Question

Which class of vertebrates is characterized by the presence of a cartilaginous skeleton?

• A. Osteichthyes
• B. Chondrichthyes
• C. Amphibia
• D. Reptilia

Answer 1

Answer:

B. Chondrichthyes

Understanding Chondrichthyes (Cartilaginous Fishes)

• Chondrichthyes is a class of fish that includes sharks, rays, skates, and chimaeras. The name itself, Chondrichthyes, is derived from Greek words: 'chondros' meaning cartilage and 'ichthyes' meaning fish, directly referring to their primary skeletal composition.
• The most distinguishing characteristic of this class is their entirely cartilaginous skeleton, which is strong yet flexible, unlike the ossified (bony) skeletons found in most other vertebrates.

• Key Features of Chondrichthyes for Competitive Exams:

  • Skeletal Structure: Their skeleton is composed of cartilage, a tissue that is lighter and more flexible than bone. This adaptation contributes to their agility and buoyancy in water.
  • Placoid Scales (Dermal Denticles): Their skin is covered by unique, tooth-like placoid scales, which give them a rough, sandpaper-like texture. These scales are structurally similar to teeth.
  • Gills: They typically possess 5 to 7 pairs of gill slits that are exposed directly to the exterior, unlike bony fish which have gills covered by an operculum.
  • Mouth Position: Their mouth is usually located on the ventral (underside) side of the head.
  • Paired Fins: They have well-developed paired fins (pectoral and pelvic fins) that aid in stability and maneuverability.
  • Buoyancy: Unlike bony fish, they lack a swim bladder. Instead, they rely on a large, oil-rich liver (which is less dense than water) and continuous swimming to maintain buoyancy and prevent sinking.
  • Reproduction: Fertilization is internal. They can be oviparous (egg-laying), ovoviviparous (eggs hatch inside the mother, live birth), or viviparous (live birth with placental connection).
  • Sense Organs: They possess highly developed sensory systems, including:
    • Lateral Line System: Detects vibrations and water movements.
    • Ampullae of Lorenzini: Specialized electroreceptors that detect weak electrical fields, crucial for locating prey.
  • Tail Fin (Caudal Fin): They typically have a heterocercal tail, where the vertebral column extends into the larger upper lobe, providing powerful propulsion.

• Comparison with Osteichthyes (Bony Fishes):

  • In contrast to Chondrichthyes, the class Osteichthyes (bony fishes) possesses a skeleton primarily made of bone. This group includes the vast majority of fish species found today.
  • Osteichthyes typically have a swim bladder for buoyancy control and an operculum covering their gills.
• Examples: Prominent examples of Chondrichthyes include various species of sharks (e.g., Great White Shark, Whale Shark - the largest known fish), rays (e.g., Manta Ray), skates, and chimaeras (often called ghost sharks).