Bryozoans (Moss Animals)
Ectoprocta (ectoprocts, Bryozoa, bryozoans, Polyzoa, polyzoans, moss animals)

External Features

The ectoprocts are microscopic, sessile, colonial coelomates that live permanently fastened in exoskeletal cases or
gelatinous material of their own secretion. Each (feeding)
zooid possesses a circular or crescentic lophophore and
a recurved digestive tract with the anus near to the mouth. The lophophore is a feeding organ consisting of a crown
of tentacle extensions of the body wall, containing hollow (fluid-filled) extensions of the coelom (fluid-filled cavity
lined by mesoderm cells). The
lophophore may be circular (as in marine gymnolaemates or horsheshoe-shaped as
in freshwater phylactolaemates). The lophophore encircles the mouth, but not the anus which opens outside the
lophophore. The tentacles are ciliated. A number of other invertebrate groups possess lophophores, such as the
phoronids, and these various groups are collectively called lophophores. In some forms, the two most dorsal
tentacles fuse into a ciliated tube, the intertentacular organ, from which ripe eggs emerge. (In other froms an
opening may form beneath the mouth and bases of the dorsal tentacles, called the supraneural pore, through which
eggs are released).

Each colony is derived from a single progenitor (
ancestrula) by asexual reproduction. The colony (zoarium) is
usually immovable and attached to the substrate (shells, seaweeds, pilings, other animals, etc.) although a few
colonies are motile and some golf-ball sized colonies have been observed swimming (a dispersal stage). The colony
may be arborescent, frondose or a flat incustration, or with adherent or erect stolons bearing zooids.

The zooids are enclosed in a secreted exoskeletal case, the
zoecium. Zoecia are separate or fused together. Each
zoecium has an orifice, opening to the exterior, which may be closable.

The ectoproct lophophore is identical to the phoronid lophophore, consisting of a tentacular crown that is protrusible
through the orifice. The lophophore is a body-wall extension, subdivided distally into a single row of ciliated hollow
tentacles that are continuous with the coelomic cavity. The lophophore is circular (in marine gymnolaemates) or
horseshoe-shaped (fresh-water phylactolaemates). The lophophore embraces the mouth (but not the anus).

Circulatory System and Coelom

The trunk is permanently fastened into the zoecium along part of the body wall. The trunk coelom is lined by
peritoneum (a layer of cells of mesodermal origin, typically ciliated) and encloses the digestive tract, the muscles
and the gonads. An imperfect septum separates the trunk from the lophophore. The gut has a few attachments to
the body wall.

Ectoprocts are said to have no circulatory system. This is correct if one defines a circulatory system (rather
narrowly) as a system of blood cavities or vessels. However, in bryozoans the coelom is extensive and no doubt
performs some functions of a circulatory system. The coelomic fluid contains free connective-tissue cells, phagocytic
amoebocytes and granular cells. Protein is also found in the coelomic fluid and sometimes free calcareous spicules
(calcium reserve?). The peritoneum is described as syncytial (being a sheet of cells fused together into a single
multinucleate sheet). The coelom undoubtedly serves as a hydrostatic fluid-filled skeleton.

A small ring coelom at the lophophore base sends out a branch into each tentacle. The trunk coelom is situated
between the digestive tract and the body wall. Strands of peritoneum and muscle fibres cross between the body wall
and the pharynx wall. Muscle fibres crossing the trunk coelom are called funicular cords and possess a peritoneal
covering. The funiculus is a funicular cord connecting the stomach to the body wall and may be the only funicular
cord in some forms, connecting to the stolon cord when present.

Excretory System

Ectoprocts lack nephridia.

Nervous System

The nervous system is dorsal and consists of a plexus in the body wall and a main ganglionic mass situated between
the mouth and anus and encircling the pharynx. This ganglionic mass gives out nerves to the tentacles and the
trunk.

Reproduction

Most ectoproctans are hermaphroditic. The gonads are borne on the peritoneum and there are no coelomic ducts
or gonoducts. Eggs are shed into the water or brooded. The free-swimming larvae are modified trochophores and
when they settle they metamorphose into an ancestrula that produces a new colony by asexual reproduction.

Asexual reproduction is by budding. In the fresh-water phylactolaemates, the autumnal reproductive bodies survive
over winter and hatch in spring.

