WHAT IS A CNIDARIAN?

Cnidarians are soft bodied animals that have stinging tentacles arranged in circles around their mouth. All cnidarians exhibit radial symmetry and have specialized cells and tissues. Most have life cycles with two different stages. One of those stages is called polyp, which is sessile and flower like, and the other stage is called medusa, which is motile and bell-shaped. Some cnidarians live in groups of dozens or thousands connected together called a colony. Other cnidarians live individually.























General Anatomy
The body wall of consists of three layers. Epidermis is the layer of cells that covers the outer surface of the body. The mesoglea is a thin non-cellular membrane in polyps and a thick jellylike material that may contain amebocytes in medusae. The gastroderm is the layer of cells that covers the inner surface, lining the gastrovascular cavity. The body wall surrounds the gastrovascular cavity where food is digested.

Some Examples
Class Hydrozoa contains hydra, obelia, and physalia
ex. Portuguese man-of-war









Portugese man-of-war



Class Scyphozoa contains aurellia (jellyfish)
ex. Lion's Mane, Box Jellies, Moon Jellies





Moon Jelly



Class Anthozoa contains sea anemones and corals
ex. Sea fans, Sea Pens, Gorgonian Coral and Brain Coral












Sea Pen


Defining Characteristics
Cnidarians have stinging cells called nematocysts in their tentacles. Nematocysts are a poison-filled sac outfitted with a dart that shoots into the victims skin. Other characteristics of cnidarians is that they have no centralized nervous system, are radially symmetric, are aquatic, and have only a single opening to the body cavity.


Habitat
Cnidarians live in mostly marine environments at varying depths. A few species live in freshwater.


Adaptations to their environment
Jellyfish have the ability to swim which makes it easier to find and catch prey, instead of having to wait for the prey to come to you like in polyps. Cnidarians have also developed stinging cells called nematocysts which paralyze their victims to make them possible to eat.


Comparisons to other phyla
Cnidarians are an advancement over sponges. Sponges are totally sessile, have no gastrovascular cavity, and have no specialized tissues. More highly evolved phyla have a centralized nervous system which cnidarians lack.


Vocab
All underlined words are important and are defined within the paragraph they are found in.

REPRODUCTION

Hydrozoa
Asexual (hydra)
Hydrozoa reproduces asexually by budding. The cavity of bud is continuous with that of the parent. The bud pinches off and drops to leave a separate existence.
Sexual (hydra)
The testes and ovaries are formed in the body wall. There are many sperm released into the water. The eggs are fertilized in the ovaries. The embryo breaks loose from the parent and matures as young hydra.

Some species of hydra are hermaphroditic. i.e. One individual produces both egg and sperm.

Obelia
Medusas are produced as buds and they reproduce sexually. The fertilization takes place externally.

Scyphozoa
Asexual
Medusas are produced by budding from polyp stage.



Sexual
There are separate sexes. The fertilization occurs in the water. The zygote develops into ciliated larva which attaches to rocks and develops into polyp form.









Anthrozoa
Asexual
Many of the species reproduce by budding.
Sexual
There are separate sexes. The gonads are kept in the gastrovascular cavity. the zygote develops into ciliated larva which attaches to rocks and develops into polyp form.

MOVEMENT

The muscle movement of cnidarians are controlled by nerve cells but they do not have very much control over their movement and are often moved by the ocean's current and washed up on beaches. The majority of the cnidarian's nerve cells are found around the mouth area, on the tentacles and stem of polyps and around the screen of jellies. As a medusa (jellyfish) cnidarians mainly move by jet propulsion using their mesoglea to propel the medusa forward in small bursts. As a polyp, cnidarians glide along using their substrates and mucous secretions to slide along surfaces.

RESPIRATION AND CIRCULATION

Most of the living cells in cnidarians are in direct contact with water causing them to live in clean, constantly moving water and they therefore do not have or need true a circulatory system. Respiration in cnidarians occurs without any of the following specialized organs such as trachae, gills or lungs. To respire, the cnidarian obtains oxygen from water and it diffuses directly through the tissues of the cnidarian. Gaseous wastes are also eliminated by diffusion but solid wastes are discharged through the mouth opening as cnidarians do not have anuses. The gastrovascular cavity functions as a transport system for the animal. The cavity circulates fluid through the organism by squeezing the body wall which pushes water in and out of the body.

RESPONSE

Cnidarians nerve nets but no centralized nervous system. Nerve nets are concentrated around the mouth. Polyps and medusa forms of cnidarians have sensory cells in the epidermis that detects chemicals from food and the touch of foreign objects.
In medusae, some groups of sensory cells are organized into simple organs called statocysts and ocelli that arrange around the rim of a medusa’s bell. Statocysts help determine which way is up and ocelli or eyespots detect the presence of light. Coral colonies usually have a basic response to environmental conditions such as light, waves and currents which makes them similar to anthozoans. Take a look at figure 1B (second diagram) in this link for a diagram of a hydra nerve net. http://cas.bellarmine.edu/tietjen/images/cnidarians.htm

EXCRETION

Cnidarians can get rid of wastes by diffusion through their body walls (epidermis, mesoglea, gastroderm, gastrovascular cavity) but other that, there is no excretory system in cnidarians.

FEEDING & DIGESTION

Cnidarians have two different life stages, polyps and medusa. Both have a body wall that surrounds the gastrovascular cavity (where digestion occurs). Cnidarians are predatory carnivores and will try to eat basically anything that comes into it's path. Most members of this phylum are filter-feeding relying on the current and water that flows through them.
Almost all cnidarians capture and eat small animals by using stinging structures called nematocysts located on their tentacles. Each nematocyst has a poison-filled sac containing a tightly coiled “dart”. When an animal touches a nematocyst, the “dart” uncoils and buries itself in the animal. The “dart” carries enough poison to paralyse or kill their prey. Once the prey is captured, the cnidarian’s tentacles push through the mouth and to the gastrovascular cavity to be gradually broken up. Special cells in the gastroderm (gastrovascular cavity lining) digest those tiny pieces of food. Nutrients are then transported throughout the body by diffusion.
In many cnidarians, tiny photosynthetic protists grow right in side the living cells of the gastroderm providing cnidarians nutrients through symbiotic nutrients. Photosynthetic protists use carbon dioxide and other wastes produced by the cnidarian’s cells to manufacture oxygen and organic compounds such as carbons and proteins. Protists use some of the oxygen and organic compounds themselves and release the rest into the tissues of their hosts (cnidarians). Also, some colonial cnidarians and jellyfish have long, tube shaped, branching gastrovascular cavities that help carry partially digested food through their bodies.