Current Affairs 11th Class

Habitat Selaginella is commonly called the little club moss or spike moss. Selaginella is mainly found in damp shaded places. A few species are xerophytic and can withstand the dry conditions for months together. In dry conditions, the plant rolls up into a compact ball and root system is disorganized. During the rainy conditions the ball on absorbing moisture, becomes green again. Such plants are called resurrection plants or bird's nest moss, e.g., S.lepidophylla and S. pilifera are xerophytes and sold in the market as novelties, S.bryopteris (Sanjeevani) and S.rupestris (ornamental). The common epiphytic species are S. chrysocaulos, S. kraussiana, S. oregana, S. chrysorrhizos. Structure External structure : The plant body is sporophytic (2n), which is an evergreen and delicate herb having adventitious roots. The plants show great variation in their morphology. Some species are prostrate growing upon the surface (e.g., S.kraussiana), some are suberect (e.g., S.trachyphylla) and others are climbers (e.g., S.allegans). The stem is covered with four rows of small leaves, out of these two rows are of smaller leaves and two of large leaves species with dimorphic leaves such as S.kraussiana, S.helvetica, S.lepidophylla, S.chrysocaulos etc.     Leaves are sessile, ovate or lanceolate with acute apex. Unbranched midrib is present in the centre of each leaf. The leaves are ligulate, i.e., a flap-like outgrowth is present at the base on adaxial side called ligule. It may be fan-shaped or tongue-shaped or lobed or fringed. At the base of ligule, there is present a sheath of elongated cells called glossopodium (secretory). The leaves possess a midrib but there is no venation. At the place of bifuraction of stem, a leafless, colourless, positively geotropic, elongated, cylindrical structure grows downwards. This is called the rhizophore and is quite different from the root in that it has no root cap. Internal structure Root : In root the stele is a protostele. It has a central core of xylem surrounded by phloem which is horse shoe shaped. It has a single protoxylem element (monarch). The xylem is exarch. Stem : The stem is internally distinguishable into a single layered epidermis having no stomata. This is followed by cortex. The stele is suspended by unicelled (rarely multicelled) trabaculae (modified endodermal cell). This layer, due to presence of casparian strips is regarded as endodermis. The stele is a protostele (haplostele) surrounded by a pericycle with a central core of xylem enclosed by phloem. Protostele is diarch and exarch. Stem of Selaginella kraussiana shows distelic condition.     Leaf : The leaf displays a simple structure. The mesophyll is uniform, being composed of elongated chlorenchymatous cells with large intercellular spaces. Each measophyll cells has one (S.martensii), two (S.kraussiana) or eight (S.willdenovii) chloroplasts. Each chloroplast has several pyrenoid-like bodies similar to Anthocerotales. The single more...

