These are fishlike vertebrates with well-developed fins and teeth, and with 2 pairs of fins, one of them supported by the pectoral girdle, the other by the pelvic girdle. Their most distinctive character, as contrasted with the bony fishes (p. 80) is that their entire skeleton, including the skull, is cartilaginous, without any true bone, though it is partly calcified, especially in the vertebrae; the skull is far simpler than it is among the bony fishes; the gill filaments are attached throughout their lengths to the partitions between the gill openings instead of being free; and the rear portion of the digestive tract is modified into the so-called spiral valve by the development of a special fold from its lining layer, which only a few bony fishes have.
Fertilization is internal in all of them, and is effected by a pair of rodlike copulatory organs, each of which is developed from the inner edge of one of the two pelvic fins, and is supported by one or more cartilages.
The sharks and rays are usually looked upon as more primitive than the bony fishes.
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