In Mans, as in all the following classes, there is no external ear, but there is in this class an exterior opening, which in some, as the owls, is covered with a valve. The membrane of the tympanum in birds is convex on its peripheral surface. It is remarkable that birds have the internal cavities of the two ears connected by the air-cells of the skull. The labyrinth in this class has only one little bone, placed between the membrane of the tympanum and the Jenestra ovalis, and the eustachian tubes communicate by a common opening with the arch of the palate. There is in birds no proper cochlea, but instead of it, a short, blunt, hollow, bony process, that passes from the vestibule obliquely backwards, and is divided by a: partition into two scalx, terminating like the winding passages of the cochlea in man and mammalia.
Most of the REPTILES have a tympanum, semicir cular canals, and a eustachian tube ; but in serpents, except in the blind worm, (anguis fragilis,) both the tympanum and eustachian tube are wanting. In the crocodile there is some vestige of an external auditory passage, which is completely wanting in every other spe cies of these classes. In lizards and serpents, as well as the crocodile, there are soft stony bodies within the vestibule ; and rudiments of similar bodies are also found in frogs.
In FISHES there are three large semicircular canals projecting into the cavity of the skull, and opposite to that part of the vestibule where the auditory nerves ter minate, there are generally found two or three small bony bodies, resembling those of serpents, but generally of a harder consistence. In some of the cartilaginous
fishes, there is also a tubular cavity resembling a tym Pa1111112.
Among the CRUSTACEA, the larger species of cancer are observed to have at the root of the palpi on each side, a small bony tube, the external orifice of which is covered by a firm membrane. Its internal cavity is lined by a thinner membrane, on which are expanded nervous filaments coming from the same branch that supplies the antenna; a circumstance which has induced some anatomists to believe, that the antennae themselves are the organs of hearing.
Of the Mom.usca, the only tribe that has any preten sions to a hearing organ, is that of sepia or the cuttle-fish. In these animals there is a cartilaginous ring, to which are fixed the large tentacula or arms, and in this ring are two oval cavities, containing each a small bag, that has a bony substance within it, and receives nervous fi laments like the labyrinth in other animals.
The only part of the organs of hearing that is univer sally found in MAMMALIA, CETACEA, BIRDS, FISHES, REPTILES, SERPENTS, and in some CRUSTAL EA and