THE PINEAL SYSTEM OF FISHES 2IJ 



hollow, tubular stalk which arises immediately behind the habenular 

 commissure. It extends upwards and backwards, in close relation with 

 the posterior wall of the dorsal sac, to its posterior angle, where it bends 

 sharply forwards and becomes continuous, without any abrupt change in 

 diameter, with an elongated tubular end vesicle. This extends forwards, 

 in a slight groove on the superficial aspect of the dorsal sac, as far as the 

 level of the posterior ends of the hemispheres, where it terminates in a 

 pointed extremity. In the posterior part of this remarkable end vesicle, 

 the wall is slightly folded, while in the anterior part the walls are smooth 

 and the lumen somewhat larger. The wall of the vesicle shows the usual 

 ependymal structure, containing certain cells which resemble sensory- 

 cells. The lumen contains a few free cells, but no special syncytial 

 formation. No parietal impression or foramen was found in the roof of the 

 skull. 



Teleostean Fishes 



In adult bony fishes, as a rule, only one pineal organ has been found 

 in the adult animal, although definite indications of two organs have 

 been described by Hill in early embryos of several types of fish belonging 

 to this class, namely : Salmo (Fig. 138, C, Chap. 17, p. 196) ; Catostomus 

 teres (sucking-carp) ; Corregonus albus (Fig. 152) ; Stizostedion (perch) ; 

 and Lepomis. Of the two organs, one, the smaller, is placed in front and 

 to the left ; the other, the larger, is behind and to the right. These 

 observations are of considerable value, as they tend to confirm the view 

 held by Locy with respect to the bilateral origin of the pineal organ, 

 which was based on his observations of paired hemispherical evaginations 

 near the outer edge of the unclosed neural-plate of dog-fish embryos, 

 and also the observations recorded by Eycleshymer and Davis on the early 

 developmental stages of Amia, and that of Kingsbury on an adult example 

 of Amia calva. 



The pineal organ of the teleostean fishes is much more variable, 

 both in structure and in form, than it is in the Selachii and ganoids. 

 It may consist of a minute end vesicle at the end of a thin, elongated stalk, 

 as in Ophidium (Fig. 153), or a small solid nodule without a definite stalk. 

 In other cases the end vesicle is expanded and has the shape of a flat, 

 toad-stool fungus ; its structure is also complicated by the intergrowth 

 of vascular folds of the surrounding mesoderm and outgrowing buds or 

 folds of the neuro-epithelial wall. A good example of such is found in the 

 pineal organ of the gar-pike {Belone acus) (Fig. 155). In bony fishes there 

 is, in general, a tendency for the stalk of the pineal organ to be short 

 and the end vesicle large, as compared with the cartilaginous fishes and 

 ganoids, in which the stalk is usually long and the end vesicle small. 



