C om m u n ica tio n présentée le 28 m ai ig 6o.
THE
N E M A T O C Y S T
OF
H Y D R A
(Part I).
THE QUESTION OF CONTROL
OF THE NEMATOCYST DISCHARGE REACTION
BY FULLY FED HYDRA
by
Allison
L . B urnett,
T hom as
L
entz
and M ichael
W
arren,
D ep a rtm e n t o f Z oology, C orn ell U n iversity,
Ith a ca , N ew -Y ork, U.S.A.
D E D IC A T IO N .
T h e author w ould like to dedicate the followi?ig series of
papers to Dr. John M . A nderson w ho was the author’s major
professor during his graduate career, and w ho provided h im
w ith an atm osphere w hich was entirely conducive to the pur­
suit of free research.
I n t r o d u c t io n .
O ne of the m ost structurally com plex and certainly one of
the m ost enigm atic organelles in the anim al kingdom is the
nem atocyst of coelenterates. F or nearly a century hosts of
scientists, too num erous to m ention, have concentrated their
attentions on the mode of form ation, the m igration pathways,
th e m echanism of discharge, and the chem ical n ature of these
unusual structures which are found only in the phylum Coe­
lenterata. Today, none of these subjects of investigation has
been resolved to any degree of satisfaction. T here are three
theories in existence today w hich attem p t to explain the m echa­
nism of nem atocyst discharge; the subject of m igration p ath ­
248
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
ways, i.e., how does a nem atocyst reach its final destination in
the tentacles, is explained by two com petent b u t conflicting
schools. T h e m ode of form ation of a nem atocyst rem ains
alm ost com pletely unknow n, an d the chem ical n atu re of the
nem atocyst capsule plus its enclosed contents is still a subject
of research, although L e n h o f f et al. (1957) have contributed
greatly to our u n d erstanding of the chem ical constituents of
the capsule.
T h e present studies will report m ainly a series of investiga­
tions into subjects of the control of discharge, the m aturation
process, and the m igration pathw ays of nem atocysts in hydra.
It cannot be hoped th a t a single investigation will answer in
detail all of the problem s concerned w ith these baffling proces­
ses; however; it is desired th a t such a study will add to our
present knowledge of the nem atocyst and will stim ulate furthei
investigation into the nature of this com plex organelle.
T h e authors feei th a t it would be repetitious to give an account
of th e m orphology of the four nem atocyst types found in hydra,
because several excellent descriptions of their gross structure
( W e i l l , 1934; H y m a n , 1940) and th eir fine structure ( S e m a l V a n G a n s e n , 1954; B o u i l l o n et al., 1958; C h a p m a n and T i l n e y ,
1959 a, 1959 b) already exist in the literature. T h e classification
of nem atocysts used in the following reports will be based on
the classification of H y m a n (1940—after W e i l l , 1934).
T h e m echanism w hich controls the discharge of nem atocysts
has been a subject of controversy for m any decades. Since each
of th e nem atocysts of h y d ra contain a prom inent « trigger » or
cnidocil which projects from the nem atocyst to the exterior
of the cnidoblast cell or nem atocyte, it was naturally tho u g h t
th a t this structure m ust be influential in the discharge process.
However, as H y m a n (1940) cautions, although the cnidocil m ay
be responsible for initiating the discharge, the m echanism
involved is not a sim ple one of the cnidocil eliciting the dis­
charge reaction sim ply because it has been m echanically stim u­
lated. H y m a n brings attention to the com m ensal ciliates which
are frequently found m oving over the surface of hydra; these
anim als apparently never discharge nem atocysts in spite of the
fact th a t they ru n over an d even bend the cnidocils in their
search for food. F urtherm ore, the nem atocyst is able to dis­
charge after it has been com pletely rem oved from the cnido­
blast cell and is no longer in contact w ith the cnidocil, b u t this
m ust not be taken as a positive indication th at the cnidocil does
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
249
not control th e eversión of nem atocysts when the nem atocyst
is inside a cnidoblast cell.
T h e au th o r has repeatedly attem pted to discharge nem ato­
cysts hy stim ulating the cnidocils m echanically w ith pieces of
lens tissue, fine glass rods, hair, cotton thread, etc., b u t has m et
w ith virtually no success. W hen the same m anuever was
attem pted w ith an isolated thoracic appendage of the brine
shrim p, A rtem ia salina, there was a prom pt discharge of
« grappling nem atocysts » or desmonemes. O n the other hand,
A rtem ia « juice » (obtaining by squeezing several h u n d red ani­
m als th ro u g h fine bolting cloth) when flooded over the tentacles
of a h y d ra causes relatively no eversión of nem atocysts. T his
evidence is offered in support of H y m a n ’s (1940) statem ent,
« apparently th e cnidocils react prim arily to the general m echa­
nical features of objects as texture an d shape and secondarily
to th eir chem ical em anations ». P a n t in (1942 a, 1942 b) has fu r­
th e r dem onstrated th a t some nem atocysts w hich do not dis­
charge after stim ulation of the tentacles w ith a glass rod will
discharge if th e glass rod is applied in the presence of food
juices. It appears th a t some chem ical or chemicals from the
food ex tract has lowered the threshhold for m echanical stim u­
lation.
