Dr. Johnson's observations on the genus planaria. 439 
tractility, the body consisting of a series of annular muscles, 
they are enabled to assume almost every variety of figure. 
In traversing a smooth surface, they have usually a gliding 
or snail-like motion, leaving a viscous tract behind ; but when 
the surface is rugged, they quit this gliding motion, and 
move forward like the leech, alternately attaching the head 
and tail : they do not, however, as far as I can perceive, fix 
the tail ; they simply draw it up, when the body is firmly 
retained by the strong glutinous substance with which it is 
invested. 
The Planarice are often seen traversing the surface of the 
water in an inverted position, and with the same facility as the 
Glossoporce , described in a former paper. When gliding along, 
they move the head from side to side with rapidity, but on 
coming suddenly into contact with any hard substance, they 
immediately retract, or draw in the fore-part of the body, 
somewhat like the Limax. 
On visiting the rivulet, from which I was in the habit of 
taking these animals for the experiments I am about to relate, 
I was surprised to find a large body of them ( P. torvce ) pro- 
ceeding against the current, gliding over its sandy bottom, 
keeping the same order as ants when passing from one of 
their establishments into another, and occupying a space of 
about twelve inches in length by two in breadth. This re- 
gular movement I observed two or three days in succession. 
The weather being at this time extremely temperate, had, 
doubtless, induced them to quit their several hiding places ; 
but I could not discover the purport of this proceeding. 
The Planarice are so gelatinous and pulpy in their texture, 
as scarcely to allow of any very accurate dissection. I may 
