Introduction to Animal

Introduction to Animals

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  • All multicellular (metazoans) & eukaryotic
  • Cells lack cell walls & come in a variety of shapes
  • Ingestive heterotrophs (take in food & internally digest it)
  • Store food reserves temporarily as glycogen in the liver
  • Have some type of skeletal support
  • Exoskeletons found in arthropods cover the outside of the body but limit size
  • Endoskeletons found in all vertebrates are found inside the body & are made of cartilage &/or bone
  • Worms have fluid-filled internal cavities giving them skeletal support
  • Sponges have the simplest skeleton 
  • May be sessile (attached & non-moving) or motile (able to move around)
  • Muscular tissue provides energy for movement
  • Reproduce sexually
  • Show levels of organization including cell, tissue, organ, & system
  • Most show division of labor among cells
  • Cells are specialized for particular functions
  • Cell junctions hold individual cells in a tissue together
  • Most vertebrates have a backbone or spine made of repeating bones called vertebrae that protect the spinal cord
  • Some show cephalization (have a head with sensory organs concentrated there)

Invertebrate Groups

  • Simplest animals
  • Contains the greatest number of animal species
  • Most found in water
  • Do not have an backbone
  • Includes sponges, cnidarians, flatworms, roundworms, annelids (segmented worms), mollusks, arthropods, & echinoderms

Vertebrate Groups

  • More complex animals
  • Most have a backbone
  • Includes fish, amphibians, reptiles, birds, & mammals

Body Areas

  • Dorsal is the back or upper surface
  • Ventral is the belly or lower surface
  • Anterior head or front end
  • Posterior is the tail or hind end opposite the head
  • Oral surface in echinoderms is where the mouth is located (underside)
  • Aboral surface in echinoderms is the surface opposite the mouth (top side)





Body Symmetry

  • Symmetry is the arrangement of body parts around a central plane or axis
  • Asymmetry occurs when the body can’t be divided into similar sections (sponges)
  • Radial symmetry occurs when similar body parts are arranged around a central point like spokes on a wheel (echinoderms)
  • Most animals with radial symmetry are sessile (attached) or sedentary (move very little)
  • Bilateral symmetry occurs when animals can be divided into equal halves along a single plane (right & left sides that are mirror images)
  • Animals with bilateral symmetry are more complex, usually motile organisms, such as worms, arthropods, and all vertebrates
  • Animals with bilateral symmetry show cephalization & have anterior & posterior ends





  • Occurs whenever animal bodies are divided into repeating units or segments
  • Found in more complex animals
  • Earthworms show external segmentation, while humans show internal segmentation (vertebrae of the backbone)
  • Segments may be fused together such as cephalothorax covering chest & head of a crayfish

Tissue Development

  • All animals reproduce sexually, but some also reproduce asexually (sponges bud & flatworms fragment)
  • Zygote is the fertilized egg all animals form from
  • Zygote undergoes rapid cell divisions known as cleavage to become hollow ball of cells called blastula
  • Blastocoel is the central cavity of the blastula
  • Blastula invaginates (folds inward at one point) to form an opening & two cell or germ layers; process called gastrulation
  • New cup-shaped structure with 2 cell layers is called the gastrula
  • Archenteron is the deep cavity of the gastrula that forms the primitive gut
  • Inner germ layer called endoderm & outer germ layer called ectoderm
  • Opening may become the mouth or the anus
  • Protostomes (mollusks, arthropods, & annelids) develop mouth from blastopore, while deuterostomes (echinoderms & vertebrates) develop an anus from blastopore
  • Some animals form a third germ layer in the middle called mesoderm
  • Cells differentiation during development changing their shapes to fit their function ( neurons or nerve cells become long to conduct messages)


  • Protostomes have spiral cleavage in which embryonic cells divide in a spiral arrangement
  • Deuterostomes have radial cleavage or embryonic cell division parallel or perpendicular to the vertical axis of the embryo

  • Protostomes have determinate cleavage ( embryonic cells can’t form a new organism if separated)
  • Deuterostomes have indeterminate cleavage ( embryonic cells can form other organisms if separated such as identical twins)

Germ Layers

  • Form the tissues, organs, & systems of an animal
  • Found in the embryo of all animals except sponges (have specialized cells but no tissues)
  • Ectoderm (outer) forms skin, nerves, & sense organs
  • Endoderm (inner) forms the digestive & respiratory organs & systems
  • Mesoderm (middle) forms muscles, circulatory system, reproductive & excretory systems

germ layers

Larval Forms

  • Some animals have indirect development & go through an immature larval form that does not resemble the adult
  • Planula is the larva of cnidarians (jellyfish, corals, & sea anemones)
  • Trochophore is the larva of mollusks (squid & octopus)
  • Dipleurula is the larva of echinoderms (starfish & sea urchins)


