444 Dr. Johnson's observations on the genus planaria. 
tail generally remains attached, and only occasionally shifts 
its situation ; but if touched, it moves with nearly the same 
quickness as the anterior extremity, preserving an uniform 
gliding motion. 
This property, which is common to the several Planaria 
enumerated in this paper, is most strongly marked in the 
P. cornuta. 
The reproductive power of these animals, when divided, 
is alluded to by Muller, Shaw, and others. Indeed it could 
scarcely be expected, after the public attention had been 
excited by the astonishing reproduction of the Polype , even 
when cut or divided into several minute pieces, that the re- 
production of an animal, so open to common observation as 
the Planaria , should long have remained unnoticed. 
The following experiments show to what a degree this 
reproductive power extends. 
July 8. I divided several of the Planaria into two equal 
parts : the head and tail portions of the P. torva were repro- 
duced in 14 days ; those of the P. cornuta in 17 days ; of the 
P. brunnea in 15 days ; and of the P. lactea in 19 days. 
July 30. An equal number of the P. cornuta and P. lactea 
were divided, transversely, a little below the eyes. The heads 
of the former were renewed in 11 days, of the latter in 14 
days. The head portions were a long time in recovering the 
tail : in 16 days they were as delineated in Jig. 10 and 11. As 
the P. lactea had recently taken food, the interanea are also 
exhibited. 
August 9. The Planaria were now divided into three equal 
parts : the middle portions were observed to reproduce heads 
and tails by the 25th (16 days). Fig. 12 represents, under 
