Structural Organisation in Specific Animals (Examples)
Organ and Organ System
In complex animals, cells are organised into tissues, tissues are organised into organs, and organs are organised into organ systems. This hierarchical organisation allows for the division of labour, where different groups of cells, tissues, and organs perform specific functions, increasing the efficiency of the organism.
Organ Level of Organisation
When two or more types of tissues combine to perform a specific function, they form an organ.
Example: The stomach is an organ made up of epithelial tissue (lining), muscular tissue (for churning), connective tissue (for support), and nervous tissue (for control). All these tissues work together to perform the function of digestion.
Organ System Level of Organisation
When several organs cooperate to perform a major physiological function, they constitute an organ system.
Example: The digestive system includes the mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas, etc. All these organs work together to digest food and absorb nutrients.
Different organ systems in complex animals include:
- Digestive system
- Respiratory system
- Circulatory system
- Excretory system
- Nervous system
- Endocrine system
- Skeletal system
- Muscular system
- Reproductive system
- Integumentary system
Understanding the structural organisation at the organ and organ system levels provides insight into how complex animals perform their various life processes.
Earthworm
Earthworms are terrestrial invertebrates belonging to the phylum Annelida. They exhibit a segmented body structure and occupy diverse habitats, typically moist soil. They play a significant role in soil health as 'friends of farmers' by burrowing and increasing soil aeration and fertility.
Morphology
Earthworms commonly found in India are Pheretima posthuma and Lumbricus terrestris (though Lumbricus is more common in other regions, Pheretima is often used as the representative Indian earthworm). We will focus on the general morphology of Pheretima.
External Features:
- Body Shape: Long, cylindrical, and tapering at both ends.
- Body Colour: Dark brown due to the presence of a pigment called porphyrin in the body wall.
- Segmentation: The body is distinctly divided into numerous segments or metameres. The segments are similar externally and internally. In Pheretima, the body consists of about 100-120 segments.
- Dorsal Surface: Marked by a dark median line running along the length of the body (dorsal blood vessel visible through the skin).
- Ventral Surface: Characterised by the presence of genital openings.
Body Divisions:
The body can be broadly divided into three regions based on the presence of the clitellum:
- Preclitellar region: Includes segments 1 to 13.
- Clitellar region: A prominent dark band or girdle covering segments 14, 15, and 16 in a mature earthworm. It is formed by glandular tissue and is involved in the formation of the cocoon during reproduction.
- Postclitellar region: Includes segments from 17 to the posterior end.
Body Openings:
- Mouth: Located at the anterior end, on the ventral side of the first segment (peristomium).
- Anus: A terminal opening located at the posterior end.
- Genital Openings:
- Male genital pores: A pair of prominent openings on the ventral side of the 18th segment.
- Female genital pore: A single opening on the mid-ventral line of the 14th segment.
- Genital papillae: A pair of raised structures on the ventral side of the 17th and 19th segments in Pheretima. They help in holding the bodies during copulation.
- Spermathecal apertures: Four pairs of small openings located ventrolaterally on the intersegmental grooves of segments 5/6, 6/7, 7/8, and 8/9. They receive sperm during copulation.
- Nephridiopores: Numerous minute pores present on the body surface through which excretory waste is released. They are scattered over the body surface in Pheretima (integumentary nephridia).
- Dorsal pores: Small pores located on the dorsal side, usually in the intersegmental grooves from the 12th segment onwards. They secrete coelomic fluid, which keeps the body moist and helps in locomotion.
Prostomium and Peristomium:
- Peristomium: The first body segment, which surrounds the mouth.
- Prostomium: A fleshy lobe that projects from the dorsal side of the peristomium. It is not a segment but serves as a sensory organ (sensing light and touch) and helps in forcing open cracks in the soil during burrowing.
Setae:
- Except for the first, last, and clitellar segments, each segment bears rows of S-shaped chitinous structures called setae.
- Setae are embedded in the body wall and can be extended or withdrawn.
- Function: Aid in locomotion by providing grip on the substrate.
*(Image shows the anterior part of an earthworm highlighting prostomium, peristomium, segments, clitellum, and location of some external openings)*
Anatomy
Body Wall:
The body wall is covered externally by a thin non-cellular cuticle. Below the cuticle is the epidermis (single layer of columnar epithelial cells), followed by two layers of muscle: circular and longitudinal. The innermost layer is the coelomic epithelium.
