Animals are multicellular organisms whose bodies are organised into cells, tissues, organs and organ systems. The structural and functional unit of the animal body is the cell, but cells of similar structure and function are grouped to form tissues. In the previous chapters, the classification of animals was based partly on the level of organisation; this chapter examines that organisation in detail. The study of tissues was founded by Bichat, who proposed the term "tissue," and the microscopic study of tissues is called histology.
In this chapter, the tissues of the animal body are studied in four major types: epithelial, connective, muscular and neural. The structure and function of each tissue type is described, along with their location in the body. Special attention is given to the frog (Rana tigrina), which is a familiar amphibian, to illustrate the structural organisation of the organ systems in a typical animal. The study of tissues forms the foundation of histology, physiology and medicine, since the health of organs and organ systems depends on the health of their constituent tissues.
Animal tissues can be grouped into four basic types: epithelial, connective, muscular and neural tissues.
Epithelial tissues are the protective tissues of the animal body, which cover most organs and cavities. They are the simplest and most widespread tissues, lying as a continuous sheet. Epithelial cells rest on a basement membrane, a non-cellular layer that separates them from the underlying connective tissue. Epithelia have a free surface facing either the body fluid or the outside environment. They perform various functions such as protection, absorption, secretion, sensation and excretion.
Epithelial tissues are classified on the basis of cell shape and the number of cell layers: - Simple squamous epithelium: Single layer of flattened, scale-like cells. Found in the inner lining of blood vessels (endothelium), the air sacs (alveoli) of the lungs, and the lining of the kidney tubules. Functions: exchange of materials, filtration. - Simple cuboidal epithelium: Single layer of cube-like cells. Found in the lining of kidney tubules and the ducts of salivary glands. Functions: absorption, secretion. - Simple columnar epithelium: Single layer of tall, elongated cells. Found in the inner lining of the stomach, intestine and gall bladder. Functions: secretion and absorption. Columnar cells bearing cilia are called ciliated columnar epithelium, found in the lining of the trachea and fallopian tubes. - Glandular epithelium: Some columnar or cuboidal cells become specialised for secretion and form glands, e.g., the goblet cells of the intestine. - Stratified epithelium: Made of more than one layer of cells, providing protection against mechanical and chemical stresses. Found in the outer skin and the lining of the mouth cavity. The stratified epithelium of the skin is called keratinised epithelium. - Pseudostratified epithelium: Appears stratified because the nuclei lie at different levels, but all cells rest on the basement membrane. Found in the respiratory tract.
Connective tissues link and support other tissues of the body. They are the most abundant and widely distributed tissues, consisting of cells separated by a non-living matrix, which may be jelly-like, fluid, dense or rigid. All connective tissues originate from the mesoderm.
Muscular tissue consists of elongated, spindle-shaped cells called muscle fibres, which contract and relax to produce movement. Based on their location, structure and function, muscles are of three types: - Skeletal (striated) muscle: Attached to bones, voluntary, with characteristic stripes (striations). The cells are long, cylindrical, unbranched and multinucleate. - Smooth (non-striated) muscle: Found in the walls of internal organs such as the stomach, intestine and blood vessels. Involuntary, spindle-shaped, uninucleate cells without striations. - Cardiac muscle: Found only in the heart. Involuntary, with striations, and the cells are branched and interconnected to form a network. Cardiac muscle cells beat rhythmically and are not fatigued easily.
Neural tissue is made of nerve cells, or neurons, which are the structural and functional units of the nervous system. A neuron consists of a cell body, a long process called the axon, and shorter, branched processes called dendrites. Neurons conduct nerve impulses to and from the brain and spinal cord, and their arrangement forms the nervous tissue that controls and coordinates body activities.
Different tissues are organised into organs. The heart, for example, is an organ made of cardiac muscle, connective tissue, nervous tissue and epithelial tissue. Organs work together in organ systems, and several organ systems together form the organism. This hierarchical arrangement from cell to tissue to organ to organ system to organism represents the structural organisation of animals.
