Give one point of difference between:

i) Bony and cartilaginous fishes.

(ii) Bilateral and radial symmetry.

(iii) Notochord and nerve cord.

(i) Bony and cartilaginous fishes.
Body of a bony is covered with cycloid or ctenoid scales whereas skin of cartilaginous fish is covered with Placoid scales.

(ii) Bilateral and radial symmetry.
An animal is said to have bilateral symmetry, if it has two equal but opposite right and left halves when it is cut lengthwise in the middle vertical plane. For example, frog.

An animal is said to have radial symmetry, if it is symmetrical with respect to any plane passing through its longitudinal axis. Any plane passing longitudinally through any diameter divides the body into equal halves. For eample, Hydra shows a radial symmetry.

(iii) Notochord and nerve cord.
Notochord is skeletal rod (of large vacuolated cells packed within a firm sheath). It lies lengthwise between C.N.S. (central nervous system) and gut. It is present in some stage of development of all chordates. In adult vertebrates, it is replaced by vertebrate (vertebral column).

A nerve cord is a solid strand of nervous tissue, forming part of central nervous system, especially of invertebrates. Nerve cells are distributed all over the nerve cord and are not confined to ganglionic swellings.

The major difference between the two is that—Notochord is part of skeleton system (so known as skeletar rod) where Nerve Cord is part of nervous system.