the significance of the relationship between actinians and

334
THE SIGNIFICANCE OF THE RELATIONSHIP
BETWEEN ACTINIANS AND ZOOXANTHELLAE
BY H. G. SMITH
Carnegie Teaching Fellow, Department of Natural History,
University of Aberdeen
(Received 6 January 1939)
(With Two Text-figures)
algae (zooxanthellae) occur in certain Actiniaria. The first important
experimental work on the nature of the association was that of Brandt (1883 a, b),
who starved anemones in light and in darkness. From the results of these experiments he concluded that the animals obtained nutriment from the zooxanthellae
when deprived of animal food. Later work by Boschma (1926) on Cribina xanthogrammica supports Brandt's view. Brandt was unable to observe the digestion of the
zooxanthellae and assumed that food material was passed in soluble form from
algae to animal. By studying the mesenterial filaments of C. xanthogrammica
Boschma brought forward evidence in favour of the view that zooxanthellae are
actually digested by the anemone. In his opinion such actinians live partly on
animal matter and partly on the zooxanthellae. During starvation they depend
chiefly on the algae.
Among Madreporaria zooxanthellae are more widely distributed than in the
Actiniaria, but conditions appear to be similar. Considerable controversy about the
exact nature of the association has been reviewed at length by Yonge & Nicholls
(1931 b). The view that zooxanthellae are a source of food to the corals is upheld by
Boschma (1924,1925 a, b, 1926) but entirely opposed by Yonge & Nicholls. The latter
believe that the presence of the zooxanthellae in the "absorptive" zone of the
mesenterial filaments, as certainly occurs when corals are starved, is due to the
rejection and not to the digestion of algae. It was found by Yonge (1931 a) that the
"absorptive" zone of the mesenterial filaments is also the excretory zone. Moreover, it was found that the zooxanthellae are always rejected if the metabolism of the
coral is lowered in any other way, e.g. by exposure to high temperature (Yonge &
Nicholls, 1931 b) or to low oxygen tension (Yonge et al. 1932). The zooxanthellae,
which are always within tissue cells, are carried by wandering cells to the mesenterial
filaments, when the metabolism of the coral is lowered, where they are passed into
the enteron and finally expelled.
In view of these results, it seemed desirable to reinvestigate the role of zooxanthellae in the Actiniaria, particularly in view of the fact that Mouchet (1930) has
UNICELLULAR
Relationship between Actinians and Zooxanthellae
335
shown that in these animals the "absorptive" zone of the mesenterial filaments is
also the excretory zone. Experiments have therefore been undertaken:
(a) To compare the effect of starvation in light and darkness on species whose
pigmentation is and species whose pigmentation is not due to the presence of
zooxanthellae.
(b) To compare the output of phosphates and the oxygen consumption in the
species of either type during starvation in light and darkness.
Both brown and green varieties of the alga-containing anemone, Anemoma
sulcata, were used for this work. Actinia eqidna which does not contain algae was
also used. For microscopical examination, material was fixed in Flemming and
Bouin and sections were cut about 7-10 y. thick. The stains used were safranin and
light green haematoxylin and eosin. Oil drops in the zooxanthellae were distinguished by staining with Sudan III. Tests used for the identification of cellulose
included a dilute solution of iodine, iodine with the addition of 25 % sulphuric
acid, chlorzinc iodine, and calcium chloride iodine.
The zooxanthellae of Anemoma sulcata are similar in structure to those of the
Madreporaria described by Yonge & Nicholls (1931a). They are yellowish brown
spherical bodies varying in diameter from 7 to 12/x. In the living cell one or two
assimilation products can be recognized, and examination of stained preparations
reveals, in each alga, a granular nucleus, one or two pyrenoids with assimilation
products and a vacuolated cytoplasm with numerous oil drops. The whole cell is
bounded by a thick wall which Geddes (1882) found to consist of cellulose. This has
been confirmed. Crystals of calcium oxalate are present on the surface of the zooxanthellae, and in stained preparations obscure the underlying structures if not
removed by placing the sections in 70 % alcohol acidified with nitric acid. The
distribution of the algae in tissues of A. sulcata agrees with that in corals. They are
confined to the endoderm (Fig. 2) and do not occur in the mesogloea or the ectoderm. As in corals, the zooxanthellae are contained in the endoderm cells.
