Observations on the Stomach and Digestive Diverticula of
the Lamellibranchia
I. The Anisomyaria and Eulamellibranchia
By G. OWEN
(From the Department of Zoology, University of Glasgow)
With 3 plates (figs, i, 2, and 6)
SUMMARY
A study of the digestive diverticula of the Anisomyaria and Eulamellibranchia
revealed certain features hitherto undescribed. The diverticula consist of blind-ending
tubules which open into ciliated main ducts by way of short, non-ciliated secondary
ducts. The main ducts open into the intestinal groove. In all the species examined the
ciliated epithelium of the main ducts was restricted to a well-defined groove, the remainder of the lumen being surrounded by a non-ciliated, brush-border epithelium.
Each tubule is surrounded by a system of smooth muscle fibres. Cilia associated with
the darkly staining cells of the crypts were demonstrated in sections of the tubules.
After feeding with titanium dioxide in suspension, this substance was later found in
the large vacuolated cells of the tubules. The particles of titanium dioxide were larger
than o-i^t.
A continuous circulation is maintained within the main ducts solely as a result of
ciliary activity. The exhalant current in the ciliated portion of the main ducts produces
an inhalant counterpart current in the non-ciliated portion. It is suggested that fresh
fluid is drawn into the tubules as a consequence of the absorptive functions of the large
vacuolated cells. Indigestible material accumulates in the large vacuolated cells and is
extruded into the main ducts where it is conveyed out of the diverticula by the exhalant
ciliary current.
In both the Anisomyaria and Eulamellibranchia the flap-like major typhlosole
prevents material entering the mid-gut except by the intestinal groove, and isolates
the rejectory currents of the intestinal groove from the general circulation of particles
in the stomach. In the Eulamellibranchia the major typhlosole also acts as a valve
which controls the entry and exit of material into and out of the inhalant and exhalant
portions of the main ducts.
INTRODUCTION
M
ANY conflicting views have been expressed regarding the functions of
the digestive diverticula of the Lamellibranchia. An absorptive function has been attributed to them by many authors, while others have regarded
them as secretory organs (for details and bibliography see Yonge, 1926a).
From a comparative study of the structure and functions of the diverticula
of numerous lamellibranchs Yonge concluded that • they were organs of
absorption and intracellular digestion and this view has received general
acceptance by the majority of workers. Recently, Mansour (1946 a and b,
1949) and Mansour-Bek (1946) have questioned the occurrence of intracellular
digestion in the Lamellibranchia. They claim that extracellular proteases and
lipases are secreted into the lumen of the stomach by the digestive diverticula.
[Quarterly Journal of Microscopical Science, Vol. 96, part 4, pp. 517-537, December 1955.J
518
Owen—Observations on the Lamellibranchia
Despite this recent interest in the functions of the diverticula little has been
added to our knowledge of the structure and mode of functioning of these
organs.
Observations on the digestive diverticula of Cardium edule carried out
during the course of feeding experiments revealed structural features hitherto
undescribed, and these were subsequently investigated in other species of
lamellibranchs. In this paper the results obtained for the Anisomyaria and
Eulamellibranchia are described and their possible significance discussed.
The work was carried out on a variety of lamellibranchs with both fresh and
fixed material. Fixed material was embedded in ester-wax (Steedman, 1947)
and sections cut at 2-6,11 were in most cases stained in Heidenhain's iron
haematoxylin, alcian blue 8GS (Steedman, 1950) and orange G in clove oil.
Before embedding, the material was cleared in monochlorisothymol (Steedman, 1955).
THE STRUCTURE OF THE DIGESTIVE DIVERTICULA
The digestive diverticula surround the stomach and consist of numerous
blind-ending tubules which have the form of globular (e.g. Cardium) or
elongate (e.g. Anodonta) sacs or, as in the Mytilidae (List, 1902), of irregularly
branched tubes with numerous saccular outgrowths. They communicate with
the stomach by a system of ducts whose structure is distinct from that of the
tubules.
Previous workers described the epithelium of the ducts as completely
ciliated and resembling that of the stomach, but, as shown in figs. 1 and 2, A,
B, and c, ciliated cells are restricted to a well-defined tract which frequently
has the form of a groove or gutter, the cells round the remainder of the lumen
being non-ciliated and possessing at their free margin a well-developed brushborder. This division of the epithelium of the ducts into two distinct regions
was recognized by List (1902) in Mytilus galloprovincialis, but he failed to
distinguish cilia from brush-border and so described the ducts as 'durchaus
bewimpert, jedoch ungleichmafiig da das Epithel aus zwei vollkommen verschiedenen Elementen zusammengesetzt ist von denen jedes ungefahr die auf
dem Querschnitt betrachtet auskleidet und jedem Abschnitt ein ganz besonderes Geprage verleiht'. This division of the epithelium of the ducts is well
shown in the Mytilidae and all the Anisomyaria examined. Dissections of the
ducts of fresh specimens of M. edulis indicate that the ciliated groove is
bounded on each side by a longitudinal ridge or typhlosole while the nonFIG. 1 (plate). Sections of the ducts of the digestive diverticula of Mytilus edulis.
A and B, main ducts showing the division of the epithelium into ciliated and non-ciliated
regions. A is a section of a main duct near its junction with the stomach. The typhlosoles are
well developed while the brush-border of the non-ciliated epithelium is obscured by 'bubbling'
of the epithelial surface. B is a section of the distal region of one of the main ducts. The
typhlosoles are smaller than in A.
c, section of a secondary duct showing the well-developed brush-border and the absence of
ciliated cells.
bubbling
epithelium
FIG. I
G. OWEN
I. The Anisomyaria and Eulamellibranchia
519
ciliated portion of the duct has a metallic sheen, possibly due to refraction
from the surface of the brush-border epithelium. The typhlosoles partially
divide the ducts into ciliated and non-ciliated regions, the division being most
marked in the larger ducts near their junction with the stomach (compare
fig. i, A and B).