Ecology

There are about 4000 described species of ectoproct and a further 15000 extinct species. Most gymnolaemates are
littoral, with some occurring down to 8200 m. The phylactolaemates live in fresh-water.

Following is a more detailed description of the gymnolaemates.

Gymnolaemata (Stelmatopoda) (marine bryozoans)

External Features

Gymnolaemates have a circular lophophore. The cystid is the boxlike, fixed part of the body wall (includes the
zoecium and the attached living layers). The
polypide consists of the internal living movable parts.

Form of colony.

In stoloniferous colonies, zooids may be borne on erect or creeping stolons either singly, in pairs, clusters, regular
rows or spirals. The stolons are tubes of body wall, divided by perforated partitions (nodes or septa) into internodes
(which are highly modified zooids). Most colonies, however, are non-stoloniferous colonies, comprising more or less
fused zooids. They may be non-calcified branching and plantlike, or are gelatinous, but most are calcified and coral-
like. Most calcified colonies are flat and incrusting, forming sheets that are fanlike or radiate. Some calcified colonies
are reticulate (fenestrated or net-like) and lacelike. In incrusting forms, it is the dorsal surface of each zooid that is
cemented to the substrate.

Anchoring stolons may be present as a radicular fibre, rhizoid or root-like structure, or a contoured holdfast. These
stolons may alternatively form tendrils that entwine around objects, or they may terminate in clawlike branches.
Monobryozoon ambulans consist of single vaselike zooids anchored in the sand by short stolons bearing clusters of
adhesive cells on their tips. Such stolons enable the animal to anchor itself or to move slowly. The stolons also give
rise to new zooids that constrict off.

Most gymnolaemate colonies are small, but some reach 45 cm in length.
Alcyonidium forms gelatinous cylinders up
to 90 cm long. Frondose colonies may be up to 15 cm tall.
Membranipora membranacea forms incrusting colonies
on
Laminaria kelp of up to 2.3 million zooids and measuring up to 5 feet long and 8 inches wide.

The colonies are usually white and pale, but some are orange, red, brown, blue, or violet. The colour is in the living
zooid or the zoecium.

The zoecium encasement.

A single zoecium is usually less than 0.5 mm long, but may be up to 4 mm. In the order Ctenostomata (ctenostomes)
the zoecium is a thin case (cuticle) and is vaselike or tubular. It has a terminal orifice and is made of chitin, which
may have foreign particles incorporated into it, or it may have a sand-grain covering. Spinous projections may be
present around the orifice or along the sides. The orifice may be provided with a chitinous rim of two lips, with the
lower lip being closable by attached muscles. Ctenostomes form small colonies.

In the order Cheilostomes, the zoecium is boxlike, oval, rectangular, or polygonal. These form contiguous non-
stoloniferous colonies. The orifice is subterminal and on the ventral or frontal surface. The dorsal surface is
adherent, whilst the sides and ends are adherent to adjacent zoecia. There may be a chitinous hinged lid
(
operculum) that can close the orifice by muscle action. The operculum is semicircular or crescentic and may
have sclerite thickenings along the rim or arches on its inner surface for muscle attachment. It consists of a 2-
layered chitinous cuticle and may be calcified.
Bugula is an example of a cheilostome. Alcyonidum has boxlike zoecia
embedded in a gelatinous secretion and forms erect cylindrical colonies almost 1 m tall.

The side and end walls of the zoecium may be strengthened. The ventral wall forms the thin, pliant
frontal
membrane
. This frontal membrane permits expansion and contraction of the interior as the lophophore retracts or
extends. The frontal membrane may have protective
spines.

In the orders Cheilostomes and Cyclostomes the zoecium is calcified. There is calcium carbonate on the inner
surface of the zoecium, external to the epidermis. In these orders there may be spines on or around the frontal
membrane. In some Cheilostomes (the cribriform Cheilostomes) spines arch over the frontal membrane and fuse to
form a ribbed calcareous ventral shield, with holes between the ribs. Some Cheilostomes possess a cryptocyst or
calcareous shelf that grows from the rear edge beneath the frontal membrane to the anterior edge. Two large holes
(opesiules) within this shelf allow muscles to attach to the frontal membrane. All calcareous outgrowths are covered
externally by cuticle and lined internally by the secreting epidermis.

