THE PINEAL EYES OF CYCLOSTOMES 187 



The Pineal Eyes of Cyclostome Fishes 



The pineal eyes of this primitive class of vertebrates have been specially 

 studied by Ahlborn, Wiedersheim, Studnicka, Dendy, Beard and Gaskell. 

 The class comprises the marine and freshwater lampreys {Petromyzon) 

 and the Australian form of lamprey (Geotria) ; also the slime-eels or hag 

 fishes {Myxine and Bdellostoma), Bdellostoma being so named on account 

 of its leech-like mouth. It is described as being blind, parasitic and 

 degraded in type, and specimens of it have been taken from great depths 

 of the sea. The lampreys, on the other hand, although the median 

 eyes are probably little more than light-percipient organs, have well- 

 developed lateral eyes in the adult animal and are active in their move- 

 ments. They feed on the flesh of living fishes and are predatory rather 

 than parasitic. 



The pineal system of the adult Petromyzon is situated close behind 

 the single nasal orifice. It consists of two vesicles : one, the larger, 

 called the parietal organ, is situated more superficially ; while the other, 

 which is smaller and less differentiated, lies beneath the former (Fig. 22, 

 Chap. 3, p. 28, and Fig. 134, p. 188). The latter was termed by Studnicka 

 the parapineal organ. The larger vesicle is connected by a definite tract, 

 the pineal nerve, with the right habenular ganglion (Fig. 14, Chap. 3, 

 p. 19), and also sends fibres into the posterior commissure and right 

 bundle of Meynert (Fig. 134). The smaller vesicle is connected by a 

 few fibres with the left habenular ganglion, posterior commissure, and 

 left bundle of Meynert. Owing to its anterior attachment to the roof of 

 the thalamencephalon or interbrain, the smaller organ is sometimes 

 termed Epiphysis I, while the larger organ, on account of its posterior 

 attachment, is known as Epiphysis II. The right habenular ganglion, 

 in correspondence with the greater size of the superficial vesicle as 

 compared with the smaller deep organ, is proportionally larger than the 

 left habenular ganglion, and the right bundle of Meynert is for the same 

 reason larger than the left bundle (Dendy, A., 1907). 



Owing to the connection of the larger vesicle, by means of its pineal 

 nerve, with the large right habenular ganglion, and the similar connection 

 of the smaller vesicle by nerve-fibres with the small left habenular 

 ganglion, and also the position of the smaller organ slightly to the left 

 of the larger, along with other evidence of the bilateral origin of the 

 two organs, Dendy, Gaskell, and others regarded the two vesicles 

 as right and left members of a primarily paired organ, rather than anteriorly 

 and posteriorly arranged unpaired metameric organs arising in the median 

 plane. Professor Dendy also assumed that the smaller vesicle having 

 undergone greater regressive changes than the larger, has been displaced 



