Animal Kingdom · Lesson 6 of 11
Mollusca, Echinodermata and Hemichordata
“Compare mantle, muscular foot, water vascular system and proboscis-collar-trunk body form.”
• Identify molluscan mantle, visceral mass, muscular foot, shell and radula with their functions. • Explain exceptions to the usual molluscan body plan, especially bivalves and cephalopods. • Trace how an echinoderm water vascular system powers tube feet and supports feeding. • Compare adult echinoderm symmetry with larval symmetry and explain its evolutionary significance. • Identify hemichordate proboscis, collar and trunk and distinguish stomochord from notochord.
Three coelomate phyla with very different body plans
A snail gliding on a surface, a sea star grasping a shellfish and an acorn worm burrowing through marine sediment are all triploblastic coelomates, yet their diagnostic structures are unrelated. A mollusc has a mantle and often a muscular foot. An echinoderm has calcareous skeletal elements and a hydraulic water vascular system. A hemichordate has a proboscis, collar and trunk and a characteristic arrangement of pharyngeal structures. Classification asks how these systems are built and work.
Mollusca: mantle, foot and visceral mass
A phylum of mostly unsegmented, soft-bodied animals with a mantle and visceral mass, often a muscular foot and sometimes a calcareous shell.
Molluscs inhabit oceans, freshwater and terrestrial environments. A common snail has a head with sensory tentacles, a broad muscular foot for crawling and a visceral mass containing many internal organs. A mantle—a specialised tissue covering—lies over the visceral region and can secrete the shell. The mantle cavity provides space for gills or respiratory surfaces and often receives excretory and reproductive openings.
A fold or sheet of molluscan body tissue covering the visceral mass and often responsible for shell secretion.
The space between mantle and visceral region where gills or other respiratory structures may occur.
The muscular foot can move an animal over a surface, anchor it or become modified into very different structures. In a squid or octopus, parts corresponding to the ancestral foot are modified into arms and a funnel-like apparatus supporting movement. The presence of a muscular foot in a generic snail drawing should not cause a student to reject Octopus from Mollusca.
Many molluscs have a hard calcareous shell, but shell form varies. A snail typically has one coiled shell, a pearl oyster has two valves, and an octopus has largely lost the prominent external shell. The shell provides protection but may restrict certain movement patterns. The mantle remains a stronger diagnostic feature than the possession of one obvious shell shape.
The radula and why bivalves are an exception
A ribbon-like rasping feeding structure bearing teeth, found in many molluscs but absent in bivalves.
In many snails, the radula moves over a support to scrape food from surfaces or process material. Its small teeth may be adapted to different diets. Bivalves such as Pinctada lack a radula and often filter suspended particles through their gills. Octopuses combine specialised beaks with radular structures. Thus 'all molluscs have a radula' is an overgeneralisation: it is typical of many members but not universal.
Most molluscs have open circulation, although cephalopods such as squid and octopus have a closed circulatory system suited to their active lifestyle. Reproduction is diverse: some are separate-sexed while others can be hermaphroditic, and development may be direct or indirect. The source examples include Pila, Pinctada, Sepia, Loligo, Octopus, Aplysia, Dentalium and Chaetopleura.
Echinodermata: marine animals with calcareous ossicles
A marine animal phylum whose members have calcareous endoskeletal components and a distinctive water vascular system.
Sea stars, sea urchins, brittle stars, sea cucumbers and sea lilies are echinoderms. Their internal skeleton contains calcareous ossicles that can form rigid plates, flexible supports or spines, depending on species. All living echinoderms are marine in the source account. Their main adult body plan is often radial, commonly pentaradial, while embryos and many larval stages show bilateral symmetry.
An adult sea star may seem to have no obvious front and back, but its larva develops a left-right body axis like many other bilaterian animals. This developmental contrast is evidence that the adult radial form is derived rather than a complete absence of bilateral ancestry. A sea star's five arms do not imply that every echinoderm has five large visible arms: sea cucumbers are elongated and sea urchins are rounded.
What the water vascular system does
A specialised fluid-filled hydraulic canal and tube-foot network of echinoderms used in locomotion, attachment, feeding and exchange.
The water vascular apparatus includes fluid-filled canals and many tube feet. A tube foot can extend when hydraulic pressure changes; specialised muscles and connective tissues help it attach, retract or manipulate food. Coordinated tube-foot actions let a sea star move over a surface and exert force on prey. Fluid circulation and large exposed surfaces can also contribute to gas exchange. It is not the same as a sponge canal system, which filters food-laden water through ostia and choanocyte chambers.
