M Ed sanothimi

Minor phyla




Ctenophora



The ctenophore are biradially symmetrical organisms, lacking nematocysts, possessing ciliary plates in eight rows and possessing a gelatinous ectomesoderm containing mesenchymal muscle fibres.



Hermiphora plumosa



Systematic position

Phylum Ctenophora

Class Tentaculata - with two long aboral tentacles

Order Cydippikda - body oral or rounded, tentacles branched and retractile into

pouch

Genus Hermiphora

Species plumosa





position in animal kingdom - Ctenophores have certain characteristics in common with coelenterates. They are believed to have diverged very early from ancestral medusoid coelenterates which was a spherical animal with concentration of cilia along 8 meridional rows which later developed into comb plates, so known as comb jellies. However they represent a blind offshoot which gave rise to no higher forms. Ctenophores also present certain advancements over Radiata such as prominence of apical region, musculature derived from mesoderm, presence of gonoducts and determinate type of cleavage. This implies that Ctenophora are intermediate between Radiata and Bilataeria and have undergone considerable specialization with many striking characteristics of their own. Therefore, it seems quite logical to treat that Ctenophores should be included into a separate phylum rather than a class or subphylum of phylum of Coelenterata.



Hermiphora plumosa is slightly smaller and pear shaped organism about 5 to 20 mm in diameter. It is of glassy transparency. The species H plumosa is found in the Mediterranian sea .



External surface bears 8 equally spaced paddle plates or comb plates each made up of transverse rows of long cilia. So, they are also known as comb jellies. Near aboral end, on opposite sides, are attached two very long ( 15 cm) solid and highly extensible tentacles provided with lateral branches. Each tentacle can be completely withdrawn into a deep cavity or tentacle sheath or pouch. At the aboral end is a shallow depression with a sense organ or statocyst. A slit like mouth is present at the oral end. It leads into a long narrow pharynx or stomodaeum opening into the stomach. The stomach is connected to a system of gastrovascular canal. Hermiphora is strictly carnivore. Food is captured by tentacles with the help of colloblast. Colloblasts are adhesive cells also known as lasso cells. Undigested materials come out of mouth. The digestion is both extracellular and intracellular.



The body is covered externally by a delicate ectodermal epithelium. The epithelium of the stomodaeum is found by development to be ectodermal. The interval between the external epidermis and the canal system is filled by a soft, jelly like mesogloea. The tentacle sheath is is an invagination of the ectoderm and the tentacle itself is covered by a layer of ectoderm with which a is a core or axis formed by a strong bundle of longitudinal muscular fibres which are of mesodermal origin which serve to retract the tentacle into tits sheath.



The feeble development of the muscular system of course correlated with the fact that the swimming plates are the main organs of progression. A further striking difference between our present type and the Cnidaria is the absence of stinging capsules (nematocysts). This causes Ctenophora to be referred to as Acnidaria in older system. The place of stinging capsules is taken in a sense by the peculiar adhesive cell with branches of tentacles are covered. Adhesive cell has a convex surface produced into a small papillae which readily adheres to any object with which it comes in contact and is with difficulty separated.



The Hermiphora is hermaphrodite. Gonads develop in the meridional gastrovascular canal. Each of which has and ovary extending along the whole length of one side and testis along the whole length of opposite side. When ripe the ova and sperms are discharged into the canals, make their way to the infundibulum then into stomodaeum an finally escape by the mouth. Fertilization takes place in sea water.



Life cycle

The egg consists of and outer layer or protoplasm containing nucleus and of an internal mass of a frothy or vacuolated nature. The vacuoles contain homogenous substance which serve as a nutrient store to the growing embryo which corresponds with the yolk which occurs in a large proportion of animal eggs. Enclosing the egg is a thin vitelline membrane separated from the protoplasm by a considerable space filled with a clear jelly. Development includes a free swimming cydippid larva.

M Ed sanothimi

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.

M Ed Biolgoy

Adaptive radiation in reptiles




As competition for food and living space prevails, a single ancestral species evolves into different forms which occupy different habitats. This is called adaptive radiations or divergent evolution. Reptiles have shown the greatest evolutionary diversity and adaptive radiation of all vertebrates. Their adaptive radiation took place twice, first in the Paleozoic and secondly in the Mesozoic.



