Current Affairs 11th Class

(1) The common Indian earthworm, Pheretima posthuma belong to the class oligochaeta of the phylum Annelida. It is found in every part of the world. It lives in damp soil and burrow in lawns, fields, garden etc. rich in humus. Earthworm is nocturnal i.e., active during night. (2) The generic name Pheretima was first used by Kinberg in 1867. Our knowleage of Pheretima is mainly due to the work of Karm Narayan Bahl (1926). (3) Body is cylindrical, bilaterally symmetrical, elongated with metameric segmentation. Earthworm shows both external and internal segmentation. The number of segments is about 100-120, the length is about 150 mm.     (4) Earthworm is brown or clay-coloured. This is because of the pigment porphyrin. Numerous granules of porphyrin pigment are found scattered in the circular muscle layer of body wall. Porphyrin protects the body from the injurious effects of bright light. (5) The first segment is peristomium or buccal segment which bears mouth. Anus is located on the last segment. (6) Three regions in body of earthworm are ? Preclitellar region (1 - 13), Clitellar region (14, 15, 16)  and Postclitellar region (17 - last). (7) Nephridiopores of integumentary nephridia 200-250 per segment found in all segments except the first six. Clitellar segment contains 2000 nephridiopores per segment, so called-forest of nephridia?. (8) In the body wall 11 pores concerned with reproduction. They are - Spermathecal pores in the intersegmental grooves of 5/6, 6/7, 7/8 and 8/9 (4 pairs). Female genital pore midventral on segment 14th. Male genital pores ventrolaterally (1 pair) on segment 18th. (9) Male genital papillae are present on segments 17 and 19 (2 pairs). (10) Body wall is dermomuscular, consists of cuticle, epidermis, muscular layers and coelomic epithelium. Epidermis consists of tall, columnar cells of four types ? Supporting cells (major part), Glandular cells (Goblet and albumin), Basal cells and Sensory cells. (11) All segments except the first, last and clitellar segment contain setae (perichaetine arrangement). Setae are 'S-shaped, yellowish and chitinous, 80-120 segment. Setae and contraction of muscles help in locomotion. (12) The body cavity of earthworm is true coelom (schizocoel) as it is formed by the division of mesoderm. The coelom is filled with milky white alkaline coelomic fluid. Coelomic fluid contains different types of carpuscles. These are granulocytes (phagocytes), most numerous mucocytes, circular nucleated cells (leucocytes) and chloragogen cells (yellow cells). (13) Chloragogen cells are small, star-shaped, yellow cells concerned with storage of reserve food, deamination of proteins, formation of urea and also excretory (analogous to the liver of vertebrates). (14) The alimentary canal of earthworm is a straight tube, representing a ?tube within tube plan, Location of different part of alimentary canal are - Buccal chamber         :                               \[1-2\frac{1}{2}\] Pharynx                         :                               \[2\frac{1}{2}-4\]              Oesophagus   more...

