Download Moss_animals

Ectoprocta (ectoprocts, 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.
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. 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).
Coelom
The trunk is permanently fastened into the zoecium along part of the body wall. The
trunk coelom is lined by peritoneum 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.
Circulatory System
Ectoprocts have no circulatory system.
Excretory Systems
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 freeswimming 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.
Gymnolaemata (Stelmatopoda)
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-clacified branching and plantlike, or are gelatinous, but most
are calcified and coral or millepore like. Most calcified colonies are flat and incrusting,
forming sheets that are fanlike or radiate. Some calcified colonies are reticulate
(fenestrated) 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 x
106 zooids and measuring up to 5 feet long and 8” 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. 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 permits internal volume changes 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.
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.
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 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 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 inbetween 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.
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 no always clearly
delimited). The pharynx is ciliated and supplied by circular muscle. The oesophagus is
nonciliated 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 is stored in the stolons. 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 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 (is 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. 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.
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.
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 nonstoloniferous forms have a
colonial nervous system.
Sensory Systems
Neurosensory cells with a projecting bristle are borne on the tentacles.
Circulatory System
There is no 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.
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.
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?
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 embryo sacs. 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. (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 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. 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 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.