Phylum Urochordata: Characteristics, Anatomy, and Evolutionary Significance | Grade 10 Biology, NGSS)
Let's grip the biology of the animals body covered by tunicin test Phylum Urochordata
Introduction
- Urochordates, commonly known as Tunicates, represent a primitive group of marine chordates that serve as a crucial evolutionary bridge between complex vertebrates and simpler invertebrate organisms.
- A defining characteristic of this group is that the notochord is strictly confined to the tail region and is only present during the larval stage of development
- The adult body is enclosed in a protective, leathery outer covering called a Tunic or Test, which is uniquely composed of a cellulose-like organic substance known as Tunicin.
Evolutionary Milestone:
The Appearance of Notochord:
- The term "Urochordata" is derived from the Greek words 'ura' (tail) and 'Chorda' (cord).
- It is a big evolutionary milestones. Although it is present only in larval tail, Hence called Urochordata, but this event is a precursor of apperance of Vertebrates.
- In Urochordata, notochord is rod like structure found in tail of Tadpole larva.
- It is disappeared in adult during the Retrogressive metamorphosis.
- Urochordates are considered as transient link with the other chordates due to apperance of notochord.
- It provides the necessary skeletal support for the larva to swim and find a suitable place to settle.
- The notochord in Urochordates is composed of vacuolated cells, which provide it the necessary turgidity (stiffness) for swimming."
Advanced Pharynx with Gill Slits:
- They developed a Pharyngeal basket which contain thousands of gill slits or stigmata. This basket make them success for filter feeding pattern.
- This Pharyngeal basket extract food from sea water.
- The gills slits are arranged in rows and communicate into an atrial cavity than open exterior through the excurrent siphon.
The Tunic test - A living Armour :
- Bodies of urochordates are covered by the protective test which is made up of tunicin. Tunicin is a cellulose like Polysaccharide.
Tunicin fact
🔥"Presence of cellulose in animal body is a big evolutionary event.
- The test of tunicin provides physical protection against predators and environmental threats.
- The test is secreted by the epidermis and covers the entire body, growing with the animal.
- It is a non-living, extracellular sheath that may contain cells, blood vessels, and sometimes spicules.
- The tunic allows these organisms to exist in various habitats, from shallow water to the deep sea.
Open Circulatory System :
- They have unique open circulatory system with a Reversing Heart.
- Reversing heart signifies that Heart of Urochordates changes its direction of blood circulation. This event is consider as beginning of blood circulation in Vertebrates.
Retrogressive metamorphosis :
- It is a unique metamorphosis in which more advanced larva is developed into less advanced Adult.
- The nerve cord, notochord , nervous system , tail present in larva are lost in the adult.
Progressive Metamorphosis
🔥"In Frog, less advanced tadpole larva is transformed into more advanced Adult.
- The free swimming tadpole larva of Urochordates is transformed into sessile adult that feeds through a filter-feeding mechanism. This is an adaptation of sessile lifestyle.
- The tadpole larva of Urochordates is bridge to higher chordates and provide relationship between tunicates and vertebrates.
Body system of Urochordates:
The Pharyngeal Filter System or Branchial Basket :
- The pharynx is the branchial basket in Urochordata . It is the main organ of respiration and digestion in adult Urochordates.
- Pharynx is barrel-shaped structure that occupies most of the body cavity.
- Water enters through the inhalant or branchial siphon and then pass through numerous gill slits or stigmata in the pharyngeal wall, enters into atrial cavity, and exits through the exhalant or atrial siphon.
- The flow of water in body of Urochordates
Water enters → Branchial siphon→ stigmata → Atrial cavity → Atrial Siphon →Water outside
- The pharyngeal walls are covered in cilia and mucus that trap suspended food particles from the water.
Digestive System
- The digestive system follows ciliary-mucus mode of feeding and include Endostyle and pharynx.
- Endostyle is located along the ventral floor of the pharynx, the endostyle secretes a sticky, viscid mucus.
