Branchiostoma has:
(A) Exoskeleton
(B) Gonoduct
(C) Heart
(D) Colorless blood

Question

Branchiostoma has: 
(A) Exoskeleton 
(B) Gonoduct 
(C) Heart 
(D) Colorless blood

Accepted Answer

The correct option is 4: Only (D)

Branchiostoma, commonly known as Amphioxus or the lancelet, is a fascinating organism that belongs to the subphylum Cephalochordata, making it one of the simplest living members of the phylum Chordata. The characteristic of having **colorless blood (D)** is fundamentally correct and crucial to understanding its primitive physiology. Unlike vertebrates, whose blood is typically red due to the presence of the iron-containing pigment hemoglobin, *Branchiostoma* blood entirely **lacks any respiratory pigment** such as hemoglobin or hemocyanin. Because there is no respiratory pigment, the blood is essentially a clear, watery plasma, thus appearing colorless. Oxygen transport in this small, thin animal is primarily achieved through **simple diffusion** across the general body surface and the extensive surface area of the pharyngeal gill slits, rather than being carried by the blood itself.

The incorrect option is 1: Only (A)

This option suggests that *Branchiostoma* possesses an **exoskeleton (A)**, which is incorrect. An exoskeleton is a rigid external covering, typical of arthropods (like insects or crabs) or some mollusks, for support and protection. As an invertebrate chordate, *Branchiostoma* has a supportive structure—the **notochord**—which runs the length of its body, but this is an internal rod, not an external skeleton. It entirely lacks a true skeleton, whether external (exo-) or the complete, bony internal skeleton (endo-) characteristic of vertebrates, reflecting its basal position on the chordate evolutionary tree.

The incorrect option is 2: Only (A) and (B)

This option is incorrect because both statements (A) Exoskeleton and (B) Gonoduct are false. As established, it lacks an exoskeleton. Furthermore, it **does not possess specialized gonoducts (B)** for the release of gametes. *Branchiostoma* has numerous segmented gonads (sex organs) arranged along the body wall. When the gonads are mature, the gametes (sperm and eggs) are released when the gonad wall ruptures, and they pass into the atrial cavity before exiting the body through the atriopore. The absence of specific gonadal ducts is a feature of its simple, primitive body plan, distinguishing it from most higher animals.

The incorrect option is 3: Only (C) and (D)

While having colorless blood (D) is correct, this option is incorrect because the inclusion of a **heart (C)** makes the pairing false. *Branchiostoma* has a well-defined, closed circulatory system with blood vessels but notably **lacks a true, chambered heart** to pump the blood. Instead, the circulation is driven by the rhythmic, peristaltic contractions of the main blood vessels, particularly the ventral aorta and the bases of the gill-arch arteries. The absence of a centralized heart is another key feature that places it within the invertebrate chordates, separate from the vertebrates.