LABORATORY ELEVEN

 ANIMAL BODY PLANS

 OVERVIEW:

            Like last week's plant evolution lab, this session involves careful observation of many specimens and demonstrations including slides as well as live animals to show the incredible diversity in the Kingdom Animalia.  Although the animals you will see appear very different, they are all constructed according to a few critical design requirements that reflect a handful of important and innovative structures like shells, skeletons, and body cavities.

            Based on these simple variations of and additions to a few original body plans, you should be able to tell something about the relationships and classification of various invertebrate phyla.  You will then use these characters in a completed chart to construct a simple dichotomous key that can be used to identify all the animals you will see in lab.  Read this entire lab before beginning on the work to be done. 

OBJECTIVES OF LABORATORY:

• To learn the major characteristics of animals, and be able to identify all lab specimens to at

   least the phylum level.   

• To become familiar with major advances in the evolution of invertebrates.

• To develop a dichotomous key that can be used to identify animals in eight phyla.

• To understand the history, relationships, and life cycle of animals.

• To practice observation, note-taking, and drawing skills.

INTRODUCTION:

            The Kingdom Animalia consists of organisms with the following characteristics:

                        1. Eukaryotic cells

                        2. Multicellular organization

                        3. Motility using muscular contraction

                        4. Feeding by ingestion

            Although the fossil record of animal life goes back more than half a billion years, early traces are ambiguous and may not be animals at all, or may be animal life of a kind totally unlike what we see today.

            Modern animal life dates approximately from the beginning of the Cambrian Period, around 590,000,000 years ago.  By the middle of this geological period some 40 million years later, most of the animal phyla still in existence today had evolved their basic forms and become diverse.  Each phylum shows a body plan that is characteristic or fundamental for all its members, sometimes called a bauplan, German for "basic design".  The complexity of this plan varies among the phyla, and those with more complex plans are usually thought to be more highly evolved than those with simpler plans.  Though this is not a hard and fast rule, parasitic members of phyla that are otherwise complex can become very simplified as a result of their habits.  This laboratory will survey some of the major features of animal body plans.

Your instructor will review with you the six major features of animal body plans that are discussed here.

 1. Body symmetry:

            This refers to the general appearance and arrangement of body parts.  While some animals, such as sponges, have an amorphous appearance and may not show any symmetry, most animals fall into one of two categories.  Bilaterally symmetrical animals can be divided into two halves that are mirror images of each other only along one axis.  Humans, insects, worms and octopi are good examples of this type of symmetry.  Bilateral symmetry is associated with movement; such animals creep, swim, run, or fly about in search of food and favorable environments.  On the other hand, radially symmetrical animals may be divided into mirror-image halves along more than one axis, and often along almost any diameter of the circle drawn around them.  Examples are sea anemones, jellyfish, starfish and sea urchins.  Radial symmetry is adaptive for animals that live their lives attached to something.  Being radially symmetrical makes it possible for them to detect stimuli coming from all directions, and also to hunt food with equal facility all around them.

            The clear connection of symmetry with habit can be seen by looking at the changes in the life of an individual starfish.  The starfish begins life as a bilateral larva.  The larva briefly attaches to a shell or rock and transforms into a radially symmetrical adult.  But adult starfish do not remain attached; they move about slowly!  Experiments have shown that starfish with 5 arms almost always move with one specific arm extended ahead.  Thus we could postulate that they are on their way to becoming bilateral again the "front" arm could become a head as a result of having abandoned their attached way of life.

2. Cephalization:

            This simply means the presence of a head.  Cephalization occurs in all bilaterally symmetrical animals because there is an advantage in having the end that goes first, as the animal moves, fitted out with sense organs—on the premise that it is better to know where you are going than where you have been.  Because a lot of computational power is needed to deal with sensory stimuli, the nervous system tends to be larger in the head as well.  And finally, food can be captured more effectively if the mouth is also at the front of the body.

3. Complete gut:

            Strange as it may seem to us, many animals do not have a gut with two openings—the same opening serves as both mouth and anus.  Because the cavity into which this "mouth" opens also serves as a rudimentary circulatory system, it is called a gastrovascular cavity.  This arrangement occurs in flatworms (Platyhelminthes) and cnidarians (Cnidaria: jellyfish, anemones, etc.).  It would seem less efficient because once the gastrovascular cavity is filled with food, the animal must wait until it is all digested before expelling the wastes the same way the food came in.

