AMER. ZOOLOGIST, 5:581-589 (1965).
CAPABILITIES OF THE GOELENTERATE BEHAVIOR
MACHINE
C. F. A. PANTIN
Dept. of Zoology, University of Cambridge, Cambridge, England
SYNOPSIS. Animals are essentially predatory behavior machines. So also are insectivorous plants which have developed raptorial feeding devices. Diploblastic and triploblastic animals meet the specification of such machines in different ways. In the
Cnidaria both muscle and nerve-net seem organised on the basis of two-dimensional
continuous sheets with local specialization. The condition is simplest in Antho/oa:
in Scyphozoa, and still more in Hydrozoa, there are further complications.
This simple picture of the Anthozoan nerve-net meets difficulties. Quick and slow
contractions of the same muscle sheet are in fact operated by the same nerve-net. The
slow contraction involves muscle-conduction and recruitment. A method of directly
observing this is described. No complete explanation is yet forthcoming for reciprocal
inhibition. The preservation of functionally significant shape seems to require proprioceptive machinery not yet discovered.
It now seems well-established that both through-conduction and the original notion
of interneural facilitation are valid elements in simple reflex responses. Knowledge of
the importance of rhythmic phasic activity has, however, greatly increased in coelenterates generally. Many of these sequences of rhythmic activity seem to be based on
modifications of the similar pattern sequences to meet different functional needs.
Particularly in connection with these phasic activities, multiple action potentials
both in response to stimuli and by spontaneous occurrence have been demonstrated.
In Calliactis, 30% of records to threshold stimuli show evidence of multiple impulses.
There is reason to associate such repetitive discharges with multipolar ganglion cells.
The relation of these multiple discharges to the functional behavior is not always
apparent.
Complication of behavior in coelenterates is charactically on the motor side. Contrasted with triploblastic animals with probably the same order of number of nervecells (roughly 105), there is a striking difference in the sensory equipment: exteroceptive
information about the objects of the real world is lacking. A hunting-wasp with
about that number of cells acts as though it had abstracted a world-model of objects,
analogous to our own model, from the information received. But an anthozoan shows
no evidence of that power. The importance of key-stimuli in anthozoan behavior is
significant in connection with this. These deficiencies in complex behavior may be
related to the topographical difficulty of complex correlation of sensory input in a
two-dimensional net. The difficulty is easily overcome in the three-dimensional nets
of triploblasts. Nevertheless, recent studies of conduction in the two-dimensional
coelenterate net show striking 'pre-adaptive' features analogous to those of triploblast
central nervous networks upon which sensory abstraction of information in these
depends.
Animals are essentially predatory behavior machines. They possess sense organs
to receive information, and predictor machinery both to process it and to direct
an appropriate apparatus of moving parts
so as to catch food, to avoid harm, or reproduce, in the immediate or more distant
future. That is an engineering definition
and not a phylogenetic one. To meet these
requirements organisms have standard materials at their disposal and their functional
structures must conform to limited engineering possibilities (Pantin, 1951). Although a plant, Dionaea muscipula, catches
flies through stimulation of sensory hairs
which excite all-or-nothing impulses (first
shown by Burdon-Sanderson, 1889), these
in turn operate the rapid closure of the
trap not by an analogue of muscular action
but by the characteristic plant mechanisms
of turgor loss (Ruhland, 1959).
Within the animal kingdom proper we
find two distinct grades of organization,
diploblastic and triploblastic, in each of
which the above requirements are met in
characteristic fashion (Pantin, 1960). Anthozoa represent the simplest diploblastic
condition in the cnidarian Coelenterata.
(581)
582
C. F. A. PANTIN
In contrast with triploblasts, sense-organs,
nerve-net, and underlying muscle sheet are
essentially confined to each of two epithelia,
ectoderm and endoderm, between which is
a fibrous connective tissue, the mesogloca
large and powerful muscle is required for
a specific functional purpose. In triploblasts this requirement is easily met by
increasing the cross-section of the muscle.
