Anatomical development of mammal from fertilization to organogenesis
The egg or ovum of rabbit is 0.1 m in diameter while that of human being is 0.15 mm in diameter. It is oval in shape and on its outer surface, there is a thin vitelline membrane. The egg contains cytoplasm. In the cytoplasm, there is eccentric nucleus. The nucleus lies in the upper half or animal hemisphere. The egg of mammal is alecithal or micro lecithal with almost no yolk or lower half or vegetal hemisphere has some particles of yolk distributed evenly in the cytoplasm. The whole egg is surrounded by a striated secondary membrane called zona pellucida secreted by follicular cells. Outside zona pellucida, there are follicle cells forming corona radiata which is soon dissolved away.
The secondary oocyte is released from the Graafian follicle into the peritoneal cavity outside ovary. The wall of ovary ruptures to release the egg which is known as ovulation. In human beings, every month or every 28 days, one ovum generally is released either from right or left ovary alternating with one another. The development of ovary and development of egg or ovum within the ovary is under the influence of Follicle Stimulating Hormone FSH produced from the Pituitary gland. The developing egg or ovum starts secreting female hormone called Estrogen.
Soon after the release of ovum, the follicular cells and blood clot forms the Corpus Luteum. The Corpus Luteum acts as endocrine gland and produces important hormone known as Progesterone. Both female hormones are responsible for development of secondary sex organs like the oviducts, uterus, vagina and mammary gland etc. and tertiary sex characters like different pattern of distribution of hairs, high pitched voice and little bit of shyness in case of human beings and deposition of fat at different parts of body like thigh and broad hip in case of man and other mammals.
The egg is fertilized in the upper part of oviduct called ampulla which is slightly swollen part after the fimbriated funnel. For fertilization, entire spermatozoon penetrates the egg but soon the tail is degenerated. The penetration by sperm is brought about by an enzyme hyaluronidase present in the semen. The haploid set of chromosome present in egg and sperm arranged into homologous pairs and diploid chromosomes is restored. The restoration of diploid chromosome is called fertilization. With fertilization, the egg is changed into zygote.
The zygote undergoes second maturation division and becomes mature ovum. Two polar bodies lie within the zona pellucida near the animal pole. The fertilization ensures the division of zygote and further development of it.
Cleavage
Blastocyst
Morula absorbs fluid secreted by the uterine mucous membrane and swells up. The liquid food is collected in a cavity which separate an outer layer of smaller trophoblast cells from a solid inner mass of cells. The embryo is now called a blastocyst which appears like a blastula but is not comparable to it. Those trophoblast cells which lie above the inner cells mass are called cells of Rauber. The inner cell mass in sonly attached to the trophoblast at animal pole like a knob. It is called embryonal knob because all parts of embryo will be derived from it. Embryonal knob is comparable to the area pellucid of chick embryo. The embryo now forms a vesicular structure called blastocyst or blastosphere.
As cavity enlarges the knob like inner cell mass becomes flattened and known as germinal or embryonal disc, which gives rise to embryo proper. In case of rabbit and other mammals, the trophoblast cells (cells of Rauber) overlying embryonal disc disintegrate. As a result the embryonal disc comes to lie at the surface of blastodermic vesicle or blastoderm and its edge becomes continuous with trophoblast around it. The trophoblast is equivalent to extra embryonic ectoderm. The cavity of embryo is equivalent to sub germinal cavity of chick. However blastocyst stage is peculiar to mammalian development.
Implantation
Gastrulation
In the centre of embryonal disc, cells collect to form a primitive streak, movement of the cells of primitive streak on both sides result in the separation of mesoderm which at first are un segmented and have no cavity. Cells move forward from the primitive streak to form notochord. Formation of mesoderm and notochord is the same as in chick. Formation of mesodermal somites and splitting of lateral plate mesoderm with appearance of splanchnocoel proceed in the same manner as in chick. However in rabbit, the lateral plate mesoderm extends as far as the equator, so that the lower wall of yolk sac consists of only ectoderm. The primitive streak retreats towards hind end. After giving up the chorda mesoderm cells of the notochord and mesoderm, the remaining cells of the embryonal disc are ectoderm. The ectoderm cells form neural plate and neural folds which become a neural tube as in chick. The mammalian embryo in its later stages is called foetus.
