exam 3 Challenges and Opportunities of Hard Structures Part 3

molluscs foot

• is not encased by shell

• ciliated in primitive forms

• muscular in large forms

• mucus glands in epidermis

  • traction and slippage for crawling

gastropods evolved to

• minimize exposure of foot

• keep entrances to shell small

• add body mass over foot without getting too high

• must coil body over foot

bivalves

• large shell reduces mobility

• filter feeding/ waiting for food

• loss of cephalization

• burrowing or attached to hard substrates

cephalopods

• active or ambush predators

• shells evolved to become: flotation device, reduced (pen), or absent

• foot & mantel = tentacles and siphon

arthropods: cuticle evolved new functions

• harder and stiffer

• a true skeleton with muscles attached

• contains joints, extension, flexion, rotation

advantages of jointed legs

• lower energy cost for locomotion

• exoskeleton is strong but light weight

• movement doesnt deform body

• muscles can be in bundles attached to precise locations

• exploitation of lever principles

levers allow

large muscle contractions move tips of levers small distance = precision

1st legs in ecdysozoans

• short, fleshy appendages

• antagonistic muscle bundles

• hydrostatic skeleton and blood pressure = antagonistic muscles

• appendages similar on every segment

ancestral arthropods

• 1 pair paddle like appendages per segment

• all but head appendages similar

increase leg lenth

move faster

reduction in number of appendages used for locomotion

• longer, efficient movement, ridged body

• differences in leg length

legs project out to side, especially in small arthropods

• stability

• 3 legs down at the same time and 3 moving

divergence in appendages along body

• head- sensation and feeding

• thorax- locomotion

• abdomen- appendages for locomotion & reproduction

arthropods have 2 unusual HOX genes that control limb development

Ubx and abd-A

Ubx and abd-A evolved in arthropods which enabled

evolution of jointed limbs

Antp expressed

in thoracic and abdinubal segments

In insects

• ubx and adba only expressed in abdominal segments

• UBX represses limb development

in brine shrimp

• ubx and abda expressed in thoracic and abdominal segments

• UBX stimulates limb development

insect wings

• outgrowths of dorsal epidermis and overlying exoskeleton

  • muscles attached to base

• oldest know fossil 400 MYA

• modified extensions of legs

  • 2 genes play roles

  • vestigial and wingless