Unit 5: Mollusca and Echinodermata
1. Phylum Mollusca
Mollusca (from Latin: mollis = soft) is the second-largest phylum of invertebrate animals. They are soft-bodied animals, usually protected by a hard calcareous shell.
1.1 General Characters and Classification
General Characters
- Body Plan: Unsegmented, soft body with a distinct head, visceral mass, and a ventral muscular foot.
- Mantle: The visceral mass is covered by a fleshy fold of skin called the mantle, which secretes the shell. The space between the mantle and the visceral mass is the mantle cavity, which houses the gills.
- Shell: An external calcareous shell is usually present (it is internal in squid and absent in octopus and slugs).
- Radula: Most molluscs have a rasping, tongue-like organ called the radula in their mouth, used for scraping food.
- Circulatory System: Open type (except in cephalopods, which have a closed system).
- Respiratory Organs: Gills (ctenidia) in aquatic forms, or a "lung" (modified mantle cavity) in terrestrial forms.
- Excretory Organs: Metanephridia (often called kidneys or organs of Bojanus).
- Nervous System: Consists of paired ganglia connected by commissures and connectives.
Classification of Mollusca (Up to Classes)
| Class |
Key Characteristics |
Examples |
| Gastropoda |
Largest class. Asymmetrical body due to torsion. Usually have a single, coiled shell. Well-developed head with tentacles. |
Pila (apple snail), Helix (garden snail), Slug |
| Bivalvia (Pelecypoda) |
Body laterally compressed, enclosed in a shell with two valves. Head is indistinct, radula is absent. Filter feeders. |
Unio (freshwater mussel), Oyster, Clam |
| Cephalopoda |
Most advanced molluscs. Head is large with complex eyes. Foot is modified into arms/tentacles with suckers. Shell is internal or absent. Closed circulatory system. |
Loligo (squid), Sepia (cuttlefish), Octopus |
| Scaphopoda |
Marine, with a tusk-shaped, tubular shell open at both ends. |
Dentalium (tusk shell) |
1.2 Respiration in Mollusca (with reference to Pila)
Pila globosa, the apple snail, is amphibious and exhibits a dual mode of respiration, allowing it to survive both in water and on land.
- Aquatic Respiration (Branchial):
- Performed by a single ctenidium (gill) located in the right side of the mantle cavity.
- Water flows into the mantle cavity, passes over the gill lamellae, and gas exchange occurs between the water and the blood flowing through the gill.
- Aerial Respiration (Pulmonary):
- Performed by a pulmonary sac (lung), which is a highly vascularized chamber in the left side of the mantle cavity.
- When on land or in oxygen-deficient water, Pila extends a respiratory siphon to the surface and draws air into the pulmonary sac for gas exchange.
1.3 Torsion and Detorsion in Gastropods
Torsion
Torsion is a unique developmental process in gastropod larvae where the visceral mass and mantle rotate up to 180° counter-clockwise with respect to the head and foot.
Consequences of Torsion:
- The mantle cavity, gills, and anus are moved from a posterior to an anterior position, just behind the head.
- The digestive tract is twisted into a U-shape.
- The nervous system is twisted into a figure-of-eight.
Advantages: The primary advantage is thought to be defensive. It allows the larva to withdraw its vulnerable head into the protection of the shell first, followed by the tougher foot which closes the opening with its operculum.
Common Mistake: Do not confuse torsion with the coiling of the shell. Torsion is the twisting of the body, while coiling is the spiral winding of the shell. They are two separate evolutionary events.
Detorsion
Detorsion is a secondary, reverse rotation (90°-180°) that occurs in the evolution of some gastropod groups (like sea slugs and land slugs). It is a reversal of torsion, leading to a more streamlined body form and the movement of the anus and gills back towards a posterior position. This is often associated with the reduction or loss of the shell.
2. Phylum Echinodermata
Echinodermata (from Greek: echinos = spiny, derma = skin) are a phylum of exclusively marine, coelomate, and triploblastic animals. They are characterized by their spiny skin and unique water-vascular system.
2.1 General Characters and Classification
General Characters
- Symmetry: Adults exhibit penta-radial symmetry, while their larvae are bilaterally symmetrical.
- Endoskeleton: A calcareous endoskeleton made of plates or ossicles, often with external spines.
- Water-vascular System: A unique hydraulic system of canals and tube feet used for locomotion, food capture, and gas exchange.
- Digestive System: Usually complete, with the mouth on the oral (ventral) surface and the anus on the aboral (dorsal) surface.
li>Circulatory & Excretory Systems: Circulatory system (hemal system) is poorly developed. Specialized excretory organs are absent.
- Reproduction: Sexes are separate. Fertilization is external. Development is indirect, involving a free-swimming larval stage.
- Regeneration: Possess remarkable powers of regeneration.
Classification of Echinodermata (Up to Classes)
| Class |
Key Characteristics |
Examples |
| Asteroidea |
Star-shaped body with five or more arms not sharply set off from the central disc. Tube feet are in open ambulacral grooves and have suckers. |
Asterias (starfish) |
| Ophiuroidea |
Star-shaped body with long, slender, flexible arms sharply set off from the central disc. No suckers on tube feet. Ambulacral grooves are closed. |
Ophiura (brittle star) |
| Echinoidea |
Globular or disc-shaped body, lacking arms. Endoskeleton forms a rigid test. Body covered with movable spines. Have a complex jaw apparatus called Aristotle's lantern. |
Echinus (sea urchin), Clypeaster (sand dollar) |
| Holothuroidea |
Elongated, cucumber-shaped body, soft and leathery. No arms or spines. Endoskeleton is reduced to microscopic ossicles. |
Holothuria (sea cucumber) |
| Crinoidea |
Plant-like body, attached to the substratum by a stalk (in sea lilies). Arms are branched and feathery. Mouth and anus on the oral surface. |
Antedon (feather star), Sea lily |
2.2 Water-vascular System in Asteroidea (Starfish)
The water-vascular system is the most distinctive feature of echinoderms. It is a hydraulic system derived from the coelom.
Structure and Path of Water
- Madreporite: A sieve-like plate on the aboral surface where water enters the system.
- Stone Canal: A short, calcareous tube connecting the madreporite to the ring canal.
- Ring Canal: A circular canal located in the central disc, surrounding the mouth.
- Tiedemann's Bodies: Small pouches on the ring canal that are thought to produce phagocytic cells.
- Radial Canals: Five canals that radiate from the ring canal, one into each arm, running along the ambulacral groove.
- Lateral Canals: Arise from the radial canals, each leading to a tube foot.
- Tube Feet (Podia): Small, hollow, muscular tubes that project from the body. Each consists of an upper, bulb-like ampulla and a lower, sucker-like podium.
Function in Locomotion
- The ampulla contracts, forcing water into the podium.
- The podium extends and its sucker attaches to the substrate.
- The longitudinal muscles of the podium then contract, shortening the tube foot and pulling the animal forward. Water is forced back into the ampulla.
- The sucker releases, and the cycle repeats. Thousands of tube feet work in a coordinated fashion to produce a slow, gliding movement.
Exam Tip: Besides locomotion, the water-vascular system is crucial for prey capture (the tube feet can pry open bivalve shells), adhesion to surfaces, and respiration.