Classes of Coelenterates: Hydrozoa, Scyphozoa, Anthozoa

The phylum Coelenterate includes the familiar hydras, jellyfish, sea anemones, and corals. Compared with the hydra, the jellyfish is upside down. Instead of the tentacles and mouth being up, as in the hydra, the tentacles and mouth are down. The largest jellyfish is Cyanea, whose tentacles may be over 30 meters long.

Coelenterates live singly or in colonies. The bodies of both polyps and medusa consist of two layers of cells, the endoderm and the ectoderm, separated by the jellylike substance called mesoglea. The mesoglea of a polyp is thin. In a medusa, however, the mesoglea often makes up the major part of the body substance. The tentacles of most coelenterates circle the mouth of the animal, and the cnidocytes are in the tentacles. Inside each cnidocyte is the coiled stringer called a nematocyst.

5.3.1. ClassHydrozoa

One of the most thoroughly studied of coelenterates is Hydra. Hydras and related animals make up the class Hydrozoa but they are not typical of hydrozoans in some ways.

Hydras are only about 1 cm long. Most hydras are orange, brown, white, or gray in color, but some are green. Hydras live in fresh water streams and ponds. They attach themselves to leaves and other debris in the water by means of a flattened adhesive base called the basal disk. Hydras move by floating, gliding or somersaulting.

Like other coelenterates, hydras have no brain or central nervous system. Hydras have sensory cells that respond to chemical and mechanical stimuli. The animals have little control in their responses to stimuli.

When a hydra catches a shrimp or a water flea in its tentacles, a feeding response begins.

Hydras reproduce asexually by forming small buds on the outside of the bodies. These buds grow, and within two or three days they fall off and begin life as independent animals. Hydras also reproduce sexually, usually in autumn. When the water temperature drops, individual hydras begin to develop either egg-producing ovaries or sperm-producing testes. The sperm swims to the egg, which remains attached to the body of the hydra. After fertilization has occurred, the egg begins to divide and falls off the adult female. The young hydra emerges in spring.

Hydra lives individually and independently. Most other, however, live in colonies.

5.3.2. Class Jellyfish

From a distance, jellyfish resemble plastic bags. Jellyfish can be seen swimming by rhythmically contracting and relaxing their “bells”. Jellyfish belong to the class of Scyphozoa. The tentacles of jellyfish with their stingers may reach up to 70 m in length. Their central disks may range from 4 cm to a meter in diameter. If you were to look down at the top of a jellyfish it would look round. Also, if the jellyfish were cut in half from top to bottom and through the centre it would be divided into two halves that would be very nearly alike. Living things having this kind of round structure are said to have radial symmetry.

In the lifecycle of a jellyfish, the medusa reproduces sexually, and the polyp reproduces asexually. The male medusa releases sperm cells through its mouth, and the female releases eggs. The eggs grow into small, free-swimming larvae.

The larvae swim away and attach themselves to the sea floor. There they develop into a polyp stage that resembles the hydra. The jellyfish polyp grows and eventually produces buds. This development is asexual phase of its reproductive cycle. The buds grow and eventually begin to form medusa, which, as they build up, resembles a stack of plates. These medusas move off one by one and begin the cycle again. The polyp may repeat the process the following year.

5.3.3. Class Anthozoa

Sea Anemones and Corals. The sea anemones and corals belong to the class Anthozoa. Anthozoa means “flower animals”. These coelenterates are beautifully colored and have varied forms.

Sea anemones are marine polyps that inhabit coastal areas. A sea anemone has a basal disc like that of hydras, by which the anemone attaches itself to rocks or other objects. Sea anemones are solitary. They feed on fish and crabs that swim within reach of their tentacles. Sea anemones digest their food in much the same way as hydras do.

Corals resemble sea anemones but have skeletons and live in colonies. Soft coral species have internal skeletons. These species include the so-called precious coral which is used to make jewelry. The stony coral has an external skeleton that is almost pure limestone. The skeletons of dead corals accumulate and form the reefs often seen rising above the surface in tropical waters.