Chapter 31: Sponges, Cnidarians, Ctenophores, and Protostomes

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Sponges, Cnidarians, and Ctenophores

• The poriferans, or sponges, are characterized by flagellate collar cells (choanocytes), which generate a water current that brings food and oxygen to the cells.
  • Collar cells also trap and phagocytize food particles.
  • The sponge body is a sac with tiny openings through which water enters; a central cavity, or spongocoel; and an open end, or osculum, through which water exits.
  • The cells of sponges are loosely associated; they do not form true tissues.
• Cnidarians are characterized by radial symmetry, two tissue layers, and cnidocytes, cells containing stinging organelles called nematocysts.
  • The gastrovascular cavity has a single opening that serves as both mouth and anus.
  • Nerve cells form irregular, non directional nerve netsthat connect sensory cells \n with contractile and gland cells.
• The life cycle of many cnidarians includes a sessile polyp stage (a form with a dorsal mouth surrounded by tentacles) and a free-swimming medusa (jellyfish) stage.
• Phylum Cnidaria includes four groups.
  • Hydrozoa (hydras, hydroids, and the Portuguese man-of-war) are typically polyps and may be solitary or colonial.
  • Scyphozoa (jellyfish) are generally medusae.
  • Cubozoa, the “box jellyfish,” have complex eyes that form blurred images.
  • Anthozoa (sea anemones and corals) are polyps and may be solitary or colonial; anthozoans differ from hydrozoans in the organization of the gastrovascular cavity.
• Ctenophores, or comb jellies, are fragile, luminescent marine predators with biradial symmetry.
  • Ctenophores have eight rows of cilia that resemble combs.
  • They are diploblastic and have tentacles with adhesive glue cells.

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The Lophotrochozoa

• The Lophotrochozoa make up a clade that includes some of the flatworms, nemerteans, mollusks, annelids, the lophophorate phyla, and rotifers.
• The true coelom is a fluid-filled body cavity completely lined by mesoderm that lies between the digestive tube and the outer body wall.
  • The coelom brings about the tube-within-a- tube body plan.
  • The body wall is the outer tube.
  • The inner tube is the digestive tube.
  • The coelom can serve as a hydrostatic skeleton in which contracting muscles push against a tube of fluid.
  • The coelom is a space in which internal organs, including gonads, can develop; it helps transport materials and protects internal organs.
• Cephalization, the evolution of a head with the concentration of sense organs and nerve cells at the anterior end, increases the effectiveness of a bilateral animal to actively find food, shelter, and mates and to detect enemies.
• The flatworms are acoelomate (have no coelom) animals with bilateral symmetry, cephalization, three definite tissue layers, and well-developed organs.
  • Many flatworms are hermaphrodites: a single animal produces both sperm and eggs.
  • Flatworms have a ladder-type nervous system, typically consisting of sense organs and a simple brain composed of two ganglia.
  • The ganglia are connected to two nerve cords that extend the length of the body.
  • Protonephridia function in osmoregulation and disposal of metabolic wastes.
• Four groups of flatworms are recognized: class Turbellaria comprises free-living
  • Flatworms, including planarians; classes Trematoda and Monogenea include the parasitic flukes; and class Cestoda includes the parasitic tapeworms.
  • The parasitic flukes and tapeworms typically have suckers or hooks for holding on to their hosts; they have complicated life cycles with intermediate hosts and produce large numbers of eggs.
• Nemerteans (ribbon worms) are characterized by the proboscis, a muscular tube used in capturing food and in defense.
  • Nemerteans have a complete digestive tract with mouth and anus, and a circulatory system.
  • The coelom is reduced.
• Mollusks are soft-bodied animals typically covered by a shell.
  • They have a ventral foot for locomotion and a pair of folds called the mantle that covers the visceral mass, a concentration of body organs.
• Mollusks have an open circulatory system except for cephalopods, which have a closed circulatory system.
  • A rasplike radula functions as a scraper in feeding in all groups except the bivalves, which are filter feeders.
  • Typically, marine mollusks have a free-swimming, ciliated trochophore larva.
• Polyplacophorans are chitons, mollusks with shells consisting of eight overlapping dorsal plates.
  • The gastropods, which include the snails, slugs, and their relatives, have a well-developed head with tentacles.
  • The body undergoes torsion, a twisting of the visceral mass.
• Bivalves are aquatic clams, scallops, and oysters.
  • A two-part shell, hinged dorsally, encloses the bodies of these filter feeders.
  • Cephalopods include the squids, octopuses, and Nautilus.
  • These active, predatory swimmers have tentacles surrounding the mouth, which is located in the large head.
• The annelids, the segmented worms, include many aquatic worms, earthworms, and leeches.
  • Annelids have long bodies with segmentation both internally and externally; their large, compartmentalized coelom serves as a hydrostatic skeleton.
• Polychaetes are marine annelids characterized by parapodia, appendages used for locomotion and gas exchange.
  • The parapodia bear many bristlelike structures called setae.
  • Polychaetes also differ from other annelids in having a well-defined head with sense organs.
• Oligochaetes, the group that includes the earthworms, are characterized by a few short setae per segment.
  • The body is divided into more than one hundred segments separated internally by septa.
• Leeches belong to the group Hirudinida.
  • Setae and appendages are absent.
  • Parasitic leeches are equipped with suckers for holding on to their host.
• The lophophorates, marine animals that have a lophophore, include the brachiopods, phoronids, and bryozoans.
  • The lophophore, a ciliated ring of tentacles surrounding the mouth, is specialized for capturing suspended particles in the water.
• Rotifers are pseudocoelomates that are thought to have evolved from animals with a true coelom.
  • They have a crown of cilia at their anterior end.

