200
PAUL BRIEN
have a strange aberrant organization, but viable, because of sufficient
adjustment.
By a strange mutation, the fundamental individuality of bryozoans is
the cystid which, originating from the metamorphosis of the larva, has
none of the organs indispensable for nutrition and reproduction. But the
cystid is supplied with astonishing activity and suddenly releases a part
of its wall: the polypidian bud. In one stroke it forms all that it had
been deprived of by the metamorphosis of the cyphonautes: digestive
loop; lophophore, which, for nutritional purposes, will compensate for
the immobility which impairs it; and a new musculature, surprisingly
exact, which controls the only movements of which the cystid is capable,
retraction and extension.
This new epigenetic organogenesis compensates for the deficiencies
consequent upon metamorphosis and attachment. It is an epigenetic
response to a mutation which could have been lethal and mortal.
The regressive evolution of bryozoans, thus compensated for, from then
on was a success, if one considers the persistence of the bryozoans
through geologic time, their differentiations, and their expansion into all
the seas and freshwater. It is true that colonial budding has given them
a considerable power of proliferation and multiplication. Polypidian
budding cannot be results of small mutations appearing by chance,
filtered from generation to generation by natural selection. It is "all or
nothing" and perfect from the start.
There has been some discussion already about the large changes that
the ascidian tadpole undergoes after its attachment. If it is necessary
to consider it again, it is to provide another example of reduction and
simplification sufficiently corrected during development. Attached by the
anterior part where the mouth opens, the young ascidian, descended
from the larva, turns 180°, restoring to the upper surface the buccal and
cloacal siphons. At the same time, the pharynx, last remnant of the
endoblast, dilates and becomes organized. Enveloped by the two peripharyngeal cavities, it becomes, for the resting sessile organism, the organ
for food capture and also for respiration. It is completed by a newly
formed digestive loop. Its wall, composed of two pharyngeobranchial
layers, is perforated by numerous small ciliated openings, and becomes
the surface for the exchange of gases where a continuous current of water
flows, maintained by a complicated and new system of cilia. How many
other patterns of organization, joined together, complete the structure of
the ascidian. Its regressive evolution is compensated for so satisfactorily
that it becomes the departure for a secondary progressive evolution.
PAUL BRIEN
have a strange aberrant organization, but viable, because of sufficient
adjustment.
By a strange mutation, the fundamental individuality of bryozoans is
the cystid which, originating from the metamorphosis of the larva, has
none of the organs indispensable for nutrition and reproduction. But the
cystid is supplied with astonishing activity and suddenly releases a part
of its wall: the polypidian bud. In one stroke it forms all that it had
been deprived of by the metamorphosis of the cyphonautes: digestive
loop; lophophore, which, for nutritional purposes, will compensate for
the immobility which impairs it; and a new musculature, surprisingly
exact, which controls the only movements of which the cystid is capable,
retraction and extension.
This new epigenetic organogenesis compensates for the deficiencies
consequent upon metamorphosis and attachment. It is an epigenetic
response to a mutation which could have been lethal and mortal.
The regressive evolution of bryozoans, thus compensated for, from then
on was a success, if one considers the persistence of the bryozoans
through geologic time, their differentiations, and their expansion into all
the seas and freshwater. It is true that colonial budding has given them
a considerable power of proliferation and multiplication. Polypidian
budding cannot be results of small mutations appearing by chance,
filtered from generation to generation by natural selection. It is "all or
nothing" and perfect from the start.
There has been some discussion already about the large changes that
the ascidian tadpole undergoes after its attachment. If it is necessary
to consider it again, it is to provide another example of reduction and
simplification sufficiently corrected during development. Attached by the
anterior part where the mouth opens, the young ascidian, descended
from the larva, turns 180°, restoring to the upper surface the buccal and
cloacal siphons. At the same time, the pharynx, last remnant of the
endoblast, dilates and becomes organized. Enveloped by the two peripharyngeal cavities, it becomes, for the resting sessile organism, the organ
for food capture and also for respiration. It is completed by a newly
formed digestive loop. Its wall, composed of two pharyngeobranchial
layers, is perforated by numerous small ciliated openings, and becomes
the surface for the exchange of gases where a continuous current of water
flows, maintained by a complicated and new system of cilia. How many
other patterns of organization, joined together, complete the structure of
the ascidian. Its regressive evolution is compensated for so satisfactorily
that it becomes the departure for a secondary progressive evolution.