In the ascophoran Cheilostomes, the zoecium is completely calcified. The frontal membrane calcifies between its
cuticle and epidermis. The zoecium contains a compensation sac (ascus) in its interior, immediately beneath the
calcified front. This sac acts as a reservoir and permits internal volume changes (necessary since the zoecium is
otherwise rigid) and opens to the exterior via a notch (sinus vanna) in the proximal rim of the orifice or via a
separate pore (ascopore) or a notch in the orificial collar (if present), termed a rimule.

Living Parts of the Zooid

The main zooid body is immovably attached inside the zoecium. The anterior end is protrusible and freely movable
and bears the lophophore. The lophophore is circular, encircling the central mouth. The lophophore also bears
retractable tentacles that expand into a feeding funnel. There are 8-34 of these tentacles. Each tentacle is hollow,
containing a coelomic lumen continuous with the ring coelom in the lophophoral base. Each has a cuticle-covered
epithelial covering, underlain by a thick basement membrane and peritoneum. Each tentacle is broader on its
external side. The epidermis consists of 9 longitudinal rows of cells and each tentacle is equipped with a pair of
lateral longitudinal
ciliated tracts. Each tentacle is movable by longitudinal muscle fibres and contains 2 motor and
2 sensory nerves. The main nervous centre is in the lophophoral base.

Body Wall

There is a muscle layer present but only in the tentacle sheath and the vestibular wall. The peritoneum is net-like.
There are communication pores or
interzoidal pores between neighbouring zooids. There are single such pores in
the transverse end walls and the bases of stoloniferous forms. Each pore is a hole plugged with a rosette of
modified epidermal cells. The lateral connections between zooids may be simple pores or
pore-plates (rosette
plates) in the anterior portion of the zooid. These are derived from aborted zooids. Funicular (mesenchymal) chords
may run through the pores (along the centre in stoloniferous forms, otherwise along the sides of the zooidal
interiors).

The body wall is inturned at the orifice, forming the
vestibule, a passage that ends in a constriction called the
diaphragm. Attached muscles operate this diaphragm, and attached muscles can also alter the vestibule. In
Ctenostomes, the diaphragm bears a circular pleated collar that projects into the vestibule as riblike cuticle
thickenings. When the zooid is retracted the pleats fold together to form a cone that blocks the vestibule. Some
cheilostomes have a pair of vestibular glands (or one gland with paired ducts) also called oral glands or opercular
glands. These open into the vestibule close to the diaphragm, or are adherent to the operculum.

The
tentacle sheath is part of the body wall that extends from the lophophoral base to the diaphragm and covers
the necklike region of the protruded animal. This sheath consists of cuticle, epidermis, an outer longitudinal muscle
layer and an inner circular muscle layer and peritoneum. It bears the anal opening and encloses the anterior
digestive tract. It introverts when the lophophore retracts and then surrounds the bunched tentacles.

Some forms have a
double frontal wall. The inner wall is calcareous and clothed on both sides with epidermis and
preitoneum, whilst the outer wall consists of cuticle, epidermis and peritoneum.

Nutrition

In filter feeding, the tentacle cilia generate a current directed into the lophophore funnel and out in-between the
tentacles. Trapped particles are batted or rolled down the funnel by rapid or slow lashing of one of the tentacles.
Bugula neritina catches zooplankton by closing its tentacle tips around the prey to form a cage. The lophophore
may scan the water by rotating and swaying. The ciliated pharynx assists ingestion. Unsuitable particles may be
rejected by mouth closure, tentacle flicking, funnel closure, or by passage between the tentacles.

Blank areas on the colony occur where lophophores are tilted away from each other. These spaces are occupied by
modified zooids that function as “chimneys” for water to exit from the colony.

The
digestive tract is U-shaped. The round mouth is encircled by the lophophore base and closed by circular
muscle fibres and sometimes also by radial muscle fibres. The mouth opens into the foregut, which is equipped with
longitudinal furrows and can be divided into pharynx and oesophagus (though these two regions are not always
clearly delimited). The pharynx is ciliated and supplied by circular muscle. The oesophagus is non-ciliated and
provided with circular and longitudinal muscles.