- Cilia distribute this mucus as a sheet across the inner surface of the pharynx, trapping food particles.
Respiratory System
- Respiration is closely associated with feeding. As water flows through the pharyngeal slits (stigmata), gas exchange occurs.
- During Gaseous Exchange Oxygen is absorbed from the water passing through, and carbon dioxide is released into the water.
- The high number of stigmata provides a large surface area for respiration in the pharyngeal wall.
Nervous System
- By Retrogressive metamorphose , Urochordates larva lost larval nervous system, leaving only the adult ganglion to control the sessile, filter-feeding.
- Tadpole larvae exhibit a typical chordate nervous system, comprising a brain vesicle and a tail nerve cord.
- In adult Stage the nervous system is heavily reduced, consisting mainly of a single ganlion located between the oral and atrial siphons.
Scientific Classification:
Phylum Urochordata is scientifically divided into three classes:
Class: Ascidiacea (The Sea Squirts)
- These are mostly sessile as adults, living on the ocean floor.
- The adult body is covered by a thick, permanent tunic (test) made of tunicin.
- They possess two prominent siphons: an atrial siphon and a branchial siphon for water circulation.
- They undergo retrogressive metamorphosis, where the highly developed larva turns into a simpler adult.
- Example : Herdmania and Ascidia
 |
| Herdmania |
Class: Thaliacea (The Salps)
- These are pelagic (free-swimming) organisms found in the open ocean.
- The body is usually barrel-shaped, with circular muscle bands that help in locomotion.
- Their tunic is very thin and almost completely transparent.
- They often show a complex life cycle with an alternation of generations (asexual and sexual phases).
- Example: Salpa and Doliolum
 |
| Salpa |
Class: Larvacea (Appendicularia)
- These are unique because they retain larval features (like the tail) throughout their entire life. This is a Neoteny.
- They live inside a complex, gelatinous "house" that they secrete and use for filter feeding.
- They possess a permanent tail with a notochord, which is used to create water currents.
- They are generally very small, almost microscopic, compared to the other two classes.
- Example: Oikopleura and Fritillaria.
 |
| Oikolpeura |
Comparison table of three classes of phylum Urochordata
| Key Feature | Class: Ascidiacea | Class: Thaliacea | Class: Larvacea |
|---|---|---|---|
| Habitat & Lifestyle | Sessile (Fixed to seafloor) | Pelagic (Free-swimming) | Planktonic (Free-floating) |
| Body Form | Sack-like; thick tunic | Barrel-shaped; thin tunic | Tadpole-like with tail |
| Adult Tail | Absent (Lost in larva) | Absent | **Persistent** (Present) |
| Unique Concept | Retrogressive Metamorphosis | Muscle bands for jetting | **Neoteny** (Larval house) |
| Examples | Herdmania, Ascidia | Salpa, Doliolum | Oikopleura, Fritillaria |
*Note: Scroll horizontally to view the full table on mobile devices.
Conclusion: The Evolutionary Importance of Urochordates
- Urochordates may look like simple sea-dwelling sacs, but they hold the secrets of our vertebrate origins.
- From the unique Retrogressive Metamorphosis to the presence of the Endostyle (the ancestor of our thyroid gland), these organisms bridge the gap between simple invertebrates and complex chordates.
- Understanding their classification and unique body systems helps us appreciate the diverse paths evolution has taken over millions of years.
Q1: How do tunicates, belonging to Phylum Urochordata, demonstrate the concept of evolutionary adaptation in their larval and adult stages?
- Answer: Tunicates exhibit a unique life cycle where the larval stage has a notochord, a dorsal hollow nerve cord, and a tail, indicating their chordate ancestry. In contrast, the adult stage is sessile, lacks these features, and has a tunic-like body. This transformation shows adaptation to different environments, highlighting evolutionary changes.
Q2:What is the significance of the tunic in Phylum Urochordata, and how does it contribute to their survival?
