AMER. ZOOL., 28:1065-1075 (1988)
Neuronal Repair and Recovery of Function in the
Polyclad Flatworm, Notoplana acticola1
HAROLD KOOPOWITZ AND MOLLY HOLMAN
Department of Developmental and Cell Biology,
University of California, Irvine, California 92717
SYNOPSIS. Polyclad flatworms are unable to regenerate or replace parts of their central
nervous systems but are able to repair lesions in the cerebral ganglion or peripheral nerve
plexuses. Repair of nerve lesions is very rapid and functional activity is reestablished within
48 hr. Bilateral coordination can be reestablished in animals with either split brains or in
animals where half of the brain, on one side, has been excised. Animals with transplanted
brains display normal behavior even if the brain has been rotated and inverted. Likewise,
in animals which had the brain transplanted into the tail region, cerebral dominance was
reestablished even after the brain was rotated through 180°. A time sequence of events
during healing of cut nerves has been established. Sprouting starts approximately 4 hr
after injury. Contact between a filopodium from the proximal section and distal stump
occurs approximately 14 hr after the injury. Filopodia are subsequently retracted and the
area of contact between the two segments increased. After 72 hr there are no filopodia
and the axon appears to be intact. Healing between the two cut ends, therefore appears
to involve fusion of the cut axons. Indirect evidence suggests that neurones recognize
their own specific segments.
within 5 days, recovery of behavior that
Despite the fact that it is well known that involved connectives between the two
planarian flatworms have remarkable pow- halves reappeared. Axons could be seen
ers of regeneration and are able to replace coursing between the two halves. If the two
entire heads containing the central ner- hemispheres of the ganglion were redivous system, very little is known about the vided then behavior was lost once more.
One of the major effects of dividing the
actual mechanisms of central nervous system regeneration within that group. In fact, brain into two is the loss of coordination
basic questions such as whether or not there between the two sides of the animal. Avoidis cell constancy or identified neurones have ance behavior to an antero-lateral stimulus
not been answered for the planarians. The such as a light jab is a longitudinal conpolyclad flatworms, on the other hand, have traction with subsequent extension and
limited powers of regeneration. They are locomotion away from the stimulated side.
unable to replace the brain if it is lost and In the absence of the brain the response is
are even unable to replace sections of the a longitudinal contraction followed by a
brain if those are excised (Hyman, 1951). backwards directed shuffle (Koopowitz,
What regeneration is possible in polyclad 1973). Split brain animals respond to the
flatworms, appears to be confined to stimulus like decerebrate animals for the
peripheral parts of the body (Samuels and first 3-4 days but then recover their norKoopowitz, unpublished observations). On mal avoidance behavior (Fig. 1A).
the other hand polyclad flatworms are able
More remarkably, Faisst et al. (1980)
to repair damage to the central nervous demonstrated that if one excised half of
system in a very efficient manner.
the brain that the severed stumps of the
nerves on that side regrew back into the
remaining half of the brain, where conREPAIR IN THE CENTRAL
nections were reestablished. No discernNERVOUS SYSTEM
Faisst et al. (1980) demonstrated that if ible differences could be detected between
the brain was split in half longitudinally, avoidance turning by normal animals with
intact brains and those that only possessed
one half of a brain. In these experiments
1
From the Symposium on Nervous System Regener- it only took 4 days to partially reestablish
ation in the Invertebrates presented at the Annual Meeting of the American Society of Zoologists, 27-30 the behavior (Fig. IB). In addition to anatomical and behavioral recovery, we were
December 1986, at Nashville, Tennessee.
INTRODUCTION
1065
1066
H . KOOPOWITZ AND M. HOLMAN
100
100 r
HALF BRAIN
SPLIT BRAIN
T
CO
• t
i
i
T
50 -1
-
i
I
- i
i
_j
•-
/
50
/
/
A
10
B
20
0
10
20
POSTOPERATIVE DAYS
FIG. 1. A. Recovery of avoidance turning by N. acticola following longitudinal hemisection of the brain. The
arrow designates where the healed brain was resectioned. Data from a single animal. B. Recovery of avoidance
turning by an animal where half of the brain had been excised on day 0. (After Faisst et al, 1980)
also able to demonstrate physiologically
that conducting systems across the brain
were also functional. Despite the fact that
nothing is known about the neuronal circuitry involved in avoidance behavior other
than that the brain is required, these
experiments pointed out several different
things. In the first place, there is an apparent redundancy in the central machinery
coordinating avoidance behavior and only
half of the brain is needed despite the fact
that this behavior involves the two sides of
the body. It is possible that the sensory
neurones feed directly onto the appropriate brain neurones. Secondly, new connections between the severed nerve stumps
on one side of the brain and neurones on
the other side must be able to recognize
each other.
