Explanation

Holothuria (sea cucumber) is an echinoderm, and echinoderms are non-chordates that never develop a notochord at any life stage. Herdmania is a urochordate whose larva possesses a notochord; Exocoetus is a bony fish and Salamander is an amphibian, both vertebrates with a notochord-derived backbone. Hence Holothuria is the organism without a notochord at any stage.