• Has antagonistic muscles: circular muscle, longitudinal muscle.
• Hydrostatic skeleton: contraction and relaxation of antagonistic muscles raise hydrostatic pressure of the fluid in the coelom
• Chaetae: fine hooks which anchor to the ground

1. Posterior chaetae anchor the ground. Circular muscles in the anterior contract and the longitudinal muscles relax, producing high hydrostatic pressure. These cause the anterior segment to extend forward
2. Anterior chaetae anchor the ground. Posterior chaetae are withdrawn. Circular muscles in the anterior relax and longitudinal muscles contract, producing low hydrostatic pressure. These cause the posterior segments to be pulled forward
3. Continuos process - Peristaltic waves enable earthworms to move forward

• Scaly and slimly body to reduce water resistance
• W-shaped antagonistic muscle blocks are arranged in segments on both sides of a vertebral column named myotomes
• Swim bladder: an air sac inside the abdomen to maintain buoyancy
• Dorsal and ventral fins - prevent yawning and rolling
• Tail fin - forward thrust
• Paired fin (pelvic and pectoral fins) - steering, prevent pitching

1. Right myotomes contract, left myotomes to relax
2. Head and tail will bend to the right
3. Right myotomes relax, left myotomes contract
4. Head and tail will bend to the left
5. Continuous contraction and relaxation of muscles allow fish to move their tail
6. The tail movement creates a forward thrust

Antagonistic muscles:

1. Flexor muscles
2. Extensor muscles:

• Found inside the internal structure of the exoskeleton
• The hind legs are bigger and longer for jumping
• Movement is produced by contraction and relaxation of these muscles

Jumping:

1. At rest, flexor muscles contract, hind legs folded in the shape of "Z"
2. Extensor muscles contract and hind legs straighten 
3. Grasshopper jumps forward and upwards