chapter 26: sponges, cnidarians, and unsegmented worms section 1: introduction to the animal kingdom

Chapter 26: Sponges, Cnidarians, and Unsegmented Worms

Section 1: Introduction to the Animal Kingdom

Introduction to the Animal Kingdom

What Is an Animal?

All animals share certain basic characteristicsAnimals are heterotrophs (they do NOT make their own food)Instead, they obtain the nutrients and energy they need by feeding on organic compounds that have been made by other organisms

What Is an Animal?

Animals are multicellular, which means that their bodies are composed of more than one cellAnimal cells are also eukaryotic – they contain a nucleus and membrane-enclosed organellesAn animal is a multicellular eukaryotic heterotroph whose cells lack cell walls

Cell Specialization and Division of Labor

The bodies of animals contain many types of specialized cellsEach specialized cell has a shape, physical structure, and chemical composition that make it uniquely suited to perform a particular function within a multicellular organismFor this reason, groups of specialized cells carry out different tasks for the organism – division of labor

What Animals Must Do to Survive

In order to survive, animals must be able to perform a number of essential functionsFor each animal group we study in the next several chapters, we will examine these functions and describe the cells, tissues, organs, and organ systems that perform them

Feeding

Animals have evolved a variety of ways to feedHerbivores eat plantsCarnivores eat animalsParasites live and feed either inside or attached to outer surfaces of other organisms, causing harm to the hostFilter feeders strain tiny floating plants and animals from the water around themDetritus feeders feed on tiny bits of decaying plants and animals

Respiration

Living cells consume oxygen and give off carbon dioxide in the process of cellular respirationEntire animals must respire, or breathe, in order to take in and give off these gasesSmall animals that live in water or in moist soil may respire through their skin

Respiration

For large active animals, however, respiration through the skin is not efficientThe respiratory systems these animals have evolved take many different forms in adaptations suited to different habitats

Internal Transport

Some aquatic animals can function without an internal transport systemBut once an animal reaches a certain size, it must somehow carry oxygen, nutrients, and waste products to and from cells deep within its bodyMany multicellular animals have evolved a circulatory system in which a pumping organ called a heart forces a fluid called blood through a series of blood vessels

Excretion

Cellular metabolism produces chemical wastes such as ammonia that are harmful and must be eliminatedSmall aquatic animals depend on diffusion to carry wastes from their tissues into the surrounding waterBut larger animals, both in water and on land, must work to remove poisonous metabolic wastes

Response

Animals must keep watch on their surroundings to find food, spot predators, and identify others of their own kindTo do this, animals use specialized cells called nerve cells, which hook up together to form a nervous system

Response

Sense organs, such as eyes and ears, gather information from the environment by responding to light, sound, temperature, and other stimuliThe brain, which is the nervous system’s control center, processes the information and regulates how the animal respondsThe complexity of the nervous system varies greatly in animals

Movement

Some animals are sessile, which means that they live their entire adult lives attached to one spotBut many animals are motile, which means that they move aroundTo move, most animals use tissues called muscles that generate force by contractingIn the most successful groups of animals, muscles work together with a skeleton, or the system of solid support in the body

Movement

Insects and their relatives wear their skeletons on the outside of their bodiesexoskeletonsReptiles, birds, and mammals have their skeletons inside their bodiesendoskeletonsWe call the combination of an animal’s muscles and skeleton its musculo – skeletal system

Reproduction

Animals must reproduce or their species will not surviveSome animals switch back and forth between asexual and sexual reproductionMany animals that reproduce sexually bear their young alive

Reproduction

Others lay eggsThe eggs of some species hatch into baby animals that look just like miniature adultsThese baby animals increase in size but do not change their overall formDirect development

Reproduction

In other species, eggs hatch into larvae, which are immature stages that look and act nothing like the adultsAs larvae grow, they undergo a process called metamorphosis in which they change shape dramaticallyIndirect development

