Animals Without Backbones: An Introduction to the Invertebrates / Edition 3|Paperback (2024)

Ralph Buchsbaum was professor emeritus of biology at the University of Pittsburgh. Mildred Buchsbaum has collaborated on previous editions of Animals Without Backbones. John Pearse, a professor of biology at the University of California, Santa Cruz, and Vicki Pearse, a research associate in biology at the University of California, Santa Cruz, are coeditors with A. C. Giese of the multivolume Reproduction of Marine Invertebrates and have published many papers in invertebrate zoology.

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Animals Without Backbones

By Ralph Buchsbaum, Mildred Buchsbaum, John Pearse, Vicki Pearse

The University of Chicago Press

Copyright © 1987 The University of Chicago
All rights reserved.
ISBN: 978-0-226-07874-8

CHAPTER 1

Introduction: Sorting Out Living Things

Anyone can tell the difference between a tree and a cow. The tree stands still and shows no signs of perceiving your presence or your hand upon its trunk. The cow moves about and appears to notice your approach. This striking difference in the behavior of plants and animals is related to the fundamental difference in plant and animal nutrition.

Plants make their own food from simple constituents in air and soil, or in water. By means of a green pigment, chlorophyll, the leaves of a tree capture solar energy and use it to combine carbon dioxide and water into sugar—a process known as photosynthesis ("putting things together with the aid of light"). The energy stored in the sugar can later be released and used by the tree to combine simple substances into the complex organic substances of which all living things are made.

Animals cannot stand in the sun, soak up energy, and store it in chemicals such as sugar. A cow must get its energy by eating plants. To find a constant supply of energy-giving food, a cow must be able to move from place to place and must react to other animals and to changes in the environment. A pasture offers few threats to cows, except those posed by humans, but any small animals such as rabbits or mice must be alert to bigger animals such as dogs or coyotes that might view them as a tasty source of energy.

Not all animals move about. Corals, for example, grow firmly attached to the sea bottom and depend on water currents to bring a steady supply of small animals within reach of their tentacles. The feeding activities of corals and many other stationary animals were not apparent to early naturalists, who classified them as plants. And in some ways stationary or sedentary animals do resemble plants. Instead of fleeing from predators, such animals protect themselves by developing hard, often spiny skeletons and distasteful or toxic substances in their tissues. Comparable defenses in plants may be tough bark, thorny stems, or tissue toxins.

There are many strategies for obtaining energy, and some stationary organisms neither capture solar energy through photosynthesis nor directly ingest the bodies of other organisms. Instead they absorb energy-rich organic substances. Mushrooms, for example, must live on or near the decaying organic matter that provides their nourishment. Mushrooms and other fungi thus differ sharply from both plants and animals in their mode of nutrition.

Plants and other organisms that supply themselves from simple, inorganic chemicals are called autotrophs ("self-nourished) or producers (because they produce food). Animals and fungi are called heterotrophs ("nourished by others") or consumers (because they take in food). Animals and fungi are further distinguished from each other by the mode in which they consume their food: animals are typically ingestors while fungi are absorbers.

As we examine more and more kinds of life, distinctions of behavior and nutrition grow less and less obvious. Eventually we find microscopic forms that exhibit characteristics possessed by plants, fungi, and animals. Most of these organisms are single cells, or unicells, with bodies that are not divided up by cell membranes or walls. Some unicells carry on photosynthesis like plants, but they move about and show the same sensitivity and rapidity of response as do typical animals. Some feed like animals, actively ingesting other small unicells, but also photosynthesize. And as extra insurance against hard times, most unicells can also absorb energy-rich organic matter as fungi do.

Thus, the characteristics of nutrition and behavior commonly used to distinguish between the familiar many-celled organisms, the multicellular plants and animals, are of little use for sorting out the unicellular organisms. Rather, unicells are characterized in part by their internal cellular organization. Many are organized like the cells of multicellular organisms: each unit has distinctive little structures called organelles, one of them a nucleus (pleural: nuclei), a discrete body containing most of the hereditary material (genes) of the cell. Whether unicellular or multicellular, organisms in which each cellular unit contains one or more nuclei, and other organelles, are said to be eukaryotic ("with a true nucleus"). In contrast, many other unicells, including the bacteria and cyanobacteria ("blue-green algas"), contain few or no organelles and the hereditary material is dispersed in the cell; these are said to be prokaryotic ("before a nucleus").

Mostly on the basis of cellular organization and also, for multi-cellular forms, mode of nutrition, organisms can be divided into five kingdoms, as shown in the table below. One kingdom, the Monera, includes all prokaryotic organisms, mainly bacteria, which may be autotrophs or may absorb nutrients from other organisms, and photosynthetic cyanobacteria, which are important producers, especially in stagnant water. The second kingdom, the Protista, includes eukaryotic organisms such as protozoans that are unicellular or form only simple colonies of cells. The members of the remaining three kingdoms are all eukaryotic multicellular organisms that are distinguished from each other mainly on the basis of nutrition: Plantae are primarily photosynthetic autotrophs, Fungi are absorbing heterotrophs, and Animalia are predominantly ingestive heterotrophs.

