The middle ear makes its first appearance in the amphibian class, where we find a tympanum, tympanic cavity, and Eustachian tube; these animals, and all terrestrial Vertebrates, certainly have the faculty of hearing. All these essential parts of the middle ear originate from the first gill-cleft and its surrounding part; in the Selachii this remains throughout life an open squirting-hole, and lies between the first and second gill-arch. In the embryo of the higher Vertebrates it closes up in the centre, and thus forms the tympanic membrane. The outlying remainder of the first gill-cleft is the rudiment of the external meatus. From its inner part we get the tympanic cavity, and, further inward still, the Eustachian tube. Connected with this is the development of the three bones of the mammal ear from the first two gill-arches; the hammer and anvil are formed from the first, the stirrup from the upper end of the second, gill-arch. (FIGURE 2.324. The rudimentary muscles of the ear in the human skull. a raising muscle (M. attollens), b drawing muscle (M. attrahens), c withdrawing muscle (M. retrahens), d large muscle of the helix (M. helicis major), e small muscle of the helix (M. helicis minor), f muscle of the angle of the ear (M. tragicus), g anti-angular muscle (M. antitragicus). (From H. Meyer.)) Finally, the shell (pinna or concha) and external meatus (passage to the tympanum) of the outer ear are developed in a very simple fashion from the skin that borders the external aperture of the first gill-cleft. The shell rises in the shape of a circular fold of the skin, in which cartilage and muscles are afterwards formed (Figures 2.313 and 2.315). This organ is only found in the mammalian class. It is very rudimentary in the lowest section, the Monotremes. In the others it is found at very different stages of development, and sometimes of degeneration. It is degenerate in most of the aquatic mammals. The majority of them have lost it altogether--for instance, the walruses and whales and most of the seals. On the other hand, the pinna is well developed in the great majority of the Marsupials and Placentals; it receives and collects the waves of sound, and is equipped with a very elaborate muscular apparatus, by means of which the pinna can be turned freely in any direction and its shape be altered. It is well known how readily domestic animals--horses, cows, dogs, hares, etc.--point their ears and move them in different directions. Most of the apes do the same, and our earlier ape ancestors were also able to do it. But our later simian ancestors, which we have in common with the anthropoid apes, abandoned the use of these muscles, and they gradually became rudimentary and useless. However, we possess them still (Figure 2.324). In fact, some men can still move their ears a little backward and forward by means of the drawing and withdrawing muscles (b and c); with practice this faculty can be much improved.