Download molluscs which Norman put together

SHELLS
part U3A
9th January 2015 Fossils, Shells & Stones
Introduction
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Modern definition - Molluscs describe the animal and (usually hard) shell combined.
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Not all molluscs have shells
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They belong to the “phylum” (major classifactory division) of Mollusca which is divided into 6 classes:
1.
AMPHINEURA – Chitons,
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Shells usually of eight pieces or valves held together by fleshy girdle.
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Have a large sucker like foot as wide as the shell.
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There is a head but no eyes or tentacles
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Two series of gills either side of foot
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Exclusively Marine.
2.
MONOPLACACOPHORA – Gastroverms (thought to be extinct until 1952) only 5 marine species
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One piece, limpet like shells.
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Wedge shaped foot
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Have a mouth but lack eyes and gills
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Exclusively marine
3.
SCAPHODA - Tusk Shells
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One piece shells, tubular, tapering & open at both ends.
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Wedge shaped foot
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Have a mouth but lack eyes and gills
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Exclusively Marine.
4.
GASTROPODA – Limpets, snails, slugs, whelks etc
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One piece shells, usually coiled and external but sometimes absent.
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Vary enormously in appearance usually have a large muscular foot, a well developed head with eyes & tentacles.
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Have either gills or modified “lungs”
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Short or long siphons present
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Marine, freshwater and terrestrial
5.
BIVALVIA – Cockles, mussels, oysters, clams etc
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Animals with two piece shells hinged at one edge.
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Large elongate or hatchet-shaped foot and a pair of siphons posteriorly
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No head or organ associated with a head
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They have gills
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Marine & Freshwater
6.
CEPHALOPODA – squids, nautiluses, cuttlefish, octopuses
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Animals without shells or one piece shells which may be coiled or straight and are usually internal.
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Usually have a series of large tentacles surrounding a mouth in which is situated a beak (like a parrots)
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The eyes are large and well developed.
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Exclusively marine
Biology and Growth
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Sexes are separate in a large number of types but vary from group to group; many gastropods (almost all non marine) &
some bivalves and cephalapods are hermaphrodite with male and female sex organs in the same animal.
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Some molluscs develop as male animals, pass through a hermaphrodite stage and end up as females.
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Reproduction is achieved in a variety of ways, fertilisation may be external or internal
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After larval development the mollusc feeds & grows adding shelly material, the rate depends on environmental conditions.
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The appearance at maturity is often greatly modified by the final growth which terminates in the production of a thicker shell
with pronounced apertural and other features. Food-rich environments help produce thicker shells & features.
Shell Form, Ornament and Colour
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There are endless variety of shell forms.
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Ecological diffences also produce differences in size, ornament and colour.
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The mollusc must continually add new shell growth to old to ensure adequate protection for itself.
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Shells may be smooth externally or have their surfaces cut into or elevated in various ways.
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Most ornamental features are produced at the same time as the additions made to the growing edge of the shell – the various
spines, scales, tubercles and ridges are not superimposed on a previously formed smooth surface.
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The only exception to the above is the production of callus deposit which overlies previously formed shell features (e.g.
Gastropod: Ancilla in which the spire is covered by a thick glaze laid down long after the spire is formed.
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Conversely some molluscs absorb previously formed ornaments such as spines. The appearance of a shell may be modified by
growth or absorption throughout its life
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Colour: very thin shells are sometimes translucent and colourless while thicker opaque shells are always coloured although
many of them are entirely white (which is the colour of crystals of the main constituent - calcium carbonate derived from
Chalk or Limestone).
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Pigment cells ranging from pale to dark brown, pink to crimson, pink to crimson, violet to purple, yellow to green are
concentrated in the mantle distributed over the shell as rays, streaks, spots, triangles, stripes, blotches or other configurations.
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Sunlight and warmth are essential to the production of bright and varied colours as they are to the growth of thick and well
ornamented shells
Distribution and Location
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Molluscs are present in all of the worlds seas, also in brackish and fresh water and some Gastropods (snails, slugs etc) are
abundant on land.
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Climate, temperature, ocean currents, depth of water, breeding habits and other factors prevent molluscs from spreading
haphazardly over the globe.
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Marine molluscs predominantly inhabit relatively shallow water although many are adapted the total darkness of the abyss.
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Floating (pelagic) molluscs are found in the ocean sea - pteropods – (tiny creatures with thin & glassy shells) are among most
abundant of all marine invertebrates
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Sandy shores or seabeds are ideal for molluscs adapted for a burrowing existence.Tusk Shells, Most bivalves & many
gastropods inhabit these areas.
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Rocky shores Bivalves, Chitons clamp down firmly with their sucker like foot & girdle, Top shells & limpets also live here some
in very heavy battering seas.
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Coral provides a food rich environment for a host of different molluscs including the worlds most beautiful
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Mangrove swamps – brackish water & oozy mud house many burrowing and crawling molluscs – oysters and other bivalves
(which often provide food for predatory gastropods) many bivalves bore into or attach themselves to living coral.
Misc.
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The study of marine sea shells is termed Conchology, the study of the whole mollusc including the shell is termed Malacolgy.
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There are currently estimated to be about 100,000 species worldwide.
Ref:
The Collectors Encyclopaedia of Shells – Edited by S.Peter Dance
Sea Shells of the West Indies – by Michael Humfrey published by Collins
Marine Fish and Sea Creatures – by Amy-Jane Beer & Derek Hall published by Lorenz Books
http://en.wikipedia.org/wiki/Seashell
http://xahlee.org/xamsi_calku/xamsi_calku.html (for link to photographs)