Endoskeleton three phyla and one subclass of animals, endoskeletons of various complexity are found: Chordata, Echinodermata, Porifera, and Coleoidea. An endoskeleton may function purely for support (as in the case of sponges), but often serves as an attachment site for muscle and a mechanism for transmitting muscular forces. A true endoskeleton is derived from mesodermal tissue. Such a skeleton is present in echinoderms and chordates. The poriferan 'skeleton' consists of microscopic calcareous or siliceous spicules or a spongin network. The Coleoidae do not have a true endoskeleton in the evolutionary sense; there, a mollusk exoskeleton evolved into several sorts of internal
Nerve net with nerve nets lack a true brain, they have the ability to display complex movements and behaviors. The presence of a nerve net allows an organism belonging to the aforementioned phyla of Cnidaria, Ctenophora, and Echinodermata to have increased fitness as a result of being able to respond to their environment. Developmental neurogenesis of nerve nets is conserved between phyla and has been mainly studied in cnidaria, especially in the model organism Hydra. The following discusses the development of the nerve net in Cnidaria, but the same mechanism for the differentiation of nervous tissue is seen in Ctenophora and Echinodermata.