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 are the first organisms to have tissues organized into organs and the first to demonstrate 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. It is characteristic of active, crawling, or swimming organisms and usually results in the formation of a distinct head (cephalization) where accumulation of nervous tissue and sensory structures occurs. 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. The Platyhelminthes and all phyla above them on the evolutionary tree are bilaterally symmetrical or have evolved from bilaterally symmetrical ancestors.
In the Platyhelminthes, different tissues cooperate in any given function. This results in the organ level of organization. Three major sets of organs characterize the phylum. The excretory system consists of flame cells and their associated ducts. The nervous system consists of a pair of anterior ganglia, usually with two nerve cords winning the length of the organism. Nerve cords are interconnected by transverse nerves to form a ladder-like structure. The digestive tract is incomplete (a single opening serves for ingestion of food and elimination of wastes).
The Platyhelminthes are triloblastic 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. The term acoelomate refers to the fact that them is no body cavity (fluid-filled space) between any of the primary germ layers.
The class Turbellaria is a primitive class, in that it retains the defining features of the phylum, such as anteriorly located sense organs and a well-developed muscular system. The remaining classes in this phylum are composed of specialized parasites that have lost many features seen in free-living animals.
Dugesia - a planarian flatworm
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. Draw the specimen and label its head, auricles, and eye spots.
2. Use a compound microscope at low power to observe a specially prepared preserved specimen of Duglesia that prominently displays the digestive tract. Draw this specimen and label its pharynx, phayngeal chamber, and highly branched gut.
CLASS TREMATODA - the digenetic flukes
Flukes are all parasitic, primarily attacking 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 structures well-developed in free-living flatworm types are found to be considerably reduced or even absent in these parasitic forms.
3. Examine a prepared slide of Clonorchis sinensis, a 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.
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 hard cuticle protects them from the host's digestive enzymes. The anterior region of a tapeworm's body is modified as a simple holdfast, 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, therefore the proglottids closest to the scolex are the least mature. As the proglottids mature, the male reproductive system develops followed by 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 whole mount of Taenia pisiformis. Find the scolex and proglottids in different stages of development - immature, mature, and gravid proglottitids. 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.
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