E w e r ( 1 9 4 7 ) presents a fine account of the stim uli required
to discharge the different types of nem atocysts in hydra. F or
instance, he found th a t the atrichous isorhizas, presum ably
fun ctio n in g in locom otory processes, discharge w hen the ten­
tacle comes into contact w ith a sm ooth surface such as a glass
slide. F u rth erm ore, he has shown th a t the presence of a food
extract from copepods will in h ib it the discharge of atrichous
isorhizas while augm enting the discharge of stenoteles. E w e r
speculated th a t a chem ical in the food m ig h t b rin g about the
solation of the cnidocils of one type of nem atocyst while effect­
ing th e gelation of another type at the same time. Solation
would ten d to m ake the cnidocil m ore supple and m ake it pos­
sible for it to deform the capsule and u n latch the operculum .
T h e au thors have conducted a sim ple experim ent, the results
of w hich, contradict E w e r ’s theory. It was found th a t if hydra
are kept in the cold (40 C) for 1 Y hours, they are able to hold
prey, such as A rtem ia , to th eir tentacles, b u t are no t able to
kill the prey. M icroscopic exam ination of the A rtem ia has
revealed th a t alth o u g h several desm onem es have been released
from the h y d ra ’s nem atocysts batteries, stenotele discharge was
2J 0
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
altogether lacking.
A fter five m inutes when the w ater sur­
ro u n d ing the h y d ra has been w arm ed slightly, the A rtem ia
w hich previously were w riggling actively on the tentacles are
im m ediately killed. A t this tim e their bodies are found to be
pierced w ith num erous stenoteles. These results cannot be
explained in the light of E w e r ’s theory which would interpret
the prevention of stenotele discharge as being effected by a sola­
tion of the cnidocil; norm ally reduced tem peratures would bring
about a gelation of the cnidocil.
It is obvious th a t there is a different threshhold of stim ula­
tion for the desm onem e and stenotele. A fter these prelim inary
experim ents w ith « cold » hydra, the authors came to the con­
clusion th a t these reduced tem peratures were either inhibiting
some nervous reaction in the h y d ra or blocking a chem ical reac­
tion in the cnidocil or nem atocyst m em brane.
T oday it is
generally assum ed th a t the nem atocyst is an independent effec­
tor and is not dependent on the nervous system for its eversión.
W h en an A rtem ia or D aphnia strikes the tentacle of a hydra,
nem atocysts are discharged only at the point of contact; there
is never a general discharge th ro u g h o u t the tentacle. C h a p m a n
an d T i l n e y (1959 a), em ploying the electron microscope, have
not been able to dem onstrate the presence of nerves in contact
w ith the nem atocyst capsule. I w a n z o f f (1896) an d P a r k e r and
V a n A l s t y n e (1932) strongly supported the hypothesis th a t the
reactions of the nem atocyst were non-nervous. F u rth e r support
came from P a n t in and P a n t in (1943), E w e r (1947), P i c k e n (1953),
an d R o b s o n (1953) who believe th a t a local stim ulation of the
cnidocil is responsible for the discharge.
However, several early investigators, notably C h u n ( 1 8 8 1 ),
L e d e n f i e l d (1887), and M u r b a c h (1893) believed th a t there was
a nervous innervation of the nem atocyst. M ore recently, G l a ­
s e r an d S p a r r o w (1909) an d J o n e s (1947) have shown th a t n ar­
cotization of the cnidoblast cell prevents the discharge of its
enclosed nem atocyst; thus, the nem atocyte itself becomes im por­
tan t as a regulator of the discharge m echanism .
T hus, we have two m ajor schools, each w ith radically diffe­
rent theories, to explain the m echanism controlling discharge,
and w ithin these schools there is also a vast disagreem ent as to
th e precise m an n er in w hich this m echanism operates. T he
problem becomes even m ore com plex when one considers
H y m a n ’s (1940) statem ent w hich states th a t neither school
explains the fact th a t when coelenterates are satiated w ith food
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
25'
the nem atocysts apparently fail to explode against the usual
food anim als.
It was this statem ent of H y m a n ’s th a t stim ulated the present
experim ents. If a fully fed coelenterate does not discharge
nem atocysts after rich feeding, then it appears th a t the anim al
is able to exert some control upon the process of nem atocyst
discharge. O n the o ther hand, if the nem atocyst is tru ly an
independent effector it should discharge whenever it is properly
stim ulated, and, theoretically, it should be possible to com ple­
tely elim inate a h y d ra ’s nem atocyst reserve by flooding the ten­
tacles w ith food anim als over a prolonged period. Some of the
experim ents conducted yielded results w hich are not com pletely
understandable at the present tim e, b u t the results will be listed
in this paper in their entirety w ith the hope th a t other investi­
gators m ay have m ore plausible explanations for observations
th a t have baffled us.
M
a t e r ia l s
and
M
eth o ds.
T h e anim als used for these experim ents were specimens of
P elm atohydra oligactis (Pallas), originally obtained from W a rd ’s
N a tu ral Science E stablishm ent, Rochester, New York. T hey
were cultured by the thousands in our laboratories after the
m ethods of L o o m i s and L e n h o f f ( 1 9 5 6 ) w ith m odifications by
B u r n e t t ( 1 9 5 9 ).
A pproxim ately 4 0 0 - 5 0 0 anim als were kept in
each of 7 finger bowls, 4 y2 inches in diam eter.
O ne bowl of starved anim als was available at all tim es, so
th a t a ready supply w ould be available for experim ental work.
