Final answer:
The structures in the arm of a sea star include the b. radial canal, which is part of the water vascular system and works in conjunction with the tube feet to enable movement and feeding.
Step-by-step explanation:
The structures in each arm of a sea star mentioned in the question include the gonads, the system that works the tube feet, and several other components that are part of the water vascular system. One of the key choices given, the radial canal, is part of the water vascular system and extends along each arm, connecting to a double row of tube feet which are used for movement and feeding. The water vascular system is an intricate network that provides sea stars with their unique movement capabilities and also facilitates gas exchange and nutrient circulation.
The structure in each arm of a sea star include the gonads, the system that works the tube feet, and the madreporite.
The madreporite is a structure located on the top of the sea star's body that regulates the amount of water in its water vascular system. This system consists of a central ring canal and radial canals that extend along each arm. Water circulates through these structures, allowing for gas exchange, nutrient circulation, and locomotion.
The tube feet, unique to echinoderms like sea stars, assist in locomotion, feeding, and chemical sensations. They can also be used to attach some sea star species to the substratum.