Maybe you think that snails, clams, mussels, squid, and
Octopods are very different. Yet, they are all in the
same category of animals known as mollusks and are
structurally similar. Mollusks are some of the most well
known of invertebrate sea creatures (there are over
50,000 species). Some are very rare and are only found
in very deep-water.
Mollusks have three body regions.:
The head contains the “brain” and sense organs
The “visceral mass” contains the internal organs
The “foot” is the muscular part of the body
Mollusks usually, but not always, have a shell, which is
secreted by a body wall called the mantle. Many mollusks
have a tongue of sorts, called a radula. The radula is
rough like sandpaper. Mollusks have well developed body
organs that are used in the respiratory, circulatory and
nervous systems.
The Stomach-Foot
The stomach-foots (class Gastropoda) contains about 70%
of all mollusk species. The stomach-foots include
snails, limpets and abalones, which have shells. Slugs
and nudibranchs are also stomach-foots, but do not have
shells. A few stomach-foots are found on land. Some
Examples below:-
Elongate
tubercles projecting on dorsum. Pattern of
polygonal rings on dorsum either white or pink.
Cymatium: common name the hairy triton, is a
species of medium-sized predatory sea snail, a
marine gastropod mollusc in the family
Ranellidae, the tritons.
The Many-Plated or Chitons
The many-plated or chitons (class Polyplacophora) have
eight plates and look like pill-bugs (but pill-bugs are
not chitons). Chitons crawl along rocks looking for
food (usually algae). A chiton uses its radula (tongue)
to scrape algae off rocks. It also has very hard teeth
that are also used to scrape algae of rocks. These teeth
are hard enough to etch glass! Embedded within their
shells are primitive "eyes" that can detect light.
Chitons are very, very slow moving. During a year, a
chiton may move only ten feet!
How Are Pearls Made?
Pearls are made by most bivalves (not just oysters) and
even some snails (such at the conch). When an irritant,
such as a grain of sand, becomes embedded in the mantle
of a bivalve, the creatures coats the irritant with the
same material used to produce the lining of its shell.
This makes the irritant smooth and less painful to
bivalve. Over time, the irritant gets covered with more
of the material, making a pearl!
The Bivalves
The bivalves (class Bivalvia) are very well known. They
include clams, mussels, oysters and scallops. All
bivalves have two shells (the name means "two shells"),
and there are about 15,000 species. Most bivalves are
marine, but about 20% are found in fresh waters. Most
bivalves do not have radula because they eat by
filtering water through their gills to obtain organic
particles.
Squid & Octopus
The class cephalopoda, meaning "head-footed," includes
squids, octopods, cuttlefish and nautiluses. The feet,
or arms, of these creatures are connected to their
heads, not their bodies. The rest of the body is in
front of the head. That's why they're called,
"head-footed." The "feet" of cephalopoda are called
"arms," not tentacles.
The cephalopods appear to be very different from other mollusks, but
physiologically they are similar. Cephalopods, like most mollusks, have
a mantle, a mantle cavity, a radula, and a U-shaped digestive tract.
Cephalopods have two kidneys and three hearts, which pump blue blood.
They are carnivores that feed on fish, shrimp, crabs and other
cephalopods.
Cuttle Fish Eating
a Painted Comber...
The most obvious difference between most cephalopods and
other mollusks is the apparent lack of a shell.
Octopuses do not have shells at all, while squids have a
small internal shell. (Nautiluses, which are found in
the South Pacific and Indian oceans, are the only
cephalopods with an external shell. They are also the
only cephalopods with four gills instead of two.)
Cephalopods have a more developed nervous system than
other mollusks. They also have very well developed
eyesight that is used in finding prey. Once prey is
found, it is grasped firmly and eaten with a mouth
located at the base of the arms. Cephalopods also have a
parrot-like beak which is used in biting into prey.
Family
Aplysiidae
-
BURSATELLA LAECHII - HAIRY SEA HAIR
Body long,
fleshy with a short triangular tail. It is covered with
lots of projections and blue spots. It has two pairs of
tentacles, oral tentacles and rhinophores about the same
size (you have to look carefully among the hairy bits to
distinguish the tentacles). The parapodia appears to be
a hole in the centre of the body, rather than 'wings' or
flaps as in other large sea hares. It may come in
different colours but is usually well camouflaged and
blends in perfectly with among seaweeds and seagrasses.
Like some other sea hares, it produces a purple ink when
disturbed. It eats cyanobacteria, in particular, the
mat-forming Lyngbya majuscula, which was formerly known
as the filamentous blue-green alga Microcoleus
lyngbyaceus. Apparently, Bursatella sea hares swallow
large amounts of sand in the process of eating, somewhat
like earthworms do.
