HEARING. 557 



which move slowly and have all their organs of sense dull, like the latter, it is 

 dense, and in the crab distinctly ossified. The fixed and slow moving mollusca, 

 as the oyster, barnacle, muscle, slug, snail, have dull senses in general ; and their 

 organ of hearing has not been detected, but some appear to hear, and one, the 

 tritonia arborescens, emits audible sounds, intended no doubt to be heard by its 

 own kind. The cephalopods, however, as the cuttle-fish and nautilus, ap- 

 proaching to fishes in complexity of structure, quickness of motion, and acute- 

 ness of sight, have also a higher development of the organ of hearing. We first 

 find a calcareous substance in the fluid of the vestibule, acting probably l^ke the 

 clapper of a bell. " In passing up through the vertebrated classes, we observe 

 the organ gradually developing the semicircular canals and cochlea, and becoming 

 enveloped in the solid parietes of the cranium ; it acquires a tympanic cavity 

 communicating with the fauces by the Eustachian tube, and containing the ossicula 

 auditus, which convey the vibrations of the membrana tympani to the vestibule 

 and the whole internal labyrinth ; and in the highest forms of the organ a still 

 more exterior meatus auditorius, and complicated moveable concha are added to 

 complete the instrument." Thus, although in the lowest cyclostome or cartilagi- 

 nous fishes, as the lamprey, the ear is of no higher order than in the cephalepodous 

 mollusca, without canals or calcareous substance, the osseous fishes have cal- 

 careous bodies in the vestibule, and large semicircular canals ending in considerable 

 ampullae. Still the organ is in the common cranial cavity, and not enclosed in 

 the temporal bone, nor are there usually a meatus and external opening. In the 

 large cartilaginous fishes, as the sturgeon and the rays, the ear is imbedded in 

 the cartilaginous temporal bone : in the former the semicircular canals only, the 

 vestibule being still in the cranial cavity ; in the latter the whole : and the 

 vestibule has sacs which are the rudiments of a cochlea. The lowest reptiles 

 resident in water, the perenni-branchiate species, as the newt, have ears like 

 those of fishes, and sounds are equally communicated through the solid walls 

 only of their cranium. The same structure exists in the larva of the caduci- 

 branchiate, as tadpoles ; but, when the animal loses its gills and becomes a frog, 

 the semicircular canals are imbedded in a distinct cavity of the temporal bone, 

 a tympanum, Eustachian tube, and three soft ossicula united are seen, and the skin 

 forms a membrana tympani on a level with the surface of the head. In the serpents 

 the ear is much the same. In the saurian reptiles the tympanum is much larger. 

 In the crocodile there is an appendage like a rudiment of a cochlea, and on the 

 margin of the membrana tympani two folds of skin, like eyelids, are found, which 

 appear like the rudiments of a concha. In the chelonia, as the tortoise, the 

 tympanum and united bones are of greater length, and a cochlea is more dis- 

 tinctly developed. In birds, the organ is greatly advanced, and large in pro- 

 portion to the head. It is completely enveloped in bone of rocky hardness; the 

 semicircular canals are smaller than in fishes, but with larger ampulla ; the 

 vestibule is lengthened ; the cochlea begins to assume a spiral form, though it 

 still has a remnant of the calcareous bodies found in the labyrinth of fishes ; the 

 tympanum is lengthened, and numerous cranial cells communicate with it; 

 a short meatus externus appears ; and in nocturnal predaceous birds, which much 

 require the sense of hearing, a high crescemic fold of skin is found at the upper 



