Introduction to Digenean Flukes
Adult Fluke
Adult Bodyplan
Digenean Tegument
Reproductive System
Lifecycle
Other Flukes
The Basic Body Plan of a Digenean Fluke
1) General Features
The parasitic digeneans have many features common to the platyhelminthes as a group (for more details go to the Platyhelminth Biology pages within this site). They mostly have leaf-like dorso-ventrally flattened bodies with no body cavity, (i.e. they are acoelomates). Briefly they have a syncytial outer tegument beneath which there are two layers of muscle (circular and longitudinal), a simple gut, a protonephridial excretory system, and the body mass consists of a network of mesodermal parenchyma within which the internal organs and muscle layers are embedded. The parenchyma consists of fibrous interstitial material and large polymorphic parenchymal cells. The digenean muscle appears to be of the smooth type, but (at least in the schistosomes) also exhibit some differences from vertebrate smooth muscles, the thin filaments being anchored in dense plaques, analogous to the Z discs of striated muscles. They are usually hermaphroditic organisms, possessing both male and female reproductive organs, the appearance and position of which are very important taxonomically. Two groups of digeneans however do have separate sexes, the members of the family Didymozoidae (an unusual group parasitic in fish) and members of the family Schistosomatidae. Finally the digeneans possess suckers to aid attachment to their host substrate. There are usually two suckers, oral and ventral, the position and size of which are of importance taxonomically. However in some groups (e.g. those of the family Notocotylidae, parasites of aquatic birds and mammals) the ventral sucker may be absent, there may be no suckers at all (e.g. those of the family Sanguinicolidae, parasites of fish), whilst in others an additional genital sucker may be found (e.g. those of the family Heterophyidae, including the human parasite Heterophyes heterophyes). In some there may be a additional type of attachment structure, the adhesive organ, (see the adult parasite section of this page)
2) The Outer Tegument
The digenean tegument is a syncytial layer, typical of that found in the parasitic platyhelminthes, often equiped with spines or scales. The specifically digenean tegument is discussed in more detail in another section of this page.
3) The Organisation of Digestive System
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These parasites live on cells, tissue fragments and fluids and blood derived from their hosts. A mouth, enclosed by the oral sucker, at the anterior end of the parasite leads into a muscular pharynx, transporting these host derived materials into a short esophagus, and then into the digestive tract. In contrast to the digestive tract of most other platyhelminthes, this consists of two caecae (occasionally one, as with those conforming to the 'Gasterostome' body form). These are commonly cylindrical, but in some species (e.g. Fasciola hepatica) may be highly branched. Lining these caecae secretory and absorptive epithelial cells and it is thought that digestion is extracellular, at least in part. Waste material must be regurgitated as there is usually no anus (although one may appear in a few aberrant species). |
4) The Excretory System
There are three routes by which metabolic waste products of the trematode metabolism may be removed from the parasites body. Firstly the tegument may be involved to some extent in removal of metabolic waste products, although this is probably not a major function of the tegument. Secondly the caecal epithelium lining the parasites digestive tract has been shown to exhibit secretion as well as taking up material. However the most important method of removal of waste products is via the well developed excretory system present in these organisms. The overall organisation of this system follows to a large extent that of the basic platyhelminth system, (see the platyhelminth biology pages within this site). This is a protonephridial sytem embedded within the parasites mesodem (M), consisting of flame cells (F) linked to protonephridial tubules (PNT), which in turn open into ducts, leading to a muscular bladderThis last structure appearing either in the metacercarial stage of the lifecycle, or in those groups such as the schistosomes which lack this developmental stage, during maturation of the juvenile adult (e.g. the schistosomulum). |
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5) The Nervous System
This again conforms to the general features of a platyhelminths as a group. Briefly there is a cerebral ganglion, situated close to the parasites pharynx, from which extend a number of major nerves, traversing the parasite longitudinally, which are themselves joined together by traverse commisures, the whole structure resembling a ladder. Trematodes have been shown to have a number of sensory organs associated with this nervous system, which may play important roles in different stages within ther lifecycle, (see particularly the free swimming miracidial and cercarial stages). Sensory organs probably also play a role in the adult form of the parasite as well. For example in the schistosomes, there is some evidence that the process by which the male and female worms locate each other before pairing is mediated by some form of chemotaxis. In addition a sensory structure that appears to be responsive to touch has been found within the tegument of a number of trematodes. In schistosomes this structure may be important in helping to maintain to parasites position within the blood vessels, by detecting the flow of fluids (i.e. blood) over the parasites surface.