Trends in Animal Evolution

The levels of organization become higher as animals become more complex in formThe essential functions of less complex animals are carried out on the cell or tissue level of organizationAs you move on to more complex animals, you will observe a steady increase in the number of specialized tissuesYou will also see those tissues joining together to form more and more specialized organs and organ systems


Some of the simplest animals have radial symmetry; most complex animals have bilateral symmetrySome of the simplest animals have body parts that repeat around an imaginary line drawn through the center of their bodyRadial symmetryAnimals with radial symmetry never have any kind of real “head”Many of them are sessile, although some drift or move in a random pattern


Most complex invertebrates and all vertebrates have body parts that repeat on either side of an imaginary line drawn down the middle of their bodyOne side of the body is a mirror image of the otherThese animals are said to have bilateral symmetry


Animals with bilateral symmetry have specialized front and back ends as well as upper and lower sidesAnterior = front endPosterior = back endDorsal = upper sideVentral = lower side


More complex animals tend to have a concentration of sense organs and nerve cells in their anterior (head) endThis gathering of sense organs and nerve cells into the head region is called cephalizationNerve cells in the head gather into clusters that process the information gathered by the nervous system and control responses to stimuli


Small clusters of nerve cells are called gangliaIn the most complex animals, large numbers of nerve cells gather together to form larger structures called brains


Section 2: Sponges

Sponges

Sponges are among the most ancient of all animals that are alive todayMost sponges live in the sea, although a few live in freshwater lakes and streamsSponges inhabit almost all areas of the sea – from the polar regions to the tropics and from the low-tide line down into water several hundred meters deep

Sponges

Sponges belong to the phylum PoriferaLiterally means pore-bearersTiny openings all over their bodySponges were once thought to be plantsSponges are sessile and show little detectable movementSponges are heterotrophic, have no cell walls, and contain several specialized cell types that live together

Sponges

Sponges are very different from other animalsSponges have nothing that even vaguely resembles a mouth or gut, and they have no specialized tissues or organ systemsMost biologists believe that sponges evolved from single-celled ancestors separately from other multicellular animalsThe evolutionary line that gave rise to sponges was a dead end that produced no other groups of animals

Form and Function in Sponges

Very simple body planThe body of a sponge forms a wall around a central cavityIn this wall are thousands of poresA steady current of water moves through these pores into the central cavityThis current is powered by the flagella of cells called collar cells


The water that gathers in the central cavity exits through a large hole called the osculumThe current of water that flows through the body of a sponge delivers food and oxygen to the cells and carries away cellular waste productsThe water also transports gametes or larvae out of the sponge’s body


Many sponges manufacture thin, spiny spicules that form the skeleton of the spongeA special kind of cell called an amebocyte builds the spicules from either calcium carbonate or silicaThese spicules interlock to form beautiful and delicate skeletonsThe softer but stronger sponge skeletons that we know as natural bath sponges consist of fibers of a protein called spongin


Sponges are filter feeders that sift microscopic particles of food from the water that passes through themAll digestion in sponges is intracellular; it takes place inside cellsThe water flowing through a sponge simultaneously serves as its respiratory, excretory, and internal transport systemAs water passes through the body wall, sponge cells remove oxygen from it and give off carbon dioxide to it


The water that flows through the body of a sponge also plays a role in sexual reproductionAlthough eggs are kept inside the body wall of a sponge, sperm are released into the water flowing through the sponge and are thus carried out into the open waterIf those sperm are taken in by another sponge, they are picked up by amebocytes and carried to that sponge’s eggs, where fertilization occurs


The zygote that results develops into a larva that swims and can be carried by currents for a long distance before it settles down and grows into a new spongeSponges can also reproduce asexuallyFaced with cold winters, some freshwater sponges produce structures called gemmulesSphere-shaped collections of amebocytes surrounded by a tough layer of spiculesCan survive long periods of freezing temperatures and droughtWhen conditions become favorable, gemmules grow into new sponges


Sponges can also reproduce asexually by buddingIn this process, part of a sponge simply falls off the parent and grows into a new spongeRemarkable powers of regeneration

How Sponges Fit into the World

Sponges provide housing for many other marine animalsSponges are also involved in symbiotic relationships with other organismsHumans have used the dried and cleaned bodies of some sponges in bathingSome chemicals that sponges secrete are being used as powerful antibiotics that are used to treat bacteria and fungi


Section 3: Cnidarians

Cnidarians

The phylum Cnidaria includes many animals with brilliant colors and unusual shapesJellyfish, sea anemones, etc.These beautiful and fascinating animals are found all over the world, but most species live only in the sea

What is a Cnidarian?