In M. edulis, the brush-border epithelium consists of cells of varying height
containing spherical nuclei and possessing at the free margin a well-defined
darkly staining region (fig. 1, B). In sections of fixed material the epithelium
frequently includes numerous, apparently empty, vacuoles and the brushborder is often obscured by a characteristic 'bubbling' of the epithelial surface
(fig. 1, A). The outline of the lumen of the non-ciliated portion is undulating
owing to variation in height of the epithelium, and the bubbling cells are
arranged in longitudinal bands coinciding with the taller epithelial cells.
Knight-Jones (1953) described similar bubbling cells in the gut wall of
Saccoglossus but he concluded that they were probably artifacts due to violent
contraction on fixation. In contrast to the brush-border cells, the nuclei of
the ciliated cells are elongate and only occasional vesicular outpushings are
present at the epithelial surface (fig. 1, B). The typhlosoles are formed of tall
ciliated cells, there being a gradual decrease in height of the epithelium toward
the depth of the groove.
The ducts of the digestive diverticula of all the Anisomyaria examined are
similar in cross-section to those of Mytilus edulis but members of the Eulamellibranchia exhibit variation in form both of the ciliated groove and of the brushborder epithelium. In Spisula subtruncata (fig. 2, c) the ciliated groove is well
defined but small relative to the size of the ducts. Numerous mucous glands
staining with alcian blue 8GS (Steedman, 1950) are present in the brushborder epithelium; they are not found in the majority of eulamellibranchs. In
Zirphaea crispata (fig. 2, A) numerous villi project into the lumen of the ducts,
the ciliated tract being situated between two villi. As in Mytilus edulis, however, dissections of these ducts show only two longitudinal typhlosoles, one
on each side of the ciliated tract. In many Eulamellibranchia the ciliated
groove is not as well defined as in the above species and the brush-border
of the non-ciliated cells is frequently obscured by the bubbling appearance
of the epithelial surface. Nevertheless, in all species of Eulamellibranchia
examined ciliated cells were restricted to a well-defined tract.
FIG. Z (plate), A, section of a main duct of the digestive diverticula of Zirphaea crispata
showing the well-developed ciliated tract between two villi.
B, section of a secondary duct of Zirphaea crispata. Cilia are absent. Compare with A.
c, portion of main duct of the digestive diverticula of Spisula subtruncata showing the
ciliated tract.
D, portion of a tubule of Aloidis gibba showing the cilia of the darkly staining cells.
E, portion of a tubule of Ostrea edulis showing the basal granules associated with the darkly
staining cells. Staining of the cilia has been lost in order to differentiate the basal granules
from the cytoplasm of the darkly staining cells.
F, section through the tip of a tubule of Cardium edule showing the network of smooth
muscle fibres. See fig. 6.
Owen—Observations on the Latnellibranchia
520
The transition from the epithelium of the ducts to that of the tubules has
been described as gradual in the Anisomyaria and as sharply defined in the
Eulamellibranchia. A feature overlooked in previous descriptions of the
digestive diverticula, however, is the existence in both the Anisomyaria and
Eulamellibranchia of short, unbranched secondary ducts connecting the main
ducts with the blind-ending tubules. These secondary ducts are non-ciliated,
the cells being similar to those of the brush-border epithelium of the main
ducts, but rarely is there any bubbling of the epithelial surface; the numerous
vacuoles present in the brush-border epithelium of the main ducts are also
brush-border
epithelium
of tubule
epithelium of
secondary duct
FIG. 3. Mytilus edulis, section showing the sharply defined junction of a secondary duct
with a tubule.
absent (figs. 1, c and 2, B). Fig. 3 represents the junction between the epithelium of a secondary duct and of a tubule in M. edulis. The junction of the
brush-border cells with the characteristically vacuolated cells of the tubules is
sharply defined. Similar junctions were found in all the Anisomyaria examined.
Thus, the duct system of the digestive diverticula is made up of main ducts
and secondary ducts differing from one another both in their microscopic
anatomy and their disposition relative to the tubules (fig. 4). The main ducts
possess a ciliated tract. They leave the stomach and ramify through the mass
of the diverticula but never communicate with the blind-ending tubules
except by way of short, unbranched secondary ducts; each secondary duct
leads into a cluster of tubules.
The structure of the tubules of the diverticula has been described by Yonge
(1926a). Prior to this Potts (1923), as a result of observations on living material
of Teredo, described the presence of cilia in both the normal tubules and in
those specialized for the ingestion of wood fragments. He also described a
ciliation similar to that of the normal tubules (these tubules are found exclusively in other lamellibranchs) in both Xylophaga and Pholas. Later, Yonge
(1926 a) observed cilia beating in the tubules of six other species of lamelli-
/ . The Anisomyaria and Eulamellibranchia
521
branchs, but in most cases he was unable to determine whether they were
present or not. Despite these clear indications of the presence of cilia in the
tubules of the diverticula in fresh material, neither Potts nor Yonge was able
to demonstrate them in sections of fixed material. Thus the cilia were described by Potts (1923) as 'easily retractile so that, when ordinary reagents are
used for fixation of material it is impossible to demonstrate them in sections'.
During the present investigations, living material of as many species of
Lamellibranchia as possible was examined. In addition to the three species
diqestive tubules
non-ciliated secondary
ducts
stomach
ciliated main
ducts
FIG. 4. The duct system of the digestive diverticula of the Anisomyaria and Eulamellibranchia shown diagrammatically.
in which Potts (1923) and the six species in which Yonge (1926a) observed
cilia actively beating within the tubules, they were observed in Venerupis
pullastra, Cardiutn edule, C. echinatum, Glossus humanus, Arctica tslandica,
Venus fasciata, V. cassina, Astarte sulcata, Tellina tenuis, Abra alba, Cultellus
pellucidus, Ensis siliqua, Aloidis gibba, Mya arenaria, M. truncata, Lutraria
lutraria, Hiatella arctica, Zirphaea crispata, and Pholas dactylus. In fact, cilia
were observed in all the Eulamellibranchia examined. In species belonging to
the Anisomyaria it was impossible from an examination of living material to
determine whether cilia were present or not. Cilia were observed, however, in
sections of fixed material in both the Anisomyaria and Eulamellibranchia.
