Sunrise, Wizard Islet, British Columbia
Sunrise, Wizard Islet, British Columbia

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Dr. Brian K. Penney

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Lecture 3. Molluscs, Annelids and Arthropods

Updated: 3/5/12

February 23, 2012. Reading: 18.9-18.12

  1. Describe the characteristics of molluscs including several example animals, unique features that distinguish them from other phyla, and how they impact humans.

    Phylum Mollusca shows variations on a common body plan, with a muscular foot, a mantle that secretes the shell, a visceral mass, and a radula, a feeding organ unique to the phylum.

    • They are bilateral protostome coelomates, and comprise the second-most diverse animal phylum (150,000 sp.).
    • Three major classes are:
      • Gastropoda (snails, slugs), crawling grazers with a broad foot and spiral shell
      • Bivalvia (clams, mussels), sedentary filter feeders with large gills and two shells
      • Cephalopods (squid, octopuses). Active, swimming predators with a foot modified to tentacles, and a reduced/lost shell
    • Molluscs show the greatest variation in body form, with both closed- and open circulatory systems (depending on activity level), highly specialized organs, and loss of characters among groups (shell, radula).
    • Important economically as food species, ecologically as components of marine food chains, and scientifically for studies of learning and behavior.

  2. Define segmentation, explain its function, and note the animal phyla where it occurs

    • True segmentation involves repetition of body regions. Must also include associated organ systems.
    • segmentation is an easy way to build a larger animal and allows greater flexibility and mobility; the form probably evolved to facilitate movement
    • Some organ systems (digestive, reproductive) differ among segments.
    • In the evolution of some groups, segments become joined into larger body regions, with internal structures also combined.
    • Annelida, Arthropoda and Chordata are the phyla that show obvious segmentation

  3. Describe the characteristics of segmented worms, including several example animals, unique features that distinguish them from other phyla, and how they impact humans.

    Phylum Annelida

    • Characteristics: represent the truly segmented worms, and most possess short hairs (chaetae).
    • They are bilateral protostome coelomates, with probably the most obvious coelom of any phylum.
    • They are moderately diverse, with about 15,000 sp.
    • Three major groups are the polychaetes (Polychaeta: clam- and tube worms, giant hydrothermal vent worms), the earthworms (Oligochaeta) and the leeches (Hirudinea).
    • A few leeches are parasitic and important in medicine, but most annelids are important decomposers or bottom feeders.

  4. Describe the characteristics of arthpropods, including several example animals, unique features that distinguish them from other phyla, and how they impact humans.

    Arthrpoda is themost diverse animal phylum, comprising over half of all known species (> 1.2 million spp.)!

    • They are protostomes, with a highly reduced coelom.
    • They have an exoskeleton that must be molted in order to grow, both compound and simple eyes and jointed legs.
    • They are segmented, but groups of segments are often combined into body regions (called tagmata), often specialized for particular tasks, such as movement or feeding. Organs and sometimes limbs are lost in particular segments as functions are divided among formerly independent segments.

  5. Compare the characteristics of the four major arthropod lineages, and give examples of each.

    • Chelicerata include horseshoe crabs and arachnids, such as spiders, scorpions and mites.
      • Chelicerates are named for specialized front legs (chelicerae), which form the "fangs" of spiders.
      • Chelicerates are important ecologically as predators and scavengers, and horseshoe crabs are important economically for medical testing.
    • Crustacea are nearly all are aquatic and include shrimp, crabs and barnacles.
      • Crustaceans typically have tougher, calcified shells and have the most diverse body forms of the arthropods, i.e. more highly varied tagmata.
      • Crustaceans are important economically as a food source and ecologically as predators and scavengers.
    • Myriapoda have similar segments over most of their body and superficially resemble annelids
      • includes millipedes and centipedes and some smaller groups that serve as scavengers or predators in terrestrial food webs.
    • Hexapoda (Insects) are the most diverse, speciose group known, with over 1 million species. They are also the most economically important invertebrate group.
      • They have successfully invaded all habitats except for saltwater.
      • Insects are important ecologically as predators, herbivores and scavengers, and form an important link in most food webs. Some insects are also important medically due to direct injury or as vectors of disease or economically due to crop damage.
      • The study of insects is the field of entomology.

  6. Describe the common characteristics of insects. Describe the process and significance of metamorphosis.

    • All have a three-part body plan (head, thorax, abdomen), modified mouthparts, and usually two pairs of wings and three pairs of legs, all on the thorax.
    • Major orders include grasshoppers, cockroaches and mantises (Orthoptera), dragonflies (Odonata), true bugs (Hemiptera), beetles (Coleoptera), butterflies and moths (Lepidoptera), flies (Diptera) and wasps, ants and bees (Hymenoptera).
    • A major division exists between insects that have complete metamorphosis versus incomplete metamorphosis. Complete metamorphosis allows a radical change in body form from larva to adult, so each form can occupy vastly different ecological niches.

A printable syllabus, with course dates, required materials, grading and other policies can be found here.

A one page printable version of the schedule can be found here.

Copyright 2007-2012, Brian K. Penney

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