208 



COMPARATIVE ANATOfilt 



stages of Teleosts. A large, paired, dorsal lobe is present on the 

 medulla. 1 



In Petromyzon the pineal apparatus is represented by two 

 vesicles, which are probably the displaced members of a pair, each 

 connected with the dorsal surface of the diencephalon (ganglion 

 habenulae) and lying one above the other just beneath the roof of 

 the skull ; the integument immediately above these vesicles is 

 pigmentless. The cells on the ventral side of the larger dorsal 

 vesicle (epiphysis or pineal organ probably representing the organ 

 of the right side) are arranged radially and contain pigment, form- 



t* 

 *^\ 



v^*> 



^ ^ 



i^ 



"Pellucida ~ f^/, 



' Z XV 



.~ Pineal organ 



Parapineal 

 organ 



FIG. 156. TRANSVERSE SECTION OF PINEAL APPARATUS or Petromyzon marinm. 



(After Studnicka.) 



The connective tissue roof of the skull (s) is seen above, the roof of the third 



ventricle (di) below. 



ing a kind of retina, but they show signs of degeneration ; the 

 lower vesicle or parapineal (left) organ, which probably corresponds 

 to the parietal organ of Lizards (q.v.) is simpler and more variable, 

 and is without pigment. The pineal stalk, arising just in front of 

 the posterior commissure, passes directly into the dorsal vesicle, 

 and the ventral vesicle is also connected with the roof of the 

 diencephalon (habenular ganglion) by a stalk. In Myxinoids the 

 pineal apparatus is evidently much degenerated, and nothing is 

 known of an epiphysis proper. 2 



Elasmobranchs. The brain of these Fishes, like that of 

 Cyclos tomes, is in many respects of a specialised form, character- 



1 The homology of the parts as given above has recently been questioned : 

 it is possible that the division described as the telencephalon corresponds to the 

 diencephalon, and that the " cerebellum " belongs to the mid-brain. 



2 For the hypophysis of Cyclostomes, see under Olfactory Organ. 



