Lophotrochozoa protostomes {mollusc}| (Mollusca) {mollusk} can have hard carbonate shells. Molluscs are second largest phylum: oyster, clam, octopus, squid, snail, slug, and giant squid. Land snails are molluscs with lungs. Squid seem to feel pain. Molluscs have ganglia groups, each controlling one activity, in ring around gut. Snails have simple eyes. Squid and octopus have compound eyes. Other organ systems are like those in arthropods.
Molluscs have broad flat appendages {foot, mollusc} for creeping.
Molluscs have sheaths {mantle, mollusc} covering visceral mass and foot.
Pharynx has hard parts {radula}|, to break plants or shells. Oysters and clams have no radula.
Molluscs have body-organ masses {visceral mass}.
Bivalves have tubes {siphon, bivalve}| that send water out.
Eye microvilli can lie parallel, exhibit dichroism {rhabdom}, and detect polarized-light polarization plane.
Molluscs {bivalve}| (Pelecypoda) can have two shells, hinged at one side, no foot, one tube and valve that takes in water, and siphon tubes that send water out. Gills filter flowing water. Mucus carries food to mouth. Bivalves include oysters, clams, mussels, cockles, and scallops. Foot comes out of shell for movement. Clams and mussels burrow. Oysters do not move. Scallops move by clamping shells shut.
Mussels have sticky threads {byssus}.
Large clams {giant clam} {tridacna clam} can burrow.
Molluscs {cephalopod} (Cephalopoda) include squid, cuttlefish, octopus, and nautilus.
evolution
Cephalopods began 500,000,000 years ago.
anatomy
Head and foot combine. Eight tentacles in octopus, or ten tentacles in squid, have suckers. Two beaks are in mouth. Mantle can fill with water and eject water for jet propulsion movement after mantle receives signals from giant axons. Ink sac squirts to confuse enemies.
anatomy: shell
Cephalopods have little or no shell, as in squid and octopus, or chambered shells, as in nautilus. Nautilus secretes gas into chambers, to float.
anatomy: eye
Eyes develop from skin folds. Octopus rapidly learns visual and tactile discriminations by trial-and-error and can learn complex landscape, using same visual cues that people do. Nautilus eyes are pinholes, with statocysts and eye muscles. Other cephalopods have eyes with photoreceptors in microvilli at right angles, to detect plane-polarized light.
anatomy: brain
Statocysts can detect three-dimensional movement. A cephalopod brain region acts like cerebellum. Cephalopods have visual-memory brain structures. They have no myelin.
blood
Hemocyanin copper protein, which has low oxygen-carrying capacity, causes green blood.
Cephalopods {chambered nautilus} {nautilus} can be tropical and have shells. Nautilus has visual pits, which are indentations with pigmented cells and focus light like pinhole cameras.
Flat spiral-shelled, octopus-like sea animals {ammonite}| are extinct.
Molluscs {gastropod} {univalve} {gastropoda} include snail, limpet, abalone, and slug. During development, they twist so anus is above head. They have one heart, one gill, one kidney, one gonad, one valve, and one muscular foot.
Gastropods {abalone} can have large colorful shells.
Tropical gastropods {conch} can have spiral shells.
Small shelled gastropods can stick to tidal rocks {limpet}.
Large gastropods {triton} can have spiral shells.
Marine slugs and marine snails {nudibranch}| have no shells.
Sea snails {cone snail} can make peptide toxin {cone snail venom} {conantokin} that paralyzes fish or molluscs by affecting calcium-ion channels. Cone snails shoot out one tube with poison at end. 500 species have 50,000 different peptides.
Marine gastropods can have no shells {sea slug} {marine slug}.
Marine gastropods {pteropod} {sea snail, mollusc} {marine snail} are small.
Land nudibranchs {slug, mollusc} have no shells.
Nudibranchs {snail, mollusc} can live on land or in water. Land nudibranchs have shells.
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Date Modified: 2022.0225