40 ZOOLOGY SECT. 



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 within 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 also lines 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 usually possesses 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. 31). 

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

 sperms in an organ called the spermary or testis. Sometimes 

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

 arrangement is termed monoecious or hermaphrodite ; sometimes 

 the ovaries occur in one set of individuals the females and the 

 testes in another set the males, when the term unisexual or 

 dioecious is employed. Very frequently the male differs from the 

 female in other respects besides the nature of the reproductive 

 elements in size, colour, and the like ; when such differences are 

 strongly marked the animal is said to be sexually dimorphic. The 

 ova and sperms are usually conveyed to the exterior by canals 

 or ducts the ovarian ducts or oviducts, and the testicular ducts, 

 spermiducts, or vasa deferentia. In some instances the ova are 

 impregnated after being discharged from the oviducts, and the 

 development of the young takes place externally ; in other cases 

 the impregnation takes place in the oviduct, and the young 

 become fully developed in the interior of a special enlargement 

 of the oviduct termed the uterus. In the former case the animal 

 is said to be oviparous, in the latter viviparous ; but there are 

 numerous intermediate gradations between these two extremes. 