(Gk. Amphi = both; bios = Life) General characters (1) Aquatic or semi aquatic (freshwater), air and water breathing, carnivorous, cold–blooded, oviparous, tetrapod vertebrates. (2) Head distinct, trunk elongated. Neck and tail may be present or absent. (3) Limbs usually 2 pairs (tetrapod), some limb less Toes 4-5 (pentadactyle) or less. Paired fins absent. Median fins, if present, without fin rays. (4) Skin soft, moist and glandular. Pigment cells (chromatophores) present. (5) Exoskeleton absent. Digits claw less. Some with concealed dermal scales. (6) Endoskeleton mostly bony. Notochord does not persist. Skull with 2 occipital condyles. i.e. Dicondylic skull. (7) Mouth large. Upper or both jaws with small homodont teeth. Tongue often protrusible. Alimentary canal terminates into cloaca. (8) Respiration by lungs, skin and mouth lining. Larvae with external gills which may persist in some aquatic adults. (9) Heart 3–chambered (2 auricles + 1 ventricle). Sinus venosus present. Aortic arches 1-3 pairs. Renal and hepatic portal systems well developed Erythrocytes large, oval and nucleated. Body temperature variable (poikilothermous). (10) Kidneys mesonephric. Urinary bladder large. Urinary ducts open into cloaca. Excretion ureotelic. (11) Brain poorly developed. Cranial nerves 10 pairs. (12) Nostrils connected to buccal cavity. Middle ear with a single rod-like ossicle, columella. Larval forms and some aquatic adults with lateral line system. (13) Sexes separate. Male without copulatory organ Gonoducts open into cloaca. Fertilization mostly external. Females mostly oviparous. (14) Development indirect. Cleavage holoblastic but unequal. No extra–embryonic membranes. Larva a tadpole which metamorphoses into adult. Classification of Amphibia : The living amphibians belong to only 2,500 species, a very much smaller number than that of other principal classes of vertebrates. Ranging from mid-Palaeozoic (Devonian) to early Mesozoic (Triassic). They dominated the World during Carboniferous, but most of them have become extinct since long. The classification most generally followed nowadays was provided by G. Kingsley Noble (1924). (a) Subclass I. Stegocephalia (Extinct) : Limbs pentadactyle. Skin with scales and bony plates. Skull with a solid bony roof leaving openings for eyes and nostrils. Permian to Triassic.   Order 1. Labyrinthodontia : Oldest known tetrapods called stem Amphibia. Carboniferous to Triassic. Example : Eryops. Order 2. Phyllospondyli : Small salamander-like. Carboniferous to permian. Example : Branchiosaurs (Ichthyostega). Order 3. Lepospondyli ­– Small salamander or eel-like.Carboniferous to Permian.  Examples – Diplocaulus, Lysorophus. (b) Subclass II. Lissamphibia (living) : Modern Amphibia lacking dermal bony skeleton. Teeth small, simple. Order 1. Gymnophiona or Apoda : (Gk. gymnos = naked ; ophioneos = serpet-like). (1) Limb less, blind, elongated worm like, burrowing tropical forms known as caecilians or blind worms. (2) Tail short or absent, cloaca terminal.   (3) In some dermal scales embedded in skin which is transversely wrinkled. (4) Skull compact, roofed with bone. (5) Limb girdle absent. (6) Males have protrusible copulatory organs, Examples : Ichthyophis, Uroaeoryphlus, Rhinatremam, Typhlonectes.

• Ichthyophis is a Limb less amphibian showing parental care. It has no tongue.

Order 2. more...