In anthozoans and indeed in all Cnidaria,
FIG. I. Transverse section (2~> fi) of mesentery of Holmes Silver. Preparation by E. ). Bathan. Scale
Metriilium .senile near oral disc, showing muscle line =100 /i\ e, endoderni; n-n, nerve net; m-s,
muscle-sheet; m, mesogloea.
sheet, and nerve-net running in intercellular space.
(Batham and Pantin, 1951; Batham, Pantin
and Robson, I960; Pantin, 1952). Mesogloea is invaded by amoebocytes, but these
are indifferent to its boundaries and also
invade ectoderm and endoderm (Fig. 1).
These statements about anthozoan organization are well supported by various
histological methods in certain species. To
prove the negative, that is that there arc
no departures from it, is necessarily more
difficult. On the principle of economy of
hypothesis, it may be provisionally accepted
unless it is gainsaid by future evidence.
But it gains strength from the manner in
which this simple layered structure meets
functional requirements. Thus in animals
of all sorts it commonh happens that a
the need is met by folding the muscle sheet,
as in the mesenteric retractor. Such folds
may be pinched off so that they become
embedded in the mesogloea, as in mesogloeae sphincters, but they still betray their
origin from a single muscle sheet. The
muscle fibers do not run individually and
freely through the mesogloea.
]n this way a number of specific effectors
are elaborated in what appears to be a
simple and continuous muscle sheet. They
include the mesenteric retractors, the marginal sphincter, the musculature of pedal
disc, and the parieto-basilars, the importance of which has been brought to our
notice by the swimming reaction of Stumphiu (Robson, 19fi3).
CAPABILITIES OF A BEHAVIOR
MACHINE
583
In the Anthozoa there is at present no
unequivocal evidence for more than a single
nerve-network in each epithelial layer,
modified as that network may be in special
regions, as in the through-conduction tracts
of the retractor faces of the mesenteries.
This at once raises the question of nervous
communication between the ectoderm and
endoderm, which in the usual formal pictures of a polyp only meet at the pharynx.
But there are other points of contact. In
the first place, pores through the body wall
at the tips of the tentacles, or at the center
of the foot, or through cinclides in the
body wall, provide ecto-endodermal contact in those anthozoans that possess these
structures. But more significant than these
is the tendency for the epithelial layers to
form tubes which can sink into or even
through the mesogloea. Such tubes are
common at the base of the mesenteries
(Batham and Pantin, 1951; Robson, 1957)
where they can be seen to carry fibers from
the column nerve-net. They also can extend through the pedal disc to the basal
ectoderm (Robson, 1965). But the most
interesting of such connections is in the
oral disc. Stephenson (1935) notes that the
radial muscular epithelium of the disc can
sink into the mesogloea. Figure 2 is an
example of the radial ectodermal muscle
sheet of the disc sinking inwards on the
exocoelic side of a pair of mesenteries till
it opens into the coelenteron and passes
continuously into the radial muscle of the
exocoelic face of the mesentery. Many years
ago Pantin (1935b) noted the polarized
radial connection of the excitable system
of the disc and tentacles with the throughconduction system of the mesenteries and
marginal sphincter, and Figure 2 illustrates
a possible avenue for the necessary connection.
In general, then, we may say that the
FIG. 2. Horizontal section of Melridium oral disc
showing radial muscle of the ectoderm passing continuously into radial exocoelic muscle of mesenter\
in the endoderm. Scale l i n e = 100 /j.; b, base of
mesentery; re, radial exocoelic face: rm, radial
muscle of ectoderm.
584
C. F. A. PANTIN
Anthozoa, which embody many primitive
features in their organization, seem to adhere to aingle epithelial muscle sheets and
single nerve-nets. Local differentiation and
various "ingenious" morphological tricks
seem to enable them to meet special requirements with this simple system. I shall
deal with certain physiological features of
their systems which still demand explanation. But I want to point out that if we
pass from the Anthozoa to what one may
reasonably call the next most highly organized cnidarians, the Scyphozoa, we at
once find a more advanced system.