A constriction, that may be equipped with a valve, opens into the stomach. The stomach can be divided into the
anterior tubular cardia, the medial sac-like caecum occupying the bend of the U, and the posterior tubular pyloris.
The cardia has circular and longitudinal muscle layers and in its anterior section may form a
denticulated gizzard
supplied by thick circular muscle. The caecum has mostly circular muscle. The pyloris is ciliated and supplied with
circular muscle and opens, via the pyloric sphincter, into the intestine. The intestine has circular and especially
longitudinal muscle fibres and is glandular and leads to the anus. The pH of stomach contents is 6.5-7.0, whilst that
of the intestine is the same as seawater (8.2).

Fat, glycogen and protein are stored in the stolons (in stoloniferous forms). These can survive adverse conditions
and regenerate the colony. Lipid is transported along the funiculus and away from actively feeding zooids.

Coelom and Coelomocytes

The coelom is divided by an incomplete septum into a small ring coelom in the lophophoral base and the body
coelom between the gut and body wall. The ring coelom extends into the tentacles. The septum extends from the
distal end of the tentacle sheath to the pharynx and has an opening near the nerve ganglion.

Radiating strands of peritoneum and muscle fibres cross the ring coelom from the tentacle sheath to the pharynx.
The trunk coelom is crossed by muscle fibres, funicular cords, and mesenchyme and contains coelomocytes. The
funicular cords are composed of connective tissue with a peritoneal covering. The peritoneal lining is a syncytial
network. The
funiculus is a funicular cord attaching the stomach caecum to the zooidal wall and is the only
funicular cord in cylclostomes and stoloniferous ctenostomes, in which it is continuous with the stolon cord.

The coelom contains free connective-tissue cells,
coelomocytes, protein and fluid. It may also contain free
calcareous spicules (a reserve of calcium carbonate?). The coelomocytes are of two types, phagocytic
amoebocytes and fusiform granular cells. The spaces between double walls (when present) and in the cavities of
alveoli are also coelomic.

An exit for the eggs may form on the lophophore at breeding time. This
supraneural pore is beneath the mouth
and the base of the dorsal tentacles and connects to the ring coelom. Alternatively, this temporary gonopore may
comprise the
intertentacular organ, in which the 2 most dorsal tentacles are fused into a ciliated tube from which
the eggs emerge from the ring coelom.

Muscular Systems

Gymnolaemate muscles may be of the striated and/or smooth type. Three to 40 pairs of parietal muscles (absent
in cyclostomes) connect the lateral walls to the frontal membrane, in the anterior part of the zooid. Two muscle-
bundles pass through the opesiules in the cryptocyst (if present).

One pair of
occlusor muscles closes the operculum. These insert on a ridge or thickening on the operculum via a
common tendon or else into a pair of pits (muscle dots). They originate on the lateral or dorsal walls. The pressure
of the advancing lophophore usually opens the operculum or else by one pair of divaricator muscles that originate
on the wall (behind the occlusor muscles) and insert by a tendon on the frontal membrane just behind the
operculum hinge. Depressor muscles situated behind the divaricators may also assist opening.

Powerful
lophophore retractors insert on the lophophoral base (on the coelomic septum or the basement
membrane of the foregut) and originate on the posterior wall of the zooid, or else on the sidewalls.

The
diaphragm consists of circular muscle fibres continuous with those of the tentacle sheath and forms a
sphincter. There may also be a sphincter at the distal end of the vestibule. Dilation of the vestibule, diaphragm and
the distal end of the tentacle sheath is by
dilator muscles attached to the zooid wall.

Stolons are often devoid of muscle fibres. In those zooids that are movable on stolons, this is achieved by
longitudinal muscle fibres.

Lophophore Protrusion and Retraction

Evagination of the tentacle sheath pushes out the tentacular crown. There are three alternative mechanisms
employed to bring this about.

Mechanism One. One mechanism by which this is achieved is by contraction of the parietal muscles, which pull the
ventral and dorsal walls of the zoecium together. (The walls contain flexible chitin). This increases coelomic pressure
forcing out the lophophore. At the same time expansion of the diaphragm and vestibule, the latter by contraction of
the dilator muscles, opens the pleated collar. Vestibule dilators may be absent and the vestibule may evaginate.

