168
B. K. DELL
brooded on the peristome or in the vicinity of the periproct. In the
spatangoids, especially the genus Abatm, the eggs develop in the sunken
dorsal petals.
The brooding habit in asteroids is well known but the statistics for
Antarctic species are not well recorded. Ludwig (1903) listed 11 Antarctic and Subantarctic species. Fisher (1940) believed it Iikely that all
species of Pteraster, Euretaster, Diplopteraster and Hymewter brood
their young, together with many species of Leptychmter and Henricia.
The habit has been observed in the genera Kampylaster, Rhopiella,
Odinella, Anasterias, Lysasterias, Diplasterias, Cryptasterias, Neosmilaster and Crranaster, and probably occurs in Mirmtrella, Anteliaster
and Pealidaster (Fisher, 1940).
Mortensen (1937) concluded that at least 50% of the ophiuroids of
the Antarctic and Subantarctic regions are viviparous. At that time
only 52 species anywhere in the world had been shown to be viviparous,
and 31 of these came from the Antarctic-Subantarctic. Mortensen also
pointed out that the overwhelming majority of the viviparous ophiuroids were hermaphrodite, and that not a single non-viviparous ophiuroid
was known to be hermaphrodite. There is also a strong tendency in
these viviparous Antarctic ophiuroids, for development to take place in
the ovary rather than in the bursae (as is more usual elsewhere). An
explanation for these facts is difficult to find. It can hardly be a direct
response to a cold-water environment since there are relatively few
viviparous forms in the Arctic regions.
Fell (1945) has, however, shown that direct development is much
commoner in the echinoderms in general than had been believed, and
that 63% of the New Zealand ophiuroid fauna then known showed
direct development. He also argued that judging by the evidence of
large, yolky eggs, perhaps 70% of the Antarctic ophiuroids have direct
development. Fell’s general conclusion was that indirect development
was not the “ typical ” development in echinoderms, “ but that the
kind of development followed depends on the particular conditions
obtaining in each species ”. From the evidence available to him, Fell
believed that, “ a yolky egg is a condition frequently associated with
viviparity ”. The young brittle star produced by viviparous species is
able to pursue its development to a much more advanced stage before
it is released into the general environment. As Fell showed it is highly
probable that additional food material may be absorbed from the adult
while the developing young are retained in the bursa.
Granted therefore that viviparity may be much commoner in
echinoderms generally than has been believed, it seems highly
probable that species exhibiting viviparity would be at a selective
B. K. DELL
brooded on the peristome or in the vicinity of the periproct. In the
spatangoids, especially the genus Abatm, the eggs develop in the sunken
dorsal petals.
The brooding habit in asteroids is well known but the statistics for
Antarctic species are not well recorded. Ludwig (1903) listed 11 Antarctic and Subantarctic species. Fisher (1940) believed it Iikely that all
species of Pteraster, Euretaster, Diplopteraster and Hymewter brood
their young, together with many species of Leptychmter and Henricia.
The habit has been observed in the genera Kampylaster, Rhopiella,
Odinella, Anasterias, Lysasterias, Diplasterias, Cryptasterias, Neosmilaster and Crranaster, and probably occurs in Mirmtrella, Anteliaster
and Pealidaster (Fisher, 1940).
Mortensen (1937) concluded that at least 50% of the ophiuroids of
the Antarctic and Subantarctic regions are viviparous. At that time
only 52 species anywhere in the world had been shown to be viviparous,
and 31 of these came from the Antarctic-Subantarctic. Mortensen also
pointed out that the overwhelming majority of the viviparous ophiuroids were hermaphrodite, and that not a single non-viviparous ophiuroid
was known to be hermaphrodite. There is also a strong tendency in
these viviparous Antarctic ophiuroids, for development to take place in
the ovary rather than in the bursae (as is more usual elsewhere). An
explanation for these facts is difficult to find. It can hardly be a direct
response to a cold-water environment since there are relatively few
viviparous forms in the Arctic regions.
Fell (1945) has, however, shown that direct development is much
commoner in the echinoderms in general than had been believed, and
that 63% of the New Zealand ophiuroid fauna then known showed
direct development. He also argued that judging by the evidence of
large, yolky eggs, perhaps 70% of the Antarctic ophiuroids have direct
development. Fell’s general conclusion was that indirect development
was not the “ typical ” development in echinoderms, “ but that the
kind of development followed depends on the particular conditions
obtaining in each species ”. From the evidence available to him, Fell
believed that, “ a yolky egg is a condition frequently associated with
viviparity ”. The young brittle star produced by viviparous species is
able to pursue its development to a much more advanced stage before
it is released into the general environment. As Fell showed it is highly
probable that additional food material may be absorbed from the adult
while the developing young are retained in the bursa.
Granted therefore that viviparity may be much commoner in
echinoderms generally than has been believed, it seems highly
probable that species exhibiting viviparity would be at a selective