Question

Chondrichthyes can be differentiated from osteichthyes externally by:
A. Naked gills
B. Heterocercal tail
C. Ventral mouth
D. All of above

Answer 1

Hint: The subphylum Vertebrata is divided into Agnatha and Gnathostomata. Agnatha superclass consists of the organisms that do not bear jaws, Hagfishes and Lampreys are its good examples whereas Gnathostomata consist of organisms that bear true jaws. It is divided into superclass Pisces and Tetrapoda which are further divided into subgroups.

Complete answer:
Pisces are the fishes characterized by the presence of scales/plates. These are cold blooded organisms with two chambered hearts and are oviparous. Some of these have skeletons made of cartilage (classified as chondrichthyes) such as shark, dog fish and stingrays. Others have their skeleton made of both bones and cartilage (named as osteichthyes) like flying fish, seahorse.
The major differences between chondrichthyes and osteichthyes depending on their external features are:
> The chondrichthyes have gills that are exposed to the environment whereas osteichthyes have gills covered with operculum.
> Placoid scales are present on the body of chondrichthyes and cycloid scales are on osteichthyes.
> Mouth is present on the ventral side of the head in chondrichthyes and at the dorsal side of head in osteichthyes.
> Chondrichthyes have heterocercal tails and osteichthyes possess homocercal tail fins.

Therefore, all of the given alternatives are true for the chondrichthyes different from osteichthyes. Thus, the correct answer is option d- all of these.

Note: The Homocercal is a type of tail in which the last vertebra adjoins the bony parts which support the fins in such a way that they appear to be symmetrical. The Heterocercal fins are made of two asymmetrical fins.