Mechanism Two. An alternative mechanism exists in ascophoran cheilostomes, which have rigid calcified frontal
walls, but possess a
compensation sac called the ascus. The parietal muscles contract and pull on the sac,
causing the sac to expand. Water enters the expanding ascus via the
ascopore. The resultant increase in internal
pressure forces the lophophore to extend.
The ectoprocts, bryozoans or moss animals, are frequently overlooked, despite being common inhabitants of marine
and freshwater. They are very unusual and remarkable animals with a great diversity of form and well worth close
study. The individuals (zooids) are more-or-less microscopic, living enclosed in a case which is often box-like. Many
such individuals form a colony, which is readily visible to the trained eye, which may be encrusting or branching and
tree-like or consisting of stolon tubes bearing the individual zooids.
Typically many zooids connect together to form colonies with a variety of forms. The pore-plates in the side-walls
(derived from modified zooids) connect the zooids together into a continuous system. A number of individual pores
(not grouped into pore plates) also connect the adjacent end-walls of individual zooids together.
Above: anatomy of a bryozoan zooid. The frontal membrane is flexible and allows for
volume changes when the zooid extends and retracts its tentacle crown out of and into the
zoecium 'box'.

Note that in this 3D model the geometry of the zoecium has been kept simple (for ease of
coding). -The frontal surface if often ornamented with ridges and spines.
Locomotion and Movement

Most gymnolaemates are attached and sessile. Monobryozoon consists of an unattached autozooid with basal
kenozooids making up the stolons. These stolons have adhesive tips and contain muscle fibres and allow slow
locomotion.

Some gymnolaemates have been seen as golf-ball sized colonies swimming freely in the water by means of their
feeding currents. It is not yet clear whether this is a normal mode of dispersal, or whether these colonies had been
forcibly detached from their substrate.

Attached stoloniferous ctenostomes can move their autozooids on their attachments by about 10 degrees, and can
cause the zooid to droop when it is retracted and to erect when it is expanded, as in
Mimosella. The colony
branches are capable of movement in some forms and may curl-up when the zooids are retracted.

The lophophore is capable of rotating and swaying from side-to-side as it scans for food sources. Lophophores of
neighbouring zooids may be tilted away from each other to form a blank area or “chimney” of outward current. Large
particles may be passed from zooid to zooid until they reach an excurrent chimney where they are rejected.

Nervous System

The ganglion nervous centre resides in the ring coelom, dorsal to the pharynx and continuous with a nerve ring
around the pharynx. This ring is thick laterally, but reduced to a fibrous strand, lacking nerve cells, ventrally. The
nerve ring sends out 2 ganglionated motor fibres and 2 sensory fibres into each tentacle. The ring also sends out
one pair of sensory nerves and one pair of motor nerves to the tentacle sheath (and possibly onto the digestive
tract). Nerves from the ganglion innervate the digestive tract.

There is a
subepithelial nerve network throughout the zooid wall, except walls cemented to the substrate. There
are stellate neurons at the network nodes. This network is concentrated at the orifice and joins the nerve ring at the
diaphragm. There is also a
tentacle sheath plexus. The nerve plexus may continue along stolons or pass through
interzooidal pores in the poor plates. At least some non-stoloniferous forms have a colonial nervous system.

Sensory Systems

Neurosensory cells with a projecting bristle are borne on the tentacles.

Circulatory System

There is no specialised circulatory system, as it is not necessary since the zooids are very small. Coelomic fluid
circulation is known only in the fresh-water gymnolaemate
Paludicella, in which cilia produce a current that runs
anterior on the dorsal side and posterior on the ventral side. The funiculus transports nutrients (from feeding zooids
along the joining stolon).

Respiration

There is no special respiratory system, since it is not needed due to the small size of the zooids. The tentacles
provide a large surface area: volume ratio for gas exchange to occur across the body surface.

Excretory System

No specialised excretory system is present. The stomach wall, coelomocytes and other mesenchyme tissues, and
the tentacles accumulate dyes. The tentacles may expel the dye to the exterior. Growing points also accumulate
dyes. The intertentacular organ, when present, may also be excretory.

Luminescence

In Acanthodesia serrata, compact masses of spherical gland cells on the frontal membrane emit a bluish light when
the animal is disturbed. Other species? [Check for updates in literature].

