BIOL 2030 Chapter Notes - Chapter 7: Pleurobrachia, Body Plan, Octocorallia
Document Summary
Contain phyla cnidaria and ctenophora which have radial (good for sessile) or biradial symmetry. Have cells called cnidocytes which contain the stinging organelles (cnidae). Five classes: hydrozoa, scyphozoa, cubozoa, anthozoa, and staurozoa. Mostly sessile or slow moving or slow swimming. Are in forms of polyp or medusa. Have an incomplete gut (gastrovascular cavity) and no respiratory or excretory system. There is dimorphism (polyp and medusa forms) and sometimes polymorphism (when a colony contains several body forms) displayed. Some hydras move by bending over and attaching their tentacles to the substratum. Life cycle: zygote turns into free-swimming planula larva. The polyp may reproduce asexually or (in case of hydrozoa and scyphozoa) the polyp makes a medusa. Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells. Polyp body wall: outer epidermis (contains gland cells, cnidocytes, sensory and nerve cells) , inner gastrodermis (lines with gastrovascular cavity), and mesoglea is in-between them. Water in gastrovascular cavity serves as hydrostatic skeleton.