Mollusca Phylum

• Mollusca is a phylum of soft-bodied animals
• 85,000 to over 100,000 species exist
• Chitons, snails, slugs, clams, squids, and octopuses are examples of molluscs
• Molluscs can be herbivores, predators, filter feeders, or parasites

Mollusca Habitats

• Marine (benthic, pelagic)
• Brackish-water
• Fresh-water
• Terrestrial

Molluscan Body Plan

• Triploblastic: Possessing three germ layers
• True coelom: A body cavity completely lined by mesoderm
• Molluscs have mesoderm-derived membranes
• Small cavity surrounds only the heart
• Protostome: A lineage of animals whose blastopore becomes the mouth, and whose body cavity forms by schizocoely

Molluscan Body - Head-Foot

• Feeding, cephalic sensory, and locomotor organs are found here
• Most have well-developed heads with photoreceptors ranging from light sensors to image-forming eyes
• Muscular foot or tentacles are also present

Molluscan Body - Visceral Mass

• It is a solid mass of tissue
• It contains digestive, circulatory, respiratory, and reproductive organs

Molluscan Body - Mantle

• Formed by two folds of skin
• Mantle cavity is the space between the mantle and the body wall
• It houses gill or lung
• It secretes a shell over the visceral mass in many species

Molluscan Body Structures - Radula

• It is a chainsaw-like structure for rasping
• It has tiny teeth made of chitin, a cellulose polymer
• Used to scrape food from surfaces
• Unique to molluscs
• Present in most molluscs, except bivalves and some gastropods

Molluscan Reproduction

• Most reproduce sexually
• Most are dioecious, having separate sexes
• Some bivalves and snails are monoecious, possessing both male and female reproductive organs
• Some snails can reproduce asexually through parthenogenesis, development from an egg without fertilization

Major Classes

• Polyplacophora
• Bivalvia
• Scaphopoda
• Gastropoda
• Cephalopoda

Class Polyplacophora

• Chitons, with "khiton" meaning tunic in Greek
• Around 800-950 species
• Characterized by a dorsoventrally flat body and a dorsal shell with 8 plates
• Exclusively marine animals preferring rocky intertidal habitats
• This is the most primitive of the extant molluscs
• Open circulatory system with a three-chambered heart
• Blood travels through an aorta and sinuses to the gills
• They feed by scraping algae and small animals using the radula
• Dioecious with external fertilization
• Have an anterior nerve ring and paired longitudinal nerves

Class Bivalvia

• Includes clams, mussels, oysters, and scallops, with around 10,000 species
• All aquatic types, most are sedentary filter feeders
• Characterized by a laterally flattened body and the absence of a head or radula

Bivalve Body Structures

• Shells articulate dorsally via a hinge ligament
• Two adductor muscles hold the shells closed
• Umbo is the oldest part of the shell
• Shells grow ventrally and form concentric rings
• Siphons are paired water channels (incurrent and excurrent)
• Cilia on gills move water through the mantle cavity
• Incurrent siphon brings in oxygen and food
• Excurrent siphon carries out wastes and gametes

Bivalve Feeding and Digestion

• Gland cells on gills and labial palps secrete mucous to trap food particles
• Ciliary tracts move the mucous mass into the mouth
• Crystalline style is a gelatinous rod in the stomach that releases enzymes and churns the contents

Bivalve Reproduction and Development

• External fertilization characterizes most.
• Mobile larval stages include trochophore and veliger
• Spat refers to the settled larval stage
• Freshwater clams perform internal fertilization
• Glochidium is a specialized larva that parasitizes fish gills

Class Scaphopoda

• "Boat foot"
• Known as tusk shells or tooth shells
• They have a tubular shell with two openings
• The mantle wraps around viscera and forms a tube
• Foot protrudes through the larger opening to burrow into substrate
• Exhibit loss of gills and exchange gas across the mantle
• Sedentary, benthic marine habitat
• Dioecious with internal fertilization

Class Gastropoda

• "Stomach foot"
• Snails, slugs, limpets, conches, whelks, periwinkles, sea slugs, sea hares, and sea butterflies
• Over 75,000 extant species

Gastropod Body Plan

• Torsion occurs, twisting during the ontogenetic development
• One side of the larva grows faster than the others
• The visceral mass rotates up to 180° counterclockwise
• Anus and mantle cavity move to an anterodorsal position

Gastropod Features

• Torsion moves gills to the anterior part of the body, improving respiration
• Affords better defense against predators by allowing the animal to retract its head into the shell
• Single shell with a single opening
• Snails often have an operculum

Gastropoda - Coiling

• Spiral winding of the shell and visceral mass evolved earlier than and separately from torsion
• All gastropods descended from coiled and torted ancestors, including slugs
• Highly cephalized
• Pair of sensory tentacles, often with simple eyes, on the head
• Radula is present
• Terrestrial snails have a vascularized mantle cavity
• Functions as a rudimentary lung

Class Cephalopoda

• "Head foot"
• About 700 species
• Two extant subclasses: Nautiloidea (nautiluses) and Coleoidea (squids, octopuses, cuttlefishes)
• A funnel derived from the foot, opening of the mantle cavity
• Circumoral arms derived from the head-foot, with suckers
• Beak is bird-like and is made of chitin
• Radula inside of the mouth
• Well-developed eyes

Cephalopod - Respiration and Circulation

• Coleoids have one pair of gills
• Nautiloids have two pairs
• Muscle pumps circulate water through the mantle cavity
• Closed circulatory system
• Systemic heart is the main pumping organ
• Branchial hearts are at gills

Cephalopod - Brain and Eye

• Highly cephalized, with complex sensory organs
• Largest brain relative to body mass among all invertebrates
• Image-forming eyes are similar to vertebrate eyes, with retina and lens
• Homoplastic to the vertebrate eye

Cephalopod - Locomotion

• Most can expel water from the mantle cavity through the funnel to propel themselves
• Squids and cuttlefishes have lateral fins to aid swimming
• Octopuses crawl on substrates using arms
• All are active predators

Cephalopod - Reproduction

• They are dioecious
• Males encase spermatozoa in spermatophores, stored in the mantle cavity
• Hectocotylus is a modified arm that the male uses to deliver spermatophore into the female's mantle cavity

Subclass Nautiloidea

• A single family, Nautilidae
• Have six extant species but many more fossil species
• Found only in tropical Indian and Pacific Oceans
• Numerous tentacles with adhesive ridges but no suckers
• Have two pairs of gills
• Multi-chambered shell provides protection and buoyancy
• Body in the largest outer chamber
• Smaller inner chambers store and release air

Subclass Coleoidea

• Superorder Octopodiformes are eight-armed, and octopuses belong here
• Superorder Decapodiformes have eight arms and two long tentacles (squids and cuttlefishes)
• They have suckers on arms
• Cuttlefish has a curved shell enclosed in the mantle
• Squid has a thin strip of vestigial shell (pen)
• Octopus has no shell
• Most coleoids have an ink sac, with a gland that secretes ink, expels it through the anus, and it is used for defense