Oxygen and phosphates experiments were carried out at the Scottish Marine
Biological Station, Millport. The micro-Winkler method of Fox and Wingfield was
used for oxygen determinations and the method of Deniges for phosphates.
I. THE EFFECT OF STARVATION ON ANEMONES IN
LIGHT AND DARKNESS
Exp. 1. Starvation in light
Two glass tanks of capacity 40 1. each were used for this experiment. Eight
specimens of A. sulcata (four brown and four green varieties) and, as controls, eight
A. equma were placed in each tank with 20 1. of fresh, filtered sea water. At the end
of the fourth week of the experiment the anemones of each species were taken from
one tank for the second experiment so that the entire experiment deals with the
contents of the remaining tank. Great care was taken, throughout this experiment
and the succeeding ones, to keep the water and the tanks clean. Solid matter,
H. G. SMITH
ejected by the anemones, was pipetted out each day. As this did not prevent a
deposit from settling on the bottom and the sides of the tanks, it was necessary to
clean out the tanks periodically. The water was kept well aerated and changed every
4 weeks.
At the beginning of the experiment algae were expelled from the coelenteron in
large numbers and individuals of the species A. sulcata became paler in colour. The
ejected algae appeared to be healthy. After several weeks the expulsion of the algae
decreased and, for the remainder of the experiment, only a small number were
ejected from time to time. At the end of 32 weeks the experiment was stopped and
Fig. 1. Diagram of the aerating and circulating apparatus and arrangement of tanks for the starvation of anemones in light and in darkness. The reservoir jar (R) is exaggerated. For explanation of
lettering see text.
the animals were fixed. No deaths had occurred and all the anemones were in a
very healthy condition. At the termination of the experiment individuals of A.
sulcata were not only very much paler owing to the loss of the algae, but also considerably smaller. The controls showed no apparent change in colour but were
reduced in size.
Exp, 2. Starvation in light and in darkness
For a second experiment, four tanks, each with a glass top, were arranged as in
Fig. 1. A and D were clear but B and C were completely coated with black enamel
several times and in addition were completely covered with a black cloth. They
were thus made lightproof. To make certain that the light factor would be the only
difference between A, D and B, C a circulating apparatus which is a modification of
that described by Cannon & Grove (1927) was used as shown in Fig. 1. Air was
supplied to an air tube (X) by a small pump worked by an electric motor and the
pressure in each tank regulated by means of screw-clips ta,tb)te, and td. A reservoir
jar (R) was fitted with a four-holed stopper through which passed glass tubes.
Y was connected with the end of the air tube, the pressure of the air being
Relationship between Actinians and Zooxanthellae
337
controlled by a screw-clip (tx). Inside R the end of the tube Y was tapered and bent
round so that it fitted into the end of tube Z which led into tank A, a system of
siphons, Si, S2, S3 and 5 4 being arranged between A and B, B and C, C and D,
and D and R. The tube P served to control the pressure of air in the reservoir jar.
The fourth tube Q was connected with the return tube W. Before starting the
apparatus R was filled a quarter full of fresh sea water. Then, as indicated by arrows,
air passed down Y and forced the water in the reservoir through Z into tank A, air
and water passing alternately along Z. As the level of the water rose in A, it was
siphoned into B and so on to D and from D along H^to the reservoir jar R. The water
in the tanks was aerated and circulated in this manner for about 12 hr. each day.
