Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes - Digestive System
The digestive systems of different platyhelminth groups vary in complexity, and may be used to classify different groups of these organisms (in conjunction with differences in their reproductive systems). They all however have a number of features in common. In all of these groups, where the organism has a gut, this has only one opening, (i.e. they have a mouth, but do not have an anus).
Parasites of the Class Cestoda lack a digestive system entirely, nutrients being absorbed through the outer tegument. In this case it is probable that the ancestral groups of cestode had digestive systems, but these have degenerated and been lost over time.
The simplest digestive systems are those found in organisms of the Order Acoela (Class Turbellaria), with an acoel digestive system. Here there is a mouth, which opens, occasionally through a pharynx, into a mass of loosely packed cells. This group is very primitive, the majority of them being very small (only a few millimetres long), lack an excretory system, and live among sand grains in the sea, where they behave in a similar way to the ciliated protozoans that share they environment. 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.
The next order of complexity is found with the mouth and pharynx attached to a straight unbranched gut. This is referred to as a rhabdocoel gut, and is represented in three Orders within the Class Turbellaria. These are the Order Catenulida, a mainly freshwater (some marine) group of flatworms, the order Macrostomida, found in both freshwater and marine environments, and the Order Neorhabdocoela. This last Order is highly diverse in its required habitats, with free-living forms found in both freshwater and marine environments, as well as terrestrial forms, and many commensal and ectocommensal representatives.
The alleoecoels have a similar type of gut, although in this case the cavity is larger, and in some groups may have many outpockets. These are mostly free-living organisms, belonging to three orders within the Class Turbellaria, the Orders Lecithoepitheliata, Prolecithophora and Seriata.
Next in order of complexity are those Platyhelminthes with guts that have two branches only. This level of organisation is found in the tree Classes of parasitic Platyhelminthes, the Monogeneans, the Aspidogastreans and the Digeneans. Often the gut is unbranched, although in some species, such as Fasciola, the gut may have many diverticulae.
The planarian turbellarians all belong to the Order Tricladida, and have a highly diverticulated, three branched gut, which is connected to the mouth by a highly muscular, tubular pharynx. These planarians feed either on the dead bodies of other animals, or in many cases may be active predators. In the case of the predators, they attach to their prey, entangling it in slimy secretions, and press it down on the substrate using their highly muscular bodies. The pharynx is then protruded through the mouth, releasing digestive fluids that aid the planarian as it tears the body of the prey into pieces, which are then swallowed. The planarians are highly effective predators, and introduced species such as Bipalium kewense and Artioposthia triangulata are important predators of earthworms in Great Britain and Ireland.
Finally the polyclads have highly branched guts. These organisms belong to the single Order Polycladida. These are mostly free-living marine organisms, though there are a few commensal groups.