Ecology

Gymnolaemates live permanently attached to rocks, shells, seaweeds, pilings, other animals, etc. They are very
successful and exploit all types of hard surface, including mangrove roots. Some, however, live on soft bottoms and
rest freely on the substrate. Some forms burrow into live or dead mollusc and barnacle shells by secreting
phosphoric acid. Polychaete tubes (
Terebella, Chaetopterus) may host Hypophorella expansa. These zooids project
into the tube lumen and utilise the water currents maintained by the host worm. Originally they inhabit the inner tube
surface, but as newly secreted layers bury them they bore through using a denticulate boring apparatus above the
orifice on the upper lip.

Harmeriella terebrans bores into the ectoproct Tubiporella, destroying living host tissues and taking over the zoecia.
The vestibule is armed with 16-22 longitudinal toothed cuticular ridges. Presumably, the vestibule is alternately
elongated and shortened in a rasping action (?). Three thick and rough cuticular rings on the zooid enable it to
adhere to the host cystid wall.

Watersiana paessleri lives in the tunic of a compound ascidian, but is nonboring. Bulbella abscondita bores into
wood or twigs, especially that of conifers, in brackish water.

Reproduction

There are no gonoducts. Germ cells may differentiate from the peritoneum during the early stages of budding. The
gonads are borne on the peritoneum of the body wall, digestive tract, or funiculus. Most gymnolaemates are
hermaphroditic with 1-2 ovaries and one or more testes. The ovaries are enclosed in peritoneum and are usually
situated distally in the zooid. The testes are usually situated proximal in the zooid and give off balls of sperm into the
coelom. Some species may exhibit protandry or protogyny. In
Alcyonidium duplex, the males zooids degenerate after
releasing sperm and then female zooids regenerate in the same zoecia.

Some species are dioecious with separate male and female colonies, or male and female (and some
hermaphroditic) zooids on the same colony, but mostly one or the other gender (monoecious colonies). Autozooids
may be sterile and male and female zooids may be reduced and may be feeding or non-feeding.

Many shed eggs (usually less than 20) into the sea via the
supraneural pore or the intertentacular organ. Some
brood, internally or externally. In internal brooders, the embryo starts developing inside the maternal coelom, either
free or in an embryo sac. In
Penetrantia the embryo then passes into the space between the double cuticle where it
forms a conspicuous bulge. This bulge makes the zooid, now a
gonozooid, distinguishable from non-brooding
autozooids. The polypide may degenerate to provide nutrition and space for the embryo. The embryo escapes
through a rupture in the body wall.

In external brooders, a row of arched spines covering the frontal membrane may form the brood chamber.
Alternatively the discharged eggs may attach to the outer surface of the diaphragm or to the outer surface of the
body wall. In
Victorella pavida, the eggs emerge from the supraneural pore and are caught in a depression in the
adjacent body wall and passed to the coelom of the vestibule (containing 3-4 embryos) and escape by rupture of
the body wall.

In most cheilostomes brooding takes place in an ovicell or oecium, which is a helmet or hood on the anterior end of
some or all zooids that houses the egg. The supraneural pore is brought opposite the ovicell entrance by zooid
retraction and then the egg is squeezed into the ovicell. The developing egg apparently derives nutrition from its
parent zooid, since the egg dies if removed. Eventually the ciliated larva escapes.

Some species produce one larva per zooid in the breeding season. In non-brooding species, up to 20 eggs per
zooid are released. In
Membranipora membranacea, numerous small eggs are released. All cyclostomes brood and
some have compound brood chambers formed by the fusion of a fertile autozooid with surrounding kenozooids and
sterile autozooids, or by the fusion of several alveoli. See
polymorphism below for a description of zooid types. (In
some round incrusting forms, in which zooids radiate from a central point, the roofed-over spaces between the
zooids are coelomic chambers, called
alveoli, that communicate by pores in their walls).

Polyembryony occurs in cyclostomes. The eggs attach to a gonozooid (which has an enlarged zoecium) and a
follicle attaches the egg to the polypide. As it disintegrates the follicle provides nutrition to the egg. The embryo
divides into secondary and tertiary embryos, producing up to 100 embryos from one egg.

The sperm possibly enter the recipient zooid via the supraneural pore, intertentacular organ or interzooidal pores.
The eggs are fertilised in the coelom before they are released.

The breeding season usually lasts for 2-6 months per year. The brooding eggs are often coloured red, orange or
yellow and so are conspicuous due to their yolk.