(L. avis = bird) or (Gk. ornis = bird) General Characters (1) Feather-clad, air-breathing, warm-blooded, oviparous, bipedal flying vertebrates. (2) Limbs are two pairs. Forelimbs are modified as wings for flying. Hind limbs or legs are large, and variously adapted for walking, running scratching, perching, food capturing, swimming or wading, etc. (3) Exoskeleton is epidermal and horny. (4) Skin is dry and devoid of glands except the oil or preen gland at the root of tail. (5) Pectoral muscles of flight are well developed.             (6) Skull smooth and monocondylic, bearing a single occipital condyle. Cranium large and dome-like. Sutures indistinct. (7) Vertebral column short. Centra of vertebrae heterocoelous (saddle-shaped). (8) Sternum large, usually with a vertical, mid ventral keel for attachment of large flight muscles. (9) Ribs double-headed (bicephalous) and bear posteriorly directed uncinate processes. (10) Both clavicles and single inter clavicle fused to form a V–shaped bone, called furcula or wishbone or merry-thought bone. (11) Heart completely 4–chambered. There are neither sinus venosus or truncus arteriosus. Only right aortic (systemic) arch persists adult. Renal portal system vestigial. Blood corpuscles nucleated. (12) Birds are the first vertebrates to have warm blood. Body temperature is regulated (homoiothermous). (13) Respiration by compact, spongy, nondistensible lungs continuous with thin air-sacs. (14) Larynx without vocal cords. A sound box or syrinx, producing voice, lies at or near the junction of trachea and bronchi. (15) Kidneys metanephric and 3–lobed. Uterus open into cloaca. Urinary bladder absent. Birds are urecotelic. Excretory substance of urates eliminated with faeces.        (16) Sexes separate. Sexual dimorphism is well marked in some birds like peacock and parrot. (17) Fertilization internal, preceded by copulation and courtship. Females oviparous. (18) Eggs develop by external incubation. Cleavage discoidal, meroblastic. Development direct, Extra-embryonic membranes (amnion, chorion, allantois and yolk-sac) present (Amniota). (19) Parental care is well marked. Classification of Aves : Birds show less diversification than any other group of vertebrate animals. 25 to 30 avian orders are recognized depending on the taxonomist. Class Aves is first divided into two subclasses. (a) Sub-Class I. Archaeornithes : (Gk. archios = ancient ; ornithos = bird) (1) Extinct, archaic, Jurassic birds of Mesozoic Age, about 155 million years ago. (2) Wings primitive, with little power of flight. (3) Vertebrae amphicoelous. (4) Sternum without a keel. (5) Thoracic ribs slender, without uncinate processes. In Archacopteryx beak in toothed. This sub-classes includes a single order Order Archaeopterygiformes : Example : Archaeopteryx lithographica, from Jurassic or Bavaria, Germany; one specimen lying in the British museum, London, the other lying in the Berlin. (b) Sub-class II. Neornithes : (Gk. neos = modern ; ornithos = Birds) (1) Modern as well as extinct post-Jurassic birds. (2) Wings usually well-developed and adapted for flight, with few exceptions. (3) Teeth absent except in some fossil birds. (4) Vertebrae heterocoelous in living forms. (5) Sternum usually with a keel. (6) Thoracic ribs usually with uncinate processes. (7) Abdominal ribs absent This sub-class is divisible into 4 super-orders: Super-order 1. Odontognathae : (Gk. odontos = teeth) more...

(L. mamma = breast) General characters (1) Hair-clad, mostly terrestrial, air-breathing, warm blooded, viviparous, tetrapod vertebrates. (2) Limbs 2 pairs, pentadactyle, each with 5 or fewer digits. Hind limbs absent in cetaceans and sirenians. (3) Exoskeleton includes lifeless, horny, epidermal hairs, spines, scales, claws, nails, hoofs, horns, bony dermal plates, etc. (4) Skin richly glandular containing sweat, sebaceous (oil) and sometimes scent glands in both the sexes. Females also have mammary glands with teats producing milk for suckling the young. (5) Endoskeleton thoroughly ossified. Skull dicondylic having 2 occipital condyles. Cranium large. A single zygomatic arch present. Pterygoids small, scale-like. Otic bones fused into periotic which forms tympanic bulla with tympanic. Each half of lower jaw made of a single bone, the dentary, articulating with squamosal of skull Vertebrae with terminal epiphyses and flat centra (acoelous). Cervical vertebrae usually 7. Ribs bicephalous. Coracoid vestigial. (6) Teeth are of several types (heterodont), borne is sockets (thecodont) and represented by two sets (diphyodont). (7) Respiration always by lungs (pulmonary). Glottis protected by a fleshy and cartilaginous epiglottis. Larynx contains vocal cords. (8) Heart 4-chambered with double circulation. (9) Kidneys metanephric. (10) Brain highly evolved. Both cerebrum and cerebellum large and convoluted. Optic lobes small and 4 in number called corpora quadrigemina. Corpus callosum present connecting both cerebral hemispheres. Cranial nerves 12 pairs. (11) Senses well developed. Eyes protected by lids, the upper of which is movable. External ear opening protected by a large fleshy and cartilaginous flap called pinna. Middle ear cavity with 3 ear ossicles?malleus, incus and stapes. Cochlea of internal ear spirally coiled. (12)  Sexes separate. (13) Fertilization internal preceded by copulation. (14) Except egg-laying monotremes, mammals are viviparous, giving birth to living young ones. (15) Development uterine. Classification of Mammalia :  Mammals have been thoroughly described and adequately classified. The main characters forming the basis of their classification into orders include :  (1) Mode of caring of their young, (2) Nature of dentition (3) Foot posture, (4) Nails, claws and hoofs, (5) Complexity of nervous system and (6) Systematics.   (a) Subclass I prototheria : (Gk. protos = first ; therios = beast). Primitive, reptile-like, oviparous or egg-laying mammals. Order 1. Monotremata : (Gk. monos = single ; trema = opening), Cloacal opening present Confined to Australian region. Examples : Monotremes. Platypus or duckbill (ornith orhynchus) spiny anteater (Tachyglossus = Echidna).   (b) Sub class II. Theria : (Gk. ther, = animal) Subclass Theria is divided into two infraclasses : Metatheria and Eutheria. Infraclass 1. Metatheria : (Gk. meta = between or after) (1) Metatherians are pouched mammals; young born in very immature state. (2) Corpus collosum absent. (3) Epipublic bone present. Order 2. Marsupialia : (Gk. marsypion = pouch). (1) Born in a very immature state, and complete their development attached to teats or nipples in the abdominal pouch or marsupium. (2) Usually 3 premolars and 4 molars in each jaw on either side. more...

