Odd one out: Agnatha

Explanation:
The given options are Urochordata, Cephalochordata, Vertebrata, and Agnatha. We need to identify the odd one out among these options.

- Urochordata: Urochordata, also known as tunicates or sea squirts, are marine invertebrates. They are characterized by a sac-like body enclosed in a tough outer covering called a tunic. Urochordates exhibit a notochord during their larval stage but lack it in their adult stage. They filter-feed by drawing water in through an incurrent siphon and expelling it through an excurrent siphon. Urochordates are considered to be the closest relatives of vertebrates.

- Cephalochordata: Cephalochordates, also known as lancelets or amphioxus, are small, fish-like marine animals. They have a slender, elongated body with a notochord that extends from the tip of the head to the posterior end. Cephalochordates are filter-feeders, using their oral hood to capture food particles from the water.

- Vertebrata: Vertebrates are a diverse group of animals that possess a backbone or vertebral column. They are characterized by bilateral symmetry, a segmented body plan, and a well-developed head. Vertebrates include fishes, amphibians, reptiles, birds, and mammals. They exhibit various adaptations for locomotion, respiration, reproduction, and feeding.

- Agnatha: Agnatha, also known as jawless fishes, are a group of primitive fishes that lack jaws. They include lampreys and hagfishes. Agnathans have a cartilaginous skeleton, paired fins, and a notochord that persists throughout their life. They are typically parasitic or scavengers, feeding on the blood or tissues of other animals.

The odd one out among the given options is Agnatha (jawless fishes) because it is the only group that does not belong to the subphylum Vertebrata. Urochordata, Cephalochordata, and Vertebrata are all subphyla of the phylum Chordata, while Agnatha is a superclass within the subphylum Vertebrata.