L A K E D W E LLIN 0 TRI C LA D S
25
shifts in the direction of How of rivers and correlated changes in watersheds which have permitted dispersal in directions which today appear
improbable. But it must be admitted that, as for many other problems
of zoogeography, we do not know the answer.
Any hypothesis which suggests the survival of organisms in the icefree areas of southern Ireland and England during the last glacial phase
must take cognizance of the climatic conditions which prevailed and the
ecology of the organisms concerned. I t seems likely (Corbet, 1962; Efford,
1962) that on the edge of the glaciated areas, tundra conditions prevailed
with summer temperatures around 9" C! (Ullyott, 1936). This climate
would enable the survival of Polycelis nigra, P . tenuis and Dendrocoelum
lncteum which can complete their life-cycles over a temperature range
of3.5-5.0" C up to 15-20" C (Table V). A!though by comparison Dugesia
is a genus more associated with warmer water, Dug. lugubris can breed
at a temperature of 10" C and upward (Reynoldson et al., 1965), also
it is found in Finland and in Russia near Leningrad (Luther, 1961)
with their long, severe winters. It is possible that it could have survived
in small numbers a t the extreme southern limit where conditions were
probably less severe. In fact, survival of all these triclads may have
depended more on the presence of suitable food organisms than on temperature. In their ability t.0 tolerate low temperature triclads differ
markedly from the Lusitanian fauna, and persistence during the last
glacial period is much more €easible. Survival of P. felina and Crenobia
alpina in Southern Ireland was also the basis of O'Rourke's argument
( 1 946) in refuting Ullyott's (1 936) hypothesis concerning t,he use of
TABLE V
Temperature Limits in "C of tlbe Various Stages and of the Complete Life-cycle
in Dendrocoelum lacteum, Pol ycelis nigra, P. tenuis and Dugesia lugubris
(Reproduced with permission from Reynoldson et al., 1965)
Stage
Deiadrocoelum Polycelis
Polycelis
Dugeaia
lactezim
d g r a
tenuis
lugubris
Cocoon production Lower
1 -5
3 -5
5 *O
10.0
Egg development Lower
1.5
1.5
3 -5
5 .O
Upper
18.5
18.5
20.0
23 -0
Upper
23.0
23.0
23.0
23.0
Maturat,ion
Lower
3 -5
3-59
5.0
10.0
Upper
15.0
18.5
20.0
23 -0
Life-cycle
Lower
3.5
3.59
5 *0
10.0
Upper
15.0
18.5
20.0
23.0
25
shifts in the direction of How of rivers and correlated changes in watersheds which have permitted dispersal in directions which today appear
improbable. But it must be admitted that, as for many other problems
of zoogeography, we do not know the answer.
Any hypothesis which suggests the survival of organisms in the icefree areas of southern Ireland and England during the last glacial phase
must take cognizance of the climatic conditions which prevailed and the
ecology of the organisms concerned. I t seems likely (Corbet, 1962; Efford,
1962) that on the edge of the glaciated areas, tundra conditions prevailed
with summer temperatures around 9" C! (Ullyott, 1936). This climate
would enable the survival of Polycelis nigra, P . tenuis and Dendrocoelum
lncteum which can complete their life-cycles over a temperature range
of3.5-5.0" C up to 15-20" C (Table V). A!though by comparison Dugesia
is a genus more associated with warmer water, Dug. lugubris can breed
at a temperature of 10" C and upward (Reynoldson et al., 1965), also
it is found in Finland and in Russia near Leningrad (Luther, 1961)
with their long, severe winters. It is possible that it could have survived
in small numbers a t the extreme southern limit where conditions were
probably less severe. In fact, survival of all these triclads may have
depended more on the presence of suitable food organisms than on temperature. In their ability t.0 tolerate low temperature triclads differ
markedly from the Lusitanian fauna, and persistence during the last
glacial period is much more €easible. Survival of P. felina and Crenobia
alpina in Southern Ireland was also the basis of O'Rourke's argument
( 1 946) in refuting Ullyott's (1 936) hypothesis concerning t,he use of
TABLE V
Temperature Limits in "C of tlbe Various Stages and of the Complete Life-cycle
in Dendrocoelum lacteum, Pol ycelis nigra, P. tenuis and Dugesia lugubris
(Reproduced with permission from Reynoldson et al., 1965)
Stage
Deiadrocoelum Polycelis
Polycelis
Dugeaia
lactezim
d g r a
tenuis
lugubris
Cocoon production Lower
1 -5
3 -5
5 *O
10.0
Egg development Lower
1.5
1.5
3 -5
5 .O
Upper
18.5
18.5
20.0
23 -0
Upper
23.0
23.0
23.0
23.0
Maturat,ion
Lower
3 -5
3-59
5.0
10.0
Upper
15.0
18.5
20.0
23 -0
Life-cycle
Lower
3.5
3.59
5 *0
10.0
Upper
15.0
18.5
20.0
23.0