T h e work of Romanes (1885), Bozler
(1926 a, b, 1927), and Horridge (1956)
shows beyond doubt that, whilst the muscular system may be based on epithelial
sheets, suitably folded, there is certainly
evidence within the epithelium for two,
or perhaps three, more or less independent
nerve-nets, linked together at the marginal
ganglia, and often serving the same muscle
in different ways. Horridge's (1956) work
on Aurelia ephyrae, though as he points
out not yet absolutely conclusive on the
histological side, leaves little room for
doubt of the independence of a net governing local feeding responses and the socalled "giant fiber" net responsible for the
contractions of the swimming bell within
the same epithelium. Compared with a
simple nerve-net, the individual developing
nerve cells of the scypho/.oan nets appear
to need extra "instructions," so that they
shall contact iieuritcs from cells of their
own kind but avoid others.
T h e most advanced condition of the cnidarian nerve-net is that seen in the medusae
of the Hydrozoa. Here, the neurites collect together to form two well-defined marginal nerve-rings, recalling the simpler
nerve cords of triploblasts. The upper and
under nerve-rings in this case touch each
other in places by abolition of the intervening layer of mesogloea (Hertwig and
Hertwig, 1878). Moreover, Hyde ' (1902)
illustrates neurites directly penetrating
through the mesogloea, and though the
figure is diagrammatic, this penetration
stems supported by the Hertwigs' exact
figures. Actual passage of a ncurite out ol
its appropriate epithelium into the underlying mesogloea would seem to demand
still further morphogenetic "instructions"
to nerve cells if the neurite is to reach its
goal.
The only unequivocal instances of neurites penetrating the mesogloea in Anthozoa seem to be in the innervation of the
marginal sphincter (Robson, 1965). In
that case the connection might be the result
of the nipping off of the epithelial muscular
tubes from the columnar muscle sheet, leaving their nervous connection with the columnar through-conduction system intact.
In tracing the development of the nervous system, the cnidarian coelenterates
seem to show a most valuable series of increasing development. That of the hydras
is simple, but the evident relation of these
to Hydrozoa with much more complex
medusoid organization suggests that their
simplicity is secondarily developed from a
system in which the more complex morphogenetic instructions originally in its parts
have been suppressed—and might reappear
unexpectedly even in the polyps.
The Anthozoa present the simplest condition of the sensory-neuro-muscular system on the histological side. There is no
unequivocal evidence against the view that
their muscle system in each of its two
epithelial layers is essentially a two dimensional sheet, modified locally to give specific effectors and in places folded into the
mesogloea, though still remaining epithelial, and antagonized simply by the coelenteric pressure. The same two-dimensional
character may be said to characterize the
nerve-net. It is unquestionably locally differentiated as in the through-conduction
system and in the oral disc. And the ectodermal and endodermal nets may be accessible to each other where the two epithelia
pierce or extinguish the intervening mesogloea. The nets do not yet seem to be
multiple within the epithelium, as in the
medusae.
Much of the physiological evidence conforms with this picture. But there are certain features which require consideration.
1) All the muscle fields of actinians investigated can give very slow contractions.
CAPABILITIES OF A BEHAVIOR MACHINE
Some fields, as in the retractors, can give
quick facilitated responses as well. Those
fields that only give slow contractions have
insufficient neurites to make contact with
each of the numerous small muscle cells
(Batham, Pantin, and Robson, 1960).
Muscular conduction as well as nervous
must be involved in the activation of these
slow muscles. Indeed, many of the features
of slow contraction in all parts of the muscle
field suggest muscular conduction as opposed to the quick facilitated contractions
which can also be elicited in the richly
innervated retractor fields. Slow contraction has an exceedingly long and variable
period. It develops in a slow sigmoid
fashion which quick-releases both during
the rise of tension and during its fall show
to be a recruitment phenomenon (Pantin,
1965). Redevelopment of tension is very
rapid during the rising phase, and is still
to be seen during relaxation. It is often
visibly transmitted as a wave of contraction
far slower than any rate of conduction
identifiable with the nerve-net (Batham
and Pantin, 1954).
Preliminary experiments by Dr. Robson
and myself show these features clearly in
the isolated mesenteries of Metridium.
