intro to animals

Introduction to Kingdom Animalia

  • Total Described Species: There are approximately 2×1062 \times 10^6 described species on Earth.

  • Animal Representation: Animals comprise approximately 75%75\% of all described species.

  • Insect Dominance: More than half (>50%> 50\%) of all animal species are insects.

  • Evolutionary Origins: Animals evolved from a colonial marine ancestor approximately 1×1091 \times 10^9 years ago.

  • Size Diversity: The kingdom ranges from microscopic organisms like tardigrades (water bears) to massive organisms like the blue whale.

Defining Characteristics of Animals

  • Eukaryotic: Cells contain a nucleus and membrane-bound organelles.

  • Multicellularity: All animals are multicellular. This trait evolved independently multiple times among different branches of eukaryotes.

  • Heterotrophic: Animals must ingest other organisms or organic material to obtain energy.

  • Absence of Cell Walls: Unlike plants and fungi, animal cells do not have cell walls.

  • Diploid (2n2n): Most animals are diploid for the majority of their life cycle.

  • Motility: Animals are motile at least during one stage of their life cycle.

  • Sexual Reproduction: Most animals are capable of reproducing sexually.

  • Organizational Complexity: Most animals have cells organized into tissues (with the notable exception of sponges).

  • Body Plan: Most animals exhibit some form of body symmetry.

Concepts of Multicellularity and Development

  • Colonial vs. Multicellular: The primary difference between a colonial organism and a multicellular one is that multicellular organisms possess specialized cells integrated for specific functions.     * Unicellular Choanoflagellate: A single-celled protist ancestor.     * Colonial Choanoflagellates: A group of individual cells living together.     * Sponges: Representative of early animal life where feeding cells (choanocytes) strongly resemble colonial choanoflagellates.

  • Early Embryonic Development Process:     1. Meiosis: Produces haploid (nn) gametes (Sperm and Egg).     2. Fertilization: Fusion of gametes creates a diploid (2n2n) zygote.     3. Mitosis: Rapid cell division from the zygote.     4. Blastula: A hollow ball of cells (seen in cross-section during early development).     5. Gastrula: Formed via gastrulation, creating the early digestive tube called the blastopore.     6. Larva: An immature form that undergoes metamorphosis to become an adult (2n2n).

Major Animal Lineages and Key Innovations

  • Lineage Breakdown:     * Ancestral Protists: The root of the animal phylogenetic tree.     * Invertebrates: Comprise approximately 1×1061 \times 10^6 species (e.g., Sponges, Cnidarians, Flatworms, Mollusks, Annelids, Roundworms, Arthropods, Echinoderms).     * Vertebrates: Comprise approximately 6×1046 \times 10^4 species (Chordates).

  • The Five Key Innovations:     1. Tissues.     2. Symmetry.     3. A body cavity (coelom).     4. Different development patterns.     5. Segmentation (repeated body units).

Key Innovation 1: Tissues

  • Definition: Cells of the same type organized together for a specific function.

  • Specialization: Tissues allow for greater complexity and eventually the development of organs.

  • Evolutionary Appearance: Tissues show up for the first time among the Cnidarians.

  • Sponges (Porifera) Exception:     * Sponges have specialized cells but lack true tissues and organs.     * Cells in sponges can differentiate and redifferentiate (one cell type can change into another).

  • Germ Layers (Embryonic Tissues):     * Diploblastic: Animals with two tissue layers (Ectoderm and Endoderm). Examples include Cnidarians.     * Triploblastic: Most animals have three tissue layers.         * Ectoderm: Becomes outer coverings and the nervous system/cells.         * Endoderm: Becomes the digestive tract and intestines.         * Mesoderm: Becomes the skeleton, muscles, circulatory system, and many internal organs.

Key Innovation 2: Symmetry

  • Radial Symmetry: Parts are arranged around a central axis (e.g., Cnidarians). These animals are often circular or cylindrical.

  • Bilateral Symmetry: The body can be divided into two mirrored halves (right and left) along a single plane. Most animals are "Bilaterans."

  • Symmetry Evolution:     * Sponges: Most have no symmetry (irregular bodies).     * Cnidarians: Exhibit radial symmetry.     * Echinoderms (e.g., Seastars): While they appear radial as adults, they are grouped with Bilaterans because they exhibit bilateral symmetry in larval stages or have a bilateral ancestry.

Key Innovation 3: Body Cavity (Coelom)

  • Definition: A fluid-filled space located between the digestive tube (gut) and the outer body wall. It originates from the mesoderm in triploblastic animals.

  • Fluid Content: In most animals, the fluid is water; in vertebrates, the cavity is often filled with air.

