Protostomes Deuterostomes Sponges Nephrozoan animals are divided into 2 major lineages that are distinguishable by features of their embryos: Cnidarians acoel flatworms Protostomes Deuterostomes

- open circulatory system - exoskeleton for muscle attachment Protostomes Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods In 2 most speciose groups, coelom has been greatly reduced Replaced by: - open circulatory system - exoskeleton for muscle attachment Echinoderms Chordates

Phylum Mollusca ~100,000 species Soft body divided into 3 parts: a) foot, for locomotion b) visceral mass = internal organs c) mantle, a flap of tissue that secretes the shell - Open circulatory system spouts blood through a body cavity called hemocoel (replaces coelom, not needed due to shell) - Ctenidia = flaps used as gills, for respiration (getting oxygen) - Radula = toothed ribbon used to scrape food - Develop through a larval stage called a trochophore, like annelid worms, to a unique veliger larva

Phylum Mollusca Class Aplacophora - Small + wormlike, primitive (no shell) Class Monoplacophora - Deep sea, cap-like shell; segmented? Class Polyplacophora - Chitons Class Gastropoda - Snails + slugs Class Bivalvia - Clams, mussels, oysters Class Cephalopoda - Octopus, squid, cuttlefish, nautilus Class Scaphopoda - Tusk shells

Molluscan Body Plan Coelom Visceral nerve Pedal nerve Mantle Ventricle Coelom Mantle Mantle cavity Visceral nerve Pedal nerve Mantle cavity encloses gills; receives waste, gametes; water may be pumped through it by cilia, or muscle action

Shells Mantle tissue expresses proteins that cause calcium carbonate to crystallize out of the ocean water as calcite or aragonite, different crystalline forms - form layers of minerals + conchin, a cross-linked protein - inner layer of pearly nacre in primitive groups Shell = physical defense against predators Also acts as a type of exoskeleton, to which muscles can attach and work off of - molluscs no longer need a coelom as as internal skeleton; coelom is very reduced, to a sac around heart

Shells vary among classes Chitons: 8 separate plates Bivalves (clams): 2 shells, hinged at dorsal umbo Gastropods (snails): 1 twisted shell with an operculum (trapdoor) Cephalopods: nautilus 1 shell squid internal shell octopus no shell  

Gastropod Shell aperture each turn = 1 whorl most spiral to the right (clockwise) trapdoor = operculum Made of: (1) outer organic layer (2) chalky calcium carbonate layer (3) inner pearly nacreous layer (missing in most higher gastropods)

Columella Spire = all the old whorls added up together Body whorl = most recent whorl, where most of the body is located

Shell Diversity Siphonal opening Look for spines or ridges, which prevent predatory crabs from getting their claws around shell to crack it open

Radula Unique molluscan characteristic: ribbon of chitonous teeth - projects from bucchal cavity in throat - anchored over odontophore structure - moved by protractor (out) and retractor (in) muscles - tips of teeth may be hardened by iron deposits May add up to 5 rows of teeth per day Can be used for herbivory, scraping algae, diatoms off rocks Can be used for predation (rasping sponges, tissue from other animals, carrion); may be highly modified (poison-injecting teeth of cone snails)

Radula Marginal teeth Median tooth Vetigastropods: Caenogastropods: - primitive condition, w/ many marginal teeth - these sweep particles into central furrow, where they’re caught on median teeth Caenogastropods: - advanced condition, w/ no marginal teeth - median teeth used for tearing, punching holes

- protractors extend the radula, odontophore - retractors withdraw, scraping surface with teeth as radula pulled back inside its sac

Digestive system Molluscs have complete guts: (1) buccal cavity (mouth) containing radula (2) esophagus links foregut to stomach - salivary glands to lubricate radula - gizzard to help grind up vegetation in some herbivores (3) stomach with finger-like extensions called digestive ceca or diverticula that weave into many regions of the body (4) intestine leading to anus, which opens into mantle cavity

Ctenidia - also used in filter feeding Ctenidia (gills) grow from wall of mantle cavity O2-poor blood enters through afferent vessel O2-rich blood leaves through efferent vessel Ciliary beating moves water past incoming O2-poor blood

Veliger larva Velum Eggs develop into a trochophore, like annelids Trochophore may develop in the plankton (abalone, limpets) or within a benthic egg capsule (all others) produced by mom Next stage is veliger larva, unique to molluscs Eye Velum Mantle cilia Statocyst Propodium Operculum

Velum shell eyespots

Bands of cilia Food groove Ciliary currents used for: -- swimming -- catching phytoplankton cells for food Bands of cilia Food groove

Phylum Mollusca Class Aplacophora - Small + wormlike, primitive (no shell) Class Monoplacophora - Deep sea, cap-like shell; segmented? Class Polyplacophora - Chitons Class Gastropoda - Snails + slugs Class Bivalvia - Clams, mussels, oysters Class Cephalopoda - Octopus, squid, cuttlefish, nautilus Class Scaphopoda - Tusk shells

Class Aplacophora Class Monoplacophora No foot, head, tentacles Rodlike spicules, instead of shell Split off early, before shell evolved... or lost it later? Single shell, “segmented” arrangement of organs?

