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Biology 1215 Outline

Biology 1215 Lecture Notes

Chapter 33: Invertebrates

Outline
  • Sponges (phylum Porifera) are sessile animals lacking true tissues
  • Cnidarians (Phylum Cnidaria) are radiate, diploblastic animals with gastrovascular cavities
  • Flatworms (phylum Platyhelminthes) are acoelomate, bilateral, triploblastic animals
    • Planarians
    • Flukes
    • Tape worms
  • Nematodes (Phylum Nematoda) are pseudocoelomates with complete digestive tracts and blood vascular systems.
  • Mollusks, Annelids, and Arthropods are among major variations on the protostome body plan.
    • Phylum Mollusca
    • Phylum Annelida
    • Phylum Arthropoda
      • Trilobites
      • Spiders and other Chelicerates
      • Uniramians
      • Crustaceans
  • The deuterostome lineage includes echidoderms and chordates
  • Phylum Echinodermata

 

Animals are grouped into about 35 phyla and inhabit nearly all environments on earth. Over 95% of all animals are aquatic and invertebrate (animals without a backbone). Their diversity is the main subject of this chapter.

Sponges (phylum Porifera) are sessile animals lacking true tissues

  • Least complex of all animals ( Fig 33.2 and 3)
  • Range in height from 2 meters to 2 cm.
  • About 9,000 species known
  • Simple body. Resembles sac perforated with holes.
  • Sponges are suspension feeders. Water is drawn to a central cavity (spongocoel) and then flows out through osculum. Lining the inside of the body are flagellated choanocytes (collar cells). Colar traps food, flagella generates a water current.
  • Body wall consists of two layers separated by gelatenous layer (mesophyl). Amoebocytes wander through mesophyl, picking up food entrapped by choanocytes and distribute it to other cells.
  • Amoebocytes form tough skeletal fibers in mesophyl: secrete spicules (calcium carbonate or silica). In some sponges, a protein (spongin) is secreted making the organism flexible and spongy.
  • Most are hermaphrodites (both male and female). Zygotes develop into larvae (dispersal stage) covered with flagella. Eventually find a suitable substrate and begin sessile existence.
  • Cell layers of sponges are loose federations of cells, not really tissues because cells are relatively unspecialized.
  • Not much coordination between cells. Extensive regeneration capabilities (asexual reproduction).

Cnidarians (Phylum Cnidaria) are radiate, diploblastic animals with gastrovascular cavities

  • Include hydras, jellyfishes, sea anemones and coral animals.
  • Most primitive eumetazoa. Evolved very early.
  • Lack of mesoderm has limited cnidarians to relatively simple body construction.
  • Diverse. Over 10, 000 species known.
  • Basic body plan is sac with central digestive compartment (gastrovascular cavity; functions as both mouth and anus). Radially symetric. (Fig 33.4)
  • Two variations in body plan
    • 1. Sessile polyp (e.g. corals, sea anemones)
    • 2. Floating medusa (e.g. jelly fishes)
  • Cnidarians are carnivores. Use tentacles armed with cnidocytes (function in defense and capture of prey) (Fig 33.5).
  • Have simplest muscles and nerves. Do not have true muscles since they have no mesoderm. Use gastrovascular cavity as hydrostatic skeleton.
  • Have no brain. Behavior is rigid. Noncentralized nerve net used to detect stimuli from all directions.
  • Three major classes (Table 1):
    • 1. Hydrozoa (alternate between polyp and medusa forms ; Hydra is an exception) (Life cycle in Fig 33.7).
    • 2. Scyphozoa (medusa prevails in lifecycle; e.g. jellyfishes).
    • 3. Anthozoa (only polyp form; e.g. corals and anemones).

