PHYLUM MOLLUSCA snail giant clam Nautilus Characterised by a soft body. Very large (second largest animal phylum next to the arthropods) and extremely diverse group, including clams, snails and octopuses.

As with the Annelids, Molluscs are true coelomates, with a schizocoelomate body plan.

Characteristics Soft body Unsegmented Bilateral symmetry Usually definite head Marine, freshwater and terrestrial habitats Ventral body wall forms muscular foot Locomotion Dorsal body wall forms mantle, enclosing mantle cavity Shell (secreted by mantle) Coelom limited mainly to area around heart (pericardial cavity)

Coelom Body cavity surrounded by mesoderm Major step in evolution of larger, more complex life forms Benefits: More stable arrangement of organs Alimentary canal more muscular and highly specialised without interfering with other organs

Characteristics Body Plan Foot, mantle, visceral mass

Head-Foot Region Well developed head Muscular foot Bears mouth and specialised sensory organs Radula inside mouth Muscular foot Variously modified and adapted for locomotion, attachment to a substrate or a combination of functions Often secretes mucous

Visceral Mass Region Visceral mass contains digestive tract, circulatory organs, paired kidneys, reproductive organs Mantle Protective sheath; outgrowth of dorsal body wall Encloses a space (mantle cavity) Cavity usually houses respiratory organs, which develop from the mantle Products from digestive, excretory and reproductive systems empty into cavity Mantle secretes the shell Gaseous exchange Mantle surface also used for gaseous exchange Jet propulsion (cephalopods)

Characteristics Radula (tongue-like rasping organ) usually present

Circulatory System Open circulatory system (closed in cephalopods) Heart, blood vessels and sinuses Respiratory pigments in blood

Respiration Excretory System Gaseous exchange by: Gills (one or two) Lung Mantle (exposed surface) Body surface Excretory System One or two kidneys (metanephridia) Open into coelom (pericardial cavity) Usually empty into mantle cavity

Nervous System Several pairs of ganglia with connecting nerve chords Sensory organs of touch, smell, taste, balance and vision (in some); eyes highly developed in cephalopods

Generalised Mollusc All systems present and well developed.

Reproduction Mostly dioecious; some monoecious Spiral cleavage of egg to produce free-swimming larva Trochophore or veliger larva (aquatic molluscs) Some with direct development

Trochophore Free-swimming larva that emerges from egg in primitive molluscs prototroch mouth anus

Veliger In many molluscs the trochophore is passed into the egg, and a veliger hatches to become the only free-swimming stage Has the beginnings of foot, shell and mantle

Class: Caudofoveata Class: Solenogastres Wormlike Marine Reduced head Have no shell but covered with calcareous scales or spicules Mostly burrowers Mostly free-living on bottom sediments Sexes are separate Hermaphroditic (monoecious) Radula present but may be reduced Usually radula is absent Gills present Gills absent, secondary respiratory structures may be present

Class: Monoplacophora Thought extinct until 1952, when living specimens were dredged up from bottom of ocean Small with rounded shell and creeping foot (limpet-shaped) Mouth with characteristic radula foot mouth gills shell mantle anus

Class: Polyplacophora Chitons Flattened dorsoventrally Convex dorsal surface with 8 plates Head with sensory organs is reduced Radula projects from the mouth and scrapes algae from rock surfaces Sexes separate Trochophore develops into juvenile

Class: Scaphopoda Tusk shells or tooth shells Marine; benthic; sedentary Slender body covered with a mantle and a tubular shell open at both ends Foot protrudes through larger end of shell – used to burrow into mud or sand Small end of shell exposed Gills absent, gaseous exchange via mantel Long tentacles extend from the head region Captacula- feeding Dioecious; trochophore larvae (ancestral)

Class: Gastropoda Largest and most diverse class naudibranchs garden slug limpet whelks snail abalone Class: Gastropoda Largest and most diverse class Marine, freshwater or terrestrial Includes snails, slugs, limpets, whelks, nudibranchs and others Head well developed with one or two pairs of tentacles, which may bear eyes Shell, when present, is always one piece and may be coiled or uncoiled

Class: Gastropoda Both trochophore and veliger larval stage Bilaterally symmetrical, but because of torsion, visceral mass has become asymmetrical

Torsion Only in gastropods Twisting process that moves the mantle cavity (originally posterior) to the front of the body, thus twisting the visceral organs through a 90° to 180° rotation Occurs in veliger stage

Practical

Torsion Before torsion, embryo’s mouth is anterior and anus and mantle cavity are posterior Change brought about by uneven growth of right and left muscles that attach the shell to the head-foot After torsion anus and mantle cavity are anterior and open above the mouth and head

Coiling Coiling of shell and visceral mass is not the same as torsion May occur in the larval stage at the same time as torsion

