Phylum : Cephalochordata: Characteristics, Anatomy, and Evolutionary Significance | Grade 10 Biology, NGSS)
Let's grip the biology of The Blue print of Vertebrates Phylum Cephalochordata
Introduction: The Blueprint of Vertebrates
- Cephalochordates (popularly known as Amphioxus or Lancelets) are small, fish-like marine animals. They are often called the "Blueprint of Vertebrates" because they possess all the primary chordate characteristics throughout their entire life.
- Unlike Urochordates, their Notochord is not lost but extends from the tip of the snout to the tail.
Table of Contents
- 🔹 Introduction: Cephalochordata
- 🔹 Evolutionary Milestone: The Phylogenetic Bridge
- 🔹 Body Systems of cephalochordates
- 🔹 Example of Cephalochordata : Amphioxus or Brachiostoma
- 🔹 USA Grade 10 Practice Test Papers
- 🔹 Critical Thinking Challenges
- Unlike Urochordates, Cephalochordates possess a notochord that extends from the head to the tail throughout their entire lifespan.
- They are Often called "Lancelets" or Amphioxus. These small, fish-like marine animals are characterized by their transparent bodies and lack of a true bony skeleton or brain.
- They are sedentary filter-feeders that typically remain buried in sandy marine sediments, using their oral cirri to create water currents for feeding.
Evolutionary Milestones of Cephalochordata
- Cephalochordates represent a significant leap in animal evolution.
- Here are the key milestones that bridge the gap between primitive protochordates and advanced vertebrates:
- This is the first group where the Notochord extends beyond the brain to the tip of the snout hence they Acraniata.
- Notochord is present in Cephalochordata from head to tail in throughout the life.
- It provides a rigid axis for the body, allowing for more powerful swimming—a precursor to the vertebral column in humans.
- It is composed of series of single cells and muscles to provide stiffness .
- The body muscles are arranged in V-shaped segments called Myotomes.
- The myotomes are arranged in blocks of muscles which are varies in number.These may be 56 to 64.
- Myotomes are divided into three types- Pre atriopore , atriopore anal and post atriopore anal myotomes.
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| Diagram showing notochord & Myotomes |
- This segmentation (Metamerism) is a huge evolutionary milestone, as it allowed animals to move with greater agility and speed, a trait seen in all modern fishes.
- They possess a hollow, dorsal nerve cord that persists throughout life.
- It is located dorsal to notochord and ventral to dorsal fin.
- Nerve cord bears some sensory eyespot or ocelli for detecting the light intensity.
- In evolution, this is the foundational structure that eventually developed into the complex Brain and Spinal Cord of higher vertebrates.
- Pharynx is specialised structure for filter feeding and respiration, located behind the mouth.
- The Endostyle release mucus along with gills slit. This mucus captures food by the movement of cilia.
- Their pharynx is large and has numerous gill slits. The presence of an Endostyle (which traps food) is an evolutionary link to the Thyroid Gland in humans.
- It shows how a feeding organ evolved into a vital endocrine gland.
- For the first time, we see a closed system of blood vessels.
- Although they lack a "true heart," the blood is pumped by the Ventral Aorta and move blood ti different parts of body.
- Ventral aorta is contractile in nature and marks the beginning of the evolution of the vertebrate heart.
- Blood is colourless and lack respiratory pigments and red blood cells. Due to absence of haemoglobin, gaseous exchange is depend on passive diffusion.
Body system of Cephalochordates
Endostyle :
- Endostyle is a groove in the floor of pharynx in Cephalochordata (like amphioxus).
- It secretes mucus to trap food particles.
- Ciliated cells help move food towards digestive tract.
- Endostyle is considered homologous to thyroid gland in vertebrates.
- It plays role in iodine uptake and food filtering.
Digestive system :
- Amphioxus has a simple digestive system.
- Food enters through mouth, passes into pharynx.
- Pharynx has gill slits for filter feeding.
- Food trapped by mucus in endostyle moves to gut.
- Digestion occurs in midgut (liver-like organ aids digestion).
- Absorption in posterior gut. Waste expelled through anus.
Excretory system:
- Amphioxus has an excretory system with nephridia which are segmentally arranged, paired structures.
- They remove waste from coelomic fluid.
- Excretory pores open near atrium (body cavity).
- Main excretory products in Amphioxus are ammonia and urea.
Table of comparison between Urochordata and Cephalochordata
| Characteristic | Urochordata | Cephalochordata |
|---|---|---|
| Body Structure | Sessile, sac-like | Long, fish-like |
| Notocord | Present in larva, lost in adult | Present throughout life |
| Pharyngeal Slits | Present in adult | Present |
| Feeding | Filter feeding (adult) | Filter feeding |
| Nerve Cord | Absent in adult | Dorsal hollow nerve cord |
| Example | Ascidia (sea squirt) | Amphioxus (lancelet) |
Example of Cephalochordata
Amphioxus :
- It is also known as Lancelet or Branchiostoma.
