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.
The object of a preposition is the noun or pronoun governed by a preposition. The object of a preposition is usually (but not always) the noun or pronoun immediately to the right of the preposition. Here are some examples of objects of prepositions.