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The Ecdysozoa

• Ecdysozoa is one of the three major animal clades; its validity is based on many types of evidence, including molecular data.
  • Members of this group go through the process of ecdysis, or molting, during which an animal sheds its outer covering; the covering is then replaced by the growth of a new one.
• Nematodes, or roundworms, have a pseudocoelom.
  • The body is covered by a tough cuticle that helps prevent desiccation.
  • Parasitic nematodes that infect humans include Ascaris, hookworms, trichina worms, and pinworms.
• Arthropods are segmented animals with paired, jointed appendages and an armorlike exoskeleton of chitin.
  • Molting is necessary for the arthropod to grow.
  • Arthropods have an open circulatory system with a dorsal heart that pumps hemolymph.
  • Aquatic forms have gills for gas exchange; terrestrial forms have either tracheae or book lungs.
• The arthropods along with the onychophorans (velvet worms) and tardigrades (water bears) make up the clade Panarthropoda.
  • Based on molecular and other data, arthropods are currently assigned to five main groups: extinct trilobites and extant Myriapoda, Chelicerata, Crustacea, and hexapoda.
• The trilobites are extinct marine arthropods covered by a hard, segmented shell.
  • Each segment had a pair of biramous appendages, appendages with two jointed branches: an inner walking leg and an outer gill branch.
• Subphylum Myriapoda includes Chilopoda, the centipedes, and Diplopoda, the millipedes.
  • Members of this subphylum have uniramous appendages, that is unbranched appendages, and a single pair of antennae.
• Subphylum Chelicerata includes the merostomes (horseshoe crabs) and the arachnids (spiders, mites, and their relatives).
  • The chelicerate body consists of a cephalothorax and abdomen; there are six pairs of uniramous, jointed appendages, of which four pairs serve as legs.
  • The first appendages are chelicerae, and the second are pedipalps.
  • These appendages are adapted for manipulation of food, locomotion, defense, or copulation.
  • Chelicerates have no antennae and no mandibles.
• Crustaceans include lobsters, crabs, shrimp, pill bugs, and barnacles, and their many relatives.
  • The body typically consists of a cephalothorax and abdomen.
  • Crustaceans vary greatly in the appearance and in the number of biramous appendages.
  • Crustaceans have two pairs of antennae that sense taste and touch, and a pair of mandibles used for chewing.
  • Two pairs of maxillae, posterior to the mandibles, manipulate and hold food.
  • The decapod crustaceans typically have five pairs of walking legs.
• Subphylum Hexapoda includes Insecta.
  • An insect is an articulated, tracheated hexapod; its body consists of head, thorax, and abdomen.
  • Insects have uniramous appendages, a single pair of antennae, tracheae for gas exchange, and Malpighian tubules for excretion.
• The biological success of the insects can be attributed to their many adaptations, including a versatile exoskeleton, segmentation, specialized jointed appendages, highly developed sense organs, and ability to fly.
  • Complete metamorphosis, transition during the life cycle from one developmental stage to another, includes egg, larva, pupa, and adult stages.
  • Complete metamorphosis reduces competition within the same species
  • Effective reproductive strategies, effective mechanisms for defense and offense, and the ability to communicate have evolved in insects.

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