Fig. 2, D is a photomicrograph of the cilia of the tubules of Aloidis gibba.
The following description of the cells of the tubules is taken from Yonge
(1926a):
The smaller darkly-staining cells are always present. They are found in small
groups round the lumen in Nucula and all Filibranchs examined. They are often low,
and lie between large vacuolated cells which meet above them and shut them off
from the lumen. In the remaining lamellibranchs the lumen of the tubules is not
5
Owen—Observations on the Lamellibranchia
22
regular, as in the species just considered, but is elliptical, tripartite or in the form of
a cross, with crypts (using the term employed by Gutheil, 1912) at the extremities
of the two, three or four arms respectively in which lie the groups of dark cells.
In all the Eulamellibranchia and in Ostrea edulis in the Anisomyaria, these
groups of darkly staining cells extend the length of the tubules to meet at the
apex. Yonge concluded that they were nests of young cells which, by dividing,
darkly staining cells
cilia
collogenic
layer
crypt
basal granules
nucleus of
amoebocyre
muscle fibres
FIG. 5. Venerupis pullastra, transverse section through a tubule of the digestive diverticula
showing the disposition of cilia and their association with the darkly staining cells of the
crypts.
were able to make good the loss resulting from the casting off of the old cells.
Mitotic figures were frequent in starved animals and always in the crypts.
From an examination of sections of the tubules of numerous species there can
be no doubt that the cilia of the tubules are associated with these darkly
staining cells. It is the dark-staining property of these cells which makes the
cilia so difficult to observe, but with careful differentiation it is possible to
make out either the basal granules, or cilia, or both. Fig. 2, E is a photomicrograph of a portion of a tubule of O. edulis. The crypt has been cut transversely
and a small number of basal granules can be seen near the free border of the
FIG. 6 (plate), A, B, and c are sections of the digestive diverticula showing the network of
smooth muscle fibres surrounding the tubules in A, Cardium edule; B, Ostrea edulis; c, Aloidis
gibba.
D, section of a tubule of Ostrea edulis showing particles of titanium dioxide which have been
ingested by the cells of the tubules.
B, 'excretory' spheres of Cardium edule containing spherules packed with colloidal graphite.
20)j
A-C
D
FIG.
6
G. OWEN
IOJJ
E
/ . The Anisomyaria and Eulamellibranchia
523
darkly staining cells. Staining of the cilia has been lost in order to differentiate
the basal granules from the dark-staining cytoplasm, but the section demonstrates the association of the cilia with the darkly staining cells of the tubules.
A section through a. single tubule of Venerupis pullastra is shown in fig. 5.
In this species the tubules are elliptical with a group of dark cells at each of the
extremities of the long axis. Long, fine cilia extend into the lumen of the
tubule while the basal granules can be seen near the free border of the darkly
staining cells. It is extremely difficult to determine the length of the cilia
accurately since the entire length of a cilium rarely lies in the plane of a single
section, but in V. pullastra and Aloidisgibba they are at least 30fi long. In most
species they are probably somewhat shorter.
In addition to the above observations it was discovered that a system of
smooth muscle fibres surrounds each tubule—a feature which although well
known in the Crustacea does not appear to have been previously described in
the Lamellibranchia. Portions of the diverticula were fixed in Bouin's fluid
and in modified Bouin-Dubosq (Atkins, 1937), and sections cut at 4-6/x were
stained by one of the following methods: Heidenhain's iron haematoxylin,
Heidenhain's Azan, Gurr's (1953) trichrome stain for elastin, smooth muscle,
and collagenic fibres, and Linder's (1949) method for collagen and reticulin.
The results indicated the presence of a variously developed skeletal sheath of
collagen round each tubule immediately beneath the basement membrane
(fig. 5); this layer of collagen is probably comparable with the tunica propria
round the tubules of the digestive gland of Crustacea. Surrounding the
skeletal sheath and partially embedded in it is a system of smooth muscle
fibres forming a meshwork rather like a string bag. This 'string bag' effect is
well shown in photographs of the fibres of Cardium edule (figs. 2, F and 6, A),
Aloidis gibba (fig. 6, c), and Ostrea edulis (fig. 6, B), and these serve better than
any description to illustrate their arrangement. Similar fibres, differing only in
degree of development, were present in all species examined. As will be seen
from fig. 6, A, the fibres are arranged circularly and longitudinally although it
may well be that they have a helicoidal arrangement, the circular fibres being
wound in a close spiral while the longitudinal fibres form an open spiral. It is
interesting to note that the circular fibres in O. edulis (fig. 6, B), unlike the
other two species figured, are better developed than the longitudinal fibres.
FUNCTIONING OF THE STOMACH AND DIGESTIVE DIVERTICULA
Most workers are agreed that digestion in the Lamellibranchia is a combination of intracellular and extracellular processes. The crystalline style projecting
from the style sac into the stomach is the source of extracellular carbohydrases
(Yonge, 19266), and recent work by Mansour-Bek (1948) and George (1952)
suggests that small amounts of extracellular proteases and Upases are also
present although their source is uncertain. The tubules of the digestive diverticula are concerned with the absorption of soluble matter and the ingestion
and intracellular digestion of fine particles; larger particles are ingested and
524
Owen—Observations on the Lamellibranchia
digested by amoebocytes (Yonge, 1926 a and b). Thus, fluid together with fine
particles must be conveyed from the stomach to the digestive diverticula while
waste material, consisting of rejected particles and excretory products, must be
conveyed in the opposite direction, i.e. from the diverticula to the stomach.
The role of the digestive diverticula as organs of intracellular digestion has
recently been questioned (Mansour, 1949). Support for the view of intracellular
digestion is based largely on the results of feeding experiments with Indian
ink, carmine (List, 1902; Vonk, 1924), and iron saccharate (Yonge, 1926a).
