Which of the following is the only well-muscled chamber in the heart of a fish?

Correct option is B

Correct Answer: (b) Ventricle
Explanation:
→ The ventricle is the strongest and heavily muscled chamber in the fish heart, responsible for pumping blood forcefully into the circulatory system.
→ Generates the pressure required to push blood through the gills and body.
→ Its thick myocardium differentiates it from other chambers.
Information Booster:
→ Sinus venosus collects blood; atrium transfers it; bulbus arteriosus smoothens flow.
→ Fish heart operates in a single-circuit system.
→ Ventricle size correlates with swimming activity.
Additional Information (Other Options):
(a) Sinus venosus – thin-walled.
(c) Atrium – moderately muscular but not force-pumping.
(d) Bulbus arteriosus – elastic, not muscular.