Here's why:

* Lack of a nucleus: Prokaryotes do not have a membrane-bound nucleus, while eukaryotes do. Bacillus anthracis has its genetic material (DNA) located in a region called the nucleoid, but it's not enclosed within a membrane.

* Lack of membrane-bound organelles: Prokaryotes lack other membrane-bound organelles like mitochondria, Golgi apparatus, and endoplasmic reticulum, which are present in eukaryotes. Bacillus anthracis has simpler internal structures.

* Smaller size: Prokaryotes are generally much smaller than eukaryotes. Bacillus anthracis is a rod-shaped bacterium, typically around 1-10 micrometers in length, which falls within the typical size range for prokaryotes.

Therefore, the absence of a nucleus and other membrane-bound organelles classifies Bacillus anthracis as a prokaryote.