In this article we will discuss about Phylum Ectoprocta:- 1. Characteristic Features of Phylum Ectoprocta 2. Classification of Phylum Ectoprocta 3. Affinities.
Characteristic Features of Phylum Ectoprocta:
1. Phylum Ectoprocta are mostly marine and some are freshwater animals.
2. Un-segmented, bilaterally symmetrical, sessile, colonial coelomate animals of trimeric construction except Monobryozoon which lives in solitary state.
3. The individuals or zooids of the colony are connected by organic substance.
4. Each zooid is covered by a cup-shaped exoskeletal case, called zoecium which is calcareous or chitinous and gelatinous in freshwater species.
5. Usually each zooid of the colony is about 0.5 mm in length.
6. V-shaped digestive tract with anus at the outside of the lophophore. Digestion in both extracellular and intracellular.
7. Respiratory, circulatory and excretory systems are absent in Phylum Ectoprocta.
8. Nervous system includes a sub-epidermal plexus with circumpharyngeal nerve ring.
9. Polymorphism is a common feature and physiological exchange takes place through the pores of the walls among the colony zooids.
10. Bryozoans are hermaphrodites.
11. Gonoducts are lacking in Phylum Ectoprocta.
12. Sperms are released into the coelom and exit through the pores in the lophophore tentacles.
13. Fertilization takes place both in sea water or in the metacoel.
14. Development takes place in sea water or in the special brood chambers of the colony (e.g., Cheilostome sp.).
15. Cleavage is biradial in Phylum Ectoprocta.
16. A lecithotrophic larva, called cyphonautes is typically present in Phylum Ectoprocta.
Classification of Phylum Ectoprocta:
The Phylum Ectoprocta is divided into two classes.
The classes are:
(1) Gymnolaemata or Stelmatophoda and
(2) Phylactolaemata or Lophophoda.
Class Gymnolaemata or Stelmatophoda:
Features:
1. The members of this class are characterised by having a circular lophophore and the absence of epistome.
2. The musculature in the body wall is absent.
This class includes five orders.
Order Ctenostomata:
1. Zoecia are membranous.
2. Ovicells and avicularia are absent.
Examples:
Nolella, Victorella, Zoobotryon, Clavopora, Paludicella.
Order Cheilostomata:
1. Box-like zoecia which are chitinous or calcareous.
2. Ovicells and avicularia are present.
Examples:
Labiostomella, Membranipora, Conopeum, Callopora, Bugula.
Order Cyclostomata or Stenostomata:
1. Tubular zoecia which are calcareous in nature.
2. The avicularia and operculum are absent.
The representatives are Tubulipora, Stomatopora, Diplosolen, Berenicea.
Order Trepostomata:
1. This order includes the fossil forms where the zoecia are elongated, tubular and traversed by horizontal partitions.
2. They are colonial forms with massive bodies.
The examples are Heteropora, Batostoma.
Order Cryptostomata:
1. This is also an extinct order of the class Gymnolaemata where the zoecia remain hidden at the bottom of the vestibule.
Examples:
Rhombopora, Fenestella.
Class Phylactolaemata or Lophophoda:
Features:
1. This class includes exclusively the freshwater forms which are provided with horse-shoe-shaped lophophore.
2. Epistome and body musculature are present.
The members of the class are Fredericella, Pectinatella, Lophopus, Cristatella, Plumatella, Stolella.
Recently D. L. Pawson, J. S. Ryland and W. D. Williams have divided the phylum Ectoprocta (Bryozoa or Polyzoa) into three classes.
The classes are:
I. Stenolaemata,
II. Gymnolaemata and
III. Phylactolaemata.
Affinities of Phylum Ectoprocta:
Like Phoronids and Brachiopods, the systematic position of the Ectoprocts is also controversial. The controversy lies due to their structural alliances with other groups of animals.
Relationship with Phoronida:
Caldwell (1888) emphasised the relationship between Phoronida and Ectoprocta.
