Introduction to Animals

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Characteristics

• 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)

DORSAL

VENTRAL

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

Segmentation

• 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)

Cleavage

• 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

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)

Metamorphosis

• 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)

• 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