Introduction to Animal Evolution and Organization

• The evolution of animals began in the Cambrian period, over 600 million years ago, in the ocean
• Approximately 6.8 million animal species exist
• Animalia is one of Earth's most diverse groups of organisms, found in almost every habitat
• Animals are multicellular heterotrophs without cell walls, belonging to the kingdom Animalia
• Most animals' bodies include epithelial, connective, and nervous tissues
• Animals reproduce sexually, with specialized body tissues and organs developing from embryonic tissue layers
• Animals are informally grouped into vertebrates (with a backbone) and invertebrates (without a backbone)

Cell Specialization and Organization

• Animal cells have evolved to become specialized for different functions like movement, digestion, and nervous systems
• Specialized cell groups form tissues, which join to create organs and organ systems, working together for complex functions

Levels of Organization in Organismal Complexity include five grades between unicellular and metazoan organisms

• Protoplasmic grade: Protoplasm differentiates into organelles for specialized functions, exemplified by Amoeba
• Cellular grade: Division of labor is evident, with some cells specializing in reproduction and others in nutrition, like Sponges
• Cell-Tissue grade: Similar cells form definite patterns and layers organized to perform a common function, forming a tissue, like Jellyfish
• Tissue-Organ grade: Organs are composed of more than one kind of tissue with more specialized functions than tissues, like Platyhelminthes
• Organ-System grade: Organs work together to perform some function; this is the highest organization level, like Earthworms

Animal Body Plan

• An animal body plan is the structure of organs and tissues within the body
• Body plans vary in the level of organization, the number of embryonic germ layers, body cavity form and number, and body symmetry
• Body symmetry is generally determined from an animal's external appearance but requires detailed study

Body Symmetry

• Symmetry is the balance or correspondence in size and shape on opposite sides of a medium plane
• Asymmetrical bodies: Bodies are not balanced
• Spherical symmetry: Any plane passing through the center divides the body
• Radial symmetry: Can be divided into similar halves by more than two planes passing through the longitudinal axis
• Bilateral symmetry: Animals are divided along a sagittal plane into two mirrored portions

Cephalization

• Bilateral symmetry and forward head movement is associated with cephalization or differentiation of a head
• A greater concentration of sense organs and nerve cells exists at the front of an animal's body
• Cephalized organisms can respond to the environment in more sophisticated ways
• Cephalization promotes brain development and survival; the mouth is closer to sense organs and the brain
• Examples of organisms with cephalization include ants, lobsters, spiders, birds, snakes, humans, and squids

Early Embryonic Development

• An animal's body plan forms through an inherited developmental sequence
• The sequence starts after sperm fertilizes an egg, creating a zygote
• The zygote, a very large cell, divides into smaller cells called blastomeres via cleavage
• The cells then rearrange themselves to form a hollow ball called the blastula

Germ Layers

• Cells in the blastula form three layers, known as gastrulation
• Each of the cell layers are called a germ layer, and it differentiates into different organ systems
• Diploblastic organisms have two germ layers, ectoderm and endoderm
• Triploblastic animals have mesoderm in addition to ectoderm and endoderm

Body cavity includes three major types of body cavity

• Acoelomate: The mesoderm completely fills the blastocoel
• An example of an acoelomate is a flatworm
• Eucoelomate/Coelomate: A true coelom arises entirely within the mesoderm layer lined by an epithelial membrane
• Pseudocoelom (false coelom): Partly derived from the mesoderm and endoderm and found between the two layers.

Embryonic Development of the Mouth

• Bilaterally symmetrical, triploblastic eucoelomates are divided into two groups depending on their early embryonic development differences
• Protostomes: Mesoderm separates to form the coelom in a process called schizocoely
• Deuterostomes: Mesoderm originates as a series of punches and pinches off before enlarging to form the coelom in a process called enterocoely

Types of Gut

• Incomplete digestive system: One opening that serves as both the mouth and the anus
• Complete digestive system: Two openings: the mouth and anus

Segmentation

• Segmentation (metamerism): The organisms contains with several repeating parts, called a metamere (body segment/somite) forms in sequence within the embryo from anterior to posterior
• Segmented animals have repeating organs or bodies consisting of similar units
• Segmentation is evident in annelids, arthropods, and chordates