Life cycle of Balanoglossus
Life cycle
Sexes are separate. Male and female Balanoglossus live in separate burrows. The male and female are indistinguishable externally. The gonads occur in one or more longitudinal rows along the alimentary canal within genital pleurae in the anterior part of the trunk. Each gonad is a sac which continues into a ductile and opens to the exterior through a gonopore. The saccular gonads are lined with germinal epithelium. By proliferation of cells from the germinal epithelium sperms or ova are produced. They shed gametes in sea water. The egg is microlecithal. The eggs are of two types. The small ovum measures about 0.06 mm in diameter and undergoes indirect development with a pelagic larva known as tornaria larva. The larger one is about 0.4 mm in diameter and undergoes direct development without larval stage. The sperm has a rounded head and a flagella like tail. The mature sperms and eggs are shed into the burrows where fertilization takes place. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. The cleavage is complete and is almost equal. A blastula is formed and later as a result of invagination, a gastrula is developed.
The development is indirect with a tornaria larva in Balanoglossus clavigerus or direct without a tornaria larva in Saccoglossus kowalevskii. After about 18 hours the animal –vegetal axis begins to elongate again and a larva is formed which is known as tornaria larva. It was first of all discovered by J Muller(1850) and was considered by his as the larva of echinoderms. It was Metschenikoff (1869) who established that is a larva of Saccoglossus. Tornaria larva is a free swimming larval stage of Balanoglossus.
Structure of Tornaria larva
The fully grown tornaria larva is usually ovoid in shape and is transparent. The size of tornaria larva varies from below one mm to 9 mm. It is provided with tow ciliated bands. At its anterior end a pair of pigmented eye spots. The mouth is situated on the ventral surface while the anus lies at the posterior terminal end. The alimentary canal is very simple having a mouth, oesophagus, stomach and intestine. A water sac is also found in the body of the larva which opens outside through dorsal pore the hydropore. There are three ciliated band which encircle the body of the larva. A pre-oral ciliated band lies anterior to mouth, post oral ciliated band lies behind mouth and a circum anal ring or posterior ciliated band lies around the anus. The oral bands help in the nutrition by directing water current towards the mouth. The circum anal ring regarded by some as a telotroch develops especially long and powerful cilia and is the chief locomotory organs of the tornaria. The interior of the tornaria contains a complete digestive tract like that of echinoderm larvae and one more coelomic vesicles. The heart vesicle develops in the later stages of tornaria and its pulsation were noticed by early observers.
Metamorphosis
After swimming for some time the tornaria larva sinks down the bottom. Its transparency is lost and the ciliated bands are disintegrated. Eyes also lost. The body elongates and is distinguished into the proboscis, collar and trunk and simultaneously the notochord, gill slits and coelomic sacs are also formed. Thus the larva gradually changes into the adult.
The metamorphosis of tornaria is accompanied by a great diminution in size probably due to loss of water. By this and thickening of skin, larva loses its transparency. Numerous gill slits are developed as outgrowth of the alimentary canal. Reproductive organs make appearance probably from mesoderm, the trunk meanwhile elongates so that the proportions of the adult are acquired.
There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.
Aves (flight and perching mechanism)
Flight is a highly spontaneous action. Larger birds either run or swim rapidly to gather enough forward momentum for a takeoff. Smaller birds s\usually take a quick jump by means of their legs followed by the beating of their wings. A bird flies on the principle of an aero plane, or heavier - than - air machine.
A bird flies on the principle of indirect movement. It moves the air, which by its displacement moves the bird. Air, displaced by the beating of wings, sets up current that keep the animal aloft and move forward, resulting in flight. According to Newton’s third law, the force of reaction of air is equal and opposite to that exerted by the moving wing on the air.
The wing is not a simple airfoil or plane. It functions both as an airfoil (lifting surface) and as a propeller for forward motion. It is thick in font, thin and tapering behind and presenting profile a convex streamlined upper and a flat or slightly concave lower surface. As the air flows across the somewhat tilted wing, the air stream moves faster along its upper convex surface that the concave lower surface. In accordance with Bernoulli’s law in physics, this differential in air speed results in a decrease in air pressure above the wing surface relative to the underside. Bernoulli’s law states that in fluid stream the pressure is least where the velocity is greatest. This basic physical principle involved in flight was first worked out by the Swiss mathematician Bernoulli in 1738. The two forces thus generated, suction above and upward thrust below the wing tend to life, keeping the bird aloft and moving forward and upwards. The air also pushes the wing horizontally backwards and tends to drag or slow the bird down. Thus the force of air on the wing can be resolved into a vertical life component perpendicular to the air steam and a backward drag component parallel to the air stream. For the bird to fly, the lift force must equal the force of gravity on the bird that is the weight of the bird. Various factors increase the lift force such as increase in the surface area of the wing and in the speed of air flow across the wing. If the angle of wing becomes too great that is when the wings become tilted sharply air swirls into the low pressure area above causing turbulence (induced drag). As a result lift is lost and wings stall movement slows down and the plummets toward the earth.
