In this article we will discuss about Brachiopoda:- 1. Shape and Symmetry of Brachiopoda 2. Structure of Brachiopoda 3. Body Wall 4. Body Cavity 5. Muscular System 6. Digestive System 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System 11. Reproductive System 12. Development 13. Affinities
Shape and Symmetry of Brachiopoda:
Brachiopoda are marine animals with a large lophophore consisting of a pair of coiled or folded arms bearing ciliated tentacles. The animal is enclosed in a bivalved shell. So they are commonly known as ‘Lamp shells’. The name Brachiopoda was coined by Dumeril (1806) (brachion-arm, podos-foot). This refers to arms arising from the lophophore. Majority of brachiopods, numbering about 30,000 are extinct, only about 200 species are still alive.
Shape:
Body may be oval, spherical, flattened or elongated.
Size:
Varies from 5 to 8 mm in living types and upto 375 mm in extinct types.
Symmetry:
Bilateral
Colour:
Some are transparent, glassy shells, others show bright orange, pink or red radiations, but most possess a yellowish grey colour.
Structure of Brachiopoda:
Shell:
The body is enclosed in a bivalved shell. The valves are separate, bilaterally symmetrical dorsal and ventral in position. In articulate brachiopods (e.g. Magellania) the two valves are hinged together posteriorly by a tooth and socket arrangement. In inarticulate branchiopods (e.g. Lingula) the valves are held together by muscles only and can be opened very widely. The gape of shell is anterior.
The ventral valve is usually larger and is produced above the dorsal valve into a short, curved, spout like beak or funnel with a foramen for the passage of pedicle.
Pedicle:
Most of the brachiopods, possess a cylinderical muscular pedicle arising from the posterior end. It may be long, flexible and passing directly between the proximal ends of the two valves Eg. Lingula. It may be short and passing through a perforation of the ventral valve Eg. Discina. In some forms like Crania a pedicle or peduncle is absent.
Mantle:
Mantle is present beneath the shell valves as two folds of integument, dorsal and ventral lobes. The lobes enclose a wedge-shaped mantle cavity. The main mass of body occupies only the posterior part of the mantle cavity. The anterior part of the mantle cavity is occupied by a large and complex lophophore.
Body Wall of Brachiopoda:
It consists of an epidermis formed by a single layer of epithelial cells, a connective tissue of varying thickness containing fibrils and a thin, flat and ciliated peritoneum lining the body cavity.
Lophophore:
Lophophore is an outgrowth of the anterior body wall. It occupies the anterior part of mantle cavity and surrounds the mouth. Shape of lophophore varies in different brachiopods. The simplest lophophores are discoid type. Tentacles are present on the lophophore and number of tentacles is increased by lobulation of the lophophore.
Additional space is obtained by adding lobes or by extending the lateral lobes of the lophophore. Other types of lophophore are schizolophus, ptycholophus, zygolophus, plectolophus and spirolophus.
In a section, each arm shows a ciliated brachial or food groove, bounded on the inner side by a wavy ridge, the brachial fold or lip. Between the two folds in a median depression is brachial gutter.
Body Cavity of Brachiopoda:
It is a true coelom divided transversely by septa into anterior protocoel, middle, mesocoel and posterior metacoel. The mesocoel and metacoel are only partially separated by a diaphragm at the level of oesophagus. Branches of the mesocoel extend into the tentacles and of the metacoel extend into the mande.
A longitudinal mesentery divides coelom incompletely into two lateral halves, a right and left half. Coelomic fluid flows throughout the coelomic system. Peritoneal cilia or flagella circulate the coelomic fluid.
Muscular System of Brachiopoda:
The shell valves both open and close by muscular action. Three types of muscles associated with shell valves are adductors, divaricators and adjustors. Speciallized muscles are also present in the body wall, lophophore and mantle edges.
Digestive System of Brachiopoda:
Alimentary canal is usually V-shaped and lined with ciliated epithelium. The mouth lies in the centre of the base of the lophophore in a transversely directed food groove. It is a narrow, transversely elongated or cresentic aperture. Mouth leads in to gullet or oesophagus which runs forwards and dorsal wards to open into a dilated stomach. Conspicuous paired diverticula arise from the stomach wall. They are called digestive glands or the liver.
Their function is not known exactly, probably they are sites of intracellular digestion. The stomach passes into a narrow intestine which is directed downwards. In some brachiopods intestine is long and coiled and opens by an anus in the mantle cavity. In others the intestine is short and terminates blindly, there being no anus.
