Structural Organisation in Animals

Introduction

Just as plants have specialised tissues and organs, animals are also organised into tissues, organs, and organ systems. This chapter covers the four basic types of animal tissues and provides a detailed study of two model organisms — the Cockroach ( · Periplaneta americana · ) and the Earthworm ( · Pheretima posthuma · ) and the Frog ( · Rana tigrina · ) — to illustrate organ-system level organisation.

Animal Tissues

1. Epithelial Tissue

Covers body surfaces, lines cavities and tubes. Cells tightly packed with little intercellular matrix. Functions: protection, secretion, absorption, excretion, sensation.

• Types (based on cell layers and shape):
• Simple squamous: flat cells; lining of blood vessels (endothelium), alveoli
• Simple cuboidal: cube-shaped; kidney tubules, salivary glands
• Simple columnar: tall; stomach lining, intestine; may have microvilli (brush border) or cilia
• Pseudostratified columnar: appears layered but is single layer; respiratory tract
• Stratified squamous: multiple layers; skin (keratinised) and oral cavity (non-keratinised) — protection
• Transitional epithelium: can stretch; urinary bladder

• Special epithelial types:
• Glandular epithelium: secretory; goblet cells produce mucus; glands (unicellular or multicellular)
• Sensory epithelium: neuroepithelium in sense organs

2. Connective Tissue

Most abundant tissue; cells scattered in an extracellular matrix (ground substance + fibres). Functions: binding, support, packing, transport, defence.

• Loose connective tissue (areolar): collagen and elastin fibres; fibroblasts, macrophages, mast cells; beneath skin and around organs
• Dense connective tissue: compactly arranged collagen fibres. · Dense regular · (tendons — muscle to bone; ligaments — bone to bone). · Dense irregular · (skin dermis)
• Cartilage: semi-solid matrix (chondrin); chondrocytes in lacunae; no blood vessels. · Hyaline · (ribs, nose, larynx — most common); · Elastic · (pinna, epiglottis); · Fibrous · (intervertebral discs)
• Bone: hard matrix (ossein + calcium phosphate); osteocytes in lacunae; Haversian canals with blood vessels
• Blood: fluid connective tissue; plasma (matrix) + formed elements (RBCs, WBCs, platelets)

3. Muscular Tissue

Contractile tissue; cells called muscle fibres (myocytes); contain actin and myosin (myofilaments).

• Skeletal (striated/voluntary): cross-striations due to regular arrangement of actin and myosin; multinucleate; voluntary control; fast, fatigable. Attached to bones.
• Cardiac muscle: involuntary striated; uninucleate; interconnected by intercalated discs (contain gap junctions for synchronised contraction); found only in heart; does not fatigue.
• Smooth (visceral/non-striated/involuntary): no cross-striations; spindle-shaped, uninucleate; slow, sustained contractions; walls of hollow organs (stomach, intestine, blood vessels, uterus).

4. Neural Tissue

Excitable tissue; conducts electrical impulses. Neurons (structural and functional unit) + Neuroglia (supporting cells — Schwann cells, astrocytes).

Neuron structure: Cell body (cyton/soma) with Nissl granules → Dendrites (receive impulses) → Axon (conducts impulse away from cell body) → Axon terminals (synaptic knobs). Covered by myelin sheath (in myelinated fibres); nodes of Ranvier between Schwann cells.

Morphology and Anatomy of Earthworm (Pheretima posthuma)

• External features:
• Elongated, cylindrical, brownish; body divided into 100–120 segments (metameres)
• Anterior end: mouth (prostomium — lobe over mouth), peristomium (1st segment)
• Clitellum: glandular, saddle-like; segments 14–16; produces cocoons
• Setae: S-shaped chitinous bristles; 8 per segment (except 1, 2, last, and clitellum); locomotion
• Dorsal pores: from 12/13 onwards; release coelomic fluid for moistening
• Genital pores: female (14th segment), male (18th segment), spermathecal (intersegmental grooves of 5/6, 6/7, 7/8, 8/9)

• Internal structure:
• Digestive system: mouth → buccal cavity (1–3) → pharynx (4–5) → oesophagus (6–7) → muscular gizzard (8–9) → stomach (9–14) → intestine (15 onwards). Intestine has typhlosole (dorsal fold increasing absorptive surface from segment 26 onwards)
• Circulatory: closed; blood red (haemoglobin dissolved in plasma). Dorsal vessel (blood flows anteriorly), ventral vessel (posteriorly). Lateral hearts in segments 7–11 (in 7,9,12,13 – 4 pairs of lateral hearts)
• Excretory: nephridia (three types: septal, integumentary, pharyngeal)
• Nervous: ventral nerve cord; cerebral ganglia (suprapharyngeal — brain); subpharyngeal ganglia; ganglion per segment
• Reproductive: hermaphrodite (monoecious); cross-fertilisation (not self-fertilisation)

Morphology and Anatomy of Cockroach (Periplaneta americana)

• External features:
• Yellowish-brown; 3 segments: head (6 fused segments), thorax (3 segments: pro-, meso-, meta-thorax), abdomen (10 segments in male; 7+genital pouch in female)
• Head: compound eyes, antennae, mouthparts (chewing — mandibles, maxillae, labium, labrum, hypopharynx)
• 3 pairs of jointed legs; 2 pairs of wings (tegmina — leathery forewings; membranous hindwings)
• Abdomen: 10 spiracles (2 on thorax, 8 on abdomen); male has anal cerci + anal styles; female has anal cerci only (no anal styles)

• Internal systems:
• Digestive: mouth → salivary glands → oesophagus → crop (food storage) → gizzard (grinding) → stomach (mesenteron, with 8 hepatic caecae for enzyme secretion) → ileum → colon → rectum → anus. Malpighian tubules (100–150, at stomach-intestine junction) for excretion.
• Circulatory: open; haemolymph (colourless); 13-chambered tubular heart (13 ostia); haemocoel
• Respiratory: network of tracheae and tracheoles; spiracles (10 pairs) regulated by sphincters; no blood pigment for gas transport
• Excretory: Malpighian tubules + fat body + uricose glands (uric acid — uricotelic)
• Nervous: supra-oesophageal ganglion (brain); sub-oesophageal ganglion; 3 thoracic + 6 abdominal ganglia; double ventral nerve cord
• Reproductive: dioecious. Male: testis (segment 4–6), vasa deferentia, accessory glands, phallomere (external genitalia). Female: ovarioles → oviduct → common oviduct → genital pouch. Oothecae (egg cases) produced by females.