THE FINER ANATOMY OF THE BRAIN OF BDELLOSTOMA 



DOMBEYI. 



1. THE' ACUSTICO-LATERAL SYSTEM. 



BY 



HOWARD AYERS AND JULIA WORTHINGTON. 

 With Eight Plates. 



In keeping with its position near the lower end of the vertebrate 

 phylum, Bdellostoma possesses a brain relatively simple in its structure, 

 yet much more complex than that of Amphioxus. It is much less 

 complicated than that of even the lowest fishes, and represents an early 

 and most important stage in the phylogeny of the vertebrate brain. The 

 vertebrate brain plan, already clearly and^ firmly outlined in Amphioxus, 

 is developed so much farther that all the fundamental brain organs 

 found in higher vertebrates are present in Bdellostoma, and the funda- 

 mental divisions and fiber tracts, persistent in all other vertebrates, 

 including man, are clearly defined. The connections between different 

 parts of the brain are, as we would expect to find them, simpler and 

 more direct than in higher forms. ~0f especial importance is the fact 

 that the fundamental relations are not obscured by the intrusion of the 

 many secondary tracts and centers which are present in the higher 

 representatives of the phylum. Hence, it seemed to us that a careful 

 study of the brain of Bdellostoma, a survey of its paths, and a charting 

 of its relay stations, would throw much light not only on the many 

 intricate questions of brain anatomy, but on that still more intricate 

 and baffling problem, the origin and development of the vertebrate head. 



The gross anatomy of the brain of Bdellostoma, together with the 

 peripheral distribution of the cranial nerves, has been worked out in a 

 previous paper (Worthington, '05), and reference will be made thereto 

 for macroscopic relations. 



We recognize two main divisions of the brain of Bdellostoma, the 

 Hindbrain, including the medulla and cerebellum, and the Forebrain, 

 Amkric.^n Journal of Anatomy. — Vol. VIII. 



