Reading Essentials for Biology Glencoe (794133), страница 69
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Different animals have different types of body structures. Imagine that you are standing and someone drops astring from the middle of your forehead to the ground. Think about each side of your body. Whatlimbs and facial features appear on both sides of the string? List them below.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnHighlight MainIdeas Highlight the names ofeach type of symmetry listedin this section.
As you readthe text, find an example ofeach type of symmetry andhighlight those examples in adifferent color.1. What is the commoncharacteristic of sessileorganisms?________________________________________________________________________302Chapter 25You have learned that animals share certain characteristics.When you look at a sponge and a leopard, it’s hard to see whatthey have in common.
Shape is not something all animals havein common. All animals do have some kind of shape, however.Each animal can be described in terms of symmetry. Symmetry(SIH muh tree) describes how an animal’s body structures arearranged. Different kinds of symmetry allow animals to move indifferent ways.What is asymmetry?Many sponges have irregularly shaped bodies. An animal thatis irregular in shape has no symmetry. It has an asymmetricalbody plan.
Animals with no symmetry are often sessile organisms,meaning they do not move from place to place. Most adult spongesare sessile organisms.The bodies of most sponges consist of two layers of cells.Unlike all other animals, a sponge’s embryonic development doesnot include the formation of an endoderm and mesoderm, or thegastrula stage.
Fossil sponges first appeared in rocks dating backmore than 700 million years. They represent one of the oldestgroups of animals on Earth. This is evidence that their two-layerbody plan suits their aquatic environment.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is symmetry?NameDateClassSection25.2Body Plans and Adaptations, continuedWhat is radial symmetry?An animal with radial (RAY deeuhl) symmetry can be divided alongany plane, through a central axis, intoalmost equal halves.
Radial symmetryis an adaptation that enables an animal to detect and capture prey coming toward it from any direction.Hydra are examples of animals withradial symmetry.SpongeAsymmetricalSea starRadial symmetryHumanBilateral symmetryCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is bilateral symmetry?An organism with bilateral (bi LA tuh rul) symmetry can bedivided down its length, along one plane, into similar right andleft halves. Butterflies have bilateral symmetry.
In animals withbilateral symmetry, the anterior, or head end, often has sensoryorgans. The posterior of these animals is the tail end. The dorsal(DOR sul), or upper surface, also looks different from the ventral(VEN trul), or lower surface. In animals that are upright, ornearly so, the back is on the dorsal surface and the belly is on theventral surface. Animals with bilateral symmetry can find foodand mates and avoid predators because they have sensory organsand good muscular control.Bilateral Symmetry and Body PlansAnimals that are bilaterally symmetrical also share other important characteristics. All bilaterally symmetrical animals developedfrom three embryonic cell layers—ectoderm, endoderm, andmesoderm.
Some bilaterally symmetrical animals also have fluidfilled spaces inside their bodies called body cavities. The internalorgans are found inside the body cavities. These cavities makeit possible for animals to grow larger because they allow for theefficient circulation and transport of fluids, and support fororgans and the organ systems.2. What type of symmetrydoes a butterfly have?________________________________________________________________________3. What are the names ofthe three embryonic celllayers that are present inbilaterally symmetricalanimals?________________________________________________________________________What are acoelomates?Animals that develop from these same three cell layers but donot have body cavities are called acoelomate (ay SEE lum ate)animals. They have a digestive tract that extends throughoutthe body.
Acoelomate animals may have been the first group ofREADING ESSENTIALSChapter 25303NameDateClassSection25.2Body Plans and Adaptations, continued4. What group of animalsmay have been the firstto develop organs?________________________animals in which organs evolved. Flatworms are bilaterally symmetrical animals with solid, compact bodies. Like other acoelomate animals, the organs of flatworms are surrounded in the solidtissues of their bodies. A flattened body and branched digestivetract allow for the diffusion of nutrients, water, and oxygen tosupply all body cells and to eliminate wastes.________________________What are pseudocoelomates?________________________The roundworm is another animal with bilateral symmetry.Unlike the flatworm, the body of a roundworm has a space thatdevelops between the endoderm and mesoderm.
