ON THE FEEDING MECHANISM OF THE COPEPODS,
CALANUS FINMARCHICUS AND DIAPTOMUS
GRACILIS
BY H. GRAHAM CANNON,
Professor of Zoology, Sheffield University.
(Received June 28th, 1928.)
(With Eight Text-figures.)
INTRODUCTION.
IN 1925, Storch and Pnsterer described at great length the feeding mechanism of
the freshwater copepod, Diaptotmis gracilis. They maintained that food was filtered
from a current produced by the activities of the head swimming limbs. Their
analysis of the mechanics of this process appeared to me inaccurate so that I
decided to re-investigate the problem.
I examined first Calanusfinmarchicns,a form so similar to Diaptomus that I
assumed its feeding mechanism would be essentially the same. My observations,
however, on the actual currents produced were totally different from those described by Storch and Pnsterer and I decided to obtain Diaptomus gracilis itself.
This I obtained through the kindness of Mr R. Gurney and I found that its feeding
mechanism and the currents it produced agreed closely with those of Calanus.
My observations agree with those of Storch and Pfisterer in that I describe
food particles as being retained by the maxilla from a current which is caused to
pass through it. I agree further that this feeding current results from the swimming
activities of the anterior limbs. It is the swimming current that I consider these
workers have described inaccurately and this is of vital importance to their argument, as their analysis of the mechanics of the process depends primarily on its
correct interpretation.
Part of the observations on Calanus were made while occupying the table of
the Royal Microscopical Society at the Marine Biological Laboratory at Plymouth.
METHODS.
The currents produced by the copepods were observed under the microscope
by placing coloured starch grains in the water in which they were swimming.
The movements of the limbs are so rapid that it is impossible to analyse them
by ordinary methods. Storch and Pfisterer (1925, p. 347) estimate a frequency of
300 beats a minute for the movements of the head swimming limbs of Diaptomus.
In the specimens I observed, a rough estimate was 1000 a minute. I succeeded,
9-2
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H. G R A H A M C A N N O N
however, in observing them with complete accuracy by using a stroboscopic source
of illumination for the microscope.
The source of light, an ordinary "opalite" bulb, was placed behind a rotating
disc which was pierced at equal distances by narrow radial slits whose width could
be adjusted. The disc was 9 inches in diameter and was pierced at the edge by four
slits about 1J inches long. The best results were obtained when the slits were about
J inch wide. The disc was rotated by a motor whose speed could be controlled
by a variable resistance.
The copepod was placed in a compressorium in a small drop of water, as deep
as possible, to allow complete and unhindered movements of the limbs. It was
then focussed under the microscope and the disc caused to rotate at gradually
increasing speed. At first only irregular images of the animal were obtained, but,
as the frequency of the flashes of light gradually approached that of the limb movements, the limbs appeared to move regularly and, for a copepod, very slowly.
By increasing the speed still further I found it possible to obtain an image of the
limbs apparently moving as slowly as desired. At the critical point, when the
frequency of the light coincided with that of the limbs, the latter appeared to stand
still, but never quite still. I assume that this means that the limb movement is
never absolutely regular. By increasing the speed of rotation beyond the critical
point the limbs appeared to move slowly but in the reverse direction. In making
observations care has to be taken that the real direction of movement is being
observed and not the reverse. This was extremely important in studying the movement of the maxilliped whose tip describes a rotary movement. It is, however,
quite easy to settle whether the correct movement is being studied, because, in
this case, a slight diminution in the frequency of the light makes the limbs move
apparently faster, while if it is the reversed movement, the limbs appear to move
more slowly or else reverse their apparent motion.
By placing the source of light so that only half is covered by the rotating disc,
it is possible to obtain the field of the microscope illuminated, on the one side, by
continuous light, and on the other, by intermittent light. In this way I have viewed
a copepod with the limbs of one side moving at their normal speed while those
on the opposite side appeared to move extremely slowly. This is of great use in
studying such a form as a copepod whose movements are apt to be spasmodic.
By shifting the slide from the stroboscopic to the continuous half of the field it
can be settled at once whether or not the copepod is moving normally.
ANATOMY.
