Download Body organization

Phylum Molluscs
Prepared by : Nada H. Lubbad
 Introduction:
o Soft bodied animals.
- name from Latin word “ mollus” meaning soft.
o The second largest phylum.
o The scientific study of Mollusks is
known as Malacology.
 Structure:
o Most mollusks are protected by an external, hard shell made of calcium carbonate.
except: - in the octopus & sea slugs is completely absent.
- in squids, it’s located internally.
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Body type : Bilateral symmetry.
Body organization : Triploblastic.
Body cavity : reduced coelom contains only heart & gonads.
Body plan
three distinct body zones.
1.
2.
3.
Head- Foot : contains sensory and motor organs.
Visceral mass : contains internal organs used for digestion, excretion, and reproduction.
Mantle : a fold of tissue that drapes over the visceral mass and secretes a shell .
o Mantle cavity : water-filled chamber that houses the gills, anus and excretory pores.
* space between the mantle and the visceral mass (body organs).
Functions of mantle:
- Protection of underlying loose structures.
- Gas exchange through it’s external surface.
- in aquatic types, it has receptors for testing the surrounding waters.
A pearl formation
Shell structure & formation
 Digestive system:
o Complete, one-way digestive tract with a mouth & anus.
o Specialized feeding structures : Gills, Beak, Radula .
o The radula :
 It is a rasping, tongue like feeding structure found in most mollusks
except bivalves.
 Has tiny rows of teeth for scraping.
 The radula is used to scrape algae off rocks or to eat the soft
tissues of plants.
o Food is taken up by cells in the lining of the stomach &
passed into the blood.
 undigested materials are compressed and packaged and discharge
through the anus to the mantle cavity and carried into water
currents
Gastropod Feeding Apparatus
 Excretion system
o Excretory functions are carried out by a pair of nephridia.
 nephridia – tubular structures that collect fluids from coelom and exchange salts with body
tissues as fluids pass along the tubules.
 Nervous system
o Relatively complex.
 Most complex - octopus.
o 2 pairs of main nerve cords.
 Visceral – serve the internal organs.
 Pedal – control the foot.
o Brain encircles the esophagus.
Generalized Mollusc Anatomy
 Respiration and circulation
o Open circulatory system ( Gastropoda, Bivalvia, Polyplacophora ).
 The blood does not circulate entirely within vessels but is collected by the gills, pumped into
the heart, and released in spaces in the tissue ( hemocoel- blood cavity ) and returns to the
gills, then the heart.
 Slow moving lifestyle has lower oxygen demands.
o Closed ( Cephalopoda ).
 Heart pumps blood within blood vessels.
 Active lifestyle, large size has higher oxygen demand and requires more efficient circulation.
o Respiration pigment – hemocyanin.
o Respiration is through gills, ctenidia, located in the mantle cavity.
 Reproduction:
o All Mollusks reproduce sexually.
 Depending on the species of mollusk, some reproduce externally or internally.
 Dioecious- Have male and female.
 Some are hermaphroditic.
o Molluscan Larval Stages.
 Most Molluscs produce a free-swimming ciliated larvae called the trochophore larvae.
 In some Molluscs the trochophore develops into the adult, but in other Molluscs (e.g., gastropods)
there is a second larval stage called the veliger.
Trochophore larvae
 Mollusk Larvae
o Trocophore Larvae.
 Free-swimming larval stage (first form)
o Veliger larvae
 Free-swimming larval stage with foot, eyes, tentacles, and shell.
Trocophore Larvae
Veliger larvae
 Major classes:
Phylum Mollusca
Class
Polyplacophora
Class
Gastropoda
Class
Bivalvia
Class
Cephalopoda
 Class Gastropoda
o Name comes from Greek – Gaster = “gut” , podos = “foot”.
o Gastropods make up 80 % of Molluscs.
o Most gastropods have an external shell that protects their bodies.
o When threatened, they can pull completely into their shells.
o They can either be herbivores, scavengers, and carnivores.
o They usually live in oceans, on rocks and land, and in freshwater.
o They use their muscular foot to creep along a carpet of mucus that they ooze out
in order to crawl.
Examples:
slug
Sea hare
Snail
Sea butterfly
Sea slug
 Class Bivalvia
o Name comes from Latin- Bis =“ twice” , valve= “ leaf “.
o Second largest class of mollusks.
o Bivalves have two shells that are held together by one or
two powerful muscles.
o Many are edible.
o Marine or freshwater.
o Eat via filter feeding- no head or radula.
o Incurrent and excurrent siphons are used to pump water
through the organism for:
 Gas exchange
 Filter feeding
 Jet propulsion
o Bivalves move around by extending the muscular foot
between the shells.
o Scallops and file shells swim by clapping their shells
together to create jet propulsion.
o Like other mollusks, bivalves have a coelom and an open
circulatory system.
o They breathe through gills and filter feed
Examples:
Mussels
scallops
clams
 Class Cephalopoda
o Name comes from Latin & Greek – Cephalic= “head” , podos = “ foot “.
o Cephalopods are typically soft-bodied mollusks in which the head is attached to a
single foot. The foot is divided into tentacles or arms.
o Cephalopods have eight or more tentacles equipped with sucking disks that grab
and hold prey.
o They all live in ocean.
o They are able to move through jet propulsion ( squeezing a current of water out
of their mantle cavity ) .
o Their closed circulatory system makes them the fastest, smartest, and the biggest
of all mollusks.
o Most cephalopods have complex eyes with cornea,
lens, chambers, and retina.
o Largest invertebrate brain.
Protection
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Color changes effected by chromatophores (pigment cells).
Allows them to blend into their background.
Squirting out water by jet propulsion helps escape predators.
Squids also release an inky substance into the water.
Examples:
nautilus
octopus
squid
Anatomy of squid