Characteristics of Animals

Four Characteristics of Animals

• All animals have multiple cells.
• Cell specialization is an adaptation that allows cells to perform specific functions; specialized cells working together form tissues.
• Tissues are formed when cells connect at cell junctions.
• The organization of cells: Cells > tissues > organs > organ systems > organisms.
• Multicellularity allows for an enormous variety of animal species.
• Animals are heterotrophic, requiring the ingestion of other organisms for energy.
• Most animals reproduce sexually; some reproduce asexually.
• A zygote is the first cell of a new individual.
• The zygote divides and differentiates, allowing cells to become different (e.g., blood cells vs. bone cells).
• Most animals move via the coordination of two tissue types.
• Nervous tissue detects stimuli using neurons in its environment and within its body.
• Muscle tissue contracts to exert a force, moving specific regions of the body.

Origin and Classification

• The earliest animals originated in the seas and were invertebrates.
• Animals evolved from protists that lived in colonies, demonstrating early specialization.
• Division of labor among cells allowed some for movement and others for reproduction.
• Taxonomists classify animals by evolutionary history, grouping them into phyla.
• Animals are organized into approximately 11 phyla, although the exact number is debated.
• Phylum Chordata includes all vertebrates.

Body Structure and Symmetry

• Asymmetrical animals lack structural patterns, like sponges, which also lack true tissues.
• Radially symmetrical animals have similar parts branching out in all directions, such as cnidarians (sea anemones, jellyfish, hydra).
• Bilateral symmetrical animals can be divided into two equal halves and have:
  • Dorsal (top)
  • Ventral (bottom)
  • Anterior (head)
  • Posterior (tail)
• Most bilateral symmetrical animals exhibit cephalization, having a head with sensory organs.

Germ Layers

• Germ layers are fundamental tissue types found in embryos of all animals except sponges.
• Ctenophore and cnidarian embryos have two germ layers, while all other animals have three.
• Each body feature, organ, and tissue originates from one of these germ layers.

Body Cavities

• Most animals contain a fluid-filled space formed between the digestive tract and the outer body wall during development.
• Flatworms possess three germ layers but lack body cavities, having solid bodies.
• In roundworms, the body cavity aids movement by providing a firm structure for muscles, and these false cavities are called pseudocoelomates.
• True body cavities, called coelomates, store and transport nutrients and wastes that diffuse into and out of the animal's body cells.

Animal Diversity

• Biologists classify animals based on similarities in body plans and development patterns.
• Phylogenetic trees illustrate relationships between taxonomic groups based on morphology.
• Sponges are characterized by multicellularity and limited cell specialization.
• Cnidarians and ctenophores have true tissues arranged in two layers.
• True tissues in three layers combined with bilateral symmetry distinguish all other animal phyla.

Invertebrates

• There are 10 invertebrate phyla, displaying great diversity in body plan.
• Invertebrates are characterized by the absence of a backbone.
• They make up the majority of animal species.

Chordates

• The term Chordate refers to the notochord, a flexible rod of tissue in the dorsal part of the body.
• Key features of chordates: a notochord, dorsal nerve cord, pharyngeal pouches, and a post-anal tail, present at some development stage.

Vertebrates

• In subphylum Vertebrata, the dorsal nerve cord develops into the brain and spinal cord.
• Pharyngeal pouches in fish have evolved into gills.
• The notochord appears during the embryonic stage but is replaced by a backbone.

Comparing Invertebrates and Vertebrates

• Invertebrates display either radial or bilateral symmetry.
  • Radial symmetry allows stimulus reception from all directions, as seen in jellyfish.
  • Bilateral symmetry allows for cephalization.
• Some invertebrates exhibit segmentation, a body composed of repeating units, such as in earthworms.
• Arthropods demonstrate varied segment forms and functions, including fused segments.

Support Systems in Invertebrates

• Sponges have simple skeletons, while roundworms rely on fluid-filled bodies for support.
• Arthropods have a rigid exoskeleton that protects soft tissues.
• Exoskeletons limit size and movement, requiring periodic shedding.

