120 
THE EAR AND HEARING. 
II. A solid body, or bodies, of greater specific gravity 
than the surrounding substance. 
The whole course of the development of the ear will be 
found to consist in a progressive series of adaptations for 
rendering more perfect the propagation of the sonorous 
vibrations, for their multiplication by resonance, and tlicir 
more delicate discrimination. 
The simplest form of ear hitherto identified is found in 
aquatic animals. It is marked only by the first of the two 
modifications noted above, and seems capable of receiving 
sonorous vibrations but imperfectly, and quite incapable of 
resolving them into tones. The next advance is marked 
by the addition of the second of the above modifications, 
thereby increasing the sensibility to vibratory influence. 
A still greater advance is the suspension of the whole sac 
in a more liquid material (the perilymph), followed succes¬ 
sively by the evolution of an aperture, covered by a membrane 
upon which the external medium can at once act—the 
differentiation of the appendages of the vestibule, the cochlea, 
and, lastly, by the appearance of an external ear. 
The lowest forms in which I am aware of the discovery of 
an ear are the Medudda , amongst the Coelenterates. In the 
edge of the umbrella of the common jelly fish (.Rhizostoma 
Pulmo ), the microscope, with a half-inch objective, shows 
small vesicles at the bases of the tentacles. In each of these 
vesicles may be seen a minute body, closely resembling a 
bell-clapper, suspended by a ligamentous neck, and vibrating 
about one hundred times a minute. These oval cells are 
thought to be the ears of the creature, and the solid particles 
contained in them to be otoliths (ear stones). In the next 
group— Echinozoa —I am not aware of the discovery of any 
auditory apparatus, although a true nervous system is 
present in the higher forms, amongst which eye spots are 
found (e.fj., some of the Entozoa and Botifera). 
Coming to the next sub-kingdom— Annulosa— we find that, 
although the sense of hearing in insects seems evident, little 
is known about their auditory apparatus. Bamdolir has 
placed it in the jaws, Strauss and Durckheim in the antenme, 
De Blainville in the tracheal tubes on the sides of the body, 
and Agassiz in the legs. Professor von Graber finds what he 
terms “ chordotonal sense organs” in the rod-like secretory 
structures of the nerves of various parts (chiefly legs or 
wings) of insects. He states that the general type of rod is 
pencil-like (scolopal), being pointed at its proximal end, and 
hollow, with extremely refractive walls. In some genera 
these rods are fastened to the integument by a special 
