The Phylum Platyhelminthes is composed of both free-living and parasitic species. The origin of these diverse lifestyles plays a significant role in the evolution of the Platyhelminthes. We demonstrate that parasitism developed into two distinct clades within the phylum. Our interpretation of the phylogram challenges the hypothesis that parasitic species arose independently of each other. We studied 12 parasitic genera and 8 free-living genera. We identified each one with 20 characters. From a cladistic analysis of the 20 genera studied, we have divided the Platyhelminthes into two parasitic classes and one free-living class.

Platyhelminthes are invertebrate Bilateria without a coelom or a definitive anus (Hyman, 1951; Russel-Hunter, 1968). Free-living, commensal, and parasitic organisms exist within the phylum. The dominant taxonomic classification for the phylum Platyhelminthes includes three classes, Turbellaria, Trematoda, and Cestoda (Brown, 1956; Bullough, 1960; Kaestner, 1967; Kershaw, 1983; Hyman, 1951). Other sources such as Margulis and Schwartz (1998) and Crespi et. al (2000) discuss the possibility of a four-class system in the phylum, but this is not as widely accepted as the three-class system. Holt (2000) and Littlewood et al. (1999) also suggest a separate taxonomy that contains five classes.

Predominantly free-living animals comprise the Class Turbellaria, but the class also includes a few ecto- and endocommensals (Kershaw, 1983; Hyman, 1951). Class Trematoda and Class Cestoda contain strictly parasitic Platyhelminthes (Corwin, 1997; Kershaw, 1983; Hyman, 1951). Through our investigation of the Platyhelminthes, we have questioned the origin of parasites within the phylum.

Kershaw (1983) proposes that the two parasitic classes evolved independently from the turbellarians early in the evolution of the phylum. An early separation of the two phyla would result in many extensive departures from the normal morphology that can be seen in some of the more advanced parasites (Hyman, 1951; Kershaw, 1983). Hyman (1951), and Kershaw (1983), noted morphological changes such as the presence of a cuticle, organs of adhesion, reduced digestive systems, and a loss of sense organs as an indication of an early branch made by the parasitic organisms.

Although a majority of biologists have concluded that each class of parasites evolved from a single branch, Barrington (1967) states that the parasitic forms may have evolved more than once within the phylum. Similar modifications to those listed above, including creeping progression, and organs of adhesion, may have evolved in the free-living forms and contributed to the individual evolution of the parasitic Platyhelminthes (Barrington, 1967; and Russel-Hunter, 1968).

The purpose of this paper is to investigate the evolutionary relationship between the free-living and parasitic organisms in the Phylum Platyhelminthes.

Organisms Examined -
We performed a cladistic analysis on 20 genera from the Phylum Platyhelminthes (Table 1). Our selected genera include several examples from each of the three classes of Platyhelminthes, Turbellaria, Trematoda and Cestoda. We obtained the descriptions of the genera examined from the sources listed in Table 1.

Table 1. The following table identifies the 20 genera used in our cladistic analysis of the Phylum Platyhelminthes. The table lists the particular literature sources used to designate the character states of each individual genera. Literature sources used for reference are as follows: Br = Brown (1950), Bu = Bullough (1958), C = Corwin (1997), Ka = Kaestner (1967), Ke = Kershaw (1983), RH = Russell-Hunter (1968).

Table 2. The following table identifies the characters used to perform our cladistic analysis of the Phylum Platyhelminthes. Each of the states for the individual characters are identified by a specific number, used to represent that state in the character-taxon matrix. The characters and character states listed here were used in the construction of our phylogram (Figure 1).

After altering Figure 1 to produce the most parsimonious tree, the resulting phylogram produced three clades. Clade A includes the Trematodes, Clade B includes the Cestodes, and Clade C includes the Tubellarians. The most parsimonious tree is displayed in Figure 1 with an actual tree length of 55. The statistical maximum and minimum tree lengths were 116 and 30, respectively.

Figure 1. This phylogram represents the most parsimonious tree derived from our character x taxon matrix. Clades A and B represent the evolution of the two parasitic classes, Trematoda and Cestoda respectively, from the primitive ancestor. The third clade, Clade C, shows the free-living Turbellarians and their close relation to the primitive ancestor.
 

Proposed Phylogeny -
The results of our phylogram show the evolutionary development of the Phylum Platyhelminthes. Three separate clades appear to have developed from an original ancestor. While this ancestor can not be determined to be free-living or parasitic, it was most likely a free-living ancestor (Margulis and Schwartz, 1992; Kershaw, 1983; Hyman, 1951).

The free-living Platyhelminthes were most likely reduced into the parasitic species of this phylum. It is not likely that the development of multiple advanced traits could develop in a single organism. This is what would have occurred if the free-living flatworms developed from the parasitic flatworms. Thus, parasitic organisms in this phylum most likely developed from the free-living species.

Clade C shows the evolution of the common ancestor into free-living and commensal Platyhelminthes. Ciliated epidermal cells, presence of eyes and a gut, direct development, a length of 2-10 mm, and a gliding form of locomotion are primitive states since they are present in all genera in Clade C. These states are most likely carried over from a common free-living ancestor.