After 4 weeks' starvation in light, a number of healthy anemones (brown and
green anemones and controls) were taken from the stock used for the first experiment and transferred to the tanks described above. Two specimens of Anemonia
(a green variety and a brown variety) and two specimens of Actinia were placed in
each tank. About the fourth week of the experiment the water in the tanks was
changed. The changing of the water and periodic examinations (every fourth day)
were always performed at night. Thus the anemones were exposed to artificial light
for a few minutes only. The extrusion of the algae, which had slowed down considerably after the animals had been starved for some time in the light, was much
accelerated on transport to darkness. As in corals (Yonge & Nicholls, 1931 a), the
zooxanthellae ejected in darkness had died and were degenerating. Soon there was
a marked difference in colour between the anemones kept in darkness and those
kept in light. After 5 weeks the experiment was stopped because of the death of a
number of specimens. All the individuals of the genus Actinia were healthy and
were used for the next experiment, but the Anemonia varied according to the tank.
Those in A (light) were living, but in B (dark) the green one had died and the brown
one was in a poor condition. All individuals of the genus Anemonia in C and D had
died. It still seemed possible that with increased care and attention, such as more
frequent changing of the water, the lives of A. sulcata could be prolonged when kept
in darkness. With this aim in view another experiment was started.
This experiment was similar to the previous one, but the order of the tanks was
A, B, D, C, so that the clear and darkened tanks alternated. The water was changed
during every third week of the experiment. A fresh stock of Anemonia was obtained
and two were placed in each tank. A and B each contained a brown variety and a
green variety while brown varieties only were placed in C and D. Actinia from the
second experiment were used again and two were placed in each tank to act as
controls. At the end of 3 weeks the Anemonia in darkness were distinctly paler than
those in light. At the end of 9 weeks all of them were healthy. In the previous
experiment a number had died after 5 weeks. The brown ones had completely lost
their colour. The green ones retained green and purple pigment at the tips of their
tentacles, but elsewhere, like the brown ones, they were uniformly white. After 25
weeks starvation in light and in darkness the anemones were still healthy. They
were reduced in size but there was very little difference in the relative size reduction
between those in light and those in darkness. In both the clear and the dark tanks
338
H. G. SMITH
the Actinia retained their dark body colour. The Anemonia in the clear tanks were
also pale. Both the brown and green varieties of Anemonia which had been kept in
darkness were uniformly snow white, except for the green pigment in the tentacles
of the green varieties. The experiment was stopped at this time and the animals were
fixed for sectioning.
D
ec.
m.
E
in.
Fig. 2. Anemonia sulcata. Longitudinal sections through a tentacle, A (normal), B (after 25 weeks'
starvation in light), C (after 25 weeks' starvation in darkness), and transverse sections through a
mesenterial filament, D (after 25 weeks' starvation in light), E (after 25 weeks' starvation in darkness).
c.t. ciliated tract; ec. ectoderm; en. endoderm; gl.c. glandular cells; m. mesogloea; n. nematocyst;
r.t. reticular tract; z. zooxanthellae; z.t. zooxanthella tract.
Microscopical examination of sections showed that the algae in the anemones
from the clear tanks A and D were few in number in the endoderm of the tentacles
(Fig. 2 B) but numerous in the zooxanthella tract of the mesenterial filaments (Fig.
2D). Compared with a normal specimen (Fig. 2 A) the endoderm of the tentacles
showed a marked reduction in size. In the case of the anemones from the dark tanks
no zooxanthellae were to be seen in the tentacles (Fig. 2C) or in the mesenterial
filaments (Fig. 2E) and the endodermal region of the tentacles was even more
Relationship between Actinians and Zooxanthellae
339
reduced. Since the ectoderm was not reduced to the same extent aa the endoderm,
the reduction must have been due mainly to the removal of the zooxanthellae. All
the Actinia equina were in a healthy condition but reduced in size.
Table I
Animals
No.
Total
volume
c.c.
