Hydra
(Coelenterata)
The most well-known animals from the Coelenterata family are the marine anemones, coral and jellyfish, and most of the family does make it's home in the oceans of the world. There are however, a number of these animals that live in freshwater. These are much smaller and less colorful than their more popular marine relatives. Most are less than an inch tall.
Hydra have a saclike body and a circle of tentacles at around the body opening. This opening is used for both taking in food and eliminating wastes. They use the tentacles for catching prey. The tentacles have special cells called nematocysts, which can either sting or stick to the prey.
Hydra eat one celled animals, small crustaceans, worms, insects and other tiny animals. They are also reported to eat small fish fry in the aquarium, where they do occasionally show up.
These animals can reproduce asexually by forming buds or sexually. When reproducing sexually they are usually hermaphroditic and lay a single egg.
Moss Animals
(Bryozoa)
This is another group of animals that mostly inhabit marine waters. There are about 3,500 species and only about 50 of them are found in fresh water.
Moss animals live in large groups and resemble a mass of very small marine feather dusters. The have small hair like tentacles that retract when they are disturbed. The tentacles bring foods to the animals. They eat algae and protozoa. These animals also sometimes appear in the home aquarium.
The picture to the right are some moss animals in one of my aquariums. I'll try to get a more clear photo in the future.
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