Cnidarians are soft-bodied animals with stinging tentacles arranged in circles around their mouthSome cnidarians live as single individualsOthers live as groups of dozens or even thousands of individuals connected into a colonyAll cnidarians exhibit radial symmetry and have specialized cells and tissuesMany cnidarians have life cycles that include two different-looking stages, the sessile flowerlike polyp and the motile bell-shaped medusa

Some cnidarians, such as sea nettles and sea anemones, are solitary. Others, such as gorgonian coral polyps, are colonial.

Both polyps and medusa have a body wall that surrounds an internal space called the gastrovascular cavityThis is where digestion takes placeThe body wall consists of three layers: EpidermisLayer of cells that covers the outer surface of the cnidarian’s bodyMesogleaLocated between the epidermis and the gastrodermGastrodermLayer of cells that covers the inner surface, lining the gastrovascular cavity

Form and Function in Cnidarians

Almost all cnidarians capture and eat small animals by using stinging structures called nematocysts, which are located on their tentaclesPoison-filled sac containing a tightly coiled spring loaded dartWhen an animal touches a nematocyst, the dart uncoils and buries itself into the skin of the animalParalyzes or kills the prey


From here, the cnidarian’s tentacles push the food through the mouth and into the gastrovascular cavityThere the food is gradually broken up into tiny piecesThese food fragments are taken up by special cells in the gastroderm that digests them furtherThe nutrients are then transported throughout the body by diffusionAny materials that cannot be digested are passed back out through the mouth, which is the only opening in the gastrovascular cavity


Because most cnidarians are only a few cell layers thick, they have not had to evolve many complicated body systems in order to surviveThere is no organized internal transport network or excretory system in cnidariansCnidarians also lack a central nervous system and anything that could be called a brainThey have simple nervous systems called nerve netsConcentrated around the mouth


Cnidarians lack muscle cells that most other animals use to move aboutMany of the epidermal cells in cnidarians can change shape when stimulated by the nervous systemCnidarian polyps can expand, shrink, and move their tentacles by relaxing or contracting these epidermal cells


Most cnidarians can reproduce both sexually and asexuallyPolyps can produce new polyps asexually by buddingWhen medusae mature, they reproduce sexually by releasing gametes into the waterFertilization occurs either in open water or inside an egg-carrying medusaThe zygote grows into a ciliated larva that swims around for some timeLater, the larva settles down, attaches to a hard surface, and changes into a polyp that begins the cycle again

Hydras and Their Relatives

Class Hydrozoa is made up of cnidarians that spend most of their lives as polyps, although they usually have a short medusa stageMost hydrozoan polyps grow in branching sessile coloniesRange in length from a few centimeters to more than a meterSpecialized polyps perform particular functionsFeedingReproductionDefense


Most common are the hydrasHydras can reproduce either asexually by budding or sexually by producing eggs and sperm in their body wallsIn most species of hydras, the sexes are separateHowever, a few species are hermaphroditesAn individual that has both male and female reproductive organs and produces both sperm and eggs


One unusual hydrozoan in the Portuguese man-of-warForm floating colonies that contain several polypsOne polyp forms a balloon-like float that keeps the colony on the surfaceSome of the polyps produce long stinging tentacles that paralyze and capture preySome polyps digest the food held by tentacles

