2)Body has more than two cell layers, tissues and organs.
3)Body without cavity.
4)Body possesses a through gut with mouth and anus.
5)Body monomeric and highly variable in form, may possess a dorsal or
lateral shells of protein and calcareous spicules.
6)Has a nervous system with a circum-oesophagal ring, ganglia and
paired nerve chords.
7)Has an open circulatory system with a heart and an aorta.
8)Has gaseous exchange organs called ctenidial gills.
9)Has a pair of kidneys.
10)Reproduction normally sexual and gonochoristic.
11)Feed a wide range of material.
12)Live in most environments.
The mollusks compose a large phylum of invertebrate animals.
Estimates of accepted described living species of molluscs vary from 50,000 to
a maximum of 120,000 species. About 200,000 living species in total are
estimated, and 70,000 fossil species, although the total number of mollusc
species ever to have existed, whether or not preserved, must be many times
greater than the number alive today.
It is estimated that there may be about 100,000 living species and
35,000 extinct species as mollusks have a long geological
history, dating back to the Precambrian to Recent.
Mollusks live in freshwater and in
terrestrial habitats. Most mollusks
are marine animals; so they are
free swimming species. Some are
found in shallow coastal areas and
others live in the deepest parts of
the ocean. Some mollusks have
been found at depths of 2,200 feet
or more, hydrothermal
communities, in regions where
molten volcanic sediments come in
contact with the cold ocean waters.
They can be found on rocky
shores, coral reefs, mud flats, and
Molluscs inhabiting littoral rocks were collected from 43 stations around
Ceylon, and 57 species belonging to 20 families were identified. An account is
given of the nature and distrbution of littoral rocks around the island, and the
stations examined. On each shore, the distribution of the molluscs was studied
in relation to the broad zonation of the shores themselves and observations
were made on plant and animal associations of each species. Analysis of the
data collected led to a classification of the rocky shores into three
groups, each characterized by particular combinations of mollusc species and
distinct patterns of shore zonation. Some molluscs were found to be
distributed over the whole island, others to be common to two groups of
shores only, and still others to be confined to a single shore type. An attempt
has been made to explain the presence or absence of species on various
groups of shores, and to explain discontinuities in distribution on the basis of
environmental factors such as the nature of the substratum, degree of wave
exposure, and weed cover. A key, based primarily on external features, has
been devised for the identification of the species listed.
Facts of molluscs
There are nine groups of molluscs today:
Caudofoveata, Solenogastres, Monoplacophora, Polyplacophora, Scaphopoda, Bivalv
ia, Gastropods and Cephalopoda
The body of the majority of shellfish consists of two parts: a head-feet, and visceral
mass: In general, the part of his anatomy that includes this mixture of head and feet
is a soft structure and stretched, in their head are pair of eyes, a mouth, and sensory
nerve structures and a radula, while using their feet to cling to power mobility and
Mollusks have a mantle that secretes a kind of hard shell that covers and protects
the visceral mass.
These animals exhibit bilateral symmetry.
Most molluscs have an open circulatory system.
The mollusks are more closely related to arthropods than to annelids.
There are about 100,000 species of molluscs, of which 60,000 to 80,000 are within
the sub-group of gastropods.
Some molluscs have no protective shell.
Some molluscs have no head.
The giant squid and giant clam shellfish are among the largest in the world.
EVOLUTION The evolutionary history both of mollusks'
emergence from the ancestral
Lophotrochozoa that is an organisms that
has true tissues organized into germ
layers, and of their diversification into the
well-known living and fossil forms from
more than 541 million years ago.
Some fossils have showed that even in
a shell is of few millimeters
long, specimens a few centimeters long
can also be found. Tiny specimens
have been suggested to be young and
the larger ones, the adults.
These are the places with more state
fossil molluscs or alive.
From Polony to Spain
PROS & CONS
A striking feature of mollusks is the use of the same organ for multiple functions.
A risk of food poisoning from toxins that accumulate in mollusks is very big. Blue-ringed octopus the
animal that belongs to cephalopod class, it means it has tentacles or arms, can make a bite very strong
that can cause unpleasant symptoms. Stings from a few species of large tropical cone shells can also kill.
However, the sophisticated venoms of these cone snails have become important tools
in neurological research.
All species of cone snails are poisonous, although many species are too small to pose much of a risk to
humans. Their venom is based on a huge array of toxins, some fast-acting and others slower but deadlier;
they can afford to do this because their toxins require less time and energy to be produced compared
with those of snakes or spiders.
These creatures have been important to humans throughout history as a
source of food, jewelry, tools, and even pets. The best natural pearls are
produced by marine pearl oysters which live in
the tropical and subtropical waters of the Pacific Ocean. Mollusks also
have been outlandish forms that have helped conjure up tales of
mythological sea monsters, such as the kraken.
Schistosomiasis also known as snail fever is
transmitted to humans via water snail hosts,
and affects about 200 million people. A few species
of snails and slugs are serious agricultural pests.