No. 5. — The Mesenteries and Siphonoglyphs in Metridium mar- 
ginatum Milne- Edwards. By G. H. PARKER.! 
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 zoölogy, 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 study 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 the 
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 Zoölogical Laboratory of the Museum of Comparative 
Zoölogy at Harvard College, E. L. Mark, Director, No. LXXV. 
VOL. XXX. — NO. 5. 1 
