Model Answer
0 min readIntroduction
Branchiostoma (also known as Amphioxus) is a cephalochordate, representing a crucial link in understanding the evolution of vertebrates. Its simple yet functional body plan exhibits key chordate characteristics. The feeding and digestive system of Branchiostoma are remarkably adapted to its filter-feeding lifestyle. This system efficiently extracts food particles from the water column, demonstrating a unique combination of mechanical and enzymatic processes. Understanding this system provides insights into the early evolution of digestive mechanisms in chordates.
Schematic Diagram of Feeding Current in Branchiostoma
Key: 1. Mouth, 2. Buccal funnel, 3. Oral hood, 4. Wheel organ, 5. Pharynx, 6. Pharyngeal slits, 7. Endostyle, 8. Midgut, 9. Liver, 10. Intestine, 11. Anus.
Mechanism of Feeding
Branchiostoma is a filter feeder, relying on a continuous flow of water through its body for both respiration and food acquisition. The feeding process involves several key steps:
- Water Intake: Water enters the body through the oral siphon (buccal funnel) and is directed into the buccal cavity.
- Mucus Secretion: The oral hood and endostyle secrete copious amounts of mucus, which traps food particles (phytoplankton, zooplankton, and detritus).
- Ciliary Action: Cilia lining the buccal cavity and pharynx generate a current that carries the mucus-laden water towards the pharyngeal slits. The wheel organ, a ciliated structure, further aids in creating the water current and concentrating food particles.
- Filtration: The pharyngeal slits, which are perforations in the pharyngeal wall, act as filters. Water exits the body through the atrial siphon, while food particles are retained in the pharynx.
Mechanism of Digestion
Digestion in Branchiostoma is relatively simple, involving both mechanical and enzymatic processes:
- Pharyngeal Digestion: Initial digestion begins in the pharynx. The endostyle secretes digestive enzymes (proteases, amylases, and lipases) that initiate the breakdown of food particles.
- Intestinal Digestion: The food particles, along with digestive enzymes, pass into the intestine. The liver secretes bile, which aids in the emulsification of fats.
- Absorption: The intestinal wall contains villi-like projections that increase the surface area for absorption of nutrients. Nutrients are absorbed into the bloodstream and distributed throughout the body.
- Waste Elimination: Undigested waste material is eliminated through the anus.
Unique Features
Branchiostoma exhibits several unique features in its digestive system:
- Endostyle: The endostyle is a thyroid hormone-producing gland homologous to the vertebrate thyroid gland, and also plays a crucial role in mucus and enzyme secretion.
- Pharyngeal Slits: These are precursors to the gill slits found in fish and other vertebrates.
- Simple Gut: The gut is a straight tube with limited regional specialization, reflecting its relatively simple digestive needs.
Conclusion
In conclusion, the feeding and digestive system of Branchiostoma is a highly efficient adaptation to its filter-feeding lifestyle. The combination of ciliary action, mucus secretion, and enzymatic digestion allows it to effectively extract nutrients from the water column. Its system provides valuable insights into the evolutionary origins of digestive processes in chordates, particularly the development of pharyngeal slits and the endostyle, which are homologous structures found in more complex vertebrates. Further research into Branchiostoma’s digestive physiology can illuminate the evolutionary trajectory of vertebrate digestive systems.
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.