T h e bowl was replenished daily from stock anim als just before
feeding tim e; thus, the starved anim al culture contained indi­
viduals w hich h ad no t fed for at least tw enty-four hours.
D u rin g th e course of these experim ents it was necessary to
inject th e gastrovascular cavities of several h y d ra w ith various
chem icals an d food extracts. T o accom plish this, a m odified
syringe was em ployed. A standard % cc tubercular syringe was
attach ed to a 2 foot m etal stand. A 24 G, ^ inch needle was
placed on th e syringe, an d a 12 inch piece of neoprene tubing
(size — I.D., . 0 3 0 ” ; O.D. . 0 4 8 ” ) was placed on the needle. T h e
connection was sealed w ith nail polish to insure an air tight
fit. A special needle was constructed to pass th ro u g h the
m o u th of the hydra. A glass capillary tube (size .8 - 1.1 m m
diam eter) was draw n out at bo th ends until fine points were
U.B.GENT
252
SOCIÉTÉ ROYATE ZOOLOGIQUE DE BELGIQUE
m ade. One end was placed in the free end of the plastic tubing
while the other end was forced th ro u g h the center of the hypostom e of the anim al to be injected (See Fig. 1). Following this
procedure, an y th in g w ith a liquid consistency can be injected
into or w ithdraw n from the gastrovascular cavity.
P IO .
1.
T h is p h o to g ra p h reveals th e m a g n itu d e o f a cap illary n eed le d esign ed
for in tr o d u c in g su b sta n c e s in to or rem ovin g su b sta n c e s from th e gastro­
v a scu lar c a v ity o f a hydra.
In those cases where it was necessary to inject h y d ra with
the body fluids of another anim al such as A rtem ia, the follow­
ing procedure was em ployed. T he m aterial to be injected was
placed in the barrel of a 1 cc syringe. A piece of bolting cloth
(m esh size 200/inch) was w rapped around the end of the barrel;
th e plunger was then placed in the norm al opening and pushed
th ro u g h forcing the m aterial in the barrel th ro u g h the bolting
cloth. A rtem ia , when subjected to this procedure, were squeez­
ed into a fine paste w hich could easily be draw n into the injec­
tion needle. In order to facilitate the passage of m aterial
th ro u g h the cloth, the norm ally draw n out end of the syringe
was broken off before it narrow ed to a th in tip.
D u ring the course of these experim ents several starved hydra
were filled w ith a non-digestible m aterial in order to sim ulate
m echanical conditions of a h y d ra which had just devoured
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
253
several A rtem ia. T o accom plish this, glass beads were fed to
starved anim als in the following m anner. C apillary tubes (size
.8 - 1.1 m m diam eter) were draw n to a fine point. W hen cooled,
th e tip of each tube was placed into a Bunsen b u rn e r flam e;
after a second’s contact w ith the flam e, a sm all glass bead was
form ed a t th e end of the th in tube. T his bead, along w ith a
sm all stalk of the tube, was broken off. T h e stalk attached to
the bead served as a useful handle for the attach m en t of m icro­
forceps when th e bead was fed to the hydra.
fig . 2.
P h o to g ra p h o f a h yd ra w h ic h h a s d evou red a s in g le large glass bead.
T h e a n im a l w as fir s t s tim u la te d w ith a s o lu tio n o f red u ced g lu ta th io n e .
T h e glass beads were then dipped in a weak solution of reduc­
ed glutathione. R educed glutathione causes h y d ra to open its
m o u th and begin a feeding reaction. T he anim als, u n d er this
stim ulus, will attem p t to ingest an y th in g th at comes into con­
tact w ith the hypostom e ( L o o m i s , 1 9 5 6 ). W hen the beads were
placed into contact w ith the hypostom e of hydra, the m outh
opened an d th e bead was prom ptly ingested. If the investiga­
tor is patient, h y d ra will devour several heads over the period
of a half-hour an d will stretch to enorm ous proportions (See
Fig. 2).
V arious am ino acids were also injected into the gastrovascular
cavities of approxim ately 1 0 0 anim als in order to determ ine
254
SOCIÉTÉ ROYALE ZOOI.OGIQUE DE BELGIQUE
w hether the digested products of proteins were influential in
in h ib iting nem atocyst discharge. Each am ino acid tested was
injected in hig h concentrations into the guts of five anim als.
tentacles
hypostome
growth
r e g i on
gastric
or
stomach
regi on
budding
region
peduncle
or
basal
disk
FlO. 3.
A sc h e m a tic illu s tr a tio n rev ea lin g th e 7 body region s o f a n orm al hydra.
T h e am ino acids em ployed were d-i threonine, d-i valine, d-i
leucine, d-i iso-leucine, d-i m ethionine, d-i phenylalanine, ihistidine, d-i tryptophan, d-i lysine, i-tyrosine, i-cysteine, gly­
cine, i-alanine, d-i serine, d-i aspartic acid, d-i glutam ic acid,
i-proline, i-argenine. These chem icals m ay all be obtained from
General Biochemicals Inc., L aboratory Park, C hagrin Falls,
Ohio. Fresh solutions of the chem icals were prepared im m e­
diately before use.
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
255
T here will be m any references in the following series of
papers to the 7 body regions of hydra, described in detail by
B u r n e t t ( 1 9 5 9 ). F igure 3 indicates the position of these regions
in th e body of a hydra.
O
b s e r v a t io n s
and
D
is c u s s io n .
