loaw 



No. 3. — Studies from the Newport Marine Laboratory. Com- 

 municated by Alexander Agassiz. 



XLII. 



Longitudinal Fission in Metridium marginatum Milne-Edwards. 



By G. H. Parker. 



From early times double specimens of different species of Metridium 

 have been observed and recorded. Thus in the last century Dicque- 

 mare ('75, p. 229, Tab. VI. Fig. 2) described and figured a specimen of 

 M. diunthus with two complete oral disks. Similar specimens were ob- 

 served by Johnston ('47, p. 233), who called them monstrosities, and 

 interpreted them as cases of coalescence brought about by the gregarious 

 habit of the species. Thorell ('59, p. 10) and Gosse ('60, p. 20) like- 

 wise mentioned specimens with two disks as monstrosities, though Gosse 

 also expressed the opinion that they were due to a tendency to spon- 

 taneous division. Foot ('63, p. 64), who confirmed the observations of 

 his predecessors on the occurrence of animals with two oral disks, also 

 recorded the discovery of a specimen with two mouths on one disk and 

 further stated that these aberrations from the normal form are merelv 

 to be considered as monstrosities. G. Y. and A. F. Dixon ('91, p. 20), 

 in reporting the occurrence of specimens with two disks as well as those 

 with two mouths on one disk, mention them likewise as monstrosities. 

 Carlgren ('93, p. 109), after stating his belief that longitudinal division 

 is not uncommon in M. dianthus, adds : " Mehrere Forscher scheinen 

 Formen mit zwei Mundscheiben als monstrose ansehen zu wollen. Es 

 kann doch wohl als eine besonnene Langsteilung, die nicht zu Ende ge- 

 fiihrt ist, betrachtet werden." 



Double specimens of M. marginatum were observed as early as 1847 

 by Professor Louis Agassiz, among whose unpublished drawings are three 

 figures, two • of which (Plate I. Figs. 2, 3) bear this date and probably 

 represent two views of the same animal ; the third figure (Fig. 1) bears 

 the date of 1 860. These figures were unaccompanied by any notes bearing 



VOL. XXXV. — NO. 3. 1 



