Animal Characteristics: Biology

Nutritional Mode

• Animals are heterotrophs, meaning they obtain nutrition by ingesting their food.

Cell Structure and Specialization

• Animals are multicellular eukaryotes.
• Animal cells lack cell walls.
• Structural proteins hold animal bodies together.
• Tissues form from groups of similar cells functioning as a unit.
• Nervous and muscle tissues uniquely define animals.

Reproduction and Development

• Most animals have at least one larval stage.
• Larva are sexually immature and morphologically distinct from the adult form.
• Larva undergo metamorphosis to become juveniles.
• A juvenile resembles an adult, but is not yet sexually mature.
• Only animals possess Hox genes, which regulate body form development.

Animal History

• Over 1.3 million animal species have been named.
• The common ancestor of living animals existed between 700 and 770 million years ago.
• Choanoflagellate cells and sponge collar cells are morphologically similar.
• Collar cells have been observed in cnidarians, flatworms, and echinoderms, but never in non-chonoflagellate protists, plants, or fungi
• DNA sequence data indicates that chonoflagellates and animals are sister groups.

Neoproterozoic Era

• Early members of the animal fossil record include the Ediacaran biota from about 560 million years ago.
• Early animal embryos and predation evidence have been found in Neoproterozoic rocks.

Paleozoic Era

• The Cambrian explosion occurred 535 to 525 million years ago.
• The Cambrian explosion marks the earliest fossil appearance of major animal groups.
• Fossils from the Cambrian explosion are primarily bilaterians, characterized by:
  • Bilateral symmetry
  • A complete digestive tract
  • A one-way digestive system
• Hypotheses regarding the Cambrian explosion and the decline of Ediacaran biota include:
  • New predator-prey relationships
  • A rise in atmospheric oxygen
  • The evolution of the Hox gene complex
• 450 million years ago is when animals began to significantly impact life on land.
• Vertebrates transitioned to land around 365 million years ago.

Mesozoic Era

• Coral reefs emerged, creating essential marine ecological niches.
• Plesiosaurs, marine reptiles, evolved from terrestrial ancestors.
• Dinosaurs were the dominant terrestrial vertebrates.
• The first mammals emerged.
• Flowering plants and insects diversified.

Cenozoic Era

• The Cenozoic Era began with mass extinctions that affected terrestrial and marine animals.
• The mass extinctions included large, nonflying dinosaurs and marine reptiles.
• Mammals increased in size, exploiting vacated ecological niches.
• The global climate cooled.

Symmetry

• Animals can be categorized by body symmetry.
• Radially symmetrical animals lack front, back, left, and right sides.
• Radially symmetrical animals are often sessile or planktonic (drifting or weakly swimming).
• Bilateral symmetry defines two-sided symmetry.
• Bilaterally symmetrical animals possess:
  • A dorsal (top) and a ventral (bottom) side
  • A right and left side
  • Anterior (front) and posterior (back) ends
• Bilateral animals generally move actively and have a central nervous system.

Tissues

• Tissues are collections of specialized cells separated by membranous layers.
• Germ layers emerging during development give rise to the tissues and organs of the animal embryo.
• Ectoderm is the germ layer that covers the embryo's surface.
• Endoderm is the innermost germ layer that lines the developing digestive tube, known as the archenteron.
• Sponges, and some other groups, lack true tissues.
• Diploblastic animals have ectoderm and endoderm, cnidarians belong to this group.
• Triploblastic animals have an intervening mesoderm layer.
• Flatworms, arthropods, and vertebrates are triploblastic animals.

Body Cavities

• Most triploblastic animals have a body cavity.
• A coelom is a true body cavity derived from mesoderm.
• Coelomates encompass animals with a true coelom (e.g., Brachiopoda, Mollusca, Annelida, Arthropoda, Echinodermata, Hemichordata, and Chordata).
• A pseudocoelom comes from the mesoderm and endoderm.
• Triploblastic animals witth a psuedocoelom are pseudocoelomates (e.g., Rotifera, Nematoda, and Loricifera).
• Triploblastic animals lacking a body cavity are called acoelomates (e.g., Porifera, Cnidaria, Ctenophora, Plattyhelminthes).

Protostome and Deuterostome Development

• Based on early development, animals can exhibit protostome or deuterostome development.

Animal Phylogeny

• Five important points about the relationships among living animals:
  • All animals share a common ancestor.
  • Sponges are basal animals.
  • Eumetazoa ("true animals") is a clade including animals with true tissues.
  • Most animal phyla belong to the Bilateria clade.
  • Bilaterian animals can be divided into three major clades: Deuterostomia, Lophotrochozoa, and Ecdysozoa.
• Deuterostomia includes hemichordates (acorn worms), echinoderms (sea stars and relatives), and chordates.
• This clade consists of vertebrates and invertebrates.
• Ecdysozoa encompasses invertebrates that shed their exoskeletons via ecdysis.
• Lophotrochozoa represents another diverse clade of bilaterian invertebrates.
• Some lophotrochozoans have a feeding structure called a lophophore.
• Others have a distinct developmental stage called the trochophore larva.

Ongoing Research

• Key questions that remain in animal systematics include:
  • Are sponges monophyletic?
  • Are ctenophores basal metazoans?
  • Are acoelomate flatworms basal bilaterians?