Main->Readings->5th Grade Readings->Animals Without Backbones->Part 2
Vocabulary |
Sponges were probably the earliest type of animal. They are almost not animals at all. Sponge bodies are hollow tubes. Their cells are found in simple layers, as if a few types of single-celled organism had gotten together in a group. The cells of a sponge still act like single-celled organisms, because if a sponge is pulled apart into individual cells, it will re-form.
Sponges live in the water and feed by filtering water through pores, or openings, in the surface. The pores trap small bits of food and the water is sent back out. A newly born sponge larva can move around, but adult sponges stay in one place all their lives. (A larva is the newly hatched young of any animal in which the young are very different in appearance from the adults.) They can be many shapes and sizes, and there are about 5,000 species of sponge.
Sponges, even if they do not have a nervous system, can be beautiful. Their skeleton can be made of soft, rubbery spongin, or it can be made of glass or calcium carbonate. The skeletons of the rubbery type used to be used for bathing, though the sponges we use today are usually made artificially.
If you have ever seen a jellyfish in the ocean or washed up on the beach, you have seen a cnidarian, or stinging-cell animal. Stinging cell animals include jellyfish, but they also include such creatures as corals and sea anemones, as well as hydras and the Portuguese Man-of-War. There are about 10,000 species of cnidarians.
Like sponges, stinging-cell animals live in the water and have a hollow sac-like body, but they are more complicated than sponges. They have nerves and muscles, for instance, and they are more organized Yet they have no head, no brain, and no special organs for breathing.
There are two facts that stand out about stinging cell animals. First, they all have stinging cells which contain a long barbed thread, sometimes poisoned. These threads are used to sting prey and bring it to the jellyfish. Second, stinging-cell animals have an unusual life cycle which alternates between the medusa (floating) and polyp (fixed) stage. An example of a medusa is the familiar jellyfish. When a jellyfish produces eggs, the eggs hatch into polyps which attach themselves to the sea floor. These polyps spend their lives on the sea floor, making more medusas. The mother of a polyp is a medusa, and the mother of a medusa is a polyp.
The deadly Portuguese man-of-war is a stinging-cell animal, but it is not a jellyfish. It belongs to the same group as the hydra, and is a colony of small organisms like the hydra. A colony is a group of the same kind of organisms living and growing together.
Flatworms, like stinging-cell animals and sponges, have a hollow sac-like body with one opening. The body of a flatworm tends to be long and flattened, however. There are three types of flatworms, two of which are mostly parasites and one which is free-living.
The free-living type includes the planarian. The planarian is a freshwater flatworm which feeds on other small organisms. It has an arrow-shaped head and two small light-sensitive eye-patches which make it look cross-eyed. It is not uncommon to see tiny planaria in ponds and streams, and they are fun to look at under microscopes. If a planarian is cut in half, each half will grow a new planarian.
The other two types are much less pleasant. They are the flukes and the tapeworms. Both types of flatworm are parasites: they live in and feed on other animals.
The tapeworm is a peculiar animal, not only in the way it is built but in its life cycle. It has a small head with hooks which allow it to attach itself to its host, and a long string of segments for a body. It has no real digestive system. It does not need one, because it lives in the host's intestine and eats the host's digested food.
Each segment in a tapeworm's body is a bag of tapeworm eggs. One by one, each segment drops off the end of the worm and passes out of the host. If the eggs are eaten by pigs or cattle, the eggs move into the muscles and hatch. If someone eats the meat of infected pigs or cattle without cooking it properly, the larvae move into the digestive system and start the life cycle over again.
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Go on to Lesson 3: Roundworms and Mollusks
This page last modified November 2, 2002