(1) Ascaris lumbricoides, the common roundworm belong to the class Rhabditea of the phylum Nemathelminthes. It is the most common endoparasite in the small intestine of human beings. It is monogenetic, i.e., without any secondary host. The worm is more common in children. (2) The body is elongated, unsegmented, cylindrical with tapering ends and four streaks-two lateral, one ventral and one dorsal. (3) Sexes are separate with sexual dimorphism. Male is smaller than female with curved tail, two penial setae (copulatory organs) and cloaca. Female is with straight posterior end of the body and posterior transverse anus and separate gonopore situated ventrally 1/3 from the anterior end. In both the excretory pore is situated mid-ventrally, a little behind the mouth. Ventral surface of male bears fifty pairs preanal and five pairs postanal papillae. These sensory papillae are absent in female. (4) Mouth both in male and female is terminal, triradiate surrounded by three denticulate lips. One median dorsal and two ventrolateral. Dorsal lip bears two sensory double papillae (tangoreceptors). Both sensory papillae and amphids (chemoreceptors) are present on ventrolateral lips. (5) Body wall consists of outer cuticle, middle epidermis and inner longitudinal muscle layer. Circular layer is absent. Cuticle is thick which is protects the body of the parasite from mechanical injury and also is resistant to action of digestive enzymes of the host. The epidermis is syncytial (coenocytic) with scattered nuclei and without partition walls. (6) The body cavity of Ascaris is pseudocoel formed by vacuoles originated from persistent embryonic blastocoel. (7) There is no alimentary canal and digestive gland. The parasite absorbs digested food of the host so their is no need of digestive organs. Absorption occurs through the general body surface. Salivary glands do not occurs in Ascaris. (8) Respiratory system is absent, respiration is anaerobic. (9) Excretory system is H-shaped. It is consists of a single excretory cell or renette cell. Excretory products are ammonia and urea. (10) Sense organs are simple like labial papillae, cervical papillae, anal papillae, amphids and phasmids. (11) Ascaris is dioecious or unisexual. Testes is single and median, so male Ascaris is monarchic (monodelphic). Only anterior part of testis is functional, so testis (also ovary) is telogonic. (12) Ascaris sperm is peculiar without flagellum, tail less, asymmetrical and amoeboidal. (13) Female Ascaris has paired ovaries so female Ascaris is didelphic. (14) Copulation occurs in the intestine of host. Fertilization in the lower part of uteri. The egg is mammilated, oval, m-shape with three protective covering?outer protein layer, middle chitinous shell and inner membrane made of esterified glycosides. (15) Embryonic development takes place only outside the body of human host in soil because it requires low temperature, more oxygen and suitable moisture. (16) Inside the shell the zygote develops into rhabditiform larva or first stage juvenile in 10-14 days. (17) The larva of first stage is not infective. It rests for a week and completes first more...

(1) Dugesia (Planaria) is found commonly in freshwater ponds, lakes, streams and shallow rivers. (2) Planaria are gregarious, i.e., they live in groups. (3) The head bears a pair of lateral projections called auricles. (4) The mouth opens on the mid ventral surface near the middle of the animal. (5) The pharynx is a tubular structure that can be everted beyond the mouth. (6) Planarians have remarkable power of regeneration. (7) If an individual is cut transversely into two parts, the anterior fragment will regenerate a new tail and a posterior piece will develop a new head. (8) Neoblast cells found in planarians which is help in regeneration.         (1) Fasciola hepatica, commonly known as sheep liver fluke is an endoparasite of sheep which reside in the liver and bile duct. (2) The liver fluke has a dorsoventrally flat, unsegmented body with two suckers, oral sucker (anterior sucker) and acetabulum (ventral sucker). (3) Liver fluke is covered with a cuticle, lacks ciliated epidermis. (4) There are three parmanent apertures on the body-mouth (surrounded by oral sucker), genital pore (located between the two suckers), excretory pore (At the extreme posterior end). During breeding season a temporary opening, the aperture of laurer’s canal is also developed. Laurer’s canal is present between the genital aperture and the uterus.     (5) Suctorial pharynx with bifurcated intestine. A large number of caeca or diverticulae arise from each branch of intestine. (6) Digestion is holozoic. The parasite obtains nourishment from bile, blood, lymph and epithelial cells. (7) Respiration is anaerobic. (8) Excretion occurs with the help of flame cells. (9) Fasciola is a digenetic endoparasite which are its primary host is sheep causing ‘liver rot’ and the secondary or intermediate host is the snail of genus Limnaea and Planorbis. (10) Fasciola hepatica is a hermaphrodite. In male has a pair of testes and female has an ovary, vitteline gland for yolk formation and mehlis’s gland for lubrication. (11) Fertilization is internal. Cross fertilization commonly occurs. (12) Different larval stages of Fasciola hepatica according to development sequence are : miracidium-sporocyst-Redia-Cercaria-Metacercaria. (13) Stage in the life cycle of Fasciola when it infects intermediate host (snail) is miracidium and primary host is metacercaria. (14) Miracidium and cercaria larva are free swimming form in water. Redia and sporocyst are formed in snail. (15) Fasciola exhibits both alternation of generation and alternation of host.   (1) Schistosoma is commonly known as human blood fluke and it is found in the blood vesseles and hepatic portal system of man, cat, pig, dog, etc. (2) Phenomenon of sexual dimorphism occurs. Thus male and female are separate but they live in close association. (3) Male is flattened while female is slender. Both possess oral and ventral suckers. (4) The ventral more...

