|
Vertebrates constitute the vast majority of living chordates, and they have evolved an enormous variety of forms. The backbone of vertebrates protects the nerve cord and serves as the axis of the internal skeleton. The skeleton provides strength and rigidity to the body and is an attachment site for muscles. The vertebrae in the middle region of the trunk give rise to pairs of ribs, which surround and protect the internal organs. A cartilaginous or bony case encloses the brain. Bone is a substance unique to vertebrates. It was formerly thought that vertebrates with cartilage skeletons (cyclostomes and sharklike fishes) were descended from early vertebrates that had not yet developed bone. However, very primitive fishes with bone skeletons are known from the fossil record, so lack of bone is now believed to be a degenerate rather than a primitive feature. All but the most primitive vertebrates, known as jawless fishes, have jaws and paired appendages. The fishes and, to a lesser extent, the amphibians and reptiles show a segmental arrangement of the muscles of the body wall and of the nerves leading to them.
There are eight vertebrate classes. Four are aquatic, and may be grouped together as the superclass Pisces, or fish; four are terrestrial or (in the case of amphibians) semiterrestrial, and may be grouped as the superclass Tetrapoda, or four-footed animals. Fishes breathe water by means of gills located in internal passages, although they may also have lungs as supplementary air-breathing organs. Most move through the water by weaving movements of the trunk and tail. All have fins, and most have two sets of paired fins (pelvic and pectoral). Tetrapods breath air, usually by means of lungs, and never have gills as adults, although the amphibians go through a gilled, water-breathing stage. Except where the appendages have been lost, as in snakes, all have two pairs of limbs, generally used for locomotion; these are homologous to the pelvic and pectoral fins of fish.
Class Agnatha
The Agnatha, or jawless fishes, are the oldest known vertebrates. The only surviving members of this class are the hagfish and lampreys, known as cyclostomes. Cyclostomes have long, slender bodies with dorsal, ventral, and caudal (tail) fins, all in the median plane. Although in their lack of jaws or paired lateral appendages they represent a very primitive stage of vertebrate development, the modern cyclostomes are highly adapted for their particular ways of life. The hagfish is a specialized scavenger, and the lamprey is a parasite on other fishes. The lamprey has a round mouth without skeletal supports, a rasping tongue, and a single, dorsally located nostril. The gill passages are enlarged to form pouches and are lined with gill filaments that serve as a surface for the exchange of respiratory gases; in vertebrates the gill passages have acquired a respiratory function. In cyclostomes, as in all fishes, water is taken in through the mouth and expelled through the gill passages; as water passes over the thin-walled gill filaments, dissolved oxygen diffuses into the blood, and carbon dioxide diffuses out. The lamprey has a notochord extending from the head to near the tip of the tail. A few cartilaginous blocks around the notochord constitute the bare rudiments of a backbone; a cartilage framework supports the gill region, and there is a rudimentary cartilage braincase. The meagerness of the skeleton is considered a degenerate, not a primitive condition. The larva of the marine lamprey is a small animal, resembling a lancelet, that uses the pharynx and gill passages for filter-feeding. It metamorphoses into the adult form before migrating to the sea. The extinct relatives of the cyclostomes, called ostracoderms, were jawless fishes with bony armor and in some cases a well-developed bony skeleton.
|