  • May be complete or incomplete
  • Usually found in arthropods
  • Incomplete metamorphosis (egg –> nymph –> adult)
  • Complete metamorphosis ( egg –> larva –> pupa –> adult)





Body Cavities

  • Coelom is an internal body cavity lined with mesoderm

  • Animals with a coelom are called coelomate animals (annelids, mollusks, arthropods, & vertebrates)
  • Acoelomate animals do not have a body cavity but have solid bodies (sponges, flatworms, & cnidarians )
  • Pseudocoelomate animals have a body cavity only partially lined with mesoderm (roundworms)

  • Schizocoely occurs in protostomes where the coelom develops when mesoderm masses split
  • Enterocoely occurs in deuterostomes

Body Layers

  • Sponges have specialized cells but no tissues or organs
  • Cnidarians (jellyfish, coral, sea anemone) have 2 body layers (ectoderm & endoderm) with a jellylike layer called mesoglea between for support
  • Cnidarians have one body opening into a large cavity called gastrovascular cavity
  • All worms, mollusks, arthropods, echinoderms, & vertebrates have 3 cell layers (ectoderm, mesoderm, & endoderm)

Mouth & Anus Development

  • Blastopore is the opening in the gastrula formed when blastula folds inward
  • Protostomes are animals that the blastopore develops into the mouth ( earthworms, mollusks, arthropods)
  • Deuterostomes are animals that the blastopore develops into the anus (echinoderms & vertebrates)

Support Systems

  • Sponges are supported by spicules, while limestone cases support corals

  • Hydrostatic skeletons in worms consist of a fluid-filled body cavity surrounded by muscles
  • Arthropods have external exoskeletons that prevent water loss but must be molted for growth to occur
  • Echinoderms & vertebrates have internal endoskeletons that grow with the organism

Digestive Systems

  • All animals are heterotrophs
  • Sponges have specialized cells to capture & digest their food
  • Cnidarians have one opening into their gastrovascular cavity where food enters & wastes leave; called a two-way digestive system
  • Annelids, arthropods, & vertebrates have a one-way digestive system in which food enters the mouth, is digested,  & wastes leave through the anus

Circulatory System

  • Transports oxygen and nutrients to cells & carbon dioxide and wastes away from cells
  • Sponges, cnidarians, & flatworms don’t have a circulatory system
  • In closed systems, blood remains in blood vessels at all times until it reaches cells (earthworms & vertebrates)
  • In open systems, blood isn’t always contained in blood vessels (arthropods)

Respiratory System

  • Oxygen is needed & carbon dioxide must be eliminated
  • Sponges, cnidarians, flatworms, & roundworms exchange gases by diffusion
  • Mollusks & fish use gills to exchanges gases, while terrestrial vertebrates use lungs

Nervous System

  • Cephalization occurs in animals that have a distinct head at the anterior end where sensory organs are concentrated
  • Cephalization is found in more complex animals 
  • Sponges have specialized nerve cells, while cnidarians & flatworms have a nerve net
  • Ganglia are clusters of nerve cells found in more complex animals
  • Nerve cells may specialize to detect, light, sound, etc.
  •  Brain interprets nerve impulses & sends a response

Body Coverings

  • Integument is the outer covering of an animal
  • Terrestrial vertebrates have water-tight outer coverings
  • Integuments of amphibians allow gas exchange through the skin
  • Adaptations of integuments include scales, fur, hair, & feathers to protect and insulate the body

Excretory System

  • Rid animals of wastes, help conserve water, & filter wastes from the blood
  • Ammonia is a toxic waste that must be gotten rid of by an animal’s body
  • Kidneys filter blood in vertebrates

Reproductive System

  • All animals reproduce sexually, but some also use asexual reproduction
  • Budding is asexual reproduction in which an outgrowth on the parent organism breaks off to form a new individual (hydra)

hydra with bud 3.JPG (83198 bytes)

  • Sponges, flatworms, & cnidarians asexually reproduce by fragmentation (separating into pieces & each piece making a new organism)
  • Some insects develop from unfertilized eggs by parthenogenesis
  • Hermaphrodites are animals that produce both sperm & eggs (earthworms – cross fertilize & tapeworms self fertilize)
  • Echinoderms, arthropods, mollusks, & vertebrates have separate sexes & exchange sperm
  • Internal fertilization occurs inside the body of the female & larger numbers of sperm & eggs are produced
  • External fertilization occurs inside the body of the female & fewer eggs & sperm are produced