Muscular Daphnia

Look at huge adductors and levator muscle visible through the head – (together) they lift the antennae and participate in swimming. 

Their activity is essential for the animal because its body is denser than the water, so if they made no effort, Daphnia would sink. Nevertheless, they cannot be mechanically used (contracted) continuously during swimming, respectively the gap between every one beat of the large antenna causes a jump-like movement – changing of sinking (resting) and refloating (pulling) phase.

Muscles of branchiopods, to which Daphnia belongs, are attached to the outer skeleton (ev. to the abdominal lamina) and don't form a compact muscle mass as in humans. Many of them also can be seen due to body transparency.

Woltereck, R. (1932): Races, associations and stratification of pelagic daphnids in some lakes of Wisconsin and other regions of the United States and Canada. Wisconsin Academy of Sciencies, Arts and Letters 27: 487–522.