LABORATORY # 7
INVERTEBRATE ANIMALS – I
LABORATORY OBJECTIVES
Upon completion of this laboratory the student will:
REFERENCE
Textbook: chapter 21, pp 421 -- 441, and 444 – 445
Photo Atlas: chapter 7, pp 137 – 150, and 156 – 161
INTRODUCTION
Animals are organisms that are multicellular, eukaryotic, and heterotrophic. In addition, most animals reach the system level of organization and have at least one stage of the life cycle that is motile. For purposes of study, the animal kingdom is often separated into those earlier, more primitive forms that do not have a vertebral column, the invertebrates, and those that have such a vertebral column, the vertebrates. Though most of us are much more familiar with vertebrate animals like birds and mammals, the invertebrates are much more numerous, varied, and ancient. In this exercise and in exercise #8 you will be familiarized with several phyla of invertebrate animals; the vertebrates laboratory will follow. In this exercise we will examine six invertebrate phyla. They are presented, more-or-less, in the order of their appearance and their complexity.
Phylum Porifera – The Sponges
What can be said of the sponges, even by an enthusiastic biologist? As a group, they are not beautiful animals; they have little effect on health; they do not represent a significant economic resource; they are not even “cute”! We can give them credit for being persistent! Sponges evolved from some protistan ancestor nearly a billion years ago and they remain little changed. These primitive animals found themselves a niche on the sea floor and they have held tenaciously to it.
Phylum Porifera holds the world's simplest animals. Sponges have only reached the tissue level of organization. Though most are somewhat more elaborate, the basic sponge can be thought of in terms of an urn or vase. Water enters the sponge through tiny pores (Porifera means "pore bearer") and exits the large opening in the top. The sponge is lined with tiny, flagellated collar cells, which propel the water with their flagella and remove food particles from it. The sponge is supported by a "skeleton" of tiny, pointed rods called spicules, or by a network of spongin fibers. These animals reproduce both asexually and sexually. Sexual reproduction produces a motile larval form, which may swim some distance before settling down to become an adult.
Activity:
Specimens of Complex and Simple Sponges
Examine the cluster of tiny simple sponges within the small vial. Are they symmetrical? Describe their skeleton. Why are these sponges said to be “simple?” Look at the dried specimens of complex sponges. See if you can locate some oscula. These sponges have skeletons of sponging fibers, rather than spicules. These are the common “bath sponges” of commerce.
Phylum Cnidaria - Jellyfish, Corals and Their Kin
Members of Phylum Cnidaria are characterized by radial symmetry, lack of a body cavity or coelom, a gut with only one opening, and tentacles with tiny stinging cells. Most are marine animals, either floating about in the water or firmly attached to the bottom. The cnidarians, too, are simple animals, consisting of only two layers of cells with a layer of jelly-like material in between.
The phylum includes organisms with either or both of two life forms. The jellyfish typify the free-floating medusa form, looking like an inverted bowl with tentacles hanging down from the edge. Corals and anemones spend their entire lives in the polyp form, anchored to the bottom with their tentacles at the top, rather like a thick mop with the head up. Others have both polyp and medusa in their life cycles, with the former being the asexual stage and the latter sexual. A ciliated, motile planula larva aids in distributing these often sessile (stuck in one place) animals.
Activities:
1. Slide and Living Specimens of Hydra
Hydra is one of the most common animals observed in the biology laboratory. Select a slide of Hydra and examine it on 40X. Is this a polyp or a medusa? Locate the tentacles, basal disc, mouth, gastrovascular cavity, and bud. The bud is an infant polyp produced asexually by the parent. Hydra does not have a medusa stage. Your instructor may have live hydra for you to look at; if so, make a wet mount using a depression slide.
2. Preserved Specimens of Aurelia, a Jelly-fish
Look at the preserved specimen of the medusa Aurelia. Note the umbrella shape, the tentacles and the dark-stained gonads. Do you now feel that you can differentiate medusae from polyps?
3. Preserved Specimens of the Sea Anemone and Coral
Examine the preserved specimens of the sea anemone and the several specimens of corals. These taxa do not employ medusae in their life cycles. The coral polyps live in the tiny openings in their hard exoskeletons of calcium carbonate. These tiny animals create the great coral reefs of our tropical seas. Be able to identify these organisms by common name, phylum and kingdom.