Flatworms (phylum Platyhelminthes) are acoelomate, bilateral, triploblastic animals

  • Over 20,000 species known
  • Live in marine, fresh water, moist habitats. Some are parasitic (e.g. flukes and tapeworms)
  • Range from microscopic to 20 m.
  • Evolutionary developments relative to cnidarians
    • bilaterally symmetrical
    • unidirectional movement
    • moderate cephalization
    • produce embryonic mesoderm (contributed to more complex organs and organ systems)
  • However, still possess gastrovascular cavity with one opening.
  • Planarians (Fig 33.10)
    • lack organs for gas exchange and circulation
      • wastes diffuse directly into environment. Existing simple excretory apparatus functions mainly in osmotic balance.
    • some cephalization; eyespots (light ) and lateral flaps (smell).
    • nervous system more complex than cnidarians. Modify behavioral responses to stimuli.
    • Hermaphrodites, but cross-fertilize
    • reproduce asexually through regeneration.
  • Flukes
    • live as parasites on other animals (Fig 33.11)
    • have life cycles with alternation of sexual and asexual stages (intermediate hosts)
    • 200,000,000 people infected around the world.
  • Tape worms
    • parasitic flatworms of vertebrates. Lack digestive sysntem. Rob nutrients and cause nutritional deficiencies in host.(Fig 33.12)
    • some may reach 20 m.

Nematodes (Phylum Nematoda) are pseudocoelomates with complete digestive tracts and blood vascular systems.

  • Pseudocoelomate condition arose independently a number of times.
  • Nematodes are cylindrical worms with tapered ends.
  • Species rich and numbers rich.
  • Found in most aquatic habitats, wet soils, moist tissues of plants, and body tissues and fluids of animals.
  • 90,000 species known
  • range from 1 mm to 1 m.
  • Pseudocoelom with fluid serves as a blood vascular system and hydrostatic skeleton. Have cuticle exoskleton (must mold to grow).
  • Have only longitudinal muscles.
  • Many are important agricultural pests, attacking the roots of plants.
  • Humans are host to 50 nematode species (pin-worms and hookworms, trichinosis)

Mollusks, Annelids, and Arthropods are among major variations on the protostome body plan.

Phylum Mollusca

  • Includes snails and slugs, oysters and clams, octopus and squid.
  • Over 150,000 known species.
  • Most are marine; some fresh water; snails and slugs have colonized land.
  • Molluscs are softbodied; some produce shell of calcium carbonate for protection.
  • Despite apparent differences, all mollusks share same body plan
    • body has 3 main parts (Fig 33.16):
    • muscular foot (movement)
    • visceral mass (contains internal organs)
    • mantle ( heavy fold of tissue draping visceral mass)
  • Life cycle of many include ciliated larvae (trochophore)
  • Lack segmentation.
  • Basic body plan has evolved in various ways in the 8 different classes. 4 of these considered here
    • Class Polyplacophora ( chitons have shells divided into 8 dorsal plates; body not segmented)
    • Class Gastropoda ( largest molluscan class; most are marine; most distinctive feature is Torsion (Fig 33.18).
    • Class Bivalvia (have shells divided into two; e.g. clams, oysters, mussels, scalops); most are suspension feeders; sedentary)
    • Class Cephalopoda ( built for speed; carnivorous; have beak-like jaws; some squids reach 17 m long, largest and most complex of invertebrates; only mollusk to have closed circulatory system, i.e. blood always contained in vessels; well developed nervous system; show ability to learn and behave in complex manner).

Phylum Annelida

  • See Fig 33.23
  • Have segmented bodies.
  • More than 15,000 species known
  • Coelom partitioned by septa; but digestive tract, longitudinal blood vessels and nerve cords run length of animal.
  • Digestive system has specialized regions.
  • Have closed circulatory system
  • Abundant blood vessels in skin function as respiratory organ.
  • Muscles work against hydroskeleton in movement.
  • Each segment has excretory tubes called metanephridia.
  • Have brain-like cerebral ganglion (cephalization).
  • Earthworms are hermaphrodites, but outcross.

