The Acanthocephala are an unusual group of organisms, with characteristics which do not make phylogenetic relationships very clear. The name means "Spiny headed worms", which refers to the spines on the proboscis, as shown in the following figure:

 Here are some of the characteristics of this phylum:

• The sexes are separate
• Absorption of nutrients and gas exchange are directly through the tegument
• Excretion is via protonephridia
• The intermediate hosts are arthropods
• There are numerous examples of parasite-induced changes in intermediate host behavior, which make the intermediate host more susceptible by the definitive host.

Here is an example of Leptorhynchoides thecatus.