            Having an anus, on the other hand, is a big plus.  It means the animal can eat more or less continuously, since there is one-way traffic in the gut.

4. Body cavity:

            As animals grew larger in body size, they faced a problem that occurs at all levels of living things — the surface/volume ratio.  Everything that enters a cell or an organism has to come through a surface.  But as size goes up, volume increases much faster than surface area.  Volume grows as the third power (cubed) of length, while surface only as the second power (squared), so as an animal's length increases from 2 to 4 units, its surface area increases from 4 to 16 square units and its volume from 8 to 48 cubic units.

            Relying on a simple, tubular gut to supply food would mean that the animal would have to stay thin but grow longer and longer to get the right surface/volume ratio.  Long thin bodies are very fragile though they certainly do exist in such animals as the horsehair worm, which may be hundreds of times as long as wide.  A better solution might be to coil a long gut up inside a short body; other tubular organs could also be packed in a smaller volume by coiling.

            The coiling of tubular organs requires that the body be hollow.  Thus the formation of a body cavity, or coelom (pronounced see-lum) represents an important advance. Technically, there are two basic types of cavity, the so-called pseudocoelom and the eucoelom, or "true coelom," but the distinctions between them are technical and there is little evidence to suggest that one works better than the other.

            A fluid-filled coelom of some invertebrates provides other advantages.  The fluid, as it sloshes around, can circulate food and oxygen and pick up wastes.  Because a fluid cannot be compressed, if an animal clamps down the muscles of its body wall on the coelomic fluid, it becomes hard and rigid—the coelom can function as a hydrostatic skeleton.

5. Skeletons:

            Many animals that swim or creep have no skeletons at all, not even a hydrostatic one.  A flatworm crawls by using tiny projections of the cells of its underside, called cilia, against the surface.  A jellyfish swims by partially closing its bell against the resistance of the water.  Effective movement using limbs to walk or swim, however, requires a skeleton, some hard structure for internal or external support.

            How a hydrostatic skeleton works in this system will be explained in the next section.  Right now we can recognize two kinds of skeletons by their positions— endoskeletons are like ours, and are on the inside of the body forming a structural framework, while exoskeletons are on the outside and look like a suit of armor.

            Endoskeletons occur primarily in chordates, where they are composed of living cartilage and bone, and in echinoderms, starfish and their relatives, where they are made of massive crystals of calcite.  Exoskeletons are found in arthropods.  They are not living and are secreted by the outermost layer of cells.  Some exoskeletons contain minerals, primarily calcite, but most are made of tough proteins and a sugar polymer, chitin.

            Which is best, an endoskeleton or an exoskeleton?  There are advantages to both.  An exoskeleton doubles as a protective layer and as waterproofing.  However, because an exoskeleton is not alive, if the animal is to grow, it must be periodically shed and re-secreted.  During this time an arthropod may be immobile, soft, and very vulnerable.  Chitin and protein are also not terribly strong, so most animals with chitinous exoskeletons are limited to small sizes.  Bone, on the other hand, is alive and can grow.  It is strengthened by mineral deposits, primarily calcium phosphate, and is quite strong.  So it is no surprise that chordates number amongst them the largest animals that ever lived.  And also because it is alive, an endoskeleton of bone can repair itself.

            Distinguishing between a skeleton and a shell can be a problem.  Generally, a shell is only for protection and does not provide either a structural framework or a system of levers which muscles operate as does the skeleton.  The most obvious and important group of animals with shells are the molluscs—clams and oysters, snails, and squids and octopi.  In the latter two groups, the confusion typical of biological systems comes in:  some squids have brought their shell to the interior of the body, and it really does act as a skeleton!

6. Segmentation:

            A segmented animal has a body that consists of repeating identical or similar units, called segments.  The division of a fundamentally worm-like, bilateral body into segments may be thought of as the final major adaptation.  This major characteristic occurs only in three of the 35 or so animal phyla.  Therefore it is likely that segmentation only arose two or three times in the evolutionary past of animals.

            In the chordates, segmentation arose as an adaptation to efficient swimming, and is primarily a characteristic of the muscles, skeleton, and nervous system.  The muscles are arranged in V-shaped bands, and as a wave of contraction passes down them, alternating on each side, the body sweeps into graceful curves which push against the water and propel it forward.  Quite naturally, the backbone and nervous system followed this plan of muscular units.

