Here's why:
* Chordata is a phylum that includes animals with a notochord (a flexible rod that supports the body), a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their development.
* Chihuahuas are mammals, and mammals are vertebrates.
* Vertebrates are a subphylum of Chordata, meaning they have a backbone (spinal column) made of vertebrae.
So, chihuahuas, like all other mammals, are part of the Phylum Chordata.