i STRUCTURE AND PHYSIOLOGY OF ANIMALS 37 



the integument to such general impressions may be increased by 

 the presence in it of a variety of tactile papillae or corpuscles 

 having nerve-fibres terminating in them. In most animals, how- 

 ever, there are certain organs, the organs of special sense, 

 adapted to receiving impressions of special kinds eyes for the 

 reception of the impressions produced by light, ears for the recep- 

 tion of those produced by the waves of sound, olfactory organs or 

 organs of smell, and gustatory organs or organs of taste. The most 

 rudimentary form of eye is little more than a dot of pigment 

 which absorbs some of the rays of bright light these producing 

 a nerve-disturbance in certain neighbouring nerve-cells. To this 

 may be added clear, highly-refracting bodies which intensify the 

 effect. In the higher types of eye there are the same essential 

 parts the clear, highly-refracting substance, the pigment and the 

 nerve-cells ; but each has undergone a development resulting in 

 the construction of an organ adapted to the reception of light- 

 impressions of a very definite character. The highly-refracting 

 body assumes the form of a lens for the focussing of the light-rays ; 

 the nerve-cells are arranged in a regular layer, the retina, from 

 which nerve-fibres pass to the central part of the nervous system ; 

 the pigment is so arranged as to absorb the light rays and prevent 

 their passage beyond the retina, and in certain cases lines also a 

 diaphragm, the iris, with a central aperture through which the 

 rays of light are admitted to the central parts of the eye. In 

 some animals (Insects, Crustacea) the eye consists of a very large 

 number of independent elements, each with its refracting apparatus, 

 its nervous element, and its absorbing pigment. 



The ear in its simplest form is a membranous sac or otocyst with 

 internally projecting stiff cilia, and containing a liquid in which 

 there lie a number of particles of carbonate of lime. The sound- 

 waves evidently set in vibration the liquid and its contained cal- 

 careous particles, and by means of these vibrations acting on the 

 cilia, an impression of a definite character is produced in the cells 

 of a neighbouring nerve-ganglion. In higher forms the apparatus 

 for receiving the vibrations becomes extremely complex, and there 

 is elaborated a nervous mechanism by which sounds of different 

 pitch and intensity produce impressions of a distinct character. 

 The organ of hearing frequently assumes the additional function 

 of an organ ministering to the sense of rotation and thus has an 

 important part to play in the maintenance of the equilibrium of 

 the body. 



The essential elements of the reproductive organs the ova 

 and spermatozoa have already been briefly alluded to (p. 28). 

 The ova are developed in an organ termed the ovary, and the, 

 sperms in an organ termed the spermary or testis. Sometimes 

 ovaries and testes are developed in the same individual, when the 

 arrangement is termed monoecious or hermaphrodite ; sometimes 



