472 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



vertebrates; and their number is, of course, proportional 

 to their size. Most bony fishes have two only, but 

 these are rather to be looked upon as calcareous 

 masses than as separate otoliths ; in Elasmobranchs 

 such otoliths are often grouped into masses of various 

 sizes and forms. In the Teleostei they are crystal- 

 line, but in Chimsera and the sturgeons they are 

 more chalky in character. Their function is, as in 

 invertebrates, that of aiding the vibrating fluid in 

 its action on the sensory cells of the auditory crest. 



CHAPTER XIII. 



ORGANS OF REPRODUCTION. 



IN the preceding chapters we have considered the 

 various organs of the body by means of which an 

 animal is enabled to sustain or defend its own exis- 

 tence, to obtain information of what is happening around 

 it, and to adapt itself more or less successfully to the 

 course of events. So far as an individual animal is 

 concerned, no other organs than those with which we 

 have already dealt are necessary for the maintenance 

 of its own existence ; indeed, there are, we know, 

 individuals which do pass through the whole of their 

 lives, are born, grow, and die, without once putting 

 into active function the set of organs that remain to 

 be considered. 



Unimportant as they may be to the individual, they 

 are of prime importance for the species to which 

 that individual belongs ; for they are the means by 

 which individuals are enabled to reproduce their 

 kind ; and they are of the more importance inasmuch 

 as, so far as we know, living matter never arises or is 

 formed except from pre-existing living matter. In 



