ON TETRACOTYLE PETEOMYZONTIS. 491 



almost inclined to regard it as the same. Seeing, however, 

 that species of these Trematodes are determined a great deal 

 by the structure and arrangement of the generative organs, it 

 is, of course, possible that two forms might bear a close 

 resemblance to one another in their early stages, and yet 

 belong to different species. On the whole, a separate name 

 seems the most desirable. A list and descriptions of these 

 immature forms is very much needed as a supplement to 

 G. Braudes' admirable work on the Holostomese. 



Occurrence. — Tetracotyle petromyzontis is found in 

 great numbers in the brain cavities of Amraocoetes. I have 

 examined many specimens from the river near Oxford, and in 

 only one case have found it absent. Last April I found a 

 number of young lampreys in a stream in Sussex, but not a 

 single parasite was present in them. Gulliver states that 

 they are never absent from the brain either in the young or 

 adult conditions, but he does not say whence he obtained his 

 lampreys. Kupfer, Shipley, Gaskell, and others make no 

 mention of any parasites in the brain, and it seems to me 

 probable that Gaskell would have seen them if present. I 

 believe a good many of his lampreys came from Surrey. 

 For myself I have never found Tetracotyle petromyzontis 

 in the adult lamprey nor in any lamprey of greater length 

 than six to seven inches. Its distribution therefore does not 

 seem to be as universal as Gulliver thougiit, and I can hardly 

 believe that many competent observers would have over- 

 looked it if present in the lampreys examined by them. 



In many Amraocoetes the brain is bulged out completely by 

 these tiny animals, and I have sometimes seen them squirted 

 eighteen inches into the air w^hen the roof of the brain is 

 divided suddenly with a sharp knife. They are found mostly 

 in the region of the fourth ventricle, and especially under 

 the choroid plexuses between the foldings of the roof of the 

 brain, whence they apparently draw their nourishment. 



In spite of all this, the Ammocoetes appear to live on quite 

 comfortably and to suffer no great inconvenience, altiiough 

 the choroid plexuses of the brain often have an inflamed 



VOL. 41, PART 4. NEW SERIES. M M 



