ZOOLOGY BOTANY CHEMISTRY

1. Early Development Development of the zygote begins within the worm and by the time the proglottid is shed, has already changed into an embryo. The zygote (Fig. 7.11 A, 7.11 B) undergoes complete but unequal cleavage. It produces a mass of blastomeres of 3 types : 2 or 3 macromeres , 3 to 5 mesomeres and numerous micromeres (Fig. 7.11 C). The micromeres form the embryo with 3 pairs of claw-like hooks at the posteiror end. The 6-hooked embryo is called hexacanth . The mesomeres and macromeres form (Fig. 7.11 G) the inner and outer embryonic membranes respectively round the hexacanth. The inner (Fig. 7.11 D, E) membrane later forms a thick, chitinous, radially striated secondary shell or embryophore (Fig. 7.11 F). The hexacanth with embryophore, outer embryonic membrane and shell or capsule wall is known as the onchosphere (Fig. 7.11 G). (Gr. onchos = hook). It is about 40 μm in diameter. It is actually the onchospheres which are set free when the proglottides disintegrate in

POLYMORPHISM Definition One of the important characters of Eumetazoa is the division of labour. In the vast majority, i.e., from flat worms to chordates, the division of labour involves the assignment of different functions to different parts or organs of the individual. In coelenterates, low organization and lack of organs do not permit any great degree of specialization for different functions within the limits of an individual. Therefore, in these animals the different vital functions are assigned to different individuals. This form of the division of labour is known as polymorphism. Polymorphism is, thus, the phenomenon of occurrence of an animal in more than one morphological and functional form. The individuals of polymorphic animals are known as the zooids or persons. 2. Grades Some coelenterates possess only two types of zooids, others have three types, and still others several types. These forms are respectively described as dimorphic, trimorphic and polymorphic. There

Flight and Flight Adaptation in Birds I. Mechanism of Flight. Three distinct types of flight may be considered gliding, soaring and flapping. 1. Gliding. This is the simplest type of flight. In it a bird, after attaining a certain velocity or after reaching a certain height, planes through the air without moving the wings. This type is also used for landing. In gliding (Fig. 7.32A), the wing is moved against the air with its strong leading (front) edge tilted upward. Air passing over the convex upper surface of the wing encounters less resistance and, therefore, speeds up and tends to pull away. This creates above the wing a drop in pressure (semivacuum), producing a 'suction zone' there. The air flowing over the lower concave, temporarily impermeable surface of the wing encounters greater resistance and is retarded. This causes under the wing a rise in pressure, producing an upward thrust. These two forces, suction above and upward thrust below, cau

Locomotion In Protozoa 1.5. LOCOMOTION IN PROTOZOA • Locomotor Organelles. The locomotor organelles of protozoans may be long filaments called flagella , short hair-like processes termed cilia , or flowing extensions of the body known as pseudopodia . The pseudopodia are of four types: lobopodia filopodia, axopodia or actinopodia and reticulopodia. (a) Lobopodia. These are thick, finger-like outgrowths with rounded, blunt tip. They consist of both ectoplasm and endoplasm. These are found in Amoeba, Entamoeba and Arcella. (b) Filopodia. These are slender processes with pointed tip, and consist of ecoplasm only. They have a tendency to branch and radiate in all directions. These are seen in Acanthometra. (c) Axopodia. These are stiff, ray-like processes from the ecotplasm supported by a firm axial filament (bundle of microtubules) from the endoplasm. They radiate fom the spherical body in all directions. These occur in Actinophyrs. (d) Reticulopodia. These are f

CLASSIFICATION Division : Bryophyta Class     : Hepaticopsida Order     : Marchantiales Family   : Marchantiaceae Genus   : Marchantia MORPHOLOGICAL FEATURES OF THE THALLUS- 1. The main plant body is gametophytic thallus. Gametophyte is small, well developed and  autotrophic. 2. The plant body is dorsiventral, the dorsal surfaces is dark green having a distinct midrib and  dichotomously branched. 3. The ventral or the lower surface bear numerous rhizoids and scales. 4. The rhizoids are unicellular and branched structures meant for anchorage and absorption. They  may be smooth walled or tuberculated (bearing peg-like projections). 5. The scales are purplish multicellular structures which protect the thallus from the dessication  by secreting the mucilage. 6. The dorsal surface also bears small gemma cups on the mid ribs. Inside the gemma cups many  erect stalked gemmae are present which helps in vegetative reproduction of Marchantia. 7. Sexual reproduction is oogamous. The male reproduc