Balanoglossus (tongue worm)
Hemichordata ( Gr. Hemi = half, chorde = cord) has been treated as subphylum of Phylum Chordata ( or Protochordata). Some authors regarded it to be independent phylum close to phylum Echinodermata. But for convenience of comparative study of protochordates, hemichordate has been retained in the subphylum of Phylum choradata. Sub phylum Hemichordata includes a group of soft, vermiform, marine and primitive chordates. The most popular hemichordate genus is Balanoglossus commonly known as tongue worm or acorn worm. Other genus is Saccoglossus. They belong to class Enteropneusta (Gr. Enteron, gut; pheustos, breathed), some characteristics in the animals of this class are body large and worm like, gill slits numerous, intestine straight.
Systematic position
Phylum Chordata
Group Protochordata ( Acraniata)
Subphylum Hemichordata
Class Enteroneusta
Family Ptychoderidae
Type Balanoglossus ( tongue worm)
Delie Chaije in 1829 named and recorded Balanoglossus clavigerus. The genus Balanoglossus was derived from two Greek words, Balanos and glossus. The term balanos means an “acorn” ( fruit of oak) and refers to the proboscis projecting from collar looking like acorn nut, therefore the common name acorn worm is given. The other term glossus means tongue which refers to the shape of its proboscis, collar and genital wings bearing a close resemblance to an ox tongue so, tongue worm is given to it. The fisher men call Balanoglossus by the name of ox tongue.
Geographical distribution
Balanoglossus and other hemichordates are marine animal. It is cosmopolitan in distribution. Balanoglossus is found in tropical and sub tropical seas of world. Some other species of Balanoglossus are B australiensis ( Australia), B carnosus ( Indo Pacific) B misakiensis ( Japan), B jamaicensis (West Indies), B gigas (Brazil), B capensis ( South Africa), B clavigerus ( Mediterrannean and British isles).
Habits and Habitat
Balanoglossus is a marine, tubiculous or burrowing hemichordates. It inhabits shallow waters of intertidal zone and few occur in deep water also.
Burrow
The animal may hide under stones, or sea weeds or excavate its own burrow in bottom sand or mud. B clavegerus makes a U shaped tube or burrow having two vertical limbs of about 50 to 70 cm deep and two openings are 10 to 30 cm apart. In Saccoglossus, the body of the animal and tube are twisted but the anterior and posterior extremities are straight. The anterior opening of the burrow is funnel shaped and wide. The anterior vertical limb may give out branches each having funnel like opening. The posterior opening of the burrow is round and concealed below the spirally coiled faecal matter of the animal.
Protective device
The inner wall of the fragile burrow is smoothly lined by sand particles cemented together into a tough cast with sticky mucus secreted by the mucous gland present in the skin of the animal. Due to this the walls of the burrow are not collapsed and protects the delicate body of animal from burial in loose sand. They also secret foul smelling odour like iodoform for protection.
Movements
The tongue worm is sluggish in movement. It is little affected by the external stimuli. It can move inside burrow with the help of cilia covering its body surface. The proboscis is the most active part of its body. The proboscis can be elongated or shortened by the action of muscle. The proboscis can help in burrowing. When the tide is over, Balanoglossus protrudes its anterior end out of the burrow to explore the surrounding. The worm can extend the posterior end out of the burrow to cast the faecal material.
Feeding
Acorn worm takes a lot of sand or mud to obtain diatoms, protozoans and other microorganisms and organic detritus present in it. Sand or mud is not the food but as it cannot separate food and sand or mud they consume both. They take lot of sand or mud because the sand or mud contains little amount of food.
External structure
- Shape – the shape of its body is cylindrical, worm like and bilaterally symmetrical.
- Size – the body measures about 10 to 50 cm in length but it may differ according to species. B gigas attains about 1.8 m in its body length according to Sawaya 1951 or 2.5 m according to Spengel 1893.
- Color – color of its body is bright or drab with reddish or orange tint.
- Locomotory organ – the body is uniformly covered by short hair like cilia. Exoskeleton and external appendages are absent.
Division of body
The body of acorn worm is unsegmented. It is divisible into three distinct regions or parts like proboscis, collar and trunk.
Proboscis
Proboscis is also known as protosome which is the anteriormost part the body. Proboscis is club shaped or conical. It is circular in cross section. It has thick muscurlar wall and cavity inside is proboscis coelom. The proboscis coelom communicates to the exterior through minute proboscis pore situated mid dorsally near its base. In some proboscis pore ends blindly or there are two pores. Posteriorly the proboscis narrows into a slender neck or proboscis stalk which is attached to the collar. Below the stalk, the base of proboscis bears U shaped ciliated epidermal depression called the preoral ciliary organ which tests the quality or food and water entering the mouth.
