No. 5. — The Mesenteries and Siplionoglyphs in Metridium mar- 

 ginatum Milne- Edwards. By G. H. Parkek.^ 



Introduction. — Since the publication of the Hertwigs' ('79) paper on 

 the anatomy of the actinians, the attention of investigators has been 

 more and more directed toward the details of the internal structure of 

 these organisms. This new departure has been conducted in the main on 

 the lines of systematic zoology, and, though its advocates in the begin- 

 ning may have been somewhat Utopian in their expectations, it has 

 certainly carried our understanding of the natural relations of this group 

 of animals a long step forward. The new features thus introduced into 

 the classification have, however, been subject to frequent modification, 

 and every actinian newly investigated may be expected to exert some 

 influence on the classification finally adopted. It is to be regretted that 

 much of this kind of investigation has been of necessity carried out on a 

 limited, often a very limited, number of specimens, so that the possible 

 error of regarding individual variations as characteristic of large groups 

 is not always eliminated. 



The following pages contain a record of certain structural peculi- 

 arities in a single species of actinian, the common Metridium marginatum. 

 Milne-Edwards of our coast, as represented by a considerable number 

 of specimens. As this record shows, uniformity of structure is by no 

 means a general characteristic of this species ; hence these observa- 

 tions are to some extent a contribution to the ^tudy of the variability 

 of this animal. 



The material on which the following observations were made consisted 

 of 131 adult specimens of Metridium marginatum. These were collected 

 in part by myself and in part by my laboratory assistant, Mr. J. I. 

 Hamaker, to whom I am under obligations for this kindness. All tlie 

 specimens came from the neighborhood of Newport, R. I., and were 

 prepared and to some extent studied in Mr. Alexander Agassiz's Labora- 

 tory at that place. I here wish to express my thanks to Mr. Agassiz for 

 the privilege of carrying on this work at the Newport Laboratory. 



1 Contributions from the Zoological Laborator}' of tlie Museum of Comparative 

 Zoology at Harvard College, E. L. Mark, Director, No. LXXV. 



VOL. XXX. NO. 5. 1 