(L. reptare = to creep) General Characters (1) Predominantly terrestrial, creeping or burrowing, mostly carnivorous, air 'breathing, cold' blooded, oviparous and tetrapodal vertebrates. (2) Body bilaterally symmetrical and divisible into 4 regions-head, neck, trunk and tail. (3) Limbs 2 pairs, pentadactyle. Digits provided with horny claws. However, limbs absent in a few lizards and all snakes. (4) Exoskeleton of horny epidermal scales, shields, plates and scutes. (5) Skin dry, cornified and devoid of glands. (6) Mouth terminal. Jaws bear simple conical teeth. In turtles teeth replaced by horny beaks. (7) Alimentary canal terminates into a cloacal aperture. (8) Endoskeleton bony. Skull with one occipital condyle (monocondylar). A characteristic T-shaped inter clavicle present. (9) Heart usually 3-chambered, 4-chambered in crocodiles. Sinus venosus reduced. 2 systemic arches present. Red blood corpuscles oval and nucleated. Cold?blooded. (10) Respiration by lungs throughout life. (11) Kidney metanephric. Excretion uricotelic. (12) Brain with better development of cerebrum than in Amphibia. Cranial nerves 12 pairs. (13) Lateral line system absent. Jacobson's organs present in the roof of mouth. (14) Sexes separate. Male usually with a muscular copulatory organ. (15) Fertilization internal. Mostly oviparous. Large yolky meroblastic eggs covered with leathery shells, always laid on land. Embryonic membranes (amnion, chorion, yolk sac and allantois) appear during development. No metamorphosis. Young resemble adults. (16) Parental care usually absent Classification of Reptilia  : According to Bogert, there are more than 7,000 living and several extinct species of reptiles, grouped into approximately 16 orders of which only 4 are living. (a) Subclass I Anapsida - Primitive reptiles with a solid skull roof. No temporal openings. Order 1. Chelonia or Testudinata : (Gk. chelone = turtle) or (L. testudo = turtle) (1) Body short, broad and oval. (2) Limbs clawed and or webbed, paddle-like. (3) Body encased in a firm shell of dorsal carapace and ventral plastron, made of dermal bony plates. Thoracic vertebrae and ribs usually fused to carapace. (4) Skull anapsid, with a single nasal opening and without a parietal Foramen.  Quadrate is immovable. (5) No sternum is found. (6) Teeth absent. Jaws with horny sheaths. (7) Cloacal aperture a longitudinal slit. (8) Heart incompletely 4-chambered with a partly divided ventricle. (9) Copulatory organ single and simple. (10) About 400 species of marine turtles, freshwater terrapins and terrestrial tortoises. Examples : Chelone, Chrysemys, Testudo, Trionyx, Dermochelys.     (b) Subclass II Euryapsida (extinct) : Skull with a single dorso-lateral temporal opening on either side bounded below by postorbital and squamosal bones. (c) Subclass III Parapsida (extinct) : Skull with a single dorso?lateral temporal opening on either side bounded below by the supra temporal and post frontal bones. (d) Subclass IV Synapsida (extinct) : Skull with a single lateral temporal opening on either side bounded above by the postorbital and squamosal bones. (e) Subclass V Diapsida (Living) : Skull with two temporal openings on either side separated by the bar of postorbital and squamosal bones. Order 2. Rhynchocephalia : (L. rhynchos = snout more...

