Model Answer
0 min readIntroduction
Metamorphosis, a biological process of post-embryonic development, involves a striking alteration of an animal’s body structure through cell growth and differentiation. It can be progressive, where the larval form is simpler than the adult, or retrogressive, where the adult is simpler than the larva. Retrogressive metamorphosis is a fascinating phenomenon observed in some urochordates, notably *Herdmania* (formerly *Saccoglossus*). This process involves the reduction and loss of larval structures, leading to a sessile, simplified adult form. Understanding this process provides insights into the evolutionary relationships and adaptations of these marine invertebrates.
Retrogressive Metamorphosis: A Definition
Retrogressive metamorphosis is a type of metamorphosis in which the adult form is structurally simpler than the larval form. This involves the absorption or degeneration of certain larval organs and systems, resulting in a less complex adult. It is commonly observed in tunicates (Urochordata) like *Herdmania*, ascidians, and some other invertebrates.
Illustrated Account of Retrogressive Metamorphosis in Herdmania
The life cycle of Herdmania exhibits a clear example of retrogressive metamorphosis. It can be divided into several stages:
1. Fertilization and Early Development
Fertilization is external, and the zygote develops into a free-swimming larva. The early larval stages include a blastula, gastrula, and eventually a ciliated larva.
2. Tornaria Larva
The larva of Herdmania is a characteristic tornaria larva. It is bilaterally symmetrical, transparent, and possesses the following key features:
- Ciliated bands: Numerous ciliated bands aid in locomotion and food collection.
- Preoral loop: A prominent preoral loop containing a mouth and sensory structures.
- Postoral loop: A postoral loop containing an anus.
- Neural gland: A rudimentary nervous system.
- Muscular system: Well-developed muscular system for swimming.
3. Settlement and Attachment
The tornaria larva eventually settles on a suitable substrate. It attaches itself using adhesive papillae located on its anterior end.
4. Metamorphosis – Regression of Larval Features
This is the core of retrogressive metamorphosis. The following changes occur:
- Absorption of the tail: The posterior part of the larva, including the tail, is gradually absorbed.
- Reduction of the preoral loop: The preoral loop is internalized and contributes to the formation of the pharyngeal basket.
- Development of the mantle: A thick, protective mantle develops, enclosing the body.
- Formation of the siphons: Incurrent and excurrent siphons are formed for water intake and waste expulsion.
- Reduction of the nervous system: The nervous system becomes greatly simplified, consisting of a cerebral ganglion and visceral nerve.
- Development of branchial basket: The pharyngeal slits develop into the branchial basket, used for filter feeding.
5. Adult Herdmania
The adult Herdmania is a sessile, sac-like organism. It lacks the bilateral symmetry and complex features of the tornaria larva. The adult is adapted for a filter-feeding lifestyle and is primarily composed of the mantle, branchial basket, and digestive system.
[Ideally, a diagram illustrating the stages of metamorphosis in Herdmania would be included here.]
Affinities of Herdmania
Herdmania, belonging to the subphylum Urochordata (Tunicata), exhibits affinities to both invertebrates and vertebrates, making its phylogenetic position crucial in understanding chordate evolution.
- Invertebrate Affinities:
- Notochord is transient: The notochord, a defining feature of chordates, is present only in the larval stage and is lost during metamorphosis.
- Lack of a true vertebral column: Adults lack a vertebral column.
- Simple nervous system: The adult nervous system is greatly reduced and lacks a distinct brain.
- Open circulatory system: Possesses an open circulatory system, a characteristic of many invertebrates.
- Chordate Affinities:
- Presence of notochord: The presence of a notochord, even if transient, places it within the Chordata.
- Dorsal hollow nerve cord: A dorsal hollow nerve cord is present in the larva.
- Pharyngeal slits: Pharyngeal slits are present in both larval and adult stages, although their function changes.
- Post-anal tail: A post-anal tail is present in the larva.
Based on these characteristics, Herdmania is considered a primitive chordate, providing valuable insights into the evolutionary origins of vertebrates. Molecular phylogenetic studies further support its position as a sister group to vertebrates.
Conclusion
Retrogressive metamorphosis in <em>Herdmania</em> is a remarkable example of developmental adaptation, showcasing the simplification of body plan from a free-swimming larva to a sessile adult. This process highlights the evolutionary link between invertebrates and chordates, with <em>Herdmania</em> exhibiting a unique combination of both invertebrate and chordate characteristics. Studying this phenomenon provides crucial insights into the evolution of chordate body plans and the adaptive strategies employed by marine invertebrates.
Answer Length
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