Jellyfish

Class ScyphozoaGo through the same life-cycle stages as hydrozoansSome jellyfish, such as the lion’s mane, often grow up to 2 meters in diameterThe largest jellyfish ever found was more than 3.6 meters in diameter and had tentacles more than 30 meters longThe nematocysts of most jellyfish are harmless to humans, but a few can cause painful stingsOne tiny Australian jellyfish has a toxin powerful enough to cause death in 3 – to 20 minutes

Sea Anemones and Corals

Class AnthozoaMost beautiful and ecologically important invertebratesHave only the polyp stage in their life cycleAdult polyps reproduce sexually by producing eggs and sperm that are released into the waterThe zygote grows into a ciliated larva that settles to the ocean bottom and becomes a new polypMany anthozoans also reproduce asexually by budding


Sea anemones are solitary polyps that live in the sea from the low-tide line to great depthsAlthough they can catch food with the nematocysts on their tentacles, many shallow-water species depend heavily on their photosynthetic symbiontsSome sea anemones can grow up to a meter in diameter


Corals grow in shallow tropical water around the worldCorals produce skeletons of calcium carbonate or limestoneMost corals are colonialAs a coral colony grows, new polyps are produced by budding


Coral colonies grow very slowly, but they may live for hundreds, or even thousands, of yearsTogether, countless coral colonies produce huge structures called coral reefsSome of these reefs are enormous and contain more rock and living tissue than even the largest human citiesThe Great Barrier Reef off the coast of Australia is more than 2000 km long and some 80 km wide

How Cnidarians Fit into the World

Certain fish, shrimp, and other small animals live among the tentacles of large sea anemonesCorals and the reefs provide shelter for thousands of species of marine lifeReefs protect the land from erosionJewelry and decorationsMedical research


Section 4: Unsegmented Worms

Unsegmented Worms

Unsegmented worms have bodies that are not divided into special segmentsPhylum PlatyhelminthesConsists of simple animals called flatwormsPhylum NematodaConsists of long, thin worms called roundworms

Flatworms

The members of the phylum Platyhelminthes are the simplest animals with bilateral symmetryMost members of this phylum exhibit enough cephalization, or development of the anterior end, to have what we call a headMany flatworms are no more than a few millimeters thick, although they may be up to 20 meters longFlatworms have more developed organ systems than either sponges or cnidarians

Form and Function in Flatworms

Flatworms feed in either of two very different waysWorms may be carnivores that feed on tiny aquatic animalsFree-living flatworms have a gastrovascular cavity with one opening at the end of a muscular tube called a pharynxThey use the pharynx to suck food into the gastrovascular cavityThe gastrovascular cavity forms an intestine with many branches along the entire length of the worm


In the intestines, enzymes help break down the food into small particlesThese particles are taken inside the cells of the intestinal wall, where digestion is completedLike Cnidarians, flatworms expel undigested material through the mouth


Many other flatworms are parasites that feed on blood, tissue fluids, or pieces of cells inside the body of their hostIn many parasitic flatworms, the digestive tract is simpler than in free-living formsTapeworms, which live within the intestines of their host, do not have any digestive tract at allThey have hooks and/or suckers with which they latch onto the intestinal wall of the host


From this position, they can simply absorb the food that passes by – food that has already been broken down by the host’s digestive enzymesFlatworms lack any kind of specialized circulatory or respiratory systemFreshwater flatworms such as planarians have structures called flame cells that help them get rid of extra water


Free-living flatworms have nervous systems that are much more developed than those of cnidarians and spongesThey have a definite head in which a simple brain is locatedOne or more long nerve cords run from the brain down the length of the body on either sideMany flatworms have one or more pairs of light-sensitive organs called ocelli, or eyespots


The nervous system of free-living flatworms allows them to gather information from their environment – information that they use to locate food and to find dark hiding placesParasitic flatworms often do not have much of a nervous system


Free-living flatworms usually use two means of locomotion at onceCilia on their epidermal cells help them glide through the waterMuscle cells controlled by the nervous system allow them to twist and turn so that they are able to react to environmental conditions


