Electron-microscope investigations on the outer layers of parasitic platyhel-minths have so far neglected the monogeneans and have been confined to the endoparasitic digeneans and cestodes (Braten, 1968; Burton, 1966; Charles & Orr, 1968; Erasmus, 1967; Lumsden, 1966; Morris & Threadgold, 1967; Race, Larsh, Esch & Martin, 1965; Rothman, 1968; Threadgold, 1965). For this reason the covering layer of monogeneans is still referred to as a ‘cuticle’ even though it is unlikely to be so since the surface of the endoparasitic platyhelminths has been shown to be a living cytoplasmic ‘epidermis’ (see Lee, 1966) which, as pointed out by Erasmus (1967) and Lumsden (1966), constitutes a metabolically active inter-face with host tissues and fluids. Two main considerations prompted the present investigations of the covering layer of monogeneans. First, it was hoped that a comparison of the covering layer of these ectoparasites with that of the digeneans and cestodes might help to define particular specializations of this layer associated specifically with the ectoparasitic or endoparasitic habit; secondly, because ectoparasitism can, in an evolutionary sense, be considered as a transitional state between a free-living and an endoparasitic existence (see Llewellyn, 1965), it was felt that this work on monogeneans could assist an understanding of the evolution of the platyhelminth outer layer.
Not one hormone or another: Aggression differentially affects progesterone and testosterone in a South American ovenbird