BIOL 1101 Lecture Notes - Lecture 91: Cnidaria, Eumetazoa, Gastrodermis
27 Jun 2016
School
Department
Course
Professor
Document Summary
The cnidaria includes about 10,000 species of hydras, jellyfish, sea anemones and corals. Unlike the porifera, the cnidaria are truly multicellular eumetazoans. They have specialized tissues and primitive organs, radial symmetry, and a digestive system. However, the cnidaria are the most primitive of the eumetazoa. Because they are radially and not bilaterally symmetrical, they do not show any tendency towards cephalization (an important role for a head containing a brain). Their digestive system is incomplete, and they are diploblastic (containing two rather than three germ layers). Perhaps because of their lack of mesoderm, the cnidarian body plan is simple. A cnidarian is basically a sac two cell layers thick. The outside cell layer is the epidermis, and the inside one is the gastrodermis. A mouth at one end is fringed with tentacles. The space inside the sac is used for digesting food and for circulation of water that transports food and oxygen.
