THE BIOLOGY OF ASCIDIAXS
61
as a result of the shorter pelagic life of ascidians. The remainder comprises mainly small numbers of widespread species, and species occurring throughout warm waters. Only one species is common to thc
Mediterranean and thc Rcd Sea.
Tropical and subtropical American Pacijic
From the Gulf of California in the north to about 4"S, on the coast
of Peru, a warm-water fauna exists, part of which is tropical. The
difficulty of fixing the northern limit of the region, for ascidians as for
other groups, has been noted by Van Name (1945), and is due to the
presence of cool water quite close to the coast far south on the Pacific
side of the United States. However, it is established that the Gulf of
California has a tropical ascidian fauna (Ricketts and Calvin, 1939).
The southern limit of the region is not precisely known because the
fauna is insufficiently described. The total of about 45 species indicates
a poor fauna. Few of them other than circumtropical species occur also
in the Atlantic American area. In other animal groups, including crabs
and echinoderms, although the percentage of species common to the
Atlantic and Pacific sides of Central America is also small, there is a
substantial number of shared genera. Such comparisons of genera have
little significance in ascidians, since most genera are of very wide
occurrence.
Boreal European Atlantic
The whole temperate North Atlantic could be taken as a faunistic
unit, but the eastern and western sides are sufficiently distinct to be
considered separately. On the European coast the northern and
southern limits are approximately the North Cape in Norway and the
western end of the English Channel.
The papers of Hartmeyer (1923, 1924) and hback-Christie-Linde
(1922, 1923, 1928, 1934) remain the major zoogeographical accounts of
the European boreal ascidians, and subsequent work (Thompson, 1930,
1931, 1932, 1934; Millar, 1966c) has done little more than fill in certain
details. The total now stands at about 74 species, and although new
species will doubtless be found, particularly from specialized habitats
not hitherto examined in detail, it is not to be expected that the list will
be greatly lengthened. The ascidian fauna is considerably poorer than
that of the Mediterranean with its 130 species, although as we have seen
the Mediterranean is not faunistically sharply cut off from the Atlantic.
Few ascidians are strictly endemic to the European boreal region,
and most of those which are centred there extend either northwards
some way into the Arctic or southwards as far as the western basin of
61
as a result of the shorter pelagic life of ascidians. The remainder comprises mainly small numbers of widespread species, and species occurring throughout warm waters. Only one species is common to thc
Mediterranean and thc Rcd Sea.
Tropical and subtropical American Pacijic
From the Gulf of California in the north to about 4"S, on the coast
of Peru, a warm-water fauna exists, part of which is tropical. The
difficulty of fixing the northern limit of the region, for ascidians as for
other groups, has been noted by Van Name (1945), and is due to the
presence of cool water quite close to the coast far south on the Pacific
side of the United States. However, it is established that the Gulf of
California has a tropical ascidian fauna (Ricketts and Calvin, 1939).
The southern limit of the region is not precisely known because the
fauna is insufficiently described. The total of about 45 species indicates
a poor fauna. Few of them other than circumtropical species occur also
in the Atlantic American area. In other animal groups, including crabs
and echinoderms, although the percentage of species common to the
Atlantic and Pacific sides of Central America is also small, there is a
substantial number of shared genera. Such comparisons of genera have
little significance in ascidians, since most genera are of very wide
occurrence.
Boreal European Atlantic
The whole temperate North Atlantic could be taken as a faunistic
unit, but the eastern and western sides are sufficiently distinct to be
considered separately. On the European coast the northern and
southern limits are approximately the North Cape in Norway and the
western end of the English Channel.
The papers of Hartmeyer (1923, 1924) and hback-Christie-Linde
(1922, 1923, 1928, 1934) remain the major zoogeographical accounts of
the European boreal ascidians, and subsequent work (Thompson, 1930,
1931, 1932, 1934; Millar, 1966c) has done little more than fill in certain
details. The total now stands at about 74 species, and although new
species will doubtless be found, particularly from specialized habitats
not hitherto examined in detail, it is not to be expected that the list will
be greatly lengthened. The ascidian fauna is considerably poorer than
that of the Mediterranean with its 130 species, although as we have seen
the Mediterranean is not faunistically sharply cut off from the Atlantic.
Few ascidians are strictly endemic to the European boreal region,
and most of those which are centred there extend either northwards
some way into the Arctic or southwards as far as the western basin of