BIOL 108

• Characteristics of animals


  • Multicellular eukaryotes; lack cell walls; extracellular structural proteins; differentiation of cells
• Nutritional mode


  Chemoheterotrophs; ingest and digest food within their bodies (internal chamber)
• Sexual reproduction


  • Diploid stage dominates the life cycle; motile, haploid sperm fertilizes a larger, non-motile haploid egg
• Asexual reproduction


  • Fission/fragmentation, budding, and parthenogenesis
• Development


  Early embryonic development (blastula, gastrulation); animal development controlled by conserved Hox genes; direct development or indirect development (larvae stages); animals are motile at one stage of the life cycle
• Early embryonic development


  Zygote undergoes cleavage to form multicellular, hollow blastula; blastula undergoes gastrulation; gastrula with two cell layers (ectoderm, endoderm)
• Animal body plan


  • Symmetry: asymmetrical; radial; bilateral (incl. cephalization)
  • Tissues: diploblastic (ecto- and endoderm) are radial; triploblastic (ecto- , endo-, and mesoderm) are bilateral
  • Triploblastic body cavity: coelom; hemocoel; no internal body cavity; function of the coelom
• Protostome development


  Spiral and determinate cleavage; mesoderm splits to form coelom; blastopore becomes the mouth
• Deuterostome development


  Radial and indeterminate cleavage; mesoderm folds from archenteron wall to form coelom; blastopore becomes the anus
• Features of animal phylogeny

  • Single common ancestor (ancestral colonial flagellate); Animalia: clade Metazoa
  • Sponges are basal animals
  • Clade Eumetazoa have true tissues
  • Most Eumetazoan phyla members part of the monophyletic clade Bilateria
  • Three major clades of bilaterians: Deuterostomia, Ecdysozoans, Lophotrochozoans
• The common ancestor of animals evolved

  700-670 mya
• Choanoflagellates


  Unicellular, heterotrophic protists with characteristic single flagellum surrounded by a collar of microvilli
• Evidence that choanoflagellates are closely related to animals

  • Cell morphology of Choanoflagellates and sponge collar cells indistinguishable
  • Collar cells are found in other animal phyla, but never in non-choanoflagellate protists nor in plants or fungi
  • DNA sequence data indicate that choanoflagellates and animals are sister groups
• Multicellular eukaryotes evolved in multiple lineages (~1.7 bya), giving rise to algae, plants, fungi, and animals
• Ediacaran biota (whole-body animal fossils from the late Proterozoic (Neoproterozoic era))

  565-550 mya
• Cambrian explosion (the origin of most major phyla of living bilaterian animals)

  535-525 mya
• Hypotheses for Cambrian explosion

  • Evolution of predation
  • Increase in oceanic O2
  • Evolution of the Hox genes
• Metazoan shared, derived traits


  • Clade Metazoa: Multicellularity and cell differentiation; Cell adhesion (extracellular matrix); Sperm and ova; Embryonic blastula
  • Clade Eumetazoa: True tissues; Gastrulation
  • Clade Bilateria: Bilateral symmetry; Triploblastic; Complex organs
• Phylum Porifera (sponges)


  • Sessile marine animals that lack true tissues; motile larval stage
• Sponge cell types


  • Epidermal cells
  • Porocytes
  • Choanocytes (collar cells) generate a water current through the sponge and ingest suspended food
  • Gelatinous acellular mesohyl contains mobile amebocytes and skeletal elements (spicules or spongin)
• Sponge feeding


  Water is drawn (by the beating of choanocyte flagella) through pores (ostia) into a cavity called spongocoel and out through an opening (osculum); food particles trapped in mucus-covered microvilli and engulfed by phagocytosis and digested or transferred to amoebocytes
• Sponge reproduction


  Most sponges are hermaphrodites and use internal fertilization (eggs retained in mesohyl); free-swimming ciliated larvae disperse from the parent sponge; sponge embryonic development is highly variable
• Phylum Cnidaria (jellyfish, hydras, sea anemones, corals)

  • Simple diploblastic, radial body plan: a sac with a central gastrovascular cavity with a single opening (functions as mouth and anus) surrounded by tentacles with stinging cells (capture prey and/or defence); epidermis (ectoderm), gastrodermis (endoderm), and mesogloea (acellular 'jelly' matrix)
• Cnidarian body plans


  • Polyp (sessile form, e.g. sea anemone)
  • Medusa (bell-shaped motile form, e.g. jellyfish)
• Cnidarian feeding


  Tentacles armed with cnidocytes, unique cells that function in defence and capture of prey, that contain secretory organelles called nematocysts that deliver a sting to other organisms
• Cnidarian clades


  • Medusozoans produce a medusa stage in their life cycle
  • Hydrozoans alternate between polyp and medusa forms
  • Scyphozoans (jellyfish) and Cubozoans (box jellyfish) have medusa as the dominant stage in the life cycle
  • Anthozoans occur only as sessile solitary or colonial polyps (corals and sea anemones)
• Majority of animals exhibit bilateral symmetry, and are in clade Bilateria
• Bilateria are triploblastic (endo, ecto & meso)
• Bilateria are protostomes & deuterostomes
• Most bilaterians possess a coelom
• Bilateria


  • Strongly differentiated along the anterior-posterior axis; Sensory and feeding structures are concentrated in the anterior region (cephalization), involving the concentration of neural ganglia forming a brain; Digestive and reproductive tracts typically discharge posteriorly; Hox genes regulate anterior-posterior differentiation during embryonic development
• Phylum Acoela


  • Smallest, flattened body with minimal cephalization & no brain; No body cavity (no coelom or hemocoel) or complex organ systems; Simple digestive system with a mouth, but no gut cavity or anus
• Clade Lophotrochozoa includes 1/2 of all animal phyla (17 phyla)
• Phylum Platyhelmines (flatworms)


  • Triploblastic but lack fluid-filled body cavities; Incomplete digestive tract, has mouth & gut but no anus; No circulatory or gas exchange system. Gas exchange occurs across the body aided by dorso-ventral flattened shapes (lots of surface area)
• Platyhelmines lineages


  • Catulenida (chain worms): low diversity, found in freshwater & asexually reproduce
  • Rhabditophora: free-living and parasitic
• Free-living Rhabditophora (planarians)


  Inhabit freshwater and prey on smaller animals; Exhibit anterior cephalization, with light-sensitive eyes, and have a gastrovascular cavity with one opening (mouth only; no anus); Hermaphrodites and can reproduce both (a)sexually
• Parasitic Rhabditophora


  Live in/on others and can use suckers/hooks for attachment to hosts, have tough outer covering to protect themselves inside the host and have complex life cycles with two or more hosts → intermediate host (asexual reproduction occurs) & definitive host (sexual reproduction); Important groups: trematodes and tapeworms
• Phylum Ectoprocta


  • Sessile colonial animals that superficially resemble coral: most colonies are encased in a hard CaCO3 exoskeleton; filter feed using a retractable lophophore; U-shaped gut with the anal opening outside of the lophophore
• Phylum Brachiopoda (lamp shells)

  • Shelled Lophophorates that superficially resemble bivalve molluscs: filter feed using paired lophophores; dominant reef-building animals of the Paleozoic era
• Phylum Mollusca


  • Soft-bodied animals, but most protected by a hard calcareous shell; Unsegmented body plan with three main parts: a muscular ventral foot, a visceral mass containing organs, and the mantle that secretes the shell and forms a water-filled mantle cavity (respiration and excretion)