Phylum Platyhelminthes - The Flatworms
Like most animals, the flatworms are bilaterally symmetrical. They share with the cnidarians, however, a body, which lacks a coelom and a complete digestive system (two openings to the gut; a mouth and an anus). These animals are dorsoventrally flattened, giving them a thin, leaf-like appearance. The phylum contains the first animals to develop a head; a region in which the nervous system and sensory organs are concentrated. There are three classes of Platyhelminthes:
1. The planarians are free-living (non-parasitic) worms, which live in small streams and ponds or under moist leaves and rocks. They have ear-like lobes and two black, photosensitive eye spots at the anterior (front) end of their flattened body. On the ventral (lower) surface, about midway back, is a tubelike pharynx, which terminates in the mouth. The digestive system consists of three blind pouches. Planarians are harmless.
2. The flukes are mostly internal parasites of vertebrates. They are small worms, which use one or two suckers to attach themselves to the host. A very complex life cycle may make use of at least one intermediate host, in addition to the primary one. In some parts of the world, flukes constitute a serious threat to human health. The Chinese liver fluke, inhabiting the bile ducts of humans, is an example, as are the schistosomes or blood flukes, which are small enough to live in the veins.
3. The tapeworms are all extremely specialized internal parasites. These ribbon-like worms spend their lives anchored to the intestinal wall by the scolex, an attachment organ with suckers and hooks. Behind the scolex is a narrow neck, followed by a series of proglottids, which become progressively larger and more mature. The tapeworm body has been reduced to the essentials. It requires no digestive, nervous, circulatory or respiratory system, due to its habitat and life style. In fact, a tapeworm is little more than a series of reproductive "units" with an excretory system.
The tapeworm life cycle is quite complex, often involving two intermediate hosts. Humans are usually infected by eating improperly cooked meat and/or fish. While most tapeworms do not approach its size, the broad fish tapeworm has been recorded at one inch wide and 60 feet long!
Activities:
1. Slide and Live Specimens of Planaria
Select a slide of Planaria. Locate the animal’s head, auricles, eyespots, pharynx and gastrovascular cavity. Your instructor has set up a dissecting scope for your examination of living Planaria. Describe how the Planaria moves about.
2. Preserved Specimen of the Fasciola, theSheep Liver Fluke
Examine the preserved specimen of the fluke. Can you recognize these creatures as flukes, flatworms, and animals? What type of symmetry do they have?
3. Preserved Specimen of the tapeworm
See the preserved specimen of a tapeworm. Can you tell which end the scolex is on? Can you see its segments (proglottids)?
4. Slide of Echinococcus granulosus, the Minute Dog Tapeworm
Carefully examine the slide of the tapeworm. This is the minute dog tapeworm. See if you can locate the scolex, neck and proglottids of this tiny worm. How does a scolex differ from a head?
Phylum Rotifera - The Wheel Animals
The rotifers constitute the most numerous and most characteristic animals inhabiting freshwater. They derive their name from the cilia typically found on the anterior end, the motion of which gives the impression of a rotating wheel. They are not only bilaterally symmetrical, but they have a complete digestive system and a body cavity. The body cavity of the rotifers is termed a pseudocoelom, since it is not lined with the mesodermal tissue typical of most animals. A unique feature, found only in the rotifers, is the muscular mastax, an organ located in the digestive tract and which bears tiny jaws used by the animals in a variety of ways. Most rotifers are female and the usual method of reproduction involves the hatching of unfertilized eggs, a type of parthenogenesis ("virgin birth").
The more than 1500 rotifer species are mostly cosmopolitan (distributed world wide) and show a variety of shapes and modes of life. Rotifers typically inhabit the waters of lakes and ponds, but may live in moist moss and on beaches among the grains of sand. Some rotifer taxa can withstand near total desiccation (drying out) and be "reconstituted" by the addition of water. They are remarkable little animals indeed!
Activity:
Slide of the Common Rotifer
Select a prepared slide of the common rotifer. Examine it on high power and identify the wheel organ (corona), foot and mastax.
