3454chapter13 - Chapter 13 Cnidaria and Ctenophora Phylum...

Chapter 13, Cnidaria and Ctenophora
Phylum Cnidaria—knide – nettle, aria – pl. of like; like a nettle
Cnidocytes—stinging cells after which the group is named
9,000-10,000 species, marine or
freshwater
Radial symmetry with oral and
aboral ends
Especially useful for sessile
animals
Tissue level of organization with
some simple organs
Diploblastic—ectoderm and
endoderm (some have an ectoderm-derived mesoderm)
Develop into epidermis and gastrodermis from these germ layers
Mesoglea—matrix between these tissues
Non-living, having a few cells, or with connec. tiss. + muscle

Gastrovascular cavity—central cavity with single opening (mouth)
Digestion (extracellular, in cavity) and respiration
Cavity extends in radial and ring canals in larger cnidarians and may
have many septa
Tentacles—extensions on the oral end for food capture
Nerve cells—simplest animals with true nerve cells, arranged in a
“nerve net” -- again, “simple” refers to the level of complexity, not the
evolutionary status. ..
Structure and
function
Polyp (hydroid)
and medusa
(jellyfish)
morphological types
Some species
are polyps, some are dimorphic, some are medusae

contraction
Colonial species may have polymorphism in its members
Different selective forces acting on polyp and medusa forms of same
species
Cnidocytes—stinging cells
Nematocysts—stinging
organelle within the cnidocyte,
chitinous barbs
Cnidocil—trigger on outside
of cnidocyte
Operculum—cap over
nematocyst
Triggering cnidocil allows water
to rush into capsule (140 atm.
osmotic pressure)