phylum CNIDARIA
The cnidaria phylum is divided into three major classes. The Hydrozoa (hydras), the Scyphozoa (jellyfish) and the Anthozoa (sea anemones and corals). Most cnidarians are marine, although a few reside in freshwater.
ECOLOGICAL ROLE
Anthozoans are important breeding areas for fish in terms of biodiversity because of the wide range of creatures that live near or in coral reefs.
Anthozoans are important breeding areas for fish in terms of biodiversity because of the wide range of creatures that live near or in coral reefs.
MOVEMENT STRUCTURES/TYPE
Cnidarians lack certain tissue types, such as muscle cells, but they do have fibers that can contract which makes them able to move about and capture prey. Cnidarians move through the contraction of a bell and tentacles. The Anthozoans are sessile, whereas the Scyphozoans are free-swimming. The Hydrozoans are not completely sedentary; in fact they move in unusual ways such as turning in somersaults.
Cnidarians lack certain tissue types, such as muscle cells, but they do have fibers that can contract which makes them able to move about and capture prey. Cnidarians move through the contraction of a bell and tentacles. The Anthozoans are sessile, whereas the Scyphozoans are free-swimming. The Hydrozoans are not completely sedentary; in fact they move in unusual ways such as turning in somersaults.
BODY COVERING; CELL LAYERS; LEVEL OF ORGANIZATION; PROTECTION
Cnidarians are multicellular organisms with few tissues and some organelles. Their bodies consist of two tissue layers, an outer protective epidermis and an inner layer called the gastrodermis. Although they do not have a mesoderm, inbetween some cnidarias is a non-living middle layer called the mesoglea, which is a jelly-like substance found in jellyfish. Cnidarians are equipped with tentacles and or stinging nematocysts that are useful for protection and for capturing prey.
SUPPORT; MAINTAINING BODY SHAPE
All cnidarians are characterized by having radial symmetry, and have no skeletel system. Their support systems consist of hydrostatic, or muscular support. There are body types of cnidarians; polypoid and medusoid. Polypoid cnidarians have tentacles and a mouth that face up (ex. Anemone or coral). Medusoid cnidarians bodies umbrella shaped with four-part symmetry. The mouth is located on the concave side with the tentacles hanging down from the rim of the umbrella. (ex. jellyfish)
OBTAINING NUTRIENTS
Most species are carnivorous or omnivorous filter feeders. Cnidarians feed using tentacles that are embedded with stinging nematocysts. The nematocysts are springing barbs with small hairlike triggers that are activated when in contact with prey. As nematocysts fire, barbs unfold and become embedded in the tissue of the prey. The nematocysts also inject the prey with an immobilizing toxin through a long hollow thread within the barb. Once the prey has been captured, the tentacles are used by the cnidarians to bring the prey into the gastrovascular cavity (digestive system). The food is then broken down into small particles by the enzymes secreted by the gastrodermal cells and the digestion is completed through small cavities within these cells.
RESPIRATION
Cnidarians do not have respiratory organs but both cell layers absorb oxygen from the water and expel carbon dioxide back into the water (diffusion). Some Anthozoa have ciliated grooves on their tentacles allowing them to pump water out of and into their digestive cavity without having to open their mouth.
CIRCULATION
Similar to Porifera, Cnidarians carry out circulation throughout the body through the process of diffusion. Sea anemones and corals stomachs serve as their circulatory system.
Similar to Porifera, Cnidarians carry out circulation throughout the body through the process of diffusion. Sea anemones and corals stomachs serve as their circulatory system.
EXCRETION
Cnidarians possess only one digestive opening which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to the internal digestive cavity called the gastrovascular cavity. Once the food has been completely digested, the indigestible parts of the prey are expelled from the gastrovascular cavity through the mouth.
Cnidarians possess only one digestive opening which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to the internal digestive cavity called the gastrovascular cavity. Once the food has been completely digested, the indigestible parts of the prey are expelled from the gastrovascular cavity through the mouth.
RESPONSE
Cnidarians lack a central nervous system. Instead, their nerves are organized into two simple nerve nets that cover their entire body; one epidermal the other gastrodermal that help coordinate muscular and sensory functions. Cnidarians have nematocysts that are stimulated when more than one sensory mode is triggered. For example, a nematocyst may respond only if there is a mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. The nematocyst discharge can range from sticky tentacles to deadly poisons. Jellyfish nervous systems are more developed than those of other cnidarians. Instead of a simple nerve net, they have a nerve ring around the edge of the bell portion of their medusa. Neurons throughout the rest of the body connect to this ring which allows the jellyfish to swim with coordination contractions of the entire bell.
Cnidarians lack a central nervous system. Instead, their nerves are organized into two simple nerve nets that cover their entire body; one epidermal the other gastrodermal that help coordinate muscular and sensory functions. Cnidarians have nematocysts that are stimulated when more than one sensory mode is triggered. For example, a nematocyst may respond only if there is a mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. The nematocyst discharge can range from sticky tentacles to deadly poisons. Jellyfish nervous systems are more developed than those of other cnidarians. Instead of a simple nerve net, they have a nerve ring around the edge of the bell portion of their medusa. Neurons throughout the rest of the body connect to this ring which allows the jellyfish to swim with coordination contractions of the entire bell.
REPRODUCTION
Cnidarians can reproduce asexually by budding, or sexually in which sperm and eggs are released by male and female organisms into the water, resulting in free-swimming larvae. Hermaphrodites produce gametes for sexual reproduction and time the gamete release in order to prevent self-fertilization. Jellyfish only reproduce sexually. When the male's sperm goes into the female's mouth, the eggs develop inside the female or on the oral arms. Eventually the larvae leave the mother and settle on the seafloor for a few weeks until they are grown adult.
Classes of cnidaria
- Anthozoa (corals & anemones): polyp stage only, tubular body ending in a mouth surrounded by a ring of tentacles, mesoglea between ectoderm and endoderm is thick, skeleton lies outside of living tissues, highly developed contractile cells, thicker mesoglea, corals have hard limy skeleton, mostly colonial species connected by living tissue.
- Hydrozoa (hydroids): most have both polyp and medusa phases, not strictly sedentary moves by somersaults, colonies have interconnected gastrovascular cavities, nerve net, polyps have tentacles and nematocysts, medusa is bell shaped and swim by tightening and relaxing it.
- Scyphozoa (swimming jellyfish): Medusa stage is dominant, grow larger than Hydrozoas, more of a developed nervous system than other cnidarians, nerve ring around bell portion, neurons over the rest of the body, movement achieved by coordinated contractions of the bell.