Model Answer
0 min readIntroduction
*Herdmania*, commonly known as the sea squirt, is a sessile, marine urochordate belonging to the subphylum Tunicata. These filter-feeding organisms are found attached to rocks, shells, or other submerged surfaces in shallow coastal waters worldwide. Their simple body plan and unique developmental stages make them a fascinating subject for zoological study, offering insights into the evolution of chordates. Understanding *Herdmania*’s biology is crucial for comprehending the broader context of invertebrate zoology and chordate phylogeny. This answer will detail its habitat, habits, general features, and developmental stages.
Habitat and Habits
Herdmania typically inhabits shallow, clear, and relatively calm marine environments. They are commonly found attached to rocks, shells, pilings, and other hard substrates in intertidal and subtidal zones. Their distribution is influenced by factors like salinity, temperature, and water currents. They are cosmopolitan in distribution, found in temperate and tropical seas.
- Attachment: *Herdmania* attaches to the substrate via a stalk secreted by the larval stage.
- Feeding: They are filter feeders, drawing water in through the incurrent siphon, filtering out phytoplankton and organic particles, and expelling water through the excurrent siphon.
- Sessile Lifestyle: Adult *Herdmania* are sessile, remaining permanently attached to their substrate.
- Solitary or Colonial: They can exist as solitary individuals or form small colonies.
General Features
Morphology
The adult *Herdmania* exhibits a simple, sac-like body plan. It is covered by a tough, gelatinous tunic, hence the name Tunicata. Key features include:
- Tunic: A cellulose-rich outer covering providing protection.
- Siphons: Two openings – the incurrent siphon (for water intake) and the excurrent siphon (for waste expulsion).
- Branchial Basket: An internal structure containing gill slits used for respiration and filter feeding.
- Atrium: The cavity surrounding the branchial basket.
- Visceral Mass: Contains the digestive, reproductive, and excretory organs.
Physiology
*Herdmania* exhibits several unique physiological adaptations:
- Circulatory System: An open circulatory system with a dorsal heart that pumps hemolymph (blood) through sinuses.
- Respiratory System: Gas exchange occurs through the gill slits in the branchial basket.
- Nervous System: A simple nervous system consisting of a cerebral ganglion and a visceral nerve.
- Excretory System: Excretion occurs through the nephridia, which filter waste products from the blood.
Anatomy
The internal anatomy of *Herdmania* is relatively simple. The body cavity is spacious and contains the visceral organs. The notochord, a defining feature of chordates, is present only in the larval stage and is lost during metamorphosis. The presence of a post-anal tail is also limited to the larval stage.
| Feature | Description |
|---|---|
| Notochord | Present in larva, absent in adult |
| Post-anal Tail | Present in larva, absent in adult |
| Gill Slits | Numerous, used for filter feeding and respiration |
| Tunic | Protective outer covering made of cellulose |
Development
The development of *Herdmania* is characterized by a unique life cycle involving metamorphosis. It includes the following stages:
- Fertilization: External fertilization occurs in the water column.
- Cleavage: Radial and determinate cleavage leads to the formation of a blastula.
- Gastrulation: Gastrulation results in the formation of a gastrula with an archenteron (primitive gut).
- Larval Stage: A free-swimming larva emerges, possessing a notochord, post-anal tail, and a well-developed nervous system – exhibiting chordate characteristics.
- Metamorphosis: The larva settles on a substrate, undergoes metamorphosis, and transforms into the sessile adult form. During metamorphosis, the tail and notochord are resorbed, and the siphons and branchial basket develop.
Conclusion
In conclusion, *Herdmania* represents a fascinating example of a urochordate, showcasing a unique blend of invertebrate and chordate characteristics. Its simple body plan, filter-feeding habits, and distinctive developmental stages provide valuable insights into the evolution of chordates. The loss of the notochord and post-anal tail during metamorphosis highlights the dramatic changes that occur during its life cycle. Further research into *Herdmania* and other urochordates will continue to refine our understanding of chordate phylogeny and development.
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.