THE SYMMETRY OF ANIMAL FORMS. 87 



includes single organs placed on the median or middle 

 line. These are often spoken of as " unpaired," but 

 it must be remembered, that they are alike on both 

 sides. The median eye (spoken of on p. 79) and 

 the median fin (p. 85) may be noted as instances of 

 " unpaired " structures. More familiar instances are 

 the cock's comb and the horse's mane. 



But the distinguishing feature of the head is the 

 presence of the brain, that central part of the nervous 

 system which correlates the impact of feeling from 

 without with the impulse of action from within, and is 

 the physical seat of those synthetic mental powers of 

 the organism as a whole which, viewed from their 

 subjective side, we call consciousness and intelli- 

 gence. The brain reaches its highest development 

 in the vertebrata, and among these its culmination 

 is found in man. The peculiarity of the brain in 

 vertebrates is, that it is all placed above the mouth 

 In most segmented invertebrates, including some of 

 very high intelligence, such as the lobster, the brain 

 is supplemented by nerves which form a collar passing 

 round the mouth and throat. 