Total
wet wt.
g-
Total
dry wt.
g-
Total
nitrogen
g-
A. Anemonia tulcata
(brown)
(i) 3 animals
(ii) 3 animals
(iii) 3 animals
32
295
386
0328
B. Anemonia tulcata
(i) 3 animals
(ii) 3 animals
(iii) 3 animals
8
7
1-53
0095
(i) 3 animals
(ii) 3 animals
(iii) 3 animals
10
96
2-O2
0-184
5 animals
20
199
2-73
0-218
(white)
C. Actinia equina
D. Anemonia tulcata
(brown)
II. PHOSPHATE EXCRETION AND OXYGEN EXCHANGE
Recent work on corals (Yonge & Nicholls, 1931 a) has shown that zooxanthellae
can utilize both the phosphate excreted by the animal and phosphate contained in
the surrounding water. Special importance has been attached to this excretory
function of the zooxanthellae of corals. It is therefore necessary to investigate the
zooxanthellae of anemones from the same standpoint. A batch of A. sulcata was
kept in darkness for 9 months by Mr R. Elmhirst at the Marine Biological Station,
Table II
(1) Anemone* 3 hr. in light. Temperature range i5-2±0'O° C. Initial phosphorus in 'sea water
2 mg. per cu.m.
(2) Anemones 3 hr. in darkness. Temperature range i s - 8 ± o - 4 ° C . Initial phosphorus in tea
water 2 mg. per cu.m.
Animals
(1) (light)
phosphorus
mg. per. cu.m.
(2) (darkness)
phosphorus
mg. per cu. m.
A. Anemonia (brown) (i)
(i>)
(iii)
6
8
8
10
B. Anemonia (white) (i)
(ii)
(iii)
6
6
6
6
7
S
5
S
6
5
5
2
2
C. Actinia
(i)
(ii)
(iii)
Control (sea water)
7
7
H. G. SMITH
340
Millport. Of these ten survived and appeared to be very healthy. They were quite
white except for a tinge of pink round the base. Nine were used for the following
experiments, while the remaining one was placed with normal A. sulcata in a tank
for 3 weeks and then fixed and sectioned. No reinfection of zooxanthellae could be
detected, and the appearance of the experimental animals also gave no evidence of
reinfection after a similar period of time. At the end of the experiments the anemones
were dried and the total nitrogen was estimated by Kjeldahl's method.
In a preliminary experiment each set of three animals (see Table I) was placed in
ajar, provided with a screw top, of capacity 415 c.c. and kept in a light of maximum
intensity for 3 hr. and in darkness for 3 hr. The results exhibited in Table II show
no significant difference in the amount of phosphates added to the water. In a
second experiment the animals were kept for 24 hr. in darkness and for 24 hr. in a
place in the open protected from direct sunlight and the phosphate content of the
sea water estimated at the end of the experiment. The crude results of this experiment are shown in Table III. In Table IV the same figures are weighted by total
nitrogen to reduce them to a comparable basis and the three sets of results in each
series summarized. Anemoma containing zooxanthellae excreted more phosphate in
daylight than in darkness. To the phosphate excretion of Anemoma denuded of
zooxanthellae, light and darkness made little difference, and the same is true of
individuals belonging to Actinia equina, a species which never has zooxanthellae.
This suggests that zooxanthellae chiefly use phosphate at night.
The conclusions which emerge from a comparison of the adjusted figures
exhibited in Table IV may be summarized thus:
(1) The presence of zooxanthellae leads to a great reduction of phosphate
excretion in darkness.
(2) In the absence of zooxanthellae (whether as a species character or as a
result of previous treatment) there is no reduction of phosphate excretion in the dark.
Table III
(1) Anemones exposed for 24 hr. to normal daylight. Temperature range 15-1 ±0'2° C. Initial
phosphorus in sea water 2 mg. per. cu.m.
(3) Anemones 24 hr. in darkness. Temperature range i6-6.±o-3°C. Initial phosphorus 2 mg.
per cu.m.
Animals
A. Anemoma (brown) (i)
(")
(iii)
B. Anemoma (white) (i)
(")
(iii)
C. Actinia
(i)
(ii)
(iii)
Control (sea water)
(1) (light)
phosphorus
mg. per cu.m.