The main points in the anatomy of Diaptomus and Calanus relevant to a description of their feeding mechanism may be summarised briefly.
Motion through the water is of three types: (1) a sudden rapid jerk forwards,
produced by the activity of the trunk swimming limbs, (2) a series of much smaller
jerks produced by spasmodic irregular movements, and (3) a steady and comparatively slow forward movement resulting from continuous and rapid vibrations of
the antennae, mandibular palps, maxillules and maxillipeds. The first type serves
Feeding Mechanism of Copepods
133
as a means of escape, the second is probably a means to counteract a tendency to
sink while the third type produces the feeding current.
The uniramous antennules project laterally and act as balancers.
The biramous antennae project ventro-iaterally just in front and to the sides
of the large upper lip or labrum (see Text-figs. 1 and 5). The exopodites curve
laterally and then dorsally close against the body. The endopodite projects ventrally
AY:- w
•X
Text-fig. 1. Sketch of lateral view of Calanus finmarchicus in resting position. The antennules
have been cut off close to the body. ant. i =antennule; ant. z endop. =endopodite of antenna; ant. 2 exop. =exopodite of antenna; Ibr. =labrum; mdb. =mandible; mx. i =maxillule;
mx. 1 ex. =exite of maxillule; mx. 2 =maxilla; mxpd. =maxilliped; s.t.l. =swimrning trunk limb.
at an angle of about 300 to the sagittal plane. The endopodites and basal part of
the exopodites are armed with long setae spread out in a ventro-lateral fan. The
exopodite terminates in a group of three long dorsally directed setae.
The mandibules are wedged in between the distinctly bifid lower lip and the
massive upper lip, being slightly overhung by the latter. The biramous mandibular
palps project ventro-iaterally and carry a fan of setae spread out over the same angle
as that of the endopodite of the antenna. These setae are shorter than those of the
134
H. GRAHAM CANNON
antenna. The maxillules arise nearer the sagittal plane than the more anterior
limbs. Their main axes project antero-ventrally overhanging slightly the mandibules. Both endopodites and exopodites terminate in long setae which, however,
are shorter than those of the mandibular palps. Those of the endopodite project
directly ventrally and slightly medially, those of the exopodite project in the same
direction as those of the mandibular palp. The maxillule is armed medially with
three endites, a powerful toothed basal endite projecting obliquely forwards to
the split in the lower lip and two distal endites which project forwards, their
terminal setae lying across the mouth. On the outer side of the maxillule there is
an exite armed with very long slender setae. The most ventral of these project
laterally and then curve round posteriorly and extend as far back as the first pair
of trunk swimming feet. The more dorsal, that is those next the body wall, project
almost directly posteriorly. A section of the fan of setae thus formed would extend
over an arc of a quarter of a circle.
setae of mc.*-.lluiavy
o
/
chamber.
^
I
ot
trunk swimming limbs'
V
Text-fig. 2. Diagram of transverse section through filter and suction chambers of copepod.
•The maxillae are short uniramous limbs projecting ventro-anteriorly. They
comprise eight joints bearing long plumose setae which extend forwards to the
mouth. They thus form the walls of a median wedge-shaped space with the mouth
at its apex (see Text-figs. 2 and 5).
The maxillipeds are cylindrical uniramous limbs arising close behind the
maxillae. In the living form it is difficult, with continuous light, to see them apart
from the maxillae. Each consists of a very short basal joint, followed by two comparatively long joints of doubtful homology and (finally) a short flexible setose
portion of five joints. The first long joint lies close against the maxilla and reaches
as far as its tip. The remainder of the limb then stretches ventro-anteriorly and
laterally, the setae spreading out in a fan close underneath the tips of the maxillules.
The trunk swimming limbs extend obliquely forwards, the apex of the anterior
pair reaching as far as the mouth. They converge to a point slightly nearer the
body than the tips of the maxillae.
Feeding Mechanism of Copepods
135
The figures in Storch and Pfisterer's paper are incorrect in several important
details. The setae of the maxillulary exite are figured in a parasagittal plane. The
maxillipeds are too short and project inwards instead of outwards and the swimming
trunk limbs reach only as far as the maxillae instead of the mouth.