Coelom:
The body cavity is a true coelom, which is large and filled with coelomic fluid. The coelom is segmentally divided by septa, allowing independent movement of segments.
Digestive System:
The digestive system is a complete tube, running from the mouth to the anus. It is a straight tube located in the body cavity.
Sequence of organs:
- Mouth: (Segment 1) - opening into buccal cavity.
- Buccal cavity: (Segments 1-3) - short tube.
- Pharynx: (Segments 4-5) - Muscular, helps in swallowing. Has pharyngeal glands that secrete mucus.
- Oesophagus: (Segments 6-7) - Narrow tube.
- Gizzard: (Segments 8-9) - Thick-walled, muscular organ for grinding food particles (like soil, decaying leaves).
- Stomach: (Segments 9-14) - Contains calciferous glands which neutralise humic acid present in humus.
- Intestine: (Segments 15 to the last segment except anus) - Long tube.
- From segment 27 onwards, a dorsal fold called the typhlosole projects into the lumen of the intestine.
- Typhlosole increases the effective surface area for absorption of digested food.
- Anus: Terminal opening for the exit of undigested waste (castings).
Earthworms feed on decaying organic matter mixed with soil. Digestion is both extracellular and intracellular. Absorbed nutrients are distributed by the blood.
*(Image shows a longitudinal view of the anterior part of earthworm's digestive tract: mouth, buccal cavity, pharynx, oesophagus, gizzard, stomach, intestine with typhlosole)*
Circulatory System:
Earthworms have a closed circulatory system. Blood flows in a network of vessels, not freely in the coelom.
- Consists of blood vessels, capillaries, and heart-like structures called 'hearts'.
- There are five pairs of lateral hearts (two pairs of anterior lateral hearts in segments 7 and 9, and three pairs of posterior lateral hearts in segments 12 and 13) in Pheretima.
- Blood contains haemoglobin dissolved in the plasma (not in RBCs), giving it a red colour.
- Blood circulates from the anterior to the posterior direction in the dorsal vessel and from the posterior to anterior direction in the ventral vessel.
*(Image shows major dorsal, ventral, and lateral vessels and lateral hearts in segments 7, 9, 12, 13)*
Respiratory System:
Earthworms do not have specialised respiratory organs (like lungs or gills).
Respiration occurs directly through the moist skin. Gaseous exchange (oxygen intake and carbon dioxide release) happens by diffusion across the body surface. The skin must remain moist for gas exchange, which is why earthworms are found in moist soil.
Excretory System:
The excretory organs are segmentally arranged coiled tubes called nephridia. Nephridia are involved in the removal of nitrogenous waste (primarily urea) and osmoregulation.
In Pheretima, there are three types of nephridia:
- Septal nephridia: Present on both sides of the intersegmental septa of segments 15 to the last segment. They open into the intestine (entero-nephric).
- Pharyngeal nephridia: Present in three pairs of tufts, one pair each in segments 4, 5, and 6. They also open into the pharynx (entero-nephric).
- Integumentary nephridia: Attached to the inner lining of the body wall from segment 3 to the last segment. They open to the outside through nephridiopores (ecto-nephric). These are most numerous.
A nephridium starts as a funnel (nephrostome) which collects excess fluid from the coelom. The fluid passes through a coiled tubule where waste products are removed and useful substances are reabsorbed. Waste is then excreted.
*(Image shows diagrammatic representation of septal, pharyngeal, and integumentary nephridia and their locations/openings)*
Nervous System:
The nervous system is organised into a central nervous system, peripheral nervous system, and sympathetic nervous system.
- Central Nervous System: Consists of the nerve ring (supra-oesophageal ganglia or 'brain' above the pharynx, sub-oesophageal ganglia below the pharynx, and circum-oesophageal connectives connecting them) and the ventral nerve cord.
- The ventral nerve cord is a solid, double nerve cord located mid-ventrally, running along the length of the body up to the last segment. It bears segmentally arranged ganglia (swelling of nerve tissue).
Sensory structures are simple. They have light-sensitive receptors (ocelli) located on the dorsal surface of the anterior end, chemoreceptors (for taste and smell), and tactile receptors.
*(Image shows anterior part highlighting supra-oesophageal ganglia, sub-oesophageal ganglia, circum-oesophageal connectives, and the ventral nerve cord with segmentally arranged ganglia)*
Reproductive System:
Earthworms are hermaphrodite (bisexual), meaning both male and female reproductive organs are present in the same individual.