100
it6
104
(2) (darkness)
phosphorus
mg. per cu.m.
10
10
10
S3
57
64
SO
52
45
5°
28
40
42
37
2
2
79
Relationship between Actinians and Zooxanthellae
341
(3) Artificial removal of zooxanthellae from a species which normally harbours
them results in an increase of phosphate excretion per gram of total nitrogen both
in light and in darkness; in darkness the phosphate excretion of the white Anemoma
(no zooxanthellae) is approximately twenty times that of the brown variety (zooxanthellae present).
Table IV
Animals
Phosphorus (mg. per cu.m per g.
total nitrogen)
Light
Darkness
A. Anemoma (brown)
327
30
B. Anemoma (white)
610
635
C. Actinia
234
190
The conclusions stated are confirmed by the results of another series of experiments in which parallel determinations of the oxygen consumption of the
anemones were also made (see Table V). Three consecutive experiments were
undertaken, each of 8 hr. duration. Light intensities were measured with a Weston
photometer which was later calibrated in terms of metre candles. The procedure was
as follows:
(1) The first experiment was started in darkness (1 a.m.) and stopped at 9 a.m.
The light intensity at that time was found to be 250 metre-candles. The initial
phosphorus in the sea water in the three experiments was 2 mg. per cu.m. The seawater control at the end of each experiment showed no difference. The initial
oxygen in the sea water varied slightly for each experiment. In this experiment the
initial oxygen in the water was 473 c.c. per litre. The sea-water controls in the three
experiments differed only slightly from the initial amount of oxygen. During the
experiment the temperature range was 147+ 0-5° C.
(2) The second experiment was started at 9.30 a.m. and stopped at 5.30 p.m.
Light intensity at 1.0 p.m. was 870 metre-candles and at 5.30 p.m. 310 metrecandles. Initial oxygen in the sea water 5-35 c.c. per litre and the temperature range
(3) The duration of the third experiment was from 6 p.m. (light intensity 290
metre-candles) to darkness (2 a.m.). Initial oxygen in the sea water 4-66 c.c. per
litre. Temperature range 15-0+ 0-2° C.
Both oxygen and phosphate determinations were carried out simultaneously and
the results recorded in Table V. In the first experiment (from darkness to 9 a.m.)
the brown Anemonia took up oxygen at the rate of 1-28 c.c. per g. total nitrogen.
In the second experiment (from 9.30 to 5.30 p.m.) the zooxanthellae in the brown
variety produced in photosynthesis about twice the amount of oxygen consumed by
the animal.
The results of other experiments are shown in Table VI. In total darkness the
342
H. G. SMITH
figures for oxygen consumption of alga-free Ammonia are higher than in the experiments in light. This is due, no doubt, to the higher temperature of the surrounding water. The oxygen consumption of normal Ammonia was less than alga-free
Anemonia in darkness; under these conditions the respiration of both is higher than
Table V. Oxygen consumption is indicated by a minus sign and
oxygen output by a plus sign
Animals
Phosphate (mg. per
cu.m. per mg. total N)
Oxygen (c.c. per hour
per g. total N)
D. Anemonia (brown)
B. Anemonia (white)
C. Actinia
92
200
-128
— 2-21
-114
D. Anemonia (brown)
B. Anemonia (white)
C. Actinia
229
189
108
+ 1-42
-2-74
D. Anemonia (brown)
B. Anemonia (white)
C. Actinia
83
263
-1-28
•
•
Experiment
I
II
III
98
-As
108
• Oxygen results lost.
Table VI
O, consumption per hr. per g. total nitrogen
Darkness
Light
Temperature
range
i4'9±o-i°C.
165 ±0-5° C.
I7'9±o-o° C.
A. Anemonia
(brown)
+ 1-04 c.c.
(expanded)
— 2-84 c.c.
