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Name____________________________ Chapter 29 Class __________________ Date __________ Comparing Invertebrates Summary 29–1 Invertebrate Evolution Paleontologists have identified microscopic fossils from 570 to 610 million years ago. They identified trace fossils from the same time period. Trace fossils are tracks and burrows made by softbodied animals. Fossils of some primitive animals were discovered in the Ediacara Hills of Australia. The Ediacaran animals lived 543 to 575 million years ago. They were flat and plate-shaped. They lived on shallow sea bottoms and had soft, segmented bodies and bilateral symmetry. The animals appear to have lacked cell specialization or a front and a back end. The Cambrian Period began 544 million years ago. It is marked by many kinds of fossils. The Burgess Shale of Canada is one of the best-known sites of Cambrian fossils. These animals evolved complex body plans. Because of its great growth in animal diversity, events of the early Cambrian Period are called the Cambrian Explosion. The Burgess Shale animals typically had body symmetry, segmentation, some type of skeleton, a front and a back end, and appendages having many functions. The appearance of each animal phylum in the fossil record shows the evolution of a successful and unique body plan. Modern sponges and cnidarians have little internal specialization. As larger and more complex animals evolved, specialized cells formed tissues, organs, and organ systems. All invertebrates except sponges have some type of body symmetry. • Cnidarians and echinoderms have radial symmetry. Radial symmetry is a body plan in which the body parts repeat around the center of the body. • Worms, mollusks, and arthropods have bilateral symmetry. Bilateral symmetry is a body plan in which only a single, imaginary line can divide the body into two equal halves. A trend toward cephalization occurred with the evolution of bilateral symmetry. Cephalization is the concentration of sense organs and nerve cells in the front of the body. Cephalization lets animals respond to the environment in more sophisticated ways. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 278 Name____________________________ Class __________________ Date __________ Most complex animals are coelomates. They have a true coelom. A coelom is a body cavity lined with tissue derived from mesoderm. • Flatworms are acoelomates—they have no coelom. • Roundworms are pseudocoelomates. Their coelom is only partially lined with mesoderm. • Mollusks, annelids, arthropods, and echinoderms all have a true coelom. The zygote of most invertebrates divides to form a blastula. An opening, called the blastopore, then forms in this blastula. • In protostomes, the blastopore develops into a mouth. Worms, arthropods, and mollusks are protostomes. • In deuterostomes, the blastopore develops into an anus. Echinoderms (and chordates) are deuterostomes. 29–2 Form and Function in Invertebrates Biologists learn a great deal about the nature of life by comparing the body systems of the various living invertebrates. All animals perform the same essential tasks: feeding and digestion, respiration, circulation, excretion, response, movement and support, and reproduction. Feeding The simplest animals—sponges—break down food inside their cells. Mollusks, annelids, arthropods, and echinoderms use extracellular digestion. Food is broken down outside the cells of a digestive tract. Food enters the body through the mouth, and wastes leave through the anus. Respiration All respiratory systems share two features. (1) Respiratory organs have a large surface area that is in contact with air or water. (2) Respiratory surfaces must be moist for diffusion to occur. Aquatic mollusks, arthropods, and many annelids exchange gases through gills. In land animals, surfaces are covered with water or mucus. Such covering prevents water loss from the body. It also moistens air moving through the respiratory system. Circulation All cells need a constant supply of oxygen and nutrients. They also must remove wastes. The smallest and thinnest animals exchange materials with the environment by diffusion. More complex animals use a system of pumps and tubes for transport. There are two types of circulatory systems. • In an open circulatory system, blood is only partly contained within blood vessels. The blood moves through the vessels into a system of sinuses, where the blood comes into direct contact with the tissues. • In a closed circulatory system, blood moves throughout the body in vessels. The blood moves under force from a heart or heartlike organ. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 279 Name____________________________ Class __________________ Date __________ Excretion Multicellular animals must control the amount of water in their tissues. They also must get rid of ammonia, a toxic nitrogen waste formed during metabolism. Most animals have an excretory system to rid the body of metabolic wastes. The excretory system also controls the amount of water in the tissues. Aquatic organisms get rid of ammonia through diffusion. Many land animals convert ammonia into urea. This compound is then eliminated from the body in urine. Response The more complex an animal’s nervous system is, the more developed its sense organs are. Invertebrates show three trends in the evolution of the nervous system: centralization, cephalization, and specialization. Simple animals have nerve cells that are spread through the body, while more complex animals have centralized nerve cells. More complex animals also have more highly specialized sense organs. Support and Movement Invertebrates have one of three main kinds of skeletal systems. • Annelids and certain cnidarians have a hydrostatic skeleton. In this system, muscles surround a fluid-filled body cavity that supports the muscles. The muscles contract and push against the water. • Arthropods have an exoskeleton, or external skeleton. • Echinoderms have an endoskeleton, a structural support located inside the body. Reproduction Some invertebrates may reproduce asexually. However, most reproduce sexually at some part of their life cycle. Asexual reproduction allows animals to reproduce rapidly. Sexual reproduction maintains genetic diversity in a population by creating individuals with new combinations of genes. Most animals have separate sexes that produce eggs or sperm. • In external fertilization, eggs are fertilized outside the female’s body. • In internal fertilization, eggs are fertilized inside the female’s body. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 280