Question

What do muscles in an acorn worm work against?

Answer 1

Muscles in an acorn worm work against the organism's hydrostatic skeleton. This fluid-filled cavity surrounded by muscles creates pressure that changes the worm's shape, thus facilitating movement.

Step-by-step explanation:

The muscles in an acorn worm work against the hydrostatic skeleton that they possess. Similar to other invertebrates such as roundworms and earthworms, acorn worms utilize a fluid-filled cavity, known as the coelom, which is surrounded by muscles. When these muscles contract, the shape of the coelom changes, causing a build-up of fluid pressure that results in movement.

As seen in organisms like roundworms, muscles must work against a counterforce to facilitate movement. In the case of roundworms, this is provided by the cuticle and the resulting hydrostatic pressure within the pseudocoelom. Aquatic organisms with a hydrostatic skeleton can control the direction of their movement much more effectively, as opposed to relying on water currents. This system, however, is not as efficient in terrestrial animals compared to aquatic ones.

Other invertebrates like arthropods have a different system altogether, consisting of an exoskeleton. Their movement is achieved through the contraction of muscles that are attached to the exoskeleton, which act across joints between various segments of the rigid external structure. Meanwhile, acorn worms, much like other invertebrates with a hydrostatic skeleton, rely on the fluid pressure provided by their coelom in combination with muscle contraction to achieve movement.

Answer 2

The muscles in an acorn worm work against a hydrostatic skeleton, a fluid-filled structure that enables movement through the contraction of muscles against the fluid pressure within the coelom.

Step-by-step explanation:

The muscles in an acorn worm, which is an invertebrate, work against a hydrostatic skeleton. This type of skeleton is composed of fluid-filled compartments which provide support for muscle contraction, allowing the organism to move. As the muscles contract, they change the shape of these compartments, resulting in movement through the fluid pressure within the coelom. This system is effective for organisms like the acorn worm and is comparable to the movement mechanism observed in roundworms and earthworms that also move via a hydrostatic skeleton.

Roundworms, for example, have a tough cuticle that restricts body expansion, enabling fluid pressure in the pseudocoelom to build up. This pressure acts as a counterforce when muscles contract, promoting efficient movement along solid surfaces. Similarly, earthworms exhibit peristalsis, waves of muscular contractions that allow them to move through soil or substrate. Muscles in organisms with hydrostatic skeletons must work against the pressure of the internal fluids to facilitate locomotion.

Conversely, other invertebrates like arthropods use an exoskeleton for movement. The muscles are attached to the inner surfaces of this external framework, and muscle contractions lead to movements of the various parts of the exoskeleton. This type of locomotion is seen in animals such as crabs, where muscles attached to apodemes — ingrowths of the exoskeleton — allow the movement of appendages like claws.