Flight muscles
Birds fly by flapping the wings which are the modified forelimbs. The flight muscles are many and belong to three main categories. They are pectoral, accessory and tensor.
• Pectoral muscles- the most important flight muscles are pectoralis major, pectoralis minor. Pectoralis major is depressor muscle causing downward stroke. When it contracts, the wing is pulled downward and forward so that the body is lifted and propels itself through air. Pectoralis minor is an elevator muscle causing upstroke of the wing. During flight, pectoralis major and minor contract and relax alternately in rapid succession.
• Accessory muscle- several small accessory muscle also help in elevating and depressing wing. They mainly help to rotate the wing in the glenoid cavity.
• Tensor muscle- these deltoid muscle keep the prepatagium fully stretched when the wing is spread out during flight.
Perching mechanism
The muscles of the legs are enlarged and strong. The shank and feet have few muscles and look slender and delicate. But certain muscles in the upper part of the legs have a special arrangement and long tendons for moving the toes. As a result, when a bird sits on a perch ( branch of tree, wire or rod) and squats, its toes are mechanically flexed and firmly grasp the perch. The muscles involved are known as perching mechanism. This is quite automatic and its enables the bird even to sleep on a twig without any risk of falling down. Perching muscles are characteristic of all birds. They comprise of two sets of muscles flexor and extensor.
Flexor muscles
Gripping of the toes is chiefly accomplished by the action of 8 flexor muscles, 6 to the anterior toes and 2 to the hind toe or hallux. The important flexor muscles are
• Ambiens - small but characteristic muscle. Its tendon join upper end of the flexor muscle of the second and third toe. It has minor role in perching.
• Peroneus medius – this muscle is present on the anterior aspect of shank. Its tendon trifurcates to supply the three anterior toes.
• Gastrocnemius – it is big calf muscle present on the back of tibiotarsus. Its tendon joind those of peroneous muscle to supply to anterior toes.
• Flexor perforans – this muscle is also attached to upper part of the tibiotarsus. Its tendon passes to the hind toe. It is joined by a slip with the peroneus medius so that a pull on any tendon will flex all the toes.
Extensor muscle
Several extensor muscles are found at the front of the tibiotarsus. They become attached to the upper part of the phalanges. The contraction serves to open the toes when the bird raises the shank while taking off the perch.
Life cycle
Sexes are separate. Male and female Balanoglossus live in separate burrows. The male and female are indistinguishable externally. The gonads occur in one or more longitudinal rows along the alimentary canal within genital pleurae in the anterior part of the trunk. Each gonad is a sac which continues into a ductile and opens to the exterior through a gonopore. The saccular gonads are lined with germinal epithelium. By proliferation of cells from the germinal epithelium sperms or ova are produced. They shed gametes in sea water. The egg is microlecithal. The eggs are of two types. The small ovum measures about 0.06 mm in diameter and undergoes indirect development with a pelagic larva known as tornaria larva. The larger one is about 0.4 mm in diameter and undergoes direct development without larval stage. The sperm has a rounded head and a flagella like tail. The mature sperms and eggs are shed into the burrows where fertilization takes place. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. The cleavage is complete and is almost equal. A blastula is formed and later as a result of invagination, a gastrula is developed.
The development is indirect with a tornaria larva in Balanoglossus clavigerus or direct without a tornaria larva in Saccoglossus kowalevskii. After about 18 hours the animal –vegetal axis begins to elongate again and a larva is formed which is known as tornaria larva. It was first of all discovered by J Muller(1850) and was considered by his as the larva of echinoderms. It was Metschenikoff (1869) who established that is a larva of Saccoglossus. Tornaria larva is a free swimming larval stage of Balanoglossus.
Structure of Tornaria larva
The fully grown tornaria larva is usually ovoid in shape and is transparent. The size of tornaria larva varies from below one mm to 9 mm. It is provided with tow ciliated bands. At its anterior end a pair of pigmented eye spots. The mouth is situated on the ventral surface while the anus lies at the posterior terminal end. The alimentary canal is very simple having a mouth, oesophagus, stomach and intestine. A water sac is also found in the body of the larva which opens outside through dorsal pore the hydropore. There are three ciliated band which encircle the body of the larva. A pre-oral ciliated band lies anterior to mouth, post oral ciliated band lies behind mouth and a circum anal ring or posterior ciliated band lies around the anus. The oral bands help in the nutrition by directing water current towards the mouth. The circum anal ring regarded by some as a telotroch develops especially long and powerful cilia and is the chief locomotory organs of the tornaria. The interior of the tornaria contains a complete digestive tract like that of echinoderm larvae and one more coelomic vesicles. The heart vesicle develops in the later stages of tornaria and its pulsation were noticed by early observers.
