Which one of the following organisms does not possess notochord at any stage of its life cycle?

The notochord is a defining characteristic of the phylum Chordata, present at some stage in their life cycle. Herdmania (a tunicate) belongs to the subphylum Urochordata, where the notochord is present in the larval tail but disappears in the sedentary adult. Exocoetus (flying fish) and Salamanders are vertebrates; in these organisms, the embryonic notochord is replaced by a bony or cartilaginous vertebral column in adults. In contrast, Holothuria (sea cucumber) belongs to the phylum Echinodermata [1]. Echinoderms are non-chordate deuterostomes belonging to the clade Ambulacraria. They lack a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits at all stages of their development, including larval and adult phases. Therefore, Holothuria is the only organism among the options that never possesses a notochord.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Echinoderms > p. 155