Structural Organisation in Animals
Animal Tissues
A group of similar cells performing a specific function is a tissue. Animal tissues are of four basic types: epithelial, connective, muscular and neural.
- Epithelial tissue — covers body surfaces, lines organs and cavities, and forms glands. The cells are tightly packed with little material between them and rest on a basement membrane. Simple epithelium is a single layer (squamous — flat; cuboidal; columnar; ciliated), used for protection, secretion and absorption; compound epithelium has many layers, for protection (e.g. skin). Tight junctions, adhering junctions and gap junctions hold and connect the cells.
- Connective tissue — the most abundant and widely distributed tissue; it links and supports other tissues. Its cells are loosely spaced in a matrix (jelly-like, fluid or solid). Types: loose connective (areolar, adipose/fat-storing), dense connective (tendons join muscle to bone; ligaments join bone to bone), and specialised connective — cartilage (flexible, e.g. ear, nose), bone (hard, with calcium salts, for support) and blood (a fluid connective tissue that transports materials).
- Muscular tissue — made of long contractile cells (muscle fibres) that bring about movement. Skeletal (striated) muscle is attached to bones and is voluntary; smooth (unstriated) muscle is in the walls of internal organs and is involuntary; cardiac muscle is in the heart wall, striated but involuntary.
- Neural tissue — made of neurons (which receive and transmit impulses) and supporting glial cells; it controls and coordinates body activities.
There are four basic animal tissues.
- Epithelial, connective, muscular and neural tissue.
Both are dense connective tissues but join different parts.
- A tendon connects muscle to bone.
- A ligament connects bone to bone.
Both are striated but differ in control.
- Skeletal muscle: striated and voluntary (under our will).
- Cardiac muscle: striated but involuntary (heart works on its own).
Key Points
- Four animal tissues: epithelial (covering/lining/glands), connective (link & support), muscular (movement), neural (control).
- Connective: loose (areolar, adipose), dense (tendon muscle-to-bone, ligament bone-to-bone), specialised (cartilage, bone, blood).
- Muscle: skeletal (striated, voluntary), smooth (unstriated, involuntary), cardiac (striated, involuntary).
- Neural tissue = neurons + glial cells.
Morphology of the Earthworm and Cockroach
Earthworm (Pheretima) is a reddish-brown, segmented worm of phylum Annelida, found in moist soil; it is a 'friend of farmers' because its burrowing and castings improve soil fertility (vermicomposting). Its long body is divided into many segments (metameres); in a mature worm a prominent glandular band, the clitellum, occurs around segments 14–16. The mouth is at the anterior tip below the prostomium. Earthworms are hermaphrodite (each has both male and female organs) but practise cross-fertilisation. Locomotion is by tiny bristles called setae and muscle contraction. The body has a true coelom, a straight digestive tube, a closed circulatory system and a nerve cord.
Cockroach (Periplaneta americana) is an arthropod (insect). Its body is divided into three regions: head (with compound eyes, antennae and mouth parts), thorax (bearing three pairs of legs and two pairs of wings) and abdomen (10 segments). It has a hard chitinous exoskeleton. The digestive system has a crop and a gizzard (for grinding food); blood flows in an open circulatory system (haemolymph in body spaces); excretion is by Malpighian tubules; and breathing is through air tubes called tracheae opening at spiracles. Sexes are separate (sexual dimorphism is seen).
The clitellum is a key external feature.
- It is a prominent glandular band around the body.
- It is found on segments 14–16 and helps in reproduction (cocoon formation).
An insect body has three regions.
- Head, thorax and abdomen.
- The thorax bears three pairs of legs and two pairs of wings.
Insects have characteristic systems.
- Breathing: through air tubes (tracheae) opening at spiracles.
- Excretion: by Malpighian tubules.
Key Points
- Earthworm (Annelida): segmented (metameres), clitellum on 14–16, hermaphrodite, moves by setae; closed circulation; helps soil fertility.
- Cockroach (Arthropoda/insect): head + thorax (3 leg pairs, 2 wing pairs) + abdomen; chitin exoskeleton.
- Cockroach: open circulation (haemolymph), Malpighian tubules for excretion, tracheae for breathing; sexes separate.
Morphology of the Frog and Its Organ Systems
The frog (Rana tigrina) is a cold-blooded amphibian that lives both on land and in water. Its body is divided into a head and a trunk (no neck or tail in the adult). The skin is smooth and moist and helps in respiration. Frogs show camouflage (protective colouration) and can change colour slightly. They are able to breathe in three ways: through the skin (cutaneous, especially in water and during hibernation), the buccal cavity, and the lungs (on land). Frogs are poikilothermic and undergo hibernation (winter sleep) and aestivation (summer sleep) to survive extreme conditions.
Its organ systems are well developed:
- Digestive system — mouth, buccal cavity, oesophagus, stomach, intestine, cloaca; with a liver and pancreas. The frog captures prey with a sticky bilobed tongue.
- Respiratory system — cutaneous, buccal and pulmonary (lungs) respiration, as above.
- Circulatory system — a closed system with a three-chambered heart (two atria and one ventricle); blood has red and white cells and carries materials; lymph also occurs.
- Reproductive system — sexes are separate; males have a pair of testes, females a pair of ovaries. Fertilisation is external (in water) and development includes a tadpole larva that undergoes metamorphosis into the adult frog.
Frogs are useful to humans: they eat insects and pests (helping agriculture), are part of food chains, and are widely used for biological study.
Frogs have multiple respiratory routes.
- Three ways: through the skin (cutaneous), the buccal cavity and the lungs.
- During hibernation in water, it respires through the skin (cutaneous respiration).
The frog has a three-chambered heart.
- Two atria (auricles) and one ventricle.
Frogs reproduce in water.
- Fertilisation is external (in water).
- Development passes through a tadpole larva that undergoes metamorphosis into the adult.
Key Points
- Frog (Rana tigrina): cold-blooded amphibian; body = head + trunk; moist skin aids respiration; shows hibernation & aestivation.
- Respires in three ways: cutaneous, buccal, pulmonary.
- Three-chambered heart (two atria, one ventricle); closed circulation.
- Sexes separate; external fertilisation; tadpole larva → metamorphosis → adult.