Model Answer
0 min readIntroduction
Larval development is a crucial stage in the life cycle of many marine invertebrates, often exhibiting distinct morphologies adapted for dispersal and feeding. The tornaria larva, characteristic of enteropneust hemichordates like *Balanoglossus*, and the bipinnaria larva, found in echinoderms such as starfish, represent fascinating examples of such adaptations. While both are planktonic, bilaterally symmetrical larvae, they differ significantly in their structural organization and developmental pathways. Understanding these differences, alongside the unique features of Hemichordata, is essential for appreciating the evolutionary relationships within the deuterostome lineage.
Tornaria Larva of *Balanoglossus*
The tornaria larva is a free-swimming, bilaterally symmetrical larva of enteropneust hemichordates. It is characterized by:
- Body Plan: An elongated, ciliated body divided into three distinct regions – pre-oral lobe, trunk, and post-oral lobe.
- Ciliation: Possesses a prominent band of cilia around the anterior end (pre-oral lobe) used for locomotion and food collection.
- Alimentary Canal: A complete digestive system with a mouth and anus.
- Proboscis Sac: A characteristic feature is the presence of a proboscis sac, a precursor to the adult proboscis, which develops as an evagination from the anterior end.
- Coelomic Cavities: Three coelomic cavities are present – protocoel, mesocoel, and metacoel – which are precursors to the adult coelomic spaces.
Bipinnaria Larva of Echinoderms
The bipinnaria larva is a planktonic larva of echinoderms (starfish, sea urchins, brittle stars, etc.). It exhibits the following features:
- Body Plan: A bilaterally symmetrical, flattened, and elongated body.
- Arms: Possesses two or more arms extending from the body, bearing ciliated bands for locomotion and feeding.
- Ciliation: Ciliated bands run along the arms and around the body, creating a current to draw in food particles.
- Alimentary Canal: A simple gut with a mouth and anus.
- Hydrocoel: A unique feature is the presence of a hydrocoel, a fluid-filled cavity within each arm, which is a precursor to the water vascular system of adult echinoderms.
Comparative Analysis: Tornaria vs. Bipinnaria
The following table summarizes the key differences between the tornaria and bipinnaria larvae:
| Feature | Tornaria Larva (*Balanoglossus*) | Bipinnaria Larva (Echinoderms) |
|---|---|---|
| Body Shape | Elongated, three-lobed | Flattened, elongated with arms |
| Ciliation | Prominent pre-oral ciliary band | Ciliated bands on arms and body |
| Proboscis Sac | Present | Absent |
| Hydrocoel | Absent | Present (precursor to water vascular system) |
| Coelomic Cavities | Three (protocoel, mesocoel, metacoel) | Less defined coelomic compartments |
| Adult Relationship | Develops into the proboscis and collar of *Balanoglossus* | Develops into the pentaradial body plan of echinoderms |
Reasons for Hemichordata as a Distinct Phylum
Despite sharing some superficial similarities with chordates (e.g., presence of a stomochord, which was initially mistaken for a notochord), Hemichordata is now recognized as a distinct phylum based on several key characteristics:
- Stomochord vs. Notochord: The stomochord is a hollow, buccal diverticulum and is not homologous to the chordate notochord, which is derived from the mesoderm.
- Absence of True Vertebral Column: Hemichordates lack a true vertebral column, a defining feature of chordates.
- Unique Body Plan: The tripartite body plan (proboscis, collar, trunk) is unique to hemichordates and differs significantly from the chordate body plan.
- Developmental Differences: The embryonic development of hemichordates, particularly the formation of the coelomic cavities and the absence of a dorsal hollow nerve cord, distinguishes them from chordates.
- Molecular Evidence: Molecular phylogenetic studies, based on ribosomal RNA and other gene sequences, consistently place Hemichordata as a sister group to Echinodermata within the Deuterostomia, separate from Chordata.
The discovery and detailed study of the tornaria larva further solidified Hemichordata’s distinct phylogenetic position, revealing unique developmental features not found in chordates.
Conclusion
In conclusion, the tornaria and bipinnaria larvae, while both planktonic and bilaterally symmetrical, exhibit significant morphological and developmental differences reflecting their distinct evolutionary lineages. The unique features of Hemichordata, including the stomochord, tripartite body plan, and distinct developmental pathways, coupled with molecular evidence, firmly establish its status as a separate phylum within the Deuterostomia. Understanding these larval forms and the phylogenetic relationships they illuminate is crucial for a comprehensive understanding of invertebrate evolution.
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.