Krijgsman (1928) and G. and S. Horstadius (1940) claim that these substances
contain particles small enough to pass through the cells by some physical
means and that phagocytosis, as suggested by Hirsch (1925), should be denned
as a process of ingesting particles larger than o-i/n. To test the phagocytic
properties of the digestive diverticula Zaki (1951) fed animals with powdered
gold-fibrin. No free gold particles were found in the cells of the diverticula
and this was taken to indicate a lack of phagocytic properties on the part of
these cells. Both Mansour and Zaki (1946) claim that the digestive diverticula
are organs of secretion. There can be no doubt, however, from feeding experiments with iron saccharate, that the digestive diverticula are certainly organs
of absorption and that fluid at least must be conveyed from the stomach to the
diverticula.
To determine whether fine particles are also conveyed from the stomach to
the diverticula, there to undergo phagocytosis, animals were fed with a suspension of titanium dioxide which was specially prepared by Acheson Colloids
Ltd. It contained no particles below, 0-5^, the majority being about i-o/x with
occasional particles up to 2-o/x. Animals were placed in suspensions of titanium dioxide in filtered sea-water and fixed after definite periods. Before
fixation, fresh preparations of the diverticula were examined under the
microscope. Four species were used—Ostrea edulis, Venerupis pullastra, Cardiutn edule, and Tellina tenuis—and in each case free particles of titanium
dioxide were present in the lumen of the tubules. From an examination of
fresh material it was impossible to tell whether any particles were present within
the cells or not. In sections of fixed material, however, particles of titanium
dioxide, showing black by transmitted light and a characteristic white by
reflected light, were easily identified within the large vacuolated cells of the
tubules. Fig. 6, D shows a section of the diverticula of Ostrea edulis fixed 3
hours after feeding with titanium dioxide. It is hoped to discuss further details
of this experiment elsewhere, but in so far as it concerns the problem of the
functioning of the diverticula it demonstrates convincingly the passage of
material from the stomach into the diverticula and the ability of the cells of the
tubules to ingest particles larger than o-1 fj, in size. Thus a two-way circulation
must be maintained within the diverticula, fluid and solid particles being
conveyed from the stomach to the tubules while waste materials, with or
without secretions, are conveyed in the opposite direction. Previous workers
(Yonge, 1937) have suggested that cilia maintain a continuous circulation
within the diverticula, while recently Graham (1949), Owen (1953), and
7. The Anisomyaria and Eulamellibranchia
525
Purchon (1955) have all suggested that the part played by muscular activity
in the functioning of the gut of lamellibranchs, and in particular of the
stomach and digestive diverticula, has not been fully appreciated.
Muscular activity
In the Crustacea the tubules of the digestive gland are surrounded by a
network of striated muscles, the passage of fluid from and into the diverticula
being controlled by the pumping action of the walls of the tubules (Pump,
1914). Morse (1902) described similar pumping movements of the 'stomachal
glands' of brachiopods, and sections of Terebratulina retusa show a system of
muscle fibres round the tubules similar to that found in the Lamellibranchia
(personal observation). Thus changes in the contents of the diverticula of
lamellibranchs could be brought about by the contraction and relaxation of
the muscle fibres surrounding the tubules. But pulsations of the tubules were
never observed in living specimens and there is no direct evidence, therefore,
that in the Lamellibranchia these muscle fibres effect changes in volume of
the tubules.
Patterson (1933), working on the comparative physiology of the gastric
hunger mechanism, introduced a rubber balloon into the stomach of the large
gaper clam, Schizothoerus nuttallii, and so recorded pressure changes there.
Slight stomach contractions occurred at the rate of one per minute regardless
of the presence or absence of the crystalline style; the contractions also
continued in the absence of foot movements. Changes in pressure brought
about by contractions of the stomach would aid the entry of material into the
digestive diverticula and an attempt was made to repeat Patterson's experiment with Cardium edule and a different recording apparatus. Slight gastric
contractions were recorded but it was impossible to determine whether these
resulted solely from the irritation of the stomach epithelium or not. Direct
observations of the stomachs of dissected animals did not reveal any rhythmic
contractions and thus there is little to suggest that muscular contractions play
an important part in the circulation of material within the diverticula. In the
larval oyster, particles are drawn into the diverticula and returned to the
stomach by the rhythmic expansion and contraction of the diverticula. But
these are simple and sac-like in the oyster larva and unlike the much divided
structure of the adult (Millar, 1955). Complicated and much-branched diverticula are present in the majority of adult lamellibranchs, and it is unlikely that
rhythmic contractions similar to those which occur in the larva would result
in an efficient circulation of material. Also, it is difficult to visualize such
contractions taking place in species in which the digestive diverticula are
closely invested by the surrounding connective tissue, e.g. Mytilus edulis and
Ostrea edulis.
Ciliary activity
Three species, Mytilus edulis (figs. 1, A and B), Ostrea edulis, and Zirphaea
crispata (fig. 2, A), provided suitable material for determining the pattern of
526
Owen—Observations on the Lamellibranchia
ciliary activity within the main ducts of the digestive diverticula. In all three
species the cilia in the depth of the groove beat along the length of the ducts
and away from the tubules. The cilia on the lower sides of the typhlosoles
beat into the groove while those on the upper sides of the typhlosoles beat
obliquely out of the groove (fig. 7). There are no cilia beating from the stomach
towards the tubules. Although this pattern of ciliary activity was confirmed
in only three species, it is probable that similar currents are present in all
Anisomyaria and Eulamellibranchia where the cilia of the main ducts are
restricted to a well-defined tract.
The cilia of the tubules were observed in fresh preparations of the divertinon-ciliated inhalant
portion of duct
ciliated exhalanr
portion of duct
FIG. 7. A diagrammatic representation of a main duct of the digestive diverticula. Unbroken
arrows indicate the direction of ciliary currents in the exhalant portion of the duct and on the
typhlosoles, while the broken arrow represents the non-ciliary, inhalant, counterpart current.
cula of numerous species of Eulamellibranchia and are interesting in that they
beat with an undulating or sinusoidal movement rather than a simple flexing
or bending motion; they do not exhibit metachronal rhythm nor do they
appear to produce any directive currents. Particles present in the lumen of the
tubules appear to move solely as a result of direct contact with these cilia.
