Lab 3:Diversity: Myxozoa, Digenea and Aspidobothrea


This lab begins your examination of parasites classified as Eumetazoa. The first group are the Myxozoa, which have traditionally been classed as protozoans, but recent studies suggest that they are multicellular and have close ties to the Cnidaria.

The second group covered is the Digenea. This is one of three labs in which you will learn about the Platyhelminthes, or flatworms. These are frequently-encountered parasites (either as adults or larval forms) of most groups of animals. The higher taxa of platyhelminths are distinctive and easily recognized, but the evolutionary history of the group has been difficult to study and there are several schools of thought regarding relationships among the higher taxa. The taxonomic scheme used in this course is a grossly simplified compromise. The Digenea and Aspidobothrea (covered in this lab) and the Monogenea (Lab 5), are given the common name "fluke" or "trematode". The Cestoidea (Lab 4) are given the common name "tapeworms".

The endoparasitic Digenea are one of the major classes of "flukes", and are covered here extensively. A description is also provided for one minor class, the Aspidobothrea, which is most closely allied to the Digenea. (You are not responsible for Aspidobothrea on the exams, but they are an interesting group worth taking a look at.)

The digeneans all have indirect life cycles, requiring 2 or more hosts and involving a variety of different larval forms. Unlike the monogeneans, adult digeneans are often more difficult to place in higher taxa without the aid of a key. This point will become apparent as you glance through the "related species" listed for each of the representatives you look at. As you examine the digenean material, observe how each species has solved the problem posed by requiring multiple host species which are often found in quite different habitats (e.g. terrestrial and aquatic).