Collar
Collar is also known as mesosome. It is middle, short and cylindrical part. The flap like or funnel like anterior margin is called as collarette. It completely surrounds and conceals the proboscis stalk and posterior part of proboscis. Ventrally, below the proboscis stalk, the collarette encloses a permanently open wide aperture called as mouth. The mouth opens into a cavity called buccal cavity which lies inside collar. The collar is well demarcated from the trunk by circular constriction. The wall of the collar is highly muscular. There is a cavity called collar coelom. The collar coelom opens to the outside through a pair of collar pores into the first pair of gill pouches behind.
Trunk
Trunk is also called as metasome. It is the posteriormost and the largest part of the body. The trunk is flat and appears annulated due to circular constrictions on the surface. The trunk bears a mid dorsal ridge and mid ventral ridge which correspond the nerve cord and blood vessel. The trunk is further divisible into three regions.
Branchiogenital region
Branchiogenital region is marked by a pair of lateral, thin, flat and longitudinal flaps called the genital wings which contains gonads. The gonopores are microscopic and cannot be seen by unaided eye. The anterior half of the branchiogental region bears two longitudinal rows of branchial apertures or gill pores. The number of gill pores increases with the age of animal. Two genital wings can be curved to meet mid dorsally so as to conceal the gill pores.
Hepatic region
The middle hepatic region of the body is somewhat smaller than branchiogenital region. It is greenish in color and its dorsal surface is marked by presence of numerous irregular intestinal sacculations or hepatic caeca.
Posthepatic region
This is the posteriormost and the longest part of the trunk. It is also called abdomen or caudal region. It is more or less of uniform diameter. The posterior end tapers somewhat and bears terminal anus.
Life cycle
Sexes are separate. Male and female Balanoglossus live in separate burrows. When fully matured, they shed gametes in sea water. The egg is microlecithal. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. Embryo is formed. Embryo hatches into a free swimming planktonic larva called tornaria. It is ciliated larva. The development is indirect.
There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.
Pisces (osmoregulation and scales
Every organism in natute has a close relationship with its environment for water and salts. An optimum concentrations of water and salts is essential in the body for proper metabolism. The maintainance of such a concentration is called osmoregulation. There may be three possible conditions of the concentration of waer and salts in the body of an organism in comparision to the concentration of the same substances in the environment.
- isotonic or iso osmotic- the cell or organism kept in it experiences no change in its volume. In this situation the osmotic pressure is same both inside and outside the cell or organism.
- Hypotonic or hypo osmotic – which is more diluted as compared to the other. The solvent or water enters into the body of organism from surrounding environment.
- Hypertonic or hyper osmotic – which is more concentrated as compared to the other. The solvent or water comes out of the body of the organism into the surrounding environment.
Animals can be put in two categories according to their tolerance for osmotic changes.
- Stenohaline - this includes the animals which can tolerate the narrow range of salinity. They change the osmotic pressure of their internal fluids according to the changes in their external medium. This includes crustaceans and several mollusks.
- Euryhaline – this includes the animals which can tolerate the broad range of salinity. They do not change the osmotic pressure of their internal fluids according to the changes in their external medium. They can maintain the concentration of their body fluids at the same level in all situations.
Marine elasmobranchs like sharks, rays and skates have a special urea retention device. The migratory fishes have their own peculiarity of osmoregulation. For instance, when the fresh water eel reaches the sea for spawning, there is a tendency for the fish to lose water and to increase the salt concentration in its body. The excess salts are removed to the outside by the active transport through the chloride –secreting cells of the gills. The kidneys keep on reabsorbing water. When they return to the rivers and grow into adults, they have to face the reverse problem where they remove the excess water along with the urine and reabsorb salts into the blood. These changes in the kidneys are regulated by the endocrine secretions.
Scales of fishes
According to the mode of origin, there are tow types of scales. (1) those which are formed due to the secretary activity of both epidermis as placoid scales of elasmobranch (ii) non placoid scales that are derived from dermis only as the scales of teleosts.. structurally the scales are classified as cosmoid, genoid or rhomboid, placoid, cycloid and ctenoid. The last two are called bony ridge scales.
Cosmoid scales – the cosmoid scales are found in extinct Crossopterygii and Dipnoi.
Ganoid scales – ganoid scales are characteristic of the primitive acrinopterygians called ganoid fishes. These scales are heavy and have an outer layer of hard inorganic, enamel like material called ganoine. The middle layer is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone called isopedine.