The cilia of the main ducts and of the tubules represent the entire ciliation
of the digestive diverticula, and although there are no cilia in the main ducts
beating towards the blind-ending tubules it is possible that a two-way circulation is maintained within these ducts solely as a result of ciliary activity. In any
closed fluid system, as represented by the digestive diverticula, a ciliary current
in the one direction inevitably carries a counterpart current in the opposite
direction elsewhere. As a consequence of this, material is conveyed out of the
diverticula in the ciliated portion of the main ducts while an inhalant counterpart current carries material in the opposite direction in the non-ciliated portion. The cilia of the typhlosoles, beating into and out of the ciliated groove,
keep the two currents distinct. Fig. 7 is a diagrammatic representation of this
two-way flow within the main ducts. Although there is morphologically only a
single duct it serves functionally as two ducts, the ciliated portion carrying an
exhalant current out of the diverticula and the non-ciliated portion an inhalant
/ . The Anisomyaria and Eulamellibranchia
527
current in the opposite direction. In this way a continuous circulation could be
maintained within the main ducts solely as a result of ciliary activity. This
circulation does not, however, include the lumen of the blind-ending tubules
since between them and the main ducts are the short, non-ciliated secondary
ducts (fig. 4). Interchange of material between the tubules and the main ducts
must result from some means other than ciliary activity.
A feature which has been overlooked in considering the entry of fluid and
particles into the lumen of the tubules is the absorptive function of the large
vacuolated cells. That these cells possess an absorptive function has been
'excretory" spheres
collaqenic layer
muscle fibres
FIG. 8. Cells of a tubule of the digestive diverticula of Cardium edule 6 hours after feeding with
a mixture of titanium dioxide and colloidal graphite.
demonstrated by various workers using carmine, Indian ink (List, 1902; Vonk,
1924), and iron saccharate (Yonge, 1926a). The short, unbranched secondary
ducts open into the non-ciliated portion of the main ducts, and thus absorption
by the cells of the tubules must inevitably result in fresh fluid being drawn into
the lumen of the tubules from the inhalant current of the main ducts. Such a
mechanism explains why only very fine particles are found in the tubules,
since only those remaining in suspension in the fluid medium can enter in
this way; the rate of absorption controls the size of particle available.
A few cells of a tubule of Cardium edule 6 hours after feeding with a mixture
of titanium dioxide and Aquadag (colloidal graphite) are represented in fig. 8.
Similar experiments producing identical results have been described by List
(1902), Vonk (1924), and Yonge (1926a). Only an abbreviated account is therefore presented here. Both Aquadag and titanium dioxide have been ingested
by the cells and are concentrated in spherules 3-5/x in diameter. Some of the
spherules contain titanium dioxide particles together with relatively small
amounts of Aquadag scattered round the periphery while others are a solid
528
Owen—Observations on the Lamellibranchia
black mass of Aquadag so that it is impossible to tell whether they contain
particles of titanium dioxide or not. This concentration of ingested particles
into spherules is also indicated in the photomicrograph of a tubule of Ostrea
edulis which had been fed with titanium dioxide (fig. 6, D). Present in the
lumen of the tubules and of the ducts of Cardium edule were numerous large
spheres, 12-18/x in diameter, enclosing a varying number of spherules containing Aquadag and titanium dioxide and having an appearance similar to those
within the cells of the tubules (figs. 6, E and 8). They are produced by the
^
exhalant ciliary current
^
inhalant counterpart current
movement due to absorption
inhalant portion
of duct -
STOMACH
main duct
exhalant portion
of duct
excretory' sphere
FIG. 9. The probable circulation of fluid and particles within the digestive diverticula of the
Anisomyaria and Eulamellibranchia shown diagrammatically. Unbroken arrows represent
ciliary currents; broken arrows a non-ciliary, inhalant, counterpart current; dotted arrows,
movement due to the absorption of fluid by the cells of the tubules.
fragmentation of the tubule cells and serve to convey indigestible material out
of the diverticula.
How the spheres leave the tubules is not certain. The continual fragmentation of the cells of the tubules could result in previously formed spheres
being pushed through the short secondary ducts and so into the main ducts.
Owing to their spherical form the spheres would not prevent fresh fluid continuing to enter the tubules. Alternatively, the tubules could be emptied by
muscular activity. Dr. C. F. A. Pantin, F.R.S. (personal communication), has
suggested that 'there are strong mechanical reasons for considering that the
circular and longitudinal fibre systems [surrounding the tubules] have distinct
functions. Circular systems of this sort [fig. 6, A] must inevitably be partly, if
not wholly, concerned with tonic balancing of internal pressure while the
I. The Anisomyaria and Eulamellibranchia
529
longitudinal fibres have necessarily much less to do with this and, in species
in which they are well developed [e.g. Cardium edule], their prime function
may be the emptying of the tubules.' Thus, it could be that the longitudinal
fibres, from time to time, contract slowly to empty the contents of the tubules
into the main ducts; the circular fibres, on the other hand, merely balance the
weak pressure resulting from the absorptive functions of the cells.
In fig. 9 an attempt has been made to represent diagrammatically the
suggested circulation of fluid and particles within the digestive diverticula. A
continuous circulation is maintained within the main ducts solely as a result
of ciliary activity, the inhalant counterpart current together with suspended
particles being carried in the non-ciliated portion of the duct. As a consequence
of the absorptive activities of the large vacuolated cells, fluid enters the lumen
of the tubules by way of the short, non-ciliated secondary ducts. Soluble
matter is absorbed and fine particles ingested. Indigestible material and waste
products are concentrated in spherules and returned to the main ducts within
large 'excretory' spheres. These excretory spheres are conveyed out of the
digestive diverticula by the exhalant ciliary currents. The transfer of material
from the inhalant to the exhalant portion probably occurs in the distal regions
of the main.ducts since here the ciliated typhlosoles which separate the two
portions are not as well developed as in the proximal regions (compare fig.
1, A and B). They finally terminate near the distal ends of the main ducts
(fig. 9). Thus the main ducts can be regarded as tubes bent on themselves to
form U-shaped structures within which a continuous circulation is maintained.
