NOTES ON THE VARIATIONS OF RHEGMATODES. 



371 



variation, though when it is remembered that in the vast majority 

 of all Hydromedusae the radial canals are normally but four in 

 number, the question naturally arises as to the conditions under 

 which this extraordinary increase has arisen, and what it may 

 signify. Such an inquiry would, however, lead too far afield for 

 our present purpose. Mayer has raised and briefly discussed 



FIG. 2. 

 canals. 



Showing series of imperfect 



FIG. 3. Showing central and 

 peripheral development of radial 

 canal. 



the problem in connection with a study of " The Variations of a 

 Newly Arisen Species of Medusa." In this paper Mayer sug- 

 gests that one of the ways in which a pentamerous medusa may 

 may have arisen from an ordinary tetramerous form is by bifur- 

 cation of one of the canals, which, if it became hereditary, would 

 easily account for the condition of pentamerism. 



The present writer has discussed somewhat the same general 

 problem in an earlier paper (op. cit., p. 239). If one had only a 

 pentamerous or hexamerous condition for which to find some 

 simple explanation, that just suggested might appear adequate. 

 But when we have conditions like that of Rhegmatodes, not to 

 mention many others of a similar sort, it will be more or less clear 

 that such an account would be inadequate. It would seem far 

 more probable that these supernumerary canals have arisen under 

 the appropriate stimulus entirely independent of any special phy- 

 letic relations. 



1 "Variations in a Newly Arisen Medusa." Bull. Brooklyn Inst. Arts and Sci- 

 ences, Vol. I. 



