Triploblastic

Triploblastic refers to animals whose embryos form three primary germ layers: the ectoderm, mesoderm, and endoderm. These layers arise during gastrulation and give rise to all tissues and organs in the body. The ectoderm develops into the epidermis and nervous system; the endoderm lines the gut and gives rise to the digestive tract and associated organs; the mesoderm forms muscle, skeletal structures, the circulatory system, the excretory system, and internal organs such as the gonads and kidneys.

The presence of a mesoderm enables more complex organ systems and often a body cavity called a coelom. However, not all triploblasts have a true coelom; some are acoelomates (no body cavity) or pseudocoelomates (body cavity not completely lined by mesoderm).

Triploblasts include most bilaterian animals, such as chordates, echinoderms, arthropods, mollusks, and annelids, as well as nematodes and flatworms. Cnidarians and ctenophores are diploblastic and lack a mesoderm.

During gastrulation, three germ layers are formed. In protostomes, the blastopore becomes the mouth; in deuterostomes, the blastopore becomes the anus. This developmental pattern is central to the diversity of bilaterian body plans.