THE PINEAL GLAND 295 



structures, a conclusion which, as already pointed out, receives 

 some additional support from the slightly left-sided position of 

 the anterior pineal organ in Geotria. We shall see presently 

 that the left-sided position of the anterior pineal organ is found 

 also in bony fishes and in the tuatara (Sphenodon) (fig. 3). 



The myxinoid cyclostomes afford a startling contrast to the 

 lampreys in that no trace of pineal organs has yet been discovered 

 in them. It is true that statements have been made to the con- 

 trary effect, but these are now entirely discredited, and the 

 structure of the brain, differing as it does in such an extra- 

 ordinary manner from that of the lampreys, renders it highly 

 improbable that such organs exist. 



In the true fishes the pineal organ, supposed to be 

 homologous with the right or posterior pineal sense-organ 

 of the lampreys, never exhibits any eye-like structure at all. 

 In the adult elasmobranchs (e.g. Spinax) we find a median 

 unpaired body in the form of a long, slender tube, which, 

 arising from the brain-roof between the posterior and superior 

 commissures, passes forwards above the brain and ends in a 

 terminal enlargement lodged in a cavity in the cartilaginous 

 roof of the cranium. Histologically the walls of this pineal 

 organ exhibit a very uniform structure throughout its entire 

 length, the most noteworthy feature being the presence of 

 numerous modified ependymal cells, of columnar form, whose 

 free extremities project slightly into the lumen of the organ 

 (Studnicka). Such cells are extremely characteristic of the 

 pineal organ throughout the vertebrate series, and they have 

 been compared to the sense-cells in the "retina" of those 

 forms in which an eye-like structure is developed, but whether 

 they themselves have any sensory function must be regarded 

 as very doubtful. According to Studnicka the organ is 

 innervated by a bundle of nerve-fibres which comes from 

 the neighbourhood of the posterior commissure, while it is 

 not certain whether nerve-fibres also connect it with the 

 habenular commissure, as has been observed in Acipenser. 

 In the ganoids (e.g. Acipenser) the pineal organ is very similar 

 in form and structure to that of elasmobranchs. In teleosts 

 also we find the pineal organ composed of a " stalk " and a 

 " terminal vesicle," the former being connected with the brain- 

 roof exactly as in elasmobranchs, but remaining much shorter, 

 while the terminal vesicle is usually larger, and may have its 



