What are the characteristics of chordates? Information about the chordate divisition, anatomy, evolution and features.
CHORDATA, a phylum that includes all animals with backbones (the vertebrates) together with a few more primitive animals. At some stage of their existence, all chordates possess a rodlike structure called the notochord, from which the group gets its name.
The chordates primarily are animals that swim, although the most primitive kinds, known as the ascidians, or sea squirts, do so only when they first develop from the egg into the tadpolelike larvae. Many other chordates long ago abandoned life in the water as fish, to become creatures of the land and air. However, as animals that swim and feed in water, the chordates all have certain distinctive features.
These are: a band of muscle extending along each side of the body or tail, which supplies the power of swimming; a stiffening rod (the notochord or its replacement), which prevents the muscles from contracting the body or tail like a concertina; gill slits on each side, which allow water that has entered the mouth to escape without passing all the way through the intestine; and a central nervous system, which is essentially a hollow tube lying above the notochord and extending the length of the body. These features are found in some form at some stage in the development of all chordate animals from sea squirts to man.
The phylum Chordata is divided into three subphyla: Tunicata, or Urochordata, which includes the sea squirts and their close relatives; Cephalochordata, which has only lancelets such as amphioxus, and Vertebrata, which includes the whole vast community, present and past, of vertebrates, ranging from the primitive jawless lamprey eels to the highly developed birds and mammals.
In the adult state the sea squirts are generally inconspicuous marine animals that occur from the intertidal zone to the greatest depths of the ocean. They always attach themselves to the surface of rocks, weeds, pilings, or the sea floor itself by means of an external supporting layer of celluloselike substance. Internally, sea squirts consist largely of a complex water-sifting feeding apparatus with two siphons opening to the exterior, one for water intake and the other for water ejection.
At one time, sea squirts were classed with bivalve mollusks, such as clams and oysters, since anything less like a swimming vertebrate was hard to imagine. However, sea squirts do possess numerous gill slits, which allow the water taken in through the mouth siphon to be eliminated without passage through the gut. Also, the tadpole-shaped larva that develops from the egg possesses a swimming tail consisting of a central notochord rod, a muscle band on each side, and a fine nerve tube above the rod. Thus the sea squirts have all the qualifications for membership in the chordate phylum. However, the truly chordate tadpolelike larvae exist as such for only a few hours and serve merely to select a suitable spot on which to settle and grow as a sedentary sea squirt.
The other kind of nonverte-brate chordate, the lancelets, inhabit the shallow seas and live, much as do the sea squirts, on the sea floor. They sift food particles from a steady stream of water taken in through the mouth and passed outward again through numerous gill slits along each side of the throat. Unlike the sea squirts, lancelets are not actually attached to some solid surface but remain imbedded in loose sand with only the forward end of the body projecting into the water.
Also, they retain their structure and capacity for swimming throughout life, in contrast to the sea squirt larvae, which resorb their tails when they become attached. The lancelet retains its notochord, lateral bands of locomotory muscle, and nerve tube, and at any time can either swim out of the sand and imbed itself in a more suitable area or wriggle through the loose, wet sand without actually emerging.
Both the lancelets and the attached sea squirts, however, feed in essentially the same way. The food-laden water taken in through the mouth is sifted as it passes through the gills. A film of mucus, secreted from a midventral gland (the endostyle) in the large throat, passes over the inner walls of the gill region and entraps small food particles; the mucus and particles then pass along to the intestine. The same elaborate food-collecting mechanism is used by the young lamprey eels, which are true vertebrates and are far more advanced than the nonvertebrate chordates. The presence of the endostylar feeding mechanism in juvenile lampreys indicates almost certainly that this was the general feeding method and mechanism employed by all early chordates and ancestral vertebrates at a time before the true fishes had evolved.
Juvenile lampreys, which grow to lengths of about 6 inches (15 cm) are not parasitic predators on fish, as they are when mature; instead, they live much like the lancelets. They inhabit fresh water rather than the sea, and live imbedded in the soft mud and in shallow water along the banks of rivers and lakes. Here they draw in water through the mouth, filtering out the contained food particles and eliminating the filtered water through the gill slits at the side of the throat. By means of their muscular bodies and tails they can move freely through the soft mud and occasionally in the water alone.
But although they live in this primitive way, their body structure has all the complex and essential organs and systems common to all backboned animals. They lack jaws, which are not needed by filter feeders, and they lack paired fins, but they possess virtually all other vertebrate features. The notorchord persists as a large and substantial structure stiffening the body, and it is reinforced by rings of cartilage, which constitute a vertebral column around the notochord. Even in human embryos, at a very early stage of development, a notochord is formed first and then the vertebral column develops around it.
The chordates, as a whole, appear to have evolved in two main phases. The first phase was the evolution of marine filter feeders, which consisted entirely of soft body structures and had only a temporary or intermittent need to swim. The second phase was the evolution of the swimming stage into the true vertebrate state; this probably occurred in fresh waters and led to the subsequent evolution of fishes and their terrestrial descendants, the land vertebrates. Throughout chordate evolution the notochord has played a dominant role—as the essential stiffener of the first swimming tail, as the basis for the construction of a vertebral column, or backbone, and finally as a vitally important tissue in the early development of all higher vertebrates.