The spheres are unable to re-enter the tubules from the main ducts since they
are almost certainly too large to be affected by the relatively weak absorptive
flow. In this way, waste material is separated from the inhalant 'food' particles
and soluble matter.
The stomach
It is now possible to discuss the function of some of the more obvious
morphological features of the stomach. One of the most constant features of
the stomach of the Anisomyaria and Eulamellibranchia is the prominent flaplike major typhlosole which extends from the combined style sac and mid-gut
or, if they are separate, from the mid-gut anteriorly across the floor of the
stomach to enter the caecum or its representative (Graham, 1949). In discussing the lamellibranch stomach Graham points out that 'elaborate mechanisms
are necessary to ensure the movement of food and waste into and out of the
ducts of the diverticula, where absorption and digestion of food particles
occur. This is provided by the complex course and arrangement of the major
typhlosole.' The importance of the major typhlosole in isolating the rejectory
currents of the intestinal groove from the main stomach cavity has already
been emphasized (Owen, 1953) but how the major typhlosole ensures the
entry of material into the diverticula has not been described.
A detailed account of the ciliary currents of the stomach of Zirphaea crispata
has been given by Purchon (1955) and a similar account for the stomach of
Owen—Observations on the Lamellibranchia
530
Glossus humanus by Owen (1953). The ciliary currents of the stomach of
Cardium edule, apart from minor differences, are similar to those of Glossus
humanus. Particles entering the stomach from the oesophagus are caught up
by the revolving crystalline style and brushed on to the ridged surface of the
posterior sorting area. Coarse particles are directed to the mid-gut by way of
the rejection groove and intestinal groove while fine particles are directed first
dorsally into the dorsal hood and then ventrally over the posterior wall of the
stomach. Finally, they are caught up by the strong anteriorly directed currents
of the major typhlosole and conveyed towards the openings of the right and
A
B
aperture leodinq
to stpmach
.». aoerture of duct
C
(c^??^ J K ^
"/M
ANTERIOR
POSTERIOR
major typhlosole
VENTRAL
intestinal groove 5 *
m m
FIG. IO. The right caeca of A, Glossus humanus; B, Cardium edule; c, Zirphaea crispata. Unbroken arrows indicate the course of ciliary currents over the major typhlosole and broken
arrows the rejectory currents of the intestinal groove.
left caeca. In all three species, particles failing to enter the caeca continue
anteriorly to rejoin material entering the stomach from the oesophagus and so
circulate again (fig. 13).
In the caeca of G. humanus, the ciliary currents over the extension of the
major typhlosole are directed from the stomach to the apertures of the ducts of
the diverticula (fig. 10, A). At the entrance to each duct, however, the ciliary
currents are directed at right angles to the long axis of the duct. They do not
convey particles directly into the ducts. In Cardium edule the pattern of ciliaryactivity in this region is more confusing (fig. 10, B). While the general trend
of the currents is towards the apertures of the ducts there are a number of
currents directed away from the ducts. This results in the formation of vortices
where, at least in dissected specimens, particles entering the caecum from the
stomach tend to collect; they do not enter the ducts. In Zirphaea crispata the
pattern of ciliary activity is again a simple one, but, unlike Glossus humanus,
the currents are all directed away from the openings of the ducts and towards
the stomach (fig. 10, c). Thus in all three species ciliary currents do not convey
particles directly into the ducts of the diverticula.
In the Eulamellibranchia the form of the major typhlosole within the caeca
is that of a tube within a tube. Fig. 11 is a diagrammatic representation of the
eulamellibranch caecum of species such as Cardium, Mya, and Zirphaea. The
incomplete inner tube, formed by the major typhlosole, is continuous with the
stomach cavity, while the outer tube formed by the wall of the caecum consti-
/ . The Anisomyaria and Eulamellibranchia
531
tutes the intestinal groove and is therefore continuous with the mid-gut.
Extensions of the typhlosole project a short distance into the non-ciliated
portions of each of the main ducts, while the ciliated portion opens on to the
intestinal groove. As a consequence of this disposition of the major typhlosole
within the caecum, waste material passing out of the diverticula in the ciliated
portions of the main ducts is caught up by the rejectory currents of the intestinal groove, so preventing its return to the circulation within the stomach:
fluid and fine particles conveyed from the stomach to the proximity of the
ducts by the ciliary currents of the major typhlosole are drawn into the non-
main du
sion of typhlosole
into opening of duct
FIG. I I . A diagrammatic representation of the form of the major typhlosole within the caecum
of most eulamellibranchs. Ciliary currents are indicated by arrows. The direction of those over
the major typhlosole will vary in the different species (see fig. 10).
ciliated portions of the main ducts by the inhalant counterpart current. Thus
the extension of the typhlosole into the caecum acts as a valve, allowing fluid
and fine particles to pass from the stomach into the ducts but preventing the
passage of material in the opposite direction. The muscular properties of the
short extensions of the typhlosole into the openings of the ducts were noted
by Graham (1949) who described them as performing 'slight twistings, expansions and contractions so that the whole structure has a gently lobed edge,
the size and disposition of the lobes continuously undergoing slight changes'.
Zirphaea crispata differs from Cardium edule and Glossus humanus in that the
ciliary currents over the extension of the major typhlosole are directed away
from the apertures of the ducts, but, as shown in fig. 10, c, the typhlosole is
long and strap-shaped and an inhalant counterpart current almost certainly
operates within the caecum as well as the non-ciliated portions of the main
ducts. As suggested by Purchon (1955), the ciliary currents of Zirphaea
crispata probably prevent blocking of the ducts and caeca, particles failing to
remain in suspension in the fluid medium being returned to the stomach.
Blocking is unlikely to occur in Glossus humanus where the stomach
532
Owen—Observations on the Lamellibranchia
undoubtedly possesses a more efficient sorting mechanism and only relatively
fine particles are ingested (Owen, 1953).