The earthworm (Pheretima posthuma) is a terrestrial annelid found in moist soil, feeding on decaying organic matter. It is an important soil animal that improves soil fertility through its burrowing and feeding activities.
The earthworm is reddish-brown, elongated and cylindrical, divided into about 100 to 120 segments (metameres). The body is bilaterally symmetrical. The dorsal surface has a dark median mid-dorsal line of blood vessel (dorsal blood vessel), while the ventral surface is lighter. The anterior end has a prostomium, a small fleshy lobe overhanging the mouth. The 14th, 15th and 16th segments are covered by a prominent dark band of glandular tissue called the clitellum, which is involved in reproduction. The segments behind the clitellum bear four pairs of s-shaped, chitinous setae that help in locomotion. The dorsal pores present between segments 12-13 onwards open to the exterior and release coelomic fluid, which keeps the body moist and slippery. The female genital pore is a single median pore on the ventro-lateral side of the 14th segment, while the male genital pores are a pair, one on each ventro-lateral side of the 18th segment. Copulatory pouches open into the 17th and 19th segments.
The body wall of the earthworm is covered externally by a thin, non-cellular cuticle, followed by the epidermis, two muscle layers (an outer circular and an inner longitudinal) and the innermost coelomic epithelium. Locomotion is brought about by the extension and contraction of these alternating muscle layers, aided by the setae.
The alimentary canal is a straight tube extending from the mouth to the anus. It consists of the mouth, buccal cavity (segments 1-3), pharynx (4th segment), oesophagus (5-7th), gizzard (8-9th, muscular, grinding food), stomach (9-14th, with calciferous glands that neutralise humic acid), intestine (15th to last), and anus. Typhlosole, an internal median fold of the dorsal wall of the intestine, increases the absorptive surface area. The earthworm is a detritivore, feeding on decaying organic matter, and digestion is extracellular.
The earthworm has a closed circulatory system. Blood is confined to vessels and is red due to haemoglobin dissolved in plasma. The system has dorsal and ventral blood vessels running along the length, and lateral vessels, along with heart-like blood vessels or "hearts" (lateral hearts and loop hearts) that pump blood. Blood transports digested food, oxygen, carbon dioxide and nitrogenous wastes.
The earthworm respires through its moist skin; the skin has abundant blood capillaries, and gaseous exchange takes place by diffusion. The excretory organs are segmentally arranged coiled tubules called nephridia. There are three kinds of nephridia: septal nephridia (the most numerous, attached to the septa), integumentary nephridia (attached to the inner surface of the body wall) and pharyngeal nephridia (in the pharyngeal region). The nephridia remove nitrogenous wastes like urea and ammonia.
The nervous system of the earthworm is a typical annelid nerve cord. It consists of a bilobed cerebral ganglion (brain) above the pharynx, connected to the sub-pharyngeal ganglion by circumpharyngeal connectives. The sub-pharyngeal ganglion gives rise to a solid, double, mid-ventral nerve cord that runs posteriorly. The nerve cord bears ganglia and lateral nerves in each segment.
The earthworm is a hermaphrodite (bisexual), bearing both male and female organs in the same animal. The male organs include two pairs of testes (segments 10-11), seminal vesicles and vasa deferentia. The female organs include a pair of ovaries (segment 13), oviducts and spermathecae. Fertilisation is cross-fertilisation. During copulation, two earthworms exchange sperm, which is stored in the spermathecae. The clitellum secretes a cocoon in which eggs and sperm are deposited and fertilisation takes place. Development is direct, without a larval stage.
The cockroach (Periplaneta americana) belongs to the phylum Arthropoda, the largest phylum. It is a nocturnal, omnivorous pest that lives in warm, moist places.
The body of the cockroach is brownish or blackish-red, dorsoventrally flattened and bilaterally symmetrical, divided into the head, thorax and abdomen. The head has a pair of compound eyes, a pair of antennae and chewing mouth parts. The thorax has three segments (prothorax, mesothorax, metathorax), each bearing a pair of legs; the mesothorax and metathorax each bear a pair of wings. The abdomen is composed of ten segments, with a pair of anal cerci in males and a pair of anal styles in females. The body is covered by a chitinous exoskeleton. In females, the brood pouch is formed by the 7th, 8th and 9th abdominal segments.
