Phylum Hemichordata: Characteristics, Anatomy, and Evolutionary Significance | Grade 10 Biology, NGSS)
Let's grip the biology of the worm like marine animals Hemichordates
Introduction:
- Phylum Hemichordata consists of a small group of worm-like marine animals that were once thought to be a part of the Chordata family.
📋 Table of Contents
- Today, they are recognized as a separate phylum of invertebrates. Often called "Acorn Worms," these creatures provide a fascinating look into how complex body plans evolved in the ocean.
- Hemichordates are marine invertebrate worm like animals.
- This is phylum which has close relationship with chordates and Echinodermata.
Evolutionary Milestone: The Bridge Between Groups
- Hemichordates are often described as an evolutionary puzzle.
- They represent a critical point in the history of life on Earth because they share characteristics with two major groups:
- The Link to Echinoderms (Starfish):Believe it or not, DNA evidence or Biochemical evidence of evolution shows that Hemichordates are more closely related to Echinoderms than to any other group.
- The Milestone: Both groups have a similar larval stage (Tornaria larva is very similar to the Bipinnaria larva of starfish). This proves they share a common ancestor.
The Link to Chordates (Vertebrates):
- They possess Pharyngeal Gill Slits, which is a "Signature Feature" of all Chordates (including humans!).
- The Milestone: They show the early "blueprint" of a dorsal nervous system and respiratory slits, which eventually led to the development of fish, amphibians, and mammals.
The "Stomochord" Transition:
- The Stomochord was nature's first "experiment" at creating a support rod (Notochord).
- While it didn't become a true backbone, it paved the evolutionary path for the complex skeletal systems we see today.
Key Characteristics :
- Hemichordates have a rudimentary structure in collar region called stomochord which is similar to notochord.
- Animals are worm like, marine with organ system level organisation.
- They are bilateral symmetrical, Triploblastic and Coelomate animals.
- Their Animals are cylindrical and body is divided into proboscis , Collar and trunk.
- Circulatory system is open type.
- Respiration takes place by gills.
- Excretion take place by Proboscis glands.
- Saxes are separated. Fertilisation are external. Development is indirect.
The "Stomochord" Mystery:
One of the most important concepts for Grade 10 Biology is understanding why Hemichordates are NOT true Chordates.
- In the past, scientists thought Hemichordates had a Notochord (a flexible rod found in all chordates).
- However, further research revealed that they actually possess a Stomochord—a tube-like structure that supports the "Proboscis." Because they lack a true dorsal nerve cord and a true Notochord, they are classified as Non-Chordates.
Evolutionary Link Table:
🧬 The Evolutionary Bridge
Echinoderms (Starfish)
Common Larval Traits
Common Larval Traits
⬇️
HEMICHORDATA
(The Missing Link)
(The Missing Link)
⬇️
Chordates (Vertebrates)
Shared Pharyngeal Gill Slits
Shared Pharyngeal Gill Slits
*Hemichordates show how life transitioned from simple invertebrates to complex chordates.)
Scientific Examples:
Balanoglossus ( Acorn worm)
- It is also known as Acorn worm.
- It is deuterostome because anus is developed before mouth.
- It is worm like animals marine animals.
- Body is divided into proboscis, Collar and trunk.
- The alimentary canal is straight starting from mouth and ending with anus.
- It has U shaped paired gills slit.
- Notochord is present on upper part of body without nerve chord.
- Saxes are separated. Fertilisation are external. Development is indirect.
 |
| Acorn worm ( Balanoglossus) |
- It is soft body animals and body is divided into Proboscis collar trunk.
- It is found in spirally coiled burrow in ocean.
- It uses its proboscis to push the body on sand.
- Mouth is located internally at junction of Proboscis and collar.
- The gills are located internally in trunk region.
- It has Glomerulus in proboscis which filter and release the excretory waste.
- The egg are converted into tornaria larva which undergo metamorphosis into young animals.
🎯 Fun Fact for US Students!
🔥 Brain teaser
Did you know? Hemichordates are considered the "Sister Group" to Echinoderms (like Starfish). Even though they look like worms, their DNA is more closely related to sea stars than to earthworms!
Quick Quiz (Check for Understanding):
- What is the primary excretory organ in Hemichordates? (Answer: Proboscis Gland)
- True or False: Hemichordates have a closed circulatory system. (Answer: False - it is Open)
- Which structure was mistaken for a Notochord? (Answer: Stomochord)
Critical Thinking Questions (The "Pro" Level)
- The DNA Paradox: If Hemichordates look like worms but their DNA is closer to Echinoderms (Starfish), how does this challenge our traditional way of classifying animals based on looks alone?
- Evolutionary Experiment: Why do scientists consider the "Stomochord" an evolutionary "failed experiment" or a "precursor" to the human backbone?
- Environmental Impact: Most Hemichordates are "Filter Feeders" or "Deposit Feeders" in the ocean. How would a sudden change in ocean floor sediment affect the survival of these creatures and the marine food chain.
Section 1: Practice Test Paper - A (Focus: Anatomy & Basics)
Total Marks: 45 | Time: 45 Minutes
Part 1: Multiple Choice (5 Qs × 3 Marks = 15 Marks)
-
Which body part of the Acorn Worm is primarily used for burrowing into the sand?
- A) Collar
- B) Proboscis
- C) Trunk
- D) Tail
-
Hemichordates are classified as "Non-Chordates" primarily because they lack a:
- A) Heart
- B) True Notochord
- C) Nervous System
- D) Digestive Tract
-
The circulatory system in Phylum Hemichordata is:
- A) Closed type
- B) Open type
- C) Absent
- D) Double circulation
-
What is the name of the larval stage found in Hemichordates?
- A) Tadpole
- B) Trochophore
- C) Tornaria
- D) Caterpillar
-
The structure once mistaken for a Notochord is now known as:
- A) Endostyle
- B) Stomochord
- C) Nerve cord
- D) Proboscis gland
Part 2: Short Answer (3 Qs × 10 Marks = 30 Marks)
6. Describe the three-part body plan of a typical Hemichordates. (10 Marks)
7. Explain the function of the Proboscis Gland. (10 Marks)
8. Why is Balanoglossus commonly called the "Acorn Worm"? (10 Marks)
Section 2: Practice Test Paper - B (Focus: Evolution & Comparison)
Total Marks: 45 | Time: 45 Minutes
Part 1: True/False & Fill in Blanks (5 Qs × 3 Marks = 15 Marks)
- Hemichordates show bilateral symmetry. (True/False)
- The Stomochord is located in the Proboscis region. (True/False)
- Hemichordates are exclusively marine animals. (True/False)
- Evolutionarily, Hemichordates are more closely related to ________ than Annelids.
- Respiratory exchange in Hemichordates occurs through ________.
Part 2: Detailed Analysis (2 Qs × 15 Marks = 30 Marks)
6. Create a comparison table between Hemichordata and Chordata based on their skeletal support. (15 Marks)
7. Discuss the significance of "Pharyngeal Gill Slits" in linking Hemichordates to higher vertebrates. (15 Marks)
📝. Critical Thinking Questions (The "Pro" Level)
- The DNA Paradox: If Hemichordates look like worms but their DNA is closer to Echinoderms (Starfish), how does this challenge our traditional way of classifying animals based on looks alone?
- Evolutionary Experiment: Why do scientists consider the "Stomochord" an evolutionary "failed experiment" or a "precursor" to the human backbone?
- Environmental Impact: Most Hemichordates are "Filter Feeders" or "Deposit Feeders" in the ocean. How would a sudden change in ocean floor sediment affect the survival of these creatures and the marine food chain
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