Mollusca

Outlines

PHYLUM MOLLUSCA

CLASS Caudofoveata: Small, wormlike. Live upside down in vertical burrows on the deep-sea floor. (70 spp)

CLASS Aplacophora (Solenogastres): vermiform, shell-less. Marine, depths greater than 200m. (250 spp)

CLASS Monoplacophora: Single, caplike shell. Foot forms weak ventral disk. No eyes. Most live at considerable depths. (11 spp in 3 genera)

CLASS Polyplacophora: Chitons. Flattened, elongate. 8 dorsal shell plates. No eyes. (600 spp)

ORDER Lepidopleurida
ORDER Ischnochitonida
ORDER Acanthochitonida

CLASS Gastropoda: Snails and slugs.

SUBCLASS Prosobranchia
- ORDER Archaeogastropoda : primative forms. 3000 spp in 26 families. Includes marine limpets.
- ORDER Mesogastropoda: Many marine, terrestrial, and freshwater snails. 10,000 spp in 100 families. Includes periwinkles, moon snails.
- ORDER Neogastropoda: mostly marine snails. 24 families.
SUBCLASS Ophisthobranchia: Sea slugs and their kin. 9 orders, 100 families, but may not be monophyletic.
SUBCLASS Pulmonata: Land snails and slugs.
- ORDER Archaeopulmonata: primitive, no operculum. Mainly littoral.
- ORDER Basommatophora: includes freshwater limpets.
- ORDER Stylommatophora: Terrestrial snails. Over 15,000 spp.
- ORDER Systellommatophora: sluglike.

CLASS Bivalvia (Pelecypoda; Lamellibranchiata): Clams, oysters, etc.

SUBCLASS Protopbranchia
- ORDER Nuculoida
- ORDER Solemyoida
SUBCLASS Lamellibranchia
- SUPERORDER Filibranchia (Pteriomorphia): primative bivalves, including mussels.
- SUPERORDER Eulamellibranchia (Heterodonta): Advanced bivalves.
  - ORDER Paleoheterodonta: 1,200 spp of marine and freshwater clams.
  - ORDER Veneroida: cockles, giant clams, etc.
  - ORDER Myoida: Thin-shelled burrowing forms, eg. shipworms.
SUBCLASS Anomalodesmata (includes old Septibranchia): carnivorous marine bivalves. One order, 12 families.

CLASS Scaphopoda: Tusk shells. Marine, benthic. 350 spp in 8 families.

CLASS Cephalopoda: Nautilus, squids, cuttlefish, octopuses.

SUBCLASS Nautiloidea (Tetrabrachiata): pearly nautilus. Mostly fossil, only one extant genus with 5 or 6 Indo-Pacific spp.
SUBCLASS Coleoidea (Dibranchiata): Octopuses and squids.
- ORDER Sepioida: Cuttlefish.
- ORDER Teuthoida (Decapoda): Squids, including Architeuthis.
- ORDER Octopoda: Octopuses. About 200 spp.
- ORDER Vampyromorpha: Vampire squids.

PHYLUM MOLLUSCA

Body Plan

Triploblastic; true coelom, though often reduced in adult forms to pericardial chamber, and spaces around gonads, parts of kidney, and occasionally parts of intestine. Bilateral symmetrical. Body usually comprised of head, foot, and visceral mass. Body covered by a sheet of epidermal-cuticular skin called the mantle, forms space between visceral mass and mantle called mantle cavity, usually houses gills, gut openings, nephridial and reproductive organs.

Locomotion

Usually by ripples along foot, which usually secretes mucus. Some variations are: use of foot as a hydraulic "probe and anchor" in burrowing bivalves; modification of foot into finlike parapodia for swimming (pteropods, or sea butterflies); drifting on surface using raft of bubbles secreted by foot (violet snails); swimming by clapping shell halves together (scallops); crawling about using tentacles(octopuses); and jet propulsion (squids, octopuses).

Feeding and Digestion

Two main divisions; macrophagy (predation and herbivory) and microphagy (filter feeding). Macrophagy usually involves radula which ranges from file-like scraper (most snails) to harpoon-like, poison-teeth-equiipped radula of cone snails. microphagy generally involves modifications of gills, which are covered by sheets of mucus which trap food, then are passed into the stomach. Digestive system is well-diffenentiated with esophagus, stomach, intestine, and anus. Modicfications inclde crops, exxtensive digestive glands, and cecae. microphagous molluscs often have crystalline style to wind up mucus strings, and typhlosole or ciliary sorting area.

Sense Organs

Most of the usual complement; chemoreceptors, mechanoreceptors, statocysts, ocelli, etc. Osphradia are specialized patches of chemosensory epithelia in the mantle cavity presumably sensing the chemical composition of the mantle fluid. Aesthetes are light-sensing organs unique to polyplacophoran molluscs. Often receptors are located on cephalic (or other) tentacles. At least one nudibranch and some snails show magnetoreception. Photoreceptors range from simple ocelli to highly sophisticated image-forming eye of cephalopods.

Nervous System

Generally a circumoesophageal ganglion anteriorly ("brain") and one or two large ventral nerve cords, often connected by (nonganglionated) commissures ("ladder" form). Bivalves have well-differentiated bilateral visceral and pedal ganglia. Cephalopods can be said to have a true brain, capable of learning and complex behavior. Squid escape responses are mediated by a giant axon system.

