Pseudophyllidea

• Pseudophyllidean tapeworms have a scolex with four longitudinal grooves.

• They have a genital pore which is centrally placed on each segment.

• The eggs and life-cycle have some similarity to that of the trematodes.

• The Pseudophyllidean egg shell is thick, brown and operculate.

• The Pseudophyllidean life-cycle uses two intermediate hosts – the coracidium, which emerges after hatching, must first be ingested by a crustacean. A larval procercoid develops within the body cavity of the crustacean. If the crustacean is then eaten by a freshwater fish, the procercoid is liberated and develops into a second larval stage, a plerocercoid, within the muscles of this new host. The plerocercoid possesses the characteristic scolex – it is only this stage that is infective to the final host.

• There are no pseudophyllideans of importance in the UK, but some are of significance in tropical or sub-tropical areas (for example Spirometra), and in sub-arctic regions (for example Diphyllobothrium).

Diphyllobothrium latum

An important cestode of humans and fish-eating mammals. They are long tapeworms with an unarmed scolex. D. latum occurs in arctic and near arctic regions. Humans act as final host, but dogs may be infected. Causes anaemia because of its avid uptake of dietary vitamin B12. Eggs release a motile larval form which develops further in aquatic copepods and ascends the food chain, from fish, to humans.