Question

Chondrichthyes is characterised by
A) Ventral mouth
B) Placoid scales
C) Ventral mouth and ctenoid scales
D) Both A. and B.

Answer 1

Fishes are of two types-bony fishes or osteichthyes,cartilaginous fishes or Chondrichthyes.

Complete answer:
Chondrichthyes are a group of cartilaginous fishes that include different types of sharks, skates, rays, chimaeras etc. The other class is osteichthyes, or bony fishes.
Chondrichthyes have jaws, vertebrates, paired fins, scales, a two chambered heart and a cartilaginous skeleton. These are a few of the characters of Chondrichthian fishes:-

They have paired fins and nostrils.
They have scales that are toothlike called dermal denticles or placoid scales.
Placoid scales provide protection and streamlining.
Mucous glands exist in some species, as well.
They have paired appendages too.
A well developed electroreceptive system is also found in Cartilaginous fishes.
Their endoskeleton is completely made up of cartilage.
Lungs or swim bladders are absent.
The mouth, nostrils and gill slits of Chondrichthyes are always found on the underside or ventral side of the fish.
The eyes and spiracles are on the upper side.
groups of fishes are exclusively marine.
Operculum is also absent in them
Their nature is predatory.
The male fishes have claspers on their pelvic fins.
Many species also have spines and thorns on the upper surface and tail.

Hence the correct answer is option (D.) Both A. and B.

Note: Cartilaginous fishes or Chondrichthyes can be further classified into two groups, namely, Elasmobranchii and Holocephali. Examples of Chondrichthyes are sharks, bateoids, chimaera, stingray, holocephali etc.