Main->Readings->5th Grade Readings->Invertebrates->Part 3
Vocabulary |
If you dig up a handful of soil, you may be holding hundreds of nematodes in your hand. Though scientists have only named 12,000 species of nematodes, there may be many more. This is partly because most nematodes are tiny. Many are so small they can only be seen under a microscope.
They are simple animals, with a long gut extending through their bodies, and yet they are more complex than sponges, stinging-cell animals, and flatworms because they have more than one body opening.
Some, like the ones found in soil, are free-living. They feed on bacteria, fungi, and other soil organisms. Many other roundworms are parasites, including heartworms, pinworms, and the trichina worm.
One of the most common roundworm parasites is the hookworm. It is easily spread in places where the sanitation is not good. The eggs leave the body in the wastes, and if they are left on the ground they hatch into larvae. The larvae crawl to a moist area and extend their bodies up into the air, waiting for a new host. If someone walks barefoot over them, they enter the skin (making it itch), enter the blood, move to the lungs and up the throat to be swallowed back down into the intestines.
The mollusks (Mollusca) are a fascinating group of animals, which has as many as 12,000 different species. They include the intelligent, fast-moving octopus with its excellent eyes, and they also include the soft, nearly brainless slug as well as the tightly closed double-shelled clam.
Even though all these organisms look very different, they have many things in common. All mollusks have a soft body with three main parts: A body mass containing the main organs, a mantle which covers most of the body mass and can make the shell, and a sort of head-foot, with sense organs, which it uses to move.
There are three main types of mollusk. The snail belongs to one group, called the gastropods ("stomach-foot"). This group is one-shelled, with a spiral shell. The slug, which has no shell, belongs to this group because it is closely related to the snail.
A second group is the bivalves, which includes clams, mussels, and oysters. These mollusks have a two-part shell with a hinge. The shell is very thick and strong, and made of calcium carbonate.
The third group is the cephalopods ("head-foot"), which includes the cuttlefish, squid, and octopuses. These mollusks have no shell or else, in the case of the squid, have a sort of shell (a "pen") on the inside. Even though they have the same body plan as other mollusks, they seem much more complicated. They have eight arms coming from their heads, where other mollusks have a simple foot. They have a horny beak for eating, and excellent eyes. Cephalopods can change the color of their skin, and can escape their prey by squirting it with ink and using "jet propulsion" by forcing water out. Finally, the cephalopods have the most advanced brain of any invertebrate.
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Go on to Part 4: Segmented Worms and Spiny-Skinned Animals
This page last modified August 15, 2002
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