Parasitism: Parasitic Flatworms (1963)

and kills another for food sometimes both species actually benefit one another termites for example eat wood but cannot digest it if we examine the intestinal contents of a termite under the microscope we can see Tiny organisms living there these flaget are able to transform bits of wood into usable food a mutually beneficial relationship like this is called mutualism but there are many smaller organisms that live at the expense of another animal such as this rat in a relationship called parasitism to see some of these parasites we remove the intestines of the Rat the small intestine is placed in a salt solution the large intestine is placed in another salt solution the parasites are found in the cavities of both large and small intestines where they get their food from the host's intestinal contents if we cut the small intestine along its length the parasites and partially digested food are released into the salt solution these are tapeworms similar to those sometimes found in man's intestine from observations of living tapeworms and other parasites biologists have developed a theory to explain the origin of parasitism in flatworms it is thought that at one time all flat worms were somewhat like the modern planaria active free living animals with sense organs nerves and muscles such an animal had a mouth and digestive cavity this plenaria is feeding on a piece of fresh liver it is capable of discovering food feeding on it and digesting it at some time in the past some flat worms attached themselves to the bodies of other animals such as this king crab the worms in this case took up housekeeping on its gills in addition to transportation and shelter they were able to obtain food from material on the surface of the crab gradually over a great many generations some flatworm species such as the worm on this fish's fin developed special structures to maintain contact with the selected host suckers on the tail end of the worm securely grip the fin rays the head end of the worm still has eyes much like those of the plenaria such flat worms are external parasites but an external existence is somewhat precarious other species eventually strayed into one of the cavities of the host such as the lungs of this common grass frog to see these parasites we remove the lung and wash it in salt solution to clear away the blood in a dish of clean solution we open the lung the flukes along with some round worms become more active when released from the lung species adapted to an internal environment are internal parasites under the microscope you can see the strong sucker surrounding the mouth the problem of getting their offspring established in new hosts is much more difficult than that of external parasites this problem has been solved in a number of ways this lung fluke like most flukes contains both male and female sex organs the dark brown coil contains eggs ready to be released the light brown structure at the tail end are testes and contain sperms other flukes adapted to living in the ducts of the liver of mammals like its relatives this liver fluke has suckers for holding itself in place the nervous systems of flukes are sufficiently welldeveloped to enable the fluke to move about the male and female sex organs of the liver fluke occupy most of its interior the ovary releases eggs which are fertilized by sperms from the testes the fertilized eggs are stored in the uterus until released to man perhaps the most important of these internal parasites is the blood fluke which has separate Sexes this is the male and this is the female blood fluke living in pears these parasites flourish in China Japan and parts of South America Asia and Africa draining the energy of the people mature male and female flukes live together in the small veins of the intestinal and pelvic regions of man a fertilized egg aided by secretions of the embryo inside Works its way out of the blood vessels through the w of the bladder or intestine and into one of these organs from there it escapes with the feces or the urine once in water the embryo becomes active and frees itself from the egg membrane then swims through the water of the pond or stream if it does not find a specific kind of snail quickly the larvae dies however if it does find such a snail the embryo bores into the soft exposed foot of the snail the intermediate host inside the snail the larval fluke continues to develop eventually one of these forms the ciria bores out of the snail into the pond water here you can see thousands of these Circas so extravagant is the production of these Circas it is likely that a few will find their way to human hosts working in the shallow water of rice fields millions of people are infected this way each year a ciria touches human skin it bores in moving through the skin tissue until it finds a blood vessel if it enters the blood vessel it will be carried along with the blood throughout the body until it reaches the intestinal and pelvic veins where it develops into a mature fluke the presence of these parasites in man causes anemia and often death mice are used extensively to study these worms in the laboratory here the Tails of anesthetized mice are exposed to living circar just as the man's feet were exposed in the pond after a period of development the mouse is opened the flukes can clearly be seen in the animal's veins to remove the worms a needle is inserted in the aorta of the mouse and a salt solution is forced through its blood system the worms are washed out of the liver along with the blood the worms can then be collected for further study here again the male and female are ready to mate and produce eggs and begin this complicated life cycle once again tapeworms are another kind of internal parasite all of these tapeworms were removed from the small intestine of a single vertebrate host a typical tapeworm has a long ribbon-like body and like flukes has suckers for holding on to the lining of the intestines of its host the beef tapeworm has four of these suckers at the head end but like all tapeworms it has no mouth or digestive system it needs none for tapeworms live in the host intestines where they absorb already digested food through their thin skin or cuticle behind the head is a short growing region from which a series of body sections called PRL s are constantly budded off each mature section or proglottid has both male and female sex organs in it the male sex organs produce sperms the female sex organs produce eggs thus each mature pgd is capable of producing fertilized eggs these fertilized eggs are stored in the uterus here seen in this stained section since passive modes of transfer are very hazardous only parasites which produce enormous numbers of potential young can survive this kind of life cycle and the end proglottids of the tapeworm are hardly more than bags of eggs these end proglottids gradually become more active eventually the end proglottid breaks loose and is carried out of the host with the feces the freed proglottid crawls away from the voided feal material eventually dries up and finally ruptures releasing its eggs if eggs of the human beef tape worm are eaten by a cow they will hatch in the cow's intestine the cow thus serves as the intermediate host in the life cycle of tapeworms in the cow's intestine the embryo in the egg will develop and escape from the egg membrane this larvae enters the intestinal tissue finds a blood vessel and is carried by the bloodstream to the muscles of the cow there it establishes itself among the muscle fibers growing into a cystlike organism called a bladder worm or cyto circus this piece of fresh beef shows such a cyst or measel meat infected by these larvas is called measly beef here we see the further develop velopment of the parasites if uncooked beef containing these bladder worms is eaten by a human the mature worm will develop in the person's intestine causing disease aside from cooking the best way to prevent parasites from infecting humans is to interrupt the normal parasitic life cycle meat for example is inspected for bladder worms by allowing only meat that is free from larvas to go to market the life cycle of the tapeworm is interrupted research to learn more of the details of these relationships and the life cycles of parasites continues at a constantly increasing Pace in Laboratories around the world man's aim is to eliminate parasitism as a human disease and to control it in his domesticated animals but parasitism is an involved and important part of nature in terms of actual numbers the animal kingdom has many more parasitic organisms than Free Living individuals studies continue to help us understand the parasitic way of life in all of nature [Applause]