It is called apseudocoelom (soo duh SEE lum)—a fluid-filled body cavitypartly lined with mesoderm.Pseudocoelomates have a one-way digestive tract that hasregions with specific functions. The mouth takes in food, thebreakdown and absorption of food occurs in the middle section,and the anus expels wastes.A coelom (SEE lum) is a fluid-filled space that is completelysurrounded by mesoderm. The body cavity of an earthwormdevelops from a coelom. Humans, insects, fishes, and many otheranimals have a coelomate body plan. In fact, the greatest diversityof animals is found among the coelomates.In coelomate animals, specialized organs and organ systemsdevelop in the coelom.
The digestive tract and other internalorgans are attached by double layers of mesoderm and are suspended within the coelom. Like the pseudocoelom, the coelomcushions and protects the internal organs. It provides room forthem to grow and move independently within an animal’s body.Animal Protection and SupportOver time, the development of body cavities resulted in agreater diversity of animal species. These different speciesbecame adapted to life in different environments. Some animals,such as mollusks, evolved hard shells that protected their softbodies. Sponges and some other animals evolved hardenedspicules, small, needlelike structures, between their cells thatprovided support.304Chapter 25READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are coelomates?NameDateClassSectionCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.25.2Body Plans and Adaptations, continuedInvertebrateSome animals developed exoskeletons.
Anexoskeleton is a hard covering on the outside of the body. Exoskeletons provide aframework for support, protect soft body tissues, prevent water loss, and provide protection from predators. An exoskeleton issecreted, or formed, by the epidermis andextends into the body where it providesa place for muscle attachment. As an animalgrows, it secretes a new exoskeleton andsheds the old one.Exoskeletons are often found in invertebrates.
Invertebrates are animals thatdo not have a backbone. Crabs, spiders,grasshoppers, dragonflies, and beetlesare examples of invertebrates that haveexoskeletons.Other animals have evolved differentstructures to give them support and protection. Invertebrates, such as sea urchins andsea stars, have an internal skeleton called anendoskeleton, which is covered by layers ofcells. It provides support for an animal’sbody.
The endoskeleton protects internalorgans and provides an internal brace formuscles to pull against. Endoskeletons maybe made of one of the following:VertebrateEndoskeletonSubstanceExampleCalcium carbonateSea starsCartilageSharksBoneReptiles, birds, amphibians,mammals, bony fishesA vertebrate is an animal with an endoskeleton and a backbone. All vertebrates are bilaterally symmetrical. Examples of vertebrates include fishes, amphibians, reptiles, birds, and mammals.READING ESSENTIALS5. What three substancesmay the endoskeleton bemade of?________________________________________________________________________Chapter 25305NameDateClassSection25.2Body Plans and Adaptations, continuedOrigin of Animals6. How many years ago doscientists feel that allmajor animal body planswere in existence?________________________________________________________________________Most biologists agree that animals probably evolved from protists that lived in groups in water.
Scientists trace this evolutionback to late in the Precambrian Period. Although evidence suggests that bilaterally symmetrical animals might have appearedlater, many scientists agree that all the major animal body plansthat exist today were already in existence 543 million years ago—at the beginning of the Cambrian Period. Since then, many newspecies have evolved but all known species have variations of bodyplans developed during the Cambrian Period.◗ After You Readacoelomate (ay SEE lum ate): an animal with nobody cavitiesinvertebrate: animal that does not have abackboneanterior: head end of bilateral animals wheresensory organs are often locatedposterior: tail end of bilaterally symmetricanimalsbilateral (bi LA tuh rul) symmetry: animals witha body plan that can be divided down itslength into two similar right and left halvespseudocoelom (soo duh SEE lum): fluid-filledbody cavity partly lined with mesodermcoelom (SEE lum): fluid-filled body cavity completely surrounded by mesodermdorsal (DOR sul): upper surface of bilaterallysymmetric animalsendoskeleton: internal skeleton; provides support, protects internal organs, and acts as aninternal brace for muscles to pull againstexoskeleton: hard covering on the outside ofsome animals, including spiders and mollusks; provides a framework for support,protects soft body tissues, and provides aplace for muscle attachmentradial (RAY dee uhl) symmetry: an animal’sbody plan that can be divided along anyplane, through a central axis, into roughlyequal halvessymmetry (SIH muh tree): a term that describesthe arrangement of an animal’s bodystructuresventral (VEN trul): lower surface of bilaterallysymmetric animalsvertebrate: an animal with an endoskeleton anda backbone1.
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