(contracted)
- 3 0 7 c.c.
(contracted)
B. Anemonia
(white)
- 3 6 3 c.c.
(expanded)
— 5-00 c.c.
(contracted)
-4-48 c.c.
(contracted)
C. Actinia
- 1 97 c.c.
(expanded)
— 1'51 c.c.
(contracted)
— 251 c.c.
(expanded)
that of Actinia. The results in light are similar to those obtained in the previous
experiment. The fact that the metabolic rate of brown Anemonia is lower than that
of the white ones may be explained by the intrinsically likely assumption that algal
cells, even in darkness, consume less oxygen than the equivalent amount of anemone
tissue expressed in terms of total nitrogen. It will be seen that alga-free Anemonia
also consume about twice as much oxygen as the Actinia. It is possible, therefore,
that the zooxanthellae might be of some use as a protection against oxygen shortage
by reducing indirectly the amount of oxygen removed from the water for the
respiration of the anemone. This view is strengthened by the knowledge that
Actinia stands up to oxygen lack better than Anemonia.
Relationship between Actinians and Zooxanthellae
343
An interesting aspect of the behaviour of anemones is the tendency to contract
in the absence of light or in diminished light. In Actinia there is usually complete
contraction of the tentacles accompanied by a swelling of the body. In Anemonia the
tentacles are partially contracted but often completely hidden from view by the
main part of the body which becomes swollen and turgid with sea water. In many
of the Madreporarian corals, on the other hand, the polyps expand during the night
and contract during the day. Table VI records an experiment in darkness. For
some inexplicable reason all the Actinia remained fully expanded—the only occasion
during the course of the investigation when experimental animals remained fully
expanded in darkness. As compared with the other experiment in darkness there is
a large increase in the amount of oxygen consumed.
III. DISCUSSION
When starved in light, A. sulcata ejects masses of zooxanthellae and becomes
pale brown. The zooxanthellae, examined under the microscope immediately after
ejection, are alive and appear to be healthy. In time, starvation in darkness brings
about complete ejection of the zooxanthellae so that the anemones, formerly brown,
become snow white. In similar experiments with alga-containing anemones Brandt
(1883 a, b) was able to keep starved animals alive longer in light than in darkness. He
concluded that anemones in darkness died before those in light, because they were
unable to obtain nutriment from the zooxanthellae. There is no doubt that Anemonia
starved in darkness die more readily than when starved in light. This is due to the
vigorous expulsion of zooxanthellae in darkness resulting in greater contamination
of the water; the total expulsion of zooxanthellae may be an additional strain on the
animals. The period which elapses between when anemones are first placed in
darkness and the complete elimination of algae may be regarded as a critical period
in which greater care and attention, must be paid to them. When exhausted of
zooxanthellae specimens of Anemonia are as easy to keep in an aquarium as are
normal individuals; they are quite active and readily ingest food.
Like other Anthozoa which contain zooxanthellae Anemonia is carnivorous. In
aquaria, they feed greedily on large amphipods and even accept whelks, complete
with shells, until they became like distended bags. The carnivorous habit, together
with the fact that a cellulase has not been found in any of the Anthozoa which
harbour zooxanthellae, suggests that Brandt has overrated their importance as a
source of food. It is difficult to understand the reason for the ejection of the zooxanthellae when the anemones are starved in light if they have as much food value
as Brandt implies. The possibility cannot be overlooked that a certain amount of
nutriment may be passed in soluble form from plant-cell to animal, but if so it is
probably slight and of little significance for alga-free Anemonia can thrive quite well
without them.
