Perhaps the most complete degeneration is found in the Rhizocephala, which are parasitic on other Crustacea.
In the parasitic Rhizocephala and in a few Copepoda (Monstrillidae) the alimentary canal is absent or vestigial throughout life.
This section comprises Darwin's Apoda, the footless, Lilljeborg's Suctoria, called by Fritz Muller the Rhizocephala or rootlet-headed, and the group to which Lacaze-Duthiers gave the alternative names Ascothoracida, sac-bodied or Rhizothoracida, rootlet-bodied.
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