As the sense of feeling has been at once manifested in the vegetative animals as a peripheric nervous sense; so antithetically or vice versâ it is in the higher animals the light-sense, as being the central nervose sense, which appears.
3341. Hitherto both these senses stood altogether alone with each other upon the stage of animal existence, in order, as it were, to play their parts in mutual consent; but so soon as the sense of feeling became individualized into tactile organs, then also the eye made its appearance in a detached or isolated manner.
3342. The Fish's eye is upon the whole composed of the same structures as in the Mammalia; but it is devoid of motion and external coverings.
3343. The ear, as being the sense of motion, has scarcely withdrawn itself from the brain, has not yet become a true external organ, and that portion of it which appears externally, is subservient to inferior systems, such as the branchiæ or gills.
3344. The external auditory organ has become confluent with the branchial aperture, and the auditory ossicles have become parts of the branchial operculum.
3345. In the internal ear only the three semicircular canals have been left. The cochlea is not yet developed.
As true palpebræ or eyelids are wanting to the visual organ, so also are the conchs of the external ear.
3346. The nostrils exist, because a vertebral canal, which terminates in them, is present; stoutly-developed olfactory nerves are also present, so that the sense of smell cannot be wanting. Only this sense has not yet taken the respiratory organ into communication with itself, and both therefore live for themselves in an arrested or stunted condition. This nasal organ does not open into the mouth, admits of neither water nor air passing through it, and therefore serves not as a test-organ to the respiratory process. This is my main point for distinction of Fishes from Reptiles.
3347. Every Sarcozoon, whose nostrils do not open into the mouth, is a Fish. The Siren therefore does not belong to the present class, while the Lepidosiren, having an imperforate nasal organ, does.
3348. The tongue has remained rather an organ of touch and deglutition, than an instrument of taste. The salivary glands are scarcely developed.