There are over a hundred thousand species of mollusks in the world. Mollusks are divided into five classes; the Amphineura, the Cephalopoda, the Gastropoda, the Pelecypoda, and the Scaphopoda.
The Mollusca, common
name molluscs or mollusks, is a large phylum of invertebrate animals. There are
around 85,000 recognized extant species of molluscs. This is the largest marine
phylum, comprising about 23% of all the named marine organisms. Numerous
molluscs also live in freshwater and terrestrial habitats. Molluscs are highly
diverse, not only in size and in anatomical structure, but also in behaviour and
in habitat.
Taxonomy
The phylum Mollusca is typically divided into nine or ten taxonomic classes, of
which two are extinct. The gastropods (snails and slugs) include by far the most
classified species, accounting for 80% of the total. Cephalopod molluscs such as
squid, cuttlefish and octopus are among the most neurologically advanced
invertebrates. Either the giant squid or the colossal squid is the largest known
species of animal without a backbone.
Opinions vary about the number of classes of molluscs—for example the table
below shows eight living classes, and two extinct ones. However some authors
combine the Caudofoveata and solenogasters into one class, the Aplacophora. Two
of the commonly recognized classes are known only from fossils
Class
Organisms
Described living species
Distribution
Caudofoveata
worm-like organisms
120
seabed 200–3,000 metres (660–9,800 ft)
Aplacophora
solenogasters, worm-like organisms
200
seabed 200–3,000 metres (660–9,800 ft)
Polyplacophora
chitons
1,000
rocky tidal zone and seabed
Monoplacophora
limpet-like organisms
31
seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft)
The two most universal features of the body structure of molluscs are a mantle
with a significant cavity used for breathing and excretion, and the organization
of the nervous system. Because of the great range of anatomical diversity, many
textbooks base their descriptions on a hypothetical "generalized mollusc", with
features common to many but not all classes within the Mollusca.
Distinguishing
features
Diversity
About 80% of all known mollusc species are gastropods (snails and slugs),
including the cowry (a sea snail) pictured here.
Estimates of accepted described living species of molluscs vary from 50,000 to a
maximum of 120,000 species.[1] In 2009 Chapman estimated the number of described
living species at 85,000.[1] Haszprunar in 2001 estimated about 93,000 named
species, which include 23% of all named marine organisms. Molluscs are second
only to arthropods in numbers of living animal species[5]—far behind the
arthropods' 1,113,000 but well ahead of chordates' 52,000. It has been estimated
that there are about 200,000 living species in total,[1][16] and 70,000 fossil
species, although the total number of mollusc species that ever existed, whether
or not preserved, must be many times greater than the number alive today.
Molluscs have more varied forms than any other animal phylum. They include
snails, slugs and other gastropods; clams and other bivalves; squids and other
cephalopods; and other lesser-known but similarly distinctive sub-groups. The
majority of species still live in the oceans, from the seashores to the abyssal
zone, but some form a significant part of the freshwater fauna and the
terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate
regions but can be found at all latitudes. About 80% of all known mollusc
species are gastropods. Cephalopoda such as squid, cuttlefish and octopus are
among the neurologically most advanced of all invertebrates. The giant squid,
which until recently had not been observed alive in its adult form, is one of
the largest invertebrates. However a recently caught specimen of the colossal
squid, 10 metres (33 ft) long and weighing 500 kilograms (0.49 LT; 0.55 ST), may
have overtaken it.
Freshwater and terrestrial molluscs appear exceptionally vulnerable to
extinction. Estimates of the numbers of non-marine molluscs vary widely, partly
because many regions have not been thoroughly surveyed. There is also a shortage
of specialists who can identify all the animals in any one area to species.
However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000
endangered non-marine molluscs. For comparison, the great majority of molluscs
species are marine but only 41 of these appeared on the 2004 Red List. 42% of
recorded extinctions since the year 1500 are of molluscs, almost entirely
non-marine species.
The words mollusc and mollusk
are both derived from the French mollusque, which originated from the Latin
molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's
τᾲ μαλάκια, "the soft things", which he applied to cuttlefish. The scientific
study of molluscs is known as malacology.
Molluscs have developed such a varied range of body structures that it is
difficult to find synapomorphies (defining characteristics) that apply to all
modern groups.[ The most general characteristic of molluscs is that they are
unsegmented and bilaterally symmetrical. The following are present in all modern
molluscs:
The dorsal part of the body wall is a mantle (or pallium) which secretes
calcareous spicules, plates or shells. It overlaps the body with enough spare
room to form a mantle cavity. The anus and genitals open into the mantle cavity.
There are two pairs of main nerve cords.
Evolution
Fossil record:There is good evidence for the appearance of gastropods,
cephalopods and bivalves in the Cambrian period 542 to 488.3 million years ago.
However, the evolutionary history both of the emergence of molluscs from the
ancestral group Lophotrochozoa, and of their diversification into the well-known
living and fossil forms, is still vigorously debated.
There is debate about whether some Ediacaran and Early Cambrian fossils really
are molluscs. Kimberella, from about 555 million years ago, has been described
as "mollusc-like",[40][41] but others are unwilling to go further than "probable
bilaterian". There is an even sharper debate about whether Wiwaxia, from about
505 million years ago, was a mollusc, and much of this centers on whether its
feeding apparatus was a type of radula or more similar to that of some
polychaete worms.[42][44] Nicholas Butterfield, who opposes the idea that
Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510
million years ago are fragments of a genuinely mollusc-like radula.
However, the Helcionellids, which first appear over 540 million years ago in
Early Cambrian rocks from Siberia and China, are thought to be early molluscs
with rather snail-like shells. Shelled molluscs therefore predate the earliest
trilobites. Although most helcionellid fossils are only a few millimeters long,
specimens a few centimeters long have also been found, most with more
limpet-like shapes. There have been suggestions that the tiny specimens were
juveniles and the larger ones adults.
Some analyses of helcionellids concluded that these were the earliest
gastropods. However other scientists are not convinced that Early Cambrian
fossils show clear signs of the torsion that identifies modern gastropods twists
the internal organs so that the anus lies above the head.
For a long time it was thought that Volborthella, some fossils of which pre-date
530 million years ago, was a cephalopod. However discoveries of more detailed
fossils showed that Volborthella’s shell was not secreted but built from grains
of the mineral silicon dioxide (silica), and that it was not divided into a
series of compartments by septa as those of fossil shelled cephalopods and the
living Nautilus are. Volborthella’s classification is uncertain.[52] The Late
Cambrian fossil Plectronoceras is now thought to be the earliest clearly
cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue
that Nautilus uses to remove water from compartments that it has vacated as it
grows, and which is also visible in fossil ammonite shells. However,
Plectronoceras and other early cephalopods crept along the seafloor instead of
swimming, as their shells contained a "ballast" of stony deposits on what is
thought to be the underside and had stripes and blotches on what is thought to
be the upper surface.All cephalopods with external shells except the nautiloids
became extinct by the end of the Cretaceous period 65 million years ago.
However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant
today.
The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves.
"Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million
years ago.[60] One bivalve group, the rudists, became major reef-builders in the
Cretaceous, but became extinct in the Cretaceous-Tertiary extinction. Even so,
bivalves remain abundant and diverse.
The Hyolitha is a class of extinct animals with a shell and operculum that may
be molluscs. Authors who suggest that they deserve their own phylum do not
comment on the position of this phylum in the tree of life
Please join us in our mission to incorporate The Congressional Evolution of the United States of America discovery-based curriculum into the classroom of every primary and secondary school in the United States of America by July 2, 2026, the nation’s 250th birthday. , the United States of America: We The
People. Click Here