Phylum Mollusca Class Aplacophora - Small + wormlike, primitive (no shell) Class Monoplacophora - Deep sea, cap-like shell; segmented? Class Polyplacophora - Chitons Class Gastropoda - Snails + slugs Class Bivalvia - Clams, mussels, oysters Class Cephalopoda - Octopus, squid, cuttlefish, nautilus Class Scaphopoda - Tusk shells

Class Polyplacophora: Chitons 1,000 spp. - Marine, mostly intertidal - No eyes, tentacles, or crystalline style in gut - Paired ctenidia in mantle cavity - may coat radular teeth with iron deposits for strength Synapomorphies: - 8 valves (dorsal shell plates) with articulamentum layer - Mantle forms thick girdle: may secrete calcareous spicules

Chiton external anatomy spicules imbedded in girdle tail valve, showing inner articulamentum layer (outer layer)

Chiton internal anatomy

Water flow through mantle cavity is controlled cilia on ctenidia

Phylum Mollusca Class Aplacophora - Small + wormlike, primitive (no shell) Class Monoplacophora - Deep sea, cap-like shell; segmented? Class Polyplacophora - Chitons Class Gastropoda - Snails + slugs Class Bivalvia - Clams, mussels, oysters Class Cephalopoda - Octopus, squid, cuttlefish, nautilus Class Scaphopoda - Tusk shells

Old Systematics Class Gastropoda - Snails + slugs Subclass Prosobranchia (marine snails) Archaeogastropods – limpets + abalone Mesogastropods – snails Neogastropods – smarter snails Subclass Opisthobranchia – sea slugs Subclass Pulmonata – terrestrial snails + slugs

Modern gastropod phylogeny Eogastropoda (true limpets) Orthogastropoda Vetigastropoda (abalone, top shells, keyhole limpets) Caenogastropoda (“prosobranch” marine snails) opisthobranchs (sea slugs) Heterobranchia pulmonates (land snails + slugs)

Gastropoda: Snails + slugs ~ 70,000 spp. Asymmetrical body usually inside a coiled shell; undergo torsion during development, to fit body into spiral shell Successfully colonized dry land 1) muscular foot for movement 2) radula for herbivory on plants: ready food supply In heterobranchs, ctenidia replaced with other gill structures - sea slugs have special posterior gills - pulmonates have lungs for breathing air Head with eyes, 1-2 tentacles, complex radula Very complex reproductive systems

Torsion Developmental process that allows body to fit into spiral shell with both anus and head at the opening In late veliger stage, visceral mass rotates 180o over the foot, counter-clockwise Proceeds in 2 steps: - first a 90o twist, done by foot retractor muscle - second 90o twist by differential tissue growth on the 2 sides Combined with spiral shells, led to reduction or loss of organs on right side of adult body (ctenidium, atrium, nephridium) - sole exception is the gonad, which remains on the right (we’ll see why in a bit)

Torsion PRE-TORSION POST-TORSION mantle cavity, anus go from posterior to anterior

Torsion Molluscs have typical protostome paired, ventral nerve chords Nerve chords end up twisting around in a figure-8

Gastropod Reproduction Basal gastropods are dioecious (have separate sexes) In derived gastropods, the right nephridium (kidney) was lost during evolution of torsion, along with most right-side organs - its plumbing and pore to the outside was taken over by the gonad, however - gonad doesn’t share ducts and pore with the excretory system, as is typical for many protostomes (e.g., annelids) - allowed complex reproductive systems to evolve, including egg capsules for embryos Derived heterobranch gastropods evolved hermaphroditism

Vetigastropoda Abalone, top snails, keyhole limpets - Basal gastropods - Shell w/ nacre layer on inside - Radula for herbivory, w/ numerous teeth in rows - Mantle cavity lacking a siphon - Most organs are paired - Separate sexes; most are broadcast spawners

Vetigastropods Wavy Top Shell, Astrea undosa Giant Keyhole Limpet, Megathura crenulata Tegula sp.

Abalone (Haliotis) excurrent tremata epipodial tentacles cephalic

red abalone green abalone pink abalone black abalone

Broadcast spawning in abalone Sperm navigate owards, then bind to egg Males and females spawn gametes into the surrounding sea water In more derived (advanced) gastropods, fertilization is internal

Abalone larval settlement Abalone zygotes pass through a swimming trochophore stage, then become veliger larvae, and finally settle + metamorphose on encrusting coralline red algae – which gives the shells of red abalone their reddish coloration