WORMS, WORMS WORMS
PHYLUM PLATYHELMINTHES: FLATWORMS
Prepared slides – planaria whole mount
-planaria sections
-taenia pisiformis (tapeworm)
-dwarf tapeworm – hymenolepsis nana
Demo slides – giant liver fluke (Fasciola gigantica)
-liver fluke (Clonorchis sinensis)
-tapeworm scolex
-planaria digestive tract
-schistosoma
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 are triploblastic. In other words, their tissues are derived from the three germ layers: ectoderm (gives rise to their outer covering – the epidermis), endoderm (gives rise to the lining of the gut), and mesoderm (giving rise to the tissues in between). As in the Cnidaria, the gut is called 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. Many species of Platyhelminthese have defined structures at the head such as eyespots and auricles. There is no specialized circulatory, respiratory or excretory systems.
Flatworms are acoelomate. 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.
The Phylum is divided into
1. Class Turbellaria
2. Class Trematoda
3. Class Cestoda
Class Turbellaria – free living flatworms
Members of the Class Turbellaria are free living (not parasitic). Planaria (Genus Dugesia) lives in fresh water ponds and is a carnivore. A 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.
Exercise 1 - Planaria whole mount: Observe the Planaria prepared slides and draw what you see. In the whole mount, the animals has been fed particles of carbon, and these blacken the gastrovascular cavity. Behind the pharynx, the GV cavity divides into two lateral canals called intestines. These intestines are highly branched, because they serves also for circulation of nutrients to every cell. These smaller branches off of the intestines are called diverticula. Identify them in your slide and your drawing. Can you find the pharynx? Label it in your diagram. Identify and label the head of the Planaria, including the eye spots and auricles
Exercise 2 - Planaria representative cross sections: The most obvious section in your slide will be 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 cavityis comprised of a median tube in the body anterior to the pharynx; posterior to the pharynx it is divided into two lateral tubes. There are only lateral tubes of the gastrovascular cavity posterior to the pharynx, but no median one. The gut is continuous with the cells of the surrounding mesoderm so there is no coelom. 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.
Look at your transverse slide and identify the pharynx, the gastrodermis lining the pharynx, the median and lateral canals (i.e. intestines) of the GV cavity.
Class Trematoda – the flukes
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:
Exercise 1 – Clonorchis sinensis whole mount: this worm (shown below) is known as a 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. Look at your slide and draw what you see. Identify the structures listed above in your slide and on your drawing.
Class Cestoda – the tapeworms
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. So they do not have a gastrovascular cavity. They absorb nutrients across their body walls. Preserved tapeworms are available in jars in this lab.
Exercise 2 – Taenia pisiformis: this worm is known as a tapeworm. The tapeworm adheres to its host via a scolex. The scolex 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. 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.
Observe your slide of Taenia pisiformis and draw the head of the worm, a small immature proglottid and a more mature one further down the worm. Identify the parts of the scolex listed above. In the mature proglottid, identify the branched uterus containing hundreds of eggs.
Exercise 3 – Demo slides: Observe demo slides of the Giant Liver Fluke (Fasciola gigantica) and compare its structures to those of the liver fluke you just looked at. Identify the same parts you found in the liver fluke. Observe the demo slide of the scolex of a tapeworm and identify the suckers and the hooks. Finally, observe the slide of the trematode known as Schistosoma.
PHYLUM ANNELIDA – THE SEGMENTED WORMS
Models – earthworm
Prepared slides – earthworm (c.s)
-leech
Preserved samples – leeches
-Nereis clamworm
-sand worm
-scale worm
Demo slides – metanephridium
The Phylum Annelida, annelids, are segmented worms. They live in marine and fresh water and in moist terrestrial habitats; some are parasitic. They are bilaterally symmetrical, have a true coelom, and a segmented body. The cellular layer that lines both sides of the coelom is called peritoneum. The digestive system begins at the mouth at one end and runs to the anus at the other. The body consists of the three basic tissue layers; the nervous system has a brain; the blood vessels are closed tubes. Many kinds have gills for respiration, but some, like the earthworm, have no specialized respiratory structures. There are no hard parts, and the segmented worms are said to have a hydrostatic skeletal support.
The body of the annelid is segmented. The segments (or metamers) are easy to see on the outside of the worm, but inside these segments are formed by walls called septa.
Class Oligochaeta – the earthworm
The name refers to the few (oligo-) bristles (-chaeta) along the body of these earthworms and fresh water worms. Examine the earthworm genus Lumbricus. The dorsal or top side is slightly darker than the ventral side. Earthworms burrow through soil and eat decomposing plant and other organic material. The mouth end is pointed and conical, and the tail end is dorsoventrally flat. The anus is a vertical slit in the last segment. As you run your fingers along the sides of the preserved worm, you might feel the rasp of the setae (singular seta) or bristles. If there are living samples of earthworms, do this carefully!!
