GENERAL ANIMAL CHARACTERISTICS There are only four main characteristics in animals. First of all, they are multicellular, and as the phyla types progress, they may contain specialized cells. They are also eukaryotic, which means their cells contain a true nucleus and all organelles needed for the organism to function. For nutrients, they are classified as heterotrophic- do not make their own food, but require an organic C source, such as glucose- through ingestion. Their cells also do not have cell walls.
CLASSIFICATION There are five ways to classify an organism by: symmetry, germ layers, segmentation, body plan, and evolution.
SYMMETRY: This explains how every animal has some sort of body symmetry, with sponges as the one exception. There are two types: radial and bilateral. Organisms with radial symmetry can be separate into a variety of planes along their central axis. These may have a cylindrical shape and have no head, front, or back. One example of this is an jellyfish. Bilateral symmetry means the organism can only be cut into two equal halves along its mid dorsal line. These animals have an anterior, posterior, ventral, and dorsal planes, which allows for cephalization- meaning the concentration of nerves and tissue around the front side to form a head. One example of this is a flatworm. Sponges are the only animal with no symmetry at all, and are asymmetrical.
GERM LAYERS: The word germ itself means grow, so germ layers describes how animal cells will classify depending on their number of special tissue layers called germ layers. The first type is only one germ layer, which exists in the most simple animals without tissues or organs. The second is diploblastic, referring to two layers: the ectoderm- outer layer forming the skin and nervous system- and the endoderm- inner layer forming the digestive tract lining. One example of a diploblastic organism is a sea anemone. The last type is triploblastic, which means the organism has three germ layers, the ectoderm, endoderm, and mesoderm. The first two were already explained, but the mesoderm grows between the ectoderm and endoderm, forming muscle and internal organs. One example of this is a human being. Triploblastic is the most advanced type of the three, and usually describes advanced organisms.
SEGMENTATION: Segmentation basically describes that how an organism is segmented can lead to specialization and variety of body regions. There are three types: none, simple, and specialized. None refers to no specialized sections, simple refers to an identical repeating pattern of sections- like an earthworm- and specialized refers to specific body segments that fuse with each other to form body regions. These sections each focus on one set of specific tasks.
BODY PLAN: This refers to whether the organism has a body cavity- also known as a coelom- to store organs and organ systems during growth. There are three types: acoelomate, pseudocoelomate, and coelomate. Acoelomates describes animals with no cavity and a poorly developed organ system, such as a Portuguese man o'war. Pseudocoelomates are organism who have a false cavity between the endoderm, and mesoderm layers. As a result, only some organs may develop in this cavity. One example is a roundworm. The final type, coelomates, refers to animals with a true coelom, located between their mesoderm of their body wall. These organisms can coordinate digestion, develop advanced organs, and are overall the most advanced level of development. One example of this is a domestic dog.
EVOLUTION: There are three types of this, identifying animals based on their evolutionary history. These three are lower invertebrates, higher invertebrates, and vertebrates. Lower invertebrates are the phyla with a very simple evolution history, and a very basic body plan. This includes the Porifera, Cnidaria, Ctenophora, Platyhelminthes, Nemertea, and Nematoda. The higher invertebrates exist as either protostomes- develop mouth before anus during embryonic development- or deuterostomes- develop anus before mouth. This includes the Rotifera, Mollusca, Annelids, Arthropods, and Echinodermata. The last is the vertebrates, which represents all of the highly evolved organisms at the highest level, which includes the chordates and describes US!
PHYLA TYPES The different types of Phyla are listed below. The ones in bold are the ones I will be explaining later.
Porifera (sponges) Cnidaria (cnidarians, or "stingers") Ctenophora (comb jellies) Platyhelminthes (flatworms) Nemertea (ribbon worms) Nematoda (roundworms) Rotifera (rotters) Mollusca (molluscs) Annelids (annelids or "little rings") Arthropods (arthropods or "segmented creatures") Echinodermata (echinoderms or "spiny skinned organisms") Chordata (chordates or animals that follow four key characteristics)