Evolution of Fishes from Invertebrates 75 



these roots then in growing downward would become the 

 vertebrate infundibulum. 



In vertebrate morphology the pecuHar character and 

 relation of the downgrowth from the thalamencephalon, 

 known as the infundibulum, has often excited discussion. 

 Its history seems to be satisfactorily explained in phylo- 

 genetic continuity from the nemerteans. For Hubrecht 

 and others have shown that from the cerebral ganglia 

 two strong nerves run down to, enter, and innervate the 

 proboscis. In the anterior muscular portion of it, the two 

 nerves break up into a complex circular and longitudinal 

 system, that give extreme sensitivity to the entire proboscis, 

 specially to the anterior portion. Now, as this anterior part 

 separated, as suggested above, from the innermost or gland- 

 ular part, the main nerve threads would become more or 

 less weakened, sundered, and reduced in function, though 

 they evidently remained in contact with the glandular por- 

 tion. So we would suggest that the infundibulum represents 

 the largely abortive remnant pair of proboscis nerves that 

 still remain in contact with and innervate the pituitary 

 body. Balfour says regarding it: "in Mammalia the pos- 

 terior part of the primitive infundibulum becomes the 

 corpus albicans, which is double in man and the higher 

 apes." 



Though it must at once be acknowledged that no living 

 animal presents transition conditions that fit in with all 

 the requirements of the case, the explanations above given 

 are so well correlated with the morphological requirements 

 that such suggestions are at least permissible, specially see- 

 ing that no equally helpful ones have been forthcoming. 



The proboscis-sheath or rhynchocoel, as studied by 

 different observers, is a strong muscular tube, which in sim- 

 pler groups of nemerteans extends only one-third back- 

 ward from the head. But in most of the nemerteans it 

 stretches from behind the dorsal ganglion to the anal region 

 of the tail. Anteriorly it unites with the rim of the pro- 

 boscis by a circular zone named the rhynchodoeum by 

 Burger. It is at this zone that sudden rupture of the 

 sheath from the proboscis may occur, when an animal is 

 strongly irritated. A gradual but permanent separation of 