This idea was based on the presence of the following similar features:
1. The nerve centre is located in the mesocoel and is supraenteric.
2. Both are provided with horseshoe- shaped lophophore.
3. Presence of epistome.
4. U-shaped alimentary canal.
5. Similar disposition of the coelom and the presence of a septu separating the mesocoel and metacoel.
Dissimilarities:
But detailed study of the two groups showed many structural differences between them. They differ widely in their anatomical construction. The embryological development shows many differences.
The most important differences are:
1. The origin of coelom is different.
2. The region between the mouth and anus is dorsal in Phoronida and ventral in Ectoprocta.
3. Circulatory system and nephridia are absent in Ectoprocta, but in Phoronida both the systems are present.
Remark:
Because of these differences, the relationship between Ectoprocta and Phoronida cannot be established. Of the three lophophorate coelomates, the Phoronida is nearer to the lophophorate ancestor and the Ectoprocta occupies a subsequent stage.
Relationship with Brachiopoda:
The Ectoprocta is related to Brachiopoda and possesses many common characters.
The similar features are:
1. Both have similar body construction.
2. Bivalved shell of Cyphonautes larva of Ectoprocta is comparable to the shell of Brachiopoda.
3. Presence of a coelomic septum between the mesocoel and metacoel.
4. U-shaped alimentary canal.
Dissimilarities:
But due to the following structural differences the relationship becomes very difficult to establish.
The main differences are:
1. The nervous system is mainly supraenteric in Ectoprocta, but in Brachiopoda it is sub-enteric.
2. The brachiopod shell cannot be compared to the exoskeleton of Ectoprocta.
3. The shell is laterally placed in Ectoprocta, but in Brachiopoda the shell is dorsoventrally placed.
4. The chitinous setae are present in Brachiopoda, but absent in Ectoprocta.
Remarks:
Because of lack of specific relationship between them, the Ectoprocta and Brachiopoda are placed in two separate phyla having remote phylogenetic connections.
Relationship with Endoprocta:
Many authors, specially Nitsche (1869) placed the Ectoprocta and Endoprocta as two classes under the phylum Bryozoa or Polyzoa, because of the presence of looped alimentary canal, ciliated tentacular circlet and the similarity in the larval stages. But a thorough examination reveals that the two groups are fundamentally different.
They exhibit the following differences:
1. The Ectoprocta possesses true coelom, whereas in Endoprocta true coelom is wanting.
2. The tentacular crown surrounds only the mouth in Ectoprocta, but in Endoprocta both the mouth and anus are enclosed by the crown of tentacles.
3. The nephridia and gonoducts are absent in Ectoprocta, but in Endoprocta both of them are present.
Different theories:
1. Farmer and co-workers (1973):
Farmer and co-workers (1973) —have suggested that the Bryozoans or the Ectoprocts evolved from some line of phoronids that took up an epibenthic existence.
2. Emig (1977), Nielsen (1977):
Emig (1977), Nielsen (1977)—Consider that phoronids and Ectoprocts are not closely related. Recent authors like Parker & Haswell (1972), Barnes (1980, 1987), Ruppert & Barnes (1994) have supported the above view.
Concluding remarks:
Considering these features, it is quite apparent that the Ectoprocta is highly organised than the Endoprocta and their exclusion as separate phylum is more reasonable. The similarities in the alimentary system are due to adaptive convergence, and the larval similarities are common for all pelagic free-swimming larval forms.
Brien & Papyn (1954), Hyman (1959) regard the entoprocts as early offshoot of Protostomiate line leading to the annelida whereas the Ectoprocta are later derivatives of the same line.
From the relationship of Ectoprocta with other animals it is convenient to place the ectoprocts under a separate phylum having phylogenetic relationship with the other two lophophorate coelomates—Phoronida and Brachiopoda (Hyman, 1959; Parker a Haswell 1972; Barnes 1980, 1987).
As regards the intraphylar relationship between different groups of the phylum, the phylactolaemates are regarded to be very primitive ectoprocts because of the presence of:
(1) Cylindrical zooids,
(2) Division of the body into epistome (protosome), lophophoral region (mesosome) and trunk (metasome),
(3) Horseshoe-shaped lophophore and
(4) Lack of polymorphism.
