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Lecture 9
1/29/2008 8:58:00 AM
The Radiates
 They all have tentacles
 All gelatinous
 Cnidarians possess a specialized cell, which is a stinging cell
 They are radially symmetrical
Radial symmetry
 Animal can be folded along any axis and it will look the same
 There is also modified radial symmetry
o You can’t cut it along any axis, only a particular one
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o Bi-radial symmetry is when you can cut only in four quarters
(two planes, happens when you have paired structures)
o Tetra-radial symmetry is when you can cut only in 8 parts (4
planes)
Because they have prey and predators coming at them from all
around and because they cannot move (to catch or protect), all
sensory organs are distributed 360 degrees
Radiates are defined because all of them have some form of
bilaterality from the beginning
True Tissues
 Have 2 layers so it is diploblastic, they are the only organisms that
are
o Ectoderm (outer layer)-epidermis
o Endoderm (inner layer)-gastrodermis
 Between the two layers there is a gelatinous layer that contains no
cells (like a jelly sandwich)
 There are two forms they can exist in, polyp or medusa, in the
same life cycle
o Polyp usually has a basal disk which attaches them
o Medusa are usually involved in reproduction, can move a little
Cnidocyte
 Inside the cnidocyte (stinging cell) is the coiled nematocyte
 They allow you to capture prey that are movable (unlike you) or
protect from predators
Statocyst
 Sensory structure controlling orientation of organism
They are carnivores, prey is ingested into gastrovascular cavity (GVC)
Digestion is in extra cellular, with specialized enzymes
“Blind gut” means mouth=anus!
3 major classes of Cnidarians
 Hydrozoa
o Between the medusa and polyp form, the polyp form
dominates
o Most are colonial, formed from individual polyps
o They form a sessile colony showing polymorphism
(differentiation)
 Gonozooid contains gonads
 Gastrozooid contains stinging cells meant for eating
o Portugese man-o-war
 Contain swimming sac filled with carbon monoxide
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 Strings meant for reproduction, eating, fighting
Scyphozoa
o “true” jellyfish
o medusa is dominant body form, polyp only exists for small
time at very beginning
o really thick mesoglea (jelly sandwich)
Cubozoa
o Known as “sea wasps” or “box jellyfish”
o Have image forming eyes (5 phylla)
o They are on what are called rhopalium
o Each rhopalium contains 2 camera lenses and 4 simple eyes
(ocelli)
o Each organism has 4 rhopalium so 24 eyes!
Anthozoa
o Polyp body form only
o colonial
o “soft” corals with no exoskeleton
o “hard” corals with exoskeleton
o Sexual reproduction
 Produce larvae with fuse
o Asexual reproduction
 Fission where it splits
 Pedal laceration
o The large colonoies occur through asexual reproduction
o Corals contain endosymbiotic algae called zooxanthellae
 The algae get protection and the coral get
photosynthesis
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Ctenophora
o Diploblastic, maybe triplo
o Medusa-like only form
o 8 rows of cilia
o no cnidocyte
o
1/29/2008 8:58:00 AM
1/29/2008 8:58:00 AM