42 
The hisk Naturalist. 
February, 
insight Bergson recognizes the extraordinary interest that 
attaches to the early worms — almost the first creatures 
to have bilateral symmetry, and the first to have head- 
brains, beginning the long process which has enabled us to 
tell our right hand from our left. He gives them their 
due, those early worms — ' infinitely plastic forms, pregnant 
with an unlimited future, the common stock of Echinoderms, 
Molluscs, Arthropods, and Vertebrates.' " 
Planarians do not make themselves specially evident, 
and in searching for them it is necessary to examine the 
under surfaces of stones, or leaves and stems of weeds 
in ponds ; the shaded situations between the leaves and 
stems of such submerged plants as flags and rushes are sure 
places to find them. Nevertheless, planarians make fre- 
quent excursions to the bottom mud, over which they glide 
in search of food. They vary in size, but are never very 
large ; a specimen one and a half inches in length would, 
indeed, be a large one, and many do not exceed one-third 
of an inch, at least when at rest. Land planarians are 
also lovers of the shade, and live either on the damp humus 
of the ground, or on the under surface of old stumps, or 
between the leaves and stem of certain plants. They vary 
much in size, small ones being less than an inch in length, 
while the large ones, found principally in the tropics, may 
measure eight or nine inches or more. Marine planarians 
closely resemble their freshwater relatives, and are in all 
essentials similar to them. 
On the general form of planarians nothing beyond the 
fact that they are as a rule flat and band -like need be said ; 
but something must be said of their colour. As a rule, 
freshwater forms are fairly evenly coloured, though mottled 
and striped examples are known. The commonest fresh- 
water planarian in 1 he Belfast district is a small dark grey 
or black creature called Polycclis nv^ra, about three-tenths 
of an inch in length when fully extended, and about one- 
tenth when at rest ; brown forms are common enough, 
and belong, I believe, to the species Planaria lugubns ; 
Dendrocoelum lacteum is a beautiful milky-white creature, 
transparent enough to show the dendritic gut through 
