3431. All possess moveable eyes; fleshy noses, which stand open both externally and internally; ears opening outwards, and mostly provided with a moveable flap; a fleshy tongue, free in front, and moveable lips; with at least thoracic limbs and a skin covered with hairs.
3432. In the Bird, Reptile, and Fish the face is merely invested by tegument, nearly devoid of any muscles, and therefore immoveable. They have a tegumental face, which is incapable of producing any expressions.
3433. In the tegumental face the eyes are motionless, and very rarely both directed so forwards that they can view an object together; the nostrils are frequently devoid of a fleshy rim; the tongue often feather-like, cartilaginous, or covered with teeth; true fleshy lips are wanting, as are frequently the teeth, with even the limbs and digits, or these are divided into a number of rays, forming either feathers, or fins; in the Aistheseozoa there are never more than five toes present, and if there be fewer, then the number admits of being referred to some crippling of the normal quantity, five.
3434. It is remarkable, and serves to the discovery of many laws, that the highest sense here appears for the first time in its state of perfection. The eye in the Aistheseozoa is present in a perfect condition throughout the class, with the exception of the eyelids; the other organs of sense are, on the contrary, exhibited in all their gradations of structure.
3435. It appears, as if the whole animal was first perfected, when the eye is present with all its investment or clothing. The eye of the Aistheseozoa has not simply all its internal chambers and all its humours, but also all its muscles; it is moveable and has perfect eyelids, with very few exceptions—Ophthalmozoa.
3436. The ear now begins to suffer arrest. Its completion is indeed the formation of an external concha or flap for receiving the rays of sound, the hand being repeated in the ear, and its skeleton constituted by the auditory ossicles. This auditory hand occurs only in the Aistheseozoa, and might serve as characteristic if it were not wanting in many, while the eyelids are present. As in the Whales, where the auditory passage can be, however, closed, an act which is not possible in any Bird. The Bird must hear whether it will or no.
In all Thricozoa the interior of the ear is perfect, having cochlea, semicircular canals, tympanum, and, as brachial parts, the three conjoined auditory ossicles. The concha of the ear passes besides through all stages of development, from having a simple margin to one with most varied convolutions, lobes and opercula.
3437. Still more than the ear does the nose undergo modifications. In the Whales it seems to be less adapted for the purposes of smell than of respiration. The olfactory nerves are in them very delicate, and the apex of the nose is destitute of motion.
In other beasts, on the contrary, it is elongated into a very muscular proboscis or snout, and is endowed with the power of voluntary motion.
The form also of the nostrils is very varied, they being round, narrow, patulous, and frequently capable of being closed.