🌊 Phylum Echinodermata — Spiny-Skinned Animals
The term Echinodermata means spiny skin. These are exclusively marine animals with an endoskeleton of calcareous plates. They show radial symmetry in adults and bilateral symmetry in larvae, reflecting their evolutionary transition toward chordates.
🧬 General Characteristics
- Habitat: All are exclusively marine.
- Level of Organisation: They exhibit the organ-system level of organisation.
- Symmetry: A unique feature is that the adult echinoderms are radially symmetrical (typically pentamerous), but the larvae are bilaterally symmetrical.
- Germ Layers: They are triploblastic.
- Coelom: They are coelomate animals.
- Digestive System: The digestive system is complete, with a mouth on the lower (ventral) side and anus on the upper (dorsal) side.
- Water Vascular System: The most distinctive feature is the presence of the water vascular system.
- This system consists of canals filled with sea water that connect to numerous tube feet (podia).
- Functions: It helps in locomotion, capture and transport of food, and respiration.
- Excretory System: An excretory system is absent.
🌊 Reproduction
- Sexes are separate (dioecious).
- Reproduction is sexual.
- Fertilisation is usually external.
- Development is indirect with a free-swimming larval stage (e.g., bipinnaria larva).
💧 Unique Feature: Water Vascular System
- Network of canals aiding in locomotion, respiration, food capture, and excretion.
- Operates through tube feet that use hydraulic pressure.
Example: Asterias (Starfish), Echinus (Sea urchin), Antedon (Feather star), Holothuria (Sea cucumber).
🧠 Economic Importance
- Ecological indicators of marine health.
- Holothuria (sea cucumber) — source of medicinal compounds.
- Echinus used in zoological research.
🪶 Summary Table: Echinodermata
| Feature | Echinodermata |
|---|---|
| Habitat | Marine only |
| Symmetry | Radial (adult), Bilateral (larva) |
| Coelom | True coelomate |
| Skeleton | Calcareous endoskeleton |
| Circulation | Open |
| Locomotion | Tube feet (water vascular system) |
| Example | Starfish, Sea urchin |
🧩 Phylum Hemichordata — The Link to Chordates
Hemichordates are marine worm-like animals once thought to be primitive chordates due to presence of gill slits and rudimentary notochord-like structure (stomochord).
Relationship with Chordates: They share some features with chordates, like pharyngeal gill slits. However, the structure earlier thought to be a notochord (stomochord) is now considered different.
Example: Balanoglossus (Acorn worm), Saccoglossus.
🧬 General Characteristics
- Habitat: This phylum consists of a small group of worm-like marine animals.
- Level of Organisation: They exhibit the organ-system level of organisation.
- Symmetry: They are bilaterally symmetrical.
- Germ Layers: They are triploblastic.
- Coelom: They are coelomate animals.
- Body Structure: The body is cylindrical and composed of an anterior proboscis, a collar, and a long trunk.
Organ Systems
- Circulatory System: Is of open type.
- Respiration: Takes place through gills (pharyngeal gill slits).
- Excretory Organ: Is the proboscis gland.
- Nervous System: Primitive, consisting mainly of a nerve net.
Reproduction
- Sexes are separate (dioecious).
- Fertilisation is external.
- Development is indirect through a free-swimming larva (tornaria larva).
🧠 Significance
- Considered a connecting link between non-chordates and chordates.
- Possess gill slits (chordate feature), but lack a true notochord.
📘 Summary Table: Hemichordata
| Feature | Hemichordata |
|---|---|
| Habitat | Marine |
| Body division | Proboscis, collar, trunk |
| Symmetry | Bilateral |
| Circulation | Open |
| Respiration | Gill slits |
| Example | Balanoglossus |
💡 Questions and Answers
Q1. Why are echinoderms called spiny-skinned animals?
A1. Due to the presence of calcareous spiny endoskeleton.
Q2. What is the function of the water vascular system?
A2. It helps in locomotion, food capture, respiration, and excretion.
Q3. Which larva of echinoderms exhibits bilateral symmetry?
A3. The bipinnaria larva.
Q4. What are the three body regions in Hemichordata?
A4. Proboscis, collar, and trunk.
Q5. Name two animals that show features common to chordates and non-chordates.
A5. Balanoglossus and Saccoglossus (Phylum Hemichordata).
Q6. Describe the functions of the water vascular system in Echinoderms.
A6. The water vascular system is a unique hydraulic system in echinoderms. It is primarily involved in:
- Locomotion: By controlling water pressure in the tube feet, allowing them to extend, retract, and grip surfaces.
- Food Capture and Transport: Tube feet can be used to manipulate and capture prey, and transport food towards the mouth.
- Respiration: Gas exchange can occur across the thin walls of the tube feet and other associated structures (dermal branchiae).
Q7. Compare the symmetry of adult and larval echinoderms.
A7. Echinoderms exhibit a unique change in symmetry during development.
- Larvae: Are bilaterally symmetrical, reflecting the probable ancestry of the group.
- Adults: Are radially symmetrical, typically with pentamerous (five-part) symmetry.
Q8. What are the key characteristics of Phylum Echinodermata?
A8. Key characteristics include:
- Exclusively marine.
- Spiny skin due to an endoskeleton of calcareous ossicles.
- Radial symmetry in adults, bilateral symmetry in larvae.
- Presence of a unique water vascular system with tube feet.
- Absence of a specialized excretory system.
- Complete digestive system (usually).
- Organ-system level of organization, triploblastic, coelomate.
Q9. Why are Hemichordates no longer considered a sub-phylum of Chordata?
A9. Hemichordates were initially placed under Chordata because of the presence of structures like pharyngeal gill slits and a rudimentary structure in the collar region thought to be a notochord (called the stomochord). However, detailed studies revealed that the stomochord is structurally different from the true notochord of chordates. Since they lack a true notochord and a post-anal tail, and their nerve cord is typically ventral (though also dorsal), they are now placed as a separate phylum within the non-chordates, often considered closely related to Echinodermata.
Q10. What are the three distinct parts of a Hemichordate body?
A10. The soft, cylindrical body of a hemichordate is typically divided into three distinct regions:
- Proboscis: An anterior, often extensible structure used for burrowing and feeding.
- Collar: A short, fleshy region behind the proboscis.
- Trunk: The elongated posterior region containing most of the internal organs.
Q11. What type of circulatory system is found in Hemichordates?
A11: Hemichordates possess an open circulatory system. Blood flows through sinuses or open spaces within the body tissues, rather than being confined entirely within blood vessels.
Q12. What is the excretory organ in Hemichordates?
A12. The primary excretory organ in hemichordates is the proboscis gland, located in the proboscis region, which filters waste products from the blood.
Q13. Describe the reproduction and development in Hemichordates.
A13. Hemichordates reproduce sexually. Sexes are separate (dioecious). Fertilisation occurs externally in the surrounding sea water. Development is indirect, involving a characteristic free-swimming larval stage called the tornaria larva, which resembles the bipinnaria larva of echinoderms.