PORIFERA, CNIDARIA AND PLATYHELMINTHES

Introduction

The animal kingdom is made up of several phyla, all of which are multicellular, eukaryotic, and heterotrophic. Most animals reach the system level of organization and have at least one stage of the life cycle, which is motile. The three phyla, which will be studied in this exercise, are among the most ancient and most primitive of animal taxa.

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 (See Perry and Morton, Figure 83a). 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 (Figure 83c), 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.

ACTIVITIES

a. Secure a microscope and a slide of Scypha or Grantia spicules. First on low power, then on high, bring these spicules into view. They should resemble those in Figure 83d of your Photo Atlas. If you have difficulty seeing these spicules, try closing your iris diaphragm to increase their contrast.

b. Look at the dried specimens labeled "Hard Head Sponge" and "Sheep's Wool Sponge". Compare these to Figure 84c and see if you can locate some oscula. These sponges have skeletons of spongin fibers (Figure 84d), 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 (see Photo Atlas Figure 86f).

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. Gonionemus, shown in Figure 88a, is a good example. 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 (See Perry and Morton Figures 86a - d). 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

c. Hydra is one of the most common animals observed in the biology laboratory. Select a slide of Hydra and examine both specimens on 40X. Is this a polyp or a medusa? Using Figures 86b, d and e as guides, 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.

d. Look at the preserved specimen of the medusa Aurelia and compare it to Figure 89d in the Photo Atlas. Note the umbrella shape, the tentacles and the dark-stained gonads. Do you now feel you can differentiate medusae from polyps?

e. Examine the preserved specimen of the sea anemone and the several specimens of corals. Compare these specimens to the illustrations on page 90 in Perry and Morton. 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.

f. See the preserved specimen of Physalia (Perry and Morton, Figure 88c). This is the Portuguese man-o-war, a floating colony of polyps capable of inflicting painful and even fatal stings to humans unfortunate enough to come in contact with it.

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, leaflike 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

g. Select a slide of Planaria. Using Figure 91a as a guide, locate the animal's head, auricles, eyespots, pharynx and gastrovascular cavity.

h. The human liver fluke, Clonorchis sinensis is illustrated in Figure 92a of the Photo Atlas. Examine a slide of this animal with your lowest power lens. Try to locate the oral sucker, the pharynx, the ventral sucker and the dark-stained uterus.

i. Examine the preserved specimen of the fluke. Can you recognize these creatures as flukes, flatworms and animals? What type of symmetry do they have? An illustration of Fasciola, the sheep liver fluke, is shown in Figure 91c of Perry and Morton.

j. See the preserved specimen of a tapeworm. Compare it with Figure 93a.

k. Carefully examine the slide of the tapeworm, Echinococcus granulosus. This is the minute dog tapeworm. See if you can locate the scolex, neck and proglottids of this tiny worm. Figure 93b, though of a different species, may be of help.