One of the most plausible hypotheses advanced to account for the origin of life states that early in the earth's history, nearly 4 billion years ago, organic compounds formed and accumulated in shallow pools of water. Energy from the sun, lightning, and volcanic activity produced changes in these substances that made them combine into increasingly complex compounds. Some eventually developed the capability of self-propagation, using the accumulated organic substances around them as energy sources and building blocks.

By analogy, viruses today are complex organic compounds (nucleic acids) that occur inside cells of organisms and self-propagate using the organic compounds within the cells as energy sources and building blocks. Sometimes this activity causes severe diseases in the host organisms, including smallpox, herpes, mumps, polio, and AIDS in humans. Before leaving a cell, the viral nucleic acid is enclosed with a protein coat that protects it and enables it to identify and contact more host cells. Because they cannot propagate outside their host cells, viruses are commonly thought to be nuclear remnants and fragments of more complex cells; however, it is possible that some are relicts of the first self-propagating particles in the earliest seas. They appear to be on the borderline between the nonliving and living, and sometimes they are placed in a separate kingdom, the Archetista.

If we imagine that the earliest self-propagating substances were something like viruses that depended on organic material, it is not difficult to suppose that further development of structures such as cell membranes and complex metabolic pathways could lead to larger, more complex organisms at the moneran level, some of which could obtain energy wholly or partly from inorganic substances. Some bacteria today, for example, which live only in habitats that lack oxygen as did the primordial surface of the earth, obtain energy by combining hydrogen and carbon dioxide to form methane, or capture the energy of the sun using pigments similar to chlorophyll but not producing oxygen. Cyanobacteria, which have a fossil record in excess of 3 billion years, use chlorophyll to capture the sun's energy and produce sugars from carbon dioxide and water, leaving oxygen as a byproduct. Oxygen production paved the way for bacteria that can obtain energy by oxidizing inorganic compounds of nitrogen, sulfur, and iron—and for the evolution of more complex forms that depend on oxygen and organic food.

The development of larger, more highly organized eukaryotic protists from prokaryotic monerans is a relatively short jump in complexity, and although no one knows how it happened, or how many times it happened, fossils of protistlike cells some 2 billion years old are known. By forming complex colonies of cells, various types of protists could have given rise to various kinds of multicellular organisms, including plants, fungi, and animals; the oldest fossils of multicellular organisms are about 0.7 billion years old.

When animals are carefully studied and compared, it is found that many of them have a row of bones (vertebras) along the middle of the back, as well as bones inside the limbs and head. Animals having such internal bones, including a vertebral column, or backbone, are known as vertebrates and comprise all the fishes, the frogs, toads, and salamanders, the turtles, lizards, snakes, crocodiles, and birds, and the hairy animals known as mammals, such as elephants, lions, dogs, whales, bats, and mice. These more or less familiar animals have a highly exaggerated importance in our minds because they are mostly of large size, because they are similar to us in structure and habits, and because, like us, they often manage to make themselves conspicuous. We are members of this group and share with the others a common body plan; most of the organs and structures in the various kinds of vertebrates, including ourselves, are similar in form and function. Actually, the vertebrate body plan is only one of more than 30 in the animal kingdom. And, in terms of the number of living species, vertebrates comprise only about 3% of the animal kingdom.

The remaining 97% consists of animals without backbones. We are all aware of the difference between these two groups of animals when we indulge in fish and lobster dinners. In the fish, the exterior is relatively soft and inviting, but the interior presents numerous hard bones. In the lobster, on the contrary, the exterior consists of a formidable hard covering, but within this covering is a soft edible interior. A similar situation exists in the oyster, lying soft and defenseless within its hard outer shell. Lobsters and oysters are but samples of the tremendous array of animals that lack internal bones and that are, from their lack of the vertebral column in particular, called invertebrates.

A distinction between vertebrates and invertebrates was first recognized by Aristotle, although he did not use these terms but divided animals into those with blood (vertebrates) and those without blood (invertebrates). Unfortunately, Aristotle's neat distinction had little to do with the facts, because many invertebrates possess red blood and most of the others have colorless blood, which he did not recognize as blood at all. Aristotle did remarkably well from the scant knowledge of his time, and the limited time he had for the wealth of observations that he made himself. But the authority of his writings became so entrenched that people stopped looking for themselves, and his error was perpetuated for over two thousand years. With the rebirth of scientific inquiry in eighteenth-century Europe, people began to be skeptical of authority and they set out to investigate nature directly. By the beginning of the nineteenth century, Jean-Baptiste Lamarck and Georges Cuvier of France finally recognized a more accurate distinction between vertebrates and invertebrates, based on fundamental body plans, and Lamarck published his treatise entitled Histoire naturelle des animaux sans vertèbres. As scientific inquiry continued, the enormous variety of animal body plans became more fully appreciated, until today it is recognized that the vertebrates comprise only part of a group sharing a body plan that is only one of many.