Placoid scales – Placoid scales are characteristic of elasmobranch fishes( sharks). Each scale has a disc like basal plate embedded in the dermis and a spine projecting out through the epidermis. Placoid scale resembles the tooth. The spine has an external covering of enamel like hard transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from which several branching dentine tubules radiate in different directions. Placoid scales are closely set in skin but do not overlap each other and fiving a sand paper like quality to skin.
Cycloid scales – cycloid scales are thin flexible, translucent plates, rather circular in outline, thicker in the centre and marked with several concentric line of growth which can be used for determine the age of the fish. They are found in a large number of teleostean fishes having soft rayed fins, such as Labeo, Catla. The central part of the scale is called focus and is the first part to develop. They form protective covering over the skin and project diagonally in an imbricating pattern.
Ctenoid scales - Ctenoid scales are also circular and can be distinguished from the cycloid scales by having a more or less serrated free edge. Several spines are present on the surface of the posterior area of the scale. These scales are characteristic of modern higher teleosteans such as perch, sunfish etc. They are firmly attached and their exposed free hind parts are not overlapped and bear numerous small comb like teeth or spines.
Economic importance of Amphibia
- Scientific study
Frogs and toads are included the syllabus of Proficiency certificate level of different universities of world including Nepal from the past several years. Specially in Nepal, the frogs are dissected in the laboratory to study the general anatomy, alimentary canal, reproductive organs, arterial system, venous system and brain etc. Amphibian developmental study is also included in the syllabus of University of Nepal. The amphibians are studies mainly because they represent both lower and higher animals in many respects. In USA, mud puppies( Necturus) are dissected for the laboratory study. Newt Diemictylus viridescens is widely used in research.
- As food
Frogs are mainly consumed as food by gourmets in USA and Japan and many other countries of the world. In Nepal also, frogs (paha as local name) are eaten by the terrain people. Usually the fleshy hindlimbs are eaten. Other edible amphibians esteemed as food are Necturus and axolotls in America and giant salamander in Japan.
- Predation
Frogs and toads are destroyers of noxious insects. Toads are of great value because they live in gardens where insects are most injurious. The French gardeners even buy toads to control harmful insects . Bufo marinus has been introduced in tropical sugarcane fields to destroy injurious insects. . in Nepal, farmers gave great resects fir the frogs as they help to increase their production.
- Medicinal value
Toads are used in Chinese medicines. Skins of toad secretes a digitalis like substance that increases blood pressure when injected into humans.
- Trade, art and recreation
Skins of frogs are used for glue, book binding and making delicate purses. Amphibians and their larvae provide motifs in the art of American Indians, toads have played a role in religious beliefs of primitive people since ancient times. Aesop 560 BC included frogs among his fables. Tribal magicians in America used parts of frog and toads in their magic. Bull frogs area said to participate in a jumping frog contest held every year in USA frogs and toads are kept as pet in household aquaria.
- Poisonous Amphibia
A wide variety of irritating toxic compounds Is produced by amphibians. Poison glands are located dorsally in skin and defense postures of anurans and urodel present the dorsal glandular surface to their predators. Poisonous secretions (bufotoxins) of Bufo marinus are fatal to dogs and cats. Poison of Dendrobates a South American frog is used by tribal people to poison the points of their arrows. Some poisonous amphibians such as Salamandra salamandra are warningly colored. Helbenders (Cyptobranchus ) are said to be poisonous but not dangerous to men.
Anatomical development of mammal from fertilization to organogenesis
The egg or ovum of rabbit is 0.1 m in diameter while that of human being is 0.15 mm in diameter. It is oval in shape and on its outer surface, there is a thin vitelline membrane. The egg contains cytoplasm. In the cytoplasm, there is eccentric nucleus. The nucleus lies in the upper half or animal hemisphere. The egg of mammal is alecithal or micro lecithal with almost no yolk or lower half or vegetal hemisphere has some particles of yolk distributed evenly in the cytoplasm. The whole egg is surrounded by a striated secondary membrane called zona pellucida secreted by follicular cells. Outside zona pellucida, there are follicle cells forming corona radiata which is soon dissolved away.
The secondary oocyte is released from the Graafian follicle into the peritoneal cavity outside ovary. The wall of ovary ruptures to release the egg which is known as ovulation. In human beings, every month or every 28 days, one ovum generally is released either from right or left ovary alternating with one another. The development of ovary and development of egg or ovum within the ovary is under the influence of Follicle Stimulating Hormone FSH produced from the Pituitary gland. The developing egg or ovum starts secreting female hormone called Estrogen.
