LECTURE 12 - Ecdysozoa: Arthorpoda Flashcards
1
Q
What is chitin?
A
- Before the lower Cambrian radiation, some wormlike ecdysozoans incorporated CHITIN into their cuticles
- Chitin is a long-chain polymer of N-acetylglucosamine that provides a strong, flexible, waterprood surface layer
- Chitin is a shared derived character of Unikonta: it contrivutes to the cell wall of fungi, and the radula of molluscs as well as the cuticle of arthropods
- A thick layer of chitin provides excellent protection but creates difficulties for moulting, respiration, and locomotion
- In particular, a thick-walled, rigid body does not allow wormlike movement; such animals require appendages that can be manipulated by muscles
- Appendages evolved several times late in the Vendian, leading to the phyla collectively called the arthropods
- The cuticle of arthropods is a thick layer of a composite material containing chitin that is secreted by a basal epithelium
- It provides an excellent basis for limb construction, but introduces a major difficulty: the inelastic cuticle prevents growth
2
Q
(IN PANARTHROPODA) What are arthropods?
A
- Small and solid bodied Bilateria use cilia or ciliary tracts for locomotion
- These are incapable of generating much force
- Muscles develop much more force, but act only over a limited distance and cannot forcibly re-extend
- Worms with a spacious coelom use it as a hydrostatic skeleton that permits forceful extension through antagonizing circular and longitudinal muscles
- They can burrow through sediments
- Segmentation facilitates burrowing by digital control of pressure in partially isolated coelomic compartments
- Segmented animals can develop limbs as outgrowths of the body wall that assist in locomotion by increasing friction
- Limbs such as annelid parapodia can then be powered by muscles that create rowing or crawling locomotion
- Limbs supported by a hydrostatic skeleton alone are weak and unable to flex effectively
- They are more effective if enclosed in a stiff tube, so that they do not deform under stress
- A stiff tube in two parts with a hinge in between can be operated as a hinged lever by muscles inserted onto the inside of the tube
- This is a very effective force transducer
- Animals with a stiff exoskeleton that use their limbs as hinged levers operated by internal muscles are the ARTHROPODS
3
Q
What does “Arthropoda” mean literally?
A
“Jointed limb”
4
Q
How are segments grouped in the body plan?
A
- Different kinds of appendage derived from the ancestral limb have become specialized for different functions
- Segments bearing similar kinds of appendage are grouped together into distinct body regions
5
Q
What does the organization of body segments permit?q
A
- It permits the evolution of complex body plans through the specialization of serially homologous limbs
- One important reason is that any given segment can be modified without necessarily affecting other segments
6
Q
Example: what is the serial homology of appendages in a stomatopod?
A
FROM LEFT TO RIGHT
- Sensing
- Antennule
- Antenna
- Food Processing
- Biting/Swallowing
- Mandible
- Maxillae 1-2
- Handling
- Maxillipeds 1-5
- Locomotion
- Walking
- Walking legs 1-3
- Swimming
- Pleopods 1-5
- 5 –> gills: respiration
- Steering, escape
- Uropod
7
Q
What are Tardigrada?
A
- Minute animals living in interstitial water of mosses, etc.
- Piercing mouthparts suck sap from plant cells
- about 400 species
8
Q
What are water bears?
A
- The water bears (phylum Tardigrada) also have fleshy, unjointed legs and use their fluid-filled body cavity as a hydrostatic skeleton
- Water bears are extremely small, and lack circulatory systems and gas exchange organs
- When dry conditions occur, the animal shrinks to a small barrel-shaped object that can survive for at least a decade
9
Q
What are Onychophora?
A
- Sister group to Arthropoda
- Inhabit the litter of humid tropical forests
- Limbs are lobopods that permit slow crawling
- Predators that eat isopods, termites, and small molluscs
- About 100 species
10
Q
What are onychophorans?
A
- Recent molecular evidence links the onychophorans (phylum Onychophora) to arthropod lineages
- Onychophorans have a flexible cuticle that contains chitin, and use their fluid-filled body cavities as hydrostatic skeletons
- They have soft, fleshy, unjointed legs bearing claws that form from outgrowths of the body
- They are probably similar to ancestral arthropods
11
Q
How do Peripatus (phylum onychophora) capture their prey?
A
- Peripatus is a predator which uses a jet of slime to tangle its prey
12
Q
What does the specialization of limb-bearing segments underlie?
A
- It underlies the remarkable diversification of the arthropods
13
Q
(IN ARTHROPODA) What are Pycnogonida?
A
- Marine arthropods with long segmented legs
- Usually small, cryptic and immotile
- All are carnivores feedings on small invertebrates
- About 600 species, mainly in cold oceans
14
Q
(IN ARTHROPODA) What are Trilobita?
A
- Benthic detritivores and scavengers
- Abundant in Cambrian and Ordovician, declining thereafter, few surviving deep-water groups extinct at end-Permian
15
Q
(IN ARTHROPODA) What are Chelicerata?
A
- MEROSTOMATA
- Large marine predators with raptorial chelae to grasp prey
- Conservative morphology; few remaining species
example: Limulus –> Horseshoe crab