(1) Hydra belongs to class Hydrozoa of phylum coelenterata. (2) Trembly (1744), a Swiss biologist discovered Hydra. Linnaeus (1758) gave the name Hydra, a Greek word, means ‘Water serpent’ based on its ability to regenerate its lost parts. (3) Hydra is a solitary polyp found in freshwater (stagnant). Among coelenterates Hydra is one of the smallest polyps. (4) It is colourless carnivorous coelenterate having radial symmetry. (5) Hydra is a diploblastic and has tissue grade of organization with division of labour on morphological basis. (6) Chlorohydra viridissima is called green hydra. It is green because of symbiotic association with a unicellular green algae Chlorella vulgaris. Algae live in the musculonutritive cells of Hydra. (7) Hydra has a cylindrical body with 6-10 hollow tentacles. It help in locomotion and food capture, so analogous (correspond functionally) to pseudopodia of Amoeba. (8) Mouth is situated on a manubrium or hypostome. It is the most sensitive in the body. Hydra has no anus. (9) The body wall of Hydra consists of ectoderm and endoderm, in between a thin, delicate, transparent and non-cellular mesogloea. (10) Ectoderm consists of epithelio-muscular cells, sensory cells, nerve cells, interstitial cells (totipotent) and stinging cells or cnidocytes having nematocysts. (11) Inner gastrodermis has nutritive muscular cells, gland cells, nerve cells, sensory and interstitial cells. Nutritive muscular cells bear both flagella and pseudopodia. (12) The contraction of muscle fibres in endothelio-muscular cells or nutritive muscle cells reduces the diameter of the body and works like circular muscles. (13) Mesogloea is thin and acellular consisting of a proteinaceous matrix and it can be crossed by interstitial cells. It is neither cellular nor fibrous. (14) Cnidoblasts or nematocysts are derived from interstitial cells of epidermis. (15) Body cavity of Hydra is called coelenteron or gastrovascular cavity. Coelenteron serves the double purpose of digestion and circulation. (16) Nematocysts are found only in epidermis mainly on tentacles. Nematocysts are also known as “independent effectors”. (17) Hydra paralyses its prey by nematocyst. If all nematocysts of a Hydra are removed it would affect its capacity to capture prey. (18) Nematocyst plays an important role in locomotion, food capture both offence and defence. (19) Hydra has four types of nematocysts : Penetrants or stenoteles (largest), valvents (smallest), stereoline glutinants (small, atrichous) and streptoline glutinants (large holotrichous) (20) Digestion in Hydra is first extracellular (in gastrovascular cavity) and then intracellular (in endoderm cells). (21) Hydra has no specialized cells for respiration, it respires by means of general body surface. (22) Nitrogenous excretory product in Hydra is ammonia and it is removed through general body surface. (23) Hydra possesses a very primitive nervous system consisting of a synaptic network of bipolar and multipolar nerve cells, but brain is absent. (24) Hydra is monoecious or dioecious. Most species are dioecious or unisexual. Bisexual species of Hydra are protandrous, so avoid self-fertilization. (25) Hydra reproduces asexually by exogenous budding, a type of vegetative propagation, and sexually by formation of gametes. Hydra reproduces by more...