Thus the pattern of ciliary activity within the eulamellibranch stomach is
such that fluid and fine particles are conveyed to the proximity of the apertures of the caeca and ducts rather than directly into the ducts. Material is
drawn into the non-ciliated portion of the main ducts by the inhalant counterpart current. Although the details are different, this is also the case in the
Anisomyaria. The ciliary currents at the junction of a main duct with the
stomach in Ostrea edulis and Mytilus edulis are represented diagrammatically
in fig. 12 (in these species extensions of the major typhlosole do not project
into the apertures of the ducts). The ciliated epithelium of the stomach
/DIGESTIVE DIVERTICULAN
non-ciliated inhalant
ciliated exhalant
portion of duct
portion of duct
FIG. 12. Anisomyaria, junction of a main duct of the digestive diverticula with the stomach,
shown diagrammatically. The unbroken arrows represent ciliary currents, the broken arrows
an inhalant, counterpart current.
produces a current which sweeps across the mouth of the duct from the nonciliated, inhalant side to the ciliated, exhalant side. Thus fresh fluid and
particles are continuously drawn into the non-ciliated portion of the ducts
while waste material is carried away from the ciliated portion.
In both the Anisomyaria and Eulamellibranchia yet another function may
be attributed to the major typhlosole. At the junction of the mid-gut with the
postero-ventral region of the stomach, the flap-like major typhlosole arches
over the opening to the mid-gut, so preventing material from entering the
latter except by the intestinal groove. It is interesting to note that in many
species the opening of the oesophagus into the stomach is also protected, being
slit-like and situated between fleshy, protuberant lips; a feature which reduces
the possibility of regurgitation of the stomach contents. Thus, of the three
possible exits from the stomach (the oesophagus, the mid-gut, and the ducts
of the diverticula), two (the oesophagus and mid-gut) are 'guarded'.
A diagrammatic representation of the suggested functioning of the stomach
and digestive diverticula of the Eulamellibranchia is shown in fig. 13. Particles
entering the stomach from the oesophagus are subjected to a sorting mechanism and to the action of extracellular enzymes present in the stomach. Coarse
particles are directed to the mid-gut by way of the intestinal groove and fine
7. The Anisomyaria and Eulamellibranchia
533
particles and digested foodstuffs towards the openings of the caeca. Within
the main ducts of the diverticula a continuous circulation is maintained by
ciliary activity, material being carried out of the diverticula in the ciliated
groove of the main ducts while free particles and digested foodstuffs are
carried into the diverticula in the non-ciliated portion by the inhalant counterpart current. Fluid and fine particles are drawn into the tubules by way of the
posterior sortinq
area vv ^
dorsal hood
r
^ fine particles
ciliary currenh-|^»^^^ coarse particles
{£
^ waste material
inhalant counterpart
o
current
>[
movement due to
...r f f lne particles
absorption
"J
rejection tract
crystalline
style •
oesophagus
major typhlosole
style sac
diqestive tubules
FIG. 13. A diagrammatic representation of the probable circulation of material within the
stomach and digestive diverticula of most Eulamellibranchia. Heavy arrows represent coarse
particles; fine arrows, fine particles; tailed arrows, waste material from the digestive diverticula; broken arrows, an inhalant counterpart current; dotted arrows, the absorptive activity
of the cells of the tubules resulting in the movement of fluid and fine particles into the lumen
of the tubules.
non-ciliated secondary ducts by the absorptive activity of the large vacuolated
cells. Indigestible material and waste products are returned to the main ducts
enclosed in large spheres formed by the fragmentation of the cells and are
carried out of the diverticula by the cilia of the main ducts.
DISCUSSION
Many workers have attempted to discover the food of lamellibranchs by
examining the stomach and faecal contents. Results have been confusing
534
Owen—Observations on the Lamellibranchia
owing largely to a failure to understand the functional morphology of the
stomach of bivalves. Recently Mansour (1946a), after examining the stomach
contents of a number of bivalves, claimed that animal plankton constitutes a
much greater part of the food than is generally accepted and that this fact has
been overlooked owing to the very rapid digestion of the animal food. As
support for rapid digestion and in the belief that, as in many animals, food is
propelled along the digestive tract of lamellibranchs at a rate which permits
effective digestion, Mansour (1949) adds that the time taken for material to
pass through the whole of the gut of Unto prasidens may be as short as 2 hours.
Mansour-Bek (1948) tested filtered stomach juice for the presence of extracellular lipases and proteases. She obtained positive results but found that the
strength of the enzymes as indicated by experiments did not correspond to
the quick disintegration recorded by Mansour. Rosen (1949) also obtained
only a weak protease reaction with filtered stomach juice. He suggests that
this extracellular protease is much too weak to account for all the splitting of
proteins and that the abundant occurrence of cathepsin bound intracellularly
in the digestive diverticula indicates a considerable intracellular digestion of
proteins.
The very rapid rate of digestion recorded by Mansour and the weak proteolytic and lipolytic enzymes present in the stomach would appear to be
contradictory. The rate of passage of material through the gut of lamellibranchs, however, bears little relation to the effective duration of enzyme
action (Yonge, 1937). The flap-like major typhlosole extending over the
opening of the mid-gut prevents material entering the latter except by the
intestinal groove, and this material is derived from the sorting areas of
the stomach and the exhalant currents of the digestive diverticula (fig. 13).
Material rejected by the sorting areas passes rapidly into the mid-gut and so
to the anus and as a consequence of this may pass through the entire alimentary canal in a very short time (from personal observation material may pass
through the entire gut of Cardium edule within 45 minutes). This feature of the
lamellibranch stomach explains why several workers have observed that at
least part of the 'food' of lamellibranchs passes through the alimentary canal
unchanged. In addition to material which passes rapidly out of the stomach,
particles too large to enter either the digestive diverticula or the intestinal
groove may also be swallowed, since, under certain conditions, the gills and
palps may pass into the mouth relatively large particles (Nelson, 1933)Yonge (1949) measured the maximum size of sand grains found in the pseudofaeces, the stomach, and faecal pellets of Tellina tenuis. The measurements were
400n, 320JU., and 80fj, respectively, but particles of 80/u. were rare in the faecal
pellets, the majority not exceeding 40/x. Thus only particles a quarter of the
diameter of the largest particles found in the stomach appear in the faeces.
