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Zoology
Cnidaria and Ctenophores
I.
Introduction to Cnidaria
A. Cnidaria takes their name from cells called cnidocytes, which
contain stinging organelles called nematocysts.
B. Nematocysts are characteristic of phyla Cnidaria. They are
formed and used by only by cnidarians.
C. Considered to have originated close to the basal stock of the
metazoans, approximately 700 million years ago. In other
words, they are closely related to Poriferans, despite having a
structure and function very different from sponges (as well as
other organisms).
D. They make up a significant portion of the biomass in some
ecosystems, and are widespread in marine environments. Few
are found in freshwater.
E. Although they are mostly sessile or slow-moving (slowswimming), they are quite efficient predators of organisms that
are much swifter and more complex.
F. This phylum includes some of nature’s strangest creatures:
branching, plant-like hydroids; flowerlike sea anemones;
jellyfishes; horny corals; and stony corals.
G. They are characterized by radial or biradial symmetry:
i. Radial symmetry- when body parts are arranged
concentrically around an oral-aboral axis
ii. Biradial symmetry- Radial symmetry in which only two
planes can pass though the oral-aboral axis to divid the
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animal into mirror images because of the presence of
some part which is paired.
H. Have not evolved above tissue level organization
II.
Characteristics of Cnidaria
A. Entirely aquatic; some in freshwater but mostly marine.
B. Radial symmetry or biradial symmetry around a longitudinal
axis with oral and aboral ends; no definite head.
C. Two basic types of individuals: polyps and medusae
D. Exoskeleton or endoskeleton of chitinous, calcareous, or protein
components in some
E. Body with two layers, epidermis and gastrodermis, with
mesoglea (diploblastic); mesoglea with cells and connective
tissue (ectomesoderm) in some.
F. A gastrovacular cavity with a single opening that serves as both
mouth and anus; extensible tentacles often encircling the mouth
or oral region.
G. Special stinging cell organelles called nematocysts in either or
both epidermis and gastrodermis; nematocysts very abundant
on tentacles where they may form batteries or rings.
H. Nerve net of synaptic and nonsynaptic patterns; with some
sensory organs; diffuse conduction.
I. Muscular system (epitheliomuscular type) of an outer layer of
longitudinal fibers at base of epidermis and an inner one of
circular fibers at base of gastrodermis; modifications of this
plan in higher coelenterates, such as separate bundles of
independent fibers in the mesoglea.
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i. Gastrodermis = inner layer of cells that lines a
gastrovascular cavity
ii. Gastrovascular cavity = functions in both digestion and
gas exchange
J. Reproduction by asexual budding (in polyps) or sexual
reproduction by gametes (in all medusae and some polyps).
Sexual forms monoecious or dioecious; planula larva.
i. Monoecious = have both male a female sexual organs
within the same individual
ii. Dioecious = having male and female sex organs on
separate individuals within the same species
K. No excretory or respiratory system.
L. No coelomic cavity.
III.
The Cell Wall and Nematocysts (Cnidaria)
A. Cnidarians display diploblastic, tissue-level organization. Cells
organize into tissues that carry out specific functions, and all
cells are derived from two embryological layers
(diploblastic=two layers). The epidermis is derived from the
ectoderm and the gastrodermis is derived from the endoderm.
Between the epidermis and gastrodermis is a jelly-like layer
called mesoglea
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i.
ii.
B. Cnidocytes are the characteristic cells found in cnidarians.
These cells can be found in the ectodermis or gastrodermis and
produces special fluid-filled structures called cnida for defense,
attachment and feeding.
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C. There are over 30 types of cnida. Nematocysts are one type
used for feeding and defense. These structures may discharge a
long tube armed with spines that penetrate prey. The spines
deliver paralyzing toxins, i.e. the stuff that stings you!!!
D. Nematocysts are tiny capsules composed of material similar to
chitin and containing a coiled tubular thread or filament, which
is a continuation of the narrowed end of the capsule. These tiny
organelles, likened by Hickman to cocked guns, are both highly
efficient devices for capturing prey and extremely effective
deterrents to predators. Each contains a coiled, tubular thread,
which may bear barbs and which is often poisoned. A
nematocyst discharges when a prey species or predator comes
into contact with it, driving its threads with barb and poison
into the flesh of the victim by means of a rapid increase in
hydrostatic pressure. Hundreds or thousands of nematocysts
may line the tentacles or surface of the cnidarian. They are
capable even of penetrating human skin, sometimes producing a
painful wound or in extreme cases, death.
i.
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IV.
Body Plans
A. Two types: medusa (or medusae) and polyp
B. Medusa is the body plan displayed by adult jellyfish. This is a
free-swimming or floating body plan. They are usually
umbrella shaped have four-part (tetramerous) symmetry. The
mouth/anus is usually on the concave side with tentacles around
the rim.
C. Polyps are usually sessile and have a tubular body. One end is
attached to the substratum or hard surface and the other is end
contains the mouth/anus. Polyps can occur alone or in groups.
If they occur in groups, the individuals specialize in different
roles, such as reproduction, feeding, or defense. This is the
body plan displayed by corals, hydra, and sea anemones.
V.
Reproduction
A. Asexual reproduction is displayed by polyp body forms. Sexual
reproduction is performed primarily by medusa body forms, but
some polyps can undergo sexual reproduction as well.