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In this article we will discuss about Brachiopods:- 1. Taxonomic Retrospect of Brachiopods 2. Characteristic Features of Brachiopods 3. Classification 4. Affinities.
Taxonomic Retrospect of Brachiopods:
1. Starting from late sixteenth century the study of brachiopods has a long history behind. They were, for a long time, regarded as molluscs because of the presence of characteristic bivalve shell.
2. Linnaeus in 1758 gave the name Anomia to the brachiopods and placed them under Vermis.
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3. By studying the anatomy of Lingula, Terebratula and Orbicula. Cuvier (1797- 1802) placed them under the group Acephal under Mollusca.
4. The credit of coining the name of the group as Brachiopoda goes to Dumeril (1806) who placed it as a separate order under Mollusca.
5. Steenstrup (1847-49) tried to establish the affinity of the brachiopods with the annelids.
6. Since then the workers on this line were divided into two distinct schools—one supporting the molluscan affinity and the others inclined to support the annelidan affinity.
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7. T. H. Huxley (1869) discarded the molluscan affinities of brachiopods and placed them as one of the three classes under Molluscoidea, the other two classes being Tunicata and Bryozoa.
8. Caldwell (1882) suggested the close relationship between Phoronida, Brachiopoda, Bryozoa and Sipunculida.
9. The brachiopods, phoronids and bryozoans were grouped under Tentaculata by Hatschek (1888) and under Lophophorata by Schneider (1902).
10. Because of the controversies regarding the systematic position, the brachiopods have been given the status of a separate phylum called Brachiopoda.
Characteristic Features of Brachiopods:
1. Exclusively marine and are found in all seas from the intertidal zone to the deep sea (about 5000 meters).
2. Bilaterally symmetrical and un-segmented body encased within a bivalve shell with dorsal and ventral valves. The shells are calcific or chitinophosphatic.
3. Sedentary or sessile coelomate animals of trimeric construction. In most cases the animals are attached to some object by an elongated muscular stalk, called pedicel or peduncle.
4. The lophophore is horseshoe-shaped which surrounds the mouth and consisting of a pair of coiled or folded arms bearing ciliated tentacles.
5. A true spacious coelom is present.
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6. U-shaped gut with anus outside lophophore or absent.
7. Digestion is intracellular (e.g., Lingula) or extracellular.
8. Blood vascular system is of open type with a contractile heart vesicle and ciliated blood vessels.
9. One or two pairs of metanephridia are present which also function as gonoducts.
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10. Nervous system with an extensive nerve net and a circumoesophogeal nerve ring.
11. Sexes are separate.
12. Gametes become mature in the coelom and exit through the metanephridia.
13. Fertilization is external.
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14. Cleavage is holoblastic and radial.
15. Gastrulation by emboly.
16. Coelom formation is either enterocoelous or schizocoelous.
17. Blastopore becomes anus.
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18. A trochophore-like free-swimming larva in the life cycle.
Geological period:
Cambrian—Recent.
Classification of Brachiopods:
The phylum Brachiopoda is classified into two classes—Ecardines or Inarticulata and Testicardines or Articulata.
Class Ecardines or Inarticulata:
The representatives of this class have:
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(i) Shell valves which are not united by hinge but by muscles only;
(ii) No shelly loop supporting the lophophore and
(iii) Anus is present.
The class contains two orders:
Atremata and Neotremata.
Order Atremata:
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The shell is mostly composed of calcium phosphate. The foramen, through which the pedicle passes out, is formed by both the shell valves.
Examples:
Lingula, Glottidia.
Order Neotremata:
The shell may be composed of calcium carbonate. The pedicle foramen is usually confined to the ventral shell valve alone. The representatives are: Crania, Craniscus, Pelagodiscus, Discina, Discinisca.
Class Testicardines or Articulata:
The members have:
(i) Shell valves united by hinge apparatus,
(ii) Shelly loop is present supporting the lophophore and
(iii) Intestine terminates blindly, i.e., the anus is absent.
The examples are:
Megellania, Lacazella, Thecidellina, Rhynchonella, Terebratulina, Chlidonoyhora, Dyscolia.
Affinities of Brachiopods:
The brachiopods constitute a distinct group of lophophorate coelomates.
