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In this article we will discuss about Brachiopoda:- 1. Meaning of Brachiopoda 2. Habit and Habitat of Brachiopoda 3. External Structure 4. Internal Structure 5. Muscular System 6. Digestive System 7. Circulatory System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Affinities.
Contents:
- Meaning of Brachiopoda
- Habit and Habitat of Brachiopoda
- External Structure of Brachiopoda
- Internal Structure of Brachiopoda
- Muscular System of Brachiopoda
- Digestive System of Brachiopoda
- Circulatory System of Brachiopoda
- Excretory System of Brachiopoda
- Nervous System of Brachiopoda
- Reproductive System of Brachiopoda
- Affinities of Brachiopoda
1. Meaning of Brachiopoda:
The Brachiopoda or “lamp-shells” are coelomate Bilateria that are enclosed in a bilaterally symmetrical bivalve shell attached directly or by way of stalk (peduncle) and composed of dorsal and ventral valves lined by a mantle lobe of the body wall and that are provided with a lophophore, an open circulatory system with a dorsal contractile vesicle and one or two pairs of metanephridia, also acting as gonoduct.
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The structural organisation in all the brachiopods is more or less similar except the articulation of two shell valves. Here Magellania has been described as a typical example of the phylum Brachiopoda.
2. Habit and Habitat of Brachiopoda:
Magellania is a marine and benthonic animal like all other brachiopods. Brachiopods are found in all seas at different depths from between tide marks to 2900 fathoms. The larger number lives at moderate depths, down to 500 or 600 fathoms.
Magellania includes several species which are widely distributed but are quite abundant off the coast of New Zealand. They remain attached by peduncle to rocky ground. They are ciliary feeders with lophophore as the food catching apparatus. They comprise minute marine organisms specially diatoms.
3. External Structures of Phylum Brachiopoda:
Shell:
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The body is entirely covered by a bivalved shell of oval form and pink colour. The valves are separate, bilaterally symmetrical but more or less dissimilar in size and are dorsal and ventral in position. Both valves are deeply concavo-convex of a pinkish or brown colour outside and white within. The ventral valve is produced posteriorly into a beak terminating in a foramen for the peduncle.
The distal margin of the foramen is left incomplete by the shell proper but is closed by a small double plate, the deltidium.
Immediately anterior to the beak is the curved hinge-line along which the valve articulates with its fellow and just anterior to the hinge-line the inner surface of the shell is produced into a pair of massive irregular hinge- teeth. On the inner surface of the valve towards its posterior end, are certain shallow depressions making the attachments of muscles.
The dorsal valve has no beak, but its posterior edge forms a hinge-line which is produced in the middle into a strong cardinal process with a folded surface. When the valves are in position this process fits between the hinge-teeth of the ventral valve, the hinge-teeth in their turn being received into depressions placed on each side of the cardinal process.
The inner surface of the dorsal valve is produced into a median ridge or septum, continuous posteriorly with the cardinal process.
Attached on either side of the base of the cardinal process are the two ends of a delicate calcareous ribbon, the shelly loop which projects freely into the cavity enclosed between the two valves. The shelly loop has the form of a simple loop bent upon itself. The inside of the dorsal valve also has muscular impressions.
The valves externally present a series of concentric ridges, the lines of growth. The shell is made up of prismatic rods or spicules of calcium carbonate placed obliquely to the external surface made up of chitinous substance mixed in the carbonate of lime and called periostracum. The calcareous spicules are separated from one another by a thin layer of membrane. Further the shell is traversed by perpendicularly disposed delicate tubules closed on the outer surface.
4. Internal Structures of Phylum Brachiopoda:
(i) Mantle:
The body proper is situated inside the posterior end of the shell. Just below the shell valves or closely applied to the shell valves are present corresponding folds of integument, the dorsal and ventral mantle lobes. Between the dorsal and ventral lobes is enclosed a wedge- shaped space, the mantle cavity.
The outer surface of mantle lobes give off hollow processes which extend into the tubules of the shell. The margin of the mantle lobes is fringed with minute setae lodged in muscular sacs like those of chaetopods. The mantle lobes are formed by reduplication of body wall.
(ii) Body Wall:
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The body wall comprises three layers; an outer epidermis consisting of single layer of cells, middle layer composed of connective tissue of varying thickness which is more or less cartilaginous at many places and a layer of ciliated coelomic epithelium lining the body-cavity.
(iii) Body-Cavity or Coelom:
The body-cavity is the true coelom. It is quite spacious and is divided into three compartments protocoel, mesocoel and metacoel. These compartments are partially separated from each other. The coelom contains mesenteries and muscles. The mesocoel gives large arm canals to the lophophore.