The kingdom animalia or animal kingdom is the kingdom of consumer organisms having ingestive type of nutrition. It is the largest kingdom, with 1.2 million members. It has numerous having different type of form, structure, organisation, complexity and development. General features of animals The animals possess several general features, which, taken together, distinguish them from the members of other kingdom. (1) Animals are multicellular eukaryotes and in most cases their body cells form tissues that become arranged as organs and organ systems. (2) Animals have heterotrophic mode of nutrition. They get carbon and energy by ingesting other organism or by absorbing nutrients from them. Animals may be herbivores, carnivores, omnivores, parasites, suspension feeders or deposit feeders. (3) Animals require oxygen for aerobic respiration. (4) Animals are motile, possess active movement during some stage of their life cycle. Even the sessile sponges have free swimming larval stages. (5) The animal body cells of nearly all species have diploid chromosome number. (6) Animal cells lack a cell wall; this provides flexibility of their cells, the most striking characteristic of animals. (7) Animals are able to make rapid responses to external stimuli as a result of the activity of nerve cells, muscle or contractile tissue or both. (8) Animals can reproduce sexually. Although some exhibit remarkable diversity of reproductive behaviour, all are capable of sexual reproduction. (9) Animal life cycle includes stages of embryonic development. Mitotic cell divisions (cleavage) transform the animal zygote into a multicellular embryo. Terms related to classification (1) Anaima : Animals without red blood e.g., sponges, cnidaria, mollusca, arthropoda, echinodermata, etc. (2) Enaima : Animals with red blood e.g., vertebrates. (3) Vivipara : Animals which give birth to young ones are included in this subgroup e.g., man, dogs, cows, etc. (4) Ovipara : Animals which lay eggs are included in this subgroup e.g., frogs, toads, lizards, snakes, birds, etc. (5) Anamniotes : Vertebrates without embryonic membranes e.g., fishes, amphibians. (6) Amniotes : Vertebrates with embryonic membranes (chorion, amnion, allantois, yolk sac) e.g., reptiles, birds, mammals. (7) Acraniata or Protochordata : Chordates without cranium (brain box). It includes urochordata and cephalochordata. (8) Chordates : Animals with notochord dorsal tubular nerve cord, paired pharyngeal gill slits. All urochordates, cephalochordates and vertebrates are called chordates. (9) Craniata or Vertebrate : Chordates with cranium. It includes cyclostomes, pisces, amphibians, reptiles, birds and mammals. (10) Nonchordates : Animals without notochord (a rod like elastic structure which supports the body). Phylum Porifera to phylum Hemichordata are called nonchordates. (11) Invertebrates : Animals without vertebral coloumn (backbone). All the nonchordates, urochordates and cephalochordates are callectively called invertebrates. (12) Levels / Grades of organization : Four levels of organization are found in multicellular animals. (i) Acellular or Molecular or Protoplasmic level : It is present in protozoans. (ii) Cellular level : The body consists of many cells which may be similar or show minor division of labour. Distinct tissues are not formed, e.g., sponges. (iii) Tissue level :  The body is multicellular. The cells form poorly defined more...

