In this chapter: epithelial, connective, muscular and neural tissue; cell junctions; cockroach (Periplaneta americana) and frog (Rana): morphology and organ systems. Earthworm is no longer in the current syllabus.Animal tissues
1. Epithelial tissue
Epithelium covers free surfaces. Its cells are compactly packed, with little intercellular matrix.
| Type | Structure | Location |
|---|---|---|
| Simple squamous | Single thin layer of flattened cells with irregular boundaries | Walls of blood vessels, air sacs of lungs; diffusion boundary |
| Simple cuboidal | Single layer of cube-like cells | Ducts of glands, tubular parts of nephrons (proximal tubule has microvilli); secretion and absorption |
| Simple columnar | Tall, slender cells; nuclei at the base; free surface may have microvilli | Lining of stomach and intestine; secretion and absorption |
| Ciliated | Columnar or cuboidal cells bearing cilia; move particles or mucus in one direction | Bronchioles, fallopian tubes |
| Compound (stratified) | More than one layer; mainly protective, limited role in secretion and absorption | Dry surface of skin, moist surface of buccal cavity and pharynx, inner lining of ducts of salivary glands and pancreatic ducts |
Glandular epithelium: unicellular (goblet cells of the alimentary canal) or multicellular (salivary gland). Exocrine glands secrete through ducts: mucus, saliva, earwax, oil, milk, digestive enzymes. Endocrine glands have no ducts; their hormones go directly into the fluid bathing the gland.
Cell junctions: tight junctions stop substances leaking across a tissue; adhering junctions cement neighbouring cells together; gap junctions connect the cytoplasm of adjoining cells for rapid transfer of ions, small molecules and sometimes big molecules.
2. Connective tissue
Connective tissue is the most abundant and widely distributed tissue; it links and supports other tissues. In all connective tissues except blood, cells secrete fibres of collagen or elastin and a matrix (ground substance) of modified polysaccharides.
| Type | Key features | Location / examples |
|---|---|---|
| Loose: areolar | Fibroblasts, macrophages, mast cells in a semi-fluid ground substance; framework for epithelium | Beneath the skin |
| Loose: adipose | Specialised to store fats | Beneath the skin |
| Dense regular | Collagen fibres in rows between parallel bundles of fibres | Tendons (muscle to bone), ligaments (bone to bone) |
| Dense irregular | Fibroblasts and fibres oriented differently | Skin |
| Cartilage | Solid, pliable matrix resisting compression; cells (chondrocytes) in small cavities | Tip of nose, outer ear, between adjacent bones of the vertebral column, limbs and hands in adults |
| Bone | Hard, non-pliable matrix rich in calcium salts and collagen; osteocytes in lacunae; bone marrow produces blood cells | Main tissue of the skeleton |
| Blood | Fluid connective tissue: plasma, RBCs, WBCs, platelets | Circulation |
3. Muscle tissue
| Skeletal | Smooth | Cardiac | |
|---|---|---|---|
| Appearance | Striated, fibres in parallel bundles | Non-striated, fusiform (tapering at both ends) | Striated, branching |
| Control | Voluntary | Involuntary | Involuntary |
| Location | Attached to bones (e.g. biceps) | Walls of blood vessels, stomach, intestine | Heart only |
| Special | – | Cells held by cell junctions | Intercalated discs fuse plasma membranes so cells contract as a unit |
4. Neural tissue
Made of neurons (excitable cells) and neuroglia, which protect and support neurons and make up more than half the volume of neural tissue in our body.
Cockroach (Periplaneta americana)
Morphology
- Brown or black, 34 to 53 mm long. Body segmented into head, thorax and abdomen, covered by hard chitinous plates (sclerites): dorsal tergites, ventral sternites, joined by a thin, flexible arthrodial membrane.
- Head: triangular, formed by the fusion of six segments, very mobile due to a flexible neck. A pair of compound eyes and a pair of thread-like antennae (sensory receptors).