BRAIN TRANSPLANTS
More recently, Davies et al. (1985) were
able to transplant entire brains between
different individual animals. Once again,
behavioral scoring was used. Four behaviors that require the presence of the brain
were selected. These were ditaxic locomotion, righting, avoidance turning and
feeding behavior. All four behaviors
require the presence of a functionally
reconnected brain in order to be executed
properly. Transplantation is a simple oper-
ation. The square of tissue between the
animal's eyes is cut out. This tissue is cut
all the way through the animal's body and
contains epidermis, muscle layers, some
digestive system and parenchyma as well
as the brain. Usually the size of the tissue
is just under 1 mm square. A similarly sized
hole is made in the recipient animal. The
worm is maintained with the part that will
receive the brain out of water and the
transplant tissue positioned in the hole. The
preparation must be irrigated from time
to time with normal sea water so that it
does not dry out. The cut edges of the
donor and recipient fuse together in a few
hours. Thereafter the animals can be
returned to normal sea water. The behaviors tested recovered within 3 days of the
transplant operation. The way that recovery is scored depends on the completion of
the total behavior. If we examine recovery
(Fig. 2), we find that behavior may be elicited only a certain percentage of the time.
In order to be performed even once the
correct neuronal circuitry must have been
repaired anatomically. It may take several
days, however, to establish the circuitry so
that it works functionally all the time. In
addition the experiments suggest that individual neurones have "labels" that are constant from animal to animal within the same
species.
1067
NERVE REPAIR IN FLATWORMS
100-1
LU
50-
50-
o
DITAXIS
RIGHTING
1
2
35
10
20
0
1
2
3 5
10
20
DAYS SINCE TRANSPLANT
Fie. 2. Behavioral recovery of N. actkola, following transplantation of the brain, with normal orientation.
Four different behaviors were assessed. Data are means for six animals. Filled stars = performance of normal
animals. Circles and open stars are data from two different groups of worms with transplanted brains. Note
the X axis has two different time scales. (After Davies et al, 1985)
ORIENTATION TO THE TARGET
We found that it was possible to get reestablishment of the correct behavior after
the brain was rotated 180° so that cut
stumpsof Nerve VI leaving the brain, which
normally run along the animal's length towards the tail, were juxtaposed to Nerves
I and II that run towards the anterior margin of the worm. These animals recovered
normal behavior and were indistinguishable from those with normal brains (Fig.
3). Histological reconstructions of the
transplanted brains and nerves revealed
that the nerves had merely fused with those
nerve trunks that were closest to them.
There are several pairs of identified sensory interneurones in the brain. We examined the physiology of four different identified neurones (Davies et al., 1985) in
transplanted and rotated brains, some 21
days after transplantation. We found that
they all received their appropriate input
from peripheral sense organs, photoreceptors and mechanoreceptors and appeared
to have normal physiology. When the cells
were filled with Lucifer Yellow and examined it became apparent that their morphology had been altered. These interneurones normally have an axon that travels
posteriorly for some distance (ca. 10 mm)
along Nerve VI but get their input from
the eyes or an anterior sensory groove
wherein the vibration receptors are situated. In the transplants these cells had
axonal processes which reversed themselves and travelled posteriorly towards
their correct targets in Nerve VI. The
way, however, that axons travelled towards
the target varied with individual kinds of
cells. In one case a wBRA cell had a den-
1068
H . KOOPOWITZ AND M. HOLMAN
1OO r ,
50
50 -
DITAXIS
0
0
1
2
1
3
2
5
3 5
10
10
20
0
20
0
1
2
1
3
2
5
10
3 5
10
DAYS SINCE TRANSPLANT
FIG. 3. Behavioral recovery of N. acticola, following brain transplantation after the brain had been rotated
through 180°. Details same as Figure 2. (After Davies el ai, 1985)
drite on one side of the brain that had
become converted into an axon that travelled through the brain towards the target
while on the other side of the brain the
normal axon merely turned backwards and
ran outside the brain towards its target (Fig.
4). Clearly, parts of cells within the brain
are able to sense their position in relation
to their normal targets.