  • Types of Body Cavities:     * Acoelomates: No body cavity; the space between the mesoderm and endoderm is solid. Example: Flatworms.     * Pseudocoelomates: The body cavity is located between the mesoderm and endoderm. Examples: Roundworms, rotifers.     * Eucoelomates (Coelomates): The body cavity is completely lined by mesoderm. This includes most animals.

  • Advantages of the Coelom:     1. Organ Support: Accommodates, protects, and cushions internal organs.     2. Growth: Allows organs to grow larger and longer within a dedicated space.     3. Interaction: Organs can interact more closely.     4. Disassociation: Movement of the body is independent of the movement of internal organs.     5. Material Distribution: Helps distribute materials in animals lacking a circulatory system.     6. Hydrostatic Skeleton: In many invertebrates, the fluid-filled cavity provides structural support.

Key Innovation 4: Development Patterns

  • Blastopore: The indentation formed during gastrulation that becomes the start of the digestive tube.

  • Protostomes ("Mouth First"): The blastopore develops into the mouth.     * Examples: Marine flatworms (Platyhelminthes), Pine wood roundworms (Nematodes), Giant clams (Mollusks), Earthworms (Annelids), Cobalt blue tarantulas (Arthropods).     * Developmental Fate: Exhibit determinate development (the fate of embryonic cells is determined very early).

  • Deuterostomes ("Mouth Later"): The blastopore develops into the anus.     * Examples: Sea-cucumbers (Echinoderms), Red-pandas (Chordates).     * Developmental Fate: Exhibit indeterminate development (cells are identical and pluripotent). If a cell is separated early on, it can still form a complete organism. This is the basis for embryonic stem cell flexibility.

Key Innovation 5: Segmentation

  • Definition: The body is formed by repeated units (segments) that look alike, at least early in development.

  • Advantages of Segmentation:     1. Redundancy: Damage to one segment may not be fatal as others can duplicate its function.     2. Locomotion: Increases efficiency because segments can move independently.     3. Specialization: Different segments can evolve to perform specialized functions.

Questions & Discussion

  • Q15/Question 1: All below are characteristics of animals, EXCEPT: (A) Eukaryotic, (B) Autotrophic, (C) Multicellular, (D) Able to reproduce sexually, (E) Motile.     * Answer: B (Animals are heterotrophic, not autotrophic).

  • Question 2: Which of the below is NOT a characteristic of animals? (A) Most have cells organized in tissues, (B) Most have body symmetry, (C) Cells with cell walls, (D) Diploid, (E) Heterotrophic.     * Answer: C (Animal cells lack cell walls).

  • Question 3: The main difference between a colonial and a multicellular organism is that multicellular organisms: (A) Have specialized cells, (B) Are larger, (C) Can reproduce sexually, (D) Are motile, (E) Are diploid.     * Answer: A.

  • Question 4: All below are true about animal tissues, EXCEPT: (A) All animals have cells organized in tissues, (B) Tissues are present in all animals except sponges, (C) Tissues allow for greater specialization and complexity, (D) They show up for the first time among cnidarians.     * Answer: A (Sponges are animals but do not have true tissues).

  • Question 5: All below are true about body symmetry in animals, EXCEPT: (A) All animals, except sponges, have body symmetry, (B) Cnidarians are radial animals, (C) Radial symmetry is found only in animals without true tissues, (D) Most animals are bilateral.     * Answer: C (Cnidarians have true tissues and radial symmetry).

  • Question 6: Which of these animals have radial symmetry? (A) Sea horse, (B) Spider, (C) Snake, (D) Snail, (E) Seastar.     * Answer: E (Seastars exhibit secondary radial symmetry).

  • Question 7: All below are advantages associated with the coelom, EXCEPT: (A) It promotes the development of a head region and a brain (cephalization), (B) It protects and cushions the internal organs, (C) Organs can grow larger, (D) Movement of organs and the body are disassociated.     * Answer: A (Cephalization is more directly associated with bilateral symmetry than the coelom itself).

  • Question 8: From which of the non-human animals below can embryonic cells be used for stem cell research? (A) Starfish (echinoderm), (B) Earthworm (annelid), (C) Spider (arthropod), (D) Octopus (mollusk), (E) None, (F) All.     * Answer: A (Only Deuterostomes, like the Starfish, have indeterminate/pluripotent embryonic cells).

  • Question 9: Which of the below is NOT an advantage associated with segmentation? (A) Damage to one segment may not be fatal, (B) More space for internal organs to grow, (C) Locomotion is more efficient, (D) Different segments can specialize.     * Answer: B (Body cavity/coelom provides space for organs, not segmentation).