Brain and Pineal Structures of Polyodon folium. 301 



the organ lies, and thus serve to keep it in place. Some of 

 the fibers arise from the dorsal face of the sheath of the 

 end organ. The essential part of the organ, within the 

 sheath, looks, on ordinary examination with a low-power 

 objective, like a small whitish nervous ganglion. It is^ 

 somewhat slipper-shaped viewed from the side, but a 

 dorsal view shows it to be a little depressed, with the 

 dorsal side slightly convex. The outline is roughly oval 

 in shape, viewed from above. In the specimen studied 

 most carefully' there was a slight rounded anterior 

 projection. From my recollection of some sections pre- 

 pared by me at the Illinois Laboratory, and of a draw- 

 ing of one of them made by Mr. Westergren, I can say 

 that the organ contains a cavity, but presents nothing 

 which could be regarded as a lens. The stalk and end 

 organ are sometimes accompanied by pigment, but 

 may show only a slight trace of it, as indicated in my 

 outline figure (Fig. 9) of these structures. In one exam- 

 ple studied pigment was much more abundant than in 

 the rest, and was accumulated beneath the end organ 

 somewhat as it is back of the eyes of Vermes and 

 Crustacea. 



The channel in which the stalk and end organ lie is in 

 one of the shovel-fish* examined 18.5 mm. in length, 

 and has an average diameter of about two millimeters. 

 Distally it enlarges somewhat, and is rounded at its ex- 

 tremity. A system of smaller passages ramifying in the 

 cartilage of the cranium is in communication with it. 



In one of my specimens the cavity about the end organ 

 is more enlarged than in the others and a small passage 

 leads through the cartilage above to the exterior. The 

 passage was plugged with connective tissue similar to 

 that constituting the skin which overlies the cartilage 

 above the end organ. In another example studied I 

 found no trace of an opening to the exterior. Unfortu- 

 nately I neglected to examine thoroughly, with reference 



* This flsh was not less than three feet long. 



