An order of protozoans, subclass Holotrichia, in which all species are mouthless. All species are parasitic in other animals, typically oligochaete annelids. Many astomatids possess an elaborate holdfast organelle, of value in attachment to the cells or tissues of the host's alimentary tract. Some authorities have considered the species of this rather large and ill-defined group to represent nothing more than degenerate, astomatous members of certain other groups, such as thigmotrichs, apostomes, and possible hymenostomes. Still, there seems to be value in recognizing the evolutionary distinctiveness of the group as a whole. Anoplophyra is a typical example.