CHAPTER 7 – Radiate Animals
- Contain phyla Cnidaria and Ctenophora which have radial (good for sessile) or biradial symmetry
- Have cells called cnidocytes which contain the stinging organelles (cnidae). Nematocysts is a
kind of cnidae.
- Mostly sessile or slow moving or slow swimming. Efficient predators
- Five classes: Hydrozoa, Scyphozoa, Cubozoa, Anthozoa, and Staurozoa.
- Are in forms of polyp or medusa
- Have an incomplete gut (gastrovascular cavity) and no respiratory or excretory system
- Asexual reproduction (budding) and sexual reproduction
- There is dimorphism (polyp and medusa forms) and sometimes polymorphism (when a colony
contains several body forms) displayed. Eg. Of polymorphism is class Hydrozoa
- Some hydras move by bending over and attaching their tentacles to the substratum.
- Life cycle: zygote turns into free-swimming planula larva. The planula settles and forms a polyp.
The polyp may reproduce asexually or (in case of Hydrozoa and Scyphozoa) the polyp makes a
medusa. Medusa reproduce sexually and are dioecious.
- 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.
- Live in colonial form and have both asexual polyp and sexual medusa stage.
- Eg. Hydra, Obelia
- Catch prey with tentacles and move it close to the mouth. The chemical glutathione causes the
mouth to open
- Sometimes, during budding, the bud do not detach from the parent and form a colony (not
hydras, but Obelia). In Obelia gonangia is the reproductive polyps
- Ocelli are light-sensitive organs. Statocysts: small organs of equilibrium