Phylum Nematoda - The Roundworms
The phylum Nematoda is a ubiquitous group, which, never the less, is relatively unknown to most people. These rather unspectacular-looking worms are nearly everywhere. It has been suggested that if all matter on earth except nematodes was eliminated, a ghostly outline of the planet and most organisms would remain! These worms are mostly small and free-living, but some are important internal parasites of humans and most other animals, as well as plants. The diversity of nematodes is such that the total number of species on earth may exceed that of the arthropods!
Nematodes are characterized by bilateral symmetry, a complete digestive system, a pseudocoelom and a tough, non-living outer covering, the cuticle. Unlike the annelids, which we will examine shortly, the nematodes lack the layer of circular muscles, which allow animals such as earthworms to elongate and shorten their bodies. This gives the nematodes an inefficient, thrashing method of movement. The nematodes seem to represent an evolutionary dead end and probably did not give rise to higher forms of life.
Activities:
1. Preserved Specimens of the Ascaris, the Pig Roundworm
Select a specimen of Ascaris, the pig roundworm, and place it in a dissecting pan in a little water. Determine the sex of your specimen. Be sure you see both sexes and can identify both. Find the anterior end by locating the mouth, surrounded by three “lips.” These large nematodes live in the intestines of pigs and humans.
2. Slide of Trichinella
Take a slide of Trichinella and examine it, first on low, then high power. You will notice larval worms encysted in muscle tissue. These worms are usually contracted by eating inadequately-cooked pork.
Phylum Annelida - The Segmented Worms
To most people, the word "worm" conjures up an image of an annelid, since the common earthworm or night crawler is a member of this phylum. The annelids take their name (meaning "little rings") from the repeating units, which make up the body of the worm. These rings or segments are quite similar, both inside and out. In addition to being segmented, members of this phylum possess a coelom different from those encountered thus far. This body cavity is lined with mesodermal tissue, which makes it a eucoelom (the prefix "eu-" means "true"). All animals studied subsequent to the annelids will be eucoelomate. The annelids continue the trend toward specialization of the anterior end of the body, with some members of the phylum having a well-developed head. There are three classes in Phylum Annelida:
1. The earthworms are the annelids most familiar to us. They are commonly used as fish bait and are frequently seen along sidewalks and roads after a rain, having been flushed from their burrows. These animals seem to be quite simple, since they lack eyes or any of the other obvious structures we generally associate with a head. In fact, there seems little to distinguish one part of their body from another. If one were to examine these animals internally, however, the story would be different. The earthworms have a well-developed digestive system and a closed circulatory system reminiscent of our own. They also have two full sets of reproductive organs, a characteristic which makes them functionally bisexual (hermaphroditic).
2. The clamworms are marine worms, which do not bear much resemblance to the earthworms, other than their segments. They are predaceous worms with a well-formed head equipped with eyes, jaws, and tentacles. Most segments of the clam worm bear a pair of leg-like appendages, the parapodia, with which it swims and breathes.
3. The leeches include the blood-sucking external parasites of fact and fiction. Other taxa are free-living and function as predators or scavengers. Leeches possess one or two suckers. The segmentation of these creatures is not as complete as in the previous classes of annelids.
Activities:
1. Model of Ascaris and Lumbricus
View the dissected specimens of the roundworm, Ascaris and Lumbricus, the earthworm. Note the presence of septa that divide the earthworm coelom into its segments. Are such septa present in the roundworm? Note that Ascaris is little more than an elongate bag containing an intestine and reproductive structures. Can you see how much more complex the earthworm appears?
2. Preserved Specimens of Lumbricus, the Earthworm
Take an earthworm, rinse it briefly, and place it in a dissecting pan in a little water. Pick up the worm (it’s NOT yucky) and gently run your fingers along the ventral (bottom) surface of the worm. Can you feel the tiny, bristle-like setae? These allow the worm to move inside its burrow. Locate the following parts: mouth, anus, clitellum, and segments. The clitellum is used to secrete the cocoon which houses the fertilized eggs.
3. Preserved Specimens of the Clamworm
Now replace the earthworm with a clamworm. Locate the parapodia, setae, and tentacles. Gently squeeze the anterior end of the worm to evert the jaws.
4. Preserved Specimen of the Medicinal Leech
Examine the preserved specimen of the medicinal leech. These leeches have been used by the medical profession to lower blood pressure, treat black eyes and reduce post-operative swelling.