Introduction to Phylum Cnidaria
Phylum Cnidaria is also
known as Coelenterates. It encompasses a
fascinating array of animals like jellyfish, sea anemones, corals, and the more
familiar Hydra. These creatures are diploblastic eumetazoans, meaning their
bodies are composed of two primary cell layers and are organized at the tissue
level.
One of the unique
features of cnidarians is the presence of cnidocytes.
It is a specialized cells that function like tiny harpoons to capture prey
or defend against predators. Cnidarians also exhibit two distinct body forms:
the polyp, which is sessile (fixed
in place), and the medusa, which is
free-swimming. This phylum occupies a middle ground in terms of complexity—more
advanced than sponges but fewer complexes than bilaterians.
Both cnidarians
and their relatives, the ctenophores
(comb jellies), share advanced characteristics such as muscles, nervous
systems, and sometimes sensory organs, distinguishing them from simpler animals
like sponges.
Basic Body Forms in Cnidaria
Most cnidarians are found in one of two body forms:
1. Polyp: Sessile and cylindrical, with a mouth and
tentacles at the top.
2. Medusa: Bell-shaped
and free-swimming, with tentacles dangling from the edges.
These forms exhibit radial symmetry, meaning their bodies
are arranged like spokes on a wheel. Since cnidarians do not have distinct
heads, their body ends are referred to as:
Oral
end:
The side where the mouth is located. and
Aboral
end:
The opposite side, furthest from the mouth.
Around the edges of
their bodies, most cnidarians have tentacles
armed with cnidocytes for capturing prey. In medusae, tentacles also form an
inner ring around the mouth. Some species, like hydroids, form colonies where
different members, called zooids,
specialize in tasks like defense, reproduction, or feeding.
The mesoglea (a gelatinous layer between the outer and inner tissues) plays an important role in structure and movement. In polyps, it is soft and thin, but in medusae, it becomes thicker and springy, enabling them to propel themselves by jet propulsion.
General Characteristics of Phylum Cnidaria
1. Habitat: Mostly marine, but some, like Hydra, live in freshwater.
2. Colonial vs
Solitary: Many are colonial (e.g.,
corals), while some are solitary (e.g., sea anemones).
3. Body Symmetry: They
exhibit radial symmetry, although
some, like sea anemones, show biradial
symmetry.
4. Body Forms:
Cnidarians exist in two main forms—polyp
(sessile) and medusa (free-swimming).
5. Body Structure: The
body wall has three layers:
Epidermis
(outer layer), Gastrodermis (inner
layer), Mesoglea (a gelatinous layer between them).
6. Cnidocytes: These specialized stinging
cells contain cnida, a fluid-filled
capsule used for defense and prey capture.
7. Gastrovascular
Cavity: A central digestive cavity, also known as the coelenteron, serves both for ingestion and egestion.
8. Digestion: Begins
externally in the coelenteron and continues intracellularly in the cells of the
gastrodermis.
9. Respiration and
Excretion: These occur through diffusion across the body wall.
10. Nervous System:
Cnidarians have interconnected nerve
nets in both the epidermis and gastrodermis, allowing nerve impulses to
travel in any direction. Medusae also have nerve
rings and ganglia for more
complex coordination.
11. Reproduction:
Asexual reproduction occurs through budding, fission, or fragmentation. Most
cnidarians are unisexual, but some are bisexual. Fertilization is external, and
development usually involves a free-swimming, ciliated larval stage called the planula.
12. Regeneration:
Cnidarians possess a remarkable ability to regenerate lost body parts.
