Transmission Electron Microscopy of the Tegumentary Sense Organs of Cotylogaster occidentalis (Trematoda: Aspidogastrea)

Abstract

At least 5 morphologically distinct types of sensory receptors are present in the tegument of mature Cotylogaster occidentalis. The different types of sensilla were distinguished by the presence or absence of sensory cilia, the morphology of the rootlets, the mechanisms of attachment to the tegument and the distribution in the body. Sensory receptors were concentrated near the buccal funnel and oral opening. They were also present in the ventral haptor, but were rare in the dorsal tegument. The Type I sensillum is the only type which extended through the thickness of tegument and had a free sensory cilium. Sensory receptors of this type may be either chemo- or mechanoreceptive in function. Four of the types (II to V) were nonciliated, entirely intrategumental in location, and were probably mechanoreceptors. The Type II sensillum contained a disk-shaped, striated rootlet and was found only on the alveolar ridges of the ventral haptor. The morphology and characteristic location of this type of receptor suggested that it might mediate in the function of the ventral haptor. Types III and IV sensilla were similar in general morphology but differed in their mode of attachment to the tegument and in the appearance of their rootlets. The subtle ultrastructural differences in the Types III and IV rootlets probably reflect physiological specializations of response to different stimuli. The Type V sensillum contained an elongate, striated rootlet and was located in deep invaginations of the tegument. They were surrounded by circular and longitudinal muscles which formed numerous hemidesmosomes with the basal lamina encircling the receptors. Sensory receptors of this type may function to detect stresses associated with muscular contractions.