Introduction to Zoology: Defining Animals

Zoology - Defining and Describing Animals

• Objectives include defining what an animal is and identifying its defining traits.
• Another key objective is understanding how to differentiate between different types of animals.

What is an Animal?

• An animal is a kingdom of eukaryotes that are multicellular, lack cell walls, and are heterotrophic.

Animal Kingdom - Multicellularity vs. Colonial

• All animals are multicellular eukaryotes.
• Colonial organisms feature a group of identical organisms living together.
• Removing a member does not harm the colonial group, and the removed individual can start a new colony.
• Multicellular organisms consist of specialized cells, none of which can survive apart from the body.

Animal Kingdom - Heterotrophic Nutrition

• All animals are heterotrophic, acquiring food and energy by consuming materials from their habitat.
• Herbivores consume only plants.
• Carnivores consume only other animals.
• Filter feeders obtain nutrients by filtering out smaller organisms and organic materials from water.
• Omnivores consume both plants and animals.

Symmetry

• Symmetry describes the quality of an object consisting of similar parts that face each other or around an axis.

Animals and Symmetry - Asymmetry

• Asymmetry means there is an apparent lack of symmetry.
• The only asymmetrical phylum is Porifera, which includes sponges.

Animals and Symmetry - Radial Symmetry

• Radial symmetry allows an animal to be turned around a central axis any number of times and still look the same.
• Phylum Cnidaria, including jellies and corals, exhibits radial symmetry.

Animals and Symmetry - Bilateral Symmetry

• Bilateral symmetry means the left half of an animal mirrors the right half.
• Phylum Chordata, including mammals, reptiles, and birds, exhibits bilateral symmetry.

Animals and Embryonic Development

• Describing animal embryonic development can be done by cell layers or organ system development.

Animals and Cell Layers - Fertilization and Early Development

• Fertilization occurs when one egg (1n) and one sperm (1n) combine to form a 2n zygote.
• The zygote is the first cell resulting from fertilization.
• Mitosis leads to the development of the blastula.
• The blastula is a hollow ball of cells that develops from the zygote.

Animals and Cell Layers - Gastrulation

• Mitosis and cell specialization leads to gastrulation of the blastula.
• Gastrulation refers to the formation of the germ layers of the embryo.
• Gastrulation begins with the formation of the blastopore, an indentation in the blastula that becomes the mouth or anus.

Animals and Cell Layers - Gastrula and Germ Layers

• A gastrula is an embryo possessing fully developed germ layers as a result of gastrulation.
• Germ layers are unspecialized layers of cells that develop into the organs of the animal’s body.

Animals and Cell Layers - Germ Layers (Ectoderm)

• Ectoderm, mesoderm, and endoderm are key germ layers.
• The ectoderm is the outer germ layer that becomes the epidermis and nervous tissues.

Animals and Cell Layers - Germ Layers (Mesoderm)

• The mesoderm is the middle germ layer that develops into muscles and the lining of the body cavity.

Animals and Cell Layers - Germ Layers (Endoderm)

• The endoderm is the inner germ layer, becoming the digestive and respiratory systems.

Animals and Cell Layers - Number of Germ Layers

• The number of germ layers present is a key factor.
• Porifera has no defined germ layers.
• Cnidaria are diploblasts, possessing only endoderm and ectoderm.
• All other animals are triploblasts, possessing ectoderm, mesoderm, and endoderm.

Animals and Cell Layers - Order of Development

• The blastopore represents an indent that becomes the mouth or anus.

Animals and Cell Layers - Protostomes

• Protostomes are organisms where the blastopore becomes the animal’s mouth.
• Protostome phyla include Platyhelminthes (flatworms), Nematoda (roundworms), Arthropoda (crustaceans and insects), Mollusca (snails, slugs, and cephalopods), and Annelida (earthworms).

Animals and Cell Layers - Deuterostomes

• Deuterostomes are organisms where the blastopore becomes the animal’s anus.
• The deuterostome phyla include Echinodermata (sea stars) and Chordata (mammals, reptiles, birds, amphibians, and fish).

Animals and Cell Layers - Coelom Structure

• A coelom is the internal cavity of an animal that contains the viscera.
• Viscera refers to the internal organs.
• Coelom structure and possession are relevant only to triploblasts.

Animals and Cell Layers - Acoelomate

• Acoelomates are triploblasts that have a layer of mesoderm between the endoderm and ectoderm.
• Platyhelminthes (flatworms) are an example of acoelomates.

Animals and Cell Layers - Pseudocoelomate

• Pseudocoelomates are triploblasts with a partially lined pseudocoelom.
• A pseudocoelom is an internal cavity with mesoderm only lining the ectoderm.
• Nematoda (roundworms) are an example of pseudocoelomates.

Animals and Cell Layers - Eucoelomate

• Eucoelomates, also known as coelomates, are triploblasts with a true coelom completely lined with mesoderm.
• Mollusca, Annelida, Arthropoda, Echinodermata, and Chordata are examples.

Making Sense of Animals and Their Traits

• At least 36 animal phyla are known to exist, but most animal species belong to only nine phyla.
• The nine common phyla of animals are: Porifera (sponges), Cnidaria (jellies and corals), Platyhelminthes (flatworms), Nematoda (roundworms), Mollusca (snails, clams, and squid), Arthropoda (insects and crustaceans), Annelida (earthworms), Echinodermata (sea stars), and Chordata (vertebrates and seasquirts).
• Sponges have asymmetrical symmetry
• Cnidarians, and Ctenophorans have Radial symmetry
• Flatworms, Nematodes, Mollusks, Annelids, Arthropods, Lophophorates, Echinoderms, and Chordates have bilateral symmetry.
• Sponges, Cndarians, and Ctenophorans are diploblasts.
• Flatworms, Nematodes, Mollusks, Annelids, Arthropods, Lophophorates, Echinoderms, and Chordates are triploblasts.
• Flatworms, Nematodes, Mollusks, Annelids, and Arthropods are protostomes.
• Lophophorates, Echinoderms, and Chordates are deuterostomes.