Even if the transplanted brain was positioned upside-down in the recipient animal
normal behavior was reestablished. However, if the brain was also rotated as well
as being inverted then feeding behavior,
which is the most complex of the behaviors
tested, only recovered in two out of the
many transplants that we attempted. The
other three behaviors recovered without
any trouble. When the brain in the anterior end was inverted a supernumerary vil-
lus-like process was produced and the brain
migrated into the tip of the villus.
POSTERIORLY POSITIONED BRAINS
The extreme plasiticity of the nervous
system in Notoplana acticola can be demonstrated when the brain is transplanted
to a position not normally occupied by that
organ. We found that when the brain was
transplanted to a site posterior to the pharynx and near the tail that the brain would
still reconnect to the nervous system and
that it would reestablish its behavioral control functions. Clear evidence of the recovery of function could be seen on the third
day after the operation. Once again the
behavior of these animals was indistinguishable from that of normal unoperated
animals. Clearly the brain was able to reestablish functional connections. Exactly how
NERVE REPAIR IN FLATWORMS
1069
TARGET
FIG. 4. Camera lucida drawing of the wBRA cells from the brain of N. acticola after rotation of the brain
through 180°. Drawn from living preparation. The axons proceeding to the new target have been cut short.
Brain is 600 /im in diameter.
it does this is a mystery. There is indirect difference if the brain is rotated so that its
evidence that much of the neuronal normal anterior nerve trunks are oriented
machinery programming motor patterns is towards the tail. During the first few days
in the peripheral plexuses (Gruber and following surgery behavior was reestabEwer, 1962; Koopowitz, 1974) but we do lished as in other transplants. Righting
not know how the central neurones from behavior was monitored in these experithe brain feed into and control those ments as the average time taken to turn
peripheral motor neurones. We do not over rather than the number of times the
know much about how connections are animal executed the behavior correctly.
made with these posteriorly positioned When this is done we find a gradual
brains and for reasons discussed below, it improvement in the animal's ability to right
is unlikely that we will be able to achieve itself and this ability is lost when the villus
a clear understanding in the near future. containing the brain is extirpated (Fig. 5).
We surmise that either alternate preexist- Histological examination of the worms
ing pathways might be used, possibly in the reveals that the main longitudinal nerve
form of nerve-nets, or else cells are able to chords of the recipient (Nerve VI) had
seek out their targets over comparatively joined with nerves coming from the brain.
vast distances.
After several days a villus is produced and
the brain is positioned at its tip. In those
villi where we have tried to reconstruct
POSTERIORLY ROTATED BRAINS
brains following serial sectioning we find
As with brains positioned in the front of that the anatomical organization of the
the worm it does not seem to make much brain is lost. Known landmarks such as spe-
1070
H . KOOPOWITZ AND M. HOLMAN
100
CD
CO
40
LU
CO
LU
50
§5
LU
DC
20
g
DC
10
DAYS
20
FIG. 5. Recovery of righting behavior in worms where
the brain was transplanted and rotated through 180°.
Brain positioned in the tail of the worms. At the arrow
the brain was removed by cutting off the supernumerary villus. Note that decerebrate worms are able
to right themselves but they take much longer times
than animals with brains. Points are means and bars
standard errors, n = 33. (After Gallemore, unpublished)
10
DAYS
100
LU
CO
O 50
cific cells and areas of neuropil can no
longer be recognized and frequently the
tough outer brain sheath is not present. It
is currently impossible to identify the
"identified" neurones in brains which have
been transplanted to the rear of the worm.
When the behavior returns, the animals
with posteriorly rotated brains still prefer
to use their original anterior ends for orientation. However, in a small number of
worms, we find a subtle change in both
righting behavior and escape locomotion.
Over the course of time (up to 15 days) the
tail may start to assume an increasingly
active role, acting like an anterior end. This
is noticeable in righting behavior (Fig. 6A)
and to a certain extent in avoidance locomotion (Fig. 6B) as well. The transference
of behavior to the tail is never complete.
Even so, the fact that some transference
occurs is remarkable. In righting behavior
the animal normally grips with the anterior
margin and pulls itself over but in a number of worms with rotated transplants, the
SLU>
rr
10
20
DAYS
FIG. 6. Transference of behavior from the front to
the tail following transplantation of the brain to the
rear. The brain was rotated through 180°. A. Orientation in righting behavior. Circles are the normal
orientation using anterior margin and stars are where
the worm used its tail to grip. Abscissa is in days posttransplantation. B. Mean avoidance behavior using
anterior end (solid line) and using tail (broken line).
