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Systematic Adaptations of Phylum Mollusca
The Chitons, Gastropods, Bivalves and Cephalopods
The molluscs are a diverse group of soft bodied invertebrates
Some varieties of molluscs produce an external shell (“test” or “valve”)made of calcium carbonate
All molluscs have a fold of skin associated with the dorsal body wall called the Mantle which encloses the Mantle Cavity. The mantle cavity houses the gills, if present.
General Characteristics
Triploblastic (well developed mesoderm) Organ System level of body organization True body cavity (eucoelomate) – the
coelom is completely bound by mesodermal tissue and lined with peritoneum.
Complete gastrulation – one way digestive tract with a mouth and an anus
Protostomes – the first opening of the gastrula forms the mouth
Bilateral symmetry
Body Form & Development
Molluscs have a unique body plan with 2 general body regions – the Head-Foot and the Visceral Mass
The Head-Foot, as the name implies, contains the primary sense organs characteristic of cephalization, the mouth and feeding apparatus, and locomotor organs
The Visceral Mass contains the “guts” – the organs associated with digestion, respiration, reproduction, and circulation
Body Plan
Mollusc Body Plan
Varying degrees of cephalization occur in the molluscs. Some (particularly the cephalopods, which include squid and octopus) have very well developed eyes.
Cephalization - Photoreception
Mollusc Eyes – Scallop and Snail
Many molluscs (notably the gastropods and cephalopods) have tentacles associated with the Head-Foot region
The tentacles are used for “touch” and “taste”
Cephalization – Chemoreception and tactile pressoreception
https://www.youtube.com/watch?feature=player_detailpage&v=yE0QqxwyL_8
Octopus Senses and Behavior
Most molluscs (all gastropods, chitons and cephalopods) have a specialized rasplike feeding organ called a Radula. The radula is a muscular tongue covered with teeth for scraping off food
Feeding apparatus
Radula Teeth from a Slug
Video of a snail eating Note the antennae, eyes, and mouth Note the texture and movement of the
radula
http://www.youtube.com/watch?v=cEMK3VN8pW8
http://www.youtube.com/watch?v=mLVDwlrSq5U
Snail feeding behavior
The foot portion of the head-foot region is used for locomotion and varies greatly within the phylum
Gastropods (“stomach foot”) and Chitons have a broad, laterally flattened ventral foot
Pelecypods (“hatchet foot”) have a muscular foot elongated along the dorso-ventral midline
Cephalopods (“head foot”) have modified the foot to form a siphon to eject water for propulsion
The Head-Foot & Locomotion
Snails https://www.youtube.com/watch?v=pRmCh
G3Jf_Q https://www.youtube.com/watch?v=utxH9M
H4tOISea Slugs https://www.youtube.com/watch?NR=1&v=
EwpexfYnlDE https://www.youtube.com/watch?v=9VL_p2
A7DPg
Gastropod Locomotion
Octopus https://www.youtube.com/watch?v=rOvhs5i
cTfg https://www.youtube.com/watch?v=t-LTWFn
Gmeg&feature=player_detailpageSquid https://www.youtube.com/watch?v=Nf2iRR
WH5OI https://www.youtube.com/watch?v=EeaMO
OnmHAA https://www.youtube.com/watch?v=5o-OMa
53F1s
Cephalopod Locomotion
The Visceral Mass contains what we normally think of as the “internal organs”
Aquatic forms have Gills for respiration Gills are enclosed in the Mantle Cavity, and
covered by the Mantle. All molluscs have a mantle and mantle cavity
Some terrestrial forms have diminished gills, and perform gas exchange across the moist membranes of the mantle cavity
Visceral Mass - Respiration
Mantle and Mantle CavityChiton and Mussel
Mantle and Mantle CavityOctopus and Squid
Many molluscs secrete protein and calcium carbonate from the mantle to create a shell
The shell typically has 3 layers:◦ The periostacum is the outer layer, made of
protein secreted by the edge of the mantle◦ The prismatic layer is the middle layer, made of
densely packed prisms of calcium carbonate in a protein matrix
◦ The nacreous layer is secreted continuously by the surface of the mantle. This layer is what we call Mother of Pearl
The Mollusc Shell
The mechanism of blood circulation varies Bivalves have an Open Circulatory System.
