Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates

General characteristics

The phylum Chordata encompasses over 40,000 animal species exhibiting a vast diversity in appearance, lifestyle, and habitat. Chordates inhabit the land surface, subterranean soils, aquatic environments, and aerial spaces.

THE ORIGIN OF chordates remains a subject of ongoing debate. Toward the end of this study guide, within the broader context of animal origins, we will examine several hypotheses and assumptions regarding this issue that are most frequently cited in zoological literature.

Chordates are coelomate, bilaterally symmetrical, deuterostome animals characterized by metameric segmentation. Metamerism is more pronounced during embryonic development than in adults. In the latter, it is better preserved in primary aquatic forms than in terrestrial species. Like other coelomates, chordates possess a closed Circulatory system, metanephridial-type excretory Organs, and Gonads that develop within the coelom.

Despite their species diversity, chordates share several fundamental structural features:

✵ chordates possess an Axial Skeleton. Initially, it develops as a dorsal rod, or notochord, which is a non-segmented strand. The notochord persists throughout life only in lower Representatives of the phylum, whereas in higher forms it is replaced by THE Vertebral Column;

✵ a hollow neural tube of the Central Nervous system lies dorsal to the axial skeleton. In almost all chordates, the anterior region of the neural tube expands to form the Brain;

✵ the anterior, or pharyngeal, section of the digestive tract communicates with the external environment via two rows of openings known as gill slits. These persist throughout life only in aquatic lower chordates, while in other groups they are present only during embryonic stages;

✵ chordates are deuterostomes, meaning the Mouth forms at the end opposite to its initial embryonic position, while the blastopore eventually develops into the anus;

✵ chordates are characterized by a secondary body cavity (coelom) and bilateral Symmetry.

Chordate animals are of immense importance to humans. This phylum includes all domestic livestock, with the exception of bees and silkworms. Many chordates (fish, birds, mammals) are targets of commercial fisheries and hunting, yielding substantial amounts of meat, fat, leather, fur, and other industrial raw Materials. Some predatory species are beneficial as eradicators of agricultural pests. At the same time, many chordates cause significant damage to agriculture and livestock production, and some act as vectors for dangerous human diseases.

The phylum Chordata is divided into three subphyla: Tunicata (or Urochordata), Acrania (cephalochordates), and Vertebrata (or Craniota).