In daytime there is a great excess of oxygen production over oxygen consumption (Trendelenburg, 1909)—so much so that bubbles of gas are often formed in the
enteron, thus reducing the specific gravity of the anemone and causing it to float with
344
H. G. SMITH
its base upwards. Yonge et al. (1932) attach little importance to the production of
oxygen by the zooxanthellae except in the case of corals situated in sheltered, still
waters. They attach more importance to the removal and utilization by the zooxanthellae of phosphates, and presumably also of ammonia—both end-products of
coral metabolism. This investigation shows that the presence of zooxanthellae in
Anemonia ensures a supply of oxygen during daylight under conditions such as exist
in rock pools at low tide. That is to say, it prevents death by asphyxia due to exhaustion of dissolved oxygen by the respiratory activity of the anemone itself. The
fact that Anemonia has a higher metabolism than Actinia indicates that zooxanthellae might be of considerable importance in maintaining the high metabolism of
Anemonia, both by the automatic removal of end-products of metabolism and by the
availability of oxygen within the tissues of the animal.
IV. SUMMARY
1. Actinia equina and Anemonia sulcata are two anemones which respectively
lack and possess symbiotic algae. If kept for several months in complete darkness
Actinia retains its colour and Anemonia becomes white through loss of its algae.
2. Such alga-free Anemonia will continue to live in a healthy condition if
supplied with solid food and sufficient dissolved oxygen.
3. In darkness the oxygen consumption of Actinia is lower than that of either
normal or alga-free Anemonia being about half that of the latter.
4. In daylight the photosynthetic activity of the algae is more than sufficient to
meet the needs of normal Anemonia, so that oxygen is added to the water.
5. The phosphate excretion of Actinia is about half as much as that of algafree Anemonia and the phosphate excretion of neither is appreciably affected by
change from darkness to light.
6. In daylight the phosphate excretion of normal Anemonia is less than that of
alga-free Anemonia. There is a great reduction of phosphate excretion by normal
Anemonia in darkness.
7. Apparently the alga benefits from the association by using the anemone as a
source of phosphorus, the extent of phosphate utilization being diminished during
photosynthesis.
8. Presumably the Anemonia benefit by insurance against oxygen starvation
when isolated in small pools of stagnant water during daylight.
I wish to express my thanks to Mr R. Elmhirst for his generous help and facilities
which he so kindly provided at the Millport Marine Station and to Prof. L. Hogben
for much criticism and advice. Acknowledgement is also due to the Carnegie
Research Fund for Scottish Universities.
Relationship between Actinians and Zooxanthellae
345
REFERENCES
BOSCHMA, H. (1924). Proc. Acad. Sci. Amst. 27, 13-23.
(192s0)- Proc. nat. Acad. Set., Wath., 11, 65-7.
(19256). Proc. Amer. Acad. Arts Set. 60, 451-60.
(1925 c). Biol. Bull. Wood's Hole, 49, 407-39.
(1926). Proc. Acad. Sci. Amst. 29, 993-7.
BRANDT, K. (1883a). Mitt. Zool. Sta. Neapel, 4, 257-65.
(18836). Arch. Anat. Physiol. pp. 451-a.
CANNON, H. G. & GROVE, A. J. (1927). J. R. micr. Soc. 47, 319-22.
GBDDBS, P. (1882). Nature, Lond., 25, 303-5.
MOUCHET, S. (1930). Stat. Ocean. Salammbd, Notes, no. 15, pp. 1-15.
TRENDELBNBURO, W. (1909). Arch. Anat. Physiol., Abt. Physiol. Leipzig, pp. 42-70.
YONGE, C. M. (1931 a). Sci. Rep. G. Barrier Reef Exped. 1928-9, Brit. Mus. (Nat. Hist.), 1, 83-91.
YONGE, C. M. & NICHOLLS, A. G. (1931a). Sci. Rep. G. Barrier Reef Exped. 1928-9, Brit. Mus.
(Nat. Hist.), 1, 135-76.
(19316). Sci. Rep. G. Barrier Reef Exped. 1928-9, Brit. Mus. (Nat. Hist.), 1, 177-211.
YONGE, C. M., YONGE, M. J. & NICHOLLS, A. G. (1932). Sci. Rep. G. Barrier Reef Exped. 1928-9,
Brit. Mus. (Nat. Hist.), 1, 212-51.