Clade A and Clade B show the evolution of the common ancestor into a separate evolutionary branch of parasitic flatworms from the free-living organisms. Parasitism, represented by Clades A and B, diverged from the free-living Clade C through the reduction of the common ancestor’s primitive states. Clade A and Clade B contain genera that have lost cilia, eyes, and a gliding form of locomotion. These clades also diverged from the free-living branch through the development of suckers, a cuticle, and a larval stage in their life cycle.

Parasitic Clades A and B divided into two separate branches during their evolution. The defining divergence occurred with the reduction of the gut of a common parasitic ancestor. Clade A genera kept the primitive state of a blind gut and a pharynx while Clade B genera are a reduced form of parasitic flatworms without a gut or pharynx.

Polychoerus and Polycelis are observed to evolve from the common ancestor separately from Clade A, B, or C. A reduction of the primitive state seems to have given rise to both of these branches of Platyhelminthes. Organisms within Polychoerus do not possess a gut or a pharynx. Polycelis flatworms do not possess eyes or a pharynx, have limited nervous centralization, and are only two millimeters in length. Polycelis also deviated from the common ancestor, exhibiting a larval developmental stage and a sucker.

Figure 2. The phylogeny of the different classes of the Platyhelminthes is believed to have evolved as shown in the figure above. This figure illustrates that the classes separated at an early stage in the evolution of the phylum, and developed characteristics that best suited their lifestyles.

Proposed Taxonomy -
Our taxonomy is based on the manner in which different genera developed within the Phylum Platyhelminthes. The proposed taxonomy shows a correlation to the taxonomy of Margulis and Schwartz (1992), Kershaw (1983), Hyman (1951), and Russel-Hunter (1968) because of its division of the phylum into three classes. Figure 1 supports this classification

The phylogram separates into three major branches as simplified in Figure 2. The taxonomy reflects this branching by maintaining a three-class system. The division of the phylogram illustrates the separate evolution of the genera contained in each clade. The three clades separate into two parasitic and one free-living class supporting our proposed taxonomy. Clade A shows the separate evolution of Trematodes into a class. Clade B illustrates the separate evolution of Cestodes into another class. Clade C indicates the evolution of the Turbellarians into a third separate class.

Polycelis and Polychoerus are classified within Class Turbellaria. Turbellarians along with organisms within Polycelis and Polychoerus all possess cilia, one epidermal layer, nerve cords, flame cells, and both sets of reproductive organs (Hyman, 1951).

Table 3. Our proposed taxonomy is based on the results of our cladistic analysis of the Phylum Platyhelminthes. It consists of the 20 genera used in our experiment. (See Table 1 & 2 and Figures 1 & 2).

Kingdom Animalia
Phylum Platyhelminthes
    Class Turbellaria
        Convoluta, Dendrocoelum, Dugesia, Mesostoma, Polychoerus, Polycelis, Stenostomum, Temnocephala,
    Class Trematoda
        Aspidogatrid, Cryptocotyle, Entobdella, Fasciola, Gorgodera, Opisthrochis, Polystomum, Schistosoma
    Class Cestoda
        Diphyloobothrium, Taenia, Procerodes, Lymaea

Barrington, E. J. W. 1967. Invertebrate structure and function. Houghton Mifflin Company. Boston. 503-506.

Brown, Jr., Frank A. 1956. Selected invertebrate types. John Wiley and Sons, Inc. New York.

Bullough, W.S. 1960. Practical invertebrate anatomy. St. Martin’s Press: New York. 47-75.

Corwin, Robert M., and Julie Nahm. 1997. Phylum platyhelminthes. http://web.missouri.edu/~vmicrorc/Platyhelminths/Platyhel.htm Date accessed: 14 February 2000.

Crespi, Erica, Mark White, Melanie Shadoan, Ginger Fisher, Andrea Schwandt, Ruth Boada, Shelley Johnson, and Roy Fuller. 1996. Phylum platyhelminthes. http://www.wfu.edu/~mackgr5/platy.htm Date accessed: 10 February 2000.

Holt, Jack R. 2000. SUPPLEMENT: A CLASSIFICATION OF THE ANIMAL KINGDOM AND DESCRIPTIONS OF PHYLA. Journal of Systematic Biology. 7(1): http://www.susqu.edu/facstaff/h/holt/Systematics/Animals/ Date accessed: 24 February 2000.

Hyman, Libbie Henrietta. 1951. The invertebrates: Platyhelminthes and Rhynchocoela the acoelomate bilateria 2. McGraw-Hill Book Company, Inc. New York. 52-422.

Kaestner, Alfred. 1967. Invertebrate zoology. Interscience Publishers. New York. 154-200.

Kershaw, Diane A. 1983. Animal diversity. University Tutorial Press. Great Britain. 59-76.

Littlewood, D.T.J., K. Rohde, R.A. Bray, and E.A. Herniou. 1999. Phylogeny of the Platyhelminthes and the evolution of parasitism. Biological Journal of the Linnean Society. 68(1/2). 257-287.

Maddison, W.P. and D.P. Maddison. 1992. MacClade: Analysis of Phylogeny and Character Evolution. Sinauer Associates, Inc., Sunderland.

Margulis, Lynne, and Karlene V. Schwartz. 1998. Five kingdoms. W.H. Freeman and Company. New York. 228-229.

Marshall, A.J., and W.D. Williams. 1972. Textbook of zoology: invertebrates. The MacMillan Press Limited. New York.

Russell-Hunter, W. D. 1968. A biology of lower invertebrates. The Macmillan Company. New York. 66-80.