Levels of Organisation

Every member of the kingdom Animalia is multicellular, but multicellularity by itself does not mean that all animals build their bodies in the same way. The cells of different animals are organised to different degrees of complexity, and this level of organisation is one of the basic characters used to classify them. Four successive levels are recognised.

Cellular level of organisation. In the simplest animals, such as sponges, the cells are simply arranged as loose aggregates. There is only a limited division of labour, with some cells taking up particular activities, but the cells are not knit together into higher structures. This loose, cell-aggregate arrangement is the cellular level.

Tissue level of organisation. In coelenterates the arrangement is more advanced. Cells that carry out the same function are grouped together into tissues. Because the body is organised into tissues but not yet into distinct organs, this is called the tissue level.

Organ level of organisation. In Platyhelminthes and other higher phyla, tissues themselves are combined to build organs, each organ specialised for a particular function. This is the organ level of organisation.

Organ-system level of organisation. In still more advanced animals — annelids, arthropods, molluscs, echinoderms and chordates — organs are further associated into functional systems, each system dedicated to a specific physiological job. This is the organ-system level, the highest of the four.

Organ systems do not all have the same design across these groups; they show varying patterns of complexity. Two familiar examples make the point.

• Digestive system. In Platyhelminthes the gut has only a single opening to the outside that acts as both the mouth and the anus; such a gut is described as an incomplete digestive system. A complete digestive system has two separate openings — a mouth at one end and an anus at the other. …