Several sim ple experim ents were conducted in an effort to
determ ine why a fully fed h y d ra apparently fails to discharge
nem atocysts w hen prey strikes the tentacles. H ydra, w hen fed
daily, norm ally devour about twenty-five to th irty A rtem ia in
a single feeding. It appeared possible th a t m echanical stretch­
ing of the gastrovascular cavity, or the presence of digested food
m aterials in the cavity m ight be responsible for the inhibition
of nem atocyst discharge in the fully fed anim al. In order to
test this hypothesis, ten h y d ra were flooded individually w ith
10-15 A rtem ia. As soon as a few A rtem ia were killed, they were
rem oved from the tentacles w ith microforceps. It was found
th a t if a h y d ra was not allowed to ingest the prey after it had
killed the victim ; h y d ra was capable of killing as m any as 115
brine shrim p. A fter this time, there is no fu rth e r nem atocyst
discharge; clearly the h y d ra were capable of killing m any more
A rtem ia th an they were capable of ingesting.
In order to fu rth e r dem onstrate w hether the presence of
A rtem ia in the gastrovascular cavity inhibited nem atocyst dis­
charge, th e following experim ent was conducted. T h irty h y d ra
were fed w ith A rtem ia until their gastrovascular cavities were
full an d no additional A rtem ia adhered to the tentacles. T h en
the tentacles and hypostom es were excised from 15 anim als
while th e rem aining 15 anim als served as controls. A fter 15
m inutes, additional A rtem ia were offered to the excised distal
portions and to the control anim als. It was found th a t the
tentacles w hich h ad been separated from the gastric region took
an average of 8 additional A rtem ia while the tentacles which
were still in contact w ith the gastric region via the gastrovascu­
lar cavity took an average of one additional A rtem ia. T h e expe­
rim ent was repeated w ith another group of 30 anim als, and on
this occasion excised portions took an average of 6 anim als, u n ­
excised portions, one anim al.
These results support H y m a n ’s (1940) statem ent th a t a fully
fed anim al is inhibited from firing off excess nem atocysts, for
if the food is rem oved from the gastrovascular cavity of a hydra
256
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
im m ediately after feeding there is a fu rth e r discharge of nem a­
tocysts and several additional A rtem ia are killed. It was decid­
ed to determ ine w hether the inhibition of nem atocyst discharge
after feeding resulted from a sim ple m echanical stretching of
the gastrovascular cavity or was chem ical in origin. P resum a­
bly a break-down product resulting from extracellular digestion,
or possibly a substance, such as an enzym e, secreted by glan­
d u lar cells of the gastroderm is during digestion, could be
transported via the gastrovascular cavity to the tentacle where
the substance would be able to exert its inhibitory action upon
nem atocysts located in this region.
In an attem p t to analyze the effect of pressure alone on the
digestive cavity after feeding, glass beads were fed to 30 hydra
u n til the anim als were swelled to enorm ous proportions (See Fig.
3). Glass was chosen because it is inert chem ically and should
have no effect upon norm al digestive processes such as the sti­
m ulation of enzym e secretion etc. A fter ingestion of the glass
beads the h y d ra killed an average of 45 additional A rtem ia.
Obviously, pressure exerted on the walls of the gastro-vascular
cavity does not prevent nem atocyst discharge, b u t it m ust be
rem em bered th a t a single h y d ra norm ally kills over 100 A rtem ia
if th e prey is not allowed to pass into the gastrovascular cavity.
T h e fact th a t h y d ra filled w ith glass beads were capable of
killing only h alf this num ber takes on significance in the light
of this observation. In addition, an o th er very im p o rtan t observa­
tion was m ade during the course of these experim ents. It was
noticed th a t hydra, w hen filled w ith glass beads, were not capa­
ble of holding the prey to the tentacles once the prey h a d been
captured. If a hydra, w hich has captured m any A rtem ia , is
picked up w ith m icroforceps and shaken slightly, the A rtem ia
fall from the tentacles to the bottom of the bowl. If this
m anuever is repeated w ith a norm al anim al, it is found th a t it is
nearly impossible to dislodge the A rtem ia unless they are forcibly
pulled from the tentacles w ith microforceps. T h e significance
of this observation will be elaborated in another section of this
paper.
A n o th er set of experim ents were undertaken in order to deter­
m ine w hether the presence of partially digested food m aterials
in the gastrovascular cavity is capable of in h ib itin g nem atocyst
discharge. Several h y d ra were allowed to feed until their diges­
tive cavities were filled and no fu rth e r brine shrim p were ingest­
ed. T he tim e required for this process is usually 30-60 m inutes.
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
257
These fully fed anim als were then strained th ro u g h bolting cloth
into a fine paste. T h e paste was taken into the needle of the
injection apparatus, previously described, and injected into the
gastrovascular cavities of io starving anim als. In order to elim i­
nate variables in this experim ent, an d insure th a t the results were
due solely to the introduction of digested food into the gastro­
vascular cavity of the io starving anim als, several starved hydra
were sim ilarly strained into a paste, an d the paste was then
injected into the digestive cavities of 5 anim als w hich h ad not
fed for at least 24 hours. T h e results of these experim ents can he
seen in T able I and II.