This retention of large particles in the stomach may explain the disintegrating
zooplankton observed by Nelson (1933) and Mansour (1946a). The zooplankton organisms recorded by them were of relatively large size, e.g.
nematodes, copepods, larvae of Crustacea, &c, and the flap-like major
/ . The Anisomyaria and Eulamellibranchia
535
typhlosole 'guarding' the entrance to the intestine would prevent such large
objects entering the mid-gut (fig. 13). They must, therefore, remain in the
gastric cavity until disintegrated. This could result from the triturating effect
of the crystalline style (Yonge, 1949) aided possibly by any weak proteases and
Upases present in the stomach cavity.
Mansour and Zaki (1946) injected into the blood of starved specimens of
Unio prasidens a solution of chlorophyll, and subsequent examination revealed
the presence of coloured globules in the large vacuolated cells of the tubules;
similar globules were later found in the faeces. This was taken as evidence of
the secretory function of the diverticula, the cells fragmenting and passing,
together with their contained enzymes, into the stomach. Yonge (1946) remains convinced that the digestive diverticula are organs of intracellular
digestion. He suggests that, while true extracellular lipases and proteases are
absent from the gut of lamellibranchs, small amounts of these enzymes may
occur owing to the presence in the lumen of numerous amoebocytes.
To test the phagocytic properties of the cells of the diverticula, Zaki (1951)
fed U. prasidens with powdered gold-fibrin. No free gold particles were found
in the cells of the diverticula and he assumed that these cells did not possess
phagocytic properties. Similar techniques have been used by G. and S.
Horstadius (1940) using gold-gelatine and gold-fibrin, and Rosen (1941) using
carbon-casein, the use of carmine and Indian ink having been criticized by
Hirsch (1925) and Krijgsman (1928) on the grounds that they contain diffusible
particles. Horstadius's experiments with Helix pomatia were negative, free
gold particles being absent from the cells of the digestive gland. This experiment was repeated by Rosen (1951) with similar results, but he found that
'the gold-fibrin had not passed up into the mid-gut gland', probably because
the gold-fibrin contained particles larger than the cells themselves. Feeding
experiments with carbon-casein, whipped pigeon's blood, and edestin demonstrated that phagocytosis does take place in the cells of the digestive gland of
H. pomatia. Rosen states that there is thus 'not only a lower critical limit but
also an upper critical limit' for the size of particle ingested by phagocytosis.
Certainly this is true of the Anisomyaria and Eulamellibranchia, where only
very fine particles enter the tubules of the diverticula. Zaki (1951) in his
account of feeding Unio prasidens with gold-fibrin does not give measurements
of particle size. Furthermore, the animals were fed by mouth with a fine
pipette, so increasing the possibility of using particles larger than those
normally ingested. The positive results obtained with titanium dioxide provide clear evidence that the cells of the tubules ingest particles larger than
O-I/JL.
Irrespective of whether the digestive diverticula are organs of secretion,
feeding experiments with iron saccharate and other substances provide ample
evidence that they are organs of absorption. Thus a two-way circulation is
necessary to ensure the movement of the products of extracellular digestion
into, and waste (with or without secretions) out of, the ducts of the diverticula.
A continuous circulation is maintained within the main ducts solely as a result
536
Owen—Observations on the Lamellibranchia
of ciliary activity, but the entry of fluid into the blind-ending tubules is
possibly a consequence of the absorptive functions of the large vacuolated
cells. The entry of fresh fluid into the tubules will in this case be dependent
on the rate of absorption, and though the resultant movement of fluid will be
slow, solid particles able to remain in suspension in the fluid medium will also
enter the lumen of the tubules. To prevent blocking of the tubules indigestible
particles must be returned to the stomach on the way to the mid-gut and
prevented from re-entering the tubules. As demonstrated by feeding experiments with titanium dioxide, the large vacuolated cells phagocytose solid
particles present in the lumen of the tubules, the ingested particles being then
concentrated in spherules and returned to the main ducts enclosed in large
excretory spheres formed by the fragmentation of the cells. These spheres are
conveyed out of the diverticula by the cilia of the main ducts. Thus in addition
to an absorptive function, the large vacuolated cells of the tubules also serve
an excretory function, which is an essential feature of their phagocytic
properties.
The results obtained by Mansour-Bek (1948), Rosen (1949), and George
(1952) indicate that weak extracellular proteases and lipases are present in the
stomach although their source is uncertain. Preliminary extracellular digestion
would certainly increase the amount of finely divided material received into
the digestive diverticula for absorption and the completion of digestion intracellularly. The excretory spheres produced by the fragmentation of the tubule
cells may contain proteases and lipases, and traces of these enzymes could be
liberated into the stomach in this way, as suggested by Morton (1951) for the
gastropod Struthiolaria. In both the Anisomyaria and Eulamellibranchia,
however, such an explanation is improbable since the ciliary mechanisms of
the stomach would appear to prevent material carried by the intestinal groove
from returning to the general circulation within the stomach (Owen, 1953).
Yonge's (1946) suggestion that traces of proteolytic and lipolytic enzymes are
liberated into the lumen of the gut as a result of cytolysis of phagocytes would
appear more likely.
This account of the functioning of the stomach and digestive diverticula of
the Anisomyaria and Eulamellibranchia is intended to be of general interest,
there being little doubt that individual species vary, particularly in the relative
development and importance of muscular activity: e.g. the Anomalodesmata,
in which the diverticula are relatively simple and sac-like. There would also
appear to be some variation in the form and disposition of the major typhlosole, e.g. Pandora (Allen, 1954). There can be little doubt, however, that the
organization of the stomach and diverticula in the majority of Anisomyaria
and Eulamellibranchia is directed towards efficient intracellular digestion and
that 'secretions' by the cells of the diverticula can play little part in the
processes of digestion in these animals.
I should like to express my gratitude to Professor C. M. Yonge, F.R.S., for
his continued encouragement and guidance, to Dr. H. F. Steedman for
/ . The Anisomyaria and Eulamellibranchia
537
technical assistance, and to Dr. C. F. A. Pantin, F.R.S., who provided ideas
and criticisms of great value. Acknowledgement is also due to the Carnegie
Trust for financial assistance.
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