Structural Organisation in Animals

Epithelial tissue covers body surfaces, lines body cavities, and forms glands. Cells are packed tightly with little intercellular matrix and rest on a basement membrane.

Types of epithelium:

• Squamous (pavement): flat cells; found in alveoli (gas exchange), skin surface, blood vessels (endothelium).
• Cuboidal: cube-shaped; found in kidney tubules, salivary gland ducts (secretion/absorption).
• Columnar: tall cells; found in stomach lining and intestine (absorption); if ciliated, moves material (e.g., fallopian tubes, trachea).
• Compound (stratified) epithelium: multiple layers; skin epidermis — primarily protective.
• Glandular epithelium: secretory; endocrine (ductless, hormones into blood) or exocrine (with ducts).

Connective tissue has cells scattered in a matrix. It binds, supports, insulates, and transports.

• Loose connective tissue: areolar (packs organs) and adipose (stores fat, insulates body).
• Dense connective tissue: tendons (connect muscle to bone — collagen fibres) and ligaments (connect bone to bone — elastin fibres).
• Specialised: cartilage (chondrocytes in lacunae; no blood supply), bone (osteocytes; Haversian system; rigid), and fluid connective tissue: blood (plasma + cells) and lymph.

Muscle tissue:

• Skeletal (striated, voluntary): attached to bones; shows cross-striations; multi-nucleate.
• Smooth (non-striated, involuntary): visceral organs, blood vessels; spindle-shaped cells; single nucleus.
• Cardiac: only in heart; striated but involuntary; branched cells with intercalated discs.

Nervous tissue: neurons (excitable, transmit impulses) + neuroglia (supporting cells).

Animal body plans are compared using five criteria: level of organisation, symmetry, germ layers, body cavity (coelom), and segmentation.

Levels of organisation:

• Cellular level: Porifera (sponges).
• Tissue level: Cnidaria (Hydra, jellyfish).
• Organ level: Platyhelminthes (flatworms).
• Organ-system level: most higher phyla.

Symmetry: Radial — Porifera, Cnidaria, adult Echinodermata; Bilateral — Platyhelminthes onwards.

Germ layers: Diploblastic (ectoderm + endoderm only) — Porifera, Cnidaria; Triploblastic (+ mesoderm) — all higher phyla.

Coelom: Acoelomate — Platyhelminthes; Pseudocoelomate — Aschelminthes (Nematoda); Coelomate (true coelom) — Annelida onwards.

Cockroach (Periplaneta americana): terrestrial arthropod with body divided into head, thorax (3 segments), and abdomen (10 segments).

• One pair of antennae; biting-chewing mouthparts.
• Three pairs of walking legs; two pairs of wings (mesothorax: tegmina; metathorax: membranous wings).
• Respiration: tracheal system via spiracles (10 pairs: 2 thoracic + 8 abdominal).
• Excretion: Malpighian tubules — uricotelic.
• Circulatory system: open; haemolymph; dorsal tubular heart (13 chambers).
• Dioecious; female forms ootheca; male has anal styles.

Earthworm (Pheretima posthuma): segmented annelid; metamerism; locomotion by setae and muscles.

• Clitellum at segments 14-16 in mature worm.
• Complete alimentary canal: buccal cavity → pharynx → oesophagus → crop → gizzard → intestine (typhlosole increases absorptive area) → anus.
• Circulatory system: closed; 4 pairs of aortic arches (hearts) in segments 7-11.
• Excretion: nephridia; gas exchange through moist skin.
• Hermaphrodite but cross-fertilizes; cocoons formed.

Frog (Rana tigrina) is an amphibian adapted for both aquatic and terrestrial life.

External features: moist, glandular, smooth skin; no neck or tail; strong webbed hind limbs; tympanum (ear drum) visible behind each eye; nictitating membrane (transparent third eyelid).

Respiration: Triple mode — cutaneous (through skin, mainly in water), buccopharyngeal (moist lining of mouth), and pulmonary (lungs, on land).

Circulatory system: three-chambered heart (two atria + one ventricle); sinus venosus receives deoxygenated blood; conus arteriosus distributes blood.

Digestive system: alimentary canal opens via cloaca. Cloacal opening = common exit for digestive, excretory, and reproductive products.

Reproduction: dioecious; external fertilization in water; larva = tadpole (aquatic); metamorphosis gives adult frog. Males have vocal sacs (produce croaking sound).