This arrangement of limbs results in two spaces of importance in the feeding
mechanism (see Text-figs. 2 and 5). The first of these is the filter chamber between
the maxillae, the walls of which are the maxillae, the roof is the body wall, and the
floor the tips of the anterior trunk swimming limbs. The floor is complete laterally
except for a small split occurring between the trunk limbs and the ventral setae
of the maxillae. Anteriorly this space is closed by the large upper lip. Its only
entrance is posteriorly between the maxillipeds and the first pair of swimming trunk
limbs. The second of these spaces, which I call the suction chamber, is bounded
medially by the maxillae and laterally by the setae of the maxillulary exite.
SUMMARY AND CRITICISM OF THE VIEWS OF STORCH AND
PFISTERER ON THE FEEDING MECHANISM OF DIAPTOMUS.
The essential points in the feeding mechanism of Diaptotnus according to
Storch and Pfisterer may be briefly summarised as follows:
The activity of the swimming limbs produces a powerful antero-posterior current which runs close against the ventral side of the body. This does not agree
with "eine langsame, gleichmassige, gleitende Vorwartsbewegung" (p. 338). A
powerful current passing directly posteriorly means that a considerable amount of
water passes backwards and hence the body causing this must pass forwards with
considerable speed.
They point out, and lay considerable stress on the fact, that the head swimming
limbs diminish in length posteriorly. Each limb produces the maximum movement of water at its tip. The mandibular palp being shorter than the antenna
produces its maximum effect nearer the body wall. "So setzt also die Tatigkeit
der drei Gliedmassenpaare eine verhaltnismassig hohe Wasserschicht in Bewegung,
und zwar in der Weise, dass je das folgende Gliedmassenpaar die bewegte Wasserschicht naher an den Korper herantragt." (p. 351.) That is, each limb draws the
current produced by the limb in front, nearer the body wall. They do not attempt
to explain the mechanics of this process, which I believe to be erroneous, but which
is, however, essential to their analysis of the feeding mechanism.
Assuming, as do Storch and Pfisterer that the limbs move synchronously, the
speed of the tip of a limb will depend on the length of that limb and the arc through
which the limb swings in one vibration. Storch and Pfisterer do not consider the
latter factor, and, presumably, assume that each limb moved through the same
arc. In this case the speeds of the tips of the limbs are directly proportional to the
lengths of the limbs. The tip of the antenna moves faster than that of the mandibular
palp, and the latter faster than the maxillule. Hence the movement of the tip of
the mandibular palp can have no effect on the layer of water set in motion by the
tip of the antenna because it will be moving more slowly than the latter. If the
mandibular palp moved over a much greater arc in one vibration than did the
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H. G R A H A M C A N N O N
antenna it would move faster, in which case it might conceivably draw some of the
water set in motion by the antenna nearer to the body. Actually, however, this is
not so. The mandibular palp and maxillule both appear to move over the same
arc while the antenna moves through a much larger angle. Hence the layer of
water set in motion by the tip of the antenna moves much faster than that forced
backwards by either mandibular palp or maxillule.
According to Storch and Pfisterer then, the swimming current is drawn close
to the body by the swimming limbs. This is further enhanced by the maxillulary
exite. They say (p. 351) that because of its dorso-ventral axis of rotation, and
because it lies close against the body wall it is especially adapted to increase still
further the moving layer of water and draw it nearer to the body wall. The same
criticism applies here as above. Being very close to the body wall its speed of
movement through the water is very slow compared with the tip of the antenna
and it can have no effect whatever in drawing the current closer to the body wall.
The current having been drawn close to the surface of the body passes backwards on either side, through the space between the forwardly projecting maxilla
and the backward exite of the maxillule. The shape and arrangement of the latter,
according to Storch and Pfisterer (p. 352), indicate that its function must be to
confine the moving layer of water, thus preventing it spreading out too quickly
and so losing force. From this it might be concluded that the maxillulary exite
was a comparatively rigid plate. Actually the setae are very fine structures, rigid
only at their bases and whip-like at their extremities. If the exite did move backwards and forwards as these workers maintain, the pressure on the setae would force
them outwards. They certainly could not act as a barrier confining a stream of
water which, as Storch and Pfisterer emphasise, is of considerable strength.