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The metacoel constitutes the main body cavity containing major part of the alimentary canal, shell muscles, nephridia and gonads, etc. It is also continued into the mantle lobes as mantle canal. The coelom is lined by a ciliated epithelium. It is filled with a fluid containing several types of free coelomocytes. The coelom communicates to the exterior through a pair of metanephridia.
(iv) Lophophore:
The mantle cavity is largely occupied by a large lophophore. The lophophore is an outgrowth of the anterior body wall that projects into the mantle cavity and fills the greater part of this cavity, being usually quite voluminous relative to the size of the body of the animal. The lophophore is the most conspicuous horse-shoe-shaped structure consisting of two spirally coiled tentacular arms.
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The two arms of the lophophore curve inwards and coil to fit into the mantle cavity. The middle of the concave edge, which is dorsal in position, is produced into a spirally coiled offshoot, and lies coiled towards the dorsal side between the two arms. The lophophore is hollow internally containing a spacious cavity or sinus. Its two main arms also receive prolongations of the coelom into which project the digestive glands.
The lophophore is fringed throughout its whole extent with long ciliated tentacles which form the outer boundary of a ciliated food-groove, bounded on the inner side by a wavy ridge or lip. By the action of the cilia microscopic particles are swept along the food-groove to the mouth.
The lophophore is supported by internal skeleton called the brachidium or brachial support.
The brachidium comprises a pair of prongs called the crura. Inside the lophophore the crura are continued forward as calcareous ribbon which form a loop. This long curve loop is present on the dorsal side. The loop is not attached with the dorsal valve. The calcareous support of the lophophore develops from the inner lamina of the dorsal mantle lobe.
5. Muscular System of Brachiopoda:
The muscular system is well-developed. Two large adductor muscules arise on each, side from the dorsal valve, and passing downwards, unite with one another so as to have a single insertion on the ventral valve. The action of these muscles is to approximate the valve and so to close the shell.
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A large and a small pair of divaricators arise from the ventral valve and are inserted into the cardinal process, which they depress, as this process is situated posteriorly to the hinge-line, its depression raises the rest of the dorsal valve and so open the shell.
Two pairs of muscles arising, one from the ventral and the other from the dorsal valve, and inserted into the peduncle, are called adjusters. The peduncle being fixed, the adjustors serve to alter or adjust the position of the animal as a whole by turning it in various directions.
6. Digestive System of Brachiopoda:
The mouth is a narrow crescentic slit-like aperture situated in the middle of the lophophore. It is bounded dorsally by the brachial fold or lip and ventrally by the tentacular fringe of the lophophore. The alimentary canal is U-shaped and lined with ciliated epithelium. The mouth leads into the oesophagus which opens into a large stomach.
On each side of the stomach and opening into it by a duct is a large, branched digestive gland or liver. The stomach passes into a narrow and straight intestine which is directed downwards and backwards towards the ventral surface and ends blindly. The anus is absent.
7. Circulatory System of Brachiopoda:
The circulatory system is simple and of open type. It consists of a contractile vesicle called heart and found attached to the dorsal mesentery. The blood channels arising from the heart supply to the various parts of the body. The blood channels have no definite wall. These are merely spaces inside the mesenteries. The blood is colourless and cell free fluid. It is coagulable.
8. Excretory System of Brachiopoda:
The excretory system consists of a pair of large metanephridia situated in the metacoel. Each metanephridium is a tubular structure, one end of which opens into the coelom by a wide funnel- shaped nephrostome, while the other end extends anteriorly and opens into the mantle cavity, the nephridiopore, situated one on each side of the mouth. The metanephridia also act as gonoducts.
9. Nervous System of Brachiopoda:
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The nervous system (Fig. 57.5) comprises a supra-oesophageal ganglion in front of the mouth and a larger infra- oesophageal ganglion behind the mouth both connected by oesophageal connective. The infra- oesophageal ganglion gives off nerves to the dorsal mantle, dorsal arms, and adductor muscles and two small ganglia which supply the ventral mantle lobe and the muscles of the peduncle.
The ganglia and commissures are in immediate contact with the ectoderm. No special sense-organs are known so far in Magellania, but in other brachiopods sense organs are represented by statocysts, eyes and sensory pathches.
10. Reproductive System of Brachiopoda:
The sexes are separate. There are two pairs of gonads, one pair dorsal and one pair ventral, near the intestine. The gonads are in the form of irregular organs sending off branches into the pallial sinuses. When the gametes become mature they are discharged into the metacoel from where they are conveyed to the exterior through metanephridia which act as gonoduct.
