Structural Organisation in Animals
In multicellular animals, a group of similar cells along with intercellular substances that perform a specific function constitute a tissue. All complex animals consist of only four basic tissue types - epithelial, connective, muscular and neural - which combine in specific proportions to form organs (e.g. stomach, lung, heart, kidney). Two or more organs working together form an organ system. This division of labour ensures the survival of the body.
The four tissues
- Epithelial: sheet-like tissue lining surfaces, cavities, ducts and tubes; has one free surface and rests on a basement membrane. Simple (squamous, cuboidal, columnar, ciliated) or compound. Cells joined by tight, adhering and gap junctions.
- Connective: most abundant and widely distributed; cells secrete an extracellular matrix. Includes loose (areolar, adipose), dense (tendons, ligaments), cartilage, bone and blood.
- Muscular: skeletal (striated, voluntary), smooth (unstriated, involuntary) and cardiac (striated, involuntary, found only in the heart).
- Neural: neurons (excitable cells) and supporting neuroglia; exerts the greatest control over body responsiveness.
Model invertebrates
- Earthworm (Annelida): segmented body, setae for locomotion, clitellum on segments 14-16, typhlosole in intestine, nephridia for excretion, hermaphrodite with cross-fertilisation (protandry).
- Cockroach (Arthropoda): body in head, thorax and abdomen; chitinous exoskeleton; open circulation (haemocoel); tracheae for respiration; Malpighian tubules for excretion.