Brain and Pineal Structures of Polyodon folium. 303 



the cartilage by an extension of the dorsal sac, which 

 can be followed rather more than half the distance to 

 the end organ. The stalk is, however, throughout its 

 course an entirely independent structure, and is made 

 conspicuous by the blood vessels accompanying it, the 

 blood supply here being apparently greater than in 

 Polyodon. The end organ hes- close under the carti- 

 lage, but not quite so near the surface as in Polyodon. 

 The cartilage over it, in one example studied, is 1.50 mm. 

 thick. I found no opening through the cartilage, and 

 the bony exoskeleton of the head, which is here about 

 2 mm. thick, is without a foramen. 



The end organ in A. rubicundus is somewhat different 

 from that of Polyodon. The stalk in its sheath expands 

 gradually at its distal extremity and forms an elongate- 

 obovate or club-shaped organ which is not firmly sus- 

 pended like that of Polyodon by elastic bands. It meas- 

 ures 2 mm. in length and has a diameter of 0.66 mm. 

 near its extremity. A system of minute blood vessels 

 ramifies over the surface of its sheath in such profusion 

 that one cannot help thinking that the organ must be 

 functional. No pigment is apparent about it. 



Whether these end organs are to be considered eyes or 

 not they certainly are entirely independent not only of the 

 lateral line system, but of all other nervous structures. 

 In both of these fishes the slender stalk has a very 

 direct and evident course from its origin on the roof of 

 the brain to its end organ, and without receiving or 

 giving off branches expands in the substance of this 

 terminal enlargement. If its present condition is taken 

 into account one must conclude that the structure is, 

 or has recently been, an important sense organ to the 

 sturgeons. 



The infundibulum is verjf large in both Polyodon and 

 A. rtibicundus. It opens freely above into the posterior 

 part of the fore-brain. The cavity extends well down 

 into the organ, but the hypophysis itself, though 



