251 
on planarice. 
animal. A delineation of these as they at present appear, 
(magnified) is given in fig. 4 and 5. The light portions show 
the parts renewed. 
The planarice submitted to my experiments were, it must 
be confessed, from their long previous confinement, but ill 
adapted for the purpose ; I think it therefore more than pro- 
bable, that a different result would have followed, had these 
planariae been active or vigorous, or but recently taken from 
their native abode. 
From the number of experiments made both by Mr. 
Dalyell and myself, and from the very few instances in 
which they proved successful, it may be reasonably inferred, 
that the production in the same animal of a second or addi- 
tional head, is a circumstance of unusual and extraordinary 
occurrence, and as such may not be unworthy a record in 
the pages of the Philosophical Transactions. 
In addition to my former remarks on the P. cornuta and 
P. torva, I have to observe, that I kept a considerable number 
of each of these species the whole of the last and the former 
summer, and not having noticed, during that period, any 
other mode of perpetuating their kind, than that of their de- 
taching small fragments either from the head or tail, I am 
of opinion they do not, like the other planarise — at least those 
I have examined, propagate by eggs ; and this may suffi- 
ciently account for the reproductive power being so very 
conspicuous in these species. The P. torva, however, does 
not possess this principle in so high a degree as the P. cor- 
nuta. In one instance, I recollect one of the latter species 
casting off two fragments from the tail, the very same night 
L 1 
MDCCCXXV. 
