232 W. BALDWIN SPENCER. 



be granted that an Elasmobranch, such as Raja or Acan- 

 thi as, differs from Cyclodus inasmuch as none of its living 

 allies have the organ more highly developed, whilst in forms 

 allied to Cyclodus it is in a high state of development; yet 

 even this is by no means of so great importance, as to make us 

 conclude that living forms present us with the highest stage 

 yet reached in Elasmobranchs. If we turn to the Am- 

 phibia we find a group of animals amongst whom in no living 

 form is there a pineal eye present, and yet we may feel per- 

 fectly sure that in the great group of extinct Amphibia 

 (Labyrinthodonta) one was not only present but most probably 

 developed to its highest point. It must be admitted that we 

 have at present no direct evidence of the existence of pineal 

 eye within the group Pisces : until our knowledge is far greater 

 with regard to the development of the structure in, more 

 especially Dipnoi and Ganoidei, it will be impossible to 

 determine the question of the presence or absence of the 

 structure within the group. Meanwhile, the varied state of 

 development seen in such forms asPetromyzon on the one 

 hand, and Acanthias, Raja, and Scyllium on the other, 

 may perhaps be taken as evidence tending in favour of the 

 view that in its present form the organ is rudimentary. All 

 that may now be rightly insisted upon is that the absence of 

 the eye in living forms, either of this or of any other class, is 

 no proof that one has not been present at some period in the 

 phylogenetic history of the group. 



The conclusions, finally, to which we are brought are the 

 following : 



(1) Our present knowledge is not great enough to 

 allow us, in Amphioxus, to homologise any structure 

 either with the Tunicate azygos eye or with the 

 epiphysis. 



(2) The epiphysis of higher Chordata is the homo- 

 logue of the larval Tunicate eye. 



(3) The pineal eye is produced as a secondary 

 differentiation of the distal part of the epiphysis. 