Soon after the release of ovum, the follicular cells and blood clot forms the Corpus Luteum. The Corpus Luteum acts as endocrine gland and produces important hormone known as Progesterone. Both female hormones are responsible for development of secondary sex organs like the oviducts, uterus, vagina and mammary gland etc. and tertiary sex characters like different pattern of distribution of hairs, high pitched voice and little bit of shyness in case of human beings and deposition of fat at different parts of body like thigh and broad hip in case of man and other mammals.
The egg is fertilized in the upper part of oviduct called ampulla which is slightly swollen part after the fimbriated funnel. For fertilization, entire spermatozoon penetrates the egg but soon the tail is degenerated. The penetration by sperm is brought about by an enzyme hyaluronidase present in the semen. The haploid set of chromosome present in egg and sperm arranged into homologous pairs and diploid chromosomes is restored. The restoration of diploid chromosome is called fertilization. With fertilization, the egg is changed into zygote.
The zygote undergoes second maturation division and becomes mature ovum. Two polar bodies lie within the zona pellucida near the animal pole. The fertilization ensures the division of zygote and further development of it.
Cleavage
Cleave is complete and holoblastic but unequal. The first cleavage is vertical diving the zygote completely into two unequal blastomeres, one being smaller and more opaque than the other. The second cleavage is also vertical and at right angle to the first one. Four celled structure is formed. After the third cleavage which is horizontal little above the equator, eight cells are formed. The further cleavages are irregular diving zygote vertically and horizontally at the same time. Then a solid ball of cells is formed called Morula. The Morula is covered by dense coat of albumen. It passes down the oviduct and comes in contact with uterus of mother and absorbs liquid food from mother.
Blastocyst
A fully formd morula shows an outer or superficial layer of cells, the trophoderm or trophoblast, surrounding an inner cell mass of larger polyhedral cells. The Morula stage passes down the oviduct and implanted into the wall of uterus.
Morula absorbs fluid secreted by the uterine mucous membrane and swells up. The liquid food is collected in a cavity which separate an outer layer of smaller trophoblast cells from a solid inner mass of cells. The embryo is now called a blastocyst which appears like a blastula but is not comparable to it. Those trophoblast cells which lie above the inner cells mass are called cells of Rauber. The inner cell mass in sonly attached to the trophoblast at animal pole like a knob. It is called embryonal knob because all parts of embryo will be derived from it. Embryonal knob is comparable to the area pellucid of chick embryo. The embryo now forms a vesicular structure called blastocyst or blastosphere.
As cavity enlarges the knob like inner cell mass becomes flattened and known as germinal or embryonal disc, which gives rise to embryo proper. In case of rabbit and other mammals, the trophoblast cells (cells of Rauber) overlying embryonal disc disintegrate. As a result the embryonal disc comes to lie at the surface of blastodermic vesicle or blastoderm and its edge becomes continuous with trophoblast around it. The trophoblast is equivalent to extra embryonic ectoderm. The cavity of embryo is equivalent to sub germinal cavity of chick. However blastocyst stage is peculiar to mammalian development.
Implantation
The trophoblat cells opposite to the embryonal knob form small papillae of trophoblastic villi which penetrate into depression or crypts in the uterine wall so the blastocyst becomes attached to or implanted in the uterus of the mother, the villi absorb food from uterus.
Gastrulation
Some cells separate from lower surface of the embryonal knob, these cells migrate and form a lining of the trophoblast, the new layer of cells is endoderm. The blastocyst is now converted into a two layered gastrula. Its fluid filled cavity is a functionless yolk sac. However there is no yolk.
In the centre of embryonal disc, cells collect to form a primitive streak, movement of the cells of primitive streak on both sides result in the separation of mesoderm which at first are un segmented and have no cavity. Cells move forward from the primitive streak to form notochord. Formation of mesoderm and notochord is the same as in chick. Formation of mesodermal somites and splitting of lateral plate mesoderm with appearance of splanchnocoel proceed in the same manner as in chick. However in rabbit, the lateral plate mesoderm extends as far as the equator, so that the lower wall of yolk sac consists of only ectoderm. The primitive streak retreats towards hind end. After giving up the chorda mesoderm cells of the notochord and mesoderm, the remaining cells of the embryonal disc are ectoderm. The ectoderm cells form neural plate and neural folds which become a neural tube as in chick. The mammalian embryo in its later stages is called foetus.
