No. 3.] STUDIES ON THE HETERONEMERTINI. 383 



of the nervous system. His more comprehensive papers, those 

 to which reference will be made in the following pages, are 

 '90b, '91b, '95; the latter paper really including the results of all 

 his previous investigations, and furnishing also a number of 

 new facts. Biirger first showed that all the ganglion cells of 

 the nemerteans are unipolar and membraneless, and he divided 

 the ganglion cells into four natural categories, which I follow 

 him in adopting; he is also the discoverer of the colossal neuro- 

 chord cells, and has been the first to recognize and distinguish 

 the connective-tissue elements of the nervous system. This 

 investigator has further been the first to apply the intra vitam 

 methylene blue stain to this group, and by means of it has 

 furnished most valuable contributions to the knowledge of the 

 nervous system, especially with regard to the ganglion and 

 nerve cells of the proboscis, and the course of the axis cylinders 

 in the lateral chords and peripheral nerves. Though the results 

 of my studies partly corroborate Burger's conclusions, still in 

 certain points I have reached views not in accord with his, 

 especially in regard to the structure of the nerve tubules and 

 of the dotted substance of the fibrous core, and of the genetic 

 and structural relations of the neuroglia to the nervous elements. 

 My material was collected at Newport, where Dr. Alexander 

 Agassiz had most kindly offered me a table in his private 

 laboratory, and at Wood's Holl, where, through the kindness of 

 Colonel MacDonald, I occupied a room in the Fish Commission 

 Station. I would express my obligations to both these gentle- 

 men. Colonel MacDonald's recent death was a sad shock to 

 all who had known him personally or had experienced his 

 generosity. Lineus gesserensis I collected in large numbers, it 

 being abundant at both these localities; of Cerebratiihis lacteus, 

 however, I secured only a single specimen. In addition to 

 these species I received a large Linetis from Norway, which I 

 was unable to determine, and shall refer to simply as "Z. spy^ 



1 This undetermined species of Lineus had, after preservation in alcoholic subli- 

 mate, a diameter of about 3 mm., and was of a cylindrical form; the color, above 

 a dark olive, lighter ventrally, with a fine, median, dorsal stripe of a yellowish 

 color, and a similar stripe on each side of the body. I am indebted to my friend 

 Dr. Fritz Schaudinn of the Berlin zoological laboratory for collecting it for me at 

 Bergen. 



