256 PRACTICAL LESSONS IN SCIENCE. 



the sweat glands and sebaceous glands serve to equalize temper- 

 ature and keep the skin moist and supple. 



The systems of organs already mentioned are all concerned in 

 the process of nutrition or the vegetative functions. The organs 

 of locomotion arid sensation, however, belong to the phenomena 

 of animal life. The organs of locomotion vary from the cilia of 

 some protozoa, and the muscular walls of worms to the compli- 

 cated mechanical apparatus of the vertebrata. The bones and 

 other hard parts which assist in locomotion frequently serve as 

 protecting organs for more delicate structures. 



Among the higher animals the organs of sensation, in addition 

 to the brain and nerves, consist of certain peculiar-shaped bodies 

 of nervous tissue, which are called the end organs of certain nerves, 

 as the rods and cones of the eye, the auditory hairs, olfactory 

 cells, taste bulbs, and tactile corpuscles. Each of these has acces- 

 sory organs which serve to protect them and at the same time 

 aid them in gaming impressions of the outside world. These or- 

 gans appear in various forms among the lower animals, and 

 some of them are occasionally wanting. 



Animals of the lower orders multiply by division and by bud- 

 ding, but some form of sexual reproduction is more common 

 among animals. Sexual reproduction depends on the formation 

 of two kinds of germinal cells, the combined action of which is 

 necessary to the development of a new organism. 



One of these cells contains the material from which the new 

 individual grows and is called the ovum or egg cell ; the other 

 called the sperm cell contains the fertilizing material, which fuses 

 with the contents of the ovum and in some unexplained way 

 causes its development. The structure of the sexual organs 

 varies widely among the lower animals. 



The simplest and most primitive case is where the ovum and 

 sperm cell are produced in the same individual; this is called 

 the hermaphrodite arrangement, and while it occurs in almost 

 every group of animals, the two cells are more commonly pro- 

 duced by different individuals. Sexual reproduction is in reality 

 a special form of growth, as the egg cell sometimes undergoes 