Bars are standard errors. (After Gallemore, unpublished)
tail may be used for the original gripping.
The avoidance locomotion is not identical
to that seen in normal animals. In a normal
worm, a jab at the front end results in a
retraction of the longitudinal musculature,
1071
NERVE REPAIR IN FLATWORMS
a turn away from the site of stimulation
and then vigorous ditaxic locomotion with
the anterior end leading. Ditaxic locomotion is a complex behavior involving the
posterior progression of alternate waves of
contraction (Faisst, 1979). It is never seen
in decerebrate animals. Animals with posteriorly transplanted and rotated brains will
after a few days walk with normal ditaxic
locomotion. In response to an anterior
aversive stimulus such as a pin prick, animals with transplanted brains indulge in a
kind of retrograde motion shuffling off
backwards with the tail leading. This can
also be seen if the prick is delivered to the
rear. About half of the time the worms
indulge in this type of retrograde shuffle
(Gallemore et al., unpublished observations). It is unlikely that the animals could
produce a totally new organizational axis
for their bodies but even a partial reorganization suggests that the system is capable of profound changes.
HEALING OF CUT NERVES
The major reason for investigating repair
using the Notoplana preparation comes
from a much simpler type of preparation
where we have started to investigate the
manner by which cuts through axons are
repaired. Initially we found that if Nerve
VI, the major longitudinal nerve cord, was
severed that the portion of the body posterior to the cut acted as if it was decerebrate. Feeding behavior in that region was
by local feeding reflexes and that segment
continued to accept food as long as it was
offered. There was no satiation. Between
36 and 48 hr after the surgery the worms
regained their normal feeding patterns
(Table 1). If the cuts were prevented from
healing then recruitment of new pathways
on the contralateral side of the body could
be demonstrated. When the cut was allowed
to heal, both histological and electrophysiological examination indicated that the cut
ends of the nerve had rejoined (Koopowitz
etal, 1975, 1976).
HEALING AT THE SINGLE CELL LEVEL
The organization of Nerve VI has not
been examined in detail but can be
expected to be similar to that seen in other
TABLE 1. Recovery of feeding turning in N. acticola
following a lesion to Nerve VI on one side of the body.*
Treatment
Series 1: Normal recovery
Normal
Cut exp. side
Healed
Cut control side
Series 2: Recruitment
Normal
Cut exp. side
Healed
recut exp. side
Cut side
Control side
100
4.3
94.9
79.8
100
92.3
97.9
0.0
100
5.6
92.1
72.1
0
100
97.9
100
* Numbers given are in percentages. Ten animals
were used for each study and approximately ten readings obtained per individual (after Koopowitz et al.,
1975).
large nerves of the ventral submuscular
plexus (Chien and Koopowitz, 1974). The
cords contain a mix of cell bodies, axons
and small areas of neuropil. There appears
to be no obvious pattern to the arrangement of those components within the cord.
A number of axons in Nerve VI arise from
neurones that have their cell bodies in the
brain. For convenience they can be
arranged into two groups, those that have
large diameter axons (approximately 10 urn
or greater) and those with smaller diameter axons. Ultrastructural investigations
showed that the large diameter axons are
invested with a glial sheath that can be several layers thick. In our current series of
experiments we have been concerned with
events that happen after the large diameter
axons have been severed.
Currently, we cut Nerve VI and at set
times after the injury fill brain cells with
Lucifer Yellow dye and then examine the
extent of filopodium production and also
the events associated with repair. The cut
is made with a sliver of razor blade, making
a lesion about 2 mm wide that goes through
the entire body, cutting muscles as well as
Nerve VI itself. The cut ends of the wound
are drawn together in a matter of hours
although an obvious scar can be discerned
for several days. In many respects flatworms are ideal for these kinds of investigations. The animals are small and flat,
relatively translucent and the entire prep-
1072
H. KOOPOWITZ AND M. HOLM AN
nature of the nervous system which may
function as a possible nerve-net (Koopo300witz, 1973), severed axons may be able to
reconnect to any of several different distal
stumps and make functional connections.
