These are the Hydrozoa hydras and other colony-forming species , the Scyphozoa jellyfish , and the Anthozoa sea anemones and corals. All Cnidaria are aquatic, mostly marine, organisms. They all have tentacles with stinging cells called nematocysts that they use to capture food. Cnidarians only have two body layers, the ectoderm and endoderm, separated by a jelly-like layer called the mesoglea. Most Cnidarians have radial symmetry.
Common features of cnidarians include radially symmetrical diploblasts with true tissues and nematocysts. The presence of nematocysts indicates that all cnidarians arose from a common ancestor.
All coelenterates are aquatic, mostly marine. The bodyform is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the hypostome, surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey.
The body is radially symmetrical. The digestion is both intracellular and extracellular. The nervous system and the circulatory system is absent…. The coelenterates characteristically have two basically similar types of individuals differing in structural details, called the polyp and the medusa. The polyp is sessile. Hydras belong to the phylum Coelenterata also called Cnidaria , which includes corals, sea anemones, and jellyfish.
Hydras have four types of nematocysts on their tentacles, which are used for a variety of purposes. Hydra is also a coelenterate but fails to exhibit polymorphism, this is not the correct answer. Planaria is Platyhelminthes and is not considered as Coelenterata and these are not an answer. Their main purpose was to create advanced weapons for the Nazis. There are about 10, species of Cnidarians. Cnidocytes contain organelles called cnidea. There are several types of cnidea which include nematocysts, spirocysts, and ptychocysts.
The most notable of these is the nematocysts. Nematocysts consist of a capsule containing a coiled thread and barbs known as stylets. Nematocysts, when discharged, deliver a stinging venom that serves to paralyze prey and enable the cnidarian to ingest its victim. Spirocysts are cnidea found in some corals and sea anemones that consist of sticky threads and help the animal capture prey and adhere to surfaces. Ptychocysts are found in members of a group of cnidarians known as the Ceriantaria.
These organisms are bottom dwellers adapted to soft substrates into which they bury their base. They eject ptychocysts into the substrate which help them establish a secure hold. In hydras and jellyfish , the cnidocytes cells have a stiff bristle that projects out from the surface of the epidermis. This bristle is called a cnidocyl it is not present in corals and sea anemones, which instead possess a similar structure called a ciliary cone.
The cnidocyl serves as a trigger to release the nematocyst. Most cnidarians are carnivorous and their diet consists mainly of small crustaceans. They capture prey in a rather passive manner—as it drifts through their tentacles the cnidarian discharge stinging nematocysts that paralyze the prey. They use their tentacles to draw the food into their mouth and gastrovascular cavity. Once in the gastrovascular cavity, enzymes secreted from the gastrodermis break down the food.
Small hair-like flagella that line the gastrodermis beat, mixing enzymes and food until the meal has been fully digested. Any undigestible material that remains is ejected through the mouth with a swift contraction of the body. Gas exchange takes place directly across the surface of their body and waste is released either through their gastrovascular cavity or by diffusion through their skin. Jellyfish belong to the Scyphozoa. There are approximately species of jellyfish that are subdivided into the following five groups:.
A jellyfish begins its life as a free-swimming planula which after a few days drops to the sea floor and attaches itself to a hard surface. It then develops into a polyp which buds and divides to form a colony. After further development, the polyps shed tiny medusa which mature into the familiar adult jellyfish form which goes on to reproduce sexually to form new planulae and complete their life cycle. Corals belong to a group of cnidarians known as the Anthozoa. There are many types of coral and it should be noted that the term coral does not correspond to a single taxonomic class.
Some groups of corals include:. Stony corals make up the largest group of organisms within the Anthozoa. Stony corals produce a skeleton of calcium carbonate crystals which they secrete from the epidermis of the lower part of their stalk and basal disc. The calcium carbonate they secrete forms a cup or calyx in which the coral polyp sits.
The polyp can retract into the cup for protection. Stony corals are the key contributors to coral reef formation and as such provide the main source of calcium carbonate for the construction of the reef.
Soft corals do not produce calcium carbonate skeletons like those of stony corals. Instead, the contain tiny calcareous spicules and grow in mounds or mushroom shapes. Black corals are plant-like colonies that form around an axial skeleton that has black thorny structure.
Black corals are found primarily in deep. Some cnidarians, such as jellyfish, are even eaten. Different cnidarian species may also be collected for trade for aquariums and jewelry. Actively scan device characteristics for identification.
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Diet and Behavior. Reproduction and Offspring. Conservation Status. Cnidarians and Humans.
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