Invertebrates

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Invertebrates

• A Survey of Invertebrates

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Trends in Invertebrate Evolution

• Common ancestors of multicellular animals had
  already evolved two distinct cell layers called
  germ layers, separated by a jelly-like middle
  layer

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Germ Layers

• The ectoderm (outer layer) will develop into skin
  and other body coverings, glands, and nervous
  system tissues
• The endoderm (inner layer) grows into tissues and
  organs of the digestive tract and respiratory
  system

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Germ Layers

• Middle layers varies by animal
• Sponges and jellyfish only have a primitive layer
  of jelly-like cells
• Higher animals have an actual third cell layer
  called the mesoderm which develops into skeletal,
  muscle, and circulatory tissues, and reproductive
  system

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Trends in Invertebrate Evolution

• Second trend existence of mesoderm-lined cavity
  called coelom
• Provides space for organs to grow and function
  without being twisted or squeezed by body
  movements
• Fluid within coelom helps to carry food, wastes,
  dissolved gases

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Coelom

• Acoelomates animals without a coelom, like the
  hydra
• Pseudocoelomates have a body cavity between
  mesoderm and endoderm, like roundworm
• Coelomates have a true coelom that houses
  digestive tract and organs, like annelids
  (earthworm)

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Trends in Invertebrate Evolution

• Third trend is the evolution of a body plan,
  built of several compartments
• Compartments called segments, allow for increase
  in body size
• In higher animals, segments are specialized and
  more complex

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Trends in Invertebrate Evolution

• Many organisms also have symmetry
• Radial symmetry body parts repeated around an
  imaginary line drawn through center of the body
• Bilateral symmetry left and right sides are
  identical

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Trends in Invertebrate Evolution

• Organisms with bilateral symmetry also show
  cephalization, which means that they have a
  distinct front end and back end (anterior and
  posterior)

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Invertebrate Phyla

• Phylum Porifera (sponges)
• Phylum Cnidaria (cnidarians)
• Phylum Platyhelminthes (flatworms)
• Phylum Nematoda (nematodes or roundworms)
• Phylum Mollusca (mollusks)
• Phylum Annelida (annelids)
• Phylum Echinodermata (echinoderms)
• Phylum Arthropoda (arthropods)
• Phylum Chordata (invertebrate chordates)

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Phylum Porifera (sponges)

• Simplest multicellular animals
• Asymmetric
• Mostly independent, but can live together
• Mostly marine, some in freshwater

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Phylum Cnidaria (cnidarians)

• Jellyfish, corals, sea anemones, hydras
• Mostly marine, few freshwater
• Radial symmetry

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Phylum Plathyhelminthes (flatworms)

• Bilateral symmetry
• Show cephalization
• Mostly parasitic

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Phylum Nematoda (nematodes AKA roundworms)

• First to show a tube-like digestive system (mouth
  at one end, anus at the other end)
• Some are microscopic, some can be a meter long

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Phylum Mollusca (mollusks)

• Clams, snails, squids
• Can live everywhere, even on land
• Many have outer shells for protection
• Sea snails secrete chemicals to avoid predators
• Clams, mussels, oysters, scallops are bivalves
  (double shells) and can shut when in danger
• Squids and octopuses also emit dark ink to
  confuse predators

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Phylum Annelida (annelids)

• AKA segmented worms
• Many are detritus feeders
• Live on land and in water

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Phylum Echinodermata (echinoderms)

• Star fish, sea cucumbers, sea urchins)
• Live entirely in water
• Spiny skin
• Have five-part radial symmetry

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Phylum Arthropoda (arthropods)

• Most successful animals - largest and most
  diverse phylum
• Jointed-leg animals
• Exoskeletons
• Divided into three groups

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Phylum Arthropoda (arthropods)

• Subphylum Chelicerata includes spiders and
  scorpions
• Named by mouth parts (chelicerae)
• Mostly carnivorous, can be posionous
• Class Crustacea includes lobster, shrimp, crab
• Class Insecta contains ½ of all arthropods

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Phylum Chordata

• Invertebrate chordates
• Link between invertebrates and vertebrates
• Endoskeleton with notochord, to which muscles are
  attached
• Ex Sea squirt