Before the body was sent to the crematorium, shilling and Crump filled the casket with animal bones, meat, and a mannequin.
Object of the preposition: genius. The subject of the sentence is called a noun clause. It is a clause because it has its own subject and verb. It is also a dependent clause which cannot stand on its own because of the word that (a relative pronoun) at the beginning of the clause. A noun clause functions as a regular noun in a sentence.
A prepositional phrase consists of a preposition plus a noun or pronoun (the object of the preposition). Object of the preposition is just a fancy name for the noun or pronoun that that follows the preposition. In our apple example above, apple is the object of all of the prepositions.