92 FREDERICK TILNEY AND LUTHER F. WARREN 



6. Myxine gluiinosa (Bdellostoma). Kupffer ('04). 226 In this 

 form there is no anlage of the epiphysel complex whatsoever. 

 The roof is entirely flat, but in spite of the absence of the epi- 

 physeal complex, both habenular ganglia are present. Such 

 descriptions of the epiphysis in Myxine as appear in the litera- 

 ture seem to be an error. Andrae Retzius 33lA in 1822 described 

 the pineal body in connection with the habenular ganglion, 

 interpreting the latter to be the epiphysis. Ley dig 239 in 1896 

 believed that he had found in Myxine the pineal body, but in 

 reality mistook a large lymph space near the surface of the 

 head for this organ. Studnicka, 388 however, in his studies was 

 unable to find any evidence of the pineal body in Myxine. 



2. The comparative histology and anatomy of the epiphyseal 

 complex in selachians 



Since the pineal organ is the only part of the epiphyseal com- 

 plex to make its appearance in selachians, the structure is much 

 more simple than in cyclostomes. Furthermore, such parts of 

 the pineal organ as do develop in selachians are relatively rudi- 

 mentary. All of the three usual elements of the pineal organ, 

 however, may be identified; that is to say, a hollow end-vesicle, 

 a stalk, and a proximal portion. The end-vesicle in no instance 

 presents the two distinct walls, namely, the ventral and dorsal 

 walls distinguishable in cyclostomes, and the end-sac itself is 

 much smaller than in the forms already considered. Slight 

 differences in the thickness of the wall of the end-vesicle may be 

 observed in different places, but with no great uniformity. In 

 consequence of this lack of differentiation, there is no evidence 

 of the formation of a retina, of a pellucida, or of a white sub- 

 stance, nor do any nerve fibers make their appearance in con- 

 nection with the end- vesicle. In fact, it is a question whether 

 the pineal organ of selachians is a primitive structure or one 

 that is distinctly retrograde. In form there may be a consider- 

 able difference in the terminal vesicle; it may be wedge-shaped, 

 cylindrical, conical, or flattened, but in all instances it is hollow, 

 containing a lumen, in spite of the statement of Cattie 60 to the 

 contrary in his descriptions of Mustelus, Raia, and Acanthias. 



