Helminth Taxonomy - Other Helminths Groups

These resemble nematodes, and have lifecycles similar to the mermithids, with juvenile forms developing within the body cavity of invertebrate hosts, (usually insects). They however have more primitive features in that;

The pseudocoelome is often largely filled with mesenchyme material

They lack an excretory system

There is only a single ventral nerve cord

The digestive system is very rudimentary, (often there is no mouth), taking up nutrients through the cuticle in the juvenile parasitic stages only.

There are two Classes within this Phylum;

Class Gordioidea - A freshwater group containing most of the known species. Aquatic or terrestrial insects act as hosts for the juvenile helminths.

Class Nectonematoidea - A marine group represented by a single genus Nectonema. this uses decapod crustaceans as hosts for the juvenile helminths.

Their bodies are similar in organisation to highly the turbellarians, but have either cylindrical or flattened bodies. They are characterised by a large eversible proboscis, which is often armed with a stylet. They are mostly free living predatory marine organisms, with a few parasitic or commensal representatives, and a few terrestrial or freshwater species.

There are two Classes of Nemerteans;

Class Anopla - A marine group. The proboscis lacks a stylet

Class Enopla - A more diverse group with marine, freshwater and terrestrial representatives. Includes some parasitic and commensal species. For example Carcinonemertes sp. is a parasite of the crab Cancer magister (the Dungeness crab), whilst the nemertean Malacobdella grossa is a commensal of many European and American bivalves.

It is thought that this group evolved from free living flatworms. Again due to the hosts utilised by the parasitic representatives these have little or no veterinary or human importance.

Helminth Taxonomy - The Aschelminthes

The Aschelminthes comprise a number of groups of helminthes, and is considered in some taxonomic schemes to be a distinct phylum. The group consists of the phyla detailed in these pages with parasitic representatives such as the nematoda, the acanthocephala, the nematomorpha, the nemertea and a number of non-parasitic phyla, such as the pripulans, tardigrades, gastrotricha, rotifera, gnathostoma and kinorhyncha. As can be seen this is a very diverse group of organisms, and this page will continue to consider them as seperate phyla. However they all have a number of features in common. They are all pseudocoelomate organisms, that is they have a fluid filled body cavity, (which in some of the constituent phya may be very minimal). The pseudocoelome differs from a true coelome (body cavity) in that it represents a persistant embryonic blastocoel, and the mesoderm is mostly situated on the outer face of the cavity, (ie the internal organs are not surrounded by a peritoneum).

Helminth Taxonomy - The Minor Groups of Platyhelminthes (Flatworms)

Class Turbellaria - The majority of these are free-living such as the planarians, although some may be ecto commensals. Their teguments are ciliated, by means of which they are motile. For more information on free living planarians such as Bipalium kewense and Artioposthia triangulata (an introduced predator of earthworms in Great Britain and Ireland).

Class Trematoda

Sub Class Aspidogastrea - A small group, mainly endoparasites of molluscs and cold blooded vertebrates such as fish and reptiles. The tegument has microscopic finger like protrusions.
 
Class Monogenea - Fluke like. Mainly ectoparasites of cold-blooded vertebrates, particularly fish, although some may be endoparasites in locations directly opening to the exterior environment, and at least one species is found in mammals (Oculotrema hippopotami in the eye of the hippopotamus). They attach to their hosts by means of a posterior opisthaptor, which may be armed with hooks or hooks and suckers.

There is only one host in their lifecycles, and they are hermaphroditic with either asexual reproduction, or cross-fertilisation with other individuals if they are available. The tegument has microscopic finger like protrusions (microvilli). They may be of considerable economic importance as pathogens in fish in fish farms.

Examples:

Order Monopisthocotylea - Simple opisthaptor armed with hooks. They are mostly epidermal skin grazers.

e.g. Gyrodactylus sp. - Ectoparasites on skin and gills of fish.

e.g. Dactylogyrus sp. - Ectoparasites on gills of fish

e.g. Entobdella sp. - Ectoparasites on gills of flatfish

Order Polyopisthocotylea - Opisthaptor armed with hooks and suckers or clamps. These parasites tend to feed on the blood of their hosts, often being found attached to the highly vascularised gill chambers of their hosts. Because of this they are thus generally more pathogenic to their hosts.

e.g. Diplozoon paradoxum - Ectoparasites of gills of Cyprinid fish

e.g. Polystoma intererrinum - Endoparasites of urinary bladder of old world frogs