Answer:
Chordata.
Step-by-step explanation:
Chordata has a subphylum of its phylum which is vertebrata. The chordates here have a notochord, a flexible rod of tightly packed cells, a tubular nerve chord(dorsal), and gill slits at some stage in their life histories.
Their other subphylum is the protochordates which have some invertebrate and vertebrate features. This suggests that they are a possible evolutionary link between the invertebrate and the vertebrate.