Development

Holoblastic, usually radial (or biradial) cleavage produces a coeloblastula which gives rise to a gastrula. In the
gastrula, an equatorial ciliary girdle or corona is the main locomotory system. A circular apical tuft of rigid cilia
possibly has a sensory function. The larva escapes from the egg or brooding zooid.

From non-brooded eggs hatch a
cyphonautes larva. This has a triangular outline and is laterally compressed. The
aboral pole (opposite the oral pole) points forward when swimming and there is an apical nervous organ. A ciliary
girdle encircles the oral surface. The rest of the body is enclosed in a
bivalve shell. The cyphonautes larva lives
freely for up to two months, feeding on minute organisms with the aid of its ciliary currents.

Brooding species produce a modified cyphonautes with no functional digestive tract, is non-feeding and lives freely
for only a brief period of time. Alternatively a different type of larva (with no special name?) may be produced. This
larva has no covering valves, is round or oval and may be elongate anteroposteriorly. It has an apical organ forming
a turret set with a groove and which may have a scalloped edge. The larva has red ciliated pigment spots.

In cyclostomes, polyembryony produces an oval larva lacking a ciliary girdle and which can not feed and leads only
a brief free life.

During metamorphosis the larva hovers over a suitable spot and everts an adhesive sac, by muscular action,
adheres with the aid of secretion and spreads over the substrate. Projecting parts retract into the interior and are
histolysed (dissolved). In the cyphonautes larva, the valve adductor muscle ruptures and the valves open out flat
and are eventually cast off. The tentacles grow and the zoecium is secreted and the first zooid is formed.

Colony Formation

The progenitor zooid or ancestrula buds off zooids, which in turn bud-off more zooids, and so on. The buds
comprise part of the parent cystid cut-off by a partition. This budding follows a definite pattern, which is more or less
modified by the substrate.

Degeneration and Regeneration

Polypides are in a constant process of degeneration and regeneration. The brown body is a rounded brown mass
found in many zooids and is a degenerated polypide (it is the remainder of the digestive tract). Brown bodies may
persist or be digested in the gut of the new zooid. This degeneration process is due to natural senescence (may be
due to the accumulation of waste?) and exposure to unfavourable conditions and to sexual reproduction.

Growth zones occur at the periphery of the colony or at the tips of branches, resulting in colony zonation. The distal
parts of the colony regenerate quicker.

Polymorphism

Autozooids are the typical, fully developed, feeding zooids.

Heterozooids have a reduced polypide, which is non-feeding and sterile. There are three types of heterozooid:
avicularia, vibracula and kenozooids.

Avicularia occur in some Cheilostomata. The operculum is well developed. In larger avicularia the polypide is
developed, but in smaller avicularia it is reduced and dependent. The avicularium is sessile and adherent to an
autozooid or pedunculate (stalked) with a bendable stalk and resembling a miniature ‘bird’s head’. The cystid
contains a coelom with muscles that operate the operculum, which is modified into a chitinous mandible. The lower
jaw or beak abuts on the rostrum (upper jaw or beak) when closed. The frontal membrane is reduced. Several types
of avicularia may be present. They serve to capture small organisms and prevent larvae settling on the colony (cf.
echinoderm pedicellariae).

Vibracula occur in some cheilostomes. The operculum is modified into a long bristle or seta (up to ten times the
length of an autozooid), which is stiff or flexible and smooth or toothed on one or both sides. The seta is freely
movable. The cystid is reduced. Vibracula may serve to prevent larvae and debris from settling on the colony (?).

Kenozooids occur in all groups and consist of body wall enclosing tissue strands and constitute the stolons, stalks,
rhizoids, holdfasts, some spines, pore chambers and pore plates.

Dwarf zooids, or
nanozooids occur in Diplosolen and Trypostego. They contain one tentacle and a functionless
digestive tract, no gonads, no funiculus and no ring coelom.
Mechanism Three. In cyclostomes the zoecium is completely calcified and rigid. The coelom is completely divided by
a cylindrical partition into inner and outer sacs. This partition extends from the diaphragm to the zooid base and is
anchored to the zooidal wall just posterior to the diaphragm by 8 fixator ligaments, and also to the tentacle sheath
via additional ligaments. The outer sac is continuous with the coelom around the vestibule. Expansion of the
vestibule by dilator muscles compresses the coelom, causing the outer coelomic sac to expand, which in turn
compresses the inner coelomic sac forcing the lophophore to extrude.