Respiration and gas exchange

Molluscs use true gills, with extensive capillary network and countercurrent flow for gas exchange. Some forms use a highly vascularized portion of the mantle to do this (e.g. pulmonate gastropods).

Circulation

True heart, usually 3-chambered (two atria and a ventricle). Blood from the pared gills enters the atria, leaves the ventricle. Circulatory system (except for cephalopods) is open: the artery from the ventricle opens and releases blood into tissue-bathing sinuses, eventually draining back into gills. Blood often contains an oxygen carrier such as hemocyanin, hemoglobin, or myoglobin. Blood also contains amebocytes and carrier proteins. Cephalopods have a closed circulatory system and an extra pair of powerful gill hearts.

Osmoregulation

Paired tubular metanephridial oragans (often called kidneys). Nephridiopores generally open into pericardial sac (coelom), and discharge into mantle cavity. Urine formed by active resorption.

Reproduction

Both hermaphroditic and gonochoristic forms. Gastropods may be either, but when hermaphroditic usually have only one kind of gonad (ovotestis) and are protandrous. Hermaphroditic gastropods nonetheless copulate and exchange gametes. Most bivalves are gonochoristic, though a few are hermaphroditic. Almost aall cephalopods are gonochoristic.

Embryology

Spiral cleavage, with trochophore larva. Cell fates deterministic. Schizocoelous and protostomate. Gastropods undergo an unusual process called torsion (though some secondarily detort).

Body Plan	Triploblastic; true coelom, though often reduced in adult forms to pericardial chamber, and spaces around gonads, parts of kidney, and occasionally parts of intestine. Bilateral symmetrical. Body usually comprised of head, foot, and visceral mass. Body covered by a sheet of epidermal-cuticular skin called the mantle, forms space between visceral mass and mantle called mantle cavity, usually houses gills, gut openings, nephridial and reproductive organs.
Locomotion	Usually by ripples along foot, which usually secretes mucus. Some variations are: use of foot as a hydraulic "probe and anchor" in burrowing bivalves; modification of foot into finlike parapodia for swimming (pteropods, or sea butterflies); drifting on surface using raft of bubbles secreted by foot (violet snails); swimming by clapping shell halves together (scallops); crawling about using tentacles(octopuses); and jet propulsion (squids, octopuses).
Feeding and Digestion	Two main divisions; macrophagy (predation and herbivory) and microphagy (filter feeding). Macrophagy usually involves radula which ranges from file-like scraper (most snails) to harpoon-like, poison-teeth-equiipped radula of cone snails. microphagy generally involves modifications of gills, which are covered by sheets of mucus which trap food, then are passed into the stomach. Digestive system is well-diffenentiated with esophagus, stomach, intestine, and anus. Modicfications inclde crops, exxtensive digestive glands, and cecae. microphagous molluscs often have crystalline style to wind up mucus strings, and typhlosole or ciliary sorting area.
Sense Organs	Most of the usual complement; chemoreceptors, mechanoreceptors, statocysts, ocelli, etc. Osphradia are specialized patches of chemosensory epithelia in the mantle cavity presumably sensing the chemical composition of the mantle fluid. Aesthetes are light-sensing organs unique to polyplacophoran molluscs. Often receptors are located on cephalic (or other) tentacles. At least one nudibranch and some snails show magnetoreception. Photoreceptors range from simple ocelli to highly sophisticated image-forming eye of cephalopods.
Nervous System	Generally a circumoesophageal ganglion anteriorly ("brain") and one or two large ventral nerve cords, often connected by (nonganglionated) commissures ("ladder" form). Bivalves have well-differentiated bilateral visceral and pedal ganglia. Cephalopods can be said to have a true brain, capable of learning and complex behavior. Squid escape responses are mediated by a giant axon system.
Respiration and gas exchange	Molluscs use true gills, with extensive capillary network and countercurrent flow for gas exchange. Some forms use a highly vascularized portion of the mantle to do this (e.g. pulmonate gastropods).
Circulation	True heart, usually 3-chambered (two atria and a ventricle). Blood from the pared gills enters the atria, leaves the ventricle. Circulatory system (except for cephalopods) is open: the artery from the ventricle opens and releases blood into tissue-bathing sinuses, eventually draining back into gills. Blood often contains an oxygen carrier such as hemocyanin, hemoglobin, or myoglobin. Blood also contains amebocytes and carrier proteins. Cephalopods have a closed circulatory system and an extra pair of powerful gill hearts.
Osmoregulation	Paired tubular metanephridial oragans (often called kidneys). Nephridiopores generally open into pericardial sac (coelom), and discharge into mantle cavity. Urine formed by active resorption.
Reproduction	Both hermaphroditic and gonochoristic forms. Gastropods may be either, but when hermaphroditic usually have only one kind of gonad (ovotestis) and are protandrous. Hermaphroditic gastropods nonetheless copulate and exchange gametes. Most bivalves are gonochoristic, though a few are hermaphroditic. Almost aall cephalopods are gonochoristic.
Embryology	Spiral cleavage, with trochophore larva. Cell fates deterministic. Schizocoelous and protostomate. Gastropods undergo an unusual process called torsion (though some secondarily detort).