There is a popular but vague recognition of the difference between vertebrate and invertebrate animals in the expression "spineless as a jellyfish." In this book we shall be concerned not only with jellyfishes, which are seldom seen by most people, but also with many familiar animals without backbones, such as clams, earthworms, lobsters, and fleas. However, there are many other invertebrate animals that generally pass unnoticed because they are too small to be seen without a microscope, because they live in water or in the ground, because they inhabit remote parts of the world, or simply because they escape the unobservant eye. We wish to introduce these animals as well.

Classified Knowledge

Although most vertebrates can be conveniently distinguished by the presence of a backbone, this is only one of many things they have in common. Having determined that an animal is a vertebrate, a zoologist can, without further examination, predict that it has both striated and smooth muscles, a heart and a circulatory system with closed blood vessels, an anterior mouth, a posterior anus, and a digestive tract that includes a large liver; that its eyes and nervous system follow a certain general pattern; that its excretory organs are kidneys; and many other characteristics, including even details of the way in which it developed from its egg.

On the other hand, identifying an animal as an invertebrate, by the absence of a backbone, tells us only that it lacks a few uniquely vertebrate characters and gives us no way to predict what characteristics it has. Among the various kinds of invertebrates are dozens of body plans, each distinct from all the others.

Biologists are continually challenged to sort out our facts about both vertebrates and invertebrates in such a way as to increase our powers of prediction. This useful ability to generalize and predict with accuracy, even about animals not fully examined, depends upon a system for organizing a vast amount of knowledge. Miscellaneous facts about animal structure, physiology, biochemistry, and behavior are almost entirely useless. They lead us to useful generalizations only when we can relate these facts to each other within the context of a system of classification. Such a system would gain us nothing, and could even mislead us, if its categories were based on superficial similarities. For example, if all blue animals were classified together, they would have virtually nothing else in common as a group. Therefore, the fundamental ground rule of the system currently used by biologists is that it should reflect, as closely as possible, how different animals are related to each other. And this is judged by the number of basic similarities they share. The system is a hierarchy. At the top are a relatively few broad categories whose members share a small number of basic similarities. Each of these categories is divided and redivided into narrower categories whose members are increasingly more closely related and more similar.

The highest level in biological classification, the kingdom, is not based on relationship but, as discussed earlier, on how cells are organized and how energy is obtained. It is likely that many groups of organisms that are included within each kingdom have independent origins and that there is no single ancestral founder of any kingdom.

The highest level of classification within the animal kingdom is the phylum (plural: phyla). Animals that share a common body plan, and whose body plan is fundamentally different from those of other animals, are grouped in the same phylum. The members of a phylum may live in every kind of habitat, may vary in size and body form, and may differ in their methods of locomotion and feeding—but the distinctiveness of their body plan shows that they are all related and have descended from some common ancestor. Most of the 34 or so phyla of animals living today are well recognized and agreed upon by zoologists, although debate continues over the status of some groups. Any system of classification contains a certain amount of ambiguity, and, depending upon differences in criteria of what makes a body plan "fundamentally different" from all others, some people tend to "lump" similar groups into a single phylum, while other people "split" the same groups into separate phyla. This problem applies to groups at lower levels of classification as well. It does not mean, however, that the groups are arbitrary, artificial, or simply a human invention; rather, the problem reflects the real complexities of animal evolution.

(Continues...)

Excerpted from Animals Without Backbones by Ralph Buchsbaum, Mildred Buchsbaum, John Pearse, Vicki Pearse. Copyright © 1987 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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Table of Contents

Preface
1. Introduction: Sorting Out Living Things
2. Life Activities
3. Simple and Complex Protozoans
4. Variations on a One-Celled Theme
5. A Side Issue: Sponges
6. Two Layers of Cells
7. Stinging-Celled Animals
8. Comb Jellies
9. Three Layers of Cells
10. New Parts from Old
11. Messmates and Parasites
12. One-Way Traffic: Proboscis Worms
13. Roundworms
14. Lesser Lights
15. Soft-Bodied Animals
16. Segmented Worms
17. Lobsters and Other Arthropods: Crustaceans
18. Arthropods on Land: Arachnids
19. Airborne Anthropods: Insects
20. Annelid-Arthropod Allies
21. Spiny-Skinned Animals
22. Chordate Beginnings
23. Records of the Invertebrate Past
24. Invertebrate Relationships
25. Further Knowledge
Selected Bibliography
Appendix: Classification
Index