The body cavity is a haemocoel filled with blood (haemolymph). The digestive system has a mouth, pharynx, oesophagus, crop, gizzard, midgut (hepatic caeca), hindgut and anus. The crop stores food and the gizzard grinds it. Malpighian tubules are the excretory organs, present at the junction of the midgut and hindgut. Respiration is by tracheae, an elaborate network of tubes that branch into air sacs, opening to the outside through spiracles (10 pairs). Blood is colourless haemolymph that transports nutrients and wastes but does not carry oxygen. The nervous system consists of a brain and a ventral nerve cord with paired ganglia. The cockroach has an open circulatory system.
The cockroach is dioecious (sexes separate). The male has a pair of testes, and the female has a pair of ovaries with ovarioles. Fertilisation is internal. The female produces oothecae, which are dark, chitinous egg cases. Each ootheca contains 14 to 16 eggs and is laid every 15 days. Development is indirect, involving nymphal stages (incomplete metamorphosis).
The frog (Rana tigrina) is an amphibian found in a wide variety of habitats. It is the representative animal studied in detail in this chapter.
The frog has a dark green body with irregular dark spots on the dorsal side and a pale ventral side. The body is divided into the head and trunk, with a neck absent. The head has a mouth, a pair of nostrils, a pair of eyes and a tympanum (ear). The forelimbs are shorter with four digits, and the hind limbs are longer with five digits and webbed feet, adapted for jumping and swimming. The skin is smooth, moist and slippery, and is attached loosely to the body, being richly supplied with blood capillaries, so the frog can respire through its skin.
The frog is dioecious, and reproduction is sexual. The male frog has a pair of testes, and the female has a pair of ovaries. Fertilisation is external and takes place in water. The eggs are laid in water in a gelatinous mass. The development is indirect, passing through a free-swimming larval stage called the tadpole. The tadpole undergoes metamorphosis to become a young frog.
| Epithelium | Shape/Layers | Location | Function |
|---|---|---|---|
| Simple squamous | Single layer, flattened | Blood vessels, alveoli | Exchange, filtration |
| Simple cuboidal | Single layer, cube-like | Kidney tubules, salivary ducts | Absorption, secretion |
| Simple columnar | Single layer, tall | Stomach, intestine | Secretion, absorption |
| Ciliated columnar | Columnar with cilia | Trachea, oviduct | Movement of mucus/ova |
| Stratified | Multiple layers | Skin, mouth | Protection |
| Muscle | Location | Control | Structure |
|---|---|---|---|
| Skeletal | Attached to bones | Voluntary | Striated, multinucleate |
| Smooth | Wall of organs | Involuntary | Non-striated, uninucleate |
| Cardiac | Heart | Involuntary | Striated, branched |
| System | Earthworm | Cockroach | Frog |
|---|---|---|---|
| Excretory | Nephridia | Malpighian tubules | Kidneys (ureotelic) |
| Respiration | Skin | Tracheae | Skin, lungs, buccal |
| Circulation | Closed | Open (haemolymph) | Closed, 3-chambered heart |
| Reproduction | Hermaphrodite | Dioecious | Dioecious |
The structural organisation in animals shows how cells of similar functions group together to form tissues, and how tissues cooperate to build organs and organ systems. The four basic tissue types, epithelial, connective, muscular and neural, form the building blocks of the entire animal body. The detailed study of the earthworm, cockroach and frog reveals how the same organ systems are designed differently in different phyla, reflecting adaptation to their habitats and lifestyles. The earthworm's nephridia, the cockroach's malpighian tubules and the frog's kidneys exemplify the diversity of excretory design, while the closed circulation of the earthworm contrasts with the open circulation of the cockroach. These comparative studies provide the essential groundwork for understanding physiology, anatomy and evolutionary biology.