1. KEY CONCEPT Sponges and cnidarians are the simplest animals.

2. Sponges have specialized cells but no tissues.
Sponges are the most primitive animals on Earth.
570 million-year-old fossils
closely related to group of protists
Sponges share common characteristics.
Sessile adult, motile larvae
reproduce both sexually and asexually
filter feeders
No tissues or symmetry

3. Sponges have several types of specialized cells.
Pinacocytes- outer layers
Choanocytes- filter feeding mechanism
amoebocytes -motile
osculum
choanocyte
amoebocyte
pinacocyte
pore
spicule

4. Cnidarians are the oldest existing animals that have specialized tissues.
Cnidarians have two body forms.
1. polyps
In the polyp form
of a coral, the tentacles
and mouth face upward.

5. 2. medusas
In the medusa form of a sea jelly, the tentacles and
mouth face downward.

6. The outer tissue layer has three cell types. contracting cells
Cnidarians are made up of two tissue layers (endoderm and ectoderm) separated by mesoglea.
gastrovascular cavity
mouth
mesoglea
tentacle
oral arms
The outer tissue layer has three cell types.
contracting cells
nerve cells
cnidocytes (which contain nematocysts to )
barbs
coiled
nematocyst
discharged

7. Two anemones aggressively fight using their specialized sacs loaded with nematocysts that contain toxins.

8. The four major cnidarian classes are defined by their dominant body form.
Anthozoans such as sea anemones have a dominant polyp stage.

9. Hydrozoans such as hydra alternate between forms.

10. Scyphozoans are true jellyfish with a dominant medusa form.
Cubozoans such as sea wasps have a dominant medusa form.

11. Sponges and Cnidarians Virtual Lab
Link: