[Illustration: FIG. 19.--_Magellania [Waldheimia] flavescens_. Interior
of dorsal valve, to show the position of the labial appendages. v,
Mouth. (A portion of the fringe of cirri is removed to show the brachial
membrane and a portion of the spiral extremities of the arms.)]
Each valve of the shell is lined by a mantle which contains
prolongations of the body cavity. The outer surfaces of the mantle
secrete the shell, which is of the nature of a cuticle impregnated by
calcareous salts. These often have the form of prisms of calcite
surrounded by a cuticular mesh work; the whole is nourished and kept
alive by processes, which in _Crania_ are branched; these perforate the
shell and permit the access of the coelomic fluid throughout its
substance. These canals are closed externally and are absent in
_Rhynchonella_, where the amount of calcareous deposit is small. In
_Lingula_ the shell is composed of alternate layers of chitin and of
phosphate of lime. The free edges of the mantle often bear chitinous
bristles or setae which project beyond the shell. As in the case of the
Lamellibranchiata, the shell of the adult is not a direct derivative of
the youngest shell of the larva. The young Brachiopod in all its species
is protected by an embryonic shell called the "protegulum," which
sometimes persists in the umbones of the adult shells but is more
usually worn off. In all species it has the same shape, a shape which
has been retained in the adult by the Lower Cambrian genus _Iphidea_.
[Illustration: FIG. 20.--_Magellania [Waldheimia] flavescens_.
Logitudinal section with a portion of the animal.
d, h, Brachial appendages.
a, Adductor
c, c', Divaricator muscles.
s, Septum.
v, Mouth.
z, Exremity of alimentary tube. The penduncular muscules have been
purposely omitted.]
The body of the Brachiopod usually occupies about the posterior half of
the space within the shell. The anterior half of this space is lined by
the inner wall of the mantle and is called the mantle cavity. This
cavity lodges the arms, which are curved and coiled in different ways in
different genera. The water which bears the oxygen for respiration and
the minute organisms upon which the Brachiopod feeds is swept into the
mantle cavity by the action of the cilia which cover the arms, and the
eggs and excreta pass out into the same cavity. The mouth lies in the
centre of the anterior wall of the body. Its two lips fusing together at
the corners of the mouth are prolonged into the so-called arms. These
arms, which together form the lophophore, may be, as in _Cistella_,
applied flat to the inner surface of the dorsal mantle fold, but more
usually they are raised free from the body like a pair of moustaches,
and as they are usually far too long to lie straight in the mantle
cavity, they are folded or coiled up. The brachial skeleton which in
many cases supports the arms has been mentioned above.
[Illustration: FIG. 21.--A diagram of the left half of an _Argiope_
(_Megathyris_), which has been bisected in the median plane.
1. The ventral valve.
2. The dorsal valve.
3. The pedicle.
4. The mouth.
5. Lip which overhangs the mouth and runs all round the lophophore.
6. Tentacles.
7. Ovary in dorsal valve.
8. Liver diverticula.
9. Occlusor muscle--its double origin is shown.
10. Internal opening of left nephridium.
11. External opening of the same.
12. Ventral adjustor.
13. Divaricator muscle.
14. Sub-oesophageal nerve ganglion.
15. The heart.
16. Dorsal adjustor muscle.]
A transverse section through the arm (fig. 22) shows that it consists of
a stout base, composed of a very hyaline connective tissue not uncommon
in the tissues of the Brachiopoda, which is traversed by certain canals
whose nature is considered below under the section (_The Body Cavity_)
devoted to the coelom. Anteriorly this base supports a gurrie or gutter,
the pre-oral rim of which is formed by a simple lip, but the post-oral
rim is composed of a closely set row of tentacles. These may number some
thousands, and they are usually bent over and tend to form a closed
cylinder of the gutter. Each of these tentacles (fig. 22) is hollow, and
it contains a diverticulum from the coelom, a branch of the vascular
system, a nerve and some muscle-fibres. Externally on two sides and on
the inner surface the tentacles are ciliated, and the cilia are
continued across the gutter to the lip and even on the outer surface of
the latter. These cilia pass on any diatoms and other minute organism
which come within their range of action to the capacious oval mouth,
which appears as a mere deepening of the gutter in the middle line. In
_Terebratulina, Rhynchonella, Lingula_, and possibly other genera, the
arms can be unrolled and protruded from the opened shell; in this case
the tentacles also straighten themselves and wave about in the water.
_The Body Cavity._--The various internal organs of the brachiopod
body, the alimentary canal and liver, the excretory organs, the heart,
numerous muscles and the reproductive organs, are enclosed in a cavity
called the body cavity, and since this cavity (i.) is derived from the
archicoel and is from the first surrounded by meroblast, (ii.)
communicates with the exterior through the nephridia or excretory
organs, and (iii.) gives rise by the proliferation of the cells which
line it to the ova and spermatoza, it is of the nature of a true
coelom. The coelom then is a spacious chamber surrounding the
alimentary canal, and is continued dorsally and ventrally into the
sinuses of the mantle (fig. 21). Some of the endothelial cells lining
the coelom are ciliated, the cilia keeping the corpusculated fluid
contents in movement. Others of the endothelial cells show a great
tendency to form muscle fibres. Besides this main coelomic cavity
there are certain other spaces which F. Blochmann regards as coelomic,
but it must be remembered that his interpretation rests largely on
histological grounds, and at present embryological confirmation is
wanting. These spaces are as follows:--(i.) the great arm-sinus; (ii.)
the small arm-sinus together with the central sinus and the
peri-oesophageal sinus, and in _Discinisca_ and _Lingula_, and, to a
less extent, in _Crania_, the lip-sinus; (iii.) certain portions of
the general body cavity which in _Crania_ are separated off and
contain muscles, &c.; (iv.) the cavity of the stalk when such exists.
The great arm-sinus of each side of the lophophore lies beneath the
fold or lip which together with the tentacles forms the ciliated
groove in which the mouth opens. These sinuses are completely shut off
from all other cavities, they do not open into the main coelomic space
nor into the small arm-sinus, nor does the right sinus communicate
with the left. The small arm-sinus runs along the arms of the
lophophore at the base of the tentacles, and gives off a blind
diverticulum into each of these. This diverticulum contains the
blood-vessel and muscle-fibres (fig. 22). In the region of the mouth
where the two halves of the small arm-sinus approach one another they
open into a central sinus lying beneath the oesophagus and partly
walled in by the two halves of the ventral mesentery. This sinus is
continued round the oesophagus as the peri-oesophageal sinus, and thus
the whole complex of the small arm-sinus has the relations of the
so-called vascular system of a Sipunculid. In _Crania_ it is
completely shut off from the main coelom, but in _Lingula_ it
communicates freely with this cavity. In _Discinisca_ and _Lingula_
there is further a lip-sinus or hollow system of channels which
traverses the supporting tissue of the edge of the mantle and contains
muscle-fibres. It opens into the peri-oesophageal sinus. It is better
developed and more spacious in _Lingula_ than in _Discinisca._ In
_Crania_, where only indications of the lip-sinus occur, there are two
other closed spaces. The posterior occlusor muscles lie in a special
closed space which Blochmann also regards as coelomic. The posterior
end of the intestine is similarly surrounded by a closed coelomic
space known as the peri-anal sinus in which the rectum lies freely,
unsupported by mesenteries. All these spaces contain a similar
coagulable fluid with sparse corpuscles, and all are lined by ciliated
cells. There is further a great tendency for the endothelial cells to
form muscles, and this is especially pronounced in the small
arm-sinus, where a conspicuous muscle is built up. The mantle-sinuses
which form the chief spaces in the mantle are diverticula of the main
coelomic cavity. In _Discinisca_ they are provided with a muscular
valve placed at their point of origin. They contain the same fluid as
the general coelom. The stalk is an extension of the ventral
body-wall, and contains a portion of the coelom which, in _Discinisca_
and _Lingula_, remains in communication with the general body cavity.
[Illustration: FIG. 22.--Diagrammatic section through an arm of the
lophophore of _Crania_. Magnified; after Blochmann.
1. The lip.
2. The base of a tentacle bisected in the middle line.
3. Great arm-sinus.
4. Small arm-sinus, containing muscle-fibres.
5. Tentacular canal.
6. External tentacular muscle.
7. Tentacular blood-vessel arising from the cut arm-vessel in the
small arm-sinus.
8. Chief arm-nerve.
9. Secondary arm-nerve.
10. Under arm-nerve.]
_The Alimentary Canal_.--The mouth, which is quite devoid of armature,
leads imperceptibly into a short and dorsally directed oesophagus. The
latter enlarges into a spherical stomach into which open the broad
ducts of the so-called liver. The stomach then passes into an
intestine, which in the Testicardines (Articulata) is short,
finger-shaped and closed, and in the Ecardines (Inarticulata) is
longer, turned back upon its first course, and ends in an anus. In
_Lingula_ and _Discina_ the anus lies to the right in the
mantle-cavity, but in _Crania_ it opens medianly into a posterior
extension of the same. Apart from the asymmetry of the intestine
caused by the lateral position of the anus in the two genera just
named, Brachiopods are bilaterally symmetrical animals.
The liver consists of a right and left half, each opening by a broad
duct into the stomach. Each half consists of many lobes which may
branch, and the whole takes up a considerable proportion of the space
in the body cavity. The food passes into these lobes, which may be
found crowded with diatoms, and without doubt a large part of the
digestion is carried on inside the liver. The stomach, oesophagus and
intestine are ciliated on their inner surface. The intestine is slung
by a median dorsal and ventral mesentery which divides the body cavity
into two symmetrically shaped halves; it is "stayed" by two transverse
septa, the anterior or gastroparietal band running from the stomach to
the body wall and the posterior or ileoparietal band running from the
intestine to the body wall. None of these septa is complete, and the
various parts of the central body cavity freely communicate with one
another. In _Rhynchonella_, where there are two pairs of kidneys, the
internal opening of the anterior pair is supported by the
gastroparietal band and that of the posterior pair by the ileoparietal
band. The latter pair alone persists in all other genera.
The kidneys or nephridia open internally by wide funnel-shaped
nephridiostomes and externally by small pores on each side of the
mouth near the base of the arms. Each is short, gently curved and
devoid of convolutions. They are lined by cells charged with a yellow
or brown pigment, and besides their excretory functions they act as
ducts through which the reproductive cells leave the body.
_Circulatory System._--The structures formerly regarded as
pseudohearts have been shown by Huxley to be nephridia; the true heart
was described and figured by A. Hancock, but has in many cases escaped
the observation of later zoologists. F. Blochmann in 1884, however,
observed this organ in the living animal in species of the following
genera:--_Terebratulina, Magellania_ [_Waldheimia_]_, Rhynchonella,
Megathyris_ (_Argiope_), _Lingula_, and _Crania_ (fig. 21). It
consists of a definite contractile sac or sacs lying on the dorsal
side of the alimentary canal near the oesophagus, and in preparations
of _Terebratulina_ made by quickly removing the viscera and examining
them in sea-water under a microscope, he was able to count the
pulsations, which followed one another at intervals of 30-40 seconds.
[Illustration: FIG. 23.--_Rhynchonella_ (_Hemithyris_) _psittacea._
Interior of dorsal valve, s, Sockets; b, dental plates; V, mouth;
de, labial appendage in its natural position; d, appendage extended
or unrolled.]
A vessel--the dorsal vessel--runs forward from the heart along the
dorsal surface of the oesophagus. This vessel is nothing but a split
between the right and left folds of the mesentery, and its cavity is
thus a remnant of the blastocoel. A similar primitive arrangement is
thought by F. Blochmann to obtain in the genital arteries. Anteriorly
the dorsal vessel splits into a right and a left half, which enter the
small arm-sinus and, running along it, give off a blind branch to each
tentacle (fig. 21). The right and left halves are connected ventrally
to the oesophagus by a short vessel which supplies these tentacles in
the immediate neighbourhood of the mouth. There is thus a vascular
ring around the oesophagus. The heart gives off posteriorly a second
median vessel which divides almost at once into a right and a left
half, each of which again divides into two vessels which run to the
dorsal and ventral mantles respectively. The dorsal branch sends a
blind twig into each of the diverticula of the dorsal mantle-sinus,
the ventral branch supplies the nephridia and neighbouring parts
before reaching the ventral lobe of the mantle. Both dorsal and
ventral branches supply the generative organs.
The blood is a coagulable fluid. Whether it contains corpuscles is not
yet determined, but if so they must be few in number. It is a
remarkable fact that in _Discinisca_, although the vessels to the
lophophore are arranged as in other Brachiopods, no trace of a heart
or of the posterior vessels has as yet been discovered.
_Muscles._--The number and position of the muscles differ materially
in the two great divisions into which the Brachiopoda have been
grouped, and to some extent also in the different genera of which each
division is composed. Unfortunately almost every anatomist who has
written on the muscles of the Brachiopoda has proposed different names
for each muscle, and the confusion thence arising is much to be
regretted. In the Testicardines, of which the genus _Terebratula_ may
be taken as an example, five or six pairs of muscles are stated by A.
Hancock, Gratiolet and others to be connected with the opening and
closing of the valves, or with their attachment to or movements upon
the peduncle. First of all, the adductors or occlusors consist of two
muscles, which, bifurcating near the centre of the shell cavity,
produce a large quadruple impression on the internal surface of the
small valve (fig. 13, a, a'), and a single divided one towards the
centre of the large or ventral valve (fig. 12, a). The function of
this pair of muscles is the closing of the valves. Two other pairs
have been termed _divaricators_ by Hancock, or _cardinal muscles_
("muscles diducteurs" of Gratiolet), and have for function the opening
of the valves. The divaricators proper are stated by Hancock to arise
from the ventral valve, one on each side, a little in advance of and
close to the adductors, and after rapidly diminishing in size become
attached to the cardinal process, a space or prominence between the
sockets in the dorsal valve. The _accessory divaricators_ are,
according to the same authority, a pair of small muscles which have
their ends attached to the ventral valve, one on each side of the
median line, a little behind the united basis of the adductors, and
again to the extreme point of the cardinal process. Two pairs of
muscles, apparently connected with the peduncle and its limited
movements, have been minutely described by Hancock as having one of
their extremities attached to this organ. The _dorsal adjusters_ are
fixed to the ventral surface of the peduncle, and are again inserted
into the hinge-plate in the smaller valve. The _ventral adjusters_ are
considered to pass from the inner extremity of the peduncle, and to
become attached by one pair of their extremities to the ventral valve,
one on each side and a little behind the expanded base of the
divaricators. The function of these muscles, according to the same
authority, is not only that of erecting the shell; they serve also to
attach the peduncle to the shell, and thus effect the steadying of it
upon the peduncle. By alternate contracting they can cause a slight
rotation of the animal in its stalk.
[Illustration: FIG. 24.--_Magellania [Waldheimia] flavescens_. Diagram
showing the muscular system. (After Hancock.)
M, Ventral,
N, Dorsal valve,
l, Loop.
V, Mouth.
Z, Extremity of intestine,
c, Divaricators.
c', Accessory divaricators.
a, Adductor.
b, Ventral adjusters.
b', Peduncular muscles.
b'', Dorsal adjusters.
P, Peduncle.]
Such is the general arrangement of the shell muscles in the division
composing the articulated Brachiopoda, making allowance for certain
unimportant modifications observable in the animals composing the
different families and genera thereof. Owing to the strong and tight
interlocking of the valves by the means of curved teeth and sockets,
many species of Brachiopoda could open their valves but slightly. In
some species, such as _Thecidea_, the animal could raise its dorsal
valve at right angles to the plane of the ventral one (fig. 4).
[Illustration: FIGS. 25, 26. _Lingula anatina._
25, Interior of ventral valve.
26, Interior of dorsal valve.
g, Umbonal muscular impressions (open valves).
h, Central muscles (close valves).
i, Transmedial or sliding muscles.
b, Parietal band.
j, k, l, Lateral muscles (j, anteriors; k, middles; l, outsiders),
enabling the valves to move forward and backward on each other.
(After King.)]
In the Ecardines, of which _Lingula_ and _Discina_ may be quoted as
examples, the myology is much more complicated. Of the shell or
valvular muscles W. King makes out five pairs and an odd one, and
individualizes their respective functions as follows:--Three pairs are
_lateral_, having their members limited to the sides of the shell; one
pair are _transmedians_, each member passing across the middle of the
reverse side of the shell, while the odd muscle occupies the umbonal
cavity. The _central_ and _umbonal_ muscles effect the direct opening
and closing of the shell, the _laterals_ enable the valves to move
forward and backward on each other, and the _transmedians_ allow the
similar extremities (the rostral) of the valves to turn from each
other to the right or the left on an axis subcentrically situated,
that is, the medio-transverse region of the dorsal valve. It was long
a matter in discussion whether the animal could displace its valves
sideways when about to open its shell, but this has been actually
observed by Professors K. Semper and E.S. Morse, who saw the animal
perform the operation. They mention that it is never done suddenly or
by jerks, as the valves are at first always pushed to one side several
times and back again on each other, at the same time opening gradually
in the transverse direction till they rest opposite to one another and
widely apart. Those who have not seen the animal in life, or who did
not believe in the possibility of the valves crossing each other with
a slight obliquity, would not consent to appropriating any of its
muscles to that purpose, and consequently attributed to all the
lateral muscles the simple function of keeping the valves in an
opposite position, or holding them adjusted. We have not only the
observations of Semper and Morse, but the anatomical investigations of
King, to confirm the sliding action or lateral divarication of the
valves of _Lingula_.
[Illustration: FIG. 27.--_Lingula anatina_.
Diagram showing the muscular system. (After Hancock.) The letters
indicate the muscles as in figs. 25 and 26.
A, Dorsal,
B, Ventral valve.
p, Peduncle.
e, Heart.
a, Alimentary tube.
z, Anal aperture.]
In the Testicardines, where no such sliding action of the valves was
necessary or possible, no muscles for such an object were required,
consequently none took rise from the lateral portions of the valves as
in _Lingula_; but in an extinct group, the _Trimerellidae_, which
seems to be somewhat intermediate in character between the Ecardines
and Testicardines, have been found certain scars, which appear to have
been produced by rudimentary lateral muscles, but it is doubtful
(considering the shells are furnished with teeth, though but rudely
developed) whether such muscles enabled the valves, as in _Lingula_,
to move forward and backward upon each other. _Crania_ in life opens
its valves by moving upon the straight hinge, without sliding the
valve.
The _nervous system_ of Brachiopods has, as a rule, maintained its
primitive connexion with the external epithelium. In a few places it
has sunk into the connective-tissue supporting layer beneath the
ectoderm, but the chief centres still remain in the ectoderm, and the
fibrils forming the nerves are for the most part at the base of the
ectodermal cells. Above the oesophagus is a thin commissure which
passes laterally into the chief arm-nerve. This latter includes in its
course numerous ganglion cells, and forms, according to F. Blochmann,
the immensely long drawn out supra-oesophageal ganglion. The chief
arm-nerve traverses the lophophore, being situated between the great
arm-sinus and the base of the lip (figs. 22 and 28); it gives off a
branch to each tentacle, and these all anastomose at the base of the
tentacles with the second nerve of the arm, the so-called secondary
arm-nerve. Like the chief arm-nerve, this strand runs through the
lophophore, parallel indeed with the former except near the middle
line, where it passes ventrally to the oesophagus. The lophophore is
supplied by yet a third nerve, the under arm-nerve, which is less
clearly defined than the others, and resembles a moderate aggregation
of the nerve fibrils, which seem everywhere to underlie the ectoderm,
and which in a few cases are gathered up into nerves. The under
arm-nerve, which lies between the small arm-sinus and the surface,
supplies nerves to the muscles of both arm-sinuses (figs. 22 and 28).
Medianly, it has its origin in the sub-oesophageal ganglion, which,
like the supra-oesophageal, is drawn out laterally, though not to the
same extent. In the middle line the sub-oespphageal nerve mass is
small; the ganglion is in fact drawn out into two halves placed on
either side of the body. From each of these sub-oesophageal ganglia
numerous nerves arise. Passing from the middle line outwards they
are--(i.) the median pallial nerve to the middle of the dorsal mantle;
(ii.) numerous small nerves--the circum-oesophageal commissures--which
pass round the oesophagus to the chief arm-nerve or supra-oesophageal
ganglion; (iii.) the under arm-nerve to the lophophore and its
muscles; (iv.) the lateral pallial nerve to the sides of the dorsal
mantle. Laterally, the sub-oesophageal ganglia give off (v.) nerves to
the ventral mantle, and finally they supply (vi.) branches to the
various muscles. There is a special marginal nerve running round the
edge of the mantle, but the connexion of this with the rest of the
nervous system is not clear; probably it is merely another
concentration of the diffused sub-ectodermal nervous fibrils.
The above account applies more particularly to _Crania_, but in the
main it is applicable to the other Inarticulata which have been
investigated. In _Discinisca_ and _Lingula_, however, the
sub-oesophageal ganglion is not drawn out, but lies medianly; it gives
off two posteriorly directed nerves to the stalk, which in _Lingula_
unite and form a substantial nerve. Sense organs are unknown in the
adult. The larval forms are provided with eye-spots, but no very
specialized sense organs are found in the adult.
[Illustration: FIG. 28.--Diagram of nervous system of _Crania_; from
the dorsal side. The nerves running to the dorsal parts are white,
with black edges; those running to the ventral parts are solid black.
Magnified. (After Blochmann.)
1. Oesophagus.
2. Supra-oesophageal commisure.
3. Circum-oesophageal commisure.
4. Under arm-nerve.
5. Great arm-sinus.
6. Small arm-sinus.
7. Tentacle.
8. Lip of lophophore.
9. Infra-oesophageal commisure.
10. Chief arm-nerve.
11. Secondary arm-nerve.
12. Nerves to tentacles.
13. Sub-oesophageal ganglion.
14. Dorsal lateral nerve.
15. Sub-oesophageal portion of the secondary arm-nerve.
16. Median pallial nerve of dorsal lobe of mantle.
17. Anterior occlusor muscle.
18. Posterior occlusor muscle.
19. Obliquus superior muscle.
20. Levator brachii muscle.]
The _histology_ of Brachiopods presents some peculiar and many
primitive features. As a rule the cells are minute, and this has
especially stood in the way of embryological research. The plexus of
nerve-fibrils which underlie the ectoderm and are in places gathered
up into nerves, and the great development of connective tissue, are
worthy of notice. Much of the latter takes the form of hyaline
supporting tissue, embedded in which are scattered cells and fibres.
The lophophore and stalk are largely composed of this tissue. The
ectodermal cells are large, ciliated, and amongst the ciliated cells
glandular cells are scattered. The chitinous chaetae have their origin
in special ectodermal pits, at the base of which is one large cell
which is thought to secrete the chaeta, as in Chaetopods. These pits
are not isolated, but are connected by an ectodermal ridge, which
grows in at the margin of the mantle and forms a continuous band
somewhat resembling the ectodermal primordium of vertebrate teeth.