Metamorphosis
After swimming for some time the tornaria larva sinks down the bottom. Its transparency is lost and the ciliated bands are disintegrated. Eyes also lost. The body elongates and is distinguished into the proboscis, collar and trunk and simultaneously the notochord, gill slits and coelomic sacs are also formed. Thus the larva gradually changes into the adult.
The metamorphosis of tornaria is accompanied by a great diminution in size probably due to loss of water. By this and thickening of skin, larva loses its transparency. Numerous gill slits are developed as outgrowth of the alimentary canal. Reproductive organs make appearance probably from mesoderm, the trunk meanwhile elongates so that the proportions of the adult are acquired.
There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.
Aves (flight and perching mechanism)
Flight is a highly spontaneous action. Larger birds either run or swim rapidly to gather enough forward momentum for a takeoff. Smaller birds s\usually take a quick jump by means of their legs followed by the beating of their wings. A bird flies on the principle of an aero plane, or heavier - than - air machine.
A bird flies on the principle of indirect movement. It moves the air, which by its displacement moves the bird. Air, displaced by the beating of wings, sets up current that keep the animal aloft and move forward, resulting in flight. According to Newton’s third law, the force of reaction of air is equal and opposite to that exerted by the moving wing on the air.
The wing is not a simple airfoil or plane. It functions both as an airfoil (lifting surface) and as a propeller for forward motion. It is thick in font, thin and tapering behind and presenting profile a convex streamlined upper and a flat or slightly concave lower surface. As the air flows across the somewhat tilted wing, the air stream moves faster along its upper convex surface that the concave lower surface. In accordance with Bernoulli’s law in physics, this differential in air speed results in a decrease in air pressure above the wing surface relative to the underside. Bernoulli’s law states that in fluid stream the pressure is least where the velocity is greatest. This basic physical principle involved in flight was first worked out by the Swiss mathematician Bernoulli in 1738. The two forces thus generated, suction above and upward thrust below the wing tend to life, keeping the bird aloft and moving forward and upwards. The air also pushes the wing horizontally backwards and tends to drag or slow the bird down. Thus the force of air on the wing can be resolved into a vertical life component perpendicular to the air steam and a backward drag component parallel to the air stream. For the bird to fly, the lift force must equal the force of gravity on the bird that is the weight of the bird. Various factors increase the lift force such as increase in the surface area of the wing and in the speed of air flow across the wing. If the angle of wing becomes too great that is when the wings become tilted sharply air swirls into the low pressure area above causing turbulence (induced drag). As a result lift is lost and wings stall movement slows down and the plummets toward the earth.
Flight muscles
Birds fly by flapping the wings which are the modified forelimbs. The flight muscles are many and belong to three main categories. They are pectoral, accessory and tensor.
• Pectoral muscles- the most important flight muscles are pectoralis major, pectoralis minor. Pectoralis major is depressor muscle causing downward stroke. When it contracts, the wing is pulled downward and forward so that the body is lifted and propels itself through air. Pectoralis minor is an elevator muscle causing upstroke of the wing. During flight, pectoralis major and minor contract and relax alternately in rapid succession.
• Accessory muscle- several small accessory muscle also help in elevating and depressing wing. They mainly help to rotate the wing in the glenoid cavity.
• Tensor muscle- these deltoid muscle keep the prepatagium fully stretched when the wing is spread out during flight.
Perching mechanism
The muscles of the legs are enlarged and strong. The shank and feet have few muscles and look slender and delicate. But certain muscles in the upper part of the legs have a special arrangement and long tendons for moving the toes. As a result, when a bird sits on a perch ( branch of tree, wire or rod) and squats, its toes are mechanically flexed and firmly grasp the perch. The muscles involved are known as perching mechanism. This is quite automatic and its enables the bird even to sleep on a twig without any risk of falling down. Perching muscles are characteristic of all birds. They comprise of two sets of muscles flexor and extensor.
Flexor muscles
Gripping of the toes is chiefly accomplished by the action of 8 flexor muscles, 6 to the anterior toes and 2 to the hind toe or hallux. The important flexor muscles are
• Ambiens - small but characteristic muscle. Its tendon join upper end of the flexor muscle of the second and third toe. It has minor role in perching.
• Peroneus medius – this muscle is present on the anterior aspect of shank. Its tendon trifurcates to supply the three anterior toes.
• Gastrocnemius – it is big calf muscle present on the back of tibiotarsus. Its tendon joind those of peroneous muscle to supply to anterior toes.
• Flexor perforans – this muscle is also attached to upper part of the tibiotarsus. Its tendon passes to the hind toe. It is joined by a slip with the peroneus medius so that a pull on any tendon will flex all the toes.
Extensor muscle
Several extensor muscles are found at the front of the tibiotarsus. They become attached to the upper part of the phalanges. The contraction serves to open the toes when the bird raises the shank while taking off the perch.
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