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Flatworm Taxonomy
Phylum Platyhelminthes is made up of soft, wormlike animals with bodies that are elongated and dorso-ventrally flattened. Animals of this type exhibit several distinct characteristics that represent structural "advances" over the cnidarians. Members of this phylum and most of the rest of the phyla we will consider this quarter have:
The flatworms are very diverse in their lifestyles. Each class shows interesting specialization to a particular way of life. Class Turbellaria includes both freshwater and marine species. Most members of this class are free-living, have an unusually located mouth and a ciliated epidermis which aids in locomotion. Turbellarians are the simplest animals yet known that have a brain and central nervous system, a well-developed excretory system and a complex reproductive system. The genus Planaria is the only turbellarian that we will examine in detail, but it is very typical of the lot. The whole mount microscope slide at your table contains two planaria. One specimen is specially stained to show the pharynx and digestive cavity. Look closely at the other specimen to see the faint ganglion and nerve cords. The living Planaria that you will examine were purchased from a biological supply company, but could easily have been collected at Dow Lake, or from local ponds or creeks.
Class Trematoda are all internal parasites and commonly referred to as flukes because of their leaf shape. They have suckers for attachment and the adult usually feeds on the blood of its host. This class is characterized by a complex hermaphroditic reproductive system and a life cycle that includes both intermediate and final hosts. Trematodes, like most internal parasites, are reproductive "machines" and are made even more efficient by the fact that the larval stages are able to reproduce asexually, thereby creating even more new individuals. A type of fluke infecting humans, Clonorchis, will serve as a representative genus. You need not memorize the names of each stage of the life cycle on demonstration, but do take the time to study the life cycle from the production of the egg to the adult and the various hosts that are utilized. You will find a whole mount slide of adult Clonorchis in the box at your table. The adult anatomy is described in Buschbaum and there is a micrograph revealing its innards on p. 19 1, but you will need the sketch on the back of one of the white handouts for this week to help you label your drawing of the adult fluke.
Class Cestoda (Cestoidea), the tapeworms, are also internal parasites.
The adult is usually found in the intestine of a vertebrate, absorbing
nutrients that have already been digested by the obliging host. Because
of this, the tapeworm needs no digestive system of its own, but it does
need a specialized epidermis to resist the host's digestive enzymes that
constantly attack it. Most tapeworms are species-specific, that is, they
are particular about the type of vertebrate they utilize. The life cycle
is usually complex and involves both vertebrate and invertebrate hosts.
The tapeworm appears segmented, even without the aid of a microscope. Each
of the segments is called a proglottid, and is basically a reproductive
machine containing both male and female organs. Each proglottid will eventually
become "ripe" with hundreds of fertilized eggs. The proglottid will then
break off from the parent and be passed out of the digestive tract of the
host. The ripe proglottids are commonly seen attached to the fur around
the tail of an infected animal or in its feces and so make their way into
the world to find the next host. Dipylidium, one type of tapeworm
that infects dogs, is the representative genus in lab. Find the slide of
the adult tapeworm in the box at your table and hold it up to the light
to reveal that there are representative segments of the worm mounted on
the slide. You will need to move the slide on the stage to find the very
narrow "head " end, an immature proglottid and also a mature proglottid
to draw and label. Dipylidium is so similar to Taenia that
you can use the diagrams and photos of Taenia in Buschbaum to help
you label your drawings. A sketch of Dipylidium can be found on
one of the white handouts for this week.
Planaria Behavior
At your desk you will find a dish filled with spring water and a few live Planaria of Class Turbellaria. Get a stereoscope from the cabinet and place the dish on the stage so that you can get a magnified look at the beasts. Follow the movements of one individual and note its size and general shape. Locate the head, the eyes and the sensory lobes (lateral projections on the head). The simple eyes are light receptors only and cannot form images. The sensory lobes are equipped with both touch and chemical receptors. Observe the smooth gliding motion of the worm. It is due to the action of cilia on the ventral surface of the body, coordinated with muscular contractions of the body. Note how the head and sensory lobes behave during locomotion. Recall from Chap. 9, Buchsbaum, that there is a nerve ganglion within the head that is functioning as a primitive brain to coordinate sensory input and movement. Place the tip of a clean toothpick in the path of the worm and note its reaction. Gently turn the worm over on its back. How does it react? Compare these to the movements of a cnidarian and draw conclusions about the possible advantages of having a head end with a concentration of nervous tissue and sense organs.
In the front of the room there is a container of individuals feeding
on fresh beef liver. Observe two or three of these for 10 minutes and note
any patterns in their behaviors. Can you glimpse the very white pharynx
protruding from the mid-ventral surface while feeding?
KINGDOM ANIMALIA: Phylum Platyhelminthes
Class Turbellaria