THE OCCURRENCE OF A SUPERNUMERARY OCELLUS 

 IN PLANARIA MACULATA. 



BERTRAM G. SMITH. 



In preparing- for class use some whole mounts of Planaria. I was 

 surprised to find one with a well-developed supernumerary ocellus, as 

 pictured in Plate I, Figures 1 and 2. This specimen belonged to a lot 

 collected during the summer of 1910 in Monona Lake, Wisconsin. These 

 planarians were unusually large and abundant, and when captured 

 appeared to be in a very thriving condition. 



The chief point of interest in such a variation is of course to determine 

 whether it is a mere abnormality of development, or a heritable varia- 

 tion of germinal origin — tlie kind of variation which is of significance 

 for organic evolution. It is unfortunate that the peculiarity was not 

 observed while the animal was in the living condition, in order that it 

 might be studied by breeding experiments ; but in the absence of data 

 from this source the following considerations seem sufficient to settle the 

 (juestion. 



If the supernumerary ocellus were an abnormality due to some acci- 

 dent of development, it would mean that a portion of the anlagt- of an 

 ordinary eye had been split off, leaving the original eye reduced in size. 

 But in this specimen both of the ordinary eyes are of the usual size, and 

 normal in all respects. iMirthermore, the supernumerary ocellus is in 

 the exact position of a member of the second pair in animals in which 

 the paired ocelli are metamerically repeated (e. g., a leech or a sea- 

 worm). The chances against this position being attained through some 

 accident of development are enormous. We must conclude, I think, that 

 this variation is of germinal origin. One might also call attention to 

 tlie obvious fact that the peculiarity is not one due to a recombination 

 of characters possessed by the parents, but is something new to the 

 race — the sort of variation one would seek in looking for the raw 

 materials which make evolutionary advance possible. It is a mutation 

 in the strict sense of the word. 



l»th Midi. .\cad. Sci. Kept.. l!ii;. 



