COELENTERATES
The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea
anemones, and corals. They have achieved the tissue level of organization, in
which some similar cells are associated into groups or aggregations called
tissues, but true organs do not occur. A gastrovascular cavity has a
single exterior opening that serves as both mouth and anus. Often tentacles
surround the opening. Some cells are organized into two simple nerve nets, one
epidermal and the other gastrodermal, that help coordinate muscular and sensory
functions. Cnidarians have two basic body forms, medusa and polyp. Medusae, such
as adult jellyfish, are free-swimming or floating. They
usually have umbrella-shaped bodies and four-part symmetry. The
mouth is usually on the concave side, and the tentacles originate on the rim of
the umbrella. Polyps, in contrast, are usually sessile. They have tubular
bodies; one end is attached to the substrate, and a mouth is found at the other end. Polyps may occur alone or in groups of
individuals; in the latter case, different individuals sometimes specialize for
different functions, such as reproduction, feeding or defense. Reproduction in
polyps is by asexual budding or sexual formation of gametes . Nematocysts define cnidarians. These tiny organelles,
linked by hickmen 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.
Firing Nematocyst
A) Cnidocil
B) Nucleus
C) Nematocyst capsule
| Development: Just ectoderm and endoderm (which become epidermis and gastrodermis) with a jelly-like substance secreted (mesoglea) in between |
| Body cavity (between endo- and ectoderm): None |
| Digestive System: Saclike, incomplete gut |
| Circulatory System: Mostly absent |
| Excretory System: Absent |
| Nervous System: Network of neurons; no brain or distinct ganglia |
| Reproductive system: Wide-ranging, from asexual to sexual with separate sexes, sometimes all in the same species |
| Claim to Fame: Only phylum with nematocysts, the stinging cells on their tentacles |
Pelagia Colorata Jellyfish
Moon Jellies