Pairs of these were dissected out under
Mg++ anaesthesia. The pair was then
turned inside-out to expose the retractor
face. They were then lightly stretched, as
585
FIG. 3. Method of microstimulation of retractor
surface of Metridium mesentery. Large anode and
fine cathode, Ag/AgCl. Bath, sea water. Mesentery,
with mercury drops, pinned out and held under
light tension by spring platform.
in Figure 3. Excitation, as usual by condenser discharge, was given by a nonpolarizable Ag-AgCl-sea water cathode
with a diameter of 70-120 ^ applied locally
to the retractor surface. The muscle sheet
was observed under a microscope. Fine
drops of mercury were scattered over the
surface, after the fashion of the classical
studies of Pratt and Eisenberger (1919).
Observation was made directly and by recording on an oscillograph camera the
movement of the mercury droplets from
which light was reflected.
With this arrangement, it is easy to record quick facilitated contractions of the
retractor over the usual stimulation frequencies of about one per second, often followed by a slow contraction (Fig. 4, upper).
1 0 3 3,3 9 I 3 J 1 ) ) I I I I I I ) 1 I I t I ) i ) ) ) J ) I ) ) I ]
1 )
) J ] ) ] ) ) 1 1 I i i I 1 I
FIG. 4. Examples of camera records of response as
recorded by light reflected from mercury droplets.
The static pictures at the right show mercury droplets and cathode. On the moving film the light is
interrupted at 1 sec intervals. The first shock of
each series is coincident with the fourth light interruption. Read from right to left.
I'pper record. Four shocks given at 1 sec intervals. Note facilitated res|x>nses as successive shocks
are given.
Lower record. Ten shocks at 3 sec intervals. Heie
one has typical slow contractions which develop at
different rates and to different extents at the several
sites.
586
C. F. A. PA.NTIN
As in the intact animal, the isolated retrac- interpretation of neuromuscular action in
tor gave slow contractions, and gave these Anthozoa concerns proprioception. Over a
alone, in response to stimulation at the very wide range of extension, the muscle
same threshold at frequencies of about 1 fields have no fixed resting length (Batham
shock per 3 sec after a latent period of many and Pantin, 1950a). Yet within this wide
seconds (Fig. 4, lower). The fact of the range, the anemone has a functional well
occurrence of both facilitated contractions organized form. The form may be symand slow contractions in histologically the metrical, or there may be a strongly polarsame retractor muscle field was thus as- ized gradient of tone as in locomotion (Pansured. The observed movement of the tin, 1952).
muscle field indicated that the slow conThese are not inevitable shapes passively
traction was propagated far more slowly assumed. An anthozoan under strongly adthan the nervous conduction. Spontaneous verse environmental conditions can pass
slowly conducted slow contractions were into grotesquely malformed shapes. In the
also to be observed. Raising the intensity normal animal it would seem that there
of the shocks to about double the threshold must be some mechanism by which the tone
value for facilitated responses commonly and extension of each part is enabled to
caused a local contraction of the muscle conform to the current phase of the whole
sheet under the electrode to a single shock. animal's form. In the higher organisms this
This was sometimes followed by a con- is clone by proprioceptive machinery. How
ducted slow contraction after a delay. is it done in Anthozoa? It may be well to
These experiments indicated that the same ' remember that the anthozoan is not a pasmuscle field could be excited to give a sive animal, and that its slow successive
quick facilitated response, presumably by contractions and expansions might serve to
nervous excitation, and a much slower re- bring some uniformity of shape and tone in
cruited response apparently involving mus- the tissues. But this scarcely explains the
cular conduction. There is no need to beautiful symmetry of a partially inflated
postulate a double motor innervation of actinian. I shall refer later to the phethe muscle.
nomena of repetitive and rhythmically
2) Pantin and Vianna Dias (1952a) noted spontaneous impulses.
