chapter 33

• Invertebrates


  95% of known animal species, morphologically diverse
• Ancestral groups of invertebrates

  • Porifera
  • Cnidaria
  • Lophotrochozoa
  • Ecdysozoa
  • Deuterostomia
• Porifera (sponges)


  • Basal animals that lack true tissues
  • Sedentary, most are marine, few in fresh water
  • Size range: few millimeters to few meters
  • Suspension feeders, capturing food particles suspended in the water that passes through their body
  • Lack true tissues and organs

  
• Spongocoel


  Cavity in sponges that water is drawn into
• Osculum


  Opening in sponges that water flows out of
• Sponge structure


  • Gelatinous non-cellular mesohyl layer between two cell layers
  • Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food
  • Amoebocytes in the mesophyl have roles in digestion and structure

  
• Hermaphroditism in sponges


  Almost all sponges exhibit sequential hermaphroditism: They function first as one sex and then as the other
• Reproduction in sponges

  1. Gametes arise from choanocytes or amoebocytes

  2. Eggs reside in the mesohyl, but sperm are carried out of the sponge by the water current

  3. Cross-fertilization results from some of the sperm being drawn into neighboring individuals

  4. Fertilization occurs in the mesohyl

  5. Zygotes develop into flagellated, swimming larvae that disperse from the parent sponge

  6. After settling on a suitable substrate, a larva develops into a sessile adult

  
• Eumetazoa


  Animals with true tissues
• Cnidaria


  One of the oldest groups in the Eumetazoa clade
• Cnidarians


  • Include: Corals, Jellyfish, and Hydras- mobile and sessile forms
  • Have a diploblastic, radially symmetrical body plan
  • The basic body plan is a sac with a central digestive compartment, the gastrovascular cavity
  • A single opening functions as mouth and anus
  • Two variations on the body plan: the sessile polyp and mobile medusa
• Polyp


  Cnidarian form that adheres to the substrate
• Medusa


  Cnidarian form with a bell-shaped body and opening on the underside, moves freely
• Cnidarian feeding and defense

  • Carnivores that use tentacles to capture prey
  • The tentacles are armed with cnidocytes, unique cells that function in defense and capture of prey
  • Nematocysts are specialized organelles within cnidocytes that eject a stinging thread
• Nerve net


  Non-centralized nerve system in cnidarians associated with sensory structures and distributed around the body, coordinates movement
• Major classes of Cnidaria

  • Hydrozoa
  • Scyphozoa
  • Cubozoa
  • Anthozoa
• Hydrozoans


  • Small predatory animals
  • Most hydrozoans alternate between polyp and medusa forms
  • Hydra, a freshwater cnidarian, exists only in polyp form
• Scyphozoans


  • In the class Scyphozoa, medusae are the prevalent form of the life cycle
  • Most coastal scyphozoans go through a stage as small polyps during their life cycle, whereas those that live in the open ocean generally lack the polyp stage altogether
• Cubozoans


  • Include box jellies and sea wasps, (box-shaped medusa stage)
  • Have complex eyes embedded in the fringe of their medusae
  • Comparatively strong swimmers, less likely to be stranded on shore
  • Most live in tropical oceans and are equipped with highly toxic cnidocytes

  
• Anthozoans


  • Sea anemones and corals ("flower animals")
  • Occur only as polyps
  • Many corals secrete a hard external skeleton of calcium carbonate
  • Each polyp generation builds on the skeletal remains of earlier generations, constructing "rocks" with shapes characteristic of their species
• Coral reefs are to tropical seas what rain forests are to tropical land areas: They provide habitat for many other species
• Coral reefs are being destroyed at an alarming rate due to pollution, overfishing, and global warming raising seawater temperatures
• Bilateria


  Clade containing Lophotrochozoa, Ecdysozoa, and Deuterostomia, have bilateral symmetry and triploblastic development, most have a coelom and a digestive tract with two openings
• Lophotrochozoa


  Clade identified by molecular data, have the widest range of animal body forms, some develop a lophophore for feeding, others pass through a trochophore larval stage, but a few have neither feature

  
• Lophotrochozoan groups


  • Platyhelminthes
  • Acanthocephala
  • Mollusca
  • Nemertea
  • Cycliophora
  • Annelida
  • Ectoprocta
  • Syndermata (Rotifers)
  • Brachiopoda
• Platyhelminthes (flatworms)


  • Live in marine, freshwater, and damp terrestrial habitats
  • Acoelomates, flattened dorsoventrally
  • Gastrovascular cavity with one opening
  • Gas exchange and osmoregulation via protonephridia
• Free-living flatworms: Planarians


  • Live in fresh water and prey on smaller animals
  • Have light-sensitive eyespots and a centralized nerve net more complex than cnidarians
  • Hermaphrodites
• Feeding in planarians


  1. Pharynx spills digestive juices onto prey, sucks small piece of food into gastrovascular cavity

  2. Completes digestion in gastrovascular cavity
• Parasitic flatworms


  • Trematodes and tapeworms
  • Trematodes parasitize a wide range of hosts and have complex life cycles, some infect humans
  • Tapeworms are parasites of vertebrates and lack a digestive system, absorbing nutrients from the host's intestine
• Life cycle of a blood fluke (Schistosoma mansoni)


  1. Mature flukes live in blood vessels of human intestine, reproduce sexually there and fertilized eggs leave in feces

  2. If feces reach water source, eggs develop into ciliates which infect snails- the intermediate host

  3. Leave snail as motile larva which penetrate skin of humans in contaminated water

  
• Tapeworms


  • Scolex contains suckers and hooks for attaching to the host
  • Proglottids are units that contain male and female sex organs and form a ribbon behind the scolex
  • Fertilized eggs leave the host's body in feces
• Rotifers


  • Tiny animals that inhabit fresh water, the ocean, and damp soil
  • Have a digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom
  • Reproduce by parthenogenesis, in which females produce offspring from unfertilized eggs
• Lophophore


  Crown of ciliated tentacles around the mouth of lophophorates
• Lophophorates


  Include the phyla Ectoprocta and Brachiopoda, have a true coelom

  
• Pork tapeworm scolex


  • Contains suckers and hooks for attaching to the host
• Pork tapeworm proglottids


  • Units that contain male and female sex organs and form a ribbon behind the scolex
• Pork tapeworm reproduction


  Fertilized eggs leave the host's body in feces
• Rotifers


  Tiny animals that inhabit fresh water, the ocean, and damp soil
• Rotifers


  • Smaller than many protists but are truly multicellular and have specialized organs systems
• Rotifer anatomy


  • Digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom