🦠 Structural Organisation in Animals Study Notes

7.1 Tissues & Epithelial Tissue

A tissue is a group of similar cells plus intercellular substances that perform a specific function. All complex animals are built from just four basic tissues; these form organs, which combine into organ systems — a division of labour.

Epithelial tissue (epithelium)

Has a free surface facing a body fluid or the outside; provides a covering or lining. Cells are compact with little intercellular matrix. Two types: simple (single layer; lines cavities, ducts, tubes) and compound (two+ layers; protective, e.g., skin).

Glandular epithelium

Columnar/cuboidal cells specialised for secretion. Unicellular (goblet cells of alimentary canal) or multicellular (salivary gland).

• Exocrine — secrete through ducts: mucus, saliva, earwax, oil, milk, digestive enzymes.
• Endocrine — no ducts; secrete hormones directly into the fluid bathing the gland.

Compound epithelium & cell junctions

Compound epithelium is multilayered with a limited role in secretion/absorption; its main job is protection (dry skin, moist buccal cavity, pharynx, salivary/pancreatic ducts).

7.2 Connective, Muscle & Neural Tissues

Connective tissue

The most abundant and widely distributed tissue; it links and supports other tissues/organs. Except blood, the cells secrete fibres of collagen or elastin (strength, elasticity, flexibility) and a matrix of modified polysaccharides (ground substance).

• Loose — cells & fibres loosely arranged in a semi-fluid matrix.
  • Areolar — beneath skin; a support framework; contains fibroblasts, macrophages, mast cells.
  • Adipose — beneath skin; stores fat.
• Dense — fibres & fibroblasts compactly packed.
  • Dense regular — collagen in parallel bundles: tendons (muscle→bone) and ligaments (bone→bone).
  • Dense irregular — fibroblasts + collagen oriented differently; in the skin.
• Specialised — cartilage, bone, blood.

Key pair to remember: Tendon joins muscle to bone; ligament joins bone to bone.

Muscle tissue

Long cylindrical fibres in parallel arrays, made of myofibrils; they contract and relax to cause movement.

Neural tissue

Exerts the greatest control over the body's responsiveness. Neurons are the excitable units; neuroglia protect and support neurons and make up more than half the volume of neural tissue. A stimulated neuron sends an electrical disturbance along its membrane that, at the endings, stimulates or inhibits adjacent cells.

7.2 Organ & organ system

Tissues → organs → organ systems. The heart contains all four tissue types. Body design grows more complex up the animal kingdom. Note the two viewpoints: morphology = external features; anatomy = internal organ structure.

7.3 Earthworm — Pheretima

Pheretima (and Lumbricus) — a reddish-brown terrestrial invertebrate of upper moist soil; a 'friend of the farmer' that makes soil porous through its burrows.

Morphology

• Long cylindrical body with ~100–120 segments (metameres), all similar.
• Dorsal side: a dark mid-dorsal line (the dorsal blood vessel). Ventral side: bears the genital openings.
• Prostomium — a lobe over the mouth; sensory, and a wedge to force open soil. The first body segment is the peristomium (buccal segment) containing the mouth.
• Clitellum — a prominent dark glandular band over segments 14–16, dividing the body into preclitellar, clitellar and postclitellar regions.
• Setae — S-shaped bristles in epidermal pits in every segment (except the first, last and clitellum); aid locomotion.

Anatomy — the gut, in order

• Calciferous glands in the stomach (9–14) neutralise the humic acid of soil. Typhlosole = an internal median fold of the dorsal intestinal wall that increases the absorptive surface.
• Body wall: cuticle → epidermis (single columnar layer with gland cells) → circular + longitudinal muscle layers → coelomic epithelium.
• Circulation — closed type: blood is confined to vessels, capillaries and hearts, flowing in one direction. Blood glands on segments 4, 5, 6 make blood cells and haemoglobin (dissolved in plasma); blood cells are phagocytic. No special breathing organs — exchange is through the moist body surface.

Nervous system: ganglia on a ventral, paired nerve cord; in segments 3–4 the cord rings the pharynx and joins the cerebral ganglia (a nerve ring). No eyes — only light- and touch-sensitive receptor cells and chemoreceptors on the anterior part.

Reproduction — hermaphrodite (bisexual)

• 2 pairs of testes (segments 10 & 11); vasa deferentia run to segment 18 and join the prostatic duct; accessory glands in segments 17 & 19; common prostate + spermatic duct open at the male pores (18).
• 4 pairs of spermathecae (segments 6–9) receive and store sperm during mating.
• 1 pair of ovaries (at the 12/13 septum); ovarian funnels → oviduct → single median female pore (14).
• Mating = mutual sperm exchange. The clitellum secretes a cocoon; fertilisation & development happen inside it in soil. After ~3 weeks each cocoon releases 2–20 young (avg ~4). Development is direct (no larva).

7.4 Cockroach — Periplaneta americana

Periplaneta americana — a bright-brown, nocturnal, omnivorous insect (Class Insecta, Phylum Arthropoda) of damp places; a pest and disease vector.

Morphology

• Adults 34–53 mm; in males the wings extend beyond the abdomen. Body = head, thorax, abdomen, covered by a brown chitinous exoskeleton of plates (tergites dorsal, sternites ventral) joined by arthrodial membrane.
• Head — triangular, formed by fusion of 6 segments, very mobile on the neck. Bears a pair of compound eyes and a pair of thread-like antennae.
• Thorax — prothorax, mesothorax, metathorax; each segment bears a pair of walking legs. Forewings (tegmina: opaque, leathery) arise from the mesothorax; hindwings (transparent, used in flight) from the metathorax.
• Abdomen — 10 segments in both sexes. Both sexes bear a pair of anal cerci on the 10th segment; males additionally have a pair of anal styles (absent in females). Females have a boat-shaped 7th sternum forming a brood (genital) pouch.

Anatomy — the gut, in order

• Foregut is lined by cuticle. Hepatic (gastric) caecae (6–8) at the foregut–midgut junction secrete digestive juice. Malpighian tubules (100–150, yellow) at the midgut–hindgut junction remove wastes from the haemolymph.
• Circulation — open type: vessels are poorly developed and open into the haemocoel; organs are bathed in haemolymph (colourless plasma + haemocytes). The tubular heart along the mid-dorsal line has funnel-shaped chambers with ostia.
• Respiration: a network of trachea opening through 10 pairs of spiracles; tracheoles deliver O₂ directly to tissues by diffusion; spiracles are valved by sphincters.
• Excretion: Malpighian tubules convert nitrogenous waste to uric acid (excreted via the hindgut) — the cockroach is uricotelic. Fat body, nephrocytes and uricose glands also help.
• Nervous system: fused, segmental ganglia on a ventral cord — 3 in the thorax, 6 in the abdomen; so much is spread through the body that a cockroach can survive about a week without its head. The brain is the supra-oesophageal ganglion. Compound eyes have ~2000 ommatidia giving mosaic vision (sensitive but low-resolution — good for night).

Reproduction — dioecious

• Male: a pair of testes (segments 4–6); vas deferens → ejaculatory duct → male gonopore; a mushroom-shaped gland (segments 6–7) is accessory. Sperm are bundled into spermatophores.
• Female: two large ovaries (segments 2–6), each of 8 ovarioles; oviducts unite into a single median oviduct (vagina); a pair of spermathecae (6th segment) opens into the genital chamber.
• Fertilised eggs are encased in a dark ootheca (~8 mm); a female makes 9–10 oothecae, each with 14–16 eggs. Development is paurometabolous — nymphs resemble adults and grow by ~13 moults; the last nymphal stage bears wing pads.

7.5 Frog — Rana tigrina

Rana tigrina — Class Amphibia, Phylum Chordata; lives on land and in fresh water. A poikilotherm (cold-blooded) that changes colour to blend in (mimicry), undergoes aestivation (summer sleep) and hibernation (winter sleep) in burrows.

Morphology

• Smooth, slippery, moist skin (mucus); dorsally olive-green with dark spots, ventrally pale yellow. It never drinks — water is absorbed through the skin.
• Body = head + trunk (no neck, no tail). Nostrils sit above the mouth; bulging eyes carry a protective nictitating membrane; a tympanum (eardrum) lies on each side.
• Forelimbs have 4 digits; the larger, muscular, webbed hind limbs have 5 digits for swimming, leaping and burrowing.
• Sexual dimorphism: the male has vocal sacs and a copulatory pad on the first digit of each forelimb — both absent in the female.

Anatomy

• Digestion: a short canal (carnivore). A bilobed tongue captures prey. The liver secretes bile (stored in the gall bladder); the pancreas supplies enzymes. Bile emulsifies fat; final digestion is in the intestine; villi & microvilli absorb; waste exits via rectum → cloaca.
• Respiration — three ways: through the skin (cutaneous, in water and during aestivation/hibernation), the buccal cavity, and the paired pink lungs (pulmonary, on land).
• Circulation — closed, plus a lymphatic system. The muscular 3-chambered heart has two atria + one ventricle inside a pericardium. The sinus venosus joins the right atrium; the ventricle opens into the conus arteriosus. Special hepatic portal (liver) and renal portal (kidney) systems exist. RBCs are nucleated and carry haemoglobin; lymph lacks some proteins and RBCs.

• Excretion — ureotelic. A pair of bean-like kidneys beside the vertebral column → ureters → cloaca; a thin urinary bladder stores urine. In males the ureter doubles as a urinogenital duct; in females ureter and oviduct open separately.
• Control & coordination: nervous + endocrine. Endocrine glands include the pituitary, thyroid, parathyroid, thymus, pineal, pancreatic islets, adrenals, gonads. The nervous system has the CNS (brain + spinal cord), PNS (10 pairs of cranial nerves + spinal nerves) and ANS. The brain has a forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes) and hindbrain (cerebellum + medulla oblongata); the medulla passes through the foramen magnum into the spinal cord.
• Sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), sight (eyes) and hearing/balance (tympanum + internal ear). Only the eyes and internal ears are well-organised; there is no external ear.

Reproduction

• Male: a pair of yellow ovoid testes attached to the kidney by the mesorchium; 10–12 vasa efferentia enter the kidney and join Bidder's canal → urinogenital duct → cloaca.
• Female: a pair of ovaries near the kidneys (no functional connection to them); oviducts open separately into the cloaca. A female lays 2500–3000 ova at a time.
• Fertilisation is external (in water); development passes through a tadpole larva that undergoes metamorphosis into the adult.