T
H yd ra
N u m b er o f A rtem ia
K ille d
1
2
6
3
4
5
2
6
7
8
9
10
I.
able
T im e b e tw e e n In je c tio n
a n d F e ed in g
4
7
30
30
30
30
30
30
30
30
5
3
3
5
2
6
1
0
m in u te s
m in u te s
seco n d s
seco n d s
secon d s
seco n d s
seco n d s
seco n d s
secon d s
seco n d s
T able I . — T h is ta b le in d ic a te s th e n u m b er o f A r te m ia k illed by starved
hyd ra a fte r th e d ig e stiv e c a v itie s o f th e se hyd ra h a d b een in je c te d w ith a
p a ste c o n ta in in g t h e tissu e s o f h yd ra p lu s th e tissu e s o f p a r tia lly d ig ested
A rte m ia .
T
H ydra
1
2
3
4
&
able
II.
N u m b e r o f A rtem ia
K ille d
25
27
30
30
.78
T im e b e tw e e n I n je c tio n
a n d F eedin g
30
30
30
30
30
se co n d s
se co n d s
se co n d s
seco n d s
seco n d s
T able II. — T h is ta b le in d ic a te s th e n u m b er o f A r te m ia k illed by starved
hyd ra a fte r t h e d ig e stiv e c a v itie s o f th e se hyd ra h a d b een in je c te d w ith
a p a ste c o n ta in in g th e tis s u e s o f hyd ra b u t c o n ta in in g n o tis s u e s of
p a rtia lly d ig e ste d A rte m ia .
It appears obvious from the foregoing experim ent th a t the
presence of digested food m ateria’s in the stom ach region of
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELCIQUE
h y d ra has an inhibitive effect on the num ber of A rtem ia that
will be killed by a starving hydra. It m ight also be m entioned
th a t the num ber of A rtem ia killed in Tables I and II is also a
record, at least roughly, of the num ber of A rtem ia th at were
ingested after the killing was complete. It will be fu rth e r notic­
ed th a t the inhibitive effect registered by the introduction of
digestive m aterials in the gastrovascular cavity is an im m ediate
one since less th a n a m inute usually elapsed between injection
tim e and feeding tim e. A n explanation for this im m ediate
action will be offered later in this paper.
Finally, attention
should be paid to the fact th a t although h y d ra which contained
no A rtem ia in th eir digestive tracts devoured a norm al am ount
of A rtem ia , the total nu m b er of A rtem ia killed was far below
th e num ber th a t a norm al anim al is capable of killing. It will
be rem em bered th a t a h y d ra can kill over 100 A rtem ia if the
A rtem ia are not allowed to pass into the digestive tract after
they have been im m obilized.
T o fu rth e r test the effect of partially digested food m aterials
on the feeding reaction, several h u n d red A rtem ia were digested
with a .4 % trypsin solution at 370 C for one hour. T h e A rte ­
mia were crushed before being placed into the enzym atic solu­
tion in order to allow the trypsin to attack the internal tissues
m ore readily. A fter cooling, the digested m aterial was injected
into starved h y d ra which were subsequently offered living A rte ­
mia. T h e results of this experim ent are tabulated in T able III.
T
H ydra
1
6
10
10
10
4
5
6
7
8
9
10
III.
N u m b e r o f A rtem ia
T a k en
2
3
able
8
6
6
7
7
7
T im e b e tw e e n In je c tio n
a n d F eedin g
30
30
30
30
30
30
30
30
30
30
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
se co n d s
T able I II . — T h e ab o v e t a b le in d ic a te s t h e n u m b e r o f A r te m ia k ille d by
s ta r v e d h y d r a a f t e r t h e d ig e s tiv e c a v itie s o f th e s e h y d r a h a v e b e e n in je c te d
w i t h a p a s te c o n t a i n i n g tis s u e s o f A r te m ia w h ic h h a d b e e n d ig e s te d by
tr y p s in a t 37° C f o r o n e h o u r .
ANNALES 9 0
(1 9 5 9 -6 0 )
259
Clearly, there is a reduction in the num ber of A rtem ia taken
by starving anim als containing trypsin digested A rtem ia in
their gastrovascular cavities as com pared to the am ount taken
by n o rm al starving hydra. However, it will be seen th a t the
nu m b er of A rtem ia taken per h y d ra is slightly above th a t taken
by anim als w hich were injected w ith an « A rtem ia paste »
rem oved from the gastrovascular cavities of living anim als.
It appeared therefore, th a t some product of digestion, after
passing via th e gastrovascular cavity to the tentacles, was able
to in h ib it nem atocyst discharge to a great degree. It was con­
ceivable th a t certain am ino-acids resulting from protein diges­
tion were influential in the process, b u t none of the nineteen
am ino acids m entioned in the section, M aterials an d M ethods,
were effective after they had been introduced into the digestive
cavities of starving anim als. T his is not surprising w hen one
considers th a t there is very little food breakdow n in the gastro­
vascular cavity of hydra. B u r n e t t ( 1 9 5 9 ) has shown th a t hydra
ingests large protein droplets directly into its tissues via large
digestive cells, and m ost of the protein digestion th a t occurs
in the an im al is intracellular. In order to test w hether the
inhibitive action was due to fat droplets in the cavity, peanut
oil was injected into the cavities of ten hydra. These anim als
all killed great num bers of A rtem ia after the injection was com­
plete.