- Male reproductive system:
- Two pairs of testes in segments 10 and 11.
- Seminal vesicles (large lobes covering testes) in segments 11 and 12, where sperm mature.
- Vasa deferentia (sperm ducts) run from each testis, fuse to form a single duct on each side, which opens out through the paired male genital pores on segment 18.
- Accessory glands (prostate glands) in segments 17 and 19, secretions help in sperm transfer.
- Female reproductive system:
- A pair of ovaries in the 12th segment, attached to the intersegmental septum between 12 and 13.
- Oviducts from segment 13 open into the female genital pore on segment 14.
- Four pairs of spermathecae in segments 6, 7, 8, and 9. They store sperm received during copulation.
Cross-fertilisation occurs during copulation. Two earthworms exchange packets of sperm (spermatophores). The clitellum secretes a cocoon, into which eggs, sperm (from spermathecae), and nutritive fluid are deposited. Fertilisation and embryonic development occur within the cocoon. Development is direct (no larval stage).
*(Image shows a diagrammatic representation of both male and female reproductive organs within the anterior segments)*
Cockroach
Cockroaches are nocturnal omnivores belonging to the phylum Arthropoda, class Insecta. They are common household pests found in warm, damp places. The species typically studied is Periplaneta americana (American cockroach).
Morphology
External Features:
- Body Shape: Elongated, dorso-ventrally flattened.
- Body Colour: Reddish-brown or dark brown.
- Body Divisions: The body is distinctly divided into three regions:
- Head: Triangular, located anteriorly at right angles to the main body axis. Highly mobile due to a flexible neck.
- Thorax: Consists of three segments: prothorax, mesothorax, and metathorax. Bears legs and wings.
- Abdomen: Consists of 10 segments.
- Exoskeleton: The entire body is covered by a hard, chitinous exoskeleton, providing protection and support. Each segment of the exoskeleton is made of hardened plates called sclerites. Sclerites are joined by thin, flexible articular membranes.
Head:
- Formed by the fusion of six segments.
- Bears a pair of filamentous antennae anteriorly, arising from socket-like depressions. Antennae have sensory receptors.
- Bears a pair of compound eyes.
- Has mouthparts directed downwards, adapted for biting and chewing. Mouthparts consist of a labrum (upper lip), a pair of mandibles, a pair of maxillae, a labium (lower lip), and a hypopharynx (tongue).
Thorax:
- Prothorax: The first thoracic segment. Has a large dorsal shield called the pronotum, which covers part of the head.
- Mesothorax: The second thoracic segment.
- Metathorax: The third thoracic segment.
- Legs: Each thoracic segment bears a pair of walking legs (total 3 pairs). Legs are jointed and adapted for running.
- Wings: Cockroaches have two pairs of wings.
- Forewings (Tegmina): Arise from the mesothorax. They are tough, opaque, and leathery, covering the hindwings at rest. They provide protection but are not used for flight.
- Hindwings: Arise from the metathorax. They are thin, membranous, and used for flight (in species that fly).
Abdomen:
- Consists of 10 segments.
- In both sexes, the abdomen bears a pair of jointed filamentous structures called anal cerci, arising from the 10th segment. Anal cerci are sensory.
- In males only, the 9th abdominal segment bears a pair of short, unjointed, pointed processes called anal styles. These are absent in females. The presence of anal styles is a distinguishing feature for sexing cockroaches.
- The abdomen bears genital openings at the posterior end.
*(Image shows dorsal view of a cockroach highlighting body divisions, antennae, forewings (tegmina), hindwings, legs, and anal cerci. May also include a separate diagram showing the posterior abdomen of male with anal styles)*
Anatomy
Digestive System:
The digestive system is a complete tube, well-developed, and divided into three parts: foregut, midgut, and hindgut.
- Foregut:
- Mouth $\rightarrow$ Pharynx $\rightarrow$ Oesophagus (narrow tube) $\rightarrow$ Crop (sac-like structure for food storage) $\rightarrow$ Gizzard (Proventriculus) (inner lining has six chitinous plates called teeth, for grinding food).
- Midgut (Mesenteron):
- A short, narrow tube. At the junction of the crop and midgut, there are 6-8 finger-like projections called hepatic caeca or gastric caeca. They secrete digestive enzymes.