- Answer: The tunic, made of cellulose-like material (tunicin), provides protection and support to adult tunicates. It helps them survive in marine environments by offering defense against predators and desiccation, allowing them to thrive in various habitats.
Q3.How does the feeding mechanism of tunicates reflect their evolutionary position in the chordate group?
- Answer: Tunicates are filter feeders, using their pharyngeal slits to capture food particles from water. This simple feeding mechanism indicates their primitive chordate features, showing a transition in evolution from simple filter-feeding chordates to more complex vertebrates with diverse feeding strategies.
Q4. Why is the metamorphosis in Urochordates called "Retrogressive"?
- Answer: In most animals, metamorphosis leads to a more complex adult (Progressive). However, in Urochordates like Herdmania, the highly developed larva (with a notochord, nerve cord, and tail) transforms into a much simpler, sessile adult that loses these advanced features. Since the organism "regresses" or moves from a complex to a simpler form, it is called Retrogressive Metamorphosis.
Q5. How does the Endostyle represent an evolutionary link to Humans?
- Answer: The Endostyle in Urochordates is a ciliated groove that concentrates iodine from seawater to help in filter feeding. In higher vertebrates and humans, this same organ evolved into the Thyroid Gland. This homology proves that Urochordates share a common ancestry with vertebrates, including humans.
Q6. If a Larvacean (Class: Larvacea) failed to undergo Neoteny, how would it differ from an Ascidian?
- Answer: Larvaceans exhibit Neoteny, meaning they retain larval traits (like the tail and notochord) as adults. If they didn't, they would likely lose their tail, become sessile (fixed), and develop a thick tunic, making them look and live like Class Ascidiacea. The persistent tail is what allows Larvaceans to remain free-swimming (pelagic) and build their unique "gelatinous houses."
Test Paper 1: Urochordata Fundamentals
Total Marks: 45 | Time: 60 Minutes
Section A: Multiple Choice Questions (10 Marks)
- Urochordata are also known as: a) Lancelets b) Tunicates c) Acrania d) Vertebrates
- The Notochord in Urochordates is present only in: a) Adult tail b) Larval tail c) Adult head d) Throughout life
- The protective covering of Urochordates is made of: a) Calcium b) Chitin c) Tunicin (Cellulose-like) d) Silica
- Which process involves the loss of advanced larval features during adulthood? a) Metamorphosis b) Paedogenesis c) Retrogressive Metamorphosis d) Regeneration
- [Add 6 more MCQs related to Feeding (Ciliary) and Excretion (Neural gland)...]
Section B: Short Answer Questions (15 Marks)
- Define Retrogressive Metamorphosis with one example.
- Why are Urochordates called "Tunicates"?
- Mention the significance of the Atriopore in Herdmania.
- Draw a simple sketch showing the difference between a Urochordate larva and an adult.
- What is the function of the Endostyle in Urochordates?
Section C: Long Answer & Critical Thinking (20 Marks)
- "Urochordate larvae are more advanced than adults." Justify this statement by comparing their nervous system and chordate features.
- Describe the Branchial Sac (Pharynx) of a Urochordate and its role in filter-feeding.
Test Paper 2: Advanced Urochordata & Evolution
Total Marks: 45 | Time: 60 Minutes
Section A: True/False & Fill in the Blanks (10 Marks)
- The circulatory system in Urochordates is of the Open type. (True/False)
- The heart of Urochordates is unique because it shows ______ flow of blood.
- ______ is a common example of a colonial Urochordate.
- (Add more questions on Larval features...)
Section B: Concept Application (15 Marks)
- Explain how the Tunic helps the animal in a marine environment.
- Describe the mechanism of Ciliary feeding in Tunicates.
- Why is the Urochordate larva considered an evolutionary link to higher chordates?
Section C: Detailed Analysis (20 Marks)
- List and explain 5 characteristics that prove Urochordates are Chordates, even if adults don't look like them.
- Discuss the Excretory system (Neural gland, Pyloric gland) and Nervous system of Herdmania in detail.
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