Kingdom Animalia – Cnidaria
corals, sea anemones, jelly fish, box jellies, hydra, sea fans
Phylum Cnidaria (~3000 species); general characteristics:
Have tissue level of organization, with two tissue layers (diploblastic)
Have radial symmetry
Have specialized stinging structures called cnidae
Have only one opening to the body cavity / digestive cavity
Have no head, no centralized nervous system (only a nerve net), no
discrete gas exchange, excretory, or circulatory systems
Exhibit alternation of asexual polypoid and sexual medusoid generations.
Kingdom Animalia – Cnidaria
General body forms
2 body forms: Polyp and Medusa
2 tissue layers (diploblastic), ectoderm
to the outside (blue), and endoderm
to the inside (yellow). The layer in
between is just a jelly-like material
called the mesoglea (it is not a true
tissue)
Only one opening to the
gastrovascular cavity acts as mouth
and anus
Kingdom Animalia – Cnidaria
General body forms
Cnidarians have radial symmetry:
Kingdom Animalia – Cnidaria
All cnidarians have specialized stinging or adhesive structures called cnidae
Discharge
acceleration is
at 40,000g!!!
When triggered, the the cnidocyte cells
discharge the cnidae. A specialized form
of cnidae called the nematocyst, has a
harpoon like structure that enters the
flesh of the potential predator or prey,
releasing neurotoxins.
Need both physical and chemical stimulation to induce release. Why do you think this is?
Kingdom Animalia – Cnidaria
No matter what the size or shape of the cnidarian, they all have cnidae, and they
are all made up of one or more of polyp or medusoid body forms
Kingdom Animalia – Cnidaria
In general, cnidarians
alternate between
sexual medusoid and
asexual polypoid forms
There are a lot of
exceptions though…
Kingdom Animalia – Platyhelminthes
Phylum Platyhelminthes – flat worms (~20,000 species); general characteristics:
Have tissue level of organization, with three tissue layers (triploblastic – with
endoderm, ectoderm, and mesoderm)
Have bilateral symmetry
Have only one opening to the body cavity / digestive cavity
Have cephalization (and at least at some stage of their lives, so will all the animals that
we’ll discuss from now on.
Are hermaphroditic, with complex reproductive systems
Can be free-living or parasitic
Have great regenerative abilities!!!
Kingdom Animalia – Nematoda
Phylum nematoda – round worms (~50,000 species, or more?); general characteristics:
Very, very, very, very, very, very, common!!!
One study found 90,000 nematordes in a single rotting apple.
Maybe we’ll check this one out for ourselves in next weeks lab !!
Another study found 236 different species in about 6.7 cc of coastal mud!!
Triploblastic, bilateral, vermiform, un-segmented, pseudocoelomates
Body round in cross section and covered by a layered cuticle; growth in juveniles
Usually accompanied by molting
Have complete gut
Have no specialized circulatory or gas exchange structures, and only longitudinal
muscles. Why do you think they need a substrate to live in?
Can be free-living or parasitic
Kingdom Animalia – Nematoda
Parasitic example: Heart worms… cool eh?
Kingdom Animalia – Nematoda
Whereas cnidarians and platyhelminthes did not have a body cavity (coelom),
Nematodes do. It is however a pseudocoelom. All the other animals that we discuss
from now on will have true coelom (including us!). Can you tell what the difference
is between a coelom and a pseudocoelom? What is the advantage of having a coelom?