- The junction of the midgut and hindgut is marked by a ring of 100-150 fine yellow filamentous tubules called Malpighian tubules (excretory organs).
- Hindgut:
- Broader than the midgut. Divided into ileum $\rightarrow$ colon $\rightarrow$ rectum.
- The rectum opens outside through the anus. The rectum absorbs water from undigested food.
Food is digested by enzymes from hepatic caeca and midgut and absorbed in the midgut.
*(Image shows the digestive tract: mouth, pharynx, oesophagus, crop, gizzard, hepatic caeca, midgut, Malpighian tubules at junction, hindgut (ileum, colon, rectum), anus)*
Circulatory System:
Cockroaches have an open circulatory system. Blood (haemolymph) flows freely in the body cavity (haemocoel), bathing the organs directly.
- The blood vessels are poorly developed.
- The 'heart' is an elongated, muscular tube located mid-dorsally in the thorax and abdomen. It has 13 segmentally arranged chambers.
- Blood enters the heart chambers through paired lateral openings called ostia.
- Blood is pumped anteriorly and then flows out into the haemocoel.
- The blood (haemolymph) is colourless and does not contain respiratory pigments (like haemoglobin). It consists of plasma and haemocytes.
- Function: Transports nutrients, hormones, and waste products, but not oxygen.
*(Image shows the dorsal heart/vessel with ostia, and the surrounding haemocoel filling the body cavity)*
Respiratory System:
Respiration occurs through a network of interconnected tubes called the tracheal system.
- Consists of spiracles, tracheae, and tracheoles.
- Spiracles: 10 pairs of small openings on the lateral side of the body (2 pairs in thorax, 8 pairs in abdomen). These are external openings for the tracheal system.
- Tracheae: Tubes that branch from spiracles and extend throughout the body.
- Tracheoles: Finer branches of tracheae that reach directly to the cells and tissues.
Air enters through spiracles, travels through tracheae and tracheoles, and oxygen diffuses directly into the tissues. Carbon dioxide diffuses in the reverse direction and exits through spiracles.
Since oxygen is supplied directly to tissues via tracheoles, the blood does not need to transport oxygen.
*(Image shows spiracles on the body side connected to branching tracheal tubes that ramify through the body)*
Excretory System:
Excretion is performed by Malpighian tubules, which are located at the junction of the midgut and hindgut.
- Malpighian tubules are blind-ended tubules that absorb nitrogenous waste products (mainly uric acid) from the haemocoel.
- The waste is then transferred to the hindgut and excreted along with faeces.
- Cockroaches are uricotelic (excrete uric acid), which helps conserve water.
*(Image shows Malpighian tubules branching off from the digestive tract between midgut and hindgut)*
Nervous System:
The nervous system is distributed throughout the body. It consists of a series of segmentally arranged ganglia joined by paired longitudinal connectives. The brain is located in the head.
- Brain: Located supra-oesophageally in the head. It innervates the antennae and compound eyes.
- A nerve ring surrounds the pharynx.
- The ventral nerve cord is double, solid, and segmented, with ganglia in the thorax (3 pairs) and abdomen (6 pairs).
Since the nervous system is spread out, even if the head is cut off, a cockroach can still survive for some time.
Sensory organs include antennae, compound eyes, maxillae, labrum, and anal cerci.
Reproductive System:
Cockroaches are dioecious (sexes are separate). They show sexual dimorphism (males and females are morphologically distinct - males have anal styles).
- Male reproductive system:
- A pair of testes in segments 4-6 of the abdomen.
- Vasa deferentia (sperm ducts) arise from testes, join to form the ejaculatory duct which opens into the male genital pore.
- Accessory reproductive glands (mushroom gland in segments 6-7, phallic gland).
- External genitalia are represented by male gonapophysis or phallomeres (chitinous asymmetrical structures surrounding the male genital pore).
- Sperm are stored in seminal vesicles and are glued together in bundles called spermatophores.
- Female reproductive system:
- A pair of large ovaries in segments 2-6 of the abdomen. Each ovary consists of ovarioles (chains of developing eggs).
- Oviducts from each ovary join to form a common oviduct (vagina), which opens into the genital chamber.
- A pair of spermathecae are present in the 6th segment; they open into the genital chamber and store sperm received during copulation.
- Female external genitalia are represented by female gonapophyses.