(Gk. Porus = Pore; ferre = To bear) Brief History : Robert Grant (1825) finally proved that sponges are animals, and coined the name ‘Porifera’ for these. Schulze (1878), Butschli (1884), Sollas (1884) and Delage (1898) separated sponges from other metazoans on the basis of embryological studies, and suggested a separate group, “Parazoa” for these. General Characters   (1) All the sponges are aquatic, sedentary, asymmetrical or radially symmetrical. First multicellular organisms and have cellular grade of organization. (2) They are diploblastic. Ectoderm is formed by pinachocyte and endoderm is formed by choanocyte. Both layers are called pinachoderm and choanoderm. A gelatinous noncellular mesenchyme present in between them. (3) Mesenchyme contains free amoebocytes and skeletal elements. (4) Different types of amoebocytes are : Archaeocytes                :   undifferentiated totipotent cells. Chromocytes                 :   with pigment granules. Thesocytes                     :   with reserve food granules. Myocytes                        :   highly contractile, spindle-shaped cells. Trophocytes                  :   supply nutrients to developing cells (nurse    cells) Gland cells                     :   secrete slimy substance. Sex cells                           :   develop from archaeocytes only during breeding season. (5) The body is perforated by numerous minute pores called ostia. (6) The ostia open into a large cavity called spongocoel or paragastric cavity. (7) The spongocoel opens to the outside by a large opening called osculum. (8) Sponges have a canal system and they need a continuous current of water flowing through their bodies for respiration, excretion, nutrition and reproduction. (9) Different types of canal system in sponges are asconoid, syconoid and leuconoid. (10) The simplest type of canal system in porifera is asconoid type.    (11) The course taken by the water current way be shown as under – Ingressing water \[\xrightarrow{\text{Ostia}}\] Spongocoel \[\xrightarrow{\text{Osculum}}\] To outside (12) The sponges possess an endoskeleton in the form of calcareous spicules. (13) Excretion and respiration occur by diffusion. (14) They have greater power of regeneration due to totipotent archaeocytes. (15) Digestion in sponges is intracellular like protozoans. (16) All sponges are hermaphrodite, reproduction takes place by asexual or sexual methods. (17) Gemmules are internal buds containing archaeocytes, mostly found in fresh water sponges, concerned with asexual reproduction. (18) Development is indirect or direct. The common larval forms are parenchymula (leucosolenia and Clathrina), amphiblastula (Sycon), etc. Classification of porifera : On the basis of types of endoskeleton, phylum porifera is divisible into three classes Class 1. Calcarea or Calcispongiae (1) Skeleton is formed of Calcareous spicules.  (2) Radially symmetrical. (3) Choanocyte cells are large and conspicuous. (4) Canal system asconoid (ascon) or syconoid (sycon) type. (5) These are also known as limy sponges. Examples : Clathrina, Leucosolenia, Sycon, Grantia, etc.,    
  • Leucosolenia is a smallest sponge more...

(Gk. platys = broad or flat; helmin = worm) Brief History : Aristotle mentioned tapeworms, but scientific studies of flatworms began only in the 18th century. It was Gegenbaur (1859) who placed these in a separate group and suggested the present name of the phylum. General Characters (1) They are dorso-ventrally flattened like a leaf. (2) They show organ grade of organization. (3) They are acoelomate animals. The cavity in platyhelminthes is filled with mesenchyme or parenchyma. (4) They are triploblastic animals. The cells of the body wall are arranged in three layers.They are the ectoderm, the mesoderm and the endoderm. (5) They are bilaterally symmetrical animals. The body of the animal can be divided into two equal similar halves through only one plan. Animals with this symmetry have definite polarity of anterior and posterior ends. (6) Some members have segmented body. The segmentation in platyhelminthes is called pseudometamerism. (7) Many of the parenchyma cells give rise to muscle fibres.The muscle fibres are arranged in circular, longitudinal and vertical layers. (8) The digestive system is completely absent from Cestoda and Acoela. The alimentary canal is branched in Turbellarians. The anus is absent from them. (9) The respiratory organs are absent. In parasites respiration is anaerobic. (10) There is no circulatory system. (11) The excretory system is formed of protonephridia (flame cells or solenocytes). (12) Anus is absent like coelenterates, with blind sac body plan. (13) The nervous system is well developed. It is formed of longitudinal nerve cords with ganglia. A pair of anterior ganglia form the brain. The longitudinal nerve cords are connected together by transverse connectives. (14) They are hermaphrodites, i.e., both male and female reproductive organs are present in the same animal. (15) Fertilization is internal in them. Self or cross fertilization takes place in them. (16) Their development is direct or indirect. Endoparasites show usually indirect development with many larval stages. Their life cycle is completed in one or two hosts. (17) They are free living or parasitic. In parasitic worms adhesive organs like hooks, spines, suckers and adhesive secretions are present. Classification of platyhelminthes : On the basis of digestive tract and free living or parasitic nature phylum platyhelminthes has been divided into three classes – Class 1. Turbellaria (L. turbella, a string) (1) Most of the turbellarians are free living but some of them are ectocommensal or parasitic, commonly called planarians or flat worms. (2) The body epidermis is either cellular or syncytial and covered with cilia. Epidermis contains rhabdites. (3) Segmentation is absent. (4) Digestive system is present except in a few. (5) Suckers are absent. (6) Life cycle is simple, development direct. Example : Dugesia, Notoplana, Bipalium, Thysanozoon, etc.
