44 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



ou the question of fission, but in the " Seaside Studies/' published in 

 1865 by Elizabeth C. Agassiz and Alexander Agassiz ('65, p. 11), longi- 

 tudinal fission is stated to occur in this species. 



Finally, Torrey ('98, p. 347), who studied the Californian species 

 M. fimbriatiun, described double specimens which he believed to be in 

 process of fission. 



The material upon which the present paper is based consisted of ten 

 specimens of Metridium marginatum, collected in part by myself and in 

 part by others. The sources of this material are acknowledged in the 

 account of the specimens given with the description of the figures. Here 

 each specimen has received a distinguishing letter. To the gentlemen 

 whose names are mentioned as having obtained certain specimens fur 

 me, I wish to express my indebtedness. I am also indebted to Dr. 

 H. C. Bnmpus, Director of the Laboratory at the United States Fish 

 Commission Station at Wood's Hole for many courtesies shown me 

 while working at the station, and I am under special obligations to 

 Mr. Alexander Agassiz for the privilege of working at the Newport Lab- 

 oratory, and for the use of unpublished drawings made for Professor Louis 

 Agassiz. 



The animals collected were for % the most part stupefied by means of 

 magnesium sulphate in sea water, Tullberg's well known method, and 

 subsequently hardened in chromic acid and dissected by hand. 



The specimens naturally fall into two groups : first, those with two 

 mouths on one disk, of which there were two examples, specimens A and 

 B (Plate II. Figs. G and 7) ; and secondly, those with two complete oral 

 disks, of which there were eight, C to J (Plate II. Figs. 4 and 5, Plate 

 III. Figs. 8-14). 1 The two specimens (Plate I. Figs. 1-3) figured by 

 Professor Agassiz belong also to this group. These two groups were not 

 only distinguished by external anatomical differences, but also by certain 

 internal characteristics. In both representatives of the first group the 

 oesophageal tubes were Y-shaped, the single inner end opening into the 

 gastrovascular cavity and the two outer ends opening each through a 

 mouth. In specimen A (Plate II. Fig. 6) the bifurcation was close 

 to the oral disk and the oesophagus was for the greater part of its extent 

 a single flattened tube. In specimen B (Fig. 7) the oesophagus was 

 double excepting at its extreme inner end, so that its form is perhaps 

 more correctly described as Y-shaped. In all the specimens in the sec- 

 ond group the oesophageal tubes were entirely distinct from their snper- 



1 One of these eight, specimen J, was sacrificed in an attempt to rear it; hence I 

 have been able to study the internal anatomy of only seven such specimens. 



