Animals

• Animals are eukaryotic, multicellular organisms without a cell wall, unlike plants or fungi
• Animals have specialized tissues and organ systems that deliver nutrients and oxygen to cells and carry wastes away
• All animals are heterotrophic, meaning they eat other organisms to acquire energy and nutrients
• Animals use oxygen to metabolize food through the biochemical pathways of aerobic respiration
• All animals are motile at some point in their life, but some animals are also sessile at some point in their life
• Most animals have sensory and nervous systems that allow them to receive, process, and respond to information
• Sexually reproducing species produce haploid gametes (eggs and sperm), which fuse to form diploid zygotes
• A zygote develops into a multicelled embryo, then into an immature juvenile or free-living larva, which develops into a reproductively mature adult
• A colonial Choanoflagellate was probably the common ancestor of all animals, living at least 700 million years ago, as proposed by Ernst Haeckel
• Several important morpohological innovations were used to develop guesses about the evolutionary relationships of the major animal groups:

  • tissue structure
  • body symmetry
  • body cavities
  • developmental patterns
  • segmentation
• Tissues are groups of cells with a common structure and function
• Tissues divide animals into 2 branches:

  • Sponges (Parazoa) that lack tissues
  • All other animals (Eumetazoa) that have tissues
• A diploblastic body plan includes 2 primary cell layers
• A triploblastic body plan includes 3 primary cell layers
• The endoderm, or the innermost layer, develops into the lining of the gut and sometimes respiratory organs
• The ectoderm, or the outermost layer, forms the external covering and nervous system
• The mesoderm, or the middle layer in triploblastic animals, forms the muscles of the body wall and most other structures between the gut and the external covering
• Most sponges are asymmetrical (irregular)
• Radiata eumetazoans exhibit radial symmetry, meaning that body parts are arranged around a central axis like spokes on a wheel
• Bilateria eumetazoans exhibit bilateral symmetry, meaning their left and right sides are essentially mirror images of each other
• Acoelomate animals have no body cavity that separates the gut from the muscles of the body wall
• Pseudocoelomate animals have a pseudocoelom, a fluid-filled space between the gut and muscles of the body wall
• Coelomate animals have a body cavity, or a coelom, completely lined by a membrane, called the peritoneum, that is derived from mesoderm
• Extensions of the peritoneum, or the membrane lining the coelom, called mesenteries, surround the internal organs and suspend them within the coelom
• Incompressible fluid within the body serves as a hydrostatic skeleton, which provides support
• Protostomes are bilaterally symmetrical animals and includes most phyla of invertebrates
• Deuterostomes are bilaterally symmetrical animals and includes the vertebrates and their nearest invertebrate relatives
• Cleavage is the early mitotic divisions of the fertilized egg
• Protostomes go through spiral cleavage, and most also undergo determinate cleavage, in which the cells' development path is determined as it is provided
• Deuterostomes go through radial cleavage, and many also undergo indeterminate cleavage, in which the developmental fates of cells are determined later
• After cleavage, some cells migrate inward through the blastopore and become the endoderm, which forms the developing gut, also known as the archenteron
• A second opening at the opposite end of the embryo turns the archenteron into a digestive tube, which in protostomes develops into the mouth and the second opening forms the anus, and in deuterostomes develops into the anus and the second opening forms the mouth
• Some protostomes and deuterostomes exhibit segmentation, or the production of body parts as repeating units
• Sponges:

  • lack tissues
  • asymmetrical
  • mature sponges are sessile
  • outer layer of flattened cells is called the pinacoderm
  • inner surface is lined by flagellated collar cells called choanocytes
  • amoebocytes in the mesophyll secrete a supporting skeleton of fibrous protein and spicules of calcium carbonate or silica
  • water and food particles flow into the spongocoel through pore sin the pinacoderm, and out through oscula
  • most are hermaphroditic
  • flagellated larvae undergo metamorphosis
• 2 Eumetazoan Phyla that have radial symmetry:

  • Cnidaria
  • Ctenophora
• Radially symmetrical eumetazoans have a gastrovascular cavity with a single opening (the mouth) that serves both digestive and circulatory functions
• Cnidarians (Phyla Cnidaria):

  • have a saclike gastrovascular cavity and mouth ringed with tentacles
  • may be vase-shaped polyps that attach to a substrate or a bell-shaped medusae that float
  • simplest animals that have specialized tissues
  • gastrodermis includes gland cells and phagocytic cells
  • epidermis includes nerve cells and sensory cells in the form of a nerve net, contractile cells, as well as cells specialized for capturing prey called cnidocytes that have stinging nematocysts
• Hydrozoa (Phylum Cnidaria, Class Hydrozoa):

  • most have polyp and medusa stages
  • polyps may form sessile colonies that develop asexually from one individual that specialize for feeding, defense, or reproduction
  • live as solitary polyps that reproduce by budding when in favorable conditions, or they produce zygote encapsulate in a protective coating
• Scyphozoa (Phylum Cnidaria, Class Scyphozoa):

  • traditional jellies
  • exist primarily as bell-shaped medusae
  • nerve cells control tentacles and coordinate the rhythmic activity of contractile cells
  • specialized sensory cells are clustered at the edge of the bell, such as statocysts that sense gravity, or ocelli that detect light
  • male and female medusae release gametes into the water, where fertilization takes place
• Cubozoa (Class Cnidaria, Class Cubozoa):

  • box jellyfish
  • exist primarily as actively swimming, cube-shaped medusae
  • immobilize prey with one of the deadliest toxins produced by animals
  • nematocyst-rich tentacles grow in clusters from the four corners of the medusa
  • groups of light receptors and image-forming eyes occur on the four sides of the bell