This powerful backward current will suck water into it from still regions. Such
a region occurs only, according to Storch and Pfisterer (p. 353), between the basal
endite of the maxillule on the outside and the tips of the maxillary setae on the
inside. Why this position is chosen to the exclusion of others is not stated. Water
is therefore sucked from this region into the backwardly flowing stream and to
replace it water passes through the maxillary setae from the anterior part of the
filter space. This results in a region of low pressure close behind the mouth which,
in its turn, sucks water forwards from behind and, to supply this, water from
the ventral part of the swimming stream is sucked into the filter space behind the
maxillipeds (Storch and Pfisterer, Fig. 11).
Particles carried on the stream of water drawn in in this way are retained in
the filter space, sucked forwards and eventually filtered off b} the maxillary setae.
They are then combed off by the proximal endite of the maxillule and pushed
forwards on to the mandibles.
With regard to the maxillipeds Storch and Pfisterer state " Die Maxillipeden
bewegen sich nur in einer sehr geringen Amplitude von hinten aussen nach vorne
innen und umgekehrt" (p. 347), but can offer no explanation of their function
(P-355)The description and mechanical analysis of the feeding mechanisms as given
Feeding Mechanism of Copepods
137
by Storch and Pfisterer thus depend on two critical points, the production of a
powerful antero-posterior swimming current by the head swimming limbs, and
the suction of that current closer to the body by the more posterior of those limbs.
The latter I have shown is mechanically impossible. The former rests on an
inaccurate observation as no such current exists.
FEEDING AND SWIMMING CURRENTS.
In a ventral or dorsal view of either Calanus or Diaptomus swimming slowly
through the water, there can be seen, very readily, two large swirls1, one on either
side of the body in the angle between the antennule and the axis of the body (see
Text-fig. 3). The centres of the swirls are indefinite but occur usually in Calanus
Text-fig. 3. Diagram of ventral view of Calanus finmarchicus slowly swimming to show
water currents.
about the middle of the total length of the body, and in Diaptomus further forwards.
They rotate in such a way that the water nearest the body moves backwards. They
are continuous underneath the body so that, in side view, a marked swirl is obvious
ventrally (see Text-fig. 4). Dorsally also there is a swirl, but this is much less
marked than ventrally. The copepod thus moves steadily forwards in the middle of
a vortex of moving water, which may be termed the " swimming vortex.'* A certain
amount of water is drawn into the vortex from in front, and a certain amount passes
out posteriorly, and this represents the motive force of the copepod when swimming
1
The term "swirl" is used to indicate a rotary movement of water. I have not used it as
synonymous with " vortex." If a vortex is viewed laterally, it will appear in the microscope, provided that the plane of the image approximately bisects the vortex, as two separate swirls rotating in
opposite directions about the annular axis of the vortex.
138
H. G R A H A M C A N N O N
in this fashion. However, the drift towards the body anteriorly is not very marked
when compared writh such a form as Hemimysis where water streams towards the head
from all directions, and is thrown out posteriorly in a powerful swimming stream.
A second smaller vortex occurs inside the swimming vortex, rotating in the
opposite direction. In a ventral view it can be seen as two swirls at the sides
of the anterior ends of the suction chambers. In side view it cannot be seen
in the median plane as it is interrupted by the forwardly projecting swimming
trunk limbs, but a little to one side it can be seen to extend forwards between
the maxillae to the bases of the anterior swimming limbs and then pass ventrally
Text-fig. 4. Diagram of lateral view of Calanusfinmarchiciisslowly swimming to show water currents.
and backwards. This vortex is the "feeding vortex." It cannot be traced over the
dorsal side of the body.
