Morphology of Auditory Canal of Echidna (Tachy- glossus aculeatus)

The echidna is a monotreme a mammal that lay eggs and produces milk to nourish its young after hatching. Three short-beaked echidnas were used to study the morphology of their auditory canal. Three echidnas were dissected, two of them were x-rayed and samples for histological examination were taken from them before dissection. The echidna possesses neither an apparent ear pinna nor a tympanic bulla. The auditory canal begins at the bottom of the auditory groove, which is vertically situated and hidden between the spikes of the head. It is hyaline cartilage in nature, and extends from the bottom of the auditory groove, running cranially; then after turning caudomedially. It ends at the slit-like external auditory opening of the skull. The epidermal layer of skin that lines the auditory canal is keratinized stratified squamous epithelium. The deep dermal layer is formed of dense connective tissue of collagen and elastic fibers with hair follicles and a few associated sebaceous and ceruminous glands. The results were discussed in the light of the ear evolution from reptiles to the early mammals (monotremes) and supported by 9 images.