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28.E: Invertebrates (Exercises)

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  • 28.1: Phylum Porifera

    The simplest of all the invertebrates are the Parazoans, which include only the phylum Porifera: the sponges. Parazoans (“beside animals”) do not display tissue-level organization, although they do have specialized cells that perform specific functions. Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum.

    Review Questions

    Cnidocytes are found in _____.

    1. phylum Porifera
    2. phylum Nemertea
    3. phylum Nematoda
    4. phylum Cnidaria


    Cubozoans are ________.

    1. polyps
    2. medusoids
    3. polymorphs
    4. sponges


    Free Response

    Explain the function of nematocysts in cnidarians.

    Nematocysts are “stinging cells” designed to paralyze prey. The nematocysts contain a neurotoxin that renders prey immobile.

    Compare the structural differences between Porifera and Cnidaria.

    Poriferans do not possess true tissues, while cnidarians do have tissues. Because of this difference, poriferans do not have a nervous system or muscles for locomotion, which cnidarians have.

    28.2: Phylum Cnidaria

    Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic, that is, they develop from two embryonic layers. Nearly all (about 99 percent) cnidarians are marine species. Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells.

    Review Questions

    Annelids have a:

    1. pseudocoelom
    2. a true coelom
    3. no coelom
    4. none of the above


    Which group of flatworms are primarily ectoparasites of fish?

    1. monogeneans
    2. trematodes
    3. cestodes
    4. turbellarians


    A mantle and mantle cavity are present in:

    1. phylum Echinodermata
    2. phylum Adversoidea
    3. phylum Mollusca
    4. phylum Nemertea


    The rhynchocoel is a ________.

    1. circulatory system
    2. fluid-filled cavity
    3. primitive excretory system
    4. proboscis


    Free Response

    Describe the morphology and anatomy of mollusks.

    Mollusks have a large muscular foot that may be modified in various ways, such as into tentacles, but it functions in locomotion. They have a mantle, a structure of tissue that covers and encloses the dorsal portion of the animal, and secretes the shell when it is present. The mantle encloses the mantle cavity, which houses the gills (when present), excretory pores, anus, and gonadopores. The coelom of mollusks is restricted to the region around the systemic heart. The main body cavity is a hemocoel. Many mollusks have a radula near the mouth that is used for scraping food.

    What are the anatomical differences between nemertines and mollusks?

    Mollusks have a shell, even if it is a reduced shell. Nemertines do not have a shell. Nemertines have a proboscis; mollusks do not. Nemertines have a closed circulatory system, whereas Mollusks have an open circulatory system.

    28.3: Superphylum Lophotrochozoa

    Animals belonging to superphylum Lophotrochozoa are protostomes, in which the blastopore, or the point of involution of the ectoderm or outer germ layer, becomes the mouth opening to the alimentary canal. This is called protostomy or “first mouth.” In protostomy, solid groups of cells split from the endoderm or inner germ layer to form a central mesodermal layer of cells. This layer multiplies into a band and then splits internally to form the coelom.

    28.4: Superphylum Ecdysozoa

    The superphylum Ecdysozoa contains an incredibly large number of species. This is because it contains two of the most diverse animal groups: phylum Nematoda (the roundworms) and Phylum Arthropoda (the arthropods). The most prominant distinguising feature of Ecdysozoans is their tough external covering called the cuticle. The cuticle provides a tough, but flexible exoskeleton tht protects these animals from water loss, predators and other aspects of the external environment.

    Review Questions

    The embryonic development in nematodes can have up to __________ larval stages.

    1. one
    2. two
    3. three
    4. five


    The nematode cuticle contains _____.

    1. glucose
    2. skin cells
    3. chitin
    4. nerve cells


    Crustaceans are _____.

    1. ecdysozoans
    2. nematodes
    3. arachnids
    4. parazoans


    Flies are_______.

    1. chelicerates
    2. hexapods
    3. arachnids
    4. crustaceans


    Free Response

    Enumerate features of Caenorhabditis elegans that make it a valuable model system for biologists.

    It is a true animal with at least rudiments of the physiological systems—feeding, nervous, muscle, and reproductive—found in “higher animals” like mice and humans. It is so small that large numbers can be raised in Petri dishes. It reproduces rapidly. It is transparent so that every cell in the living animal can be seen under the microscope. Before it dies (after 2–3 weeks), it shows signs of aging and thus may provide general clues as to the aging process.

    What are the different ways in which nematodes can reproduce?

    There are nematodes with separate sexes and hermaphrodites in addition to species that reproduce parthenogentically. The nematode Caenorhabditis elegans has a self-fertilizing hermaphrodite sex and a pure male sex.

    Describe the various superclasses that phylum Arthropoda can be divided into.

    The Arthropoda include the Hexapoda, which are mandibulates with six legs, the Myriapoda, which are mandibulates with many legs and include the centipedes and millipedes, the Crustacea, which are mostly marine mandibulates, and the Chelicerata, which include the spiders and scorpions and their kin.

    Compare and contrast the segmentation seen in phylum Annelida with that seen in phylum Arthropoda.

    Arthropods have an exoskeleton, which is missing in annelids. Arthropod segmentation is more specialized with major organs concentrated in body tagma. Annelid segmentation is usually more uniform with the intestine extending through most segments.

    28.5: Superphylum Deuterostomia

    The phyla Echinodermata and Chordata (the phylum in which humans are placed) both belong to the superphylum Deuterostomia. Recall that protostome and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the digestive cavity develops first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the anus is the first to develop.

    Review Questions

    Echinoderms have _____.

    1. triangular symmetry
    2. radial symmetry
    3. hexagonal symmetry
    4. pentaradial symmetry


    The circulatory fluid in echinoderms is _____.

    1. blood
    2. mesohyl
    3. water
    4. saline


    Free Response

    Describe the different classes of echinoderms using examples.

    The Asteroidea are the sea stars, the Echinoidea are the sea urchins and sand dollars, the Ophiuroidea are the brittle stars, the Crinoidea are the sea lilies and feather stars, the Holothuroidea are the sea cucumbers.