1: Arthropods: Trilobites & Chelicerates

Question 1

What are the Arthropods segments like?

Answer 1

Metamerically segmented
Appendages on segments

Question 1

Generally describe the features of arthropods

Answer 1

Ecdysis (moulting)
No spiral cleavage
Coelomates

Question 2

Describe the evolution of segments in Arthropods

Answer 2

Evolution modification by:
Groups of segments specialised → tagmata
- Process of specialisation of segments = tagmosis
- Specialisation of appendages depending on location
= huge variability within arthropods

Question 3

Describe hox genes

Answer 3

First discovered in an arthropod (drosophila)
→ Control anterior-posterior axis
→ Hox genes ‘tell’ cells which segment they are in
→ Can suppress appendage development / modify it to create different morphology

Question 4

What is arthropods cutile primarly made from?

Answer 4

Chitin

Question 5

Describe locomotion in Arthropods

Answer 5

No peristalsis but muscles attached to cuticle (apodemes)
Appendages have joints

Question 6

Describe Arthropod growth

Answer 6

→ Ecdysis
1. Epicuticle secretes proenzymes
- Forms moulting fluid
2. Starts secreting new epicuticle
3. Takes up water/ air to expand cuticle → which new cuticle forms around allowing it to have room to grow

Question 7

Describe the Arthropod eye

Answer 7

Compound eye
Made up of ommatidia (up to 4000)
Sensitive to movement over large field of vision, magnification
But relatively poor res & image formation

Question 8

Describe the Trilobites

Answer 8

Appendages not very specialised
- Biramous (2)
- Gnathobase
- E.g Antenna at anterior end
Vary in size from around 1 mm - 70 cm

Question 9

Describe the body plan of the Chelicerates

Answer 9

2 tagmata:
- Prosoma (feeding, sensory, locomotion)
- Opisthosoma (digestive, respiratory, reproductive)
- No antenna, no distinctive head

Question 10

What are the characteristic features of the Chelicerates?

Answer 10

Characteristic features:
Chelicerae → first appendages
Pedipalps → second appendages (walking legs)

Question 11

Define a Gnathobase

Answer 11

Modified appendeage of an arthropod for carrying or chewing food

Question 12

Describe the body plan of a Horseshoe crab

Answer 12

2 tagmata: prosoma and opisthosoma
Appendages:
- Chelicera
- Pedipalp
- 4 pairs of walking legs
- Gnathobase (grinds up food)

Question 13

Describe the blood of Horseshoe crabs

Answer 13

Haemocyanin → blue blood
Harvested and bled - source of limulus amoebocyte lysate
Substance that detects endotoxins, useful for testing drugs/vaccines
Released afterwards but a lot die = pop. decline

Question 14

Describe the body plan of Sea Spiders (Pycnogonids)

Answer 14

Appendages:
Chelifore
Palps
4 pairs of legs
Proboscis → feeding
Digestive organs and gonads found on legs due to reduce bod size