The earthworm has two layers of muscles: circular and longitudinal. The circular muscle layer runs around the body; when they contract the body narrows and the segments lengthen. This action is just the opposite of the longitudinal layer. Together they produce the kind of rippling motion characteristic of your own intestine (it likewise has two muscular layers) that is called peristalsis. In order for a worm to move forward through the ground, it contracts the longitudinal muscle layer in the posterior part and sticks the setae into the soil; this end of the worm becomes fatter and serves as an anchor to push against. The circular muscle layer in the anterior part of the worm contracts to thin and extend the front end of the worm; the setae in this portion are retracted.
Tiny excretory pores from the metanephridia can be found on the lateral or ventral surfaces of all segments except those at the ends. The earthworm has both ovaries and testes and is hermaphroditic. The openings of the oviducts can be seen on the sides of segment 14 and those of the sperm ducts, which have swollen edges on segment 15. The enlarged ring that begins at segment 31 or 32 and ends at 37 is called the clitellum. It is glandular and secretes a slimy mucus around two copulating individuals.
Exercise 1 – Earthworm cross section: Observe your slide of the earthworm known as Lumbricus. Draw what you see and try and identify the following structures: Epidermis, circular muscles, dorsal blood vessel, ventral nerve cord, intestinal lumen. Look for the typhlosole – a folding of the intestinal wall that increases the animal’s absorptive surface area. Does your slide have one?
Exercise 2 – Earthworm model: Observe the model of a typical earthworm. Identify the following structures: prostomium, pharynx, esophagus, crop, gizzard, intestine with the typhlosole, dorsal blood vessel, hearts, ventral nerve cord, metanephridia, testes, seminal groove, ovaries, clitellum, setae, circular and longitudinal muscles.
Exercise 3 – Earthworm Dissection: Examine your preserved earthworm and determine the anterior and posterior ends in addition to its dorsal and ventral sides. The ventral side will be slightly flattened and will have two openings types of openings. The openings toward the anterior of the worm are the male genital pores or sperm ducts. There are two pairs. The first pair is anterior to the second pair. The second pair is called the genital setae. They are tiny and are located just above the clitellum. They may be too small to see but may be visualized using a dissecting microscope. Sperm are produced in the testes and pass out through these pores. Just above the first pair of sperm ducts are the female genital pores. Again, these may be too small to see without a dissecting microscope. Eggs are produced in the ovaries and pass out of the body through these pores.
Locate the worm's mouth and anus. Near the mouth will be a fleshy protruberance called the prostomium. The prostomium is located on the dorsal surface of the worm. Note the swelling of the earthworm near its anterior side - this is the clitellum. Label the clitellum on the drawing to the right. The clitellum is active in the formation of an egg capsule, or cocoon.
Place a preserved earthworm dorsal side up in a dissecting pan. Anchor the anterior end with a pin through the tip, stretch the worm slightly, and put a second pin through the posterior end. Before dissecting, look for the following
Starting at the posterior end near the anus, use a scalpel or fine scissors to make a longitudinal, mid-dorsal incision through the skin; don't cut deeply. Continue the cut up the worm using fine scissors. This will ensure a shallow cut. Open the incision with your fingers or forceps. Pin the skin back on both sides so the internal organs are exposed to view.
Identify the following structures in your dissection: prostomium, pharynx, esophagus, crop, gizzard, intestine with the typhlosole, dorsal blood vessel, hearts, ventral nerve cord, metanephridia, testes, seminal groove, ovaries, clitellum, setae, circular and longitudinal muscles, pygidium with anus.
Reproductive System: The first structures you probably see are the seminal vesicles containing the testes. They are cream colored and located toward the anterior of the worm. These are used for producing sperm. The seminal receptacles of the female reproductive system may be seen attached to the wall of the worm next to these as a single cream colored structure. Sperm released by one worm will travel through a seminal groove on the body of another worm and enter through the female genital pores. The sperm will be stored in these receptacles and will be used to fertilize the eggs as they are laid and pass out through the female genital pores.
Circulatory system: The dorsal blood vessel appears as a dark brownish-red vessel running along the intestine. The hearts (or aortic arches) can be found over the esophagus (just posterior to the pharynx) and will look like enlarged blood vessels. Carefully tease away the tissues to expose the arches of the heart, the run across the worm. If you are careful, you can expose all 5 of them. The ventral blood vessel is opposite the dorsal blood vessel, and cannot be seen at this time because the digestive system covers it. It is found on top of the ventral nerve cord.
Label the diagram (use the bold words from above)
QUESTION: Does the earthworm have a closed or open circulatory system? ______
Digestive System: Use tweezers to remove the seminal vesicles from over the top of the digestive system that lies underneath it. The digestive system starts at the mouth. You will trace the organs all the way to the anus and identify each on the worm.
Find the mouth opening, the first part after the mouth is the pharynx. You may see stringy things attached to either side of the pharynx (pharyngeal muscles). The esophagus leads from the pharynx but you probably won’t be able to see it, since it lies underneath the hearts. You will two structures close to the clitellum. First in the order is the crop, followed by the gizzard. These are prominent structures that were initially seen below the seminal vesicles.The gizzard leads to the intestine which is as long as the worm and ends at the anus.