The organs of special sense in arthropods are eyes. olfactory auditory organs, and organs of touch. The eyes are of two sorts, median eyes and a pair of faceted, compound eyes. The median or `nauplins' eyes sometimes occur in the larvae only, and are composed of a cuticular lens and a group of specially modified epidermal cells.
The paired, compound eyes are eovered by a tiansparent cuticula. A siugde visual element of the compound e? called an ommat idium, elenn•nt is composed of two parts. On outer vitreous and an inner sensory group of cells. The latter is known as the rhabdome and is made up of the rethink. and a refracting rod. Eaell ommatidium is in a state of optical isolation. Fibres pass from the optic ganglia to the rethink. The organs of touch are bristled cells on the appendages. especially oil the antennfe, the palpi, and on other parts of the body. The Qifaebe.e organs of the crayfish efoisist of 0 number of delieate setae borne on the antennules. The midi 1...ry organ is pit upon the exterior and lined with auditory seta:. I :rains of sand in the case of Crustacea often serve the function of otoliths. These must he renewed after each molt.
.\iiiphioxits possesses an olfactory pit. It is a ciliated depression at the level of the anterior end of the brain. It is connected with the olfac tory lobe of the brain. This pit may correspond with the 'pituitary body' of l'rocliorda and Craniata. The organ of sight is merely a pig ment spot. It is median and without a lens. (elk supposed to have a sensory function occur on the roof of the oral hood and tactile yells occur on the buccal cirri.
The organs of special sense of vertebrates com prise various kinds of tactile organs, of which may by mentioned the lateral-line organs. whieh in structure resemble the organs of elm.topods.
The organs occur in amphibians and fishes, and are supposed to perceive vibrations in the water and also to have an auditory func tion. End-bulb organs likewise oeeur here and there over the entire surface of fishes. From amphibians upward they are restricted to the mouth. Specialized tactile cells. such as the Paeinian corpuscles of the duck's beak, occur in different vertebrates.
The olfactory organ is in fishes a pair of sensory pits near the anterior end of the snout. From the Dipind onward these pits communicate with the month cavity as well as with the ex terior. From the 1)ipnoi onward also these organs are more or less inclosed either in a cartilaginous or they are included in the cranium. The internal nostrils with their glands are developed in Dipimi and Amphillia. The
nasal cavity 4.1 Laeertilia is divided into an outer0 and an inner part. From the eroeodiles; onward the nasal organs revede in correspondence with the forward extension of the facial region. Roth in reptiles and birds there is (oily one true turbinal bone in the nasal canal. Because of l'Xt 111•V(.11.1,111ent of the skull of mals, the nasal ea vity is proportionately larger. The thrbinals consequently greatly develop here. The nasal cavity 4.f mammals communicates with eertflin sinuses of the face.
The sensitive elements of the vertebrate eye develop from outgrowths, primary optic vesicle:, of the brain. The pigment epithelium develops from an inpocketing of this vesicle. The kn., develops front thiekened epidermal cells. in cyclostomes the lens and iris are absent and the ^clerotie and cornea are undifferentiated. A globular lens is present in the eyes of true fishes. Amphibian eyes are simpler than those of fishes, but a eiliary muscle is present. The sclerotic is cartilaginous with bony plates even in lizards and birds. The eye-hall of nocturnal birds is elongated and tubular. The iris of birds may be highly colored 4/Ming to the presence both of pigment and of fat globules. The eye of mam mals is more or less inclosed in the bony orbit, the sclerotic is fibrous, aml the fibres of the optic nerve pass into the retinal yells. The retina is histologically a complicated structure and consists of seven layers; the light must pass through all the layers in order to reach the innermost rod and cone layer. The region of ientest vision is the fovea centralis, where only the owes persist.
As to auditory organs. it is the sacculus and cochlea in the vertebrate ear that attain a higher and higher degree of development. In fishes the cochlea is a knob-like appendage of the sacculus. III amphibians it is constricted off from the sac culus and only remains connected by a strand of eelk. In mammals the cochlea heroines spirally coiled. There are three coils in man. A chain of three amlitory ossicles appear in the higher vertebrates.
LiTERATURE: Wiedersheirn. COM pa .1n(11- Only Of FerfrbraleS ( Eng. trans., London, 1897) ; Kingsley. Text-book of I i rye boric Zoology ( New York, 1699); Lang, Tcst-book of Comparutice Anatomy ( London. 1801-9(1) : Lang, "Leber pri mitre Aletamerie des Neuralrohres der. Vertebra ten," in Sit.;:ungsberiehte (101- koodylich baieriRc)f ell .tkarlcmie du- Wisscuschaften (Munich, IS85).