  • Bipalium is only terrestrial planarian.
Class 2. Trematoda (Gr. trema, hole) (1) Ecto or endoparasites of vertebrates; commonly called flukes. (2) Body mostly oval, unsegmented. (3) Body wall without cilia, but covered by a thick, resistant, syncytial tegument. (4) Suckers, and often hooks and spines, present for attachment to more...

 (L., Mollis or Molluscus = Soft bodied) Brief History : Aristotle described a number of molluscs. Johnston (1650) proposed the name of the phylum. General characters (1) Molluscs are multicellular organisms. (2) They are  mostly marine. (3) They have a bilateral symmetry, but snails are asymmetrical. (4) They are triploblastic animals. (5) They are coelomate animals. True coelom is reduced the haemocoel is well developed in them. (6) They have organ system grade of organization. (7) The body is soft and unsegmented. (8) The soft body is covered by a fleshy fold of the body wall. It is called mantle. (9) The molluscs are provided with one or two calcareous shells. The shells may be external or internal, univalve or bivalve. (10) Respiration is carried out by the gills or pulmonary chambers. (11) The digestive system is well developed. It contains a radula and a hepatopancreas. (12) The circulatory system is of an open type. Blood with amoebocytes, respiratory pigment is copper containing haemocyanin dissolved in plasma. (13) The excretory organ is the kidney (organ of Bojanus). (14) The nervous system is well developed with paired ganglia, commissures and connectives. (15) The sensory organs are eyes, statocysts and osphradia (a chemoreceptor to test chemical nature of water). (16) Reproduction sexual. Sexes are separate in them, or they are hermaphrodites. (17) The development in their case is either direct or indirect with free larval forms like trochophore, veliger, glochidium, etc. Classification of Mollusca : On the basis of body shape and symmetry and characteristics of foot mantle, respiratory organs, nervous system, etc. phylum mollusca are divided into seven classes : Class 1. Monoplacophora (1) The body is bilaterally symmetrical and segmented. (2) The shell is formed of a single valve. (3) The head is without eyes and tentacles. (4) The gills are external and serially arranged. (5) The nephridia are five pairs. Example : Neopilina galathea
  • Neopilina is a living fossil and connecting link between Annelida and Mollusca.
Class 2. Aplacophora or Solenogasters (1) The body is worm–like, bilaterally symmetrical and cylindrical. (2) The head, mantle, foot, shell and nephridia are absent. (3) The body is covered with spicule–bearing cuticle. (4) The digestive tract is straight with radula. (5) A mid dorsal longitudinal keel or crest is often present . Example : Neomenia, Chaetoderma, etc., Class 3. Polyplacophora (1) These molluscs are bilaterally symmetrical, and dorsoventrally flattened. (2) Head small, without eyes and tentacles.     (3) The shell is composed of a longitudinal series of 8 plates. (4) The foot is flat and ventral. (5) The radula is well developed. (6) Respiration by 8 to 60 pairs of gills. (7) Unisexual; only one gonad; trochophore larval stage. Example : Chiton, Cryptochiton, etc.
  • On the dorsal surface of chiton is a convex shell composed of 8 transversely elongated calcareous plates arranged in a longitudinal manner.
Class 4. Gastropoda (1) more...

