or have longitudinal ribs. This animals don't
have eyes or gills and they burrow in soft bottoms using a foot that
emerges from the larger opening (anterior). Tusk shells feed on
microscopic organisms in the sediment, which they capture with thin
tentacles called captacula. This type of shells are rarely
encountered alive.
Class
Cephalopoda
The
earliest cephalopods had external shells, with interconnected
chambers that allowed them to become gas-filled and buoyant. During
their evolution, the vast majority of cephalopods have lost their
external shell. Some have an internal shell that has been reduce,
like is the case of squids and sepias. Others have lost completely
any kind of shell, like octopus. Some cephalopods developed the
ability to swim by undulating their fins, as well as by jet
propulsion.
Of
the approximately 900 species of cephalopods living today , only six,
all belonging to the primitive genus Nautilus have kept the external
shell. These animals only occupy the last chamber of the bilaterally
symmetrical shell, the rest of the shell has chambers filled with gas
that ables the animal to control their buoyancy by regulating the gas
flow into and out of the chambers. Nautilus spices inhabits the deep
waters along coral reefs during the day and rise to shallower waters
at night to feed.
Cephalopods
inhabits all oceans at all depths. Their size can range from 25 mm to
more than 14 meters in length, and include both the Giant Squid and
the even larger Colossal Squid, the largest known invertebrate. All
cephalopods are predatory, with the head and mouth surrounded by
muscular tentacles that capture the prey, which is then eaten by a
parrotlike beak and radular teeth.
