Phylum Cnidaria: Hydras, Jellyfish, Coral, and Sea Anemone

• Cnidarians have to basic forms (which both have radial symmetry)
• Polyp: cup-shaped, tubular cnidarian with a mouth and tentacles at one end and a sticky basal disk for attachment at the other.

Polyp form

• Medusa: has an expanded bell-shaped body and swims freely.
• It contracts and relaxes its body, gliding through the water in jerky upward motion. 

Medusa form



• Even though, some only exist as either a polyp or medusa, many have both polp and medusa stages in their life cycle.
• Cnidarians are divided into three different classes:
• Hydrozoa (Hydras)
• Sycphozoa (Jellyfish)
• Anthozoa (Cora and Sea Anemone)

Hydras (Class Hydrozoa)

Hydra

• Hydra are a small freshwater cnidarian commonly found in quiet lakes and ponds.
• They are often white, green, or brown. Some hydras are green because they ingest algae which produces sugars which the hydra then uses as food. 
• Structure
• Hydra polyp form is a hollow tube with a single opening (or mouth). All cnidarians only have one opening with functions for both ingestion and excretion. 
• Long, movable tentacles, which the hydra uses to catch food, surround the mouth. 
• Like porifera, the hydra has two layers of cells separated by a jelly-like layer. 
• Epidermis, mesoglea, and gastrodermis (which primarily perform digestion)
• The epidermis and gastrodermis have contractile fibers that contract like muscles, allowing them to bend their body, move their tentacles, and contract into a tiny ball when disturbed.
• They can move by a strange somersaulting motion, but usually remain attached by the sticky basal disk.
• Feeding
• While feeding, the hydra hang limply from its underwater base and allow its tentacles to dangle in the water.
• They have stinging cells called cnidocytes lining their tentacles. They contain poisonous barbs (nematocysts), coiled threads, or sticky substance. 
• When a small invertebrate brush against the tentacles, the hydra releases the nematocysts paralyzing their pray and entangling them with the threads or sticky substance.
• The tentacles draw the prey into their gastrovascular cavity where it is then digested. 

• Excretion and respiration are both done on a cellular level. The waste is excreted directly into the gastrovascular cavity and leaves through the mouth.
• Responses and Reproduction
• Hydra have a nerve net that allows them to dramatically react to stimuli.
• Ex. the tentacle reaction to food or contracting into a tiny ball.
• Hydra use budding and regeneration to reproduce asexually.
• When reproducing sexually, the ovum develops and breaks through the body. Then sperm is released from another hydra and fertilizes the ovum. The hydra is dormant during the winter months and then during the spring, forms a new hydra.

Jellyfish (Class Scyphozoa)

• Free-swimming medusa (dominant form) spurts through the water and feeds upon organisms that bump into its oral arms.
• Sperm released and travels into the gastrovascular cavity of the female, fertilizing the ovum.
• Zygote leaves the cavity through the mouth and clings to the oral arms. When the zygote developes into a larva, it leaves the female and swims away, attaching itself to a base. It becomes a small polyp. After months of storing food and producing new polyps by budding, the body develops into the medusa form.

Corals and Sea Anemones (Class Anthozoa)

• Anthozoa means “flower animals” and includes some of the most beautiful cnidarians.

Coral

• Corals are common in the ocean. The actual coral is a polyp that lives in a self-made skeleton or cup. When not feeding the polyp contracts into the cup for protection. 
• They usually stack themselves on top of each other, making colonies of thousands of polyps. They can stretch for miles.

Sea anemone

• Sea anemones, the “flowers” of the sea, live on the sea floor. They attach to rocks and other submerged objects. 
• They use hydrostatic skeletons for support. It depends on two layers of muscles in the body wall and a fluid-filled interior. As the muscles contract, the water pressure in the anemonoe increase, redistributing the water and causing its body to flex and twist in response. 
• They have tentacles that use nematocysts. Most are not harmful to humans.