 (Gk. echinos = spines; derma = skin/covering) Brief History : Although Jacob Klein (1738) had earlier coined the name “Echinodermata”, yet Linnaeus included these animals under “Mollusca”, and Lamarck under his class “Radiata” as “Echinodermes”. Finally, Leuckart (1847) raised the group to the status of a separate phylum. General characters (1) Echinoderms are exclusively marine beings. (2) They are triplobalstic and coelomate (enterocoetomate) animals. (3) They have radially symmetrical body. The radial symmetry is due to sedentary or sessile mode of life and it is a secondary character in echinoderms. (4) They have organ system grade of organization. (5) They have well developed endoskeleton formed of calcareous ossicles and spines. (6) They have a water–vascular system (Ambulacral system) with tube–feet for locomotion, feeding and respiration. (7) Circulatory system is of the open–type. (8) Respiratory organs include dermal branchiae, tube feet, respiratory tree and bursae. (9) Nervous system is complex and contains both central and peripheral components, but no brain. (10) The sensory organs are poorly developed. (11) The excretory organs are absent. (12) They have pedicellariae. (13) Development is indirect. (14) The larval forms are bilaterally symmetrical. (15) Regeneration power is well developed in Echinoderms. Classification of Echinodermata : On the basis of body shape, position of madreporite and kind of larval form, echinoderms are classified into two subphylum. Subphylum (I) Eleutherozoa ­: Free-living echinoderms with ventral mouth. Class 1. Asteroidea (1) Starfishes or sea stars. (2) Arms 5 or more and not sharply marked off from the central disc. (3) Tube feet in orally placed ambulacral grooves; with suckers.   (4) Anus and madreporite aboral. (5) Pedicellariae present. (6) Free-living, slow-creeping, predaceous and scavengerous. Examples : Astropecten, Luidia, Goniaster, Oreaster (= Pentaceros), Asterina, Solaster, Pteraster, Echinaster, Asterias, Heliaster, etc. Class 2. Ophiuroidea (1) Brittle-stars and allies. (2) Body star-like with arms sharply marked off from the central disc. (3) Pedicellariae absent. (4) Stomach sac-like; no anus. (5) Ambulacral grooves absent or covered by ossicles; tube feet without suckers. (6) Madreporite oral. Examples : Ophiura, Ophiothrix, Ophioderma, Ophiopholis, Gorgonocephalus, Asteronyx. Class 3. Echinoidea (1) Body not divided into arms; globular (sea urchins), or flattened disc-like (sea-cakes). (2) Mouth at lower pole, covered by 5 strong and sharp teeth, forming a biting and chewing apparatus called “Aristotle's Lantern”. (3) Tube-feet slender with suckers. (4) Skin ossicles fused to form a rigid globular, disc like, or heart-shaped shell or test with movable spines. (5) 3–jawed pedicellariae present in skin. (6) Gut long, slenderical and coiled. Anus present. (7) Larval forms pluteus and Echinopluteus. Examples : Echinus, Clypeaster, Echinarachinus, Echinocardium, etc.
  • Members of Echinoidea are also known as Floating stone.
Class 4. Holothuroidea (1) Body massive, long and cylindrical like a cucumber; elongated in oral–aboral axis; no arms. (2) Mouth at anterior and anus at posterior ends.     (3) Mouth surrounded by many hollow retractile tentacles. (4) Tube feet usually present; sucker-like. (5) Skin more...

 (Gk. kteis = comb; pherein = To bear) Brief History : The ctenophores as a distinct group were first recognized by Escscholtz (1829).  Hatschek (1889) placed it under a separate phylum called ctenophora. General characters (1) All the ctenophores are marine. (2) They are solitary and pelagic. (3) They are transparent. (4) The have tissue-grade of organization. (5) They have biradial symmetry. (6) They are acoelomate animals. (7) They are unsegmented. (8) They body-wall is diploblastic. (9) The mesogloea contains cells. (10) Nematocysts are absent. (11) Special adhesive cells called colloblasts are present in all ctenophores. (12) The gastrovascular system is well developed. (13) Two anal openings are present. (14) Skeletal system is absent. (15) Excretion and respiration are carried out by diffusion. (16) The nervous system is in the form of nerve net. (17) An aboral sense organ is present in the form of statocyst. (18) Cilia are used for locomotion. (19) They are hermaphrodites. (20) Development is indirect. It includes a cydippid larva. Classification of Ctenophora Class 1. Tentaculata (1) The body is simple, rounded or oval or ribbon-like. (2) Two long aboral tentacles are present. (3) Mouth is narrow and pharynx is small. Examples : Pleurobrachia, Hormiphora, Mertensia Mnemiopsis, Bolinopsis, Velamen, Cestum, Ctenoplana, Coeloplana, etc.