Fertilisation is external. The cleavage is holoblastic and occurs along the radial plane. A coeloblastula is formed which eventually becomes a gastrula by emboly. Coelom is enterocoelous in this case. Mesoderm differentiates as a single sac which becomes separated off from the posterior end of the archenteron by a developing partition.
After subsequent developmental stages, a free swimming ciliated larva emerges out which resembles closely to the annelidian trochophore larva. After short free swimming phase the larva fixes itself with the substratum by the peduncular region and metamorphoses into an adult.
11. Affinities of Brachiopoda:
Brachiopoda shows affinities with Mollusca, Annelida, Ectoprocta, Chaetognatha and Phoronida.
(i) Affinities with Mollusca:
Presence of bivalved shell and mantle lobes surrounding the body and trochophore-like larval form led many zoologists to include the brachiopods within the Mollusca. But the shell valves are dorsal and ventral in Brachiopoda in contrast with the lateral valves in Mollusca. They also differ in peduncle, internal structure of arm and embryology, their affinities cannot be advocated.
(ii) Affinities with Annelida:
The Brachiopoda and Annelida have many structural similarities.
These similarities are:
(1) Indication of segmentation of body.
(2) Presence of setae.
(3) Presence of a well-developed perivisceral coelom.
(4) Presence of metanephridia which also act as gonoducts.
(5) The larva resembles a trochophore. The larval segmentation is comparable in both the Annelida and Brachiopoda.
In the light of these affinities, the Brachiopoda may be regarded as annelids with three segments marked in the embryo by the annular constriction of the integument. Thus, the Brachiopoda may be considered segmented annelids like Cephalobranch or Oligonereis which have become fixed and transformed their tubes into a pair of calcareous scales (valves) adhering to the epidermis.
Segmentation of brachiopods is only superficial and there is no segmentation of the body cavity or genital organs or nervous system or segmental organs in the adult. Therefore, the resemblances of Annelida with Brachiopoda do not carry much weight and the Brachiopoda differ from Annelida basically.
(iii) Affinities with Ectoprocta:
The Brachiopoda is related to Ectoprocta by possessing many similarities.
These similarities are:
(1) Both Brachiopoda and Ectoprocta have similar body plan.
(2) A coelomic septum is present between mesocoel and metacoel.
(3) U-shaped alimentary canal in both.
(4) The bivalve shell of cyphonautes larva of Ectoprocta is comparable to the shell of Brachiopoda.
Since these common features are due to the descendence from a common lophophorate ancestor, the above mentioned affinities are unsatisfactory due to the following main differences:
(1) The nervous centre is mainly supra-enteric in Ectoprocta, but in Brachiopoda it is sub-enteric.
(2) The Brachiopoda shell cannot be compared to the exoskeleton of Ectoprocta.
(3) The shell is laterally placed in Entoprocta, but in Brachiopoda the shell is dorso-ventrally placed.
(4) The chitinous setae are present in the brachiopods but in Ectoprocta no such setae have been observed.
(5) The coelomic septum is poorly developed in most brachiopods.
(6) The anus is lacking in some brachiopods.
(iv) Affinities with Chaetognatha:
Lemeere (1931) considered closer affinities between Chaetognatha and Brachiopoda. The affinities are based chiefly on the presence of longitudinal, dorsal and ventral mesenteries and the enterocoelic mode of origin of coelom and mesoderm in both the groups but probably without any phylogenetic significance.
Even if phylogenetic significance may be given to the development it will be seen that in the present case only the appearance of mesoderm has some similarity between the two and the further development is quite different and so is the organisation of the adult.
(v) Affinities with Phoronida:
The Brachiopoda and Phoronida have many similarities such as:
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(1) Similar lophophore.
(2) Epistome representing the anterior segment of the body.
(3) U-shaped alimentary canal.
(4) Presence of septum separating the mesocoel and metacoel.
(5) Presence of subepidermal nerve plexus.
(6) A pair of metanephridia in the metacoel acting also as gonoducts.
(7) Derivation of the mouth directly from blastopore.
(8) The dorsal surface between the mouth and the anus becomes greatly shortened.
In spite of the above-mentioned similarities, these two groups have many structural differences. The differences are as follows:
(1) The nerve centre is supra- enteric in Phoronida but it is sub-enteric in Brachiopoda.
(2) Two sets of tentacles (larval and definite) are present in Phoronida but in Brachiopoda the larval set is absent.
(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 of closed type.
(6) The pattern of cleavage is spiral in Phoronida but in Brachiopoda it is not so. Because of such differences the two groups are placed in separate phyla. The similar features are due to remote connection with the ancestral stalk. The recent knowledge of the anatomy and development of brachiopods suggests that the group may be assigned the position of an independent phylum.
