BIOL 2030 Chapter Notes - Chapter 7: Rhopalium, Strobilation, Velarium
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
- Larger jellyfishes
- The medusa have no velum (membrane on the subumbrellar surface) unlike hydrozoans
- Each notch in the bell have a rhopalium (sense organ) Aurelia has 8 notches.
- Planula larva attaches and produces scyphistoma (may bud to make other polyps). The
scyphistoma form a series of bus (ephyrae) and become strobila. This process is called
strobilation. When the ephyrae breaks loose, they grow into a mature jellyfish.
- Their life cycle does not include the medusa form
- The polyps reproduce sexually
- Have 8 “arms”
- Medusa is the dominant form and are almost square shaped
- The base of each tentacle is differentiated into a form called pedalium
- Have velarium which functions the same as a velum in hydrozoan medusa (increases swimming
- Eg. Carybdea
- No medusa stage
- Has three subclasses (zoantharia, ceriantipatharia, octocorallia)
- Zoantharia and ceriantipatharians have a hexamerous body plan (based on six or multiples of
six) while octocorallia has octomerous body plan and always have 8 tentacles.
- Sea anemones (subclass zoantharia) have pharynx which leads to gastrovascular cavity. They are
- 8 rows of comblike plates which they bear for locomotion
- “sea walnuts” and “comb jellies”
- Eg. Pleurobrachia
- Like cnidarians they are radial symmetry (the only 2 that have it, the rest are bilateral)
- Emit light at night
- The 8 rows of plates are fused cilia called comb plates or ctenes
- Have two tentacles and when they catch food they bring it to the mouth and it goes through the
pharynx to the gastrovascular cavity (intracellular digestion)
- They catch small prey with colloblast
- Have anal pores where waste is expelled
- Have a sense organ: statocyst (organ of equilibrium)
- Are monoeceious