Final answer:
The tapeworm Echinococcus granulosus enters sheep when they ingest or potentially inhale eggs from contaminated environments, leading to the formation of hydatid cysts inside their organs.
Step-by-step explanation:
The parasite that typically enters through the nasal cavity of sheep is the tapeworm Echinococcus granulosus. This parasite has a complex life cycle that starts when dogs belonging to the family of canids shed tapeworm segments containing fertilized eggs.
If these eggs are ingested by sheep or cattle, they hatch in the animal's intestines, and the resulting embryos can then burrow into the bloodstream and eventually lodge in organs like the liver or lungs, forming a hydatid cyst. While not explicitly mentioned as entering via the nasal cavity, this tapeworm's eggs could potentially be inhaled by sheep when they are around contaminated feces.