Balanoglossus (tongue worm)
Hemichordata ( Gr. Hemi = half, chorde = cord) has been treated as subphylum of Phylum Chordata ( or Protochordata). Some authors regarded it to be independent phylum close to phylum Echinodermata. But for convenience of comparative study of protochordates, hemichordate has been retained in the subphylum of Phylum choradata. Sub phylum Hemichordata includes a group of soft, vermiform, marine and primitive chordates. The most popular hemichordate genus is Balanoglossus commonly known as tongue worm or acorn worm. Other genus is Saccoglossus. They belong to class Enteropneusta (Gr. Enteron, gut; pheustos, breathed), some characteristics in the animals of this class are body large and worm like, gill slits numerous, intestine straight.
Systematic position
Phylum Chordata
Group Protochordata ( Acraniata)
Subphylum Hemichordata
Class Enteroneusta
Family Ptychoderidae
Type Balanoglossus ( tongue worm)
Delie Chaije in 1829 named and recorded Balanoglossus clavigerus. The genus Balanoglossus was derived from two Greek words, Balanos and glossus. The term balanos means an “acorn” ( fruit of oak) and refers to the proboscis projecting from collar looking like acorn nut, therefore the common name acorn worm is given. The other term glossus means tongue which refers to the shape of its proboscis, collar and genital wings bearing a close resemblance to an ox tongue so, tongue worm is given to it. The fisher men call Balanoglossus by the name of ox tongue.
Geographical distribution
Balanoglossus and other hemichordates are marine animal. It is cosmopolitan in distribution. Balanoglossus is found in tropical and sub tropical seas of world. Some other species of Balanoglossus are B australiensis ( Australia), B carnosus ( Indo Pacific) B misakiensis ( Japan), B jamaicensis (West Indies), B gigas (Brazil), B capensis ( South Africa), B clavigerus ( Mediterrannean and British isles).
Habits and Habitat
Balanoglossus is a marine, tubiculous or burrowing hemichordates. It inhabits shallow waters of intertidal zone and few occur in deep water also.
Burrow
The animal may hide under stones, or sea weeds or excavate its own burrow in bottom sand or mud. B clavegerus makes a U shaped tube or burrow having two vertical limbs of about 50 to 70 cm deep and two openings are 10 to 30 cm apart. In Saccoglossus, the body of the animal and tube are twisted but the anterior and posterior extremities are straight. The anterior opening of the burrow is funnel shaped and wide. The anterior vertical limb may give out branches each having funnel like opening. The posterior opening of the burrow is round and concealed below the spirally coiled faecal matter of the animal.
Protective device
The inner wall of the fragile burrow is smoothly lined by sand particles cemented together into a tough cast with sticky mucus secreted by the mucous gland present in the skin of the animal. Due to this the walls of the burrow are not collapsed and protects the delicate body of animal from burial in loose sand. They also secret foul smelling odour like iodoform for protection.
Movements
The tongue worm is sluggish in movement. It is little affected by the external stimuli. It can move inside burrow with the help of cilia covering its body surface. The proboscis is the most active part of its body. The proboscis can be elongated or shortened by the action of muscle. The proboscis can help in burrowing. When the tide is over, Balanoglossus protrudes its anterior end out of the burrow to explore the surrounding. The worm can extend the posterior end out of the burrow to cast the faecal material.
Feeding
Acorn worm takes a lot of sand or mud to obtain diatoms, protozoans and other microorganisms and organic detritus present in it. Sand or mud is not the food but as it cannot separate food and sand or mud they consume both. They take lot of sand or mud because the sand or mud contains little amount of food.
External structure
- Shape – the shape of its body is cylindrical, worm like and bilaterally symmetrical.
- Size – the body measures about 10 to 50 cm in length but it may differ according to species. B gigas attains about 1.8 m in its body length according to Sawaya 1951 or 2.5 m according to Spengel 1893.
- Color – color of its body is bright or drab with reddish or orange tint.
- Locomotory organ – the body is uniformly covered by short hair like cilia. Exoskeleton and external appendages are absent.
Division of body
The body of acorn worm is unsegmented. It is divisible into three distinct regions or parts like proboscis, collar and trunk.
