252 
Dr. Johnson's observations 
it was taken, which were only prevented from becoming 
perfect animals, by an accidental occurrence. 
Having found the hirudo vulgaris or common rivulet leech 
to produce its young in greater number when kept separate, 
I thought the planarice might be similarly affected. To ascer- 
tain this point, I took several of the P. cornutce and placed 
them singly in separate vessels, and in another vessel, by 
way of contrast, about an equal number together. During 
the first fortnight scarcely any fragments were ^detached 
from the latter, whilst the former, with but few exceptions, 
had each gone through this process ; some indeed throwing 
off' or detaching more than one fragment. This spontaneous 
separation occurring so soon in those planariae kept apart, 
led me to think it was owing to the necessity the?i existing of 
continuing their species. Hence it would also appear, that 
this process is at all times under command of the animal, and 
may be called into action upon any particular emergency. 
And this I think the more evident, from the circumstance of 
my having lately placed three lively planarise in the glass 
globe where the double headed planaria had been hitherto 
confined alone — that spontaneously divided within the short 
space of four days, (Dec. 24th) in the manner represented 
in fig. 6 , 7, 8. 
In regard to the planariae placed together, although at 
first extremely indolent, yet they ultimately threw off as 
many fragments as in the former case ; thus proving, that 
their being kept together or separate makes no further dif- 
ference, than that where the demand is strong upon them to 
perpetuate their kind, this process is sooner brought into 
operation. 
V 
