THE PINEAL SYSTEM OF FISHES 221 



The Pineal Region of the Dipnoi 



The primitive nature of this sub-class, the three genera of which 

 are collectively known as lung-fishes, might lead one to expect a specially 

 instructive condition of the pineal system. This anticipation is, however, 

 hardly realized, for although from many standpoints these fishes — 

 Ceratodus, Protopterus, and Lepidosiren — are of prime importance in 

 the study of comparative anatomy, the parts of the brain included in the 

 pineal system do not show a high grade of organization. No pineal eye is 

 present. The pineal organ shows signs of degeneration and all trace of 

 a parietal foramen has disappeared. Incidentally, it may be mentioned 

 that the lateral eyes of Lepidosiren are relatively small and show a pecu- 

 liarity in development which was described by Graham Kerr (1919), 

 namely, instead of the optic vesicle being formed as a hollow outgrowth 

 it is developed as a solid bud from the forebrain, which at the time when 

 the optic rudiment first appears is itself solid ; later a cavity appears 

 as a secondary formation in the primarily solid optic outgrowth, and the 

 subsequent development of the eye follows the usual course. Another 

 peculiarity which the lateral eyes of the Dipnoi share with Elasmobranchs 

 and Cyclostomes is the absence of a processus falciformis. The origin 

 of the lateral eyes as solid buds is probably a devolutionary rather than 

 an evolutionary change. Moreover, with regard to the structure of 

 the end vesicle of the pineal organ of Polypterus, Studnicka found the 

 walls markedly folded, the lumen thus being almost obliterated. He also 

 found that some of the epithelial cells contained a brown pigment, both 

 of which conditions indicate a degenerate state of an organ previously 

 more highly developed. It would seem, therefore, that we are not only 

 dealing with a class of animals which is primitive in the evolutionary 

 sense, the Dipnoi having persisted up to the present age from the early 

 Mesozoic period without having undergone much further developmental 

 organization — that is to say, have remained in a more or less stationary 

 condition — but that during or even before this period retrogressive changes 

 have taken place both in the development of the lateral eyes and in the 

 pineal organ. 



Ceratodus Fosteri 



Huxley (1876) described the pineal organ as consisting of a cylindrical 

 stalk ending anteriorly in a heart-shaped expansion, the latter lying in a 

 depression in the cartilaginous roof of the skull. He also gave an admirable 

 illustration of the bones on the dorsal surface of this remarkable skull 

 (Fig. 156), which closely resembles Krefft's figure. In neither of these 

 is there any indication of a parietal foramen on the superficial aspect of 