- Mouthparts (biting and chewing): labrum (upper lip), a pair of mandibles, a pair of maxillae and a labium (lower lip). The median flexible lobe within the cavity is the hypopharynx (tongue).
- Thorax: prothorax, mesothorax and metathorax, each with a pair of walking legs. Forewings (mesothoracic), called tegmina, are opaque, dark and leathery, and cover the hindwings. Hindwings (metathoracic) are transparent, membranous and used in flight.
- Abdomen: 10 segments. In females, the 7th sternum is boat-shaped and, with the 8th and 9th sterna, forms a brood or genital pouch. In males, the genital chamber lies at the hind end. Males have a pair of short, thread-like anal styles, absent in females. Both sexes have a pair of jointed, filamentous anal cerci on the 10th segment.
Organ systems
| System | Key points |
|---|---|
| Digestive | Foregut: mouth, pharynx, oesophagus, crop (stores food), gizzard (proventriculus: thick circular muscles and six chitinous teeth to grind food). Midgut: a ring of 6 to 8 blind hepatic (gastric) caecae at the junction of foregut and midgut, secreting digestive juice. Hindgut: ileum, colon, rectum, anus. |
| Excretory | Malpighian tubules (100 to 150) at the junction of midgut and hindgut absorb nitrogenous wastes and convert them to uric acid (uricotelic). Fat body, nephrocytes and urecose glands also help. |
| Circulatory | Open type; blood vessels open into the haemocoel. Haemolymph: colourless plasma with haemocytes. Heart: an elongated muscular tube along the mid-dorsal line of thorax and abdomen, with 13 chambers having ostia on either side. |
| Respiratory | A network of tracheae opening through 10 pairs of spiracles on the sides of the body. Spiracles are regulated by sphincters; gas exchange at the tracheoles by diffusion. |
| Nervous | Fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side: 3 ganglia in the thorax, 6 in the abdomen. Only a bit of the nervous system is in the head, so a headless cockroach can live for up to a week. The supra-oesophageal ganglion acts as the brain. |
| Sense organs | Antennae, eyes, maxillary palps, labial palps, anal cerci. Each compound eye has about 2000 hexagonal ommatidia, giving mosaic vision: more sensitivity, less resolution, useful at night. |
Reproduction
- Dioecious. Male: a pair of testes in the 4th to 6th abdominal segments; vas deferens, ejaculatory duct, male gonopore. A characteristic mushroom-shaped gland (accessory gland) in the 6th and 7th segments. Sperms are stored in seminal vesicles and glued together as spermatophores. External genitalia: male gonapophyses (phallomeres).
- Female: two large ovaries in the 2nd to 6th abdominal segments, each with eight ovarioles. A pair of spermathecae in the 6th segment opens into the genital chamber.
- Fertilised eggs are enclosed in capsules called oothecae, dark reddish to blackish brown, about 8 mm long. A female produces 9 to 10 oothecae on average, each with 14 to 16 eggs.
- Development is paurometabolous (through nymphs). The nymph moults about 13 times; the next-to-last nymphal stage has wing pads, but only adults have wings.
- Some species are pests that contaminate food and can spread bacterial diseases.
Frog (Rana)
Morphology and habits
- Cold-blooded (poikilotherm): body temperature varies with the environment. Can change colour to hide from enemies (camouflage). Undergoes summer sleep (aestivation) and winter sleep (hibernation).
- Skin smooth and slippery due to mucus, always moist. Dorsal side olive green with dark irregular spots; ventral side pale yellow. The frog never drinks water; it absorbs water through the skin.
- Body of head and trunk; no neck or tail. Eyes bulged, covered by a nictitating membrane. A tympanum (ear) on each side receives sound. Forelimbs with 4 digits; hindlimbs larger with 5 digits and webbed feet.
- Sexual dimorphism: males have vocal sacs and a copulatory pad on the first digit of the forelimbs.
