Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
The name Platyhelminthes means flat worms; there are about 15,000 living species. They live in marine and fresh water habitats, moist terrestrial environments, and inside other organisms as parasites. All have three tissue layers: ectoderm on the outside, endoderm lining the gut, and mesoderm in between. As in the Cnidaria, the gut is a gastrovascular cavity and has the functions of both digestion and circulation; the mouth serves as both entrance and exit from the gut. The body is bilaterally symmetrical, and there is a well defined rostral or head end and, opposite it a caudal or tail end. At the head end the nervous system has a coordination center which can be called a brain. There is no specialized respiratory system.
Flatworms are acoelomate, that is the outside of the gut wall is directly continuous with the mesoderm; there is no coelomic cavity and so the gut is not free inside the body. The mesoderm is muscular and is used for movement. Ectodermal cells may be ciliated and provide another means of locomotion.
Members of the Class Turbellaria are free living (not parasitic). Planaria (Genus Dugesia) lives in fresh water ponds and is a carnivore. The pharynx can be protruded from the mouth which is in the middle of the ventral side of the animal. The diet consists of such foods as insect larvae, small crustaceans, and other small living and dead animals. Planarians reproduce asexually and sexually; individuals have both testes and ovaries.
Examine the live planaria by transferring a few to spring water in a Petri dish and examining them with a low-power dissecting microscope. They glide over surfaces by the action of cilia; muscular contractions wrinkle or bend the body. There is clearly a head end with a pair of black, light sensitive spots. The color of these planaria would make them blend in and be invisible among the rotting leaves on the bottom of a pond.
Your instructor will supply a bit of yolk of a hard boiled egg. Sprinkle a little among the planaria. How quickly do they sense its location? How do they feed? When you are done, carefully transfer the planaria only, not the egg yolk, into a separate supply bottled labeled "fed."
Planaria whole mount (no high power) shows the main features of the body; one individual has been fed particles of carbon, and these blacken the gastrovascular cavity. It is highly branched, because it serves also for circulation of nutrients to every cell.
Planaria representative cross sections, has three or four slices through a planarian. Most obvious is the one from the middle with the great circular, muscular tube of the pharynx; the space around it is actually outside of the body proper. The gastrovascular cavity has a median tube in the body anterior to the pharynx; alongside and posterior to the pharynx it is divided into two lateral tubes. Posterior to the pharynx, there are lateral tubes of the gastrovascular cavity but no median one. The gut is continuous with the cells of the surrounding mesoderm; recall that the gut is lined with cells of endoderm and the outside of the animal is covered with ciliated cells of ectoderm. The mesoderm is muscular, and some bands of muscle cells run from the dorsal to ventral sides of the worm.
Trematodes are commonly called flukes, and they are parasitic. The life cycles of parasites are very complex, and there are intermediate hosts and life stages; read the description of this genus in the text book. Slide:
Opisthorchis (also called Clonorchis) whole mount (no high power) is the liver fluke The anterior end has a mouth at the end and a sucker disc for holding on; a ventral sucker is somewhat farther posterior. The intestine divides into two simple tubes near the head end. The dark colored organ in the center of the body is the uterus and it is filled with eggs; the ovary is just posterior to it and appears as a pink mass; it has a seminal receptacle at the posterior end. The branched pink organ in the posterior part of the body is the testes.
Trematode life cycle: Opisthorchis sinensis, the liver fluke, is an important human parasite that is widely distributed in the Far East. Human infection occurs where raw fish is a delicacy. The small brown eggs of the fluke exit from a person's body in the feces. The eggs hatch when they are eaten by a certain kinds of fresh water snails. The hatchling is called a miracidium; after several metamorphoses the parasite leaves the snail and infects fresh water fish, especially minnows and carp; it encysts in their flesh. When a person eats these fish uncooked or insufficiently cooked, the live parasites enter the digestive system and migrate from the small intestine via the bile duct to the liver. There they mature into adult liver flukes and in about 3 weeks begin to produce eggs. Eggs return via the bile duct to the digestive system and exit from the body in feces.
The Cestoda are parasitic tapeworms. The life cycles are complex (see the text). All tapeworms are extremely flat; the body is divided into segments, and there is no digestive system. They absorb nutrients across their body walls. Preserved tapeworms are available in jars. Slides:
Scolex (no high power) is the term for the head end of a tapeworm; it has a disc of hooks at the tip, which anchor it into the lining of the host's intestine, and four large suckers for holding on. New segments or proglottids are generated behind the scolex. As you move down the worm away from the head, these segments get larger. Each is a complete reproductive machine with testes, ovaries and uterus.
Mature proglottid (no high power), shows a branched uterus containing hundreds of eggs in each segment. The chance of any one egg hatching and completing the life cycle to parasitize the final host is so small that high reproductive capacity is a must.
Cestode life cycle: Mature proglottids of human tapeworms, Taenia, exit from the body in feces. The intermediate host of one species of Taenia is cattle, of another, pigs. When the proglottids or eggs are eaten by the intermediate host, the eggs hatch, and the larvae encyst in the flesh of the animal; the bladderlike cyst contains one or more inverted (inside out) scolecies. When a human eats infected meat that is raw or inadequately cooked, the live cysts break down during digestion, the scolecies turn right side out, and they attach themselves to the mucous membrane of the intestine by their hooks and suckers. Here they become sexually mature adults.
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Last updated 22 September 2007 (JHW)