Final answer:
The envelope, particularly the glycoprotein spikes on it, is responsible for membrane fusion between an animal virus and a host cell, allowing the virus to enter and infect the cell.
Step-by-step explanation:
The part of animal viruses involved in membrane fusion with a host cell is the envelope. Specifically, glycoprotein spikes embedded in the viral envelope are used for this process. The envelope itself is derived from the host's membrane structures, and the glycoproteins facilitate the virus's attachment to the host cell, allowing it to subsequently enter and infect the cell. These virus-encoded proteins, such as the ones found on the Human Immunodeficiency Virus (HIV), are critical for the infectivity of the virus. While not all viruses have an envelope, those that do can use it to their advantage in entering host cells.