The Cranial Nerve Components of Petromyzon. 
By 
J. B. Johnston. 
With plate V and 18 figures in Text. 
In 1902 the author published an account of the brain of Zam- 
petra Wilderi together with a description of the cutaneous sense 
organs and an incomplete account of the components in the roots 
and ganglia of the cranial nerves. Since that time repeated efforts 
have been made to obtain fresh material of ZLampetra or of its 
Ammocoetes in which to complete the study of the eranial nerve 
components. Through the great kindness of Prof. S. H. GAGE of 
Cornell University, I have received Ammocoetes material which has 
served my purpose although the identification of the species is 
uncertain. In the region about Ithaca, N. Y. the brook lamprey 
(L. Wilderi) and the lake lamprey (P. dorsatus Wilder) breed in 
the same streams and Prof. GaGE states that there is no way to 
distinguish between the Ammocoetes of the two forms until after 
the metamorphosis has begun. The specimens which he gave me, 
therefore, and which include several stages, may be those of either 
species or of both. However, the specimen used for the series of 
eross sections from which the following deseription is taken shows 
certain differences from Zampetra Wilderi in the number and arrange- 
ment of the pit organs and in the form, position and structure of 
the ganglia of the VIIT—VIII, IX and X‘ nerves which I can not 
attribute to differences between the Ammocoetes and the adult ot 
the same species. I think, then, that the species described is P. 
dorsatus. The specimen was about 60 mm in length. 
Petromyzon species offer especial difficulties in the study of 
nerve components; on the one hand diffieulty in obtaining complete 
Morpholog. Jahrbuch. 34. 11 