3>2OOUsually, within 36 to 48 hr following the
cut the nerve is able to conduct impulses
across the scarred region. Our original light
histology (Koopowitz et al., 1976) revealed
= 100that the cut ends of the axons in the region
of the scar had become swollen, but we
were unable to ascertain if cut ends of the
8
12
axons had fused to produce a continuous
Time (hours)
cylinder or if they had merely come into
FIG. 7. Growth rates of filopodia from severed axons very close proximity by some kind of gap
in Nerve VI of A', acticola. The maximum filopodium or possibly a chemical synaptic junction.
length from a single filled cell was measured. Contact
with the distal severed stump is first noted about 14
There is also the problem of how new
hr after cutting. The filopodia measured are not nec- connections are made. Chemical axoessarily the ones that actually made contact. Each point
refers to a single filled cell. Abscissa is time after axonal synapses have been found in regions
where leech axons have repaired themNerve VI was cut.
selves (Fernandez and Fernandez, 1974).
Possibly the initial contact could also be
aration can be examined on the stage of a due to gap junctions. Another question to
compound microscope. The rapidity with be answered is whether or not the nature
which they heal also allows one to carry of the junction changes with time. Our curout experiments in a relatively short period rent experiments suggest that within 2
of time and one can carry out the kinds of days the portions of the axon actually
observations in an almost intact animal that fuse together and cytoplasmic continuity is
can only be done in tissue culture in other reestablished. The major question remains:
how does a cell recognize the appropriate
situations.
segment to which it needs to become
attached?
PROBLEMS CONCERNING NERVE REPAIR
Four possibilities suggest themselves as
AXONAL REPAIR
processes that could account for the events
that take place during the healing process.
The maximum rate by which processes
(1) The severed axons could sprout and grow out from the severed stump was mearegrow to their original targets. The distal sured by filling cells at known times after
severed portions of the axons might then Nerve VI was cut. Typically, sprouting
be expected to degenerate. Electron commences approximately 4 hr after injury
microscopic examinations of the region of and sprouts appear to grow at a steady rate,
the nerve which had been cut have revealed slightly less than 30 nm per hour (Fig. 7).
myelin bodies suggesting that a certain The filopodia make contact with the distal
amount of degeneration does take place. severed stump approximately 14 hr after
(2) The animals are able to recruit alter- surgery and at that time Lucifer Yellow
nate pathways through undamaged por- dye introduced into the cell soma in the
tions of the nerve plexus. This has in fact brain will be transported into the distal
been demonstrated with regards to feeding stump, suggesting that cytoplasmic contibehavior. (3) The cut ends of the severed nuity is reestablished. Once a connection
axons are able to find each other, recon- has been made between a filopodium and
nect and become functional. In this case the distal stump, the entire stump is filled.
there would be a one to one specificity or It is quite clear that the filopodia are not
perhaps specificity for a particular class of merely growing towards the target area.
neurone. (4) Because of the plexiform Following contact filopodium length is
NERVE REPAIR IN FLATWORMS
12
1073
14
Fie. 8. Camera ludda drawings of living axons at various times after cutting Nerve VI of N. acticola. Arrows
denote points of contact between proximal and distal axon segments. The brain is positioned at the top of
the drawing. Numbers refer to hours after cut was made. Each drawing was a different cell. Cell at far right
had not made contact after 19 hr.
reduced and 60-70 hr later most signs of
filopodia are no longer evident. Examination of the distal stumps at the time that
contact is made revealed (Fig. 8) that
sprouting from the distal stump had also
taken place. Initial contact is usually
between a single very slender filopodium
and a distal segment. On four occasions,
two filopodia from the proximal side made
contact with the same distal stump. We have
never seen dye-coupling between one
proximal portion and more than one distal
segment. This suggests that there is some
specificity in the reconnections between
those neurones that we have been investigating. One case is interesting in this
regard. One neurone was filled that had
failed to make contact by 19 hr (Fig. 8) and
this neurone displayed an unusually large
number of filopodia. One cannot help but
wonder if this was because contact had not
yet been made? By inference it appears
probable that the two cut stumps of the
axon may be drawn together by the filopodia after contact has been made. The
filopodium appears to get shorter and wider
until it can no longer be discerned.
In an earlier study in our laboratory (C.
