Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes - Summary of Taxonomy
The Phylum Platyhelminthes is conventionally divided into four Classes, these being;
Class Turbellaria
The turbellarians have long been recognised as a class, but it becoming clear from recent research (based on ribosomal RNA sequences and reassessments of morphological features) that the class is not really monophylectic. However the grouping is still, and until a new taxonomy has been established, will continue to be used as a convenient way of grouping a large group of mainly free living platyhelminthes. The vast majority of the turbellarians are free living organisms, mostly living in water, either marine or freshwater. There are exceptions to this general rule however, with terrestrial representatives, particularly the planarians. In addition there are many turbellarians that have close symbiotic, commensal or mutualistic associations with other organisms, particularly with other invertebrates, although some may act as ectocommensals to marine vertebrates such as turtles. These commensal and parasitic forms are very similar to their free living cousins, but may have adaptations to their epidermis such as the formation of a syncytium, loss of rhabdoids, mucus glands and cilia, and a loss or reduction in their eyes. Apart from these parasitic and commensal representatives they are mostly predacious, although some may feed on algae, and many feed on diatoms as juveniles, before becoming fully carnivorous as adults. The Class Turbellaria are usually further classified into nine Orders. These may be divided into two types;
Firstly the are the Archoophoran turbellarians. In these vitelline glands are absent, and they are often thought of as being more primitive organisms. they include;
Order Acoela - Almost all small, (less than 2mm long) free living marine organisms. Some may act as symbionts with other invertebrates, and a few members have symbiotic algae living beneath their epidermis. The most famous example of this is found with Convoluta roscoffensis, on the channel coast of France. This species of acoel turbellarian is coloured green by the photosynthetic flagellate Tetraselmis convolutae. There is no digestive cavity or gut, and no distinct gonads, the gametes originating directly from the mesenchyme.
Order Catenulida - A rhabdocoel turbellarian (gut a simple sac). A mainly freshwater group, with some marine representatives. This is a relatively aberrant group, with a ciliated, sac-like intestine, simple pharynx, and unpaired gonads.
Order Macrostomida - A rhabdocoel turbellarian. Freshwater or marine turbellarians. These have a simple ciliated sac-like intestine, and a simple pharynx. Interestingly it is these, rather than the more primitive Acoela, that are thought to be closest to the ancestral turbellarians.
Order Polycladida - A rhabdocoel turbellarian. These are mostly free living marine flatworms, with a few commensal, and a few freshwater representatives. They have greatly flattened, oval, often brightly coloured bodies, usually ranging from between 3 to 20mm in length. They often have numerous eyes
The other main division of turbellarians are the Neoophorans. In these vitelline glands are present, and they are often thought of as more developed organisms.Order Lecithoepitheliata - An alleocoel turbellarian with a relatively simple sac-like intestine. Marine and freshwater species. Penis in the form of a stylet.
Order Prolecithophora - An alleocoel turbellarian. Marine and freshwater species. Have a bulbous pharynx.
Order Neorhabdocoela - A rhabdocoel turbellarian. A highly diverse group, with, apart from many free living representatives, many commensal species, particularly in other invertebrates and some true parasites in marine crustaceans. These have a bulbous pharynx.
Order Temnocephalida - A rhabdocoel turbellarian. Commensal flatworms of marine crustaceans and vertebrates such as turtles. They cling to their hosts using an adhesive disc on the posterior ventral surface and adhesive tentacles.
Order Seriata - This is an important group, divided into two suborders, (in some classifications these are raised to the level of orders in their own right). The pharynx is folded and the gut is alleocoel in form, with many diverticulae.Suborder Proseriata - marine and interstitial species. In these the gut is not branched.
Suborder Tricladida - The planarians. These are mostly free living, often actively predacious, flatworms. In Great Britain and Ireland introduced species such as Bipalium kewense and Artioposthia triangulata are highly efficient predators of earthworms, and may become important pests.
The remaining classes of platyhelminthes are all parasitic in nature. On the basis of a number of features including morphological features, most strikingly a syncytial tegument outer surface, (a "neodermis"), and molecular studies using RNA and DNA analysis, they have all been shown to be monophylogenetic. On this basis many taxonomist place them all within a taxon "Neodermata", but as this is not yet generally used by parasitologists, and there are still disputes as to the need for this taxon, the conventional classification will be used in these pages. Furthermore within these parasitic groups there appear to be two distinct lineage's, that of the monogeneans, cestodarians and eucestodes, and another forming the trematodes.
