Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes -The Outer Surface
The outer surface of Platyhelminthes varies to some degree, but by comparison between different Platyhelminth groups, shows a development towards the syncytial tegument seen in the Digenean and Cestode parasites.
1) The Outer Surface of a Typical Free-living Turbellarian
Here the outer surface is made up of a single layer of epithelial cells. These are equipped with many cilia, particularly on the ventral surface, where they provide motility for the organism, especially in the smaller species, although muscular undulations are probably more important for the larger land planarians. Other sensory cilia may be found, projecting from sensory cells within the epidermis. Other features present on the external surface include the openings of numerous gland cells, the cells either located within the outer epidermis, or buried within the mesenchyme, with their necks pushed through the outer layers. These gland cells secrete a number of substances, which may be used in trapping their prey, as an adhesive substrate, or to aid in the motility of the organism. Finally, in some turbellarians nematocysts (stinging cells) may be found. Interestingly these are not made by the worm itself, but are derived from hydroids which the worm has eaten. The nematocysts are in this case not digested, but are transported from the gut to the epidermis, where they are then recycled for the organisms self defence. Within the outer epidermis itself are numerous rod shaped bodies called rhabdites. These are thought to be used, either in the organisms defensive mechanisms, or to secrete a slimy coat over the animal. Finally in some marine species the outer epidermis may be armed with numerous calcareous spicules, which may have a structural role Below the epidermis are layers of outer circular and inner longitudinal muscle fibres.
2) The Outer Surface of a Parasitic Tubellarian Flatworm
ome of the parasitic and symbiotic turbellarians have an ciliated epidermis similar to their free living relatives. However in many others the features of the outer surface may be seen. These features include a syncytial outer layer, with the nuclei present in sunken epidermal cell bodies, connected to the syncytium by cytoplasmic connections. However none of the turbellarians possess outer surfaces entirely homologous to the parasites teguments. Probably the closest similarity is seen in an ectosymbiont of crustaceans, Genostoma kozloffi. Here most of the body is covered by a syncytial, non-ciliated layer with cytoplasmic connections to insunk nucleated cell bodies, whilst on the ventral surface is a layer of ciliated epidermis with insunk cell bodies very similar to the outer surface of many larval parasitic platyhelminthes.
The formation of a syncytium, with the nuclei present in the bodies of sub-tegumental cells deep with the body of the organism, may well be an adaptation to a parasitic mode of existence. Placed here they are protected against a potentially hostile environment, where the outer surface of a parasite must withstand the onslaught of its hosts immune system. The nuclei and many of the mitochondria then be able to renew the outer surface of the organism at a greatly reduced risk, especially if that outer surface is a syncytium, as any nuclei that are damaged would not necessarily result in a potentially fatal breach in the parasites defences. Some turbellarians may have become pre-adapted to this different mode of life as a way of protecting their outer layers from constant abrasions due to moving over sand and other gravel, as some free living forms have an outer epithelium equipped with nuclei present in cell bodies sunk beneath the muscle layers. Another adaptation seen here is the presence of numerous microvilli on the surface, rather than just cilia. These probably increase the parasites surface area, an important adaptation as many of the organisms nutrients are taken up across the outer syncytial layer.
The Parasitic Groups
The outer surface of the parasitic groups are all very similar, being in the form of a syncytial layer, where the nuclei are protected by being situated in sunken subtegumental cell bodies below or within the muscle layers, connected to the syncytium by cytoplasmic processes.
3) The Outer Surface of a Parasitic Monogenean
The tegument found in the monogeneans is characterised by the presence of numerous microvilli on the surface.
4) The Tegument of a Parasitic Cestode
The organisation of the tegument found in the cestodes is described in more detail in the cestode biology pages within this site. Briefly in the eucestodes the surface of the tegument is covered in numerous highly modified microvilli called "microtriches". These are characterised by having a spine like appearance, equipped with electron dense tips and internally with numerous microtubular structures. The parasites do not have a gut, and therefore to obtain nutrition the tegument acts as an absorptive surface, the microtriches greatly increasing its surface area and probably helping the parasite to attach to the intestinal wall of their hosts. The tegument of the cestodarians appears to be intermediate between that found in the monogeneans and the eucestodes, having many densely packed microvilli similar to the microtriches, but lacking the electron dense tip. This probably reflects the intermediate stage of development of the cestodarians between these two major groups of parasites.
5) The Tegument of a Digenean Trematode
The trematodes have teguments that conform to the generalised feature found throughout the phylum. The Aspidogastrean tegument has not been extensively studied but appears to be similar to that of Monogeneans, being equipped with microvilli on the surface. The organisation of the tegument found in the digeneans is described in more detail in the digenean biology pages within this site. Briefly the tegument is often equipped with electron dense spines.