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 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.
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.