ECTOPROCTA - STRUCTURE LIFE HISTORY AND AFFINITIES
- The phylum Bryozoa, or Polyzoa, or Ectoprocta are the greatest and most common of the three lophophore phyla.
- Colonial sessile animals such as Cnidaria and Ectoprocta were for a long time confused by early naturalist sea plants and named them as zoophytes. In 1827 Grant found that ectoprocta are highly organised than true polyps in having opening to digestive tract. While in 1831 Ehrenberg coined the term Bryozoa for them.
- The Ectoprocta are minute, sessile, colonial, unsegmented coelomate animals. Provided by the circular or cresentic lophophore, a u-shaped alimentary canal with anus opening near the mouth but outside of lophophore. Usually posses free swimming larva but without nephridia or circulatory system.
- Geographically they are very ancient group preserve since the Cambrian period. Their calcareous covering occurs in Palaeozoic and Mesozoic formation.
STRUCTURE OF ECTOPROCTA
- The form of the colony is variable in some extent even within the same species. A great number of colonies are encrusting forming, thin, flat sheet of zooids attached to other objects.
- The mouth is simple leads to a short and broad muscular pharynx, lined by a columnar ciliated epithelium. The pharynx passes into a long and narrow oesophagus, the lining of which is devoid of cilia. The oesophagus terminates into a large u-shaped stomach sac. Its long and conical aboral prolongation called caecum remains attach to zoecium by funiculus.The tubular part directed upward and lined by ciliated epithelium is called pylorus. In some cases thick walled gizzard lined by chitinous teeth exist in between oesophagus and stomach.
- The pylorus leads to constriction into narrow intestine usually called rectum that opens into anus. Anus is situated near the mouth but outside of lophophore.
- Ectoprocta are ciliary feeder they feed on microscopic plant and animal including diatoms. Tentacular cilia beat strongly moving water current down the tentacular funnel towards the mouth, which swallow the nutritive particles.
- Definite excretory organs are not known to be present.
- Nitrogenous waste product is probably collect and store by coelomic leucocytes and certain cells of funicular tissue.
- A general sub-epithelial nervous network exists almost throughout the wall of the zooid. The nervous centre is in the form of single bilobed ganglion placed in the ring coelom between the mouth and anus.
LIFE HISTORY OF ECTOPROCTA
- With few exception ectoprocta are hermaphrodite. Usually one ovary, sometimes two and one or more testes are found within same individual. Gonads develop on the peritoneum of body wall.
- The ovary is located distally towards the oral end or about the middle and testes generally proximally towards the base. The ovary consists of a cluster of rounded oocytes with large nuclei enclosed in the thin peritoneum.
- Often ovary and testes mature at different time tendency of protandary is not uncommon. The ripe ova escape into the coelom where fertilisation takes place.
- The supraneural pore is a simple aperture between the mouth and base of tentacles. They communicates with lophoral coelom with exterior and serve for exit of eggs and probably also of sperms. Cross fertilisation is wide spread among the bryozoans.
- More often early fertilisation takes place under the protection of parents
- Brooding chamber makes place in the coelom or in special diverticulum of it serving as a brood chamber. Other structure serve as a brood chamber may be tentacular sheath.
- Larvae cleavage is holoblastic and approximately equal. A coeloblastula results which gastrulates by delamination at the vegetal pole leading to modified trochophore larva.
- The larva varies considerably in forms. It is bell shaped looking triangular in side view but is strongly compressed laterally. It is encircled by ciliary girdle or corona, forming a thickened edge. It is chief locomotory organ of larva and it divides the surface into two distinct regions oral and aboral. The aboral surface is enclosed in bivalve shell at aboral apex there is an opening between the two valves through which it projects knob like ciliated retractile disc, apical organ or calotte. It is carried forward in swimming. The anterior end of ciliary cleft usually bears a tuft of cilia called vibratile plume.
- The posterior invagination of thickened ectoderm is adhesive organ or the sucker by which larva attaches itself. In between the pyriform organ and adhesive sac lies a spacious non ciliated cavity, or vestibule at bottom of which is the mouth.
- The larva attaches itself by the ventral adhesive sac which is suddenly spread out over the substrate as a flat disc of ectoderm. The two shell valves are shed and alimentary canal is disappears in fact all larval organs undergo histolysis so that the larva becomes reduced in a layers of epithelium surrounding a loose mass of cells and debris. The ectoderm secretes outer cuticles.
- The aboral ciliated disc begins to proliferate and invaginates inward to form vesicle or primordium of polypide. The vesicle is surrounded by mesenchyme, which gives rise to peritoneum organs of mesoderm of the future polypide. The ectoderm of vesicle also contribute to the mesoderm beside forming the lining of alimentary canal, the ganglion covering of the tentacles and the tentacular sheaths, etc. The primary zooids also called ancestrula thus formed gives rise to future colony of asexually budding.
AFFINITIES OF ECTOPROCTA
Affinities with entoprocta
Formerly ectoproctas and entoproctas were linked together as a class under the phylum Bryozoa. Because of the following similarities-
- Presences of crown of ciliated tentacles
- Presences of looped digestive tract
But such features are common in all sessile animals. The tentacular circlet act as good catching device.
- The cyphonautes larva of ectoprocta bears a marked superficial resemblance to larva of entoprocta. But all pelagic larvas bear certain similarities.
There are sharp difference between entoprocta and ectoprocta that are following
- Entoproctas are without true coelom
- In ectoprocta the tentacular crown embraces only mouth not anus.
- Nephiridia are absent in ectoproctas while present in entoprocta.
- Gonoduct are also absent in ectoprocta while present in entoprocta.
Thus pseudocoelomate entoprocta are much lower in organisation than coelomate ectoprocta.
Affinities with other lophophorate phyla
They are found to be closely related with phoronida and Brachiopoda. Bearing the following close resemblance.
- Presence of horse shoe shape tentacular lophophore.
- Lack of cephalisation
- Reduced dorsal body surface
- Coelomate animals
- Presence of looped digestive tract
- Presence of sub-epidermal nervous plexus.
- Chitinous secretion is common
- The free swimming and ciliated larvae.
But differences are enough to separate and classify them independently. The difference are mainly occur in their anatomy and development.
With all information of anatomy and development available for ectoproctas it seems advisable to state that it is an independent phylum with closer affinity to other lophophorate phylum.
The Phylactolaemates must be regarded as most primitive ectoproctas because of :-
- Preservation of fundamental cylindrical shape of zooids
- Lack of polymorphism
- Simple horseshoe shaped lophophore
- Lack of elaborated devices for operating the lophophore and closing of orifice.
- Regionation of body into proteosome, mesosome and metasome with definite, though incomplete septum between mesocoel and metacoel
- Muscular body wall constituting a simple mechanism for protrusion of the polypide
On the other hand, the shortened and altered embryology of Phylactolaemates may be due to their being fresh water representative of a predominantly marine group.