The textbook uses a complete digestive system as the broad echinoderm pattern, with mouth and anus, but lineages vary in anatomy. Many echinoderms have separate sexes and release gametes into seawater, so fertilisation is often external and development may include free-swimming larvae. Their waste excretion relies largely on diffusion and associated body surfaces rather than a specialised excretory organ comparable to a vertebrate kidney.
Examples include Asterias, Ophiura, Echinus, Antedon and Cucumaria. Asterias is a typical sea star; Ophiura a brittle star; Echinus a sea urchin; Antedon a sea lily; and Cucumaria a sea cucumber. These forms differ in movement and body geometry while retaining core echinoderm traits.
Hemichordata: proboscis, collar and trunk
A phylum of mostly marine, bilaterally symmetrical, triploblastic coelomate animals with a body differentiated into proboscis, collar and trunk.
Hemichordates such as Balanoglossus and Saccoglossus are elongated marine animals often living in sediment or on the sea floor. The body is divided into an anterior proboscis, a collar and a longer trunk. Their pharyngeal slits and developmental characters are important for understanding their relationships within deuterostome animals, but they are classified separately from Chordata.
The source describes an open-type circulatory system, gill or pharyngeal structures for respiratory exchange and a proboscis gland involved in excretion. Separate sexes and external fertilisation are common descriptions, often followed by an indirect developmental stage. The proboscis is a feeding and burrowing region; it is not a fishlike snout or a segmented arthropod limb.
Stomochord is not the chordate notochord
Older classifications associated hemichordates with chordates because a structure in the collar region, called the stomochord, superficially resembles a supportive rod. Modern comparative anatomy shows it is not equivalent to the chordate notochord. True chordates possess a notochord at least during some life stage, usually along the dorsal body axis in a distinct developmental context. A hemichordate does not become a vertebrate or chordate merely by possessing a stomochord.
| Phylum | Diagnostic structures | Symmetry / habitat | Examples |
|---|---|---|---|
| Mollusca | Mantle, visceral mass, usually foot; shell often, radula in many | Usually bilateral; marine, fresh water or land | Pila, Pinctada, Octopus |
| Echinodermata | Calcareous ossicles, water vascular system and tube feet | Adult usually radial, larva bilateral; entirely marine | Asterias, Ophiura, Echinus |
| Hemichordata | Proboscis, collar, trunk; pharyngeal slits, stomochord | Bilateral; marine | Balanoglossus, Saccoglossus |
Problem
A has a mantle and muscular foot. B moves using hydraulically extended tube feet and has calcareous ossicles. C has a proboscis, collar and trunk. Name their phyla.
- 1.A is a mollusc: mantle and foot are defining features even if no obvious shell is present.
- 2.B is an echinoderm: water vascular apparatus and calcareous ossicles are characteristic.
- 3.C is a hemichordate: its proboscis-collar-trunk organisation is distinctive.
Quick Quiz
Quiz
Which molluscan structure frequently secretes the shell?
Which statement about the radula is accurate?
What is characteristic of adult echinoderms?
How does echinoderm symmetry vary during development?
Which animal is a hemichordate?
Why does Hemichordata not belong inside Chordata?
Which molluscan group is an important exception to the usual open circulation pattern?
Practice Problems
- Explain mantle, visceral mass, muscular foot, mantle cavity and radula in molluscs.
- Describe the water vascular system and compare it with the sponge canal system.
- Why must echinoderm larvae be considered in symmetry classification?
- Give two mollusc, two echinoderm and two hemichordate examples.
- Explain the stomochord/notochord distinction and classify Balanoglossus.
The mantle covers the visceral mass and commonly secretes shell. The foot provides locomotion or specialised appendages. The mantle cavity often contains gills. A radula scrapes food in many molluscs, but is absent in bivalves.
Key Takeaways
Molluscs typically have a mantle, visceral mass and muscular foot; shells and radulae have important exceptions. Echinoderms are exclusively marine with calcareous ossicles and hydraulic water vascular tube feet; larvae are commonly bilateral. Hemichordates have proboscis, collar and trunk regions and marine deuterostome features. A hemichordate stomochord is not a true chordate notochord. Identify phyla through distinctive anatomical systems rather than relying on shell shape, colour or habitat alone.