The inner part of the swimming vortex probably represents the "powerful
antero-posterior current" described by Storch and Pfisterer for Diaptomus. It
is certainly powerful but it is a vortex and this accounts for the fact that the copepod
moves forwards comparatively slowly. According to Storch and Pfisterer, a considerable amount of water is transported in an antero-posterior direction, and in
order to take its place the body would have to move forward with corresponding
speed. If, however, the water which moves backwards at the level of the tips of
the swimming limbs, the level at which the velocity must be greatest, moves at
the same high speed which Storch and Pfisterer describe, but is part of a vortical
Feeding Mechanism of Copepods
139
movement, there is comparatively very little actual transport in an antero-posterior
direction and hence the body moves slowly forward. The energy of the limbs is
expended, not on transporting water backwards, but in maintaining a vortical
movement in the water and very little energy is required for this. This may account
for the extreme rapidity of the limb movement. They have very little resistance to
overcome, simply the viscous drag on the vortex, and hence are able to vibrate
at a speed which is very considerable for a Crustacean limb.
The feeding swirl represents the suction through the maxilla described for
Diaptomus.
The swimming vortex results from the vibrations of the antennae, mandibular
palps, and distal part of the maxillules. The feeding vortex is a necessary resultant
of the swimming vortex, but is increased by the activities of the maxillulary
exite and maxillipeds. This was demonstrated clearly in a specimen of Calanus
which was nearly dead. All its limbs had ceased moving except the antennae and
mandibular palps, and the antennules had not flexed backwards as happens when
a copepod dies. The swimming vortex was almost as large as normal and there
was a very pronounced feeding vortex. This does not agree with Storch and
Pfisterer's account of Diaptomus which regards the maxillule as of vital importance
in producing the feeding current.
LIMB MOVEMENTS AND CURRENT PRODUCTION.
When Calanus and Diaptomus are swimming slowly forwards the frequency of
the head swimming limb movements is remarkably but not absolutely constant.
In the other types of movement the limbs move irregularly, and hence their vibrations cannot be analysed stroboscopically. It is the steady forward motion which
results in the feeding current and that is the type analysed here.
The maxilla shows no rhythmical movement. For the greater part of the time
it remains still, but if the mouth becomes congested with food particles, the maxillae
may be flexed ventrally throwing the accumulated food away from the body into
the swimming swirl.
The maxillipeds, maxillules, mandibular palps and antennae vibrate regularly,
in the case of Diaptomus at the rate of about 1000 times a minute and in the case
of Calanus about 600 times a minute. Their movements are synchronous but not
in the same phase, as apparently Storch and Pfisterer assumed. They exhibit a
marked metachromial rhythm of the type shown in other Crustacea such as Chirocephalus or Nebalia (Cannon 1927, 1928), that is, each limb commences its back
stroke just before the limb immediately anterior to it. The phase differences are
such that the maxilliped commences its back stroke (outward movement) just after
the antenna commences its forward stroke, so that these two limbs move almost
in opposite phase. The phase difference between mandibular palp and maxillule
is very small. The back strokes of antennae, mandibular palps and maxillules are
faster than their fore strokes. The tip of the maxilliped exhibits a rotary movement in
an obliquely frontal plane (see Text-fig. 5) such that the outward part of the rotation
H. G R A H A M C A N N O N
140
(backward stroke) is faster than the inward part. The movements of these limbs
are represented graphically in Text-fig. 6.
Cutrenis.
Lin-.l; Mouemen+s.
Text-fig. 5. Diagram of anterior region of Calanus finmarchicus. The endopodite of the antenna,
the mandibular palps and the distal parts of the maxillules have been removed. The position of
the swimming trunk limbs is indicated by the shaded area inside the dotted line. On the right
side of the figure the limb movements are indicated, on the left, the water currents, ant. 1 =antennule; ant. 2 =antenna; ant. 2 ex.r. =rotation path of tip of exopodite of antenna ;f.ch. =filter
chamber; Ibr. =labrum; mdb. =mandible; mx. 1 =maxillule; mx.x ex.r. =rotation path of tips
of setae of maxillulary exite; mx.z =maxilla; mxpd. =maxiliiped; mxpd.r. =rotation path of tip
of maxilliped; s.ch. =suction chamber.
The antennae move through by far the greatest arc. The innermost setae of
the endopodite, at the end of the back stroke, reach the tips of the njaxillules. The
range of movement decreases in the more dorso-lateral setae. This results from the
fact that, as the endopodite moves backwards the exopodite swings forward. The
Feeding Mechanism of Copepods
141
former moves practically in a straight line sloping posteriorly towards the sagittal
plane. The tip of the exopodite rotates in an ellipse passing nearer to the sagittal
plane on its forward stroke (Text-fig. 5).
