124 PLANARIA. 
various minute beings of Planarian form and habits, may ordinarily be 
seen crawling on the glass, and frequently containmg one or more ova. 
In general they suddenly disappear, or perish. But I have not been able 
to discover that they undergo any metamorphosis ; besides, were it into 
fresh-water Zoophytes, these are so very few, that many would remain 
unaccounted for, 
It is not improbable, however, that some may be the young of such 
worms as attain much larger dimensions, both in the fresh and in the 
salt waters, for until much better acquainted with the subject, and in 
particular, with the history of individual specimens, which constitute the 
main elements of Zoology, there is no absolute test whereby to discri- 
minate them. 
I doubt not that I may have erred by comprehending a few animals 
among the Planariz, which do not actually belong to them ; but, at the 
same time, the reader is provided with the means of detection, and oc- 
casionally of correction, by the best figures that could be procured, being 
presented to him, with such descriptions and explanations as experiment 
and observation warranted. 
In a genus with external characters so scanty and obscure, and with 
an internal conformation to be so seldom discovered, there will be long an 
opportunity afforded of improving any group into which naturalists may 
attempt to associate them. Nor do I presume to assert that any of the 
preceding will remain for permanent adoption. 
There must be some unknown secret principle promoting the mul- 
tiplication of this race of animals. Specimens of one kind are remark- 
ably rare, for example, in places not far distant from others where they 
are remarkably abundant. While particularly occupied by the subject 
from the year 1802 downwards, I could not by any means discover a 
specimen of the Planaria lactea ; and at the same time the Planaria pan- 
niculata, a brown species, was hardly to be seen. At this present period, so 
many years after, I find the latter im multitudes in a pond called the Marl 
Pits, I believe, which is near Craigcrook, about three miles west of Edin- 
burgh, but not one of the Planaria lactea among them. On the other 
hand, I find corresponding multitudes of the Planaria lactea in a pond 
