THE PINEAL SYSTEM OF FISHES 223 



Lepidosiren 



The development of the South American lung- or mud-fish was 

 specially studied by Graham Kerr (1919). The pineal organ arises in the 

 usual situation between the habenular and posterior commissures and 

 extends forwards on the roof of the diencephalon as a tubular, " carrot- 

 shaped " diverticulum, which reaches a point just behind the hemispheres 

 and well above the paraphysis (Fig. 157). On either side of it are the well- 

 developed habenular ganglia, which as they enlarge become connected 

 by a transverse bridge of nerve fibres, the habenular commissure. The 

 roof of the mesencephalon, which lies close behind the pineal organ, 

 becomes slightly thickened on each side of the mesial plane, forming the 

 arched tectum opticum, but " correlated with the small size of the eyes in 

 Lepidosiren the thickening never becomes so great as to produce projecting 

 optic lobes such as are formed in most vertebrates." 



Protopterus 



The pineal organ of the South African lung-fish was described by 

 Burckhardt in 1892 and another specimen was described by Studnicka in 

 1905. In both these specimens the organ was divisible into three parts, 

 a club-shaped end vesicle which was subdivided into several compartments 

 by folds of its wall ; an elongated stalk which in Studnicka's case was 

 hollow throughout, whereas in Burckhardt's specimen the central part was 

 solid, only the third or proximal part having a lumen. The lumen, 

 however, was closed at its base, there being no communication with the 

 ventricle. This proximal part probably corresponds to the " pineal gland " 

 or epiphysis of mammals ; the stalk and end vesicle may fail to develop as 

 in the higher vertebrates, or the latter may be cut off by the growth of 

 the skull from the proximal part, the end vesicle persisting as Stieda's 

 organ, e.g. in the frog, while the proximal part differentiates as the 

 epiphysis. 