A single observation recorded early in the course of the pre­
sent investigation prom pted the authors to repeat earlier feeding
experim ents. It will be rem em bered th a t a hydra, when filled
w ith glass beads, is able to capture an d kill several A rtem ia, bu t
is unable to hold the prey to the tentacles. It appeared possible
th a t hydra, after devouring several A rtem ia, m ig h t exhibit a
sim ilar behavior. Several h y d ra w hich h ad no t fed for twentyfour hours were flooded w ith A rtem ia. These anim als were
observed steadily for a period of two hours under a binocular
dissecting m icroscope giving a m agnification of 20 x . It was
noticed th a t once the gastrovascular cavity of an anim al was
full of food th e m o u th opening closed. A rtem ia w hich were
still clinging to the tentacles of the h y d ra at this tim e began
to fall to th e b ottom of the dish. In several cases the hydra
rubbed th eir tentacles over one another or over the surface of
the dish in w hat appeared to be an obvious attem p t to dislodge
the A rtem ia . A fter the prey h ad been dislodged, fu rth e r A rte ­
m ia w hich came into contact w ith the tentacles appeared to
2ÖO
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
swim away unharm ed. A superficial observation of this beha­
vior would induce an investigator to assume th a t h y d ra was no
longer discharging its nem atocysts. However, it was noticed
th a t several A rtem ia w hich struck the tentacles and swam away,
apparently unharm ed, were capable of only sw im m ing a few
m illim eters. A fter this tim e they would sink to the bottom of
the dish either com pletely paralyzed or tw itching slightly.
W hen these A rtem ia were exam ined under an oil im m ersion
objective, their bodies were found to be pierced w ith one, some­
tim es two, stenoteles. O ther individuals were seen to come into
contact w ith the tentacles, struggle a few seconds, then swim
away com pletely unharm ed. Finally, num erous A rtem ia con­
tacted the tentacles and were im m ediately killed, bu t these ani­
m als did not adhere to the tentacles. A fter a m inute or so
the A rtem ia would he dislodged, often w ithout active move­
m ents on the part of the hydra.
T hus, it was realized th a t a h y d ra is perfectly able to dis­
charge its nem atocysts after it is fully fed. Again, it m ust be
stressed th a t this process is very easily overlooked since the
hy d ra does not retain the A rtem ia for a long period of time.
F urtherm ore, a fully fed h y d ra exhibits a behavior pattern
quite unlike th a t of a norm al anim al. N orm ally, w hen a starv­
ed h y d ra receives a stim ulus indicating th a t prey has contacted
the tentacles, the tentacle containing the prey quickly bends
into a loop in such a m anner th a t the struggling prey is quickly
b rought into contact w ith other nem atocyst batteries. Several
additional stenoteles are fired off and in a m atter of seconds
the prey is com pletely im m obilized. A fully fed anim al, how­
ever, rem ains perfectly still after the prey has contacted the ten­
tacles. If an A rtem ia contacts a tentacle in an area where there
are no stenoteles, i.e., an area w hich has h ad its stenotele supply
depleted by previous contact w ith A rtem ia in this region, the
A rtem ia, after struggling for a few seconds w ith the grappling
desmonem es, is capable of sw im m ing away unharm ed. O ther
A rtem ia striking the tentacles m ay cause the discharge of one
or two stenoteles at the point of contact, since, however, the ani­
m al is not pulled into contact w ith m ore nem atocyst batteries,
it often has the power to break from the tentacles, and m ay be
quite distant from the h y d ra when paralysis ensues.
O ther
A rtem ia receive a sufficient dose of poison from the stenoteles
to be killed on the point of contact with the tentacles. Close
exam ination w ith a water im m ersion objective reveals th a t cnido-
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
261
blast cells containing the discharged nem atocysts are slowly
squeezed out from the epiderm al layer and released entirely from
the tissues of the hydra. T h e A rtem ia , plus its attached steno­
teles th en sinks to the bottom of the dish. T h e m echanism
w hich controls this release of cnidoblast cells is not know n at
the present.
It has been shown th a t m ost h y d ra are capable of killing or
partially im m obilizing as m any as 45-50 A rtem ia after full feed­
ing. T h e tim e required for this process m ay be as long as one
an d a h alf hours. It has been concluded th a t h y d ra preserves its
nem atocyst supply after full feeding sim ply by not responding
physically when prey comes into contact w ith its tentacles.
»
%
F ig . 4.
P h o to g ra p h sh o w in g tw o hydra a t d iffe r e n t in te r v a ls d u rin g t h e feed in g
p rocess. N o te t h a t w h en th e n a n im a l is fu ll o f fo o d th e r e is a great
c e n tr ifu g a l s t r e t c h in g in t h e area j u s t u n d er t h e h yp ostom e. T h e a n im a l
w h ich is h a lf fu ll o f food does n o t sh ow t h is stretch in g .
F u rth er investigation into this m atter has revealed th a t once
a h y d ra’s m o u th closes after full feeding, it cannot be induced
to open again. F ully fed h y d ra were placed in dilute solutions
of reduced g lutathione along with starved control individuals.
2Ó2
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
L o o m i s ( 1 9 5 6 ) has dem onstrated th a t this chem ical is released
from the body cavities of h y d ra ’s prey through the hole pro­
duced by the stenotele after it penetrates the tissues of th e prey.