It is difficult to say what is the meaning of the recurved exopodite. It may
function in assisting the feeding swirl but it also may serve to diminish the centrifugal drag on the base of the limb. The tissues of a copepod must be relatively
heavy compared with water, especially in such forms as Diaptomus and Calanus
which reduce their total specific gravity by the production of a drop of light oil.
Consequently the mass of the moving limb must be considerable. The position
of the exopodite does not reduce the inertia of the limb but it shifts the centre of
gravity almost to the axis of rotation, and this must diminish the centrifugal drag
on the body.
anienna.
mandible.
maxiflule.
OuiLiord
StrakP OT
= Stichon from filler chamber
maxiiii.'i ned.
outward siroke = suction
info filter chamber.
Text-fig. 6. Graphic representation of the movements of the head swimming limbs of
Calanus finmarchicus.
The mandibular palps and maxillules move backwards and forwards in a parasagittal plane over a comparatively small arc.
The swimming swirl is produced by these three limbs, and, of these, the antennae are the most effective. The feeding swirl results primarily from the swimming
swirl. This can be best understood by considering the path of a jet of water when
squirted into a volume of still water. If, from a cylindrical jet, a small mass of
water is suddenly expelled it does not move forwards as a moving cylinder of fluid
but spreads out into the form of a vortex, and this is the more marked the greater
the velocity of the moving jet (Text-fig. 7, a). Considering two points, one just inside
the cylinder of fluid immediately it has left the nozzle, and another point just outside in the still water, there will be marked discontinuity between the velocities
at these two points and discontinuity in velocity in a fluid leads to vorticity. Or
H. GRAHAM CANNON
142
again, the viscous drag of the moving cylinder immediately it has left the nozzle,
will drag in still water from behind while the viscous resistance at the front end
of the cylinder will flatten it out and these two effects together will produce the
vortex.
If now, instead of a simple jet, an annular cylindrical jet is considered, the same
reasoning applies. The moving water will tend to spread out. It will move outwards
from the axis of the jet but, at the same time, it will spread inwards and so produce
an inner vortex (Text-fig. 7, b).
This latter type of jet is produced by the steadily swimming copepod. The
head swimming limbs move at greatest speed at their tips while their bases, being
attached to the body, are stationary. Their setae spread out in fans extending
almost half way round the body. The result is that they produce a moving layer
of water in the form of half an annular jet (Text-fig. 7, c). This spreads outwards
as the swimming swirl but at the same time spreads inwards and produces the
feeding swirl.
a
Text-fig. 7 (a) Diagram of swirl produced by jet from simple tube.
(h)
„
„
„
annular tube.
(c)
„
„
„
antennae of copepod.
In the copepod there is an additional effect which increases this vortex production. In the annular jet there is a continuous supply of water being forced through
the nozzle. In the copepod this supply comes from the spaces between the swimming limbs. As the antenna moves backwards it obliterates the space between it
and the mandibular palp and forces out the water into the swirls. On extending
forwards this space is opened out and water must pass in again. It will not pass
in from the tip of the limb to any great extent as water in this region is moving
rapidly backwards. There is, however, a slight tendency foi water to pass in, for
if particles are watched passing over the tips of the swimming limbs they are seen
to pass slightly inwards towards the base of the limbs but are immediately
thrown out again on the backstroke. The main mass of water will naturally be
sucked in from the bases of the limbs where the water is relatively still. That is, a
region of low pressure must exist at the bases of the swimming limbs. This will
suck in water partly from in front (Text-fig. 5) and partly from behind. The latter
suction will serve to increase the feeding swirl,
Feeding Mechanism of Copepods
143
The action of the maxiliulary exite and maxilliped is to force part of the feeding
swirl through the comb of filter setae on the maxillae. The two limbs co-operate.
The maxilliped sucks water into the filter chamber while the maxiliulary exite
sucks it out of this chamber through the maxillary setae.