The ovary and testes are heaped-up masses of red or yellow cells due
to a proliferation of the cells lining the coelom. There are four of
such masses, two dorsal and two ventral, and as a rule they extend
between the outer and inner layer of the mantle lining the shells. The
ova and the spermatozoa dehisce into the body cavity and pass to the
exterior through the nephridia. Fertilization takes place outside the
body, and in some species the early stages of development take place
in a brood-pouch which is essentially a more or less deep depression
of the body-wall median in _Thecidea_, while in _Cistella_ (_?
Argiope_) there is one such pouch on each side, just below the base of
the arms, and into these the nephridia open. The developing ova are
attached by little stalks to the walls of these pouches. In spite of
some assertions to the contrary, all the Brachiopods which have been
carefully investigated have been found to be male or female.
Hermaphrodite forms are unknown.
[FIG. 29.--Three larvae stages of _Megathyris_ (_Argiope_). A, Larva
which has just left brood-pouch; B, longitudinal section through a
somewhat later stage; C, the fully formed embryo just before
fixing--the neo-embryo of Beecher. Highly magnified.
1. Anterior segment.
2. Second or mantle-forming segment.
3. Third or stalk-forming segment.
4. Eye-spots.
5. Setae.
6. Nerve mass (?).
7. Alimentary canal.
8. Muscles.]
_Embryology._--With the exception of Yatsu's article on the
development of _Lingula_ (_J. Coll. Sci., Japan_, xvii., 1901-1903)
and E.G. Conklin's on "Terebratulina septentrionalis" (_P. Amer. Phil.
Soc._ xli., 1902), little real advance has been made in our knowledge
of the embryology of the Brachiopoda within recent years. Kovalevsky's
researches (Izv. Obshch. Moskov, xiv., 1874) on _Megathyris_
(_Argiope_) and Yatsu's just mentioned are the most complete as
regards the earlier stages. Segmentation is complete, a gastrula is
formed, the blastopore closes, the archenteron gives off two coelomic
sacs which, as far as is known, are unaffected by the superficial
segmentation of the body that divides the larva into three segments.
The walls of these sacs give rise at an early stage to muscles which
enable the parts of the larva to move actively on one another (fig.
29, B). About this stage the larvae leave the brood-pouch, which is a
lateral or median cavity in the body of the female, and lead a free
swimming life in the ocean. The anterior segment broadens and becomes
umbrella-shaped; it has a powerful row of cilia round the rim and
smaller cilia on the general surface. By the aid of these cilia the
larva swims actively, but owing to its minute size it covers very
little distance, and this probably accounts for the fact that where
brachiopods occur there are, as a rule, a good many in one spot. The
head bears four eye-spots, and it is continually testing the ground
(fig. 29, A, C). The second segment grows downwards like a skirt
surrounding the third segment, which is destined to form the stalk. It
bears at its rim four bundles of very pronounced chaetae. After a
certain time the larva fixes itself by its stalk to some stone or
rock, and the skirt-like second segment turns forward over the head
and forms the mantle. What goes on within the mantle is unknown, but
presumably the head is absorbed. The chaetae drop off, and the
lophophore is believed to arise from thickenings which appear in the
dorsal mantle lobe. The Plankton Expedition brought back, and H.
Simroth (_Ergeb. Plankton Expedition_, ii., 1897) has described, a few
larval brachiopods of undetermined genera, two of which at least were
pelagic, or at any rate taken far from the coast. These larvae, which
resemble those described by Fritz Muller (_Arch. Naturg._, 1861-1862),
have their mantle turned over their head and the larval shell well
developed. No stalk has been seen by Simroth or Fritz Muller, but in
other respects the larva resembles the stages in the development of
_Megathyris_ and _Terebratulina_ which immediately precede fixation.
The cirri or tentacles, of which three or four pairs are present, are
capable of being protruded, and the minute larva swims by means of the
ciliary action they produce. It can retract the tentacles, shut its
shell, and sink to the bottom.
[FIG. 30.--Stages in the fixing and metamorphosis of _Terebratulina_.
