Animals which are marine, bilaterally symmetrical, have a coelom and a notochord, but never form a vertebral column are placed under which one of the following groups ?

Key Points

• Animals belonging to the phylum Chordata are basically characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. 
• These are bilaterally symmetrical, triploblastic, coelomate with the organ-system level of organization.
• The notochord is a flexible, rod-shaped structure that is found in the embryonic stage of all chordates. It is located between the digestive tube and the nerve cord and provides support.
• The blood vascular system is present and it is closed type.
• They possess a post-anal tail and a closed circulatory system.
• Common Examples are- Branchiostoma, Ascidia, Lizards.

Explanation:

Phylum Chordata can be divided into 3 sub-phyla -

1. Urochordata - 
   • Notochord is present only in larval tail
   • Examples - Ascidia, Doliolum.
2. Cephalochordata - 
   • Notochord extends from head to tail and is present throughout life.
   • Example - Branchiostoma.
3. Vertebrata - 
   • Notochord is present in the embryonic stage and is replaced by a vertebral column in adult.
   • Examples - Fishes, amphibians, reptiles, birds, mammals.

• Urochordata and Cephalochordata are together known as Protochordata and they are exclusively marine animals.
• Therefore, chordates except the vertebrates will have all the given features, which indicates the Protochordates.

Therefore, the correct answer is Protochordata.