            In the annelid worms (such as earthworms) segmentation arose as an adaptation to burrowing in soft mud.  The coelom (hydrostatic skeleton) was divided into compartments.  Because liquids cannot be compressed, the volume of each compartment must remain the same.  Each compartment is wrapped in two layers of muscles, circular and longitudinal.  When the longitudinal muscles are contracted, they squeeze the segment into a broader, shorter form that can anchor a worm in a burrow.  The contraction of the circular muscles makes the segment longer and thinner, pushing the whole animal ahead of it.  By alternating these two shapes, the worm can anchor itself in the soil or mud or get a push to move further into the substrate.  The muscles require nerves to operate them, so these are segmentally arranged, too.  Since each compartment must be sealed, it must have its own set of certain organs.  Material is conducted from compartment to compartment by a circulatory system of closed vessels.

            Animals whose segments are all alike show homonomous segmentation such as the earthworm.  But in many advanced animals, the segments have become specialized for different functions — heteronomous segmentation.  The segments may also be arranged in groups called tagmata (singular tagma).  For example, in both chordates and arthropods, the segments nearest the front have become specialized for feeding, breathing, and gathering sensory information.  They have often fused with the head; most arthropods have a head that includes at least 6 segments.  Your head includes 10 segments, marked in your nervous system by 10 pairs of cranial (head) nerves.  For example in Chordates, heteronomous segmentation can be seen in the spinal column and ribcage.

            Arthropods have armored their segments with exoskeletal plates. The insects show the greatest level of heteronomous segmentation and tagmatization; each insect body consists of three multisegmented units.  The head collects information and feeds, the thorax is concerned with movement, walking and flying, and the abdomen contains most of the internal organs, including those involved in reproduction.

            From this listing of body plan characteristics, you can see that the simplest animals have the smallest number of these features, and the most complex the largest number.  Since the characteristics were probably acquired by animals in the order in which they are listed, they can also be used to show the course of evolution, as in the phylogenetic tree attached to this lab.

            In lab today you will examine a variety of animals to determine their body plan features, and learn the names and characteristics of some of the major animal phyla.

Procedure:

EARLY DEVELOPMENT IN ANIMALS:

            The similarities in early development among all animals can be seen by examining the slide of starfish embryos going through the early stages of this process.  Observing these early stages will help you to understand several of the features of the body plan that have been presented—bilateral symmetry, cephalization, gut, and coelom development. 

1.        Obtain a slide of starfish development.  Locate the cells with the clear nucleus, which contains another structure called a nucleolus.  This is the unfertilized egg of the starfish.  Now find single cells that are exactly the same size but that have no visible nucleus.  You are likely to see the fertilization membrane around such cells.  The fertilization membrane looks like a cellophane wrap around the cell.  This is the fertilized egg or zygote.  If you cannot find a fertilization membrane here, you might see it more clearly as you move to the next stage of development.  Draw and label (the items in bold) the unfertilized egg and the zygote on the sheet provided.  Estimate the size of the zygote in micrometers.  If you can’t remember how to do this, consult the directions for the first laboratory in this manual.  Keep your measurement in mind as you observe later stages of starfish development.  Do the embryos get larger, smaller, or stay the same size as they continue to develop? Be prepared to explain your observations.

2.         The zygote divides to form the two-cell stage.  Cell division in embryos is referred to as cleavage.  The cells are called blastomeres.  Draw the 2-cell stage, labeling the fertilization membrane.

3.         Find an eight-cell embryo and draw that.  Notice that even though the embryo contains more cells, blastomeres, that the overall size of the embryo is essentially the same as the zygote.

4.         The next stage to look for is the morula, 16 to 32 cells in a tight ball.  Draw an embryo at this stage.

5.         Find an embryo that appears as a hollow ball of cells.  The space in the center of the embryo is the blastocoel.  An embryo at this stage is referred to as a blastula.  Sketch the blastula.

6.         After the formation of the blastula, one surface of the hollow ball begins to move into the space.  With this inward sweep of cells, two tissue layers are established:  the outer surface layer called the ectoderm and the inner layer called the endoderm.  The opening is called the blastopore and leads to the primitive gut tube, or the archenteron.  Find an embryo at this stage, called the gastrula, and draw and label the structures:  blastocoel, gut or archenteron (also called the gastrocoel), endoderm, ectoderm, blastopore, and mesoderm.