Fertilisation is internal. During copulation, the male transfers spermatophores into the female's spermathecae. The female then forms a dark reddish-to-blackish brown capsule called an ootheca, which contains fertilised eggs (typically 14-16 eggs in Periplaneta). Oothecae are dropped or glued to a suitable surface. Development is paurometabolous (incomplete metamorphosis), involving nymphal stages that resemble the adult but are smaller and lack wings. The last nymphal stage moults to become an adult.
*(Image shows separate diagrams for male and female reproductive organs, including testes, vasa deferentia, ejaculatory duct, accessory glands, phallomeres (male); ovaries with ovarioles, oviducts, common oviduct/vagina, genital chamber, spermathecae (female))*
Frogs
Frogs are amphibians belonging to the phylum Chordata, class Amphibia. They are vertebrates adapted to live both on land and in freshwater habitats. The species commonly found in India is the Indian Bullfrog, Rana tigrina.
Morphology
External Features:
- Body Shape: Dorso-ventrally flattened.
- Body Colour: Dorsal side is typically olive green with irregular spots; ventral side is pale yellow. Colour can change for camouflage.
- Body Divisions: Head and Trunk. The neck and tail are absent in adult frogs.
- Skin: Moist and slippery due to mucus secretion. The skin is porous and serves as a respiratory organ (cutaneous respiration).
Head:
- Broad and short.
- Bears a pair of bulging eyes covered by a nictitating membrane (transparent third eyelid) which protects them in water.
- Just behind the eyes is a membranous tympanum (eardrum), which receives sound signals.
- The snout is the anterior pointed part.
- The mouth is a large opening at the anterior end.
- A pair of nostrils (external nares) are present above the mouth, connected to the bucco-pharyngeal cavity.
Trunk:
- The trunk is the main body part.
- Bears two pairs of limbs:
- Forelimbs: Shorter, with 4 digits (no claws). Used for support and cushioning during landing. In male frogs, the first digit of the forelimb bears a nuptial pad (a thickened patch on the inner side) which helps in holding the female during mating (amplexus).
- Hindlimbs: Longer and muscular, with 5 digits connected by webs. Adapted for swimming and jumping.
- The hind feet have webs between the digits, aiding in swimming. The forefeet lack webs.
- The posterior end of the trunk terminates in the cloacal aperture, a common opening for the digestive, urinary, and reproductive systems.
Sexual Dimorphism:
- Male frogs can be distinguished from females by the presence of vocal sacs (on the sides of the head) and a nuptial pad on the first digit of the forelimb.
- Vocal sacs help amplify the croaking sound produced by males to attract females during the breeding season.
*(Image shows dorsal and possibly ventral views of a frog, highlighting body divisions, limbs, webbed feet, eyes, tympanum, and showing or indicating vocal sac and nuptial pad in a male)*
Anatomy
Digestive System:
The digestive system is a complete tube.
Sequence of organs:
- Mouth $\rightarrow$ Buccal cavity (large, with a bilobed sticky tongue for catching insects) $\rightarrow$ Pharynx $\rightarrow$ Oesophagus (short tube) $\rightarrow$ Stomach (J-shaped) $\rightarrow$ Small intestine (coiled) $\rightarrow$ Large intestine $\rightarrow$ Cloaca $\rightarrow$ Cloacal aperture.
- The stomach and intestines are lined with glands that secrete digestive juices.
- Associated digestive glands are the liver (produces bile, stored in gall bladder) and pancreas (produces pancreatic juice).
Digestion occurs in the stomach and intestine. Bile (from liver) and pancreatic juice (from pancreas) are released into the small intestine via a common duct. Digested food is absorbed in the small intestine. Undigested waste passes into the large intestine and is then eliminated through the cloacal aperture.
*(Image shows the digestive tract of a frog including mouth, oesophagus, stomach, small intestine, large intestine, cloaca, and associated organs like liver, gall bladder, pancreas)*
Respiratory System:
Frogs respire in multiple ways:
- Cutaneous respiration: Respiration through the moist skin. This is the primary mode of respiration both on land and in water. Oxygen dissolves on the moist skin surface and diffuses into the blood; carbon dioxide diffuses out.
- Pulmonary respiration: Respiration using lungs. Occurs when the frog is on land. Air enters through nostrils, passes through buccal cavity, glottis, larynx, trachea (short), and into the lungs. Lungs are a pair of simple sac-like structures.