Proboscis
Proboscis is also known as protosome which is the anteriormost part the body. Proboscis is club shaped or conical. It is circular in cross section. It has thick muscurlar wall and cavity inside is proboscis coelom. The proboscis coelom communicates to the exterior through minute proboscis pore situated mid dorsally near its base. In some proboscis pore ends blindly or there are two pores. Posteriorly the proboscis narrows into a slender neck or proboscis stalk which is attached to the collar. Below the stalk, the base of proboscis bears U shaped ciliated epidermal depression called the preoral ciliary organ which tests the quality or food and water entering the mouth.
Collar
Collar is also known as mesosome. It is middle, short and cylindrical part. The flap like or funnel like anterior margin is called as collarette. It completely surrounds and conceals the proboscis stalk and posterior part of proboscis. Ventrally, below the proboscis stalk, the collarette encloses a permanently open wide aperture called as mouth. The mouth opens into a cavity called buccal cavity which lies inside collar. The collar is well demarcated from the trunk by circular constriction. The wall of the collar is highly muscular. There is a cavity called collar coelom. The collar coelom opens to the outside through a pair of collar pores into the first pair of gill pouches behind.
Trunk
Trunk is also called as metasome. It is the posteriormost and the largest part of the body. The trunk is flat and appears annulated due to circular constrictions on the surface. The trunk bears a mid dorsal ridge and mid ventral ridge which correspond the nerve cord and blood vessel. The trunk is further divisible into three regions.
Branchiogenital region
Branchiogenital region is marked by a pair of lateral, thin, flat and longitudinal flaps called the genital wings which contains gonads. The gonopores are microscopic and cannot be seen by unaided eye. The anterior half of the branchiogental region bears two longitudinal rows of branchial apertures or gill pores. The number of gill pores increases with the age of animal. Two genital wings can be curved to meet mid dorsally so as to conceal the gill pores.
Hepatic region
The middle hepatic region of the body is somewhat smaller than branchiogenital region. It is greenish in color and its dorsal surface is marked by presence of numerous irregular intestinal sacculations or hepatic caeca.
Posthepatic region
This is the posteriormost and the longest part of the trunk. It is also called abdomen or caudal region. It is more or less of uniform diameter. The posterior end tapers somewhat and bears terminal anus.
Life cycle
Sexes are separate. Male and female Balanoglossus live in separate burrows. When fully matured, they shed gametes in sea water. The egg is microlecithal. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. Embryo is formed. Embryo hatches into a free swimming planktonic larva called tornaria. It is ciliated larva. The development is indirect.
There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.
Pisces (osmoregulation and scales
Every organism in natute has a close relationship with its environment for water and salts. An optimum concentrations of water and salts is essential in the body for proper metabolism. The maintainance of such a concentration is called osmoregulation. There may be three possible conditions of the concentration of waer and salts in the body of an organism in comparision to the concentration of the same substances in the environment.
- isotonic or iso osmotic- the cell or organism kept in it experiences no change in its volume. In this situation the osmotic pressure is same both inside and outside the cell or organism.
- Hypotonic or hypo osmotic – which is more diluted as compared to the other. The solvent or water enters into the body of organism from surrounding environment.
- Hypertonic or hyper osmotic – which is more concentrated as compared to the other. The solvent or water comes out of the body of the organism into the surrounding environment.
Animals can be put in two categories according to their tolerance for osmotic changes.
- Stenohaline - this includes the animals which can tolerate the narrow range of salinity. They change the osmotic pressure of their internal fluids according to the changes in their external medium. This includes crustaceans and several mollusks.
- Euryhaline – this includes the animals which can tolerate the broad range of salinity. They do not change the osmotic pressure of their internal fluids according to the changes in their external medium. They can maintain the concentration of their body fluids at the same level in all situations.
Marine elasmobranchs like sharks, rays and skates have a special urea retention device. The migratory fishes have their own peculiarity of osmoregulation. For instance, when the fresh water eel reaches the sea for spawning, there is a tendency for the fish to lose water and to increase the salt concentration in its body. The excess salts are removed to the outside by the active transport through the chloride –secreting cells of the gills. The kidneys keep on reabsorbing water. When they return to the rivers and grow into adults, they have to face the reverse problem where they remove the excess water along with the urine and reabsorb salts into the blood. These changes in the kidneys are regulated by the endocrine secretions.
Scales of fishes
In vertebrates, the exoskeletal covering of body is made on two types scales. They are epidermal and dermal. The epidermal scales are cornified derivatives of malpighian laer of epidermis and developed in terrestrial animals like reptiles , birds and mammals. Dermal scales are mesenchymal in origin and especially developed in fishes. They are small, thin, calcareous or bony plates which fit closely together or overlap. Primitive fossil fishes of Silurian and Devonian periods, possessed exoskeleton in the form of plates and scales which consist of three district layers. The innermost layer consisted of a compact bone, isopedine, the intermediate or middle layer of spongy vascular bone and the outer layer is of dentine.
