Example of Phylum Brachiopoda: Magellania

In this article w will discuss about Magellania:- 1. Habit and Habitat of Magellania 2. External Structures of Magellania 3. Body Wall and Masculature 4. Coelom 5. Digestive System 6. Respiratory System 7. Circulatory System 8. Excretory System 9. Nervous System and Sense Organs 10. Reproductive System and Development.

Contents:

1. Habit and Habitat of Magellania
2. External Structures of Magellania
3. Body Wall and Masculature
4. Coelom of Magellania
5. Digestive System of Magellania
6. Respiratory System of Magellania
7. Circulatory System of Magellania
8. Excretory System of Magellania
9. Nervous System and Sense Organs of Magellania
10. Reproductive System and Development of Magellania

1. Habit and Habitat of Magellania:

Magellania, like all other brachiopods, is a marine and benthonic animal. Brachiopods are present in all the seas and at all depths extending from the intertidal zones to the depth of 5000 m.

The genus Magellania in­cludes several species which are widely dis­tributed but are quite abundant in the coast of New Zealand. They remain attached by peduncle to rocky bottoms (Fig. 20.3A). They are ciliar-mucus-feeders and lophophore is the efficient food-driving apparatus. The food includes minute organisms, specially the diatoms.

2. External Structures of Magellania:

The body of Magellania is provided with shell which is divided into a dorsal and a ventral valve. The valves are more or less oval in shape and are unequal in size. The ventral valve is larger and projects over the dorsal valve and forms a short beak.

A peduncle or pedicel emerges through an oval foramen present in the beak and the animal fixes itself on the substratum by the peduncle. The two valves are bilaterally sym­metrical and are hinged posteriorly by tooth and socket arrangement.

The dorsal valve has a strong cardinal process. Lines of growth are represented by concentric rings on the surface of the shell valves. The shell has a thin external covering called periostracum. This layer is produced by organic materials which are probably chitinous in nature. The main inorganic com­ponent is the calcium carbonate.

The body proper is situated inside the posterior end of the shell (Fig. 20.3B). The mantle cavity is largely occupied by a large lophophore. The lophophore is a prolonga­tion of the anterior body wall. It is large in size and occupies the greater part of the mantle cavity. The dorsal shell valve is re­lated to the lophophore.

The lophophore is supported by internal skeleton called brachidium or brachial support. The brachidium consists of a pair of prongs called crura. Inside the lophophore the crura are continued forward as a calcareous ribbon which forms a loop. This long curved loop is present on the dorsal valve. 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. The lophophore is provided with long cili­ated tentacles which help in food capture. The lophophore shows extreme variations in different brachiopods. The type present in Magellania is called plectophous type, i.e., a median coiled arm develops between the two simple lateral arms.

3. Body Wall and Masculature of Magellania:

The body is covered by a single-layered epidermis on the outerside. The epidermal cells are cuboidal or columnar in shape. Interspersed with the epidermal cells there are gland cells.

Beneath the epidermis there is the median connective tissue layer which shows besides the fibres, a cartilaginous consistency in many parts. The inner lining comprises of a thin ciliated peritoneum. The body wall has muscle layer in the inner side of the peritoneum, in the mantle lobes and the lophophore.

Three sets of specialised muscles are present. The adductor or occlusor muscle helps to close the valves and the divaricator or diductor muscles that help to open the valves and the peduncular muscles operate the peduncle.

4. Coelom of Magellania:

The coelom is spacious and the space is divided into three compartments—protocoel, mesocoel and metacoel. These compartments are partially separated from each other. The mesocoel gives large arm canals to the lophophore.

The coelom contains mesenter­ies and muscles. The metacoel constitutes the main body cavity containing major part of the alimentary canal, shell muscles, ne­phridia, gonads, etc. It is also continued into the mantle lobes as mantle canals. This coe­lom communicates with the exterior through a pair of metanephridia.

The coelom is filled with a fluid contain­ing several types of free coelomocytes.

5. Digestive System of Magellania:

The mouth is a transverse slit-like aper­ture situated at the middle of the lophophore which leads into a U-shaped alimentary ca­nal. The mouth leads into an oesophagus which opens into a large stomach. There is a large digestive gland or liver which opens into the stomach.

The liver occupies most of the inner space and is composed of numerous spherical acini. The product of the liver is poured into the stomach by one to three large ducts. The stomach passes into an intestine which ends blindly, i.e. the anus is wanting.

6. Respiratory System of Magellania:

There is no specialised respiratory organ in Magellina, but the lophophore, in addition to food-catching function, plays the respira­tory role. The water current plays double functions of feeding and of drawing in and expelling out respiratory gases. The coelomic fluid present in the tentacular canals, helps in gaseous exchange.

7. Circulatory System of Magellania:

The circulatory system is of open type. The blood channels have no definite wall. These are mere spaces inside the mesenter­ies. The blood is a colourless and cell-free fluid. A globular contractile sac, called ‘heart’ is present. It is attached to the posterior region of the stomach. The blood channels, arising from the heart supply the various parts of the body.

8. Excretory System of Magellania:

The excretory system consists of one pair of 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 through nephridiopore, situated one on each side of the mouth.

In addition to the excretory function, the metanephridia also acts as gonoducts.

9. Nervous System and Sense Organs of Magellania:

The nervous system comprises of a circumenteric nerve ring with a large sub-enteric ganglion and a smaller supraenteric ganglion. The circumenteric nerve ring supplies nerves to the arms and tentacles.

The sub-enteric ganglion gives out several lateral nerves supplying the dorsal mantle lobe and a pair of thick trunk nerves to the posterior side which innervate the ventral mantle lobes, the adductor muscles and finally proceed to the peduncle as pe­duncular nerve.

Special organs of sense are lacking in Magellania, but in other Brachiopods they are represented by statocysts, eyes and sensory patches.

10. Reproductive System and Development of Magellania:

The sexes are separate. The gonads are localised modifications of the peritoneum and are four in number. They resemble bunches of grapes and become considerably enlarged during maturity. When the gam­etes become mature they are discharged into the metacoel from where they are conveyed to the exterior through metanephridia which act as the gonoducts.

Fertilization 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 the annelidan trochophore larva. After a short free-swimming phase the larva fixes itself with the substratum by the peduncular region and metamorphoses into an adult.