Evolution
Early in the Paleozoic Era, the mandibulate lineage divided into at least one group that continued a marine lifestyle (the crustacea), and another group that adopted a terrestrial lifestyle. This terrestrial lineage, which encompasses all present-day myriapods and insects
Crustaceans are very diverse in terms of their body shape and form, bearing anywhere from 3 to 50 pairs of limbs. However, crustaceans also share common features such as jointed, paired appendages, and two pairs of antennae. All crustaceans are enclosed in a protective exoskeleton made of chitin, which must be shed (or "moulted") to accommodate growth.
Crustaceans are very diverse in terms of their body shape and form, bearing anywhere from 3 to 50 pairs of limbs. However, crustaceans also share common features such as jointed, paired appendages, and two pairs of antennae. All crustaceans are enclosed in a protective exoskeleton made of chitin, which must be shed (or "moulted") to accommodate growth.
The arthropod's jointed appendage was the key to its success. Over time, the basic limb layout of the ancestral arthropod evolved into specialized forms that were suited for different functions, allowing arthropods to follow new niches and join new habitats.
Arthropod limbs have diversified over evolutionary time into a variety of useful shapes. This diversification was enabled by a few key characteristics of the arthropod limb:
Exoskeleton: With a hard skeleton on the outside and soft tissue on the inside, arthropod legs can be adapted to do tough jobs (like prying open clam shells and slicing up other arthropods).
Joints: Joints allow appendages to flex, pinch, sweep, flap, probe, strike and more. This increased range of motion provided by joints allows the limbs to evolve a variety of functions.
Multiple copies: Having many legs means that there are more legs to handle specialized tasks. Some legs can be used for walking, while others can evolve to probe, grasp or swim.
More Examples:
The Barnacle's Rake
A barnacle uses its delicate legs to gather food particles in the same way that a rake collects leaves. The barnacle sweeps its legs through the water, trapping particles in tiny hairs of the legs. The barnacle then pulls these legs inside its shell and scrapes off the food particles.
The Stomatopod's Sledgehammer
Some stomatopods, crustaceans that look a bit like praying mantises, use their massive forelegs as sledgehammers, smashing the life out of any tasty-looking mollusk or crab that happens to pass by.
Caprella's C-clamp
The tiny crustacean, Caprella, clamps onto seaweed with its hooked rear legs and from there, uses its grasping forelegs to snag any edible morsels that drift by.
The Krill's Leafblower
Some krill feed on small particles on the seafloor. To get to those edible tidbits, the krill use their abdominal appendages as fans to create a dirty cloud of sediment. The krill then backs up into the cloud and uses its other legs and mouthparts to filter bits of food out of the cloud.
The Remipede's Oar
Remipedes use their series of paddle-like legs as oars, propelling themselves through the water in a kind of backstroke (they swim belly up!). The movement of their legs resembles a wave that travels down their bodies as they swim.
The Crab's Crowbar
The coconut crab doesn't let its size slow it down: these 3 - 5 kg (7 - 11 lb) land-dwelling behemoths can climb trees! The crabs' legs are encased in an extra-thick exoskeleton that prevents drying out and equips them for the serious work of digging burrows, catching other crabs, and even prying open fallen coconuts!
Click Here for an excellent visual
Arthropod limbs have diversified over evolutionary time into a variety of useful shapes. This diversification was enabled by a few key characteristics of the arthropod limb:
Exoskeleton: With a hard skeleton on the outside and soft tissue on the inside, arthropod legs can be adapted to do tough jobs (like prying open clam shells and slicing up other arthropods).
Joints: Joints allow appendages to flex, pinch, sweep, flap, probe, strike and more. This increased range of motion provided by joints allows the limbs to evolve a variety of functions.
Multiple copies: Having many legs means that there are more legs to handle specialized tasks. Some legs can be used for walking, while others can evolve to probe, grasp or swim.
More Examples:
The Barnacle's Rake
A barnacle uses its delicate legs to gather food particles in the same way that a rake collects leaves. The barnacle sweeps its legs through the water, trapping particles in tiny hairs of the legs. The barnacle then pulls these legs inside its shell and scrapes off the food particles.
The Stomatopod's Sledgehammer
Some stomatopods, crustaceans that look a bit like praying mantises, use their massive forelegs as sledgehammers, smashing the life out of any tasty-looking mollusk or crab that happens to pass by.
Caprella's C-clamp
The tiny crustacean, Caprella, clamps onto seaweed with its hooked rear legs and from there, uses its grasping forelegs to snag any edible morsels that drift by.
The Krill's Leafblower
Some krill feed on small particles on the seafloor. To get to those edible tidbits, the krill use their abdominal appendages as fans to create a dirty cloud of sediment. The krill then backs up into the cloud and uses its other legs and mouthparts to filter bits of food out of the cloud.
The Remipede's Oar
Remipedes use their series of paddle-like legs as oars, propelling themselves through the water in a kind of backstroke (they swim belly up!). The movement of their legs resembles a wave that travels down their bodies as they swim.
The Crab's Crowbar
The coconut crab doesn't let its size slow it down: these 3 - 5 kg (7 - 11 lb) land-dwelling behemoths can climb trees! The crabs' legs are encased in an extra-thick exoskeleton that prevents drying out and equips them for the serious work of digging burrows, catching other crabs, and even prying open fallen coconuts!
Click Here for an excellent visual