According to the mode of origin, there are tow types of scales. (1) those which are formed due to the secretary activity of both epidermis as placoid scales of elasmobranch (ii) non placoid scales that are derived from dermis only as the scales of teleosts.. structurally the scales are classified as cosmoid, genoid or rhomboid, placoid, cycloid and ctenoid. The last two are called bony ridge scales.
Cosmoid scales – the cosmoid scales are found in extinct Crossopterygii and Dipnoi.
Ganoid scales – ganoid scales are characteristic of the primitive acrinopterygians called ganoid fishes. These scales are heavy and have an outer layer of hard inorganic, enamel like material called ganoine. The middle layer is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone called isopedine.
Placoid scales – Placoid scales are characteristic of elasmobranch fishes( sharks). Each scale has a disc like basal plate embedded in the dermis and a spine projecting out through the epidermis. Placoid scale resembles the tooth. The spine has an external covering of enamel like hard transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from which several branching dentine tubules radiate in different directions. Placoid scales are closely set in skin but do not overlap each other and fiving a sand paper like quality to skin.
Cycloid scales – cycloid scales are thin flexible, translucent plates, rather circular in outline, thicker in the centre and marked with several concentric line of growth which can be used for determine the age of the fish. They are found in a large number of teleostean fishes having soft rayed fins, such as Labeo, Catla. The central part of the scale is called focus and is the first part to develop. They form protective covering over the skin and project diagonally in an imbricating pattern.
Ctenoid scales - Ctenoid scales are also circular and can be distinguished from the cycloid scales by having a more or less serrated free edge. Several spines are present on the surface of the posterior area of the scale. These scales are characteristic of modern higher teleosteans such as perch, sunfish etc. They are firmly attached and their exposed free hind parts are not overlapped and bear numerous small comb like teeth or spines.
Economic importance of Amphibia
Most of the amphibians are beneficial to mankind. They are of great economic importance. Specially frogs and toads are of economic importance.
- Scientific study
Frogs and toads are included the syllabus of Proficiency certificate level of different universities of world including Nepal from the past several years. Specially in Nepal, the frogs are dissected in the laboratory to study the general anatomy, alimentary canal, reproductive organs, arterial system, venous system and brain etc. Amphibian developmental study is also included in the syllabus of University of Nepal. The amphibians are studies mainly because they represent both lower and higher animals in many respects. In USA, mud puppies( Necturus) are dissected for the laboratory study. Newt Diemictylus viridescens is widely used in research.
- As food
Frogs are mainly consumed as food by gourmets in USA and Japan and many other countries of the world. In Nepal also, frogs (paha as local name) are eaten by the terrain people. Usually the fleshy hindlimbs are eaten. Other edible amphibians esteemed as food are Necturus and axolotls in America and giant salamander in Japan.
- Predation
Frogs and toads are destroyers of noxious insects. Toads are of great value because they live in gardens where insects are most injurious. The French gardeners even buy toads to control harmful insects . Bufo marinus has been introduced in tropical sugarcane fields to destroy injurious insects. . in Nepal, farmers gave great resects fir the frogs as they help to increase their production.
- Medicinal value
Toads are used in Chinese medicines. Skins of toad secretes a digitalis like substance that increases blood pressure when injected into humans.
- Trade, art and recreation
Skins of frogs are used for glue, book binding and making delicate purses. Amphibians and their larvae provide motifs in the art of American Indians, toads have played a role in religious beliefs of primitive people since ancient times. Aesop 560 BC included frogs among his fables. Tribal magicians in America used parts of frog and toads in their magic. Bull frogs area said to participate in a jumping frog contest held every year in USA frogs and toads are kept as pet in household aquaria.
- Poisonous Amphibia
A wide variety of irritating toxic compounds Is produced by amphibians. Poison glands are located dorsally in skin and defense postures of anurans and urodel present the dorsal glandular surface to their predators. Poisonous secretions (bufotoxins) of Bufo marinus are fatal to dogs and cats. Poison of Dendrobates a South American frog is used by tribal people to poison the points of their arrows. Some poisonous amphibians such as Salamandra salamandra are warningly colored. Helbenders (Cyptobranchus ) are said to be poisonous but not dangerous to men.
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