Turning to the invertebrata, we find, even in creatures as low down in the scale of life as jelly-fish, around the margin of the umbrella in certain medusa, simple auditory organs. In some cases they are pits containing otoliths (minute calcareous or other bodies, which are supposed to be set a-dance by the sound-vibrations); in others there is a closed sac with one or more otoliths; in others, again, they are modified tentacles, partially or completely enclosed in a hood. All these are generally regarded as auditory, there being specially modified cells of the nature of hair-cells. We shall see, however, that another interpretation of organs containing otoliths is at any rate possible. For the present, we will follow the usual interpretation, and regard them as auditory. Vesicular organs containing otoliths are found near the cerebral ganglia in some of the worms and their relations. But the common earthworm, though it appears to be sensitive to sound, does not appear to have any such organs. Molluscan shell-fish are generally provided with auditory organs. In the fresh-water mussel it is found in the muscular foot. It can be more readily seen in the _Cyclas_, if the transparent foot of this small mollusc be examined under the microscope. It is a small sac containing an otolith. Mr. Bateson found that the mollusc _Anomia_ "can be made to shut its shell by smearing the finger on the glass of the tank so as to make a creaking sound. The animals shut themselves thus when the object on which they were fixed was hung in the water by a thread." In the snail and its allies the auditory sac is found in close connection with the nerve-collar that surrounds the gullet. In the cuttle-fishes it is found embedded in the cartilage of the head. [Illustration: Fig. 26.--Antennule of crayfish. i.j., inner joint; o.j., outer joint; ol., olfactory setæ; ol'., the same, enlarged; au.op., auditory opening in the basal division, which has been cut open to show au.s., the auditory sac; au.n., auditory nerve branching to the two ridges beset with auditory hairs; au.h., auditory hair, enlarged. (After Howes.)] [Illustration: Fig. 27.--Diagram of ear. t.m., tympanic membrane, to which is attached a chain of small bones stretching across the cavity of the drum, the innermost of which, st., fits into the "oval window." The vibrations are transmitted up one side and down the other side of the cochlear canal, c.c., and thus reach the "round window," f.r.; s.c. is one of the semicircular canals, the other two are omitted; e.t. is the Eustachian tube connecting the cavity of the drum with the mouth-cavity.]