PHYLUM PLATYHELMINTHES – TRIPLOBLASTIC ACOELOMATES
The Platyhelminthes include free-living flatworms, like the planarians, and the parasitic tapeworms and flukes. The term flatworm refers to the fact that the body is dorsoventrally flattened. Flatworms have tissues organized into organs, and they exhibit bilateral symmetry. Bilateral symmetry means that one plane passing through the longitudinal axis of an organism divides it into right and left halves that are mirror images. Bilaterally symmetrical animals typically move about actively and exhibit cephalization, which is the accumulation of nervous tissue and sensory structure at the anterior end of the animal. This reflects the importance to the organism of monitoring the environment it is meeting - rather than that through which it has just passed - and results in the presence of definite anterior and posterior ends. However, the digestive tract is incomplete, with a single opening that serves for both ingestion of food and elimination of wastes.
The Platyhelminthes are triploblastic and acoelomate. There are three primary germ layers: ectoderm, endoderm, and mesoderm. As with the Cnidaria, the ectoderm gives rise to the outer epithelium and the endoderm gives rise to the lining of the gut tract. The third germ layer, the mesoderm, gives rise to the tissue between the ectoderm and the endoderm, including muscle, excretory structures, and undifferentiated cells referred to as parenchyma. Typically, different tissues will come together to form organs and cooperate to perform body functions. The term acoelomate refers to the fact that there is no coelom (fluid-filled body cavity) between the gut and outer body wall.
Class Turbellaria – the free-living flatworms
The class Turbellaria contains free-living flatworms (such as planarians) that retain the defining features of the phylum, such as anteriorly located sense organs and a well-developed muscular system. The other classes in this phylum are composed of specialized parasites that have lost many features seen in free-living animals.
1. Place a live planarian (Dugesia) in a watch glass with a small amount of water. Observe its locomotion and behavior as it explores its environment. The planarian may respond to adverse stimuli (such as touch, vibration, or bright light) by contracting its body. As you observe your specimen, identify the head, auricles, and eyespots. Also look for the pharyngeal tube projecting off the ventral surface of the animal.
2. Use a compound microscope at low power to observe a prepared slide of Dugesia that prominently displays the digestive tract. Draw this specimen and label its pharynx, pharyngeal chamber, and highly branched gut. Label your drawing with the total magnification at which your observation was made.
Class Trematoda – the digenetic flukes
Flukes are all parasitic, primarily infecting vertebrates, including man. The flukes in the class Trematoda are digenetic, meaning they have a life cycle requiring two or more hosts. In such a cycle, the final host is termed the primary or definitive host, while the other (or others) are intermediate hosts. Flukes of this group are typically endoparasites (living inside the host). They have highly specialized reproductive systems, very high reproductive capacity, and complex life cycles in which most of the intermediate stages are capable of (and exhibit) asexual reproduction. Some of the body structures that are well-developed in free-living flatworms are considerably reduced or even absent in these parasitic forms.
3. Examine a prepared slide of Clonorchis sinensis, the human liver fluke. This specimen demonstrates typical parasite features, including the absence of sensory organs, reduction of locomotor and digestive systems, expanded reproductive system, and presence of holdfast organs. Sketch the specimen and label the gut, the prominent reproductive structures (uterus, testes, and yolk gland), and the oral sucker and ventral sucker. Label your drawing with the total magnification at which your observation was made.
Figure 1: Anatomy of a turbellarian flatworm, a representative platyhelminth.
Class Cestoda – the tapeworms
These are highly adapted endoparasites which absorb their nutrients directly through their body walls from the host gut. They have lost their own digestive system and increased their reproductive capacity. A resistant cuticle protects them from the host's digestive enzymes. The anterior region of a tapeworm's body is modified as a simple attachment structure known as the scolex. The rest of the tapeworm body is composed of a series of segments called proglottids. Proglottids are produced continuously behind the scolex, and therefore the proglottids closest to the scolex are the least mature. As the proglottids mature, the male reproductive system develops before the female reproductive system. The most mature proglottids are little more than a uterus filled with eggs, and are called gravid proglottids.
4. Examine a prepared slide of Taenia pisiformis. Find the scolex and also proglottids in different stages of development: immature, mature, and gravid proglottids. Draw the scolex and label the hooks and the sucker discs. Draw a mature proglottid and label the testes, ovary, uterus, and genital pore. (A diagram of a mature proglottid can be viewed here.) Also draw an immature proglottid and a gravid proglottid. Label your drawings with the total magnification at which your observations were made.