Model Answer
0 min readIntroduction
Balanoglossus, a marine hemichordate, occupies a pivotal position in understanding the evolutionary link between invertebrates and chordates. Often referred to as a ‘tongue worm’, it exhibits a unique blend of characteristics that have led to its long-standing consideration as a forerunner of the chordates. The debate surrounding its phylogenetic position stems from the presence of certain structural features, particularly in its larval stage, that bear a striking resemblance to the primitive chordate body plan. This essay will justify the statement that Balanoglossus is the forerunner of chordates by elaborating on these shared characteristics and their evolutionary significance.
Balanoglossus: An Overview
Balanoglossus is a burrowing marine animal found in shallow coastal waters. It is a tube-like organism characterized by a proboscis, collar, and trunk. Its body plan, though seemingly simple, reveals several features that are crucial in understanding its evolutionary relationship with chordates.
Features Suggesting Chordate Ancestry
1. Similarities in Early Embryonic Development
The early embryonic development of Balanoglossus exhibits remarkable similarities to that of chordates. Specifically, the blastocoel, the fluid-filled cavity formed during cleavage, is quite prominent. Furthermore, the fate map of early blastomeres shows a degree of correspondence with the development of chordate structures.
2. Presence of a Notocord-like Structure
The most compelling evidence supporting Balanoglossus’s role as a chordate forerunner is the presence of a ‘stomochord’. This is a hollow, rod-like structure projecting into the buccal diverticulum. While not homologous to the true notochord of chordates (being of endodermal origin rather than mesodermal), it provides skeletal support and is functionally analogous. The stomochord is considered a precursor to the notochord, demonstrating a primitive form of axial support.
3. Pharyngeal Slits and Gill Clefts
Balanoglossus possesses numerous gill slits in its pharynx, used for filter feeding. These slits are formed by the breakdown of the pharyngeal wall and are structurally similar to the gill slits found in chordate larvae. In chordates, these slits are modified during development into various structures, including the jaws and middle ear. The presence of these slits in Balanoglossus suggests a common ancestral origin.
4. Dorsal Tubular Nerve Cord – Precursor
Although not a fully developed nerve cord, Balanoglossus exhibits a dorsal nerve cord that runs along the length of its body. This structure, while simpler than the chordate nerve cord, represents a precursor to the central nervous system found in chordates. It is located dorsally, a key characteristic of chordates.
5. Post-Anal Tail – Rudimentary Form
Balanoglossus exhibits a short, post-anal tail in its larval stage. While not as prominent or muscular as the tail in chordates, it represents a rudimentary form of this characteristic feature. The post-anal tail is important for locomotion in chordates and its presence, even in a primitive form, in Balanoglossus suggests a shared ancestry.
Differences and Limitations
Despite these similarities, it’s crucial to acknowledge the significant differences between Balanoglossus and chordates.
- Origin of the ‘Notocord’: The stomochord is endodermal, while the true notochord is mesodermal.
- Circulatory System: Balanoglossus has a diffuse circulatory system, unlike the closed circulatory system of chordates.
- Lack of Cranium and Vertebral Column: Balanoglossus lacks the defining skeletal features of vertebrates.
These differences suggest that Balanoglossus is not a direct ancestor of chordates but rather represents a side branch in the evolution of deuterostomes, sharing a common ancestor with chordates. It showcases features that were likely present in the ancestral chordate.
Phylogenetic Position
Modern phylogenetic analyses, based on molecular data, suggest that hemichordates (including Balanoglossus) and echinoderms form a clade called the deuterostomia, which is sister to the chordates. This indicates a shared evolutionary history, but not a direct ancestor-descendant relationship.
Conclusion
In conclusion, while Balanoglossus is not a direct ancestor of chordates, its anatomical and embryological features provide valuable insights into the evolutionary origins of chordate characteristics. The presence of a stomochord, pharyngeal slits, a dorsal nerve cord precursor, and a rudimentary post-anal tail in its larval stage strongly suggest that it represents a crucial link in understanding the transition from invertebrate to vertebrate life. It serves as a living model demonstrating the primitive features that were likely present in the earliest chordate ancestors, solidifying its position as a significant forerunner in the evolutionary history of chordates.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.