W h en a h y d ra receives a reduced glutathione stim ulus, it res­
ponds by opening its m o u th and attem pting to devour anything
th a t comes into contact w ith the hypostom e. T h e tentacles
squirm actively d u rin g this period and repeatedly are brought
towards the m o u th opening exhibiting hydra's typical « feeding
reaction ». F ully fed anim als, however, never open their m ouths
after a reduced glutathione stim ulus, whereas starved control
anim als open their m ouths im m ediately. M oreover, there is no
tentacle response from fully fed anim als after a glutathione sti­
m ulus.
T e n s i o n on
longitudinal
m u s c l e s of
hypostome
P ig . 5.
T h is fig u r e is an a tte m p t to illu s tr a te th e force exerted u p o n th e w alls
o f t h e g a strovascu lar c a v ity o f a n a n im a l w h ic h h a s re c e n tly fed . T h e
p ressu re in t h e low er reg io n s w o u ld exert a te n sio n u p o n t h e lo n g itu d in a l
m u s c le s su r r o u n d in g th e m o u th th u s p rev en tin g th e m from co n tra ctin g .
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
263
T h e m echanism w hich inhibits a fully fed anim al from open­
ing its m outh appears to be a simple one. Once the gastrovas­
cular cavity of a h y d ra has been swelled by the intake of food
m aterials, there is a great pressure exerted against the walls of
the cavity. T h is pressure m akes it im possible for the m outh to
open because the longitudinal muscles surrounding the m outh
are not able to overcome the resistance of the pressure acting
centrifugally on the walls ju st proxim al to the m outh opening.
L ongitudinal muscles located in the hypostom e open the m outh
by contracting. A n y th in g which would tend to stretch the
muscle fibers would prevent m outh opening. Figures 4 and j
represent an attem pt to illustrate this idea.
Dr. B k ie n at Brussels U niversity has suggested another theory
w hich explains the inability of the m outh to open. It is quite
possible th a t the m outh of hydra is not opened by a contraction
of longitudinal muscles surrounding the m outh, bu t ra th e r by
a relaxation of circtdar muscles in the same region. Dr. B r ie n
has suggested th a t d u rin g the swelling of the gastrovascular
cavity after rich feeding, impulses m ay be set up in the ecto­
derm al « nerve net » of h y d ra w hich stim ulate the circular
muscles su rro u n ding the m outh to contract. These forces would
act antagonistically to any force w hich norm ally would cause to
m o u th to open. Since the relationship existing between the
nervous and m uscular systems of h y d ra have never been clearly
defined, it is difficult at the present to determ ine w hich of the
two m uscles layers is m ore influential in the opening an d clos­
ing of th e m o u th of hydra.
In order to fu rth e r confirm these hypotheses, several fully fed
h y d ra were placed in a glutathione solution. N one of these ani­
m als was able to open its m outh. T h e regions distal to the
grow th region were then severed in 5 anim als. These anim als
im m ediately opened their m ouths while th e rem aining intact
anim als showed no response. F urtherm ore, the anim als w hich
opened th eir m ouths began to capture additional A rtem ia im m e­
diately; th e tentacles responded in a m an n er identical w ith th a t
of n o rm al starving anim als.
T h e pressure exerted against the walls of the gastrovascular
cavity after feeding is not entirely due to the fact th at food
m aterials are pressing against the body wall of the anim al.
Once protein m aterials, neutral fats, etc. are liberated from the
tissues of the partially digested prey, there is a great intake of
water into the gastrovascular cavity w hich causes the h y d ra to
264
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
« bloat ». It appears th a t d u rin g digestion the tissues of the
h y d ra act as a kin d of sem i-perm eable m em brane w hich allows
water to pass into the cavity faster th a n it can escape. T he
swelling or bloating resulting from the water intake causes the
m o u th to close. Once the m outh closes, tentacle m ovem ent
stops, an d any A rtem ia th a t are ad hering to the tentacles are
quickly dislodged. T h e inactivity of the tentacles w hen prey
contacts them is not easily explainable. Perhaps, an increased
pressure in their cavities, sim ilar to the pressure produced in
the cavity of the gastric region, prevents the longitudinal m us­
cles of the tentacle from contracting freely.
In the light of recent findings it is now possible to interpret
the observations recorded after feeding h y d ra glass beads and
the paste consisting of the partially digested tissues of A rtem ia.
Injection of digested products of ingested A rtem ia into the gas­
trovascular cavity of a h y d ra caused the h y d ra to swell through
a rapid intake of water into the digestive cavity. T his swelling
essentially prevented fu rth e r m outh opening and fu rth e r m ani­
pulation of food m aterials by the tentacles. H ence, fewer A rte ­
m ia th a n norm al adhered to and were killed by the tentacles.
T he fact th a t a few of these A rtem ia were actually ingested can
readily be explained by the fact th a t it was impossible by the
injection of a food extract to exactly sim ulate the conditions
of a fully fed hydra. Swelling after injection procedures was
probably not as great as one would find in a norm al, previously
starved anim al. It is quite likely th a t m any more A rtem ia were
killed in these experim ents th an is recorded in T able I, how­
ever, at this tim e the investigators were not aware th a t m any
A rtem ia were capable of sw im m ing away from a hydra after
th ey h ad been pierced by a stenotele, and th at several A rtem ia
were soon dislodged from the tentacles after they had been
killed.
