Biology 625
ANIMAL PARASITOLOGY
Fall semester lecture note outline

Updated: 24 September 1999

Characteristics of the phylum Platyhelminthes

1. bilaterally symmetrical; dorso-ventrally flattened
2. acoelomate
3. cannot synthesize fatty acids or sterols de novo
4. tegument (living external layer)
5. digestive tract incomplete or absent
6. excretory system protonephridia

The phylum Platyhelminthes consists of four classes (Note: not all taxonomists would agree with this simplified and traditional classification. In some schemes, the monogenes are not grouped separately and are more closely aligned with the cestodes. Different names can also be found for some taxa. However, the scheme below is highly useful for an undergraduate course)

1. CLASS: Turbellaria (you will not need to know species within this taxon)
   1. mainly free-living flatworms
   2. some as symbionts associated with echinoderms, molluscs, fish, cnidarians, etc. Although the majority are thought to be commensals, some are truly parasitic.
   3. few are commensals or parasites of invertebrates
   4. free-living examples include Planaria spp. in aquatic environments and the giant terrestrial land planaria in the genus Bipalium spp.; commensal/parasitic forms include Syndesmis spp. in the intestines of sea urchins, Bdelloura spp. on the gills of horseshoe crabs, and Stylochus frontalis in the valves of oysters.

2. CLASS: Monogenoidea (Monogenea)
   1. all parasitic
   2. most species on fish gills or skin; however, some internal in urinary bladder, nasal passages, cloaca
   3. one host life-cycles
   4. evolutionarily, appears to be more closely related to cestodes than trematodes
   5. newer revision of monogenes has been proposed based on cladistics (1993, Syst Parasitol 26(1): 1-32)
   6. two main "traditional" subclasses for this class
      1. Subclass: Monopisthocotylea (predominately the Polyonchoinea)
      2. Subclass: Polyopisthocotylea (predominately the Polystomatoinea and Oligonchoinea)

3. CLASS: Trematoda
   1. all parasitic
   2. mainly in digestive tract
   3. most with suckers
   4. most with 2-more host life cycles
   5. 3 subclasses
      1. Subclass: Digenea (typical flukes)
      2. Subclass: Aspidogastrea (no need to learn this taxon)
      3. Subclass: Didymozoidea (no need to learn this taxon)

4. CLASS: Cestoidea
   1. all parasitic; tapeworms
   2. most with 2-more host life cycles
   3. no digestive tract
   4. most segmented
   5. systematics of this group has been changing rapidly; many taxonomic schemes and varying names/status of higher taxa (especially if monogenes are added)
   6. Main groups
      1. Subclass: Gyrocotylidea (no need to learn this taxon)
      2. Subclass: Cestoidea
         1. Superorder: Amphilinidea (no need to learn this taxon)
         2. Superorder: Eucestoda
            1. Infracohort: Pseudophylla
            2. Infracohort: Saccouterina