| Jellyfish--fish or cnidarian?
These fragile, translucent stingers are mostly mistaken by their name, jellyfish. But, unlike fish, jellyfish are lacking in brains, scales, eyes, or gills. Instead, they have nervous system throughout their bodies that helps them detect light, odor, and other differences in thier surroundings. The body itself is structured in layers. The outer layer is called the epidermis. The inner layer is called the gastrodermis. In between these layers is a jelly-like substance called mesoglea. The whole body of the jellyfish is called the bell. The mouth is located under the bell, along with many tentacles filled with stinging cells. Some tentacles have openings in the end used in digestion. Most jellyfish digest through a digestive cavity, which combines a stomach, gullet, and intestine. The mouth works as an entrance and exit for food/waste.
Jellyfish mate in a simple, mindless way that is similar to the sea anemone. When the reproductive organs have formed in the lining of a jellyfish's gut, the male release sperm out of its mouth. The female then recieves the sperm (via mouth) and fertilization begins inside the female. After development, the tiny swimming larva leave the mother through her mouth in a water column. They find a sheltered area on the ocean floor, where they slowly develop into polyps, and then into adult jellyfish, which are also known as medusae.
The moon jellyfish (pictured above) is one of the only type of jellyfish that can survive in freezing climates.
The Portuguese man-of-war (below) is an example of a colony of hydrozoans. For a better picture of a Portuguese man of war, see Lord Mr.'s Gallery.
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Hydrozoans--the miniscule, little-known cnidarian
Freshwater hydrozoans are made from individual polyps, and never change form as the jellyfish does. One side of their body is a sticky basal disk, and the other is composed of tentacles. They move either by gliding or by tumbling along the ocean floor. Marine hydrozoans are more complex than freshwater ones, and group together to form colonies. An example of one of these colonies is the Portuguese man-of-war, and is composed of up to 1,000 polyps. Atop the colony is a gas-filled float, which is guessed to be a highly modified polyp. Most hydrozoan colonies reproduce asexually (without another of its kind) from tiny buds lining the inside of its body. These buds develop into polyps that later leave the colony. Many hydrozoans are also able to reproduce sexually, but are usually not seen doing so.
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