Which of the following character is not found in all the chordates?

• The characteristic features of the phylum Chordata are:
  

1. A dorsal, hollow nerve cord.

2. A notochord at some time during development.

3. Pharyngeal gill slits at some time during development.

4. A muscular post-anal tail at some during development.

• The Diaphragm is a dome-shaped muscular partition that separates the thorax from the abdomen. Its presence is a characteristic feature of only mammals.
  

So, the correct option is 'Diaphragm'.