Study Notes on Structural Organization of Animals

Study Notes on Structural Organization of Animals


  • The structure of the cells vary according to their function. Therefore, the tissues are different and are broadly classified into four types: (i) Epithelial, 2) Connective, (iii) Muscular and (iv) Neural.
  • Epithelial Tissue We commonly refer to an epithelial tissue as epithelium- This tissue has a free surface.
  • These faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body.
  • The cells are compactly packed with little intercellular matrix.
  • There are two types of epithelial tissues namely simple epithelium and compound epithelium.
  • Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts, and tubes.
  • The compound epithelium consists of two or more cell layers and has protective function as it does in our skin.
  • The squamous epithelium is made of a single thin layer of flattened irregular boundaries. They are found in the walls of blood vessels Boundary.

Study Notes on Structural Organization of Animals

  • The cuboidal epithelium is composed of a single layer of a)squamous b) Cuboidal (c) Concave-like cells.
  • This is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption.
  • The epithelium of proximal convoluted tubule (PCT) of nephron in the kidney has microvilli. The columnar epithelium is composed of a single layer of tall and slender cells.
  • Their nuclei are located at the base. Free surface may have microvilli.
  • They are found in the lining stomach and intestine and help in secretion and absorption.
  • If the columnar or cuboidal cells bear cilia on their free surface they are called Ciliated epithelium.
  • Their function is to move particles or mucus in a specific direction over the epithelium.
  • They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.
  • Some of the columnar or cuboidal cells get specialised for secretion and are called glandular epithelium.
  • They are mainly of two types: unicellular, consisting of isolated glandular cells. And multicellular, consisting of cluster of cells (salivary gland).
  • On the basis of the mode of pouring of their secretions, glands are divided into two categories namely exocrine and endocrine glands.
  • Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products.
  • These products are released through ducts or tubes. In contrast, endocrine glands do not have ducts.
  • Their products called hormones are secreted directly into the fluid bathing the gland.
  • Compound epithelium is made of more than one layer (multi-layered) of cells and thus it has a limited role in secretion and absorption.
  • Their main function is to provide protection against chemical and mechanical stresses.
  • They cover the dry surface of the skin, the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts.
  • Three types of cell Junction are found in the epithelium and other issues. These are called as it adhering and gap Junctions.
  • Tight Junctions help to stop substances from leaking across a tissue.
  • Adhering Junctions perform cementing to keep neighbouring cells together.
  • Gap junctions facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.


  • Connective tissues are most abundant and widely distributed in the body of complex animals.
  • They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body.
  • The range from soft connective tissue to specialised types which include cartilage bone adipose and blood.


  • These are cells and fibers loosely arranged in a semi fluid ground substance
  • Adipose tissue is one of example of loose connective tissues located mainly beneath the skin.
  • Areolar tissue is another example of these tissues.


  • These are fibres and fibroblasts are compactly packed in the dense connective tissue.
  • In the dense regular and dense irregular, connective tissue the collagen fibers are present in rows between many parallel boundaries.


  • Bones have a hard and non- pliable ground substance rich in calcium salt collagen fibers.
  • These provide strength and frame work to the body.

Study Notes on Structural Organization of Animals


  • Blood is a fluid connective tissue containing plasma, red blood cells, white blood cell, and platelets.
  • The main circulating fluid that helps in the transportation of various substance.



  • The muscle is made of many long cylindrical fibres arranged in parallel arrays.
  • These fibres are composed of numerous fine fibrils, called myofibrils.
  • Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion.
  • Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body.
  • In general, muscles play an active role in all the movements of the body.
  • Muscles are of three types, skeletal, smooth, and cardiac.
  • Skeletal muscle tissue is closely attached to skeletal bones.
  • In a typical muscle such as the biceps, striated (striped) skeletal muscle fibres are bundled together in a parallel fashion.
  • A sheath of tough connective tissue encloses several bundles of muscle fibres also stimulated to contract.
  • The smooth muscle fibres taper at both ends (fusiform) and do not show strations Cell functions hold them together and they are bundled together in a connective tissue sheath.
  • The wall of internal organs such as the blood vessels, stomach and Intestine contains this type of muscle tissue Smooth muscles are involuntary as their functioning cannot be directly controlled.
  • Cardiac muscle tissue is a contractile tissue present only in the heart.