Respiratory System of Brachiopoda:
No specialized respiratory organs are present. Lophophore and mantle lobes probably serve for gaseous exchange.
Circulatory System of Brachiopoda:
Brachiopods have two circulating body fluids, blood and coelomic fluid. Blood circulates in an open circulatory system. The main blood vessel is middorsal and contains a pulsating vesicle. The middorsal vessel divides into anterior branches. Each branch communicates with an extensive sinus around the gut continuing to the lophophore, where branches reach each tentacle.
The middorsal vessel bifurcates posteriorly, forming dorsal and ventral mantle vessels which branch extensively in the mantle. The ventral mantle vessel also communicates with sinuses in the nephridia and gonad. The pulsating vesicle beats slowly and circulation is sluggish.
Blood channels are without definite walls. The blood is colourless, but coelomic fluid contains the respiratory pigment haemerythrin.
Excretory System of Brachiopoda:
Usually consists of a pair of metanephridia lying in the metacoel. Internally each nephridium opens into the metacoel by a large funnel-shaped nephrostome. The tubular part of nephridium narrows and it opens into the mantle cavity by nephridiopore on each side of the mouth.
Particulate material is taken up by coelomic phagocytes and by peritoneal cells that detach and disintegrate. Streams of particle move to nephridium and are discharged to outside by nephridiopores. Besides discharging an excretory function, the metanephridia also serve as gonoducts.
Nervous System of Brachiopoda:
A circumenteric ring connects larger subenteric and smaller supra enteric ganglia. Nerves to the lophophore issue from the supraenteric ganglion and nerves to the mande axis from the subenteric ganglion. The inarticulate nerve system appears to be in the epidermis and the articulate nervous system below the epidermis.
Sense Organs:
Different sense organs described in brachiopods are statocysts in Lingula, rudimentary eyes in Magellania and patches of sensory epithelium in Cistella.
Reproductive System of Brachiopoda:
Generally dioecious, four pairs of gonads are usually present on the peritoneum. Gametes come out of body cavity through nephridia. In majority of species development is external taking place in sea water. Few species brood young in the mantle cavity, nephridia or the lophophore arms.
Development of Brachiopoda:
Cleavage holoblastic and radial giving rise to coeloblastula. Gastrulation is by invagination. The blastopore closes and the mouth arise from the anterior end. The coelom is enterocoelous in articulates and schizocoelous in inarticulates. In the free swimming larva the head region develops a broad cephalic umbrella bordered with cilia.
Metamorphosis:
The larva attaches to the substratum by the peduncular segment. The mantle lobes secrete the shell valves. The permanent setae develop in the place of provisional setae. Finally lophophore with the tentacular arms develops on the inner side of the dorsal mantle lobes and becomes an adult.
Affinities of Brachiopoda:
The affinities of brachiopods can be studied under following heads:
1. Affinities with Annelida
2. Affinities with Mollusca
3. Affinities with Chaetognatha
4. Affinities with Lophophorate coelomates.
1. Affinities with Annelida:
Brachiopods show following resemblances with annelids:
1. Segmentation of the body
2. Presence of Setae
3. Presence of perivisceral schizocoelous coelom
4. Presence of metanephridia
5. Presence of circumoesophageal nerve ring
6. Presence of trochophore like larval stage, and
The above resemblances of brachiopods with annelids are superficial.
They differ from annelids in following:
1. Segmentation external
2. Unsegmented coelom
3. Usually 4 pairs of gonads
4. The detailed plan of nerve system different, and
5. Larva of brachiopods differ from trochophore in many respects.
2. Affinities with Mollusca:
Brachiopods show following resemblances with Mollusca:
1. Presence of shell, and
2. Presence of mantle lobes.
They differ from Mollusca in following:
1. Shell valves are unequal
2. Shell valves are dorso-ventral
3. Presence of peduncle
4. Presence of lophophore, and
5. Enterocoelous nature in some species.
Brachiopods also differ from Mollusca in other internal structures and embryonic development.
3. Affinities with Chaetognatha:
According to Lemeere (1931) Brachiopods show affinities with “arrow worms”. In both 3 pairs of chambers arise from coelom which are separated by transverse septa. Further, grasping spines present in Chaetognatha are compared with the lophophore of brachiopods. The above view of Lameere is not acceptable to other authors.
4. Affinities with Lophophorate Coelomates:
Brachiopods resemble to other lophorate phyla namely Ectoprocta and Phoronida. All these three phyla are characterized by a crown of ciliated tentacles, the lophophore used for food capture. The lophophore is a complex structure and provides strong evidence of relationship.