Final answer:
The ureters are the structures that enter the urinary bladder at oblique angles to form valvelike openings, which are essential for preventing the backflow of urine.
Step-by-step explanation:
The structure that enters the urinary bladder at oblique angles, forming valvelike openings that prevent backflow of urine as the bladder fills, is the ureters.
The ureters are tube-like structures that connect the kidneys to the urinary bladder. They are designed to transport urine in one direction, ensuring that when urine enters the bladder, it does not reflux back into the ureters. The oblique entry of the ureters into the bladder creates a one-way valve effect that is crucial for proper urinary system function. This design helps prevent urinary tract infections (UTIs) by disallowing urine from flowing back to the kidneys and possibly spreading infection.