Class Monogenea
In common with the Trematodes and Cestodes, these are thought to have evolved from rhabdocoel-like turbellarian ancestors, similar to the Neorhabdocoela and Temnocephalidans. They are fluke-like, parasitic (usually ectoparasitic) organisms. They have a single host in their lifecycle, the group deriving its name from the one generation present in its lifecycle. This is host is usually a fish, but other organisms such as amphibians, squid, copepods, and higher vertebrates such as turtles and even the hippopotamus may be affected. This Class is further divided into two Sub Classes, based on the morphology of their attachment organs. These are;
Sub Class Monopisthocotylea - a more "primitive" group, feeding by browsing on the skin of their hosts. Their attachment organ, or opisthaptor, is equipped with simple hooks.
Sub Class Polyopisthocotylea - These parasites feed on the blood of their hosts, often being found attached to highly vascularised tissues such as that found in the gill chambers of fish. Their opisthaptors are much more complex, being equipped with suckers and clamps as well as hooks.
For more information go to the taxonomy page within this site. These appear to have arisen from ectoparasitic rhabdocoel turbellarians, and some monopisthocotyleans still possess a protrusible pharynx for feeding on the epidermis of their hosts.
Class Cestoda
These are endoparasites, and lack either a mouth or gut. They usually have two or more hosts, and the adult form of the parasite is found in the vertebrate intestine. There are two Sub Classes of Cestodes;
Subclass Cestodaria - These are non-segmented parasites, with only a single set of sexual organs and no scolex. They are parasites of fish and other cold blooded vertebrates. Although they are grouped within the Cestodes, the evolutionary connection between the Cestodarians and Eucestodes is not clear, and they may only be very remotely related. However on the basis of a number of features such as the type of tegument, and a free swimming larvae possessing hooks (a lycophore), it is thought by many that they form an intermediate group between the monogeneans and the eucestodes. There are two Orders of parasites placed within this group, although in fact they may not be even that closely related. These are the order Amphilinidea and the order Gyrocotylidea.
Subclass Eucestoda - The tapeworms. These are segmented parasites, each segment (proglottid), containing both male and female sexual organs, (though these may mature at different rates). They can be divided into a number of Orders on the basis of such features as the structure of the scolex and arrangement of sexual organs. The adult forms are found in the intestine (or more rarely bile duct) of, mainly, vertebrates. Only the Pseudophyllidean and Cyclophyllidean parasites are of major interest as tapeworm parasites of man. For more information on this group, go to the Cestode Biology page within this site.
Class Trematoda
These are the flukes. All are parasitic with complex lifecycles utilising one or more host organisms. If more than one host is involved, the first host in the lifecycle is always an invertebrate mollusc. In contrast to the monogenean/cestodarian/cestode lineage of parasites the attachment organ is a sucker or suckers, without hooks or hooklets. The Class Trematoda may be further divided into two Sub Classes;
Sub Class Aspidogastrea - Similarly to the digeneans the first, and in most cases only, host is a mollusc, some species having second hosts, a fish or turtle that may be infected buy ingestion of the first host. The attachment organ is either a large divided sucker, or a row of suckers, arranged on the parasites ventral surface. These organisms are considered by many to be the most primitive of the parasitic platyhelminthes (Neodermata). This is shown by their relatively poor adaptation to parasitism, as evidenced by low host and organ specificity's, highly complex nervous system including many sensory organs, and very long survival time outside their hosts.
Sub Class Digenea - Usually with two or more hosts in their lifecycles. For more information on this group, go to the Digenean Biology page within this site. They probably evolved from rhabdocoel turbellarian commensals or parasites of molluscs, as the first intermediate host of these organisms is always a mollusc. It has been noted that one of the lifecycle stages of the digenean trematodes, the redia, bears some resemblances to some of the turbellarians, and it has been hypothesised that the trematodes as a group evolved from a turbellarian ancestor that had a single molluscan host.