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Cnidarians have two basic body types, a medusa and a polyp.

The body symmetry of a cnidarian is radial in both the medusa and polyp stages. In a hydra, the gastroderm and the epidermis are separated by a jellylike mesoglea. Polyps differ from medusa forms because polyps are cylindrical and usually sessile, and medusas are bell-shaped and motile. The nerve cells of cnidarians make up a nerve net.

In a cnidarian, cells that sense gravity are statocysts In a cnidarian, cells that sense gravity are statocysts. In jellyfish, both polyp and medusa are diploid. The class Scyphozoa contains jellyfishes. Coral reefs occur in areas where there are high levels of light. Coral bleaching is a serious ecological threat because it causes corals to die.

Coral Bleaching

An extremely diverse coral reef can live in water that contains few nutrients because reef-building corals live symbiotically with photosynthetic algae. Unlike plant cells, animal cells do not have cell walls. Many small aquatic organisms move oxygen and carbon dioxide through their skin by the process of diffusion. When an animal’s environment changes, sexual reproduction improves a species’ ability to evolve.

Muscle tissue exerts force by becoming shorter Muscle tissue exerts force by becoming shorter. A parasite is a type of symbiont. The upper side of an organism is its dorsal side. An animal that has distinct left and right sides shows bilateral symmetry. A blastula is a stage in the development of an embryo that consists of a hollow ball of cells. Some biologists believe that sponges evolved independently of all other animals.

The mesoderm gives rise to muscles and much of the circulatory, reproductive, and excretory systems in animal embryos. Organisms that spend their entire adult lives attached to one spot are said to be sessile. Sponges contain choanocytes, archaeocytes, pore cells, spicules, and gemmules.

Sponges reproduce sexually by a process called internal fertilization Sponges reproduce sexually by a process called internal fertilization. Many sponges protect themselves from predators by producing toxins. Sponges feed by filter feeding. The benefit of a sponge’s producing gemmules is that gemmules allow the sponge to survive long periods of harsh conditions. Some sponges are green because they have photosynthetic organisms in their tissues.

Flagella are important to the essential functions within a sponge because flagella help move water through the organism’s body. Sponges benefit some marine animals by providing a habitat. Sponges play an important role in the primary productivity of coral reefs because they have symbiotic relationships with photosynthetic organisms.

Cnidocytes help a cnidarian survive by paralyzing prey.

Cnidarians are the simplest animals to actually have body symmetry Cnidarians are the simplest animals to actually have body symmetry. Sponges are the simplest animals, but have no symmetry. Nematocysts function in defense and capturing prey. A characteristic of cnidarians is that they are carnivorous animals.

Fossilized Sponge near Miami, FL http://soundwaves.usgs.gov/2007/07/

Sea Anemone