Sponge animals are animals that belong to the phylum Porifera. (Recent molecular research shows that sponges are monophyletic, and this is the phylogeny we describe here; nevertheless, other studies imply that sponges are paraphyletic.) Sponges are among the most basic creatures, and the ancient Greeks mistook them for plants.

The majority of species are marine, with sizes ranging from a few millimeters to a few meters.

Sponges are filter feeders, meaning they filter out food items floating in the surrounding water as they pull it through their body, which resembles a sac perforated with holes in certain species. Water is pulled into a central cavity through the pores.

Cnidarians come in polyp and medusa forms.

A cnidarian's body wall is made up of two layers of cells: an epidermis (darker blue; derived from ectoderm) layer on the outside and a gastrodermis layer on the inside (yellow; derived from endoderm). Digestion starts in the gastrovascular cavity and ends inside food vacuoles in the gastrodermis cells. The mesoglea is a gelatinous layer that sits between the epidermis and the gastrodermis.

Most sponges are hermaphrodites, which means that each individual performs both male and female sexual reproductive activities by generating sperm and eggs. Almost all sponges display sequential hermaphroditism, which means they operate as one sex first and subsequently as the other.

Cross-fertilization can occur when sperm discharged into the water stream by a male person is attracted into an adjacent individual who is acting as a female. The zygotes that form grow into flagellated, swimming larvae that spread from the parent sponge. A larva grows into a sessile adult after landing on a suitable substrate.

Sponges generate a wide range of antibiotics and other defense chemicals.

A cnidarian's fundamental body design is a sac containing a central digestive compartment, the gastrovascular cavity. This cavity's single entrance serves as both the mouth and the anus.

This body design has two variants: the mainly sessile polyp and the more motile medusa. Polyps are cylindrical forms that attach to the substrate with their aboral end (the end opposite the mouth) and stretch their tentacles in search of prey. Hydras and sea anemones are two examples of polyps.

Despite being largely sedentary, many polyps may move slowly across their substrate by utilizing muscles near the aboral end of their bodies. Some sea anemones may separate from their host when threatened by a predator.

A hydra's cnidocyte. The nematocyst, which includes a coiled thread, is found in this kind of cnidocyte. When a "trigger" is triggered by touch or specific chemicals, a thread shoots out, puncturing and injecting poison into prey.

Nematocysts are specialized cnidae with a stinging thread that may pierce the body wall of the cnidarian's victim. Other cnidae have lengthy threads that cling to or entangle tiny animals that come into contact with the cnidarian's tentacles.

Cnidarians have contractile tissues and nerves in their most basic forms. Bundles of microfilaments organized into contractile fibers are seen in the epidermis (outer layer) and gastrodermis (inner layer) cells.

The gastrovascular cavity serves as a hydrostatic skeleton against which the contractile cells can function. When a cnidarian closes its mouth, the volume of the cavity remains constant, and the contraction of certain cells causes the animal to chafe.

Sea stars (also known as starfish) and the majority of other echinoderm groupings (from the Greek urchin, spiny, and derma, skin) are slow-moving or sessile aquatic creatures. Coelomates are echinoderms.

A thin skin covers a hard calcareous plate endoskeleton, and most species are prickly due to skeletal ridges and spines.

The water vascular system is unique to echinoderms, consisting of a network of hydraulic canals branching into tube feet that act in movement and feeding. Echinoderm sexual reproduction often involves distinct male and female individuals who release their gametes into the sea.

Echinoderms originated from bilaterally symmetrical ancestors, although most species seem to be radially symmetrical at first glance. Most adult echinoderms' internal and exterior components radiate from the center, generally in the form of five spokes. Echinoderm larvae, on the other hand, exhibit bilateral symmetry. Furthermore, mature echinoderm symmetry is not really radial.

The entrance (madreporite) of a sea star's aqueous circulatory system, for example, is not central but displaced to one side.

There are five clades of living echinoderms.