(a) Chondrichthyes have cartilaginous endoskeleton and hence they are also called cartilaginous fish. Their skin is covered with placoid scales. Air bladder is absent in them.
Examples: Scoliodon, Pristis etc. Osteichthyes have bony endoskeleton and hence they are also called bony fishes. Their skin is covered with cycloid scales. Air bladder is present in them. Example; Rohu, Catla, etc.

(b) In Urochordata, notochord is present only in the larval tail in larval stage. In Cephalochordata, notochord extends from head to tail region and is present throughout their life. Ascidia and Salpa are examples of urochordate.
Branchiostoma is an example of cephalochordates. Both of these are referred to as protochordates. These are exclusively marine animals.