in a Brazilian Bunodactis that, whereas the
The behavior patterns, not only of anthoparietal musculature only gave a slow and zoans but of other coelenterates as well,
symmetrical response to excitation by the appear to be built up of:
through-conduction system, a mechanical
1) Simple reflexes, with through-conducstimulus at one side caused a \ery rapid lo- tion. Horridge's (1957) analysis of conduccal bending of the column towards the tion in colonies of coral polyps showed that
stimulus. At the time it was supposed that through-conduction does not require perthese were both parietal responses. II so, manent ability to transmit across every
they set a difficult problem for explanation nerve-net connection. Transmission at a
by a simple nerve-net. On the other hand, sufficient percentage is all that is required,
this genus possesses well developed parieto- and it need be but temporary at any one
basilar muscles. In view of the rapid uni- synapse. Through-conduction seems to be
lateral contractions of these in Stompliin, a a secondary simplification of the coelenterre-investigation of the reflex and ot the ate conduction system, but the case of
muscles responsible in Bunodactis is neces- Dionaea warns us that it may be very easy
sary.
to evolve.
3) Batham and Pantin (1954) showed oc2) Local reflexes with "interneural" facasional evidence of reciprocal inhibition cilitated spread of excitation, as in the disc
between the adjacent parietal and circular and tentacles of anthozoans (Pantin,
muscle sheets of Metridium.
There is as ]935a). The realitv and generality ot this
\et no adequate explanation ol this.
phenomenon seems to have become appai}) But the outstanding difficult) in our ent (fosephson. 1961).
CAPABILITIES OF A BEHAVIOR MACHINE
3) Rhythmic activity and phases. Much
of the behavior of anthozoans is brought
about by well defined patterns of activity,
which may be spontaneous or "released" by
appropriate stimuli (Batham and Pantin,
1950a, b). Highly complex and purposive
sequences of these appear, particularly in
the sequence of activities recorded by Ross
(Ross and Sutton, 1961) and his colleagues,
by which Calliactis actively transfers itself
to a gastropod shell of the sort normally
inhabited by its appropriate hermit crab.
Several points of special interest are to
be noted here. First, the existence of
rhythms with indications of origins from
local pace-makers, as in the normal parietalcircular contraction sequences of Metridium. Secondly, the manner in which purposive phasic activity is built up from appropriate sequences of activity. Thus, in
the feeding sequence of Metridiwn, specific
chemical food-stimuli lead to peristaltic
elongation of the column and expansion of
the disc. Then comes swaying of the column; here re-examination of cinefilms suggests that unilateral contractions of the
parieto-basilars may perhaps play a part.
Contact with food leads to nematocyst discharge and ingestion of food. Then follows
peristalsis, expansion of the coelenteron,
and finally peristaltic defecation. Now,
some elements of this same sequence can
perhaps be detected in the phasic swimming response sequence of Stomphia to
specific echinoderm and molluscan chemical stimuli, and also in the attachment behavior of Calliactis; nor is the sequence
wholly remote from the locomotor sequence
of Hydra as described by Ewer (1947).
We still do not know what determines a
particular phasic activity in anthozoans, or
links its successive parts. It may sometimes
involve chemical action, such as the presence of specific external chemical stimuli.
On the other hand, a phase change may
result from simple electric excitation. If
chemical, the action here must be endocrine, presumably in the coelenteron since
diffusion through the tissue sheets would
be exceedingly slow.
In some cases, as in feeding, there certainly appear to be links in the phasic se-
587
quence analogous to those of a chain reflex.
The discharge of nematocysts into food
through a contact-chemical stimulus (Pantin, 1942) leads to the feeding reaction in
Anemonia (Pantin and Pantin, 1943). The
transference response of Calliactis to Buccinum shells is initiated by nematocyst discharge; or possibly a spirocyst discharge in
view of the part these structures play in
adhesion, and by analogy with the part
played by the atrichous isorhizas in the attachment of the tentacles of Hydra to the
substratum during locomotion (Ewer,
1947).
Particularly important in connection
with phasic rhythmic activities has been the
discovery of electrical action currents in hydroids. Sequences of these may appear
spontaneously or in response to stimuli
(Josephson, 1961, 1962; Passano and McCullough, 1962). The ability of the nervenet to respond to a single electrical stimulus by more than one nerve impulse has
long been known in actinians, though its
functional significance in these is not clear.
