Invertebrate Characteristics

Although it may be difficult for us to see many similarities between a clam and an octopus, they are classified in the same phylum. Adult invertebrates show a tremendous amount of morphological diversity.

Symmetry

Most invertebrates display radial or bilateral symmetry. The radial symmetry of a jellyfish, which drifts rather than swims, allows the animal to receive stimuli from all directions. Most invertebrates have bilateral symmetry, which is an adaptation to a more motile lifestyle. Bilateral symmetry allows for cephalization, which is present in varying degrees in different animals. Some bilaterally symmetric invertebrates, such as the sea hare shown in Figure 32-7, are not highly cephalized. Members of Aplysia do not have a true brain and are capable of only basic responses to the environment. Other invertebrates, such as squids and octopuses, are highly cephalized and have a distinct head and a nervous system dominated by a well-organized brain.

Segmentation

Segmentation in animals refers to a body composed of a series of repeating similar units. Segmentation is seen in its simplest form in the earthworm, an annelid in which each unit of the body is very similar to the next one. Within the phylum Arthropoda, however, segments may look different and have different functions. In the arthropod shown in Figure 32-8 on the next page, fusion of the anterior segments has resulted in a large structure that includes the animal's head and chest regions.

objectives

• Compare symmetry, segmentation, and body support in invertebrates and vertebrates.

• Describe the differences in the respiratory and circulatory systems of invertebrates and vertebrates.

• Compare the digestive, excretory, and nervous systems of invertebrates and vertebrates.

• Contrast reproduction and development in invertebrates and vertebrates.

vocabulary segmentation exoskeleton gill open circulatory system closed circulatory system hermaphrodite larva endoskeleton vertebra integument lung kidney figure 32-7

The California sea hare, Aplysia californica, is a shell-less mollusk that has a simple nervous system.

figure 32-7

The California sea hare, Aplysia californica, is a shell-less mollusk that has a simple nervous system.

figure 32-8

In animals such as this crayfish, Procambarus sp., segments are fused, producing larger structures. The head and chest structure in this crayfish results from the fusion of several segments. Segments may also give rise to other structures, such as limbs. The crayfish's exoskeleton is also clearly visible.

figure 32-8

In animals such as this crayfish, Procambarus sp., segments are fused, producing larger structures. The head and chest structure in this crayfish results from the fusion of several segments. Segments may also give rise to other structures, such as limbs. The crayfish's exoskeleton is also clearly visible.

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Support of the Body

Invertebrate bodies have diverse means of support. Sponges have a simple skeleton that supports their soft tissue; the dried, brown, irregularly shaped "natural sponge" found in stores is this skeleton. The bodies of some other invertebrates, such as roundworms, are supported by the pressure of their fluid-filled body cavity.

An exoskeleton is a rigid outer covering that protects the soft tissues of many animals, including arthropods, such as crustaceans, which include crayfish, shown in Figure 32-8. An exoskeleton limits the size and may impede the movement of the organism. Also, an exoskeleton does not grow and must be shed and replaced as the animal grows.

Respiratory and Circulatory Systems

Animals produce carbon dioxide, CO2, as a byproduct of metabolism. Therefore, carbon dioxide in the blood must be exchanged with oxygen, O2, from the environment. This process, called gas exchange, occurs most efficiently across a moist membrane. In the simplest aquatic invertebrates, gas exchange occurs directly across the body covering. Aquatic arthropods and mollusks, however, have gills, organs that consist of blood vessels surrounded by a membrane and are specialized for gas exchange in water.

In most animals, the circulatory system moves blood or a similar fluid through the body to transport oxygen and nutrients to cells. At the same time, carbon dioxide and wastes are transported away from the cells. Sponges and cnidarians have no circulatory system, so nutrients and gases are exchanged directly with the environment by diffusion across cell membranes. Arthropods and some mollusks have an open circulatory system, in which circulatory fluid is pumped by the heart through vessels and into the body cavity and is then returned to the vessels. Annelids and other mollusks have a closed circulatory system. In a closed circulatory system, blood is pumped by a heart and circulates through the body in vessels that form a closed loop. The exchange of gases, nutrients, and wastes occurs between body cells and very small blood vessels that lie near each cell.

Digestive and Excretory Systems

In sponges, digestion occurs within individual cells. In cnidari-ans, a central chamber with one opening serves as the digestive system. Most other invertebrates, however, have a digestive tract, or gut, running through their body. In these animals, food is broken down in the gut, and the nutrients are absorbed by specialized cells that line the gut.

In simple aquatic invertebrates, wastes are excreted as dissolved ammonia, NH3. In terrestrial invertebrates, specialized excretory structures filter ammonia and other wastes from the body cavity. The ammonia is then converted to less toxic substances, and water is reabsorbed by the animal before the waste is excreted.

Nervous System

The extraordinary degree of diversity among invertebrates is reflected in their nervous systems. Sponges have no neurons, although individual cells can react to environmental stimuli in much the same way that protozoa can. Neurons evolved in cnidari-ans, which have a very simple, loosely connected nervous system. Within a single invertebrate phylum, Mollusca, we can trace a step-wise progression of cephalization and the evolution of the brain.

The mollusks have very diverse nervous systems. Recall the sea hare, shown in Figure 32-7. Although its head is not well defined and its nervous system can perform only simple information processing, the sea hare can learn to contract a part of its body in response to certain stimuli. Contrast this simple behavior with that of a highly cephalized mollusk, such as the octopus. The octopus shows very complex decision-making behavior, and it can build a shelter from debris it finds on the ocean floor.

Reproduction and Development

Invertebrates are capable of some form of sexual reproduction, and many can also reproduce asexually. Some invertebrates, such as earthworms, are hermaphrodites. A hermaphrodite (huhr-MAF-roh-DlET) is an organism that produces both male and female gametes, allowing a single individual to function as both a male and a female.

Invertebrates may undergo indirect or direct development. Animals that undergo indirect development have an intermediate larval stage, as is shown in Figure 32-9. A larva (plural, larvae) is a free-living, immature form of an organism that is morphologically different from the adult. Larvae often exploit different habitats and food sources than adult organisms do. As a result, organisms in each stage are more likely to survive. Many insects, which constitute a class of arthropods, have indirect development.

In contrast, in direct development, the young animal is born or hatched with the same appearance and way of life it will have as an adult; no larval stage occurs. Most invertebrates undergo indirect development. A few, such as grasshoppers, undergo direct development.

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