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Questions and Answers
What are echinoderms commonly known as?
What types of habitats do echinoderms primarily occupy?
Marine habitats, from intertidal to deep sea.
Most deuterostomes have __________ cleavage.
radial
Most protostomes have __________ cleavage.
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What is the first opening in protostomes?
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What is the first opening in deuterostomes?
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Match the following classes of Echinodermata with their examples:
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Which of the following are main characteristics of Echinodermata?
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What does pentamerous radial symmetry mean?
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What are ossicles?
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What is the purpose of the water vascular system in echinoderms?
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What does the madreporite do in echinoderms?
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What are tube feet?
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What is one hypothesis regarding the classification of echinoderms?
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Study Notes
Phylum Echinodermata
- Comprises spiny-skinned marine animals with around 7,000 existing species and numerous extinct forms registered in the fossil record.
- Exclusively marine habitat, ranging from intertidal zones to deep-sea environments, primarily benthic.
- Includes notable members like starfishes, brittle stars, sea urchins, sea cucumbers, and sea lilies.
Cleavage Patterns
- Deuterostomes, including echinoderms, exhibit radial cleavage, while most protostomes demonstrate spiral cleavage.
Gastrulation Differences
- In protostomes, the first embryonic opening becomes the mouth, whereas in deuterostomes, it becomes the anus.
Classification of Echinodermata
- Divided into five classes:
- Crinoidea (sea lilies, feather stars)
- Ophiuroidea (brittle stars, basket stars)
- Asteroidea (starfishes or sea stars)
- Holothuroidea (sea cucumbers)
- Echinoidea (sea urchins, sand dollars)
Controversial Hypotheses
- There are two main hypotheses regarding echinoderm classification:
- Asterozoan hypothesis
- Cryptosyringid hypothesis
Main Characteristics of Echinodermata
- Defined by four unique characteristics known as synapomorphies:
- Pentamerous radial symmetry evident in adult forms.
- Calcareous skeleton made of ossicles.
- Mutable collagenous tissue allowing flexibility.
- Unique water vascular system for locomotion and feeding.
Pentamerous Radial Symmetry
- Adults display five-part symmetry around a central axis, evolving from a bilateral ancestor; larvae retain bilateral symmetry.
Calcareous Skeleton
- The mesodermal skeleton is composed of calcium carbonate ossicles, which form various structural configurations from flexible to rigid, contributing to the phylum's name due to spiny texture.
Mutable Collagenous Tissue
- Ligaments composed of collagen are neural-controlled, allowing for adjustments in body rigidity for different species, such as flexible bodies in sea cucumbers and rigid forms in sea urchins.
Water Vascular System
- A fluid-filled canal system originally for respiration and food transport; also aids in locomotion via tube feet. No excretory organs or centralized brain; generally dioecious in reproductive structure with external fertilization.
Madreporite
- A calcareous structure that filters water into the water vascular system, indicating the intake point into the system.
Tube Feet
- Muscular extensions of the water vascular system that function in locomotion and attachment; equipped with suckers, extended by hydraulic pressure from the body's muscular sacs (ampullae).
Classes of Echinoderms
- Major groups include:
- Class Crinoidea
- Class Asteroidea
- Class Ophiuroidea
- Class Echinoidea
- Class Holothuroidea
Class Crinoidea
- Comprises sea lilies and feather stars, primarily suspension feeders with long arms for catching food.
- Sea lilies are typically sessile in deep water, while feather stars can crawl and swim.
- Ancestral forms likely emerged as stalked sea lilies, with the mouth positioned on the upper surface, representing a basal group.
Class Asteroidea
- Characterized by starfishes with varied adaptations for predation and environmental interaction.
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