BIOL 2003 Lecture Notes - Lecture 5: Gastrovascular Cavity, Cnidocyte, Mesoglea
.Cnidaria
September 21, 2015
• ~10,000 aquatic species (mostly marine)
• Five main classes of cnidarians
• Two main body forms: polyp and medusa
• Unique stinging cells (cnidocytes)
• Jelly layer celled mesoglea
• Blind-ended gut with tentacles around mouth
• Asexual and sexual reproduction
Cnidarians represented the big next step in evolution.
• Tissues present (diploblastic)
• Radial symmetry
Levels of Organization:
A. In Porifera and Placozoa:
• Ensemble of cells – functionally differentiated (totipotent; archaeosite)
B. In cnidarians: ensemble creates tissue grade
Cnidarians are diploblastic and forms a gastrula from a blastula (in porifera). They do not form a gut
but folds in to create endoderm and ectoderm (two separate germ layers). Eventually forms adult
cell layers.
Symmetry:
• Radial
• Oral and aboral
• Sense environment from all sides
o No head
o Good for sessile or free-floating animals
Diploblast Cnidaria with Three Layers:
• Endoderm = gastrodermis
• Ectoderm = epidermis
• Mesoglea = jelly, not cellular in between living layers
• Creates gastrovascular cavity with a mouth opening
• Could go through each tentacle to create cavity *no anus therefore, spit back out
Two Body Forms: Dimorphic bodies!
1. Polyp – normally sessile
• Anemone form
• Solitary or colonial forms
• Mouth faces upwards
2. Medusa – swimming
• Jellyfish form
• Mouth faces downwards
OR Polymorphism
• Seen in polyps
• In one animal
• Hydrozoans
Zooids
• Gastrozooids – collect food
• Gonozooids – reproduction
• Dactylozooids – defense (stinging cell)
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