 They usually attach to the external surface of the host, in the throat-mouth cavity, or the gills.One family of poecilostomatoid copepods, however, have evolved an endoparasitic mode of life and live deep within their hosts' bodies rather than merely attaching themselves to exterior and semi-exterior surface tissue.In addition to typical marine environments, poecilostomatoid copepods may be found in such very particular habitats as anchialine caves and deep sea vents (both hydrothermal vents and cold seeps).