CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM 

 OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. E. L. MARK, 

 Director — No. 164. 



DIMORPHISM AND REGENERATION IN METRIDIUM. 



BY 



C. W. HAHN, 

 With 2 Figures. 



The experiments on which this paper is based were begun at 

 Harvard University in the winter of 1901-02 and continued at 

 the United States Fish Commission Laboratory at Woods Hole 

 in the summers of 1902 and 1903. 



To Prof. E. L. Mark, Director of the Harvard Zoological 

 Laboratory, and Drs. H. M. Smith and Francis B. Sumner, 

 directors in successive years of the Fish Commission Laboratory, 

 I wish to express thanks for the accommodations kindly provided 

 for the work. 



At the suggestion of Dr. W. E. Castle, to whom I am deeply 

 indebted for guidance and help throughout this investigation, I 

 undertook to discover if the dimorphism which occurs in Metri- 

 dium marginatum Milne-Edwards, is perpetuated in accordance 

 with some hereditary law, perhaps related to the law of Mendel. 

 So far at least as concerns asexual reproduction, it soon became 

 evident that this is not the case. Further studies have shown that 

 the dimorphism is due to a peculiar method of development in 

 asexual reproduction. 



The dimorphism of actinians was noticed and described more 

 or less completely by Thorell ('59), Dixon ('88), McMurrich ('89), 

 and Carlgren ('93), but the first full discussion of it was made by 

 Parker ('97), and the only extensive experimental study thus far 

 made with a view to discovering the exact nature of the anomaly 

 is that of Carlgren ('04). 



The dimorphic condition referred to is this: The polyps of 

 a given species may have either one or two (rarely three) siphono- 

 glyphs. With each siphonoglyph, as shown for Metridium by 

 Parker ('97), there is invariably associated a pair of directive 

 mesenteries. 