Blood pumped by the heart circulates through blood vessels in the gills, but empties into the coelom where the visceral organs are bathed by blood
Cephalopods have a Closed Circulatory System. Blood is circulated through blood vessels, including capillaries, into all body regions
Circulation
Metabolic wastes and excess water are removed by the kidneys (metanephridia)
The kidneys open on the inside to the coelom, communicating with the blood, and discharge wastes into the mantle cavity
The ducts of the excretory system are shared by the Reproductive system, providing passageways for sperm and egg to the mantle cavity
Excretion & Osmoregulation
Reproduction: Trochophore Larvae
The larval form of a mollusc is called a Trochophore See diagrams and descriptions p. 330 in your
textbook
Mollusc Diversity Mollusca is one of the most diverse and
widespread phyla in the animal kingdom, second only to the arthropods
We will focus on only a few of the major classes of molluscs◦ Polyplacophora (Chitons)◦ Gastropods (snails and slugs)◦ Pelecypods (Bivalves – Clams, Oysters, Mussels &
Scallops)◦ Cephalopods (Squid, Nautilus and Octopus)
Chitons are similar to gastropods in general body form, but lack the degree of cephalization that is characteristic of slugs and snails
Chitons are flattened in shape, with multiple prominent gills between the mantle and the foot
As the name polyplacophora suggests, the shell consists of many plates
Polyplacophora - Chitons
Polyplacophora - Chitons
Foot is flattened and tight to the belly (thus “gastro” “pod” – belly foot)
Definite cephalization with both eyes and tentacles (for tactile sense and chemoreception)
Slugs – no shell. Snails – single shell Asymmetrical deposition of calcium
carbonate by the mantle causes the shell of snails to coil (see page 335 in text)
Gastropoda – Slugs and Snails
Gastropods – Snails and Slugs
See page 334 in the textbook During embryonic development, the mantle
cavity of gastropods undergoes torsion – the mantle twists so that the posterior of the mantle is relocated over the anterior of the head-foot
Torsion allows the snail to more readily retract the head into the shell for protection
Torsion also positions the excretory openings over the gills. This problem is resolved by bringing water into the mantle cavity from the left and expelling waste water from the right
Torsion in gastropods
Due to shell coiling, most snails have the right gill diminished or absent. The whorls of the coiled shell compress the right side of the mantle cavity and visceral mass
Terrestrial forms of both snails and slugs are lacking gills entirely, using the moist skin of the mantle cavity for gas exchange. These forms are called Pulmonates
Nudibranchs are slugs with exposed gills (nudi = naked, branch = gill)
Gill variations in gastropods
Pelecy = hatchet. The pelecypods are molluscs with a hatched shaped foot
Pelecypods are bivalves (2 shells). The shell has 2 distinct parts, attached by a hinge ligament
Clams, oysters, and mussels are all pelecypods
All are filter feeders and lack a radula All have little or no cephalization (although
some have eyes at the rim of the mantle)
Pelecypoda – The Bivalves
Pelecypods - Bivalves
Many bivalves use their hatchet shaped foot to burrow into the soil
The mantle is organized to form an incurrent siphon and an excurrent siphon – which may extend significantly out of the shell
Water circulates through the mantle cavity, and ciliary action of the gill surface channels particles of food trapped in the gill mucus to the mouth
See textbook pages 340-342
Filter Feeding in the Bivalves
Bivalve shells have a distinct Umbo, the oldest part – representing the position of the embryonic shell
The two valves of the shell are connected by a protein hinge ligament. The elasticity of the hinge ligament causes the shell to open
A pair of adductor muscles (one anterior and one posterior) contract to close the shell
Sand particles trapped between the shell and the mantle may be covered with secretions of calcium carbonate, forming a pearl
Bivalve shells
Bivalve shells
Cephalopods – Squid, and Octopus
True Facts? True Facts about the Land Snail
◦ https://www.youtube.com/watch?v=VTV23B5gBsQ True Facts about the Cuttlefish
◦ https://www.youtube.com/watch?v=GDwOi7HpHtQ
True Facts about the Octopus◦ https://www.youtube.com/watch?v=st8-EY71K84
Cephalopods – Nautilus and Ammonites
https://www.fossilera.com/fossils-for-sale/ammonite-pendants
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