AP BIOLOGY

Detailed explanation-1: -In the case of cnidarians there are two layers: the endoderm and the ectoderm. The endoderm is the inner layer (’endo’ for ‘within’), and the ectoderm is the outer layer (’ecto’ for ‘outer’). If there was a third layer it would be called the mesoderm, ‘meso’ for ‘middle’. But cnidarians do not have this middle layer.

Detailed explanation-2: -During gastrulation, an embryo’s cells split into separate layers called germ layers. All animal embryos, except sponges, develop two or three germ layers. The germ layers will give rise to every tissue and organ in the body.

Detailed explanation-3: -Cnidarians don’t have true mesoderm, which is a third germ layer that develops as a result of gastrulation. Cnidaria is made up of two germ layers: endoderm and ectoderm.

Detailed explanation-4: -The phylum Cnidaria (sea anemones, corals, hydras and jellyfish) is the likely sister group of the triploblastic Bilateria. Cnidarians are generally regarded as diploblastic animals, possessing endoderm and ectoderm, but lacking mesoderm.

Detailed explanation-5: -Cnidarians nominally develop from two germ layers, the ectoderm and endoderm, whereas the presence of a third germ layer, the mesoderm, traditionally characterizes higher animals.