The movement of the maxillipeds has already been described (p. 139). On their
outward stroke the setae on the distal joints spread out into a fan, so that a suction
is produced in a ventro-lateral direction. The maxillipeds lie just underneath the
splits between the tips of the trunk swimming limbs and the maxillae (Text-fig. 2).
Hence the suction must extend into the filter chamber and draw in water from
behind.
Just after the maxillipeds have finished their outward stroke the maxillules
commence to move forwards (Text-fig. 6). The maxiliulary exites, at the end of
their back stroke lie flat against the outer faces of the maxillae, thus diminishing,
\
Diagram showing four consecutive phases in the movement of the maxiliulary exite.
and at the same time closing the suction chamber between them and the maxillae.
On their forward stroke they simply tend to enlarge this space and so produce in
it a region of low pressure. The extent of this suction can be seen from the curvature
of the setae as the exite moves forwards (Text-fig. 8). The suction effect of the setae
is increased by their armature of setules. These project laterally from the outer
faces of the setae so that, on the outward movement of the exite they spread out
and fill up the inter-setal gaps, while on the inward movement they collapse and
allow the escape of water through the setae (Text-fig. 8). The maxillipeds and
maxillules thus work together. The former sucks water into the filter chamber
and this is immediately followed by a suction of the maxillule which draws water
through the maxillary setae. In the back stroke of the exite the setae spread out
and sweep the water backwards into the swimming swirl.
In the backwards and forwards motion of the maxillule the setae on its exite,
144
EL G R A H A M
CANNON
which project at right angles to the axis of the limb, move through the same angle
as the limb itself. The tips of the setae thus move in a dorso-ventral direction.
This, combined with their in and out motion resulting from the suction activity
of the exite, results in the tips of the setae moving in a flattened ellipse lying close
against the ventro-lateral body wall (Text-fig. 5). The tips of the exites thus beat
towards the posterior opening of the filter chamber. Thus, while the anterior part
of the maxillulary exite is producing suction in the suction and filter chambers,
the posterior whip-like ends are actively sweeping particles into the latter.
Small particles sucked into the filter chamber are deposited on the setae of
the maxillae. These are heavily armed on their inner faces with laterally projecting
setules so that the whole limb forms an efficient filter (Text-fig. 2).
Particles so filtered, if they happen to be deposited near the anterior end of the
chamber are scraped off, as Storch and Pfisterer (p. 354) point out, by the endites
of the maxillule and passed directly forwards on to the mandibles. In addition,
however, there are several long setae which arise on the basal joints of the maxillipeds and project forwards on to the inner faces of the maxillae. These scrape
particles off the hinder parts of the filter plates and push it forwards on to the
maxillules. The maxilla is thus brushed clean on both its inner and outer faces.
SUMMARY.
1. Calanus finmarchicus and Diaptomus gracilis both feed automatically when
swimming slowly and steadily through the water.
2. A feeding current is produced which is filtered by the stationary maxillae.
Food so obtained is passed on to the mandibles by the maxillulary endites and
setae on the bases of the maxillipeds.
3. The feeding current is a vortex passing through the mouth parts which
results automatically from the swimming activities of the antennae, mandibular
palps and maxillules.
4. The feeding vortex is caused to pass through the maxillae by the combined
activities of the maxillipeds and the maxillulary exites. The former suck water
into the filter chamber between the maxillae while the latter suck it out through
the maxillary setae.
5. The views of Storch and Pfisterer on the feeding mechanism of Diaptomus
gracilis are criticised. There is no powerful antero-posterior swimming current
as described by these authors. The swimming current is in the form of a vortex
encircling the body and most marked at the sides in the angle between the body
and the antennules.
LITERATURE.
H. G. (1927). "On the Feeding Mechanism of Nebalia bipes." Trans. Roy. Soc. Edin.
55-3SS-7O.
(1938). "On the Feeding Mechanism of the Fairy Shrimp, Chirocepkalus diapkanus." Trans.
Roy. Soc. Edin. 55. 807-23.
STORCH, O. and PFISTERER (1935). "Der Fangapparat von Diaptomus." Zs. vergl. Physiol., Berlin,
Bd. 3. 330-76.
CANNON,