Palkowski, unpublished) it was found that
healing and reestablishment of feeding
behavior were delayed by 24 hr if the cut
surface of the wound was washed with 0.2
mg-ml"1 Concanavilin A. In the present
study we looked at the effect of Concanavilin A on filopodium sprouting in healing
neurones. While we did not see any differences in the initiation times of sprouts
between controls and treated preparations, we did find what appeared to be significant differences in orientation of filopodia towards the target (Table 2). In the
control group 78% of the filopodia produced were oriented towards the target
while the other 22% grew away from the
target area. In preparations treated with
0.2 mg-ml"1 Concanavilin A slightly over
half, 54%, of the filopodia grew towards the
target and the other 46% grew in other
directions. Differences between treated and
control groups were found to be significant
1074
H . KOOPOWITZ AND M. HOLMAN
FIG. 9. Camera lucida drawings of living axons at various times after cutting Nerve VI and following treatment
with Concanavilin A. Arrows denote position of contact between proximal and distal axon segments. The
brain is positioned at the top of the drawing. Numbers refer to hours after cut was made.
using a Chi2-test (Zar, 1984), x2 = 7.02 and
0.01 < P < 0.005. While there seemed to
be a slight increase in the total number of
filopodia produced with Concanavilin A
treatment these numbers were not statistically significant. An additional effect may
be the apparent length of time before filopodial retraction after axonal contact and
fusion has occurred but this has not been
examined in detail yet. Figure 9 suggests
that healing may be even more rapid than
in untreated controls. If surface glycoproteins are involved with target orientation
as our experiment suggests, Concanavilin
A certainly does not appear to interfere
with axon-axonal fusion although we do
not know if it interferes with contact specificity. That remains to be tested.
DISCUSSION
At this stage, the studies on nerve repair
in Notoplana acticola have contributed more
insight into the organization of the nervous system in polyclads than into the processes and problems associated with nervous system repair and regeneration. The
ability of the brain to reintegrate itself into
the nervous system irrespective of where
it is positioned suggests that the brain may
be interacting with diffusely conducting
networks and specificity involves recognizing the appropriate network. Certainly such
networks do exist with regards to sensory
systems in polyclads (Koopowitz, 1973,
1975). It is difficult, however, to reconcile
the inability to integrate information when
the brain is turned through 90° with the
presence of diffusely conducting nerve networks that it could integrate into if the
TABLE 2.
Effects of Concanavilin A on Jilopodial growth
in damaged neurones.*
Treatment
Concanavilin A
Control
# filopodia
towards target
# filopodia
away from target
Total
37
43
32
12
69
55
* Filipodia growing posteriorly were scored as orienting towards the target. Others were scored as
growing away from the target. Seven cells each were
scored for experimental and control treatments. Data
significant x ! = 7.02 and 0.01 < P < 0.005.
NERVE REPAIR IN FLATWORMS
brain is twisted through 180° and/or even
inverted.
There also does appear to be a fair
amount of specificity. Repair and fusion in
individual axons occur on a one to one basis
and it is suggested that there may be
"search" conditions with profuse sprouting if the appropriate axon cannot be
encountered. This in turn suggests that
some kind of feedback mechanism must be
involved that regulates sprouting and perhaps is also involved with the withdrawal
of filopodia once contact has been made.
Intriguing questions still remain to be
answered. How do cells within the brain
"know" their position in relation to that
of their targets and how do they restructure existing neurites such as dendrites into
axons? How do cells recognize the target
once it has been reached and how is the
sequence of events involved with fusion
regulated? Finally, is repair to damaged
nerves carried out using the same basic
processes in all animal groups or is there
a diversity in the ways that different taxa
repair themselves? We think that the polyclad flatworms may be instrumental in
helping us understand and perhaps answer
some of these questions.
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Davies, L., C. L. Keenan, and H. Koopowitz. 1985.
Nerve repair and behavioral recovery following
1075
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Faisst, J. 1979. Locomotion and neuronal plasticity
in the flatworm Notoplana acticola. Ph.D. Diss.,
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Faisst, J., L. Keenan, and H. Koopowitz. 1980. Neuronal repair and avoidance behavior in the flatworm, Notoplana acticola. J. Neurobiol. 11:483496.
Fernandez, J. H. and M. S. Fernandez. 1974. Morphological evidence of an experimentally induced
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Gruber, S. A. and D. W. Ewer. 1962. Observations
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Koopowitz, H. 1973. Primitive nervous systems. A
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Koopowitz, H. 1974. Some aspects of the physiology
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Koopowitz, H. 1975. Electrophysiology of the
peripheral nerve net in the polyclad flatworm,
Freemania litoricola. J. Exp. Biol. 62:469-479.
Koopowitz, H. and P. Chien. 1974. Ultrastructure
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Koopowitz, H., D. Silver, and G. Rose. 1975. Neuronal plasticity and recovery of function in a polyclad flatworm. Nature 256:737-738.
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