Paleozoic radiation

During Paleozoic, with no competitors on land, the ancestral reptiles or cotylosaurs multiplied rapidly occupying all ecological niches available to them. Their radiation involved adaptations to different methods of locomotion and feeding. Distinct anapsid and synapsid forms dominated.



Mesozoic radiation

By the end of Paleozoic , the ancestral colylosaurs had disappeared. Their descendants produced a second and bigger radiation during Mesozoic. They dominated not only land but also sea and the air. This Mesozoic era is called as Age of Reptiles. This lasted over a great span of time about 130 million years. By comparison man is no older than 2 to 4 million years. The Mesozoic reptiles are represented by as many as 16 Orders. Of these one led to birds, one led to mammals and four to the modern reptiles. The rest disappeared. We will discuss only notable lines based on the morphology of the skull such as anapsid, synapsid, euryapsid, parapsid and diapsid .



Anapsid line

The modern Chelonia (turtles and turtoises) represent a direct and an early offshoot of cotylosaurs retaining anapsid skull. They have remained unchanged since Triassic some 160 million years ago.



Synapsid line

The mammal like reptiles or Synapsid had a single temporal cavity in skull ventral to postorbital and squamosal. Early Pelycosauria or Theromorpha were similar to cotylosaurs. Later Therapsida with differentiated dentition and improved locomotion were more mammal like. Before disappearing in Jurassic they gave rise to ancestral mammals.



Euryapsid line

The euryapsid or plesiosaurs had a single temporal fossa in skull above the joint of postorbital and squamosal. They were large marine turtle like heavy bodied and logn necked creatures. They were obviously fish eaters. All became extinct towards end of Cretaceous.



Parapsid line

There was another marine blind alley like Euryapsida represented by fish like or purpoise like ichthyosaurs. They also became extinct.



Diapsid line

Most of the today’s reptiles are diapsid with tow temporal openings on either side of skull separated by squamosal and postorbital bones. The earliest diapsids divide into two branches like Lepidosauria and Achosauria. The Lepidosauria were probably the ancestral to modern Squamata ( snakes and lizards) and Sphenodon. The Archosauria were the ruling reptiles dominating the Mesozoic Era. They represented the extinct Pterosauria, the extinct Dinosaurs and modern Crocodilia. They also gave rise to the modern birds.



A) Pterosauria

The extinct flying reptiles called Pterosauria or Pterodactyla were of light built. Their fore limbs evolved into membranous wings or patagia. Rhamphorhynchus of late Jurassic was a primitive pterosaur with 1 meter wingspan, a long balancing tail and toothed jaw. Pteranodon of Cretaceous had a 9 meter wingspan but no tail .



B) Dinosaur

At the end of Triassic, Thecodontia, the early descendents of Archosauria gave rise to the most fantastic Mesozoic reptiles the dinosaurs ( Gr. Dinos, terrible+saurus , lizard). They sub divided early into two orders Saurischia and Ornithischia.



Saurischia means “reptiles hip” they possessed a tri radiate pelvis with pubis entirely separate and anterior ischium. Sub order Thorapoda included flesh eating and bipedal carnivores. Tyrannosaurus rex from Cretaceous was 15 meters long and stood 6 meters high. Branchiosaurus was more than 25 m long and weighed over 50 tons.



Ornithischia means “ bird hip” they had a typical tetraradiate bird like pelvic girdle with pubis directed backwards parallel to ischium. They were all herbivores. Stegosaurus measures 8 meters and weight 10 tons. Fore limbs were much shorter than hind limbs.



Causes of extinction

After thriving and dominating the earth for 130 million years, the great dinosaurs and their contemporaries became suddenly extinct by the end of Cretaceous period. Various factors have been suggested for their total extinction like catastrophism, epidemic, food poisoning, racial senescence, climate change, overspecialization, intra specific warfare. Probably a combination of several factors was responsible for their extinction.

M Ed Biology

 

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.

1. 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.
2. 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.
3. 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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