Structural Organisation In Some Animals Revision Notes For NEET


1.   Earthworm

        Pheretima posthuma (the common Indian earthworm) is nocturnal, fossorial, cold-blooded animal. It is brown in colour due to the pigment porphyrin, which protects the animals from UV light. Its body is cylindrical, bilaterally symmetrical, elongated and metamerically segmented.

        The first segment is peristomium bearing mouth and fleshy lobefold called prostomium. All segments of its body are alike except the 14th, 15th and 16th segment, which are thick, dark and glandular forming clitellum. All segments except the first, last and clitellar segment contain setae (perichaetine arrangement). Setae are S-shaped, yellowish and chitinous locomotory organs. Body cavity is true coelom (schizocoel) containing coelomic corpuscles, i.e. mucocytes, phagocytes, circular cell and chloragogen cells.

        The alimentary canal is a narrow tube made up of mouth, buccal cavity, pharynx, gizzard, stomach, intestine and anus.

        Gizzard is the main grinding organ, stomach produces secretions which neutralise the acidity of soil and intestinal wall consists of glandular cells, which secretes digestive enzymes.

        In most part of the intestine, a large villi-like internal fold is visible called typhosole. It increases the surface area for absorption of digested food.

        Its closed type of circulatory system consists of blood, blood vessels and heart. Blood is red in colour due to the presence of haemoglobin as respiratory pigment.

⇨span style='font:7.0pt "Times New Roman"'>        Its excretory system is made up of segmentally arranged coiled tubes called nephridia. These regulates the volume and composition of the body fluids.

        Their nervous system is represented by ganglia arranged segment wise on the ventral paired nerve cord. In the 3rd and 4th segment there a nerve ring is present which consisting of pharyngeal ganglions along with circumpharyngeal connectives.

        It is a hermaphrodite animal, but undergoes cross-fertilisation due to protandrous condition.

        Female genital pores are present on the 14th segment whereas male genital pore on the 18th segment.

        Fertilisation and development take place inside a cocoon secreted by the glands of clitellum.

2.   Cockroach

        Periplaneta americana is a brown or black coloured, nocturnal, omnivorous animal present in damp places throughout the world.

        Body is segmented and covered by chitinous exoskeleton called sclerites (tergites dorsally and sternites ventrally) that are joined to each other by a thin and flexible articular membrane (arthrodial membrane).

        Body is divided into head, thorax and abdomen. Head bears a pair of thread-like antennae, a pair of compound eyes and a pair of fenestra or simple eyes. Anterior part of head bears biting and chewing type of mouth parts.

        Mouth parts include a labrum (upper lip), a labium (lower lip), one pair of mandibles (jaws), one pair of first maxillae and a hypopharynx (tongue).

⇨span style='font:7.0pt "Times New Roman"'>        Antennae is made up of many segments called podomers. Thorax consists of three pair of walking legs and two pair of wings (forewings and hind wings).

        Alimentary canal is well developed with a mouth surrounded by mouth parts, pharynx, oesophagus, crop, gizzard, midgut, hindgut and anus.

        The blood vascular system is of open type. Blood vessels are poorly developed and open into space (haemocoel). Visceral organs located in the haemocoel are bathed in blood (haemolymph). It is colourless plasma with haemocytes.

        The respiratory system consists of a network of trachea that opens through 10 pairs of small holes called spiracles present on the lateral side of the body.

        At the junction of midgut and hindgut another ring is present which consist of 100-150 yellow coloured thin filamentous Malpighian tubules. They help in removal of excretory products from haemolymph.

        Nervous system consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side.

        Cockroach Is a dioecious animal. Its reproductive organs are present in the abdomen region (consisting of 10 segments).

        Testes are paired and present in 4-6th segment. Ovaries are present in 2-6th segment.

        Fertilisation is internal and development occurs inside ootheca egg case.

3.   Frog

        Rana tigrina (Indian bull frog) is a cold-blooded animal, which is found in or near freshwater.

        Body of frog consists of a head and a trunk without neck and tail.

        Head bears bulged eyes covered by a nictitating membrane that protect them while in water.

        On either sides of eyes a membranous tympanum (ear) is present, which receives sound signals as external ears are absent. Frog have smooth, moist and scaleless skin consisting of mucous and poison glands.

        The alimentary canal consists of buccal cavity, pharynx, oesophagus, stomach, intestine and rectum, which opens into cloaca.

        The buccal cavity bears teeth and a tongue. Stomach secretes gastric juices containing HCl and proteolytic enzymes. The main digestive glands are liver and pancreas.

        Respiration in frog can be cutaneous, buccopharyngeal and pulmonary types. The exchange of gases occurs by diffusion in all three modes.

        Circulatory system in frog is of closed type. It comprises of heart, blood vessels and blood (containing haemoglobin). There is an incomplete double circulation in frog because of the presence of the presence of three chambered heart (two atria + one ventricle).

         There are two accessory chambers in the frog’s heart, a tubular truncus arteriosus (on ventral side) and a triangular sinus venosus (on dorsal side).

        Both hepatic and renal portal systems are present in frog. RBCs in frog are nucleated and contain haemoglobin.

        Frog is ureotelic, i.e. secrete urea. Its excretory system consists of a pair of kidney, a pair of ureters, a urinary bladder and cloaca. The ureters are also called urinogenital ducts in male frog, since they pass out sperms as well as urine.

        Nervous system is well defined in frog is divided in CNS (brain and spinal cord), PNS (cranial and spinal nerves) and ANS (sympathetic and parasympathetic). Brain is enclosed in a bony structure called brain box. Ten pairs of cranial nerves arise from the brain.

        Reproductive system in frog is well organised. Testes and ovaries are found attached to kidneys. A female lays 2500-3000 ova at a time.

        Fertilisation is external (in water) and development is indirect through metamorphosis of tadpole larva into a adult frog.

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