Stone canal is a part of ______ structure of echinoderms.

A. Digestive

B. Respiratory

C. Water vascular

D. Excretory

The stone canal is an important part of the water vascular system in echinoderms. In sea stars, water enters this system through a sieve-like structure on the upper surface called the madreporite. The madreporite connects to a small sac known as an ampulla, which in turn is linked to the stone canal. As the name suggests, the stone canal has walls typically lined with calcareous (calcium-based) material.

The water vascular system is a hydraulic system unique to echinoderms like sea stars and sea urchins. It plays a vital role in locomotion, respiration, and the transportation of food and water. This system is made up of canals connected to numerous tube feet. Movement occurs when muscles contract to force water into the tube feet, extending them to push against a surface. Relaxation follows, allowing the tube feet to retract.

Although the detailed structure may vary among the five classes of echinoderms, the water vascular system always functions as part of their coelomic cavities, along with the haemal system, perivisceral coelom, gonadal coelom, and perichaetial coelom.

Hence, the correct answer is option (C).