            In Chordates and Echinoderms, like you and this starfish, respectively, the blastopore becomes the anus of the primitive gut tube, and this tube touches the other surface of the embryo and forms the mouth.  Organisms in this line of descent, with the two openings formed in this order, are called the deuterostomes.  The other phyla are called protostomes; in protostomes the blastopore becomes the mouth.  In both of these lines of descent, a space, called a coelom, may form within a third layer of tissue called the mesoderm which arises between the outer surface of the body, the ectoderm, and the gut, composed of endoderm.  The three primary tissue layers, endoderm, ectoderm, and mesoderm, differentiate into the following structures in the mature organism:

                        endoderm:  digestive organs and lungs

                        mesoderm:  muscles, connective tissues, skeleton

                        ectoderm:  skin, brain, nerves.

7.         The next developmental stages occur relatively rapidly.  You should be able to find a multicellular larval form, that is no longer spherical, that possesses the third tissue layer, the mesoderm, which cannot be specifically detected but makes up some of the internal structures of the organism.  Draw the larva.

            This background on development, the diagram provided in this lab, your textbook (remember to use its index!), the specimens and information available in the laboratory, as well as the questions and observations concerning each phylum included here, will help you complete the chart for the major phyla.

SURVEY OF THE EIGHT PHYLA:

1.         Examine preserved specimens of jellyfish and sea anemones, both members of the Phylum Cnidaria.  What kind of symmetry do they have?  Is a complete gut present?  Do they have a skeleton?

            Name several characteristic specimens and fill in the column under Cnidaria in the chart.

2.         Obtain a live flatworm, or planaria of the Phylum Platyhelminthes, in a depression slide and watch it under a microscope.  How does it move?  Is it segmented?  Draw a sketch of the specimen.

              Observe the other platyhelminthes on display—flukes and tapeworms.  Fill in the proper column in the chart. 

3.         Make a wet-mount slide from the culture of vinegar eels, a minute nematode worm from the Phylum Nematoda.  How do the worms move?  This worm has a pseudocoelom that helps it move in this fashion.

            Observe the other "roundworms" on display.  Fill in the proper column on the chart. 

4.         Observe a live earthworm from the Phylum Annelida in the fingerbowl.  Notice how the shapes of the segments change as the worm crawls across the towels.  Does this worm show homonomous or heteronomous segmentation?

            Observe the segmented worms on display.  These worms have a true coelom.  Fill in the proper column on the chart. 

5.         Look at the trays of insects, Phylum Arthropoda and other specimens provided; identify the three tagmata.  Is this homonomous or heteronomous segmentation?  What is the nature of the skeleton?

            How many tagmata does a crawfish or lobster have?  Observe other arthropods on display.  Fill in the chart.

6.         A display of shells on demonstration suggests something of the variety of types to be found in the Phylum Mollusca.  Make careful observations concerning the other mollusks available.  Refer back to the information given earlier in this lab concerning skeletons v. shells.  Remember, a shell is not a skeleton.  

                        Give examples of a few of these shell types and other specimens of interest. 

7.         A small sea animal called a lancelet, Amphioxus or Brachiostoma, will be our example of the Phylum Chordata.  Remember that YOU are an example as well.  Look at the whole specimen mounted on a slide and note the form of the segmentation.  A stiff rod runs the length of the body; this is the notochord.  Its elasticity opposes the contractions of the muscles.  Notice that some of the anterior segments bear gill slits.  These segments become part of the head in the vertebrates, which probably descended from an ancestor much like the lancelet.

            Review the other chordate specimens on display.  Fill in the characteristics on the chart. 

8.         The last phylum to be examined is also the most peculiar, the Phylum Echinodermata.  A starfish is a typical example.  What kind of symmetry does it have?

            Examine other specimens available in this group.  Fill in the chart for this phylum. 

Assignment:

1.    Your final activity today is to make up a dichotomous key, similar to the one you used several weeks ago, that could be used to identify the eight animal phyla you have just studied.  Hand the key in as a part of your evaluation this week.

2.  Study for your final practical which includes labs 7-11.

STARFISH DEVELOPMENT