Skip to main content


Of all the invertebrates on the Earth, the largest group is composed of arthropods. Worldwide, there are about 80,000 species of these organisms. Many of the arthropods of the coral reef are familiar to nearly everyone and include animals such as shrimp, crabs, and lobsters.

Most arthropods are relatively small animals whose bodies are covered with hard, protective coats called exoskeletons. Exoskeletons give the bodies of animals structural support and protect them from predators. The tough skeletons of arthropods are primarily composed of chitin, an extremely hard, but highly flexible, material made of long chains of molecules that are similar in structure to cellulose.

Like the bodies of bristle worms and other segmented marine worms, arthropods’ bodies are divided into sections. An arthropod has a definite head region that is specialized for handling food and gathering information about the environment. Most have compound eyes, which create multiple pictures and arrange them like tiles in a mosaic. Organisms with multiple eyes cannot focus on objects as well as human eyes, but they are very good at detecting motion.

Even though they are enclosed in a suit of formidable armor, arthropods can move about quickly. Their ease of motion is due to their jointed appendages. An appendage is a leg, antenna, or other part that extends from the main body of the animal.

Sexes are separate in arthropods, and mating is usually a seasonal event with elaborate courtship rituals. In many species the male deposits sperm in the female’s body. The sperm are held here until eggs mature, then, as each egg leaves the ovary, sperm are released and fertilization occurs. Resulting zygotes mature into larvae that swim in the plankton for a short period of time before settling down on the reef floor to mature.

Popular posts from this blog

Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the animal species are equipped with a hard, outer covering called an exoskeleton. The functions of exoskeletons are similar to those of other types of skeletal systems. Like the internal skeletons (endoskeletons) of amphibians, reptiles, birds, and mammals, exoskeletons support the tissues and give shape to the bodies of invertebrates. Exoskeletons offer some other advantages. Serving as a suit of armor, they are excellent protection against predators. Also, because they completely cover an animal’s tissues, exoskeletons prevent them from drying out. In addition, exoskeletons serve as points of attachment for muscles, providing animals with more leverage and mechanical advantage than an endoskeleton can offer. That is why a tiny shrimp can cut a fish in half with its claw or lift an object 50 times heavier than its own body.
Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them …

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea.

Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows)…