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