NCERT grounding
This subtopic sits inside section 4.2.11 Phylum – Chordata of the Class 11 NCERT Biology chapter Animal Kingdom. NCERT states that animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits, and that they additionally possess a post-anal tail and a closed circulatory system. The text then makes the central organising statement of this page: "Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata."
NCERT groups Urochordata and Cephalochordata together as protochordates, describes them as exclusively marine, and contrasts the fate of the notochord in each subphylum. It then justifies why subphylum Vertebrata is separated out, with the famous line that all vertebrates are chordates but all chordates are not vertebrates. The NIOS supplement on Kingdom Animalia reinforces the same three-subphylum division and confirms that the notochord is a structure "found in embryonic stage or adults of some animals".
"In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life."
NCERT Class 11 Biology · Section 4.2.11
Subphyla of Phylum Chordata
Phylum Chordata is the most advanced phylum in the animal kingdom. Its members are bilaterally symmetrical, triploblastic and coelomate, and they show an organ-system level of organisation. Before any chordate can be slotted into a subphylum, it must first qualify as a chordate by displaying the diagnostic chordate features at some stage of life. The internal subdivision of the phylum then turns almost entirely on a single question: what happens to the notochord, and how far through the body does it run.
NCERT recognises exactly three subphyla. Urochordata (also spelt and named Tunicata) and Cephalochordata are the two protochordate subphyla; both are exclusively marine, both lack a vertebral column, and both keep the notochord rather than replacing it. Vertebrata is the third and largest subphylum, distinguished by the replacement of the embryonic notochord with a cartilaginous or bony vertebral column in the adult. This three-way split — and the notochord logic behind it — is the single most examined idea on this page.
The notochord rule. Each subphylum is defined by where the notochord is found and how long it lasts. Memorise the three positions and the subphylum names follow automatically.
Urochordata
Larval tail
notochord location
Also called: Tunicata
Status: protochordate, exclusively marine
Examples: Ascidia, Salpa, Doliolum
Cephalochordata
Head to tail
notochord location
Persistence: throughout life
Status: protochordate, exclusively marine
Example: Branchiostoma (Amphioxus)
Vertebrata
Embryo only
notochord location
Replaced by: vertebral column in adult
Status: vertebrate, not protochordate
Includes: fishes, amphibia, reptilia, aves, mammals
The four fundamental chordate features
Every chordate, regardless of subphylum, must show four diagnostic structures at some point in its life cycle. NCERT names three of them as the fundamental characters — the notochord, the dorsal hollow nerve cord and the paired pharyngeal gill slits — and adds two further shared chordate traits: a post-anal tail and a closed circulatory system. These features are the entry ticket into the phylum; the subphyla are only meaningful once an animal has already cleared this checklist.
The notochord is a mesodermally derived, rod-like structure formed on the dorsal side during embryonic development. The dorsal hollow nerve cord is single — a point NCERT stresses, because non-chordates have a ventral, solid and double nerve cord. The pharyngeal gill slits are paired perforations in the wall of the pharynx. The post-anal tail is the part of the body that extends behind the anus. Finally, the chordate heart is ventral and the circulatory system is closed, in contrast to the dorsal heart of non-chordates.
Figure 1. The generalised chordate body plan. The notochord runs along the dorsal side, the single hollow nerve cord lies above it, the pharynx is perforated by paired gill slits, the tail extends beyond the anus, and the heart is ventral — the mirror image of the dorsal heart seen in non-chordates.
Subphylum Urochordata (Tunicata)
Urochordata, also called Tunicata, is the first protochordate subphylum. Its members are exclusively marine. The defining feature, and the one NEET tests repeatedly, is that the notochord is present only in the larval tail — the prefix uro- means tail. The free-swimming, tadpole-like larva shows all the chordate characters, but during metamorphosis into the sessile, bag-like adult the tail and its notochord are lost. The adult therefore looks distinctly non-chordate, which is exactly why this subphylum is a favourite trap.
NCERT lists three standard examples of Urochordata: Ascidia, Salpa and Doliolum. The accompanying NCERT figure of Ascidia shows the adult tunicate form. Because the chordate features appear only transiently in the larva, Urochordata illustrates the principle that a chordate is defined by what it possesses at some stage of life, not necessarily as an adult.
Subphylum Cephalochordata
Cephalochordata is the second protochordate subphylum and, like Urochordata, is exclusively marine. Here the notochord behaves very differently. It extends from the head to the tail region and is persistent throughout the animal's life. The notochord is never replaced by a vertebral column, so a cephalochordate is a chordate that keeps its notochord permanently yet is not a vertebrate.
The standard NCERT example is Branchiostoma, also known as Amphioxus or the Lancelet. Branchiostoma is a small, fish-like marine animal in which the head-to-tail notochord is clearly visible. Its persistent, full-length notochord makes Cephalochordata the cleanest illustration of a functional notochord retained for life, a point NEET often pairs against Urochordata to test whether students can keep the two protochordate patterns separate.
Figure 2. The notochord (teal) defines the three subphyla. In Urochordata it occupies only the larval tail; in Cephalochordata it runs the full body length for life; in Vertebrata it is an embryonic structure (dashed) that the vertebral column (purple) later replaces.