Pantin (1935c) showed that multiple impulses could arise under two conditions;
as in most other excitable tissues, a very
strong stimulus could initiate a battery of
impulses, but in these early experiments on
Calliactis it was noted that about 30% of
the responses to threshold stimuli included
one or more arbitrary "after-discharges" at
an interval after the primary response to
the stimulus, which was often far too long
to be due to conduction delays. Moreover,
at times a single stimulus would occasionally be followed by a regular repetitive discharge, as is not uncommon in experiments
on scyphomedusan tissues. These phenomena are of especial interest in view of
the apparent location by Robson (1961,
1963) of large pace-making multipolar cells
in the mid-column region of Stomphia,
governing the unilateral parieto-basilar
contractions during swimming. This is a
region in which transverse conduction in
the column of Metridium and Calliactis is
found to be most difficult (Parker, 1919;
Pantin, 1935b).
Passano (1963) and Passano and McCullough (1963) have pointed out the impor-
588
C. F. A. PANTIN
tance of such widely conducted pace-making centers in Hydra as an essential element
in the evolution of simple coelenterate behavior machines. This is indeed true, but
the difficulty as I see it is to provide a
neuromuscular model which can convert
these pace-maker batteries of impulses into
the complex and highly functionally-significant accompanying movements of the
body. Sometimes indeed such impulse batteries seem to have at present no functionally significant consequences, or only a
mere correlation of simultaneity with particular, body movements without as yet evidence of how these are linked together.
Perhaps the most striking feature of
what we now know of the coelenterate behavior system is the complication on the
motor side. I have pointed out elsewhere
(Pantin, 1965) the contrast between coelenterate behavior and that of the smaller organisms with well-developed exteroceptive
sense organs and complex three-dimensional nervous systems. We are very deficient
in our knowledge of the number of nervecells in animals, but there is some reason to
suppose that in those singularly "clever"
insects, the Hymenoptera, creatures such as
the hunting wasp Ammophila contain
roughly 10° nerve-cells. Yet detour experiments with Ammophila (Thorpe, 1950)
show clearly that it contains a world-model
of the region round its burrows which it
uses in its behavior. This world-model is
an image of the external world and its
properties, abstracted from the sensory information it has received.
It seems probable that many actinians
have at least 10"' nerve-cells in their nervenets. But their sources of information are
far more restricted. There are no sensory
instruments; even the eyes and otocysts of
medusae do not seem designed for the abstraction of complex information. Actinians must rely upon tactile, mechanical,
and other such simple sensory information,
at the surface of the body.
The really important first step in the
evolution of advanced behavior is the replacement ol simple .stimuli or simple patterns of stimulation for the genesis of behavior, by an abstracted model of objects
in a real world—that same real world of
objects with which our own naive realism
endows the world. An ant reacts to stone
and so do we, rather than reacting to the
very different initial sensory inputs by
which these are detected by ants and men.
The sensory deficiency of coelenterates
may account for the importance of key
stimuli in evoking behavior patterns. These
appear in the feeding of Metridium, the
shell-transference sequence of Calliactis,
the mouth-opening response of Hydra to
glutathione. The special and unique properties of well-chosen key stimuli may sometimes serve to define objects with some of
the success of abstracted visual and other
distance-receptor information. But in addition to sensory deficiency, there is another restriction: whatever their partial departures from it, their nerve-nets are two
dimensional. As Horridge (1957) and Josephson and his colleagues (1961) have
shown, there are significant parallels between the properties of these nets and those
of the central nervous system in higher animals. But an analytical machine for the
abstraction of a world-model from a twodimensional net of connections seems to be
topographically impossible: the necessary
distant correlations cannot be made; whereas in three-dimensional network this is
easy.
Nevertheless, the properties of the nervenet, as they are becoming elucidated, seem
to be exactly the pre-adaptive features
which, combined with exteroceptive sensory instruments and a three-dimensional
development of the net, could give what is
required. It is so significant that both
Ammophila and an actinian may have
about the same number of nerve-cells. What
they can do with their predictor machinery
depends upon how that number of nervecells is organized in each case.
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