H ydra w hich h ad been fed w ith glass beads had a norm al
nem atocyst supply and were capable of killing m any additional
A rtem ia. T h e swelling of the gastrovascular cavity undoubtedly
produced a sm all effect in curtailing tentacle activity b u t it
m u st be rem em bered th a t the swelling produced by feeding an
anim al w ith glass beads is no t a uniform one. T he anim als
were grossly distorted, and there was not an equal pressure
exerted on all of the walls of the gastrovascular cavity. M any
of these anim als were capable of opening their m ouths and
ingesting several A rtem ia. As long as A rtem ia were passing
ANNALES 9 0
(1 9 5 9 -6 0 )
265
into the m outh, the tentacles were actively engaged in the feed
ing reaction. W hen the m outh opening was finally closed, i.e.,
after several A rtem ia h ad been ingested, the few dozen A rtem ia
still adh erin g to the tentacles were quickly dislodged.
T hus, it has been established th a t hydra is not able to prevent
nem atocyst discharge com pletely after rich feeding. T he anim al
is m erely capable of reducing the num ber of nem atocysts dis­
charged. A gain, it appears th a t the nem atocyst of h y d ra is
truly an independent effector.
T h e m echanism w hich causes a nem atocyst to discharge is at
the present unknow n. T he fact th a t this m echanism is in no
way connected w ith the nervous system can readily be dem on­
strated by placing h y d ra in a solution containing an anaesthetic.
If h y d ra are placed in solutions of 1 % chloretone or io %
alcohol they are quickly paralyzed. P inching the body or ten­
tacles w ith m icroforceps causes no response whatsoever. H ow ­
ever, if these h y d ra are flooded w ith A rtem ia they are capable
of killing great num bers of these anim als.
T h e au thors hold the belief th a t the nem atocyst of h y d ra is
controlled by the cnidocil. E xam ination of the tentacle of a
living h y d ra u n d er oil im m ersion reveals th at the cnidocils of
desm onem es are m uch longer and th in n er th an are the cnido­
cils of stenoteles. Prey contacting the tentacles of a h y d ra will
stim ulate th e cnidocils of desm onem es before those of stenoteles.
If the cnidocil is indeed the « trigger » of the nem atocyst then
it would be expected th a t desm onem es will he discharged before
stenoteles when the prey has contacted the tentacles. T h is proves
to be the case. A rtem ia striking the tentacles are always captur­
ed before they are killed. Desm onem es hold the A rtem ia to the
tentacles. T h e A rtem ia invariably struggles an d d u rin g the
struggling process the stenotele is discharged. T h e efforts of the
A rtem ia to dislodge itself from the tentacles presum ably brings
the body of th e A rtem ia into contact w ith th e short, b lu n t cnido­
cils of th e stenoteles. ¡
It was noted in the Introduction th a t h y d ra w hich have been
subjected to tem peratures of 4 0 C for one and a half hours are
capable of discharging desm onem es when prey contacts the ten­
tacles, b u t are not capable of discharging stenoteles. T h e authors
are not able to explain these observations at the present time,
however they feei th a t cold tem peratures are not exerting their
in h ib itio n th ro u g h the nervous system, b u t by interfering w ith
a chem ical m echanism in the cnidocil proper. M ore evidence
will be necessary to elucidate these findings.
206
SOCIÉTÉ ROYALE ZOOLOGIQUE DE BELGIQUE
Before concluding, the authors would like to caution any
investigator who repeats any of the foregoing experim ents on
hydra. T he strain of P elm atohydra oligactis th a t we have em ­
ployed in the U nited States contained approxim ately 100 steno­
teles per tentacle. T hus, a hydra which has killed 100 A rtem ia
(probably em ploying about three stenoteles per killing) has
greatly depleted its supply of nem atocysts. A ny fu rth e r killing
will occur only if an A rtem ia strikes the tentacle in an area
where a stenotele is located; in o ther words, it m ust be a « direct
h it ». T he strain of h y d ra th a t the senior au th o r is presently
studying in Belgium contains approxim ately 500 stenoteles per
tentacle. Prelim inary experim ents have shown th a t these ani­
mals can easily kill as m any as 250-300 A rtem ia during a period
of one an d a half hours. T herefore, the num bers of A rtem ia
killed by a h y d ra in the present experim ents are relative, and
the nu m b er m ay greatly differ between different strains of ani­
mals.
Sum m ary
and
C o n c l u s io n .
It has heen shown, contrary to popular belief, th at a fully fed
hy d ra is capable of discharging nem atocysts and subduing prey.
However, once the h y d ra are fully fed there is no attem p t on
the part of the h y d ra to ingest prey which has heen killed after
the gastrovascular cavity of the hydra is filled. A t this tim e the
m o u th is incapable of opening due to the internal pressure
exerted on the walls of the gastrovascular cavity. T his pressure
results from a large intake of w ater into the gastrovascular
cavity d u rin g the early phases of digestion. It has been dem on­
strated th a t A rtem ia w hich are killed by the h y d ra after full
feeding are quickly dislodged from the tentacles, and m any
A rtem ia are able to recover from the initial contact w ith the
h y d ra because the tentacles are inactive at this time. T h e inac­
tivity of the tentacles after full feeding by the h y d ra is in ter­
preted as a m eans w hereby a fully fed anim al preserves its
nem atocyst supply.
A n injection apparatus designed for rem oving and introduc­
ing substances from or into the gastrovascular cavity is described.
Also, a m ethod is described whereby it is possible to feed hydra
substances which are inert chem ically, but which serve to m im ic
the physical state of a fully fed anim al.
ANNALES 9 0 ( 1 9 5 9 - 6 0 )
267
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