Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Chordates

General characteristics

Chordates are widely distributed across the Earth and inhabit a diverse range of environments: in Water, on land, and in the air.

The total number of extant chordate species is approximately 45,000.

The common features of all chordates are as follows:

✵ The Axial Skeleton develops as a longitudinal rod known as the notochord, or dorsal string, located along the main body axis between the neural tube and the digestive tract. The notochord consists of highly vacuolated Cells with thick walls. In lower chordates, the notochord persists throughout life; in higher chordates, it is replaced during embryonic development by a cartilaginous or bony Vertebral Column;

✵ The Central Nervous system develops as a hollow tube situated above the axial skeleton. In most chordates, the anterior region of the neural tube differentiates into the Brain;

✵ The anterior part of the digestive tube forms pharyngeal gill slits. In lower aquatic chordates, the walls of these slits transform into gills. In terrestrial vertebrates, gill slits disappear during early embryonic stages, and aerial respiratory Organs—the Lungs—are formed;

✵ The central circulatory organ is The Heart, located on the ventral side of the body.

Chordates share A number of characteristics that align them with certain other animal phyla: bilateral body Symmetry, the presence of a secondary body cavity (coelom) and a secondary Mouth, as well as the metameric arrangement of certain organs.

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

Class="center">***

Tunicates (Tunicata), or Urochordates (Urochordata), represent an early-diverging and highly specialized branch of chordate animals that evolved primarily along a path of morphologically regressive evolution. Typical chordate features are clearly expressed in tunicates only during the larval stage. The notochord is restricted to the posterior (tail) region of the body and is mostly present only during early developmental stages, later disappearing. Their body is covered by a tunic or test.

They feed passively by filtering large volumes of water. The Circulatory system is open, of a lacunar type. Tunicates are hermaphroditic; the majority of them are capable of asexual reproduction by budding.

Tunicates are exclusively marine. Some lead a sessile (attached) lifestyle, while others drift slowly through the water column.

The subphylum includes three classes: Appendiculariae, Ascidiae, and Salpae, comprising about 1,000 species. This subphylum is not covered in the present practical course.