Coiling Apex contains the oldest and smallest whorl Whorls become successively larger and spiral about the central axis (columella) Largest whorl presses on right side of mantle cavity, leading to a loss of organs on RHS (gill, auricle, kidney)

Shell Shell may be right handed (dextral) or left handed (sinistral) Operculum – horny plate that covers shell aperture when body is withdrawn into the shell

Class: Gastropoda Subclass: Prosobranchia E.g. periwinkles, abalone, limpets, whelks Almost all marine One pair of tentacles Sexes usually separate Shell present Operculum often present Tored Haliotus midae

Class: Gastropoda Subclass: Opisthobranchia Sea slugs, sea butterflies and others Nearly all marine two classical groups: Tectibranchs - with gill and shell Nudibranchs - no shell or true gill, but have secondary gills along the sides between the mantle and foot Usually two pairs of tentacles All monoecious Some detorsion Sea butterfly or pteropod

Class: Gastropoda Subclass: Pulmonata snail slug Class: Gastropoda Subclass: Pulmonata Terrestrial and freshwater snails and slugs Usually lack gills, but mantle wall has developed a lung Monoecious Aquatic species have one pair of non-retractable tentacles, at the base of which are eyes Terrestrial species have two pairs of tentacles with the posterior pair bearing eyes Some detorsion ctenidium - comb-like respiratory structure serving as the gill of certain molluscs. Terrestrial forms thought to be more advanced; evolved from aquatic forms.

Class: Bivalvia Mussels, clams, oysters, scallops Black mussels; Mytilus edulus Mussels, clams, oysters, scallops Mostly marine; some freshwater or brackish water Mostly sedentary filter feeders Laterally compressed 2 shells (valves) held together dorsally by a hinge ligament Giant clam Second largest class

Class: Bivalvia Head greatly reduced No radula Wedge-shaped foot Mantle may be modified into ventral incurrent and dorsal excurrent siphons in marine forms Dioecious Fertilization usually external Trochophore and veliger larvae Open circulatory system Second largest class

Class: Cephalopoda Most complex of the molluscs Squid, octopuses, nautiluses, cuttlefish All marine All active predators Anterior margin of head drawn out into a circle of arms or tentacles used to capture prey (adhesive secretions or suckers) Powerful parrot-like beak is used to tear prey apart

Class: Cephalopoda Foot modified into funnel for expelling water from mantle cavity (jet propulsion) Shell often reduced or absent Most complex of the molluscs

Class: Cephalopoda Chromatophores – Pigment cells in skin that produce colour changes Bioluminescence in deep sea forms Ink production – Ink sac with fluid containing melanin empties into rectum

Class: Cephalopoda Brain Complex nervous system Increased sensory perception and behavioural complexity Some species capable of learning (intelligence?) Some with closed circulatory system Separate sexes Direct development; no larval stages As a result of their more active predatory life styles, they have developed a nervous system of far greater complexity than any other mollusc. Associated with this more complex nervous system is an increased sensory perception and behavioural complexity. Of special interest is their ability to effect rapid skin colour changes due to the presence in the skin of several types of chromatophore which are under nervous as well as hormonal control. Experiments have shown that some species, at least, are capable of learning and may exhibit a reasonable level of 'intelligence'. As a result, many people consider that the cephalopods represent a zenith, not only of the molluscan line, but also of all the invertebrates.

Class: Cephalopoda Subclass: Nautiloidea Nautilus (chambered Nautilus) only remaining members Coiled external shell divided into gas filled chambers Relatively simple eyes 2 pairs of gills Tentacles contain no suckers and extend through the shell opening Last remaining cephalopods with an external shell. Slower moving than other cephalopods.

Class: Cephalopoda Subclass: Ammonoidea Totally extinct End Cretaceous (65 MYA) External shell First appeared about 400 MYA and went extinct about 65 MYA.

cuttlefish squid octopus Class: Cephalopoda Subclass: Coleoidea (most have 8 arms and 2 tentacles) Cuttlefish, squid and octopus Sepiidea – Cuttlefish Teuthida – Squid; Largest of the cephalopod orders

Class: Cephalopoda Order: Sepioidea Cuttlefish Rounded or compressed bulky body bearing fins Arms and tentacles bear suckers but tentacles only at their ends cuttlefish

Class: Cephalopoda Order: Teuthiodea Squid Body more cylindrical 8 arms, 2 tentacles Arms with stalked suckers with horny rims bearing teeth E.g. Colossal squid (Mesonychoteuthis hamiltoni)- largest known invertebrate in terms of mass

Class: Cephalopoda Order: Octopoda No shell; 8 Arms; no tentacles; short compact body; suckers not stalked and do not bear horny rims

Evolutionary Relationships Molluscs are allied with both annelids and arthropods Molecular evidence suggests they are more closely related to the annelids