Air bladder is a gas filled sac found in fishes. It is of great importance as it helps in maintaining buoyancy and helps them to ascend or descend and stay still in the water current. It is importance but there are organisms like Chondrichthyes which do not possess air bladder. Thus, the presence of the air bladder is not necessary in Pisces.

Parasitic platyhelminthes have the following peculiar features:
1. They have dorsiventrally flattened body and bear hooks and suckers to attach inside the body of the host. They can absorb nutrition from the host directly through the body surface.
2. Their body is covered with thick covering to protect them from the action of digestive juices of the host.

The phylum, Arthropoda, consists of more than two-thirds of the animal species on earth. The reasons for the success of arthropods are as follows.
(i) Jointed legs that allow more mobility on land. 
(ii) Hard exoskeleton made of chitin that protects the body from the harsh conditions. 
(iii) The hard exoskeleton also reduces water loss from the body making them more adapted to terrestrial conditions. 
(iv) The fertilisation is internal which provides protection to the progeny from the external harsh conditions.

Chordates are those animals which have a  notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Chordates are sub classified into four other groups of which vertebrate is one subphyla.
However, Vertebrates are characterized by the presence of the notochord in the embryonic stage, which gets replaced by a cartilaginous or bony vertebral column in adults.
Since the notochord is not replaced by the vertebral column in all the chordates, thus, it can be said that all vertebrates are chordates but all chordates are not vertebrates.

C.

Echinodermata

Echinodermata it consists of an array of radiating channels, tube feet. The water vascular system helps in locomotion, food capturing and respiration.