The distinct chordate affinity found in acorn worms (hemichordates) among the given options is the presence of pharyngeal gill slits.

Correct option: Pharyngeal gill slits

Acorn worms possess a pharynx that opens to the exterior through multiple paired gill slits on the lateral sides of the body. These structures are true pharyngeal gill slits, which are also one of the key diagnostic features of chordates at some stage of their life cycle.

Hemichordates (including acorn worms) are evolutionarily close to chordates, and they share pharyngeal gill slits as a strong chordate affinity, along with an endostyle‑like structure. This shared character is one of the reasons acorn worms are extensively studied to understand chordate origins.

Hence, the correct answer is: presence of pharyngeal gill slits.

---

Why other options are incorrect

Presence of post-anal tail

A post-anal tail is a hallmark chordate character, present at least in the embryonic stage of all chordates. Interestingly, some hemichordates (for example juvenile members of the family Harrimaniidae) do show a ventral post‑anal tail, and this has been discussed as homologous to the chordate post‑anal tail.

However, in acorn worms this trait is:

• Not universal across all groups.

• Not as clear and consistent a chordate affinity as pharyngeal gill slits, which are widespread and functionally prominent.

For exam purposes (CSIR/NEET/NET style MCQs), pharyngeal gill slits are treated as the classical, well‑established chordate affinity of acorn worms, so this option is not considered the best answer.

Radially symmetrical body plan

Chordates and hemichordates are both bilaterally symmetrical animals, not radial. Acorn worms have an elongated body with distinct anterior and posterior ends and left‑right symmetry, fitting the bilateral plan.

Radial symmetry is seen in groups like cnidarians and echinoderms (adult stage), not in acorn worms or chordates. Therefore, radially symmetrical body plan is incorrect.

Ventral hollow nerve cord

Chordates are characterized by a dorsal hollow nerve cord, not a ventral hollow one. Hemichordates such as acorn worms possess a diffuse nerve plexus with both dorsal and ventral nerve cords, and the dorsal cord can form a partially hollow tube in the collar region, somewhat reminiscent of a chordate neural tube.

However:

• Their main nervous system is not a single dorsal hollow cord like in chordates.

• The ventral cord is not hollow and is not homologous to the chordate dorsal hollow nerve cord.

Thus, ventral hollow nerve cord is anatomically and phylogenetically incorrect as a chordate affinity.

---

Introduction: acorn worms and chordate evolution

Acorn worms (hemichordates) occupy a crucial phylogenetic position as close relatives of chordates, making them ideal models for exploring how defining chordate traits evolved. They share some features with chordates, especially pharyngeal gill slits, while lacking others such as a notochord and a fully developed dorsal hollow nerve cord, which helps biologists infer the stepwise origin of chordate characteristics.

---

Key chordate characters and acorn worms

Chordates are defined by four hallmark features that appear at least once during development: notochord, dorsal hollow nerve cord, pharyngeal slits, and post‑anal tail. Hemichordates, in contrast, exhibit pharyngeal gill slits, an endostyle‑like organ, and in some cases a post‑anal tail, but they lack a true notochord and a typical dorsal tubular nerve cord.

Among these shared elements, pharyngeal gill slits are the clearest and most consistent chordate affinity in acorn worms, both structurally and functionally.

---

Option-by-option explanation

1. Presence of pharyngeal gill slits (correct)

Acorn worms possess an anterior pharynx perforated by numerous paired gill slits that open laterally, allowing water to exit after passing through the pharyngeal region. This arrangement closely parallels the pharyngeal slits of early chordates, which function in feeding and respiration and later give rise to diverse structures in vertebrates.

Because pharyngeal gill slits are both a fundamental chordate character and a prominent, well‑developed feature in hemichordates, they are recognized as the most important chordate affinity of acorn worms in standard zoology and comparative anatomy texts.

2. Presence of post-anal tail

The post‑anal tail is a defining chordate character, extending the body axis beyond the anus and present at least in embryonic stages. Some hemichordate acorn worms, especially juvenile harrimaniids, bear a ventral post‑anal tail, and developmental gene expression suggests possible homology with the chordate tail.

Despite this, the post‑anal tail in hemichordates:

• Is not universal across all acorn worms.

• Is less emphasized than pharyngeal gill slits as a classical diagnostic affinity in exam‑oriented zoology.

Therefore, although interesting evolutionarily, it is not the best single answer when asked for a distinct chordate affinity in acorn worms.

3. Radially symmetrical body plan

Acorn worms have a bilaterally symmetrical body, organized along an anterior–posterior axis with left–right mirror halves, consistent with bilaterian animals. Their body is divided into proboscis, collar, and trunk, but it does not show radial symmetry like cnidarians or adult echinoderms.

Chordates are also bilaterally symmetrical, so a radially symmetrical body plan is neither characteristic of acorn worms nor of chordates and cannot represent a chordate affinity.

4. Ventral hollow nerve cord

Chordates are defined by a dorsal hollow nerve cord, which later forms the central nervous system in vertebrates. Acorn worms instead possess a diffuse epidermal nerve plexus with longitudinal dorsal and ventral nerve cords; in the collar region the dorsal cord can become internalized and partially hollow, somewhat resembling a neural tube.

However:

• The main cord that shows hollowing is dorsal, not ventral.

• The ventral nerve cord in hemichordates is not hollow and is not homologous to the chordate dorsal tubular cord.

Thus, ventral hollow nerve cord is anatomically wrong and cannot be considered a chordate affinity.

---

Exam takeaway

Hemichordates and chordate origins, the safest and most widely accepted answer for “distinct chordate affinity found in acorn worms” is presence of pharyngeal gill slits.