Three primary germ layers Gastrulation is a key phase in embryonic development when pluripotent stem cells differentiate into the three primordial germ layers: ectoderm, mesoderm and endoderm.

There are three germ layers in an early embryo of an animal: ecto-, meso- and endo-derms: this is true for triploblastic animals. Animal phyla Porifera, Cnidaria, and Ctenophora are diploblastic, they develop only ectoderm and endoderm.

Arthropods are triploblastic (they have three germ layers) and are coelomate organisms.

phylum Arthropoda

Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell. Cnidarian body forms. Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). The sessile asexual colony has two types of polyps, shown in (Figure).

Multicellular animals are heterotrophic by ingestion of food into a body cavity (coelom) that is completely lined by the mesoderm. Primitive phyla without a true coelom include the Porifera and Coelenterata (Cnidaria). Sponges (Porifera) have a primitive cellular level of organization and lack tissues and symmetry.

Cnidarians are not considered to have a coelom because they are diploblastic, so they don’t have any mesodermic tissue. Cnidaria are a phylum consisting of aquatic animals like jellyfish, anemones, and corals.

All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the hypostome, surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey.

Type of Coelom: Porifera have no coelom. Type of Body Plan: Porifera use canals and pores (diffusion) to perform life functions. Number of body layers: Porifera have no real body layers but they do have 2 cell layers; an outer layer that makes up the epidermis, and an inner layer that makes the inner cavities.

A pseudocoelomate is an organism with body cavity that is not derived from the mesoderm, as in a true coelom, or body cavity. A pseudocoelomate is also known as a blastocoelomate, as the body cavity is derived from the blastocoel, or cavity within the embryo. Sponges do not have a coelom.

Flatworms are hermaphroditic and capable of sexual and asexual reproduction. Their bodies have only a single opening, which serves as both a mouth and an anus. They are, as their name implies, flat. They have no circulatory system or body cavity (coelom), but they do have an excretory and digestive system.

Flatworms would have lost the coelom secondarily. Since their acoelomate condition is no longer indicative of a primitive origin, most of their characters put them in the protostomes, with the spiralian phyla. This position suggests the ancestors of flatworms lost both the anus and the coelom.