Highly magnified. (From Morse.)
A, Larva (neo-embryo) just come to rest.
B, C, D, Stages showing the turning forward of the second or mantle
segment.
E, Completion of this.
F, Young Brachiopod.
1, 2, 3, The first, second and third segments.]
C.E.E. Beecher (_Amer. Jour. Sci._ ser. 3, xli. and xliv.) has
classified with appropriate names the various stages through which
Brachiopod larvae pass. The last stage, that in which the folds of the
second segment are already reflected over the first, he calls the
Typembryo. Either before or just after turning, the mantle develops a
larval shell termed the protegulum, and when this is completed the
larva is termed the Phylembryo. By this time the eyes have
disappeared, the four bundles of chaetae have dropped off, and the
lophophore has begun to appear as an outgrowth of the dorsal mantle
lobe. The protegulum has been found in members of almost all the
families of Brachiopod, and it is thought to occur throughout the
group. It resembles the shell of the Cambrian genus _Iphidea
[Paterina]_, and the Phylembryo is frequently referred to as the
_Paterina_ stage. In some orders the Phylembryo is succeeded by an
_Obolella_ stage with a nearly circular outline, but this is not
universal. The larva now assumes specific characters and is
practically adult.
[FIG. 31.--Shell of larval Brachiopod. Phylembryo stage. (From
Simroth.) 1, Protegulum; 2, permanent shell.]
_Classification_.--Beecher's division of the Brachiopoda into four
orders is based largely on the character of the aperture through which
the stalk or pedicle leaves the shell. To appreciate his diagnoses it
is necessary to understand certain terms, which unfortunately are not
used in the same sense by all authors. The triangular pedicle-opening
seen in _Orthis_, &c., has been named by James Hall and J.M. Clarke
the delthyrium. In some less primitive genera, e.g. _Terebratula_,
that type of opening is found in the young stages only; later it
becomes partly closed by two plates which grow out from the sides of
the delthyrium. These plates are secreted by the ventral lobe of the
mantle, and were named by von Buch in 1834 the "deltidium." The form
of the deltidium varies in different genera. The two plates may meet
in the middle line, and leave only a small oval opening near the
centre for the pedicle, as in _Rhynchonella_; or they may meet only
near the base of the delthyrium forming the lower boundary of the
circular pedicle-opening, as in _Terebratula_; or the right plate may
remain quite distinct from the left plate, as in _Terebratella_. The
pro-deltidium, a term introduced by Hall and Clarke, signifies a small
embryonic plate originating on the dorsal side of the body. It
subsequently becomes attached to the ventral valve, and develops into
the pseudo-deltidium, in the Neotremata and the Protremata. The
pseudo-deltidium (so named by Bronn in 1862) is a single plate which
grows from the apex of the delthyrium downwards, and may completely
close the aperture. The pseudo-deltidium is sometimes reabsorbed in
the adult. In the Telotremata neither pro-deltidium nor
pseudo-deltidium is known. In the Atremata the pro-deltidium does not
become fixed to the ventral valve, and does not develop into a
pseudo-deltidium. The American use of the term deltidium for the
structure which Europeans call the pseudo-deltidium makes for
confusion. The development of the brachial supports has been studied
by Friele, Fischer and Oehlert. A summary of the results is given by
Beecher (_Trans. Connect. Acad._ ix., 1893; reprinted in _Studies in
Evolution_, 1901).
The orders Atremata and Neotremata are frequently grouped together, as
the sub-class Inarticulata or Ecardines--the Tretenterata of
Davidson--and the orders Protremata and Telotremata, as the Articulata
or Testicardines--the Clistenterata of Davidson. The following scheme
of classification is based on Beecher's and Schubert's. Recent
families are printed in italic type.
[FIG. 32.--Diagram of the pedicle-opening of _Rhynchonella_. Magnified.
1. Umbo of ventral valve.
2. Deltidium.
3. Margin of delthyrium.
4. Pedicle-opening.
5. Dorsal valve.]