  • Cell functions fuse the plasma membranes of cardiac muscle cells and make them stick together Communication junctions (Intercalated discs) at some fusion points allow the cells to contract as a unit.
  • when one cell receives a signal to contract, its neighbours are also stimulated to contract.


  • Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions.
  • The neuroglia cell which constitute the rest of the neural system protect and support
  • neurons Neurogtia make up more than one half the volume of neural tissue in our body.
  • When a neuron is suitably stimulated. an electrical disturbance is generated which swiftly travels along its plasma.


  • The basic tissues mentioned above organise to form organs which in tum associate to form organ systems in the multicellular organisms.
  • Such an organisation is essential for more efficient and better coordinated activities of millions of cells constituting an organism.
  • Each organ in our body is made of one or more type of tissues.
  • Heart consists of all the four types of tissues, i.e., epithelial, connective, muscular and neural.


  • Earthworm is a reddish brown terrestrial invertebrate that inhabits the upper layer of the moist soil.
  • During day time, they live in burrows made by boring and swallowing the soil.
  • In the gardens, they can be traced by their faecal deposits known as worm castings.
  • The common Indian earthworms are Pheretima and Lumbricus.


  • Earthworms have long cylindrical body. The body is divided into more than hundred short segments which are similar (metameres about100-120 in number).
  • The dorsal surface of the body is marked by a dark median mid dorsal line (dorsal blood vessel) along the longitudinal axis of the body.
  • The ventral surface is distinguished by the presence of genital openings (pores). Anterior end consists of the mouth and the prostomium.
  • A lobe which serves as a covering for the mouth and as a wedge to force open cracks in the soil into which the earthworm may crawl.
  • The prostomium is sensory in function. The first body segment is called the peristomium (buccal segment) which contains the mouth. In a mature worm.
  • The segments 14-16 are covered by a prominent dark band of glandular tissue called clitellum.
  • the body is divisible into three prominent regions – preclitellar, Clitellar and postelltellar segments.
  • The Four pairs of spermathecal apertures are situated on the ventro-lateral sides of the intersegmental grooves, i.e., 5h-gth segments.
  • A single female genital pore is present in the mid-ventral line of 14th segment.
  • A pair of male genital pores are present on the ventro-lateral sides of the 18th
  • Numerous minute pores called nephridiopores open on the surface of the body.
  • In each body segment, except the first, last and Clitellum, there are rows of S-shaped setae, embedded in the epidermal pits in the middle of each segment.
  • The Setae can be extended or retracted, Their principal role is in locomotion.

Study Notes on Structural Organization of Animals



  • Cockroaches are brown or black bodied animals that are included in class Insecta of Phylum Arthropoda, Bright yellow, red and green coloured cockroaches have also been reported in tropical regions.
  • Their size ranges from 4 inches to 3 inches (0.6-7.6 cm) and have long antenna, legs and flat extension of the upper body wall that conceals head.
  • They are nocturnal omnivores that live in damp places throughout the world.
  • They have become residents of human homes and thus are serious pests and vectors of several diseases.


  • The adults of the common species of cockroach, Periplaneta americana.
  • These are about 34-53 mm long with wings that extend beyond the tip of the abdomen in males.
  • The body of the cockroach is segmented and divisible into three distinct regions – head, thorax and abdomen The entire body is covered by a hard chitinous exoskeleton (brown in colour).
  • In each segment, exoskeleton has hardened plates called sclerites (tergites dorsally and sternites ventrally) that are joined to each other by a thin and flexible articular membrane (arthrodial membrane)

Study Notes on Structural Organization of Animals