Give an example of cartilaginous fish.

Hint: Cartilaginous fishes are included in the subclass Chondrichthyes. Cartilaginous fishes inhabit large rivers like Amazon, the Ganges, and the Tigris. They may climb up to the limits of tidal water.

Complete answer:
Sharks are marine cartilaginous fishes, inhabiting oceans and seas. The body of the shark is long spindle-shaped and terminally tapering. It has three regions, head, trunk, and tail. The Head is dorso-ventrally flattened, and the trunk and tail are laterally compressed. The mouth is subterminal. It is bordered by upper and lower jaws, which are armed with delicate teeth. Just in front of the mouth ventrally, there are two oblique external nostrils. Eyes are lateral. Each eye is provided with movable upper and lower eye-lids and a transparent nictitating membrane. Trunk bears fin system, formed of paired lateral fins and unpaired median fins. Paired fins include the pectoral and pelvic fins. Unpaired fins include two dorsal fins, a caudal fin, and a ventral fin. Pelvic fins are simple in females. But, in males, each of them bears a copulatory organ called clasper. In between the two pelvic fins, is a cloacal aperture, a common passage for an alimentary canal and urinary and genital ducts. Running laterally from head to tail is a lateral line on each side. Beneath this, is a lateral line canal, which contains the lateral line sense organs. In the head region, this canal undergoes extensive branching.-Wholly cartilaginous endoskeleton, true bones being altogether absent.

Additional Information: - Exoskeleton consists of dermal denticles or placoid scales which probably represent the remnants of ancestral bony armor.
-Jaw suspension is hyostylic or Amphystylic. In the former, jaws are suspended from cranium through hyoid, and in the latter through hyoid and quadrate.
-Hyomandibular gill-cleft is reduced to a spiracle.

Note: The main features of Chondrichthyes include;-Gills are lamelliform, gill-slits are usually five pairs, and operculum is absent.
-Mouth and external nostrils are ventral. Internal nostrils are absent. Lungs and gas bladder are absent.
-Cloaca present. The copulatory organs claspers are present in male. Tail is Heterocercal.