Final answer:
If you were only able to observe seminal vesicles in the lab slide, the presence of seminal vesicles alone is not enough to determine the sex of an earthworm.
Step-by-step explanation:
The seminal vesicles are a pair of glands that lie along the posterior border of the urinary bladder and contribute to the production of semen.
However, other parts of the male reproductive system, such as the prostate gland and the bulbourethral gland, also contribute to the production of semen. Therefore, it is important to observe other reproductive structures and organs to accurately determine the sex of an earthworm.
In summary, the presence of seminal vesicles alone is not sufficient evidence to determine the sex of an earthworm. Additional observations of other reproductive structures and organs are necessary to make a conclusive determination.