Subphylum Vertebrata
Vertebrata is the third subphylum and contains every familiar backboned animal. NCERT states that the members of subphylum Vertebrata possess a notochord during the embryonic period; the notochord is then replaced by a cartilaginous or bony vertebral column in the adult. This replacement is the single feature that separates a vertebrate from a protochordate. It also drives the famous conclusion: thus all vertebrates are chordates, but all chordates are not vertebrates.
Besides the basic chordate characters, vertebrates have a set of additional features that NCERT lists explicitly: a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages which may be fins or limbs. The subphylum Vertebrata is itself divided further — into the jawless division Agnatha (class Cyclostomata) and the jawed division Gnathostomata, which contains the super-class Pisces (Chondrichthyes, Osteichthyes) and the super-class Tetrapoda (Amphibia, Reptilia, Aves, Mammalia).
| Feature | Urochordata | Cephalochordata | Vertebrata |
|---|---|---|---|
| Other name | Tunicata | — | — |
| Group | Protochordate | Protochordate | Vertebrate |
| Habitat | Exclusively marine | Exclusively marine | Aquatic & terrestrial |
| Notochord | Larval tail only | Head to tail, lifelong | Embryonic period only |
| Vertebral column | Absent | Absent | Replaces notochord in adult |
| Examples | Ascidia, Salpa, Doliolum | Branchiostoma | Fishes, frogs, birds, mammals |
Protochordates versus vertebrates
The phylum splits, at the highest level, into protochordates and vertebrates. Protochordata is not a formal subphylum but a convenient grouping of the two subphyla — Urochordata and Cephalochordata — that NCERT describes as exclusively marine and lacking a vertebral column. They retain the notochord rather than replacing it. Vertebrata, by contrast, is a true subphylum whose members convert the embryonic notochord into a vertebral column and add a chambered ventral heart, kidneys and paired appendages.
Protochordates
2 subphyla
Urochordata + Cephalochordata
- Exclusively marine
- No vertebral column at any stage
- Notochord retained, not replaced
- Examples: Ascidia, Salpa, Doliolum, Branchiostoma
Vertebrates
1 subphylum
Vertebrata
- Aquatic and terrestrial habitats
- Vertebral column replaces embryonic notochord
- Ventral 2-, 3- or 4-chambered heart, kidneys
- Paired appendages as fins or limbs
The relationship is best stated as a set inclusion. Every vertebrate possesses the chordate features at some stage, so the set of vertebrates lies entirely inside the set of chordates. But the chordate set is larger: it also contains the protochordates, which never form a vertebral column. This is precisely why NCERT and NEET both insist that the statement "all vertebrates are chordates but all chordates are not vertebrates" is correct, with the replacement of the notochord by the vertebral column as its correct explanation.
Placing an animal within Phylum Chordata
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Step 1
Confirm chordate status
Check for notochord, dorsal hollow nerve cord and pharyngeal gill slits at some stage.
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Step 2
Vertebral column?
If notochord is replaced by a vertebral column in the adult, the animal is in Vertebrata.
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Step 3
Notochord in tail only?
No vertebral column and notochord confined to the larval tail places it in Urochordata.
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Step 4
Notochord head to tail?
No vertebral column and a lifelong head-to-tail notochord places it in Cephalochordata.
Subphyla of Chordata
Urochordata, Cephalochordata and Vertebrata. The first two are protochordates and exclusively marine; the third is the vertebrate subphylum. Hemichordata is not one of them — it is now a separate phylum under non-chordata.
Worked examples
In which subphylum of Chordata does the notochord persist from head to tail throughout the animal's life?
The answer is Cephalochordata. NCERT states that in Cephalochordata the notochord extends from head to tail region and is persistent throughout life — Branchiostoma (Amphioxus) is the example. In Urochordata the notochord is restricted to the larval tail, and in Vertebrata it is present only during the embryonic period before being replaced by a vertebral column.
Why is Hemichordata not treated as a subphylum of Chordata?
NCERT explains that Hemichordata was earlier considered a sub-phylum under phylum Chordata, but is now placed as a separate phylum under non-chordata. Hemichordates possess only a rudimentary structure in the collar region called the stomochord — a structure merely similar to a notochord, not a true notochord. Because a true notochord is absent, hemichordates fail the chordate test and the three subphyla of Chordata remain Urochordata, Cephalochordata and Vertebrata only.
Classify Salpa, Branchiostoma and a frog into their chordate subphyla.
Salpa belongs to Urochordata, alongside Ascidia and Doliolum. Branchiostoma (Amphioxus or Lancelet) belongs to Cephalochordata. A frog is a backboned tetrapod and belongs to Vertebrata. Salpa and Branchiostoma are protochordates and exclusively marine; the frog is a vertebrate in which the notochord has been replaced by a vertebral column.
List the additional features that distinguish a vertebrate from a protochordate.
According to NCERT, besides the basic chordate characters, vertebrates possess a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages that may be fins or limbs. The defining difference, however, is the replacement of the embryonic notochord by a cartilaginous or bony vertebral column — protochordates never form this column.
Common confusion & NEET traps
The Chordata subphyla are a dense cluster of look-alike facts, and NEET exploits the overlaps year after year. The single most common error is mixing up the notochord fate of Urochordata and Cephalochordata. The 2020 NEET paper did exactly this, offering the statement that in Urochordata the notochord extends from head to tail throughout life — which is false, because that description belongs to Cephalochordata.