- Dorsal tubular nerve cord is extended from head to tail .
- Gill silts are present in the Pharynx.
- Post anal tail is also present.
- Presence of myotomes which are useful for locomotion.
- Dorsal, caudal and ventral fins are present.
- The excretory system contains protonephridia.
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| Amphioxus ( Brachiostoma) |
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Finished learning about Phylum Urochordata . Next, dive into our comprehensive study guide on Phylum Cephalochordata : to complete your understanding of the Animal Kingdom
Question :How does the notochord in amphioxus (Cephalochordata) provide an evolutionary advantage?
- Answer: The notochord provides flexibility and support for swimming movements, allowing amphioxus to efficiently filter feed in aquatic environments.
Question : Compare the feeding mechanisms of amphioxus and bivalve mollusks. How are they similar?
- Answer : Both amphioxus and bivalve mollusks are filter feeders. They use structures (pharyngeal slits in amphioxus, gills in bivalves) to trap food particles from water.
Question : What can the simple nervous system of amphioxus tell us about vertebrate evolution?
- Answer : The simple dorsal hollow nerve cord in amphioxus suggests that early chordates had basic nervous systems, which likely evolved into more complex systems in vertebrates.
Question : Why are Cephalochordata like amphioxus important for studying vertebrate evolution?
- Answer : Cephalochordata share key chordate features (notochord, dorsal nerve cord, pharyngeal slits) with vertebrates but are simpler, making them useful models for studying early chordate and vertebrate evolution.
Test Paper: Cephalochordata (Amphioxus) Fundamentals*
Total Marks: 45 | Time: 60 Minutes
Section A: Multiple Choice Questions (10 Marks)
1. Which structure in amphioxus provides axial support and is a characteristic of chordates?
a) Nerve cord b) Notochord
c) Endostyle d) Atriopore
2. How does amphioxus obtain food?
a) Predation b) Filter feeding
c) Parasitism d) Absorption
3. The excretory organs in Cephalochordates are:
a) Kidneys b) Nephridia
c) Malpighian tubules d) Flame cells
4. What is the function of the endostyle in amphioxus?
a) Digestion b) Respiration
c) Mucus secretion for food trapping
d) Excretion
5. Cephalochordates are classified as:
a) Vertebrates b) Invertebrates
c) Chordates d) Echinoderms
6. The circulatory system in Cephalochordates is:
a) Closed b) Open
c) Absent d) Complex
7. How does the notochord contribute to amphioxus movement?
a) Provides rigidity b) Allows flexibility c) Aids in digestion d) No role
8. What type of metamorphosis do Cephalochordates exhibit?
a) Retrogressive
b) Progressive
c) No metamorphosis
d) Paedogenesis
9. The atrium in amphioxus opens to the exterior via:
a) Mouth
b) Anus
c) Atriopore
d) Nephridium
10. Cephalochordates are evolutionary significant because they:
a) Have complex organs
b) Show vertebrate ancestry
c) Are predators
d) Live on land
Section B: Short Answer Questions (15 Marks)
1. Explain how the notochord functions in amphioxus .
2. Describe the filter-feeding mechanism in amphioxus .
3. How does the structure of the pharyngeal slits support the feeding process in amphioxus?
4. Compare the nervous system of amphioxus with that of vertebrates .
5. Why are Cephalochordates considered an evolutionary link?
Section C: Critical Thinking & Application (20 Marks)
1. *Phenomenon:* Amphioxus burrows into sand with its anterior end while filter feeding.
- Explain how its body structure (notochord, musculature) aids this behavior.
2. Analyze how the open circulatory system of amphioxus meets its metabolic needs.
3. *Design an experiment:* Test the hypothesis that water flow affects amphioxus feeding efficiency.
Test Paper 2: Advanced Cephalochordata & Evolution
Total Marks: 45 | Time: 60 Minutes
Section A: True/False & Fill in the Blanks (10 Marks)
1. Cephalochordates have a *dorsal hollow* nerve cord. (True/False)
2. The notochord in amphioxus is present *throughout life*. (Fill in)
3. Cephalochordates exhibit *progressive* metamorphosis. (Fill in)
4. The excretory organs in amphioxus are called *nephridia*. (True/False)
5. Amphioxus is a *marine* organism. (True/False)
Section B: Concept Application (15 Marks)
1. Explain how amphioxus’ body plan reflects adaptations to its environment.
2. Describe the evolutionary significance of the dorsal hollow nerve cord in Cephalochordates .
3. How does studying amphioxus help understand vertebrate evolution?
Section C: Detailed Analysis (20 Marks)
1. Compare the feeding mechanisms of amphioxus and bivalve mollusks – identify similarities and differences .
2. Discuss how the structural features of amphioxus (notochord, pharyngeal slits) provide evidence for chordate evolution .
3. *Model:* Create a simple model explaining how amphioxus regulates internal environmen.
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