Reproduction in free-living flatworms can be either sexual or asexualMost free-living flatworms are hermaphroditesThe eggs hatch within a few weeks

Planarians

The free-living flatworms belong to the class TurbellariaMost familiar members of this class are planariansTurbellarians vary greatly in color, form, and sizeAlthough most Turbellarians are less that 1 cm in length, some giant land planarians, which are found in moist tropical areas, can attain lengths of more than 60 cm

Flukes

Class Trematoda contains parasitic flatworms known as flukesMost flukes are internal parasites that infect the blood and organsThese flukes have complicated life cycles that involve at least two different host animalsBlood flukes are found primarily in Southeast Asia, North Africa, and other tropical areasHumans are the primary hosts of blood flukes

Flukes

Most flukes are hermaphrodites and undergo sexual reproduction in a manner similar to that of free-living flatwormsFlukes produce many more eggs than free-living flatwormsBlood flukes lay so many eggs that the tiny blood vessels of the host’s intestine break openThe broken blood vessels leak both blood and eggs into the intestine

Flukes

The eggs are not digested by the host and thus become part of the fecesIn developed countries, where there are toilets and proper sewage systems, these eggs are usually destroyed in the sewage treatment processBut in many undeveloped parts of the world, human wastes are simply tossed into streams or even used as fertilizer

Flukes

Once the fluke eggs get into the water, they hatch into swimming larvaeWhen these larvae find a snail of the correct species, they burrow inside it and digest its tissuesThe snail is an intermediate host for the flukeIn the intermediate host, the flukes reproduce asexually

Flukes

The resulting new worms break out of the snail and swim around in the waterIf they find a human, the worms bore through the skin and eat their way to the blood vesselsIn the blood, the get carried around through the heart and lungs to the intestine, where they live as adults

Flukes

People infected with blood flukes get terribly sickThey become weak and often die – either as a direct result of the fluke infection or because they cannot recover from other diseases in their weakened condition

Tapeworms

Members of the class Cestoda are long, flat parasitic worms that live a very simple lifeThey have a head called a scolex on which there are several suckers and a ring of hooksThese structures attach to the intestinal walls of humans and other animalsAdult human tapeworms can be up to 18 meters longTapeworms almost never kill their host

Tapeworms

Behind the scolex of the tapeworm is a narrow neck region that is constantly dividing to form many proglottids, or sections, that make up most of the body of the tapewormThe youngest and smallest proglottids are at the anterior end of the tapewormMale and female reproductive organs are contained in the proglottids

Tapeworms

If food or water contaminated with tapeworm eggs is consumed by cows, pigs, fish, or other intermediate hosts, the eggs enter the intermediate host and hatch into larvaeThese larvae grow for a time and then burrow into the muscle tissue of the intermediate host and form a dormant protective stage called a cyst

Tapeworms

If a human eats raw or incompletely cooked meat containing these cysts, the larvae become active within the human hostOnce inside the intestine of a new host, they latch onto the intestinal wall and grow into adult worms

Roundworms

Members of the phylum Nematoda, which are known as roundworms, are among the simplest animals to have a digestive system with two openings – a mouth and an anusFood enters through the mouth, and undigested food leaves through the anusRoundworms may be the most numerous of all multicellular animalsA single rotting apple can contain as many as 90,000 roundworms

Form and Function in Roundworms

Most roundworms are free-livingAll roundworms have a long tube-shaped digestive tract with openings at both endsAny material in the food that cannot be digested leaves through an opening called the anusRoundworms breathe and excrete their metabolic wastes through their body wallsThey have no internal transport system

Form and Function in Roundworms

Roundworms have simple nervous systemsThey have several ganglia in the head region but no definite brainRoundworms reproduce sexuallyFertilization takes place inside the body of the female

How Unsegmented Worms Fit into the World

Do not have a lot of positive influence on humansResponsible for some of the most painful and horrific diseases knownHookwormEye wormTrichinosis

chapter 26: sponges, cnidarians, and unsegmented worms section 1: introduction to the animal kingdom

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