The vertebrate fauna of the Outer Hebrides is typical for an archipelago, it is dominated by the aquatic species with a smaller number of the terrestrial species. The terrestrial species are post-glacial colonisers which arrived either by chance (i.e. they could swim or fly) or as immigrants with man or as a result of his activities.
Vertebrates: fish, amphibians, reptiles, birds, and mammals; are animals with backbones and have a vertebral column, which runs along the dorsal surface from head to tail and forms the main skeletal axis of the body.
Characteristics found in most vertebrates :
- internal sketelton of bone or catrilage
- integument (natural covering of an organism or an organ, such as its skin) comprising two layers, an outer epidermis and an inner dermis. It is often modified to produce hair, scales, feathers, glands, horn, etc.
- replacement of notochord by vertebral column more or less complete, depending on group
- muscular, perforated pharynx; this structure is the site of gills in fishes but is much reduced in adult terrestial forms (although it is extremely important in embryonic development of all vertebrates)
- movement provided by muscles attached to the skeleton
- digestive system with large digestive glands, liver, and pancreas
- ventral heart with 2-4 chambers
- blood with red blood cells containing hemoglobin and white corpuscles
- well developed body cavity (coelom) containing visceral systems
- paired kidneys with ducts to drain waste to exterior
- most vertebrates have two sexes
- general body plan consisting of head, trunk, 2 pairs of appendages, and postanal tail (but these structures are highly modified in many vertebrates and sometimes absent).