L A K P DWELLING TRICLADS
27
private communication) while P. felina (and P. nigra) was recorded from
the Outer Hebridean Island of Barra (Forrest et al., 1936) but the latter
identification needs confirming based on the new taxonomy. There are
also some records of Crenobia alpina in the Faroes, and Steinbock (1931)
very tentatively suggested that it may have been transported to the
island on icebergs, but this seems very doubtful. The presence of these
populations cannot be explained satisfactorily at present. Triclads appear
to be absent from Iceland (Steinbock, 1948). This general scarcity in the
outlying islands is interesting since it appears that they have had no connection with the mainland since the last glacial phase (see Corbet, 1961).
There are some striking resemblances between the distribution of
Asellus (Williams, 1962b) and lake-dwelling triclads in the British Isles
which deserve elaboration. The northern extension of both groups is
similar in their scarcity north of the Highland Fault; the few occurrences of both organisms are probably due to trout stocking. The Highland Fault is mentioned only as a convenient landmark and it is not
suggested that it has operated as a barrier. The most likely obstacle to
northward spread is the Grampian Mountains to the south of the Fault
which leave only narrow coastal plains on the east side. Another important brake on this movement is the general east-west flow of the
main rivers in Britain. The distribution of A . meridianus is very similar
to that of Polycelis nigra, each being the only representative on most of
the western islands (e.g. Skokholm, Bardsey, Isle of Man) and the common species on Anglesey. It does not however extend so far north as
P. nigra, being absent for example from Islay. Williams (ibid.) attributed this distribution to either the survival of the species in southern
Ireland and England during the last glacial phase and its dispersal to
the islands while land connections still existed by closely following the
edge of the retreating glaciers, i.e. as postulated for P.
or to its
reaching Ireland from the mainland by land connections following retreat of the glaciers. As indicated (p. 24) the latter seems doubtful and
unnecessary. Asellus aquaticus resembles the rest of the triclads in
having a more restricted distribution which Williams attributed to its
later arrival. He expressed the view that it was accidentally transported
both to England and Ireland by man. Here events seem to differ from
those suggested for triclads where cogent reasons have been advanced
against man being primarily involved. The apparently more rapid
spread of A . aquuticus compared with trriclads in recent times, probably
reflects its greater resistance to desiccation and mechanical damage
compared with triclads. A striking contrast to triclads in rate of dispersal is also afforded by the more suitably equipped Hydrobia ( = Potamopyrgus) jenkinsi which has been reported on by Hunter and Warwick
(1957). In general, the distributions of AseZlus, triclads and certain mites
27
private communication) while P. felina (and P. nigra) was recorded from
the Outer Hebridean Island of Barra (Forrest et al., 1936) but the latter
identification needs confirming based on the new taxonomy. There are
also some records of Crenobia alpina in the Faroes, and Steinbock (1931)
very tentatively suggested that it may have been transported to the
island on icebergs, but this seems very doubtful. The presence of these
populations cannot be explained satisfactorily at present. Triclads appear
to be absent from Iceland (Steinbock, 1948). This general scarcity in the
outlying islands is interesting since it appears that they have had no connection with the mainland since the last glacial phase (see Corbet, 1961).
There are some striking resemblances between the distribution of
Asellus (Williams, 1962b) and lake-dwelling triclads in the British Isles
which deserve elaboration. The northern extension of both groups is
similar in their scarcity north of the Highland Fault; the few occurrences of both organisms are probably due to trout stocking. The Highland Fault is mentioned only as a convenient landmark and it is not
suggested that it has operated as a barrier. The most likely obstacle to
northward spread is the Grampian Mountains to the south of the Fault
which leave only narrow coastal plains on the east side. Another important brake on this movement is the general east-west flow of the
main rivers in Britain. The distribution of A . meridianus is very similar
to that of Polycelis nigra, each being the only representative on most of
the western islands (e.g. Skokholm, Bardsey, Isle of Man) and the common species on Anglesey. It does not however extend so far north as
P. nigra, being absent for example from Islay. Williams (ibid.) attributed this distribution to either the survival of the species in southern
Ireland and England during the last glacial phase and its dispersal to
the islands while land connections still existed by closely following the
edge of the retreating glaciers, i.e. as postulated for P.
or to its
reaching Ireland from the mainland by land connections following retreat of the glaciers. As indicated (p. 24) the latter seems doubtful and
unnecessary. Asellus aquaticus resembles the rest of the triclads in
having a more restricted distribution which Williams attributed to its
later arrival. He expressed the view that it was accidentally transported
both to England and Ireland by man. Here events seem to differ from
those suggested for triclads where cogent reasons have been advanced
against man being primarily involved. The apparently more rapid
spread of A . aquuticus compared with trriclads in recent times, probably
reflects its greater resistance to desiccation and mechanical damage
compared with triclads. A striking contrast to triclads in rate of dispersal is also afforded by the more suitably equipped Hydrobia ( = Potamopyrgus) jenkinsi which has been reported on by Hunter and Warwick
(1957). In general, the distributions of AseZlus, triclads and certain mites