  • Cestum is commonly called “venus’s girdle”.
• Ctenoplana shows commensal with Alcyonea.   Class 2. Nuda (1) Body is large thimble-shaped or conical. (2) Tentacles are absent. (3) Mouth is wide and pharynx is large. (4) The meridional vessels are produced into a complex system of anastomosing branches. Example : Beroe
  • Befroe is commonly called “Swimming eye of cat”.

 (Gk. knide = nettle or stinging cell) Brief History : Peyssonel (1723) and Trembley (1744) proved these to be animals. Hence, Linnaeus (1758), Cuvier (1796) and Lamarck (1801) included these under ‘Zoophyta’, together with sponges. Leuckart (1847) included sponges and cnidarians under his phylum Coelenterata. Finally, Hatschek (1888) divided “Coelenterata” into three phyla–Spongiaria (= Porifera), Cnidaria and Ctenophora. General characters  (1) Coelenterates are radially symmetrical animals with tissue grade of body organization. (2) All the members of this phylum are aquatic, mostly marine. (3) They are solitary or colonial, sedentary or free swimming. (4) The body wall is diploblastic. It is made up of two layers of cells, namely the ectoderm and the endoderm with a non-cellular layer called mesogloea in between. (5) Cnidarians exhibit diamorphism with polypoid and medusoid stage (Metagenesis or alternation of generation). (6) Asexual phase is generally polyp and sexual phase is medusa. (7) Coelom is absent; Hence coelenterates are acoelomate animals. (8) A gastrovascular cavity or coelenteron is present. It can be compared to the gut of higher animals. (9) Mouth is present but anus is absent (blind-sac body plan). Mouth is surrounded by tentacles. (10) The most characteristic feature of coelenterates is the presence of nematocysts or stinging cells. (11) Digestion is extracellular as well as intracellular. (12) Respiratory, excretory and circulatory system are absent. (13) Primitive nervous system with synaptic or non-synaptic nerve net but no brain. (14) Sense organs are statocysts (tentaculocysts), ocelli and olfactory pits. (15) Reproduction both asexual and sexual. (16) Development is indirect as there are one or two larval forms, Planula (Obelia) and Ephyra (Aurelia). Classification of coelenterata : On the basis of the dominance of medusoid or polypoid phase in the life cycle, phylum coelenterata is divided into three classes – Class 1. Hydrozoa (Gr. hydros, water, zoios, animal) (1) Hydrozoa are solitary and fresh water or mostly colonial and marine, sessile and free-swimming forms. (2) They exhibit tetramerous or polymerous radial symmetry. (3) Body wall consists of an outer ectoderm and an inner endoderm separated by a non-cellular gelatinous mesogloea. (4) Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament. (5) Skeleton or horny structure is horny perisarc in some forms, while coenosarc secretes a skeleton of calcium carbonate forming massive stony structure or coral in other forms. (6) They exhibit polymorphism. There are two main types of zooids, the polyp and medusa. Medusa is provided with true muscular velum. (7) Many hydrozoa exhibit alternation of generation. (8) Reproductive products of sex cells are usually ectodermal in origin and discharged externally. (9) Cleavage is holoblastic, embryo ciliated planula. (10) Both polypoid and medusoid stages present. Examples : Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia, Companularia, Millepora, Stylaster, Geryonia, Physalia, Porpita, Velella, Pericolpa, Periphylla, Cynaea, Rhizostoma or Pilema Cassiopeia, etc.,
  • Obelia is trimorphic and marine colony.
  • Hydranth of obelia bears twenty four (24) tentacles while medusa bears sixteen (16) tentacles in addition to tentaculocysts.

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