ADULT SHARK (AFTER CUVIER AND HAECKEL).—FROM “DARWIN AND AFTER DARWIN.”
There never had been an external ear in the world till this was done, or any good ear at all. Creatures which live in water do not seem to use hearing much, and the sound-waves in fishes are simply conveyed through the walls of the head to the internal ear without any definite mechanism. But as soon as land-life began, owing to the changed medium through which sound-waves must now be propagated, a more delicate instrument was required. And hence one of the first things attended to was the construction and improvement of the ear.
MARBLE HEAD OF SATYR, IN MUNICH, SHOWING CERVICAL AURICLES.
It has long been a growing certainty to Comparative Anatomy that the external and middle ear in Man are simply a development, an improved edition, of the first gill-cleft and its surrounding parts. The tympano-Eustachian passage is the homologue or counterpart of the spiracle, associated in the shark with the first gill-opening. Professor His of Leipsic has worked out the whole development in minute detail, and conclusively demonstrated the mode of origin of the external ear from the coalescence of six rounded tubercles surrounding the first branchial cleft at an early period of embryonic life. Haeckel’s account of the process is as follows: “All the essential parts of the middle ear—the tympanic membrane, tympanic cavity, and Eustachian tube—develop from the first gill-opening with its surrounding parts, which in the Primitive Fishes (Selachii) remains throughout life as an open blowhole, situated between the first and second gill-arches. In the embryos of higher Vertebrates it closes in the centre, the point of concrescence forming the tympanic membrane. The remaining outer part of the first gill-opening is the rudiment of the outer ear-canal. From the inner part originates the tympanic cavity, and further inward, the Eustachian tube. In connection with these, the three bonelets of the ear develop from the first two gill-arches; the hammer and anvil from the first, and the stirrup from the upper end of the second gill-arch. Finally as regards the external ear, the ear-shell (concha auris), and the outer ear-canal, leading from the shell to the tympanic membrane—these parts develop in the simplest way from the skin-covering which borders the outer orifice of the first gill-opening. At this point the ear-shell rises in the form of a circular fold of skin, in which cartilage and muscles afterwards form.”[3]
