cnidaria

Cnidaria means needle from Greek word knide. There are about 10,000 species mostly marine.

Cnidarians (Cnidaria) are a group of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. The basic form of the cnidarian body is quite simple and consists of a gastrovascular cavity with a single opening through which food is ingested and waste is released. Cnidarians are radially symmetrical (a basic body plan in which the organism can be divided into similar halves by passing a plane at any angle along a central axis) and they have tentacles that encircle their mouth.

The body wall of cnidarians consists of an outer layer or epidermis, an inner layer or gastrodermis(the inner layer of cells that lines a gastrovascular cavity ), and a middle layer or mesoglea(translucent, non-living, jelly-like substance found between the two epithelial cell layers in the bodies of coelenterates).
Cnidarians lack organs and posses a primitive nervous system known as a neural net.

Cnidarians exhibit two basic structural forms, a medusa and a polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening

COLONIAL POLYPS…

located on the underside of the bell, and a gastrovascular cavity.The medusa form is most commonly associated with adult jellyfish. Although jellyfish pass through planula and polyp stages in their life cycle, it is the medusa form that is most recognized with this group of animals.

The polyp is a sessile( does not move) form which attaches to the sea floor and often forms large colonies. The polyp structure consists of a basal disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth opening located on the top of the polyp, and numerous tentacles which radiate out from around the edge of the mouth opening. Some cnidarians remain a polyp for their entire life, while others pass through the medusa body form. The more familiar polyp cnidarians include corals, hydras, and sea anemones.

Most cnidarians are carnivorous and feed on small crustacans. Prey becomes entangled in the tentacles and stinging structures embedded within the tentacles fire barbs and threads into the prey which secrete venom and paralyze the victim. The tentacles then bring the prey into through the mouth into the gastrovascular cavity where it is digested.

LAYERS AND PARTS 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.
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.

Cnidarians reproduce by both sexual and asexual means. They reproduce asexually through budding, and sexually by producing gametes which are excreted in hopes that they will meet up with gametes of the opposite sex. 

In asexual reproduction, new individuals arise from bits of tissue that are budded off from a parent, or by a parent dividing lengthwise or crosswise into two smaller individuals. Polyps that remain physically attached to one another or embedded in a common mass of tissue constitute a colony.

CNIDARIAN LIFE CYCLE…

The reproductive life cycle of a typical jellyfish illustrates both asexual and sexual reproduction. Males release sperm and females release eggs into the water. When an egg and sperm fuse during sexual reproduction, a larva develops that attaches to a rock or other object and develops into a polyp. In a type of asexual reproduction, the polyp divides to form a colony of polyps that resembles a stack of saucers. Each saucer in the stack develops tentacles and swims away from the colony as a new medusa, and the reproductive cycle repeats.