Phylum Arthropoda

  • Most successful group of organisms ever to live. Two thirds of all organisms known are arthropods.
  • Widely distributed.
  • General characteristic of arthropods
    • 1. segmentation (specialization of individual segments; division of labor among segments; )
    • 2. hard exoskeleton ( cuticle made of layers of protein and chitin; impervious to water; attachment of muscles; preadaptation for move onto land; must molt to grow; well developed sensory organs, including eyes, oldfactory receptors, antennae to touch and smell; extensive cephalization;
    • 3. jointed appendages
      • allows for rapid controlled movemen
    • have open circulatory systems (hemocoel, hemolymph, heart)
    • have specialized organs for gas exchange (most insects have tracheal systems)
  • Arthropod phylogeny and classification
    • Arthropoda diverged into 4 subphyla:
      • 1. Trilobitomorpha Trilobites (extinct)
      • 2. Cheliceriforms Spiders, ticks, scorpions, sea spiders, eurypterids (extinct)
      • 3. Uniramia Insects, Centipedes, millipedes
      • 4. Crustacea Crabls, lobsters, shrimps, barnacles
  • Trilobites
    • Extinct for 250 MY. Common in Paleozoic.
    • Pronounced segmentation, but show little variation among segments.
    • As arthropods continued to evolve, segments tended to fuse and become fewer in number, and the appendages became specialized for a variety of functions.
  • Spiders and other Chelicerates
    • Chelicerate body divided into an anterior cephalothorax and a posterior abdomen.
    • Appendages more specialized than trilobites. Most anterior segment specialized as either fangs or pincers (chelicerae).
    • Most chelicerates are in the class Arachnida (scorpions, spiders, ticks, mites)
    • Arachnids have a cephalothorax with 6 pairs of appendages(1 pair chelicerae, 1 pair pedipalps, and 4 pairs walking legs)(Fig 33.30)
    • Fang-like chelicerae , equiped with poison used to attack prey.
    • Spiders feed by sucking up enzymatically digested liquid food. Yummy.
    • Most spiders carry out gas exchange using book lungs, stacked plates in an internal chamber.
    • Many spiders spin silk. Used for catching food, transportation, "sexual encounters".
    • Chelicerates lack antennae, and most have simple eyes (single lens).
    • Exoskeleton in chelicerates was a preadaptation to move on land. Prevents desiccation, anchors muscles.
    • Chelicerates invaded land at same time as plants. Hurray!
  • Uniramians
  • Includes Millipedes, Centipedes, and insects.
    • Millipedes: worm-like; large number of walking legs (two pairs per segment); vegetarians; millipedes were among first animals on land.
    • Centipedes: terrestrial carnivores; head has antennae, and 3 pairs of appendages modified as mouthparts; each segment has 1 pair of walking legs.
    • Insects:
  • outnumber all other forms of life combined.
  • most successful group of organisms ever to inhabit earth.
  • class insecta dividded into 26 diverse orders.
  • Entomology is the study of insects.
  • oldest insect fossils date to 400 MYA.
  • Flight has been key to success of insects; dragonflies among first insects to fly.
  • Internal anatomy included complex organ systems (Fig 33.33)
    • digestive tract
    • open circulatory system
    • unique excretory organs called Malpighian tubules.
    • gas exchage involves tracheal system
    • increased cephalization. Insects capable of complex behavior, although largely innate.
    • Many insects undergo metamorphosis (have larval stages).
    • Reproduction usually sexual. Fertilization usually internal.
    • The shear abundance of insects on earth is a testament to their impact on other life forms.
  • Crustaceans
    • Restricted to aquatic environment, mostly marine. 40,000 species known.
    • Multiple appendages of crustaceans are extensively specialized. E.g. lobsters and crayfish have 19 pairs of appendages
    • Crustaceans are the only arthropods with two pairs of antennae.
    • Walking legs on thorax, but unlike insects, crustaceans have appendages in their abdomen.
    • In small crustaceans gas exchange is through thin areas of cuticle. Larger forms have gills.
    • Excrete metabolic wastes by diffusion across thin areas of cuticle.
    • Exoskeleton made of calcium carbonate.
    • Plankton includes larvae of many crustaceans. Very important as part of food chains.
    • Taste absolutely fabulous.

The deuterostome lineage includes echidoderms and chordates
Phylum Echinodermata

  • Most are sessile or slow-moving animals with radial symmetry (a secondary adaptation to sessile lifestyle).
  • Internal and external body parts radiate from center ( Fig 33.38).
  • Thin skin covers endoskeletom of hard calcareous plates.
  • Water vascular system; unique to echinoderms; its a network of hydraulic canals branching into extensions called tube feet that function in locomotion, feeding and gas exchange.
  • Radial adults develop from bilateral larvae.
    • Divided in six classes ( Fig 33.37).
      • 1. asterodea (sea stars)
      • 2. ophiuroidea (brittle stars)
      • 3. echinoidea (sea urchins and sand dollars)
      • 4. crinoidea ( sea lillies)
      • 5. holothuroidea ( sea cucumbers)
      • 6. Concentricycloidea ( Sea daisies)



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