Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes - General Features Of Lifecycles
The lifecycle of the platyhelminthese may be divided into a number of discrete stages, not all of which are found in all of the groups.
1) The platyhelminth egg
The platyhelminth egg may take a number of forms, depending on the class or species of worm. The more primitive Archoophoran turbellarians, such as the acoels have eggs similar to other organisms, in that the food reserves are found with the cytoplasm of the egg. In the Neoophoran turbellarians and the parasitic groups however this does not occur. In these platyhelminthes the eggs food reserves are located in yolk, or vitelline, cells (produced by vitelline glands) around the egg. Masses of eggs and vitelline cells are deposited together, surrounded by a thin shell to form a capsule, often attached to a substrate by a stalk.
In the parasitic groups a single egg, and accompanying vitelline cells, are surrounded by a capsule, the egg shell the formation of this egg being characteristic for the group. Briefly, as the egg enters the öotype of the parasite it becomes surrounded by a predetermined number of vitelline cells, which form the food reserve of the egg. In addition these vitelline cells produce globules of a mixture of proteins and phenols, which are extruded to the outer surface of the developing egg. Here the phenols oxidise to form quinone, which then coalesces with the protein, reacting to form scleratin, a hard inert yellowish substance, making up the egg shell. In most of the groups (monogeneans, aspidogastreans, most digeneans and many cestodes) these eggs are operculate.
In contrast some platyhelminthes have developed more divergent egg types, particularly among the cestodes. For example eggs of cyclophyllidean tapeworms are characterised by being both non-operculate, and having very thick egg shells. (e.g. Hymenolepis). Another divergent form of platyhelminth egg is found in the schistosomes. These are non-operculate, and are ornamented with spines.
The eggs of the monogeneans are characterised by having either one or two filaments attached to the ends of the eggs. These filaments may be very short, or many times the length of the egg. Occasionally the eggs of some digeneans may also bear filaments, although these are more generally oval in shape, and brown in colour due to quinone-tanning of egg shell components.
2) The emergent free swimming larvae
In the turbellarians the egg capsule opens to release juvenile flatworms, which simply grow into the adult form. One interesting observation is that the symbiotic and parasitic turbellarians produce many more eggs than the entirely free living species. This adaptation, which in mirrored in the parasitic clases which also generally produce high numbers of eggs, is probably necessary for this mode of life to compensate for the many individuals that never succeed in infecting a new host.
In contrast to the free living forms, the parasitic classes all have larval forms emerging from the egg that differ considerably from the adult parasite. One fairly common feature, and one that betrays their origin among the free living organisms, is that the outer surface of the newly hatched parasitic platyhelminth consists of a layer of cilliated epidermal cells, similar to the outer surface of the free living forms. This is clearly seen in the monogeneans, with the oncomiracidium, and both groups of trematodes, the digeneans, with the miracidium (select these links to the digenean lifecycle pages, and the schistosome miracidium as an example of a typical digenean miracidium) and the larval aspidogastreans. Among the other group, the cestodes, in the pseudophylideans, the egg hatches to release a cilliated coracidium, which is internally structurally very similar to the cyclophyllidean onchosphere except that the onchosphere is not a free swimming form and must be ingested to infect its host. In the cases of the free swimming cilliated larvae, those of the trematodes must infect a molluscan host, whilst those of the pseudophyllidean cestode must infect a copepod, and the monogenean infect a vertebrate. These preference, which are relatively highly conserved within the groups, (especially within the trematodes, which always utilise a mollusc) probably reflect the host preference of the ancestral turbellarian group from which they arrose, indicating different orgins within the turbellarians for all of the parasitic groups.
3) The first larval parasitic stages
In the parasitic turbellarians, and in the monogeneans and most aspidogastreans there are no intermediate hosts, and therefore the first parasitic stage is the adult form, brifly described below. Other groups, such as the digeneans, cestodes and a few aspidogastreans, utilise intermediate hosts in their lifecycles. These therefore have larval forms, which in cestodes may be found in many organisms, (e.g many vertebrates and arthropods). In the digeneans (and some aspidogastreans where intermediate hosts exist) molluscs are utilised as intermediate hosts.
The cestodes exhibit many forms, most of which exist encysted within the tissues of these hosts. In most cases no reproductive cycle occurs in these hosts, but this is not always the case, cestodes within the family Taeniidae (e.g. Echinococcus sp.) producing many new larvae by budding or within cysts.
In contrast, in the digeneans the intermediate host is a site of intense asexual reproduction, within the larval forms present here. The digeneans show two main forms, initially a sporocyst, which may be followed by susequent generations of daughter sporocysts or redia. The sporocyst itself is a sac-like structure, within which germinal cells give rise to the subsequent generations of asexually dividing forms within this intermediate host. Sporocysts do not have a gut, but survive on stored food reserves or nutrients absorbed across the tegument. The redia stage is similar to that of the sporocyst except that it has a developed digestive system, and may actively feed on the tissues of its intermediate host. These developmental stages are descibed in more detail in the digenean biology pages. Depending on the species of digenean germinal cells within either sporocysts or redia give rise to another free swimming larval form, the cercaria. This leaves its molluscan host, either directly infecting the parasites definitive host (e.g. the schistosome ceracia, or more commonly invading these tissues of a second intermediate host, where they encyst in a similar fashion to most of the larval cestodes, or on the surface of vegetation (e.g. as is the case with Fasciola hepatica).
4) The free living adult platyhelminth
The free living turbellarians (they are primarily aquatic, and the majority are marine organisms) are mostly predacious or feeding on other dead organisms, although some may feed on algae, and many feed on diatoms as juveniles, before becoming fully carnivorous as adults. They mostly live under stones, in mud and sand, or on the surface of aquatic vegetation. There are a few terrestrial representatives, generally in humid tropical regions, although some may be found in temperate regions, some even recently colonising parts of the United Kingdom.
5) The adult parasitic platyhelminth
In the case of the cestodes and most digeneans encysted in intermediate hosts (or on vegetation for some digeneans), infection occurs on ingestion, either of that host or of contaminated vegetation, by the definitive host.
Comparing the different groups the parasitic turbellarians, monogeneans and some aspidogastreans are ectoparasitic, feeding by grazing on the tissues (usually skin or gills) of their hosts. In contrast the digeneans and cestodes, with more complex lifecycles are endoparasites, usually found within the gastrointestinal tract. Details of these parasites are given in the pages within this site devoted to the general biology of these organisms.