 (L.annelus = ring, eidos = form) Brief History : Linnaeus (1758) included all soft–bodied worms in “Vermes”. Lamarck (1801) established phylum annelida for higher types of worms. General characters (1) Annelids are bilaterally symmetrical animals. (2) They have organ-system grade of organization. (3) They are coelomate (schizocoelomate) animals. (4) They have triploblastic body wall. (5) The muscle layers are thick in the body wall. Hence the body wall is said to be dermomuscular. (6) The body is divided into a numerous segments called the metameres or somites. The segmentation is known as metamerism. (7) The body is covered with a thin cuticle. (8) Locomotory organs are setae. (9) Digestive system is well developed. These have tube-within-a-tube body plan. (10) Blood vascular system is a closed type (11) Excretory system is formed of segmentally arranged nephridia. (12) These always show cutaneous or skin respiration. (13) Nervous system is formed of a pair of cerebral ganglia (brain) and a double ventral nerve cord. (14) Mostly annelids are hermaphrodites. Fertilization is generally cross and may be external or internal. (15) The gonoducts are formed from coelom (coelomoducts). The coelomoducts have connection with nephridia. (16) Regeneration is common character in this phylum. (17) Their development is direct or indirect and includes a free-swimming trochophore larva. Classification of annelida : On the basis of position and arrangement of setae when present, absence and presence of sense organ, phylum annelida has been divided into four classes – Class 1. Polychaeta (Gr. polus, many, chaite, hair) (1) Polychaeta are marine and carnivorous. (2) Body is elongated and segmented. (3) Head consists of prostomium and peristomium and bear eyes, tentacles, cirri and palps, etc. (4) Setae are numerous and are borne up on lateral prominances of the body wall known as parapodia. (5) Locomotory organs are parapodia. (6) Clitellum is absent. (7) Cirri or branchiae or both may be present for respiration. (8) Coelom is spacious usually divided by inter segmented septa. (9) Alimentary canal is provided with an eversible buccal region and protrusible pharynx. (10) Excretory organs are segmentally paired nephridia. (11) Sexes are separate. (12) Fertilization is external; free swimming larval stage is trochophore. (13) Asexual reproduction occurs by budding. Examples : Nereis, Aphrodite, Polynae, Chaetopterus, Glycera, Arenicola, Amphitrite, Terebella, Sabella, Eunice, etc.

• Arenicola, Amphitrite and Terebella have external gills.
• Chaetopterus exhibits luminescence and great power of regeneration. 

  Class 2. Oligochaeta (Gr. oligi, few) (1) They are mostly terrestrial or some fresh water forms. (2) Body has conspicuous external and internal segmentation. (3) Distinct head, eyes and tentacles are absent. (4) Parapodia are absent. (5) Locomotory organs are setae. (6) Setae are usually arranged segmentally. (7) Clitellum is usually present. (8) Pharynx is not eversible and without jaws. (9) They are hermaphrodites. (10) Development is direct and takes place within cocoons secreted by clitellum. (11) No free larval stage Examples : Tubifex, Dero, Pheretima, (Indian earthworms), Lumbricus more...