Organ systems
| System | Key points |
|---|---|
| Digestive | Short alimentary canal (carnivore). Bilobed tongue catches prey. Stomach digests with HCl and gastric juice; partially digested food (chyme) goes to the duodenum, which receives bile (from the liver, stored in the gall bladder) and pancreatic juice through a common bile duct. Absorption by villi and microvilli of the intestine; undigested food to the rectum and out through the cloaca. |
| Respiratory | In water: through the skin (cutaneous respiration). On land: buccal cavity, skin and lungs (pulmonary respiration). During aestivation and hibernation, only through the skin. |
| Circulatory | Closed, with a lymphatic system. Heart three-chambered (two atria, one ventricle), covered by pericardium. A triangular sinus venosus joins the right atrium; the ventricle opens into the conus arteriosus. Special venous connections: hepatic portal and renal portal systems. RBCs are nucleated. |
| Excretory | A pair of dark red, bean-shaped kidneys made of nephrons (uriniferous tubules). In males, ureters act as urinogenital ducts; in females, ureters and oviducts open separately into the cloaca. The frog excretes urea (ureotelic). |
| Nervous | CNS, PNS and ANS. Ten pairs of cranial nerves. Brain in a bony cranium: forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (a pair of optic lobes), hindbrain (cerebellum, medulla oblongata). The medulla passes through the foramen magnum into the spinal cord. |
| Endocrine | Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. |
| Sense organs | Touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (simple eyes), hearing and balance (tympanum with internal ear; no external ear). |
Reproduction
- Male: a pair of yellowish, ovoid testes attached to the upper part of the kidneys by a double fold of peritoneum (mesorchium). 10 to 12 vasa efferentia enter the kidneys and open into Bidder's canal, which connects to the urinogenital duct and cloaca.
- Female: ovaries near the kidneys with no functional connection to them; oviducts open separately into the cloaca. A mature female can lay 2500 to 3000 ova at a time.
- Fertilisation is external, in water. Development includes a larval stage, the tadpole, which undergoes metamorphosis.
- Frogs eat insects and protect crops, and are an important link in food chains.
Common traps: cockroach = open circulation, 13-chambered heart, uric acid; frog = closed circulation, three-chambered heart, urea. Anal styles are only in male cockroaches; anal cerci are in both sexes. Neuroglia are more than half of neural tissue volume.NEET focus
- Epithelium types with locations; cell junctions; tendons vs ligaments; intercalated discs.
- Cockroach numbers: 6 head segments, 13 heart chambers, 10 pairs of spiracles, 100 to 150 Malpighian tubules, 3 + 6 ganglia, about 2000 ommatidia, 8 ovarioles, 9 to 10 oothecae with 14 to 16 eggs, 13 moults.
- Frog: cutaneous respiration, portal systems, 10 pairs of cranial nerves, Bidder's canal, sexual dimorphism.
Practice questions
Which epithelium lines the fallopian tubes and bronchioles?
- Squamous
- Ciliated
- Compound
- Cuboidal
Show answer
B. Cilia move particles or mucus in one direction.
Ligaments connect:
- Muscle to bone
- Bone to bone
- Muscle to muscle
- Skin to muscle
Show answer
B. Tendons connect muscle to bone.
The heart of a cockroach has:
- 3 chambers
- 10 chambers
- 13 chambers
- 4 chambers
Show answer
C.
Mosaic vision in the cockroach is due to:
- Simple eyes
- Ommatidia of compound eyes
- Antennae
- Anal cerci
Show answer
B. About 2000 hexagonal ommatidia per eye.
Which structure is present only in male cockroaches?
- Anal cerci
- Anal styles
- Spermatheca
- Ovarioles
Show answer
B. Spermathecae and ovarioles are female structures; cerci are in both.
Bidder's canal in the frog is associated with the:
- Female reproductive system
- Male reproductive system
- Digestive system
- Nervous system
Show answer
B. Vasa efferentia open into it within the kidney.
Frogs are:
- Ammonotelic
- Ureotelic
- Uricotelic
- Aminotelic
Show answer
B. Adult frogs excrete urea.