THE CONQUEST OF THE WATER AND THE BIRTH OF THE VERTEBRATES
Although animals without back-bones (invertebrates) are more abundant numerically and more diverse in species variety, they have never been able to reach the sizes that animals possessing a backbone can (vertebrates). All animals with a back-bone and some with a stiffened cartilage rod called a notochord belong to the phylumChordata. One of the most primitive members of this group are the Tunicates or sea-squirts. Although the sessile adult phase bears a superficial resemblance to a sea-anemone (Coelenterata), the rest of the vertebrate fauna was derived from such a simple organism. Evidences for this ancestry is in the tunicate's larval stage which resembles a tadpole and has the following features which are shared with all other vertebrates:
1. Perforations in the wall of the pharynx, or pouches that suggest ancestral perforations.
2. A nerve cord dorsal to the gut that is tubular and reflects its embryonic development from a tough piece of ectoderm that became roofed over.
3. A stiff rod called a notochord that supports the nerve cord from below.
The larva is short and the animal attaches itself to a rock and loses its tail and becomes a sedentary filter-feeder.
