24 



SENSE-ORGANS 



complicated folding of this sac are elaborated a sacculus and 

 utriculus, from which spring two vertical and one horizontal 

 semicircular canals in all Craniata excepting the Cyelostomes. 



The relation these organs of special sense bear to their respec- 

 tive cartilaginous capsules, and their influence on the process of 



Fig. 13. 



cephalisation, has already been alluded to above (p. '!). Xo homo- 

 logues of these organs have been found in the Cephalechorda. 



Vet another organ of sense remains to be noticed — the pineal 

 eye. Although it may not have a claim to the all-important 

 function attributed to it by Descartes, the pineal eye, or epiphysis, 

 is of considerable interest. Leydig in 1874 described it as a sense- 

 organ ; but De Graaf and Spencer, in 18S6, were the first to demon- 