The Brachiopoda, Phoronida and Ectoprocta possess many similar features, viz., presence of lophophore, septum between mesocoel and metacoel, presence of epistome representing the anterior section (protosome) of the body, chitinous secretion, U-shaped alimentary canal and trochophore like larva.
In spite of these common features, the Phoronida, Brachiopoda and Ectoprocta possess many striking individual characteristics which demand serious consideration. Because of that, all the three groups have been given the status of separate phyla.
Relationship with Phoronida:
The Brachiopoda and Phoronida have many similar structures, such as:
1. Similar lophophore;
2. Epistome representing the anterior segment of the body;
3. U-shaped alimentary tube;
4. Presence of septum separating the mesocoel and metacoel;
5. Presence of sub-epidermal nerve plexus;
6. A pair of metanephridia in the metacoel acting also as gonoducts;
7. Derivation of mouth directly from the blastopore;
8. The dorsal surface between the mouth and anus becomes greatly shortened.
Dissimilarities:
In spite of the similarities, these two groups have many structural differences.
The differences are:
1. The nerve centre is supraenteric in Phoronida but in Brachiopoda it is sub-enteric.
2. Two sets (larval and definite) of tentacles, are present in Phoronida, but in Brachiopoda the larval set is wanting.
3. The shell of Brachiopoda cannot be correlated with exoskeleton of Phoronida.
4. The chitinous setae in Brachiopoda have no counterparts in Phoronida.
5. Circulatory system is of open type in Brachiopoda but in Phoronida it is closed type.
6. The cleavage pattern is spiral in Phoronida, but in Brachiopoda it is not so.
Remarks:
Because of such differences the two groups are placed under separate phyla. The similar features are due to remote connection with the ancestral stock.
Relationship with Ectoprocta:
The Brachiopoda is related to Ectoprocta by having many similar features.
They are:
1. Both Brachiopoda and Ectoprocta have similar body plan.
2. The bivalve shell of Cyphonautes larva of Ectoprocta is comparable to the shell of Brachiopoda.
3. A coelomic septum is present between mesocoel and metacoel.
4. U-shaped alimentary canal. But because of the under-mentioned differences, the affinities become difficult to be considered. The common features are due to descend from a common lophophorate ancestor.
Dissimilarities:
The main differences are as follows:
1. The nervous centre 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 in Ectoprocta no such setae are observed.
5. The coelomic septum is poorly developed in most brachiopods.
6. The anus is lacking in some brachiopods.
Remarks:
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Considering the relationship between the three lophophorate coelomates, it seems reasonable to assume that the phoronida is a primitive group because they have many features common to the lophophorate ancestor.
The Brachiopoda constitutes a very divergent group amongst the lophophorate coelomates. Because of the lack of specific relationship between them, the Brachiopoda, Phoronida and Ectoprocta have been treated as separate phyla. The idea of uniting them under a common phylum is not favoured nowadays.
Relationship with Annelida:
The Brachiopoda and Annelida have some structural similarities.
These are:
1. The setae in Brachiopoda are comparable with those of Annelida.
2. Presence of metanephridia which also act as gonoducts.
3. The larva resembles a trochophore. The larval segmentation is comparable in both.
The main difficulty lies in the origin of coelom in these two groups. Despite the enterocoelous origin of coelom in
Brachiopoda, the relationship cannot be established with certainly as both the groups differ fundamentally. Because of the presence of trochophore-like larval form, Brachiopoda is regarded to be related to protostomous phyla.
Relationship with Mollusca:
Presence of bivalved shell and mantle- lobes surrounding the body and presence of trochophore-like larval form led many Zoologists to include the brachiopods within the phylum Mollusca. But a closer examination reveals that the shell valves are laterally placed in mollusca, but in brachiopods the shell comprises of dorsal and ventral valves. Because of lack of convincing evidences the relationship cannot be advocated.
Concluding remarks:
After reviewing the relationship of Brachiopoda with other groups, it becomes necessary to place them under a separate phylum having relationship with protostomous and deuterostomous phyla.
As regards the interrelationship between different members of the Brachiopoda, it can be suggested that the articulate brachiopods evolved from inarticulate brachiopods along different lines in their phylogenetic history. The geological records show that the brachiopods originated in Pre-Cambrian period and the oldest brachiopods were the inarticulates belonging to extinct superfamily Obolacea.
