PHYLUM CNIDARIA AND CTENOPHORA
1. For sessile (sedentary) organisms, radial symmetry is useful
because the animalcan just "sit down" and grab food or detect threats
from all directions.
2. Cnidarians share several basic characteristics. All Cnidaria are
aquatic, mostly marine, organisms. They all have tentacles with
stinging cells called nematocysts that they use to capture
food. Cnidarians only have two body layers, the ectoderm and
endoderm, separated by a jelly-like layer called the mesoglea
Phylum Ctenophora are: Biradially symmetrical, with an oral - aboral
body axis . Eight rows of ciliated "combs" or ctenes , controlled by an
apical sense organ at the aboral pole. Incomplete gut (mouth but no
anus ).
3. Major Groups of Cnidarians. Cnidarians are divided
into three major classes. These are the Hydrozoa (hydras and other
colony-forming species), the Scyphozoa (jellyfish), and the Anthozoa
(sea anemones and corals).
The phylum ctenophora is divided into two classes: Tentaculata and
Nuda. Class 1 Tentaculata: They have tentacles and small
stomodaeum. Examples: Hormiphora (The Sea Walnut), Pleurobrachia
(The Sea gooseberry).
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5. Cnidarians are incredibly diverse in form. Yet, these diverse animals
are united because they are all armed with stinging cells
called cnidocytes and contain a structure called a nematocyst.
These stinging cells are used to capture and subdue prey.
Thenematocyst is a coiled thread-like stinger.
6. Unlike central nervous systems, where neurons are typically grouped
together, neurons found in nerve nets are found spread apart.
This nervous system allowscnidarians to respond to physical contact.
... Hydra, which are cnidarians, have a nerve net throughout their
body.