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
    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.

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.

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.