Characteristics

Characteristics that all animals have are being eukaryotic, heterotrophic, multicellular, having at least one motile phase, and the absence of a cell wall. All animals also need some type of support, which comes in the form of a hydroskeleton, exoskeleton, or endoskeleton. Some differences among animals are the presence or absence of true tissues, the type of body symmetry, and the presence or absence of a body cavity.

Diploblastic vs Triploblastic

Animals are either diploblastic or triploblastic. Diploblastic means the animal has two germ cell layers, such as Cnidarians. Triploblastic means the animal has three herm cell layers, such as chordates and mollusks. Germ layers are initial embryonic tissue layers. The ectoderm is the outermost layer, the endoderm is the innermost layer, and the mesoderm is the middle layer.

Types of Animals

Porifera are heterotrophs, asymmetrical, have various cell types, no distinct organs, and filter feeders. Filter feeders are flagellated cells that produce constant water flow through the sponge.

Cnidaria are divided into four main classes: cubozoan, scyphozoans, anthozoans, and hydrozoans. They have radial symmetry and are usually marine carnivores. They also have two body plans: medusae and polyps.

Platyhelminthes have no body cavity, bilateral symmetry, are marine and freshwater flatworms, and have primitive sensory systems

Annelida have bodies divided into segments, and those segments are divided by septa.

Mollusca have diverse body forms, a mantle that secretes the shell, and an open circulatory system (with the exception of cephalopods).

Arthropoda consist of arachnids, crustaceans, and insects. They have an external skeleton, jointed appendages, gills for respiration (crustaceans), tracheal tubes and spinal openings (insects), and an open circulatory system. Each of their body segments are paired with appendages (also seen in annelids).

Echinodermata consists of starfish, sea cucumbers, and sea urchins. They have penta-radial symmetry, deuterostomes, derma gills to respire by diffusion, oral and aboral sides, and a water vascular system consisting of water filled canals ending in tube feet topped with ampullae for locomotion.

Chordata have a notochord, pharyngeal slits, a post-anal tail, and an endostyle.