The inner ear of the echidna Tachyglossus aculeatus: the vestibular sensory organs.

The monotremes, comprising the echidnas and platypus, have more sensory organs in the inner ear than do other mammals. In addition to the organs usually seen in the mammalian inner ear, they have a lagenar macula, a sense organ found in all non-mammalian vertebrates. In the echidna Tachyglossus aculeatus, this macula consists of a narrow sensory strip at the end of the cochlea. All vestibular sensory organs are populated by two types of sensory cells, cylindrical bouton-innervated hair cells and more or less regular bottle-shaped calyceal-innervated hair cells. The two types are mixed in all vestibular organs, as in placental mammals, and their arrangement in the maculae in particular is very unlike that in reptiles. The sensory epithelia are very large relative to the size of the animals. The utricular macula contains more sensory cells than found in any other amniote so far examined.