part of 34 Flashcards
Phylum Mollusca
Second in diversity only to arthropods
Include snails, slugs, clams, octopuses and others
Some have a shell, some do not
Range in size from microscopic to huge
Giant clams may weigh 270 kg
Evolved in the oceans, and most groups have remained there
Important source of human food
Economically significant in other ways
Pearls are produced in oysters
Mother-of-pearl is produced in the shells of abalone
Pests – Zebra mussel (Dreissena polymorpha)
Mantle Thick epidermal sheet Bounds mantle cavity Secrete shell (if there is one)—used for gas exchange if it doesn’t Foot Primary means of locomotion for many Divided into arms or tentacles in cephalopods Attachment, food capture, digging
Internal organs
Coelom is highly reduced
Limited to small spaces around the excretory organs, heart, and part of the intestine
Digestive, excretory, and reproductive organs are concentrated in a visceral mass
Ctenidia – gills in aquatic mollusks
Also filter food in most bivalves
Shell
Protects against predators and adverse environments
Secreted by outer surface of mantle
Clearly not essential – repeated loss or reduction
Typical shell has 2 layers of calcium carbonate
Internal layer may be mother-of-pearl or nacre
Pearls are formed by coating foreign object with nacre to reduce irritation
No shell—cuttlefish, squids and octopuses (cephalopods) and slugs
(gastropods)
Radula
Characteristic of most mollusks
Rasping, tonguelike structure with chitinous teeth used in feeding
Used to scrape up algae
In predatory gastropods, modified to drill through clam shells
In Conus snails, modifies into harpoon with venom gland
Bivalves do not have a radula
Gills used in filter feeding
Nitrogenous waste removal – nephridia Consist of cilia-lined openings called nephrostomes Tube to excretory pore to mantle cavity Circulatory system Open circulatory system Hemolymph sloshes around hemocoel 3-chambered heart Cephalopods have a closed circulatory system
Mollusk Reproduction
Most mollusks are gonochoric A few are hermaphroditic Some oysters change sex Most engage in external fertilization Gastropods have internal fertilization Mollusk zygote undergoes spiral cleavage
Trochophore Free-swimming larval stage Veliger Second free-swimming larval stage Only in bivalves and most marine snails Both forms drift widely in the ocean
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Classes of Mollusks
There are 7 or 8 recognized classes Polyplacophora – chitons Gastropoda – limpets, snails, slugs Bivalvia – clams, oysters, scallops Cephalopoda – squids, octopuses, cuttlefishes, and chambered nautilus
Class Polyplacophora (Chitons)
Marine mollusks that have oval bodies
8 overlapping dorsal calcareous plates
Body is not segmented under the plates
Most chitons are grazing herbivores
Class Gastropoda
Limpets, snails, slugs
A primarily marine group – some freshwater, and only terrestrial mollusks
Most have a single shell – some lost it
Heads typically have pairs of tentacles (chemoreceptors or mechanoreceptors) with eyes
Torsion
Unique among animals
Mantle cavity and anus are moved from the posterior to the front
–Nudibranchs are active predators
Exposed gills
Many secrete distasteful chemicals
Feed on cnidarians and some extract nematocysts from cnidarians and transfer them to their body surface
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Class Bivalvia (Bivalves)
Includes clams, scallops, mussels, oysters, and others
Most marine, some freshwater
No radula or distinct head
Have 2 shells (valves) hinged together
Adductor muscles counter hinge ligament
Water enters through inhalant siphon and exits through exhalant siphon
Class Cephalopoda
More than 600 strictly marine species
Active marine predators
Only mollusk with closed circulatory system
Foot has evolved into a series of arms equipped with suction cups
Beak-like jaws, toxic saliva
Largest relative brain sizes among invertebrates
Highly developed nervous system
Living cephalopods lack external shell
Except chambered nautilus
Squid and cuttlefish have internal shells
Jet propulsion using siphon
Ink can be ejected from siphon
Chromatophores allow for changing skin color for camouflage or communication
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Phylum Nemertea
About 900 species of cylindrical to flattened very long worms
Most are marine; a few species live in fresh water and humid terrestrial habitats
Lineus longissimus has been reported to measure 60 m in length – the longest animal known!
Body plan resembles a flatworm
Has a complete gut
Rhynchocoel – fluid filled coleomic cavity
Gonochoric with sexual reproduction Asexual reproduction through fragmentation Belong to lophotrochozoans because Blood flows entirely in vessels Rhynchocoel
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Phylum Annelida
Segmented worms
Body built of repeated units
Allows for specialization
May not be monophyletic
Body plan Head has well-developed cerebral ganglion Sensory organs in ring like segments Many species have eyes Segments divided internally by septa Each segment has a pair of excretory organs, a ganglion, and locomotory structure Closed circulatory system Ventral nerve cord
Each part of digestive tract specialized for different function
Locomotion
Coelomic fluid creates a hydrostatic skeleton
Alternating muscle contractions allows complex movements—circular and longitudinal muscles
Chaetae – bristles of chitin found in most groups
Closed circulatory system
Gas exchange by diffusion across body surfaces
Excretory system – nephridia like mollusks—2 pair per segment
Roughly 12,000 described species of annelids occur in many habitats 2 classes Class Polychaeta Monophyly not well established Class Clitellata Oligochaeta Hirudinea
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Class Polychaeta (Polychaetes)
Include clamworms, scaleworms, lugworms, sea mice, tubeworms
Have paired parapodia on most segments
Used in locomotion or gas exchange
Chaetae on parapodia
-Deep-sea tubeworm Riftia Adults gutless Sulfur-oxidizing bacteria synthesize organic compounds used by worm Found near hydrothermal vents Most gonochoric External fertilization Trochophore larva
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Class Clitellata
Clitellum found in all members Earthworms Head not well differentiated No parapodia Few chaetae project from body wall Eat soil and digest organics Hermaphroditic but cross-fertilize Clitellum secretes mucus cocoon
Leeches Occur mostly in freshwater Usually flattened dorsoventrally Hermaphroditic and cross-fertilization Clitellum develops only during breeding season Coleom reduced, not divided into segments Suckers at both ends of body No chaetae (except for one species) Some eat detritus, others suck blood
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The Lophophorates
Include two phyla of mostly marine animals
Bryozoa and Brachiopoda
Both characterized by lophophore
Circular or U-shaped ridge around the mouth with 1–2 rows of ciliated tentacles
Functions in gas exchange and feeding
May be convergent evolution
Phylogeny a puzzle
Phylum Bryozoa
Also called Ectoprocta or “moss animals”
Bryozoans are small (0.5 mm) and only animal to live in colonies
Their anus opens near their mouth
Individuals secrete a tiny chitinous chamber called a zoecium attached to substrate
May deposit calcium carbonate (formed reefs in past)
Asexual reproduction occurs frequently by budding
Phylum Brachiopoda
Brachiopods or lamp shells
Have two calcified shells
Dorsal and ventral (not lateral as in bivalves)
Lophophore lies on the body, between the shells
Phoronids were once a separate phylum
Each individual secretes a chitinous tube
Extends lophophore to feed
