BIOEE 1780 Lecture Notes - Lecture 16: Synapomorphy, Pancrustacea, Crustacean
7 May 2018
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Ecdysozoans Indented Phylogenetic Tree
Ecdysozoans [molt cuticle]
Nematodes
Arthropods [jointed appendages]
Trilobites
Chelicerates
Myriapods
Pancrustaceans
Crustacea
Insecta
Ecdysozoans: one of the two major clades of protostomes
o Synapomorphy: organisms increase in size by molting (ecdysis) their cuticle and replacing it with a
new, larger one.
o Before molting, a new cuticle is already forming underneath the old one.
o Until the new cuticle hardens, the animal is vulnerable. Its outer surface is easy to penetrate
and the animal moves slowly or not at all.
• Cuticle: an outer layer of non-living tissue that is excreted from the epidermis
o Provides protection and structural support
o Once formed, it doesn't grow.
o Thin cuticles allow the exchange of gases, minerals, and water across the body surface.
• Examples: arthropods, nematodes
Arthropods ("jointed foot"): have appendages which aid in walking, swimming, gas exchange, food
capture/manipulation, and sensory perception.
• Synapomorphy: jointed appendages
• Internal digestion
• Muscles are attached to the inside of the exoskeleton
• Have a brain and ventral nerve cord
o Some have segmental ganglia, which are clusters of nerves in segments along the body
• These are control centers that are separate from the brain and can act independently.
• In some species, removal of the brain still allows organisms to walk.
• Most diverse group of animals
• Segmented bodies
o Three major regions:
• Head
• Thorax
• Abdomen
o Each region can be composed of multiple segments.
o One pair of appendages per body segment, although these may be lost.
o This modularization has allowed for the independent development and evolution of regions and
appendages.
o Segments are modified for different functions:
• Antennae - sensing
• Mouthparts - tasting, manipulating food
• Claws - grabbing food
• Walking legs - walking
• Abdominal apps - swim, hold eggs, mating
