Name the oldest living species of animal.
The oldest living species of animal is believed to be the glass sponge (Hexactinellida), which has been in existence for approximately 750 million years.
Complete the following task based on the instruction and optional input provided.### Instruction:
Name the oldest living species of animal.

### Output:
The oldest living species of animal is believed to be the glass sponge (Hexactinellida), which has been in existence for approximately 750 million years.