ON TETRAGOTYLE PETROMYZONTIS. 495 



The flame-cells are best seen by staining with very weak 

 methylene blue. 



I may mention that terminal vesicles of the excretory 

 system appeared surrounded in parts by subsidiary vesicles. 

 As mentioned above, the addition of acid to the animal causes 

 the calcareous bodies to disappear gradually with evolution of 

 gas which comes off at the excretory pore. The solution of 

 these bodies is far too gradual to allow of them being con- 

 sidered entirely calcareous. Probably they contain a certain 

 amount of organic matter as well. As to their formation I 

 have nothing to say. 



The physiology of this excretory system seems to me well 

 worth further study, in view of the existence of calcareous 

 bodies in the parenchyma of other Platyhelminthes. Some 

 light might be thrown on the origin of the latter, and possibly 

 even the older view that they were derived from the excretory 

 system be re-affirmed. 



Tail Region. — Reference was made earlier in this paper to 

 the tail region of the body which grows out of the posterior 

 region, and in adult forms contains the reproductive organs. 

 In the younger stage the excretory bladders seem to occupy 

 a great deal of it. 



The tail is not of the same length in all individuals examined 

 by me. In many it is hardly perceptible, whilst in others it 

 may be nearly one third the length of the animal. A study of 

 sections of the posterior end of the animal makes manifest the 

 great number of nuclei aggregated there. They are scattered 

 about pretty uniformly in individuals with an inconspicuous 

 tail, but in those with longer tails they are already becoming 

 gathered into groups — some of them very defined — and these 

 are probably rudiments of the future generative organs. 



Part, at any rate, of the development of Tetracotyle 

 petromyzontis takes place in the brain of Ammoccetes, but 

 does not proceed very far. 



I have been quite unable to trace any connection of Tetra- 

 cotyle petromyzontis with an adult form. I do not quite 

 know how to trace it till more knowledge of the animals 



