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Vertebrates are a very diverse group, ranging from lampreys to Man. It includes all craniates, except hagfishes,
and are characterized chiefly by a vertebral column, hence their name. The majority of the extant vertebrates are the jawed
vertebrates, or gnathostomes, but lampreys are jawless vertebrates. However, in Late Silurian or Early Devonian times, about
420 to 400 million years ago, the situation was reverse, and the majority of the vertebrate species were jawless fishes. The
decline of the jawless vertebrates and the subsequent rise of the gnathostomes took place about 380 million years ago.
The following are some characteristics of vertebrates in general:
Extrinsic eye muscles. These muscles are attached to the eyeball and orbital wall, and ensure eye movements
Radial muscles in fins. These are small muscles associated with each of the cartilaginous radials of the unpaired and
paired fins. They ensure the undulatory movements of the fin web.
Atrium and ventricle of heart closely-set.
Nervous regulation of heart. The heart in the embryo of the vertebrates is aneural, like the heart of adult hagfishes.
In adult vertebrates, however, the heart is innervated by a branch of the vagus nerve.
Typhlosole in the intestine. This is a spirally coiled fold of the intestinal wall. In the Gnathostomes, it can be developed
into a complex spiral valve.
At least two vertical semicircular canals in the labyrinth
True neuromasts in the sensory-line system There are many other vertebrate characteristics, both anatomical and physiological.
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