Multicellular organisms, especially chordates (most vertebrates), exhibit several different body structures which allow for more sophisticated locomotion, sensory perception, and structure than in less developed animals.
All chordates exhibit a notochord, which is a long, flexible rod that provides support, and a place for muscles push against in locomotion. The notochord is present in the embryonic stage of vertebrates such as mammals, and develops into spinal discs later on. Another important development is the muscular tail. It is important as a source of locomotion in marine chordates, and serves many purposes in terrestrial chordates. The muscular tail is significant because it extends beyond the digestive tract. This is important because a combination of the various delicate digestive organs and the constant movement involved in locomotion would result in an efficiency loss of both systems.
Finally, another important chordate innovation is the pharyngeal cleft. The pharyngeal clefts are located on the pharynx, the region of neck or head just behind the mouth. These clefts, or pouches, serve many purposes among the chordates. In fish and some amphibians, they develop into gills. In other aquatic animals, they may develop into filter-feeding mechanisms. In many terrestrial chordates, they develop into structures which aid in sensory perception, such as ears and other structures on the pharynx.
Chordates have many developments which allow them to surpass other animals on many levels. They can achieve more advanced locomotion and greater structural strength with the notochord and the muscular tail. They can develop specialized organs on the pharynx that aid in many systems out of the pharyngeal clefts. Chordates are advanced animals whose success is based on seemingly simple innovations.