Evolution of Animals 439 



that passed to the cerebral organs, which the writer would 

 view as in part an auditory apparatus. 



Two ventral ganglia, closely placed below and behind the 

 dorsal in many nemerteans, are united with each other by a 

 strong ventral commissure, that in nemerto-vertebrates appar- 

 ently became the Pons Varolii. In front of these ganglia 

 the proboscis-tube and proboscis opened into the oral or buccal 

 cavity. But as the proboscis-tube became a strengthening 

 rod of tissue, and filled its cavity from behind forward with 

 loose cells to thus constitute the notochord, the posterior or 

 basal glandular part of the proboscis grew upward above the 

 buccal cavity as the pituitary body, while its glandular middle 

 zone probably gave origin to the thyroid and thymus glands. 

 In regard to these latter glands, however, it may well be that 

 they originated from the mass of oesophageal gland- tissue 

 that Burger appropriately suggests might be named (p. 194) 

 the "drusendarm." A comparison of the structure of these 

 in existing fresh-water and land metanemerteans with the 

 andostylar or thyroid organs of vertebrates would constitute 

 a valuable and welcome study. 



During the above changes the ventral ganglia rose upward 

 through the region once occupied by the front part of the pro- 

 boscis, and became apposed behind and somewhat below the 

 bilobed dorsal ganglia, to constitute the medulla oblongata. 

 The cavity between the increasingly convergent dorsal anterior 

 lobes became the third ventricle, that between the dorsal 

 posterior lobes the iter, and that between the now uprisen 

 and converged ventral ganglia the fourth ventricle. The 

 ventral nerve ganglia, therefore, which we consider to have 

 fused dorsad to form the medulla oblongata, are largely con- 

 cerned with correlation of the simpler or bio tic functions. In 

 this connection C. J. Herrick (^4^^:668) says: "While much 

 remains to be explained in the comparative anatomy of the 

 medulla oblongata, the underlying morphological pattern has 

 been exposed, and is found to be surprisingly constant in all 

 vertebrates. This constancy of type grows out of the fact 

 that this part of the brain uniformly serves the simpler \dtal 

 functions, such as feeding, respiration, etc., whose peripheral 

 mechanisms are broadly similar in vertebrates." In contrast 

 to this the dorsal ganglia are largely cognito-cogitic in func- 

 tion, since they rapidly bring each organism into contact with 

 light, sound, gravity, and the more delicate chemic stimuli, 

 and correlate these so as to give rise to cogitic responses. Figs. 

 18a-18f illustrate the above changes. 