 (Gk. Arthron = joint; Podos = foot) Brief History : Aristotle described a few crabs and other arthropods. Linnaeus included all such animals in his group “Insecta”. Lamarck divided this group into three classes – Crustacea, Hexapoda and Arachnida. Finally, Von seibold (1845) established the phylum Arthropoda for these animals. General characters (1) Occur widely on land, in air, and in all sorts of water, from snowy tops of high mountains to the depths of ocean. Many are parasites of other animals and plants. Hence, the phylum is of great economic importance. (2) Bilateral, triploblastic, body segmented and also divided into head, thorax and abdomen. Segmentation marked only externally; number of segments or somites fixed and each has its separate exoskeleton of thick and hard,  chitinous cuticle secreted by epidermis of body wall. Head somites always fused. (3) Each segment basically bears a pair of lateral jointed appendages adapted for food ingestion, locomotion, respiration, copulation, etc. (4) Muscular system well-developed; muscle fibres always striated. (5) Digestive tract complete. Most head appendages from mouth parts with lateral jaws for chewing or sucking. Anus terminal. (6) Coelom reduced to small cavities in excretory and reproductive organs; replaced elsewhere by blood sinuses which merge together to form a large perivisceral cavity – the haemocoel–around viscera. Sinuses form an “open blood vascular system” filled with haemolymph which may contain haemocyanin. Haemocoel communicates with a long tubular and pulsatile, mid–dorsal heart. (7) Respiration by gills (aquatic forms), or trachea or book lungs (terrestrial forms); by diffusion through body surface in some. (8) Excretion by coelomoducts or specialized green or coxal glands, or by malpighian tubules. Excretory product is uric acid. (9) Nervous system basically similar to the typical annelid plan; head with a brain-ring which is connected to a double ventral nerve cord, having paired segmental ganglia which represent true metamerism. Well–developed sensory organs of various types. (10) Cilia completely absent. Muscles mostly striated and capable of rapid contraction. (11) Sexes mostly separate with sexual dimorphism. Paired reproductive organs and ducts. (12) Fertilization typically internal, in female's body. Eggs megalecithal. Oviparous or viviparous (13) Life–cycle includes one or more larval stages that metamorphose into adults. Classification of Arthropoda : On the basis of body shape, degree of segmentation and regionation, and presence or absence of certain appendages (antennae, mandibles and chelicerae), phylum Arthropods is divided into four subphyla; Biggest phylum in regard to the number of species is Arthropoda. Subphylum (I) Onychophora (Gr. Oychos = claw, phoros = bearing) (1) Terrestrial walking worms. (2) Body cylindrical with indistinct extenral segmentation. (3) Unjointed 14-43 pairs of legs. (4) Head not distinct, Oviparous/Viviparous (5) A pair of eyes, short antennae and blunt oral papillae. (6) Excretory organs are metanephridia segmentally arranged. (7) A living connecting link forming a transitional link between Annelida and Arthropoda. Example : Peripatus, Ophisthopatus, Ooperipatus, etc. Subphylum (II) Trilobitomorpha (Gr. TRIA = Three; LOBOS = lobe; MORPHE = form) (1) Most primitive, extinct, marine arthropods of Cambrian to Permian rocks. (2) 10 to more...

(Gk. nema = thread; helmin = worm) Brief History : Ancient people were familiar with certain large-sized nematode parasites of domestic animals. Minute nematodes were discovered only after the invention of microscope. Linnaeus (1758) included these in “Vermes” Rudophi (1793, 1819) included these under “Nematoidea” Gegenbaur (1859) ultimately proposed “Nemathelminthes” for these. General Characters (1) Many endoparasites of various animals and plants; others free–living and widely distributed in all sorts of water and damp soil. (2) Mostly minute or small; some large (1 mm to 25 cm); some upto several meters long. (3) Slender, cylindrical, elongated body usually tapering towards both ends, and unsegmented. (4) Body wall formed of a thick, tough and shiny cuticle, a syncytial hypodermis beneath cuticle, and innermost layer of peculiar, large and longitudinally extended muscle cells arranged in four quadrants. (5) Triploblastic, bilaterally symmetrical, pseudocoelomate, false coelom derived from embryonic blastocoel, unsegmented. (6) Straight alimentary tract terminal mouth and anus. These are first animals to have complete gut. (7) ‘Tube within a tube body’ plan, organ-system grade of body organization. (8) Circulatory system and respiratory organs absent. A simple excretory system consists of protonephridia, comparatively simpler or complicated sensory organs, and a well–developed nervous system present (9) Reproductive system well–developed. Usually unisexual with sexual dimorphism. (10) Many kinds of Nematodes are parasites of useful plants and domestic animals. Some of these are pathogenic to their hosts, causing serious diseases. Even man  is a host for more than 50 species, of which Ascaris lumbricoides and Enterobius vermicularis (pin worm) are very common. Other common human nematodes are Wuchereria which causes Filaria, Trichinella causing trichinosis, and Ancylostoma causing hookworm disease. Classification of Nemathelminthes : On basis of the presence or absence of some specialized sense organs and caudal glands, and characteristics of excretory system, nematodes are classified into two classes – Class 1.  Phasmidia or Secernentea or Rhabditea (1) Mostly parasitic. (2) Possess a pair of unicellular, pouch-like sense organs, called phasmids, near hind end of body. (3) Another pair of reduced, pore–like sense organs, called amphids, present near anterior end. (4) Excretory system with paired lateral canals. (5) Caudal glands absent. Examples – Ascaris, Enterobius, Ancylostoma, Wuchereria, Trichuris, Trichinella, Dioctophyma, Rhabditis Necator, Gnathostoma, Dracunculus, Loa, etc. Class 2. Aphasmidia or Adenophorea or Enoplea (1) Mostly small, free-living. (2) No phasmids. (3) Amphids spiral, cord like or disc like, seldom pore like. (4) No lateral excretory canals. (5) Caudal glands present. Examples : Enoplus, Dorylaimus, Mermis, Halichoanolaimus, Monohystera, Desmoscolex, etc.