- Buccopharyngeal respiration: Respiration through the lining of the buccal cavity and pharynx. Occurs when the frog is on land. The floor of the buccal cavity is raised and lowered, causing gaseous exchange across the thin, moist lining.
Gills are present only in the larval stage (tadpole).
Circulatory System:
Frogs have a closed circulatory system, but it is somewhat less efficient than in mammals due to the mixing of oxygenated and deoxygenated blood.
- The heart is three-chambered, consisting of two atria and one ventricle.
- Oxygenated blood from the lungs and skin enters the left atrium. Deoxygenated blood from the body enters the right atrium. Both atria open into the single ventricle.
- Some mixing of oxygenated and deoxygenated blood occurs in the ventricle, but a significant amount of oxygenated blood is pumped to the head and body, and deoxygenated blood to the lungs and skin.
- The circulatory system includes blood vessels (arteries, veins, capillaries) and blood (plasma and blood cells - RBCs with haemoglobin, WBCs, platelets). RBCs are oval and nucleated (unlike mammalian RBCs).
- Frogs have a portal system, including a renal portal system and a hepatic portal system.
*(Image shows a simplified diagram of the frog heart with two atria and one ventricle, and arrows indicating blood flow, showing mixing in the ventricle)*
Excretory System:
The excretory system consists of a pair of kidneys, ureters, cloaca, and urinary bladder.
- Kidneys are a pair of elongated, dark red, bean-shaped structures located on either side of the vertebral column in the posterior part of the body cavity. They are mesonephric kidneys.
- Kidneys contain structural and functional units called nephrons.
- Ureters: A pair of tubes that arise from the kidneys. In males, the ureters also serve as vasa deferentia, carrying both urine and sperm (urogenital duct). In females, ureters carry only urine.
- Ureters open into the cloaca.
- Urinary bladder: A bilobed sac located ventral to the rectum, opening into the cloaca. It stores urine temporarily.
Excretory waste is primarily urea (frogs are ureotelic in adults). Tadpoles are ammonotelic (excrete ammonia).
*(Image shows kidneys, ureters leading to cloaca, and urinary bladder)*
Nervous System:
The nervous system is well organised and includes the central nervous system (brain and spinal cord), peripheral nervous system (cranial nerves and spinal nerves), and sympathetic nervous system.
- Brain: Located in the skull. Consists of forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes), and hindbrain (cerebellum, medulla oblongata). The medulla oblongata connects to the spinal cord.
- Spinal cord: Extends from the medulla oblongata through the vertebral column.
Sensory organs: Eyes, ears (tympanum), taste buds (tongue), olfactory epithelium (nasal lining), and touch receptors (skin).
Reproductive System:
Frogs are dioecious (sexes are separate). Fertilisation is external.
- Male reproductive system:
- A pair of yellowish, oval testes are attached to the kidneys by mesentery.
- Vasa efferentia: 10-12 fine tubes arise from the testes and enter the kidneys, opening into Bidder's canal.
- Bidder's canal connects to the ureter (urogenital duct), which opens into the cloaca.
- There is no copulatory organ.
- Female reproductive system:
- A pair of ovaries are attached to the kidneys by mesentery. Ovaries produce ova (eggs).
- Oviducts: A pair of coiled tubes that arise from the anterior part of the body cavity (funnel-shaped opening) and open into the cloaca separately from the ureters. Oviducts are muscular and glandular; the glands secrete jelly-like coating around the eggs.
- There is no vagina.
During the breeding season (monsoon), males vocalize to attract females. Amplexus occurs (male clasps the female). As the female releases eggs into the water, the male releases sperm over them, leading to external fertilisation. Fertilised eggs develop into tadpoles (larva) with gills, which undergo metamorphosis to become adult frogs.
*(Image shows separate diagrams for male and female reproductive organs, including testes, vasa efferentia, Bidder's canal, kidney, ureter (male); ovaries, oviducts, cloaca (female))*
The examples of Earthworm, Cockroach, and Frog demonstrate increasing complexity in structural organisation, from segmented invertebrates with an open circulatory system and tracheal respiration (Cockroach) or closed circulation and cutaneous/nephridial excretion (Earthworm) to vertebrates with a closed circulatory system, well-developed lungs, and kidneys (Frog), showcasing the diversity of adaptations in the animal kingdom.