Arthropod Terrestrialisation & Chelicerates

Question 1

Trilobita

Answer 1

(‘three lobed’)
● Over 20,000 species described
● Very diverse, useful for dating rocks
● Varied from a few millimetres to
~70cm

Question 2

Common themes in arthropod terrestrialisation

Answer 2

Respiration - gills
Cuticle - CaCO3
Excretion - Ammonia
Reproduction - Internal fertilisation, spermatophores
Limbs - Biramous (multi-functional

Question 3

Gas exchange basics:

Answer 3

● Gills: present in larger Crustacea
● Tracheae: independently evolved in Myriapods, Hexapods and Chelicerates

Question 4

Chelicerates have also evolved their gas exchange

Answer 4

Book lungs: a series of
vascularised plates (like pages of a book) held in a pouch of the exoskeleton with a small opening

Question 5

Epicuticle

Answer 5

Hydrophobic waxy layer (can abrade easily)
Cement layer: outside the
waxy layer in some animals,
provides extra mechanical
protection for the waxy layer

Question 6

Nitrogenous waste excretion (crustaceans, chelicerates)

Answer 6

● Crustaceans (excluding hexapods): passed as
ammonium ions via gills – or antennal / green
glands (requires lots of water to flush out toxic
ammonium)
● Chelicerates, Hexapods & Myriapods: Malpighian tubules (evolved at least twice independently) use uric acid excretion (can be
crystallised in order to retain water

Question 7

Chelicerata

Answer 7

● ~93,000 species
Chelicerata body plan:
● Clawlike appendages called chelicerae
● Two sections (tagmata): Cephalothorax and abdomen
● Pedipalps but no antennae
● Four pairs of uniramous walking legs