Question

what are protochordates?why are they considered as a connecting link between invertebrates and vertebrates

Answer 1

Protochordate, any member of either of two invertebrate subphyla of the phylum Chordata: the Tunicata (sea squirts, salps, etc.) and the Cephalochordata (amphioxus).
Like the remaining subphylum of the chordates, the Vertebrata, the protochordates have a hollow dorsal nerve cord, gill slits, and a stiff supporting rod, the notochord, the forerunner of the backbone.
The protochordates differ chiefly from the vertebrates in not having a backbone.
Recent protochordates are thought to have evolved from the same ancestral stock as that which gave rise to the vertebrates.