30
T . B . REYNOLDSON
especially the proportions of t’he ions (Macan, 1950) the magnitude of
thesc in relation to the range found between lakes is small. Such a factor
would appear unlikely to be directly concerned in the pattern, and experiments described on p. 31 confirm this. The other effect of rainfall,
change in water level, does not differentiate between the species from
what we know of their reaction. Extension of the level is certainly exploited by them in feeding on drowned terrestrial organisms; lowering
of the level by evaporation is normally such a slow process that few will
be stranded.
Bearing in mind the restriction of sampling to favourable stony
shores it is reasonable to conclude that weather does not have a direct
bearing on the characteristic pattern of distribution and abundance of
these four triclad species.
B. A PLACE TO LIVE
A primary requirement of triclads is a firm substratum of leaves or
stones, preferably in shallow water, which provides shaded resting sites.
The sampling restrictions reduced variations from this source and also
such hazards as abrasion from moving sand; effects of wave action have
already been mentioned. Therefore, such physical attributes of the
habitat as these are unlikely to be involved. The chemical properties of
the lake water may be regarded as an aspect of (‘a place in which to
live” and as we have seen, they show a relationship to distribution and
abundance. The first problem to be examined here is the nature of the
correlation, whether it is direct or indirect. Macan (1963) reviewing the
relationships between calcium and freshwater organisms concluded that
a direct effect was unlikely except in special cases like Procerodes
( = Gunda, Pantin, 1931) but each case requires testing; a similar conclusion would apply for the T.D.M. relationship. It is obviously preferable to test the ability of the various triclad species to survive and
reproduce in lakes with low calcium and T.D.M. by retaining the natural
conditions as far as possible during the experiment. This is best achieved
by keeping the organisms in a suitable lake in cages which allow exchange of water between cage and lake. This avoids or reduces greatly
the possibility of unnatural conditioning of the water, shown to be important for survival in some circumstances (Alee, 1951); it also permits
the natural temperature regime to operate. For these several reasons
triclads were kept in polythene cylinders 10 cm long and 5 cm internal
diameter, closed a t each end with bolting silk of mesh fine enough to
exclude organisms such as planktonic Crustacea. The cylinders> were
clamped on to a metal frame by Terry clips and the apparatus kept in a
sheltered part of the shore at a depth of about 30 cm. Lake Ogwen was
selected for the experiments since it has a low calcium content (0-5 mg/
T . B . REYNOLDSON
especially the proportions of t’he ions (Macan, 1950) the magnitude of
thesc in relation to the range found between lakes is small. Such a factor
would appear unlikely to be directly concerned in the pattern, and experiments described on p. 31 confirm this. The other effect of rainfall,
change in water level, does not differentiate between the species from
what we know of their reaction. Extension of the level is certainly exploited by them in feeding on drowned terrestrial organisms; lowering
of the level by evaporation is normally such a slow process that few will
be stranded.
Bearing in mind the restriction of sampling to favourable stony
shores it is reasonable to conclude that weather does not have a direct
bearing on the characteristic pattern of distribution and abundance of
these four triclad species.
B. A PLACE TO LIVE
A primary requirement of triclads is a firm substratum of leaves or
stones, preferably in shallow water, which provides shaded resting sites.
The sampling restrictions reduced variations from this source and also
such hazards as abrasion from moving sand; effects of wave action have
already been mentioned. Therefore, such physical attributes of the
habitat as these are unlikely to be involved. The chemical properties of
the lake water may be regarded as an aspect of (‘a place in which to
live” and as we have seen, they show a relationship to distribution and
abundance. The first problem to be examined here is the nature of the
correlation, whether it is direct or indirect. Macan (1963) reviewing the
relationships between calcium and freshwater organisms concluded that
a direct effect was unlikely except in special cases like Procerodes
( = Gunda, Pantin, 1931) but each case requires testing; a similar conclusion would apply for the T.D.M. relationship. It is obviously preferable to test the ability of the various triclad species to survive and
reproduce in lakes with low calcium and T.D.M. by retaining the natural
conditions as far as possible during the experiment. This is best achieved
by keeping the organisms in a suitable lake in cages which allow exchange of water between cage and lake. This avoids or reduces greatly
the possibility of unnatural conditioning of the water, shown to be important for survival in some circumstances (Alee, 1951); it also permits
the natural temperature regime to operate. For these several reasons
triclads were kept in polythene cylinders 10 cm long and 5 cm internal
diameter, closed a t each end with bolting silk of mesh fine enough to
exclude organisms such as planktonic Crustacea. The cylinders> were
clamped on to a metal frame by Terry clips and the apparatus kept in a
sheltered part of the shore at a depth of about 30 cm. Lake Ogwen was
selected for the experiments since it has a low calcium content (0-5 mg/