Respiratory and Circulatory Systems in Invertebrates

• Animals perform gas exchange, expelling CO2 and taking in O2.
• Simple aquatic invertebrates exchange gases through their body covering, efficient in moist environments.
• Aquatic arthropods and mollusks use grills.
• Sponges and cnidarians lack circulatory systems; nutrients and gases diffuse across cell membranes.
• Arthropods and some mollusks have open circulatory systems where blood-like fluids are pumped from vessels into the body cavity.
• Annelids and other mollusks have closed circulatory systems where blood circulates in tubular vessels.

Digestive and Excretory Systems in Invertebrates

• Sponges digest food within cells, while cnidarians have a central chamber with one single opening for both mouth and anus.
• Most invertebrates have a digestive tract (gut), in which food is broken down and absorbed by cells.
• Aquatic invertebrates excrete dissolved ammonia, while terrestrial invertebrates filter waste from body cavities to conserve water.

Nervous systems in Invertebrates

• Sponges lack neurons, but their cells can still react to stimuli.
• Invertebrate nervous systems range from simple (snails) to complex (octopi).

Reproduction and Development in Invertebrates

• Invertebrates reproduce both sexually and asexually; some are hermaphrodites.
• Some invertebrates, like insects, undergo indirect development using a larval stage.

Key Features of Vertebrates

• Vertebrates possess an internal skeleton that grows with the animal (endoskeleton).
• Vertebrates are segmented, shown by their ribs and vertebrae.
• Limb and muscle evolution has positioned the limbs under the body, benefiting terrestrial vertebrates.
• Limb positioning allows for faster movement (longer stride and bipedalism).
• Integument is a term given to the outer covering of an Animal's body.
• Fish and amphibians have integuments adapted to moist environments.
• Integuments in terrestrial vertebrates are watertight to prevent dehydration.
• Functions of Integuments:
  • Protection (scales)
  • Insulation (feathers, fur)
  • Gas Exchange (amphibians).
• Aquatic vertebrates respire using gills, (fish and larval amphibians).
• Terrestrial vertebrates have lungs that evolved for land.
• Vertebrates have closed circulatory systems with multi-chambered hearts.
• Some vertebrates have heart that separates oxygenated from deoxygenated blood (mammals).
• Digestion occurs from mouth-to-anus within a long gut.

Digestive and Excretory Systems in Vertebrates

• Ammonia waste is processed into less toxic substances in the kidneys of vertebrates to conserve water.
• Vertebrate brains vary in complexity, from simple stimulus/response in fish to flexible behavior in animals like dogs.
• Behavior complexity is proportional to the size of brain
• Fertilization location in fish and amphibians: water, fertilization method: external.
• Fertilization location for reptiles, birds, and mammals: internal, fertilization method: efficient.
• Development location in fish, amphibians, reptiles and birds: external-egg laying.
• Some vertebrate zygote eggs are retained inside the mother until they hatch.
• Most mammals give birth to live young nourished in the placenta by the mother’s blood supply.
• Except for amphibians and some fishes, vertebrates undergo direct development.

Fertilization and Development

• Haploid gametes (sperm and egg) unite in fertilization to form a diploid zygote.
• Post-fertilization, the zygote undergoes DNA replication and cell division, known as Cleavage.
• Cleavage results in first two cells and then four, 8, 16, 32…. forming a hollow ball of cells called a blastula.
• The blastocoel, or blasto-seal, is the name given to the name given to the central cavity within the blastula.
• Gastrulation describes the process of blastula's change into a multilayered embryo called a gastrula.
• The blastula collapses inward to form a pouch known as the archenteron.
• The archenteron eventually forms the gut.
• The outer germ layer of a gastrula is called the ectoderm, and the inner layer is called the endoderm.
• Most phyla develop a third germ layer, the mesoderm, which forms between layers.
• The endoderm forms the throat passage, gills or lungs, and gut.
• The ectoderm develops the skin, hair, nails, and nervous system.
• The mesoderm forms the skeleton, muscles, a circulatory system, and the lining of the body cavity.

Body Cavities Categorized

• Acoelomates refers to body cavities with none and are typically flatworms.
• Pseudocoelomates are bodies that have a false cavity not totally surrounded by mesoderm tissue, one example being roundworms.
• Coelomates refers to bodies that have a mesoderm lining, supporting the endodermic gut and are mollusks, annelids, arthropods, chordates, and echinoderms.