General Characters (1) Aquatic, aerial or terrestrial. All free-living with no fully parasitic forms. (2) Body small to large, bilaterally symmetrical and metamerically segmented. (3) A post anal tail usually projects beyond the anus at some stage and may or may not persist in the adult. (4) Exoskeleton often present; well developed in most vertebrates. (5) Body wall triploblastic with 3 germinal layers : ectoderm, mesoderm and endoderm. (6) Coelomate animals having a true coelom, enterocoelic or schizocoelic in origin. (7) A skeletal rod, the notochord, present at some stage in life cycle. (8) A cartilaginous or bony, living and jointed endoskeleton present in the majority of members (vertebrates). (9) Pharyngeal gill slits present at some stage; may or may not be functional. (10) Digestive system complete with digestive glands. (11) Blood vascular system closed. Heart ventral with dorsal and ventral blood vessels. Hepatic portal system well developed. (12) Excretory system comprising proto-or meso- or meta-nephric kidneys. (13) Nerve cord dorsal and tubular. Anterior end usually enlarged to form brain. (14) Sexes separate with rare exceptions. Classification of chordata : Phylum chordata can be divided into two groups: Acrania (Protochordata) and Craniata (Euchordata) having contrasting characters. Group A. Acrania (Protochordata) : (Gk. a = absent; kranion = head,) or, (Gk. protos = first; chorde = cord). All marine, small, Primitive or lower chordates. Lacking a head, a skull or cranium, a vertebral column, jaws and brain. About 2,000 species. The Acrania is divided into three subphyla: Hemichordata, Urochordata and Cephalochordata, chiefly on the character of notochord present. Subphylum I. Hemichordata : (Gk. hemi = half; chorde =cord). Body divided into 3 regions: Proboscis, collar and trunk. Notochord doubtful, short, confined to proboscis and non–homologous with that of chordates. Class 1. Enteropneusta : (Gk. enteron = gut; pneustos = breathed). Body large and worm-like. Gill slits numerous. Intestine straight. Acorn or tongue worms. 70 species. e.g. Balanoglossus, Saccoglossus.       Class 2. Pterobranchia : (Gk. pteron = feather; branchion = gill). Body small and compact. Gill-slits one pair or none. Intestine U–shaped. Pterobranchs. 20 species. e.g. Cephalodiscus, Rhabdopleura. Subphylum II. Urochordata or Tunicata : (Gk. oura = a tail;) (L. chorda = cord). Notochord and nerve cord only in tadpole-like larva. Adult sac-like, often sessile and encased in a protective tunic. Tunicates. Class 1. Ascidiacea : Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-clefts numerous. Ascidians or sea squirts. 1,200 species. e.g. Herdmania, Ciona, Molgula.

• Retrograssive metamorphosis present in Herdmania.

      Class 2. Thaliacea : Free-swimming or pelagic tunicates with circular muscles in tunic. Sometimes colonial. Salps or chain tunicates. 30 species. Salpa, Doliolum, Pyrosoma. Subphylum III. Cephalochordata : (Gk. kephale = head;) or (L. chorda = cord). Notochord and nerve cord present throughout life along entire length of body. Class Leptocardii more...