Hemichordata Classification — NEET Biology

Correct answer: B — S1, S3 and S5 only

The core concept tested involves the classification rationale of Hemichordata, distinguishing features from Chordata, and characteristics of Chordate subphyla. S1: Correct. The NCERT states, 'Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called stomochord, a structure similar to notochord.' The lack of a true notochord is the key reason for its non-chordate placement. S2: Incorrect. While pharyngeal gill slits are a fundamental chordate characteristic, Hemichordates also possess gills for respiration (NCERT: 'Respiration takes place through gills.'), and thus the characteristic is not 'exclusively' observed in Chordates. Furthermore, the source's Q3 implied Hemichordates possess pharyngeal gill slits. S3: Correct. The NCERT states regarding notochord, 'Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.' For Cephalochordata, it states, 'notochord ... extends from head to tail region and is persistent throughout their life.' Branchiostoma is an example of Cephalochordata. S4: Incorrect. The stomochord is a rudimentary, notochord-like structure for support, not an excretory organ. The excretory organ in Hemichordates is the proboscis gland. Nephridia are excretory organs in Annelids. S5: Correct. The NCERT states, 'Subphyla Urochordata and Cephalochordata are often referred to as protochordates ... and are exclusively marine.' In